Earthquake Report: M 7.5 Guatemala

In 1976 there was a devastating earthquake in Guatemala with a magnitude of M 7.5.

https://earthquake.usgs.gov/earthquakes/eventpage/usp0000ex3/executive

The 2026 PATA Days (Paleoseismology, Active Tectonics, and Archeoseismology) is being held in Guatemala this year. Fifty years since the earthquake. Geologists will likely discuss this earthquake to a great extent (and this was the inspiration for me to prepare this report).

This will be the 13th International INQUA Meeting on Paleoseismology, Active Tectonics and Archaeoseismology. There is room for 100 participants.

There were many adobe style houses that were destroyed.

There were about 23,000 fatalities and 76,000 injured.

The Polochic-Motagua fault system is a left-lateral strike-slip plate boundary fault system separating the North America plate to the north and the Caribbean plate to the south.

The 7.5 slipped along this fault and has a strike-slip earthquake mechanism. Because of the mechanism and the mapped fault, we can interpret this as a left-lateral strike-slip fault.

This means that if one is standing on one side of the fault, and they look across the fault during this earthquake, things on the other side of the fault would have moved to the left. It works for being on either side of the fault.

Geologists from around the world made key observations in the days to weeks after the earthquake.

They measured fault offsets, documented building damage, described earthquake triggered landslides and other mass wasting, and interpreted earthquake intensity.

They installed a field network of seismometers to gather important aftershock records. These can help develop the geometry of the earthquake fault or faults involved.

George Plafker joined dozens of other U.S. Geological Survey geologists to map the fault rupture. I heard that David Schwartz (USGS) was thinking of attending PATA Days as he did some work on this earthquake. I hope he is there!

I noticed that Tina Niemi and Christoph Grützner have been working on fault trenching across the Motagua fault and I suspect we will learn more about their and their collaborators’ work at the meeting! They did present some of their findings at the 12th PATA Days in Chile! https://libros.uchile.cl/files/presses/1/monographs/1446/submission/proof/146/

Before the meeting, I visited Tikal. During my undergrad years, I spent some time taking classes to on the Maya hieroglyphics (about 30 years ago now!).

When I went to see the Grand Plaza, I overheard a tour guide giving an English tour. Jose described the buildings surrounding the plaza. The two temples. The cemetery on one side and the kin’s mansion on the other.

I walked up to thank him and shared with him some of what I remembered about the glyphs. We both learned from each other that day.

My classes were taught by Alex Jones, History prof at Humboldt State. He and his wife, Carolyn, took some of us (like, me) to Austin Texas where they held an annual hieroglyph workshop.

There, I met Linda Schele. Schele (rest in peace) was an artist who was key to unravelling the mysteries of the glyphs.

The glyphs have two components. They have sounds and they have symbols that have meanings. It was the combination of these two features that allowed people like Michael Coe to decipher them successfully.

The craftspersons who made the glyphs were artists. So, Linda was able to recognize the key symbology in each glyph and how the craftspeople imparted individual styles into the glyphs.

This information allowed us to fully identify almost all the glyphs.

There is a syntax to most of the carvings and identifying the glyphs was key.

The second part was the sounds that each glyph makes. There exist two codices that have, side by side, glyphs and Spanish. Using the pronunciation of colonial Spanish (e.g., where the “x” has a “sha” sound), we were able to learn the sounds of the glyphs.

Then, we learned that the 32 Maya languages today include these sounds. The Maya language is alive. And these hieroglyphs can be sounded out using (basically) the modern Maya language.

It was a collaboration between linguists, artists, and the modern Maya people, who helped unravel the Maya Hieroglyphs!

Just like we are today learning how it is important to use transdisciplinary work to push forth and advance society (e.g., combining both social and physical sciences).

I toured the two museums at Tikal. Something that was sad for me was to have been to museums around the world that had much better curated collections of Maya artifacts. The world needs to invest in these collections from where they come. Both are needed, good museums globally and good museums locally (so that the modern Maya can appreciate their wealth).

The museum at Tikal should have a real guest center, with books and other materials for sale. There were artists selling local art (and i supported them). But if the museum had a book/gift store, more money could go into supporting the museums and the ongoing restoration/maintenance efforts.

OK, now on to the 1976 earthquake…

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1925-2025 with magnitudes M ≥ 3.0 in one version.
  • I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a small scale map showing the major plate boundaries.
  • In the lower right corner is a map that shows a comparison of earthquake intensity between the USGS models and the Did You Feel It observations.
  • In the upper right corner is a map that shows the results of the USGS earthquake triggered landslide and earthquake induced liquefaction model. Read more about this model here.
  • In the upper center is a map inset and cross section. The map shows seismicity plotted as colored circles. The cross section B-B’, with the location on the main map and the map inset, shows how there are earthquakes within the subducting Cocos plate.
  • In the right-center is a map from Mann et al. (2007) that shows the tectonic setting for the entire region.
  • Here is the map with a month’s seismicity plotted.


Other Report Pages

Shaking Intensity

  • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Both data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity.
  • Below the map and the lower plot is the USGS MMI Intensity scale, which lists the level of damage for each level of intensity, along with approximate measures of how strongly the ground shakes at these intensities, showing levels in acceleration (Peak Ground Acceleration, PGA) and velocity (Peak Ground Velocity, PGV).


    • Here is the Espinosa et al. (1978) map showing shaking intensity (MMI contours). This is from the USGS report that we include later in the report.


      • Modified Mercalli intensity distribution in Guatemala from the main event. Circle indicates epicenter location of the February 4 earthquake; dashed line indicates approximate isoseismal. (Base map modified from Guatemala Institute Geografico Nacional, 1974, 1:500,000.)

  • Here is a map from Porfido et al., 2015 showing intensity observations from field observations (e.g., surface faulting, ground failure, etc.)


    • Map showing the main epicenter of 4 February 1976, Guatemala earthquake, the Motagua surface fault, the total area of secondary effects (modified after Plafker et al. 1976; Harp et al. 1981). The number indicates the main localities with the intensity value according to ESI scale (considering surface faulting, slope movements, ground cracks, liquefaction phenomena and ground settlements)

  • This map (Caccavale et al., 2019) shows intensity from the 1976 earthquake.
  • The dashed lines are called isoseismal lines. Each line represents the same intensity, like an intensity contour.
  • These researchers were developing a seismic hazard model for Guatemala and I present the results from this later in the report.


Potential for Ground Failure

  • Below are a series of maps that show the potential for landslides and liquefaction. These are all USGS data products.
    There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:

    FOS = Resisting Force / Driving Force

  • When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.


  • Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
  • Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
  • An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
  • Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.


  • Here Dr. Bohon demonstrates the phenomena of liquefaction.
    • @drwendyrocksit #Liquefaction is a process by which water-saturated sediment temporarily loses strength and acts as a fluid. This can happen during #earthquake shaking. #geophysics #geology ♬ Quicksand – Hatchie

  • And, another video demonstration.

  • Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.


  • Below is the liquefaction susceptibility and landslide probability map (Jessee et al., 2017; Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides.
  • I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. Learn more about how the USGS prepares these model results here.


    •

Some Relevant Discussion and Figures

    The Subduction Zone

    • Here is tectonic map from Franco et al. (2012).


      • Tectonic setting of the Caribbean Plate. Grey rectangle shows study area of Fig. 2. Faults are mostly from Feuillet et al. (2002). PMF, Polochic–Motagua faults; EF, Enriquillo Fault; TD, Trinidad Fault; GB, Guatemala Basin. Topography and bathymetry are from Shuttle Radar Topography Mission (Farr&Kobrick 2000) and Smith & Sandwell (1997), respectively. Plate velocities relative to Caribbean Plate are from Nuvel1 (DeMets et al. 1990) for Cocos Plate, DeMets et al. (2000) for North America Plate and Weber et al. (2001) for South America Plate.

    • These figures are from the USGS publication (Benz et al., 2011) that presents an educational poster about the historic seismicity and seismic hazard along the Middle America Trench.
    • First is a map showing earthquake depth as color (green depth > red). Seismicity cross section B-B’ is shown on the map. Today’s M=6.6 quake is nearest this section.



      •

    • Here is a map showing cross section locations in a following figure from Espinosa et al. (1978).
    • The map shows epicenters for earthquakes (their location plotted on Earth’s surface). The cross sections are labeled A, B, C, D, and E.


    • I could not connect to the AWS server from Guatemala and will upload a screenshot of the figure caption later. For now, I write a brief caption.

      • Seismicity (1,331 earthquakes) from NOAA Hypocenter database. Five cross sections are taken perpendicular to the Middle America trench.

    • Here are the cross sections showing the seismicity associated with subduction (much like the Benz figure above, though these 70’s era hypocenters are poorly constrained, so the Cocos plate is poorly defined).


    • I could not connect to the AWS server from Guatemala and will upload a screenshot of the figure caption later. For now, I write a brief caption.
    • Here is a map showing the spreading ridge features, along with the plate boundary faults (Mann, 2007). This is similar to the inset map in the interpretive poster.


      • Marine magnetic anomalies and fracture zones that constrain tectonic reconstructions such as those shown in Figure 4 (ages of anomalies are keyed to colors as explained in the legend; all anomalies shown are from University of Texas Institute for Geophysics PLATES [2000] database): (1) Boxed area in solid blue line is area of anomaly and fracture zone picks by Leroy et al. (2000) and Rosencrantz (1994); (2) boxed area in dashed purple line shows anomalies and fracture zones of Barckhausen et al. (2001) for the Cocos plate; (3) boxed area in dashed green line shows anomalies and fracture zones from Wilson and Hey (1995); and (4) boxed area in red shows anomalies and fracture zones from Wilson (1996). Onland outcrops in green are either the obducted Cretaceous Caribbean large igneous province, including the Siuna belt, or obducted ophiolites unrelated to the large igneous province (Motagua ophiolites). The magnetic anomalies and fracture zones record the Cenozoic relative motions of all divergent plate pairs infl uencing the Central American subduction zone (Caribbean, Nazca, Cocos, North America, and South America). When incorporated into a plate model, these anomalies and fracture zones provide important constraints on the age and thickness of subducted crust, incidence angle of subduction, and rate of subduction for the Central American region. MCSC—Mid-Cayman Spreading Center.

    • Here are 2 different figures from Mann (2007). First we see a map that shows the structures in the Cocos plate. Note the 3 profile locations labeled 1, 2, and 3. These coincide with the profiles in the lower panel.


      • Present setting of Central America showing plates, Cocos crust produced at East Pacifi c Rise (EPR), and Cocos-Nazca spreading center (CNS), triple-junction trace (heavy dotted line), volcanoes (open triangles), Middle America Trench (MAT), and rates of relative plate motion (DeMets et al., 2000; DeMets, 2001). East Pacifi c Rise half spreading rates from Wilson (1996) and Barckhausen et al. (2001). Lines 1, 2, and 3 are locations of topographic and tomographic profi les in Figure 6.

    • Here are 2 different views of the slabs in the region. These were modeled using seismic tomography (like a CT scan, but using seismic waves instead of X-Rays). The upper maps show the slabs in map-view at 3 different depths. The lower panels are cross sections 1, 2, and 3. Today’s M=6.6 earthquake happened between sections 1 & 2.


      • (A) Tomographic slices of the P-wave velocity of the mantle at depths of 100, 300, and 500 km beneath Central America. (B) Upper panels show cross sections of topography and bathymetry. Lower panels: tomographic profi les showing Cocos slab detached below northern Central America, upper Cocos slab continuous with subducted plate at Middle America Trench (MAT), and slab gap between 200 and 500 km. Shading indicates anomalies in seismic wave speed as a ±0.8% deviation from average mantle velocities. Darker shading indicates colder, subducted slab material of Cocos plate. Circles are earthquake hypocenters. Grid sizes on profi les correspond to quantity of ray-path data within that cell of model; smaller boxes indicate regions of increased data density. CT—Cayman trough; SL—sea level (modifi ed from Rogers et al., 2002).

    • Below is a video that explains seismic tomography from IRIS.

    • Here is the McCann et al. (1979) summary figure, showing the earthquake history of the region.


      • Rupture zones (ellipses) and epicenters (triangles and circles) of large shallow earthquakes (after KELLEHER et al., 1973) and bathymetry (CHASE et al., 1970) along the Middle America arc. Note that six gaps which have earthquake histories have not ruptured for 40 years or more. In contrast, the gap near the intersection of the Tehuantepec ridge has no known history of large shocks. Contours are in fathoms.

    • Here are some figures that show how the subduction zone varies across the Tehuantepec Ridge. More about this in my initial report, as well as in my reports for the M 8.1 earthquake.
    • This is a figure showing the location of the Tehuantepec Ridge (Quzman-Speziale and Zunia, 2015).


      • Tectonic framework of the Cocos plate convergent margin. Top- General view. Yellow arrows indicate direction and speed (in cm/yr) of plate convergence, calculated from the Euler poles given by DeMets et al. (2010) for CocoeNoam (first three arrows, from left to right), and CocoeCarb (last four arrows). Length of arrow is proportional to speed. Red arrow shows location of the 96 longitude. Box indicates location of lower panel. Bottom- Location of features and places mentioned in text. Triangles indicate volcanoes of the Central American Volcanic Arc (CAVA) with known Holocene eruption (Siebert and Simkin, 2002).

    • Here is another figure, showing seismicity for this region (Quzman-Speziale and Zunia, 2015).


      • Seismicity along the convergent margin. Top: Map view. Blue circles are shallow (z < 60 km) hypocenters; orange, intermediate-depth (60 < z < 100 km); yellow, deep (z > 100 km). Next three panels: Earthquakes as a function of longitude and magnitude for shallow (blue dots), intermediate (orange), and deep (yellow) hypocenters. Numbers indicate number of events on each convergent margin, with average magnitude in parenthesis. Gray line in this and subsequent figures mark the 96 deg longitude.

    • This shows the location of the cross sections. The cross sections show how the CP changes dip along strike (from north to south) (Quzman-Speziale and Zunia, 2015).


      • Location of hypocentral cross-sections. Hypocentral depths are keyed as in previous figures.

    • Here are the cross sections showing the seismicity associated with the downgoing CP (Quzman-Speziale and Zunia, 2015).


      • Hypocentral cross-sections. Depths are color-coded as in previous figures. Dashed lines indicate the 60-km and 100-km depths. Tick marks are at 100-km intervals, as shown on the sections. There is no vertical exaggeration and Earth’s curvature is taken into account. Number of sections refers to location on Fig. 3.

    • This figure shows thrust and normal earthquakes for three ranges of depth (Quzman-Speziale and Zunia, 2015).


      • Earthquake fault-plane solutions from CMT data. a. Shallow (z < 60 km), thrust-faulting mechanisms. b. Intermediate-depth (60 < z < 100 km) thrust-faulting events. c. Deep (z > 100 km), thrust-faulting earthquakes. d. to f. Normal-faulting events, in same layout as for thrust-faulting events.

Strike-Slip Motagua fault

  • The Motagua and related faults are connected to strike-slip/transform plate boundary faults to the east.
  • Let’s look at those fault systems first.
  • Here is the tectonic map from Garcia-Casco et al. (2011). I include their figure caption below in blockquote.


    • Plate tectonic configuration of the Caribbean region showing the location of the study cases presented in this issue (numbers refer to papers, arranged as in the issue), and other important geological features of the region (compiled from several sources).

  • This is another map showing earthquake history, fault location, and earthquake slip direction from Calais et al. (2016). Note how the relative plate motion near Puerto Rico is oriented parallel to the plate boundary (the Puerto Rico trench). This suggests that most of the plate motion would result in strike-slip earthquakes. However, the relative motion is oblique, so subduction zone earthquakes are still possible.


    • Seismicity and kinematics of the NE Caribbean. The inset shows Caribbean and surrounding plates, red arrows show relative motions in cm/yr: a: NEIC seismicity 1974–2015 is shown with circles colored as a function of depth, stars show large (M > 7) instrumental and historical earthquakes; b: red and blue bars show earthquake slip vector directions derived from the gCMT database [www.globalcmt.org], black arrows show the present-day relative motion of the NA plate with respect to the Caribbean.

    • Here is a map from Calais et al., 2023 that shows the regional tectonics of the Greater Antilles.
    • From top to bottom, the maps show the seismicity, the earthquake mechanisms, and the GPS plate velocities (see more about geodesy further down in the report).


      • Seismotectonic and plate kinematic setting of the Northern Caribbean plate boundary. Top: 1978 to 2023 seismicity along the Northern Caribbean plate boundary according to the USGS/NEIC catalog, M>3. Note the earthquake cluster north of Cuba (81oW/21oN) and the M<6 events along the western continuation of the Puerto Rico -- North Hispaniola trench round 75oW/20.5oN), which are discussed in the text. Center: source mechanisms for earthquakes with a moment magnitude greater than 6, according to the gCMT catalog. Bottom: GNSS-derived velocities with respect to the Caribbean plate. Note the transition from shallow strike-slip events and plate boundary kinematics west of ~74oW, to a combination of strike-slip and plate-boundary-shortening east of that longitude. OF: Oriente fault, WP: Windward Passage, SF: Septentrional fault, EPG: Enriquillo-Plantain Garden fault, NHF: North Hispaniola fault, PRT: Puerto Rico trench, MT: Muertos trench, CSC: Cayman spreading center, PR: Puerto Rico.

    • Here is a map from Calais et al., 2023 that shows the earthquake mechanisms associated with the Oriente fault.
    • Most of the mechanisms show the left-lateral strike-slip motion along this fault. Though there are some compressional mechanisms (reverse faults) that are in places where the strike-slip fault changes orientation in a way that causes compression.


      • Seismotectonic map of Cuba and its surrounding showing the major active and potentially active faults (discussion in the text), the location of the GNSS sites (continuous and episodic) used in this study, the M>3 seismicity from 1978 to 2023 from the USGS/NEIC catalog, and the focal mechanisms for M>5 earthquakes from the CMT database. One exception is the Mw4.7, Dec 17 2022 normal faulting on a NE-SW oriented fault plane off the North Cuban fault at the toe of the Bahamas Platform. CCB: Cabo Cruz basin, SDB: Santiago Deformed Belt.

    • Here is the map from Mann et a. (1991). Note how today’s earthquake is in an area that may have overlapping faults of different types.


      • A. Tectonic map of Cayman trough region showing strike-slip faults (heavy lines), oceanic crust (gray) in Cayman trough, and magnetic anomaly identifications (numbered bars) (after Rosencrantz et a., 1988). Arrows show relative displacement directions. Fault zones: OFZ – Oriente; DFZ- Dunvale; EPGFZ – Enriquillo-Plantain Garden; WFZ – Walton; SIFZ – Swan Islands; MFZ – Motagua. Bl. Late Miocene reconstruction of Cayman trough. C. Early Miocene reconstruction.

    • This is a map from ten Brink et al., 2002. Earthquake mechanisms are plotted for events along the Oriente, Sean Island, and Walton faults along the Cayman trough.


      • (a) Major morphologic features of the Caribbean plate and its vicinity. Bathymetric data is from ETOPO-5 database. Region inside box is the study area shown in Figs. 1(b) and 4(a),(b). Tectonic interpretation is modified from Mann, Schubert, and Burke (1990). Black arrow: Caribbean plate motion relative to North America. Red arrows: plate motion vectors relative to a hotspot reference (Gripp & Gordon, 1990). (b) General bathymetry of Cayman Trough and its vicinity from ETOPO-5 database and the major faults bounding the trough. Earthquake locations are from the NGDCs Preliminary Determination of Epicenters
      database.

    • This map shows the gravity anomaly map for the region from ten Brink et al. (2002).
    • Gravity data can reveal the subsurface structures.


      • (a) Free-air gravity anomaly map of the central Cayman Trough derived from satellite altimetry data (Sandwell and Smith, 1997). Grid spacing is 1 min. Contour interval is 10 mGal. The locations of the east-west profile along Cayman Trough in Figs. 4(c) and 5 and of profiles B–B0 , and C–C0 in Fig. 4(c) are shown as gray lines. Heavy black lines: Caribbean–North America plate boundary. O, S, and X: locations where rocks with oceanic crust affinity, serpentinized peridotite, and continental crust affinity were dredged, respectively (Perfit and Heezen, 1978). H: locations of heat flow measurements shown in Fig. 8. Numbered lines with open circles are locations of two-ship reversed seismic refraction profiles of Ewing et al. (1960). (b) Free-air gravity anomaly map of the central Cayman Trough from shipboard gravity data. Tracklines and individual points that were used to create the map are shown as thin lines and dots. Heavy lines are as in Fig. 4(a). The data were gridded at 1-min blocks. See text for further details about the processing. (c) Comparison between satellite (solid line) and shipboard (short-dashed line) free-air gravity anomalies derived from their respective gridded data sets along the profiles shown in Fig. 4(a) and (b). The actual shipboard gravity measurements along profiles B–B0 and C–C0 (long-dashed lines) are shown for comparison. The origin in profiles B–B0 and C–C0 is located at their crossings with the east–west profile.

    • Here is the focal mechanism summary map from Funk et al. (2009). The authors depict the forearc sliver as a shaded red region. Note the strike-slip focal mechanisms in the region on the northeast of the forearc sliver.


      • Tectonic setting of Central America displayed on satellite topography and bathymetry from Sandwell and Smith (1997). Subduction of the Cocos plate beneath the Caribbean plate occurs along the Middle America Trench. Global positioning system (GPS)–based plate velocities are relative to a fixed Caribbean plate, and focal mechanisms are from the Harvard Centroid Moment Tensor (CMT) catalog for all events from 1976 to 2007. Northwestward arc-parallel translation of the Central America forearc sliver (red shading) occurs at 7–8 mm yr–1 on Nicoya Peninsula of Costa Rica (Norabuena et al., 2004) and 14 mm yr–1 in Nicaragua (DeMets, 2001). The boundary between the Caribbean and North American plate occurs along the left-lateral strike-slip Swan Islands fault zone (SIFZ), Polochic fault (PF), and Motagua fault (MF).

    • Geodesy is the study of how the Earth deforms.
    • Geodesists place GPS stations in different locations. These GPS stations record their location with high precision.
    • As each GPS monument moves with time, we can measure this movement with time, then calculate and plot the velocity of each monument.
    • These are the GPS velocities for the geodetic sites in the region.
    • The arrows have direction and length (so we call them vectors), which show the direction and rate of movement. The ellipse represents the uncertainty of the tip of the vector.
    • The upper map shows motion relative to the North America plate and the lower map shows motion relative to the Caribbean plate.
    • Something we can look at is how the length of these vectors changes on either side of the Motogua/Polochic fault. This means that the GPS stations are moving faster on one side of the fault than the other side.
    • Looking at the lower panel, the stations are moving faster on the north side than the south side of the fault. This shows that the sites on the south side of the fault are not moving much relative to the Caribbean plate. And, since the sites are moving faster on the north side of the fault, there must be some reason for this (e.g., that the two plates are moving relative to each other, like along a plate boundary fault: the Motahgua-Polochic fault system).


      • GPS velocity field from Table 1 in (a) North America and (b) Caribbean plates reference frames. Euler NA/ITRF2000 and CA/ITRF2000 angular velocities as well as velocity vectors of sites GUAT and TEGU (white arrows) are from DeMets et al. (2007). Dark grey bold lines outline active faults (names as in Fig. 2). Light grey lines indicate location of profiles shown in Fig. 4. Dotted line follows volcanic arc.

    • This figure has a series of paired panels with (1) profiles of topography on top and (2) plots of GPS velocity on bottom, across transects shown on the above map.
    • Then fault parallel GPS velocity plot shows plate motion in one direction to the north of the fault and the opposite motion to the south of the fault.
    • The Eastern Profile crosses multiple strike-slip earthquake faults.
    • The far field displacement of GPS velocities tells us the overall slip rate across this fault system.
    • My estimate from the figure is that the fault system shows about 18 mm per year in the Eastern Profile and about 14 mm per year in the Central Profile. The authors calculate 20 and 16 mm per year (my estimate was close).
    • Thus, these plots confirm what we saw on the maps above, that the North America plate is moving west relative to the Caribbean plate at a rate of about 20 to 16 mm per year.
    • I located a reference after I had put most of this together and could not upload figures (I lack access to the AWS server here in Guatemala). Lyon-Caen et a. (2006) provide another slip rate estimate for the region.
    • Lyon-Caen et al. (2006) describe the overall plate boundary relative motion is about 20 mm/year (consistent with these Franco et al., 2012 data).


      • (a) Topography (top panel) and ITRF2000 GPS velocities projected onto fault-parallel components along eastern profile, E (bottom), with fit model for a locking depth of 20 km. Main active fault traces are indicated by dotted lines. Volcanic arc area is shaded. (b) χ2 = 1 contour line for locking depth and rate estimated using half-space elastic modelling (see Section 3 in text). Cross shows model for a 20-km locking depth (best-fit). (c) χ2 = 1 contour line for a two-fault model, showing the relative contribution of the Polochic Fault to the NA/CA motion, as a function of the asymmetry coefficient K across fault (Le Pichon et al. 2005). Case K = 0 corresponds to an homogeneous half-space model. (d) and (e) Same as (a) and (b), respectively, for central profile, C. Shaded area on (e) is that consistent with the locking depth range estimated from eastern profile in (b). (f) same as (a) and (d) for western profile, W. See Fig. 3 for the location of profiles

    • Burkhart (1978) studied the geologic offsets across the Polochic fault using LANDSAT satellite imagery and geologic mapping.
    • GPS Geodesy works over decadal times scales, depending how far back the GPS stations were installed or GPS sites were observed. These analyses provide decadal scale slip rates.
    • But we can look at the offsets of geologic units for a longer view of the slip rates across a fault, slip rates over millions of years.
    • We can also look at fault trench data (more on this below) to look at intermediate scale (millennial scale) slip rates.


      • Map of western Guatemala and southern Chiapas showing trace of Polochic fault and its relations to exposed Paleozoic sedimentary core (shaded) of fold belt of northern Guatemala. Arrows indicate match-up points on reconstruction shown in Figure 3. Concordia faults are probably correlative with reverse faults near Salam on the southern block.

    • Here is their geologic mapping. They back slipped the geology across the fault so that geologic units lined up (i.e., before they were offset by the fault).


      • Geologic map showing reconstruction across Polochic fault after left slip of 132+-5 km has been eliminated. Each block is bounded by its own present-day segment of the Polochi fault. Space is left between blocks.

    • Here is their schematic illustration showing how they interpret some major geologic units were offset over time.
    • These authors focused on estimating the initiation of movement on this fault system, not the slip rate.
    • However, if we take their displacement distance (132km) and divide by the time of initiation (6.5 million years), we can calculate a slip rate of 20 mm per year. This is a crude estimate with lots of assumptions but it aligns well with the modern GPS derived slip rate! (between 20 and 16 mm/yr)!!!


      • Diagrammatic representation of stages of deformation of Cenozoic fold belt of southern Chiapas and northern Guatemala. Shaded areas represent exposed Paleozoic core. Wavy symbols indicate fold trends. The following stages are represented: (1) preslip; (2) after most of the 132 km of slip was effected; (3) present configuration after 25 degrees counterclockwise rotation of eastern Guatemala. The long 90 deg ! line applies to stage 3 and southern (fixed) block of stages 1 and 2.

    • Obrist-Farner et al. (2020) use a geologic framework to look at the long term role of the Polochic-Motagua fault as a part of the North America-Caribbean plate boundary.
    • They look at the geologic mapping of features offset across the fault. They investigate the sedimentary rocks as a history of this region. They integrate “industry reports, well logs and reports, well cuttings, vintage seismic data, outcrop observations and geochronological data to constrain the initial infill and age of inception of the basin” data to interpret how a marine platform was developed, uplifted, then eroded.
    • These interpretations, along with numerical ages, allow them to constrain the initiation of the fault motion. Finally, they make interpretations about how tectonic strain is partitioned across different fault systems/structures.


      • Topographic and tectonic map of southern Guatemala showing traces of the three main faults of the Polochic–Motagua Fault System. Inset map: Tectonic and topographic map of the Caribbean region. Black box shows the study area. Modified from Mann (2007) and Authemayou et al. (2012). Slip rates on faults from Ellis et al. (2019)

    • Here is their overall geologic map for the Lake Izabal area.


      • Geological map of the Izabal area showing the location of the seismic lines and wells used in this study. Modified from Bonis et al. (1970)

    • This is the stratigraphic section for this region.


      • Simplified stratigraphic column from the Izabal region. Lithological column is based on outcrop and well observations and it is modified from Bartole et al. (2019). Absolute ages are from Gradstein et al. (2004)

    • This shows the topography here. They plot seismic lines in red. The faults are shown in black.


      • Topographic map of the eastern side of the Lake Izabal Basin highlighting the location of the outcrops studied and the extent of the units in the San Gil Hill area.

    • These are their proposed scenarios for the fault development here.
    • Pay attention to the gray geologic units that are displaced across the faults. They are all in the same location in all these maps but the things that are different are the slip rates for each scenario.
    • These authors suggest that the fault may have initiated motion earlier than Burkhart (1978): perhaps as early as 12 Ma. Thus, using the same crude method I used above, the geologic slip rate is now constrained at between 11 and 20 mm/yr (still remarkably consistent with the GPS slip rate).


      • Slip rate scenarios for the PMFS showing fault switch activity deduced from this study. (a) Left sketch shows Scenarios 1 and 4 indicating that the Polochic Fault was the only fault of the plate boundary between 15 and 10 Ma (Scenario 1) or between 12 and 7 Ma (Scenario 4).Right sketch shows the Motagua Fault as the main plate boundary fault from 10 Ma (Scenario 1) or from 7 Ma (Scenario 4). (b) Left sketch shows Scenario 2 with the Polochic Fault as the main plate boundary fault and the Motagua Fault a secondary fault from 15 to 7 Ma. Right sketch shows
      the Motagua Fault as the main plate boundary fault and the Polochic Fault a secondary fault from 7 Ma. Light grey box shows the 15 Ma magmatic arc displacement of >100 km from Ratschbacher et al. (2009) and dark grey shape shows the displacement of the Comalapa anticlinorium displacement of 132 km from Burkart (1983). Caribbean plate vector from DeMets, Gordon, and Argus (2010)

    • Authemayou et al. (2012) studied the Quaternary activity of the Polochic fault system.
    • Their goal was to constrain the Late Quaternary activity of the Polochic fault by using active structure fault geometry, slip rates, and paleoseismic event data.
    • This map shows the active faults and areas affected by historical seismicity.
    • They found evidence for 4 prehistoric earthquakes in the past 17,000 years.
    • They also provide a fault trench slip rate (over 17 ka) of about 4.8 mm/yr (and a vertical slip rate of about 0.3 mm/yr). BUT this is just for one of the faults in the fault system, the Polochic fault. The rest of the slip resides on other faults.


      • Traces of major active faults in Guatemala and historical seismicity according to White [1984].

    • Here is a map showing their fault orientation measurements.
    • Note how the Chixoý River has been offset across the fault.


      • Trace of major active faults and Quaternary fault kinematics data in Chixoý and Uspantán regions. Locations: S1-S6: studied sites, CB: Cobán, LC: Los Chorros rock avalanche, SC: San Cristóbal Verapaz, TC: Tactic, U: Uspantán. For relative location see Figure 1. Fault populations are shown in lower hemisphere stereonets.

    • Here are some geologic and geophysical observations from site 1A.
    • The upper right shows an electrical resistivity profile across the fault (location plotted on the map as a red line).


      • Geological and geophysical observations at site 1A (Agua Blanca). (a) 0.5-m-resolution aerial photograph of site 1 with superimposed 20 m contours (thin white lines) showing the Polochic fault trace and the location of the ERT (red line) and the topographic profile (blue line). The dotted black line corresponds to a minor fault. (b) ERT profile located on Figure 4a showing the measurement of the vertical offset of the formation B top surface. The black dashed line delineates the inferred boundary between formations A and B. The white dashed line indicates the effect of valley side. Red line shows the Polochic fault trace (Figure 4a). (c) Topographic profile produced from the 20-m-interval contours of the 1:50,000 topographic map and vertical offset of the topographic surface through the Polochic fault trace (Figure 4a). (d) View of a strike-slip fault plane affecting Los Chocoyos pumice (Figure 4a). (e) View of a normal fault plane affecting old alluvial Chixoy river deposits.

    • This shows an illustration and oblique images of their fault interpretations.
    • The upper illustration shows their fault offset measurements. The imagery shows how these faults are offset vertically.


      • Normal fault scarp at site 2 (La Hacienda). (a) South looking sketch of fault scarps developed on two generations of alluvial fans above the western termination of the main segment of the Polochic fault of site 2 (Figure 3). Vertical offsets are indicated in meters. (b) View of scarps in the eastern region of site 2. White dashed line shows the fault trace. (c) View of scarp in the western region of site 2. White dashed line shows the fault trace.

    • Here are more geological and geophysical observations, here at site 3.


      • Geological and geophysical observations at site 3 (Cotoxac). (a) West looking view of Cotoxac landslide scar through the normal fault scarp affecting the alluvial fan (Figures 3 and 6). (b) Sketch of view showing the damage zone of the normal fault and bedding of the alluvial deposits. (c) View of cracks associated with the fault scarp on the alluvial fan surface. (d) ERT profile across the fault scarp of the Cotoxac alluvial fan produced near the landslide scar. Black dashed square indicates the location of the landslide scar view of Figure 8a. The red line shows the fault plane associated with the fault scarp affecting the alluvial fan. Black bold number indicated the vertical offset measurement in meters associated with this fault. Red dashed lines correspond to the inferred earlier fault planes and boundaries of associated damage zone. White dashed lines delineate the inferred boundaries between formations A, B and C.

    • Here are more geological and geophysical observations, here at site 5.


      • Log of trench wall at site 5 with two photographs revealing detailed of the main fault structure (Tierra Blanca-Pericón). Trench location is denoted on Figures 3 and 6. Encircled black numbers represents the unit name. Black numbers with black and white stars indicate the age and the location of the samples dated with radiocarbon analyses. Red lines point out global aspect of the strata geometry in units.

    • This is their interpretation of the fault history at site 4.


      • Inferred sequence of deformation and sedimentation at the trench of site 4.

    • Brocard et al. (2016) studied the paleoseismicity of the Polochic fault by using fault trenching and sediment coring.
    • Using seismoturbidites (sublacustrine landslide deposits generated during strong ground shaking from earthquakes), they found evidence for 10 earthquakes in the past 1,200 years!
    • Seismoturbidite triggering has a lower magnitude threshold than do fault offsets near the surface. We can get landslides for earthquakes as small as M 5.5 (e.g. the Mineral Virginia earthquake). But surface rupture is generally only for much larger earthquakes. This makes sense, that the there would be evidence for more earthquakes in the turbidite record than for in fault trenches (the same is true comparing turbidites offshore of subduction zones with evidence for coseismic subsidence or tsunami: there should be more turbidites than tsunami deposits).
    • Based on the timing of a clustering of seismicity, they suggest that these earthquakes may have contributed to the decline of the Classic Maya period.
    • This map shows the tectonic structure and the locations of their field studies.


      • Structure of the North American-Caribbean plate boundary in Guatemala. Faults: 1: strike-slip, 2: reverse, 3: normal. 4 and 5: major earthquakes, isoseismal intensities in roman numbers, 4: 1976 CE18, 5: 181619, modified, and 830 CE6, 6: cities damaged around 830 CE, 7: hot springs, 8: sulfate-rich springs. Inset: location of the study area along the Caribbean-North American plate boundary.

    • These maps show the location of their on fault and near-fault study locations.


      • Location of on-fault and near-fault observations. (A) Location of the trenches (yellow star) and lake (red star) with respect to the Polochic fault trace (black solid lines). Orange: 2009 CE Los Chorros avalanche scar, deposits, and debris flow corridor. (B) Close up map of Lake Chichój with coring sites (dotted circles). Core used for the composite section in red, cores in yellow not used in this study but described in Fig. S2-1).

    • This map shows their fault trenching locations.


      • Geological map of the Agua Blanca area with location of trenches (red lines). Topographic contour line spacing: 10 m. Grey-shaded areas: footprint of post-2010 debris flows and earthworks. Note the shift of the Chixoy River from its pre-2010 bed (dark blue) to its corrected bed (striped light blue). White dots: 36Cl surface exposure dating24. White stars: 14C soil dating.

    • Here is a cross section in the floodplain showing the paleosols they used in their study.


      • Cross sections in the floodplain of Creek Chicochoc, showing the overall floodplain architecture, and the relationships between paleosols (dark gray), fault strands (F), and 14C-dated samples (white stars, A1–C2) with calendar ages reported at 68% confidence level (Table S1, additional information). Light gray: sand and gravel, with largest gravels in blacl.

    • Here is the sedimentary record of seismoturbidites in their study.


      • Composite section from the west basin of Lake Chichój. Layers A–J are seismo-turbidites (light brown marks); layer II (turquoise) is a flood layer (1945 CE) and layer α is a mafic ash fall. Grey-shaded area: depth range corresponding, within uncertainties (one sigma dark grey; two-sigma light grey), to the age of the youngest 14C-dated displaced soil at Agua Blanca. It is drawn to show where seismic-related disruptions should be observed, had slip produced ground shaking > MMI VI. White stars indicate age markers (137Cs peak and calibrated radiocarbon ages).

    The 1976 Earthquake

      • George Plafker sure did lots while at the USGS. A geology super hero!
      • Here are some figures from Plafker (1976).
      • This is a summary map showing the extent of his study, highlighting important features (like faults and epicenters).


        • Map showing the relation of segments of the Motagua and Mixco faults that moved during the earthquake of 4 February 1976 to the main shock epicenter, the larger aftershock epicenters, and major structural and volcanic features in northern Central America. Circled numerals along the Motagua fault indicate selected measured sinistral displacements in centimeters.

      • Here is an aerial image of an agricultural field that shows a fault scarp crossing the landscape (not the three white arrows pointing out a lineament, which is the fault rupture).


        • Oblique aerial view south toward the linear trace of the Motagua fault (arrows) in farmland west of Cabanias. Rows 1 m wide with about 70-cm sinistral offset may be seen in the field at the left side of the photograph and in the enlarged inset.

    • Here is an excellent view of surface rupture.

        • View west along Motagua fault trace where it crosses a soccer field at Gualan. Note the characteristic right-stepping en echelon fractures and the “mole track” of pressure ridges caused by 92-cm sinistral displacement.

      • This photo shows a row of trees that are offset by the fault.


        • View south along a row of trees offset about 325 cm in a sinistral sense (indicated by white bar) where it is intersected by the Motagua fault. The man is standing on the fault trace, which is a single fissure oriented perpendicular to the line of trees at this locality.

      • Here we see the fault has cut across a road.


        • View northeast along one of the larger breaks in the Mixco fault zone near Colonia El Milagro in Guatemala City. The displacement here is about 12 cm vertically down to the east and 5 cm dextrally. The break occurred at the base of a degraded scarp that was probably formed by previous late Quaternary displacement of this fault.

      • Here is Plafker’s map showing the tectonic setting, which is the basis for the following few figures.


        • Map showing the Motagua fault in relation to the boundaries of the Caribbean plate and extensional fault systems within the northwestern part of the Caribbean plate. Large arrows indicate relative plate movement directions; black dots indicate major volcanoes of the Middle American arc (36).

      • This figure has three possible interpretations for the tectonics of the Motagua-Polochic fault system, along with subduction to the southwest and the east-west extension (there exist numerous north-south oriented normal faults mapped on Plafker’s first map above). There are other interpretations that came later (some of which are in this report).


        • Schematic diagrams showing three alternative models for the present tectonics of part of Middle America. Inferred plate motion directions and relative velocities are indicated by the open arrows; relative fault displacements are indicated by conventional symbols; black dots indicate major volcanoes; the shaded pattern outlines major zones of extension faults. The locations of the 1972 Managua, Nicaragua (a), 1973 Costa Rica (b), and 1965 San Salvador (c) earthquakes (2, 32, 37) are shown in (C).

      • Here we can see how Plafker viewed the subsurface geometry of the faults in the region.


        • Block diagram showing the relation of the Motagua fault zone and the inferred zone of decoupling within the Caribbean plate to major tectonic and volcanic elements in Guatemala and contiguous countries.

      • Here is a map from Young et al. (1989) that shows the 1976 epicenter in relation to the fault mapping.
      • The numbers on the map represent measurements made by Plafker. These are left-lateral (aka sinistral) displacements in centimeters.
      • The maximum offset was 3.4 meters! Though there were some vertical displacements as well.


        • A map of the Motagua fault area, showing the locations of the 4 February 1976 main shock epicenter (star) and of locally determined aftershocks (circles) (after Langer et al., 1976). The earthquake was caused by sinistral slip of the Caribbean and North American Plates along the Motagua fault (shown as a solid line where observed and dashed where inferred). The numbers along the fault strike are the measured sinistral displacements in centimeters (from Plafker, 1976). The top left inset shows the fault model geometries that we used in our inversions for the rupture process: (a) a single fault with 75 ° strike, and (b) a composite fault consisting of two segments, one with a strike of 75° and the other with a strike of 90 °.

      • Kanamori and Stewart (1978) studied the seismic records of the earthquake to construct a focal mechanism. They used inversions of “teleseismic” waves (seismic waves recorded on instruments far away from the earthquake, that have travelled through the Earth).
      • Here is their focal mechanism, which represents a strike-slip earthquake.
      • A focal mechanism has two nodal planes, each representing a possible slip plane for the fault.
      • In this focal mechanism, it is almost purely strike-slip and could be a right-lateral or a left-lateral earthquake. One must take into account other information, such as aftershock patterns, existing mapped faults, or field evidence of coseismic rupture, to interpret which nodal plane is the correct one.
      • In this case, there is a mapped left-lateral fault and Plafker et al. mapped surface ruptures that exhibited left-lateral features (evidence of left-lateral motion).


        • a) The P wave first-motion data for the Guatemala earthquake indicating left lateral strike slip motion on the preferred fault striking N 66øE. A stereographic projection of the lower focal hemisphere is shown (b) A map of the main shock and aftershock locations. The observed displacements along the Motagua fault are plotted inside the circles (values are in meters) [after Langer et al., 1976; Plafker, 1976](c) A plot of the number of aftershocks as a function of depth [after Langer et al., 1976].

      • Here we see a comparison between observed and synthetic Rayleigh waves for the seismic station in Pasadena. The observed waves are from the seismic instruments installed in Pasadena. The synthetic waves were produced by these authors.
      • In general, there are four types of seismic waves. Primary, Secondary, Love, and Rayleigh.
      • Primary are impulsive or compressive waves. Secondary are slower and are shear waves. Love and Rayleigh waves are body waves that have more complicated motions.
      • Learn more about seismic waves at IRIS.


        • Observed and synthetic Rayleigh waves (R2) at Pasadena (ultra-long-periods instrument, number 33). Note the agreement of the phase.

      • Here are some plots comparing their synthetic waveforms with observed waveforms.


        • Observed and synthetic P waves for individual WWSSN stations obtained from the multiple-shock analysis. For each station the source time series is obtained by using the mechanism given in Figure la and the source time function shown here. The surface reflections pP and sP are included in the source time function. The resulting series is given for each station along with the moment for the first event . The height of the vertical bar is proportional to the moment of the individual event; Δ is the epicentral distance.

      • As part of the USGS effort to document this earthquake, the USGS published Espinosa et al. (1978): USGS Professional Paper 1002: The Guatemalan Earthquake of February 4,1976, A Preliminary Report. There are a number of chapters, each with a different set of authors (but i only include the main reference in the reference list below.
      • As part of this report, they evaluate the tectonic history and this figure shows their interpretations.


        • Model for the evolution of the Caribbean plate. (Used by permission from Malfaat and Dinkelman, 1972.) Arrows indicate motion relative to the North American plate. Solid triangles indicate volcanoes.

      • Here they show the possible places where the triple junction (where three plate boundary faults meet) arrives in the region.


        • Possible appearance of the Caribbean-Cocos-North American plate triple junction and nearby plate boundaries 50 m.y. from now, assuming that there will be no major changes in local plate motions. The tendency for this plate geometry to develop may explain the present day east-west elongation of the Caribbean-Cocos-North American plate triple junction. Open arrows indicate motion relative to the Caribbean plate; serrated line indicates underthrusting or subduction.

      • Here is a map that shows the main aftershocks.


        • Epicenters of main event and the principal aftershocks through March 7, 1976. These locations are based on data available as of April 5, 1976, and are subject to slight revision. Light parallel lines represent volcanic linears [lineations?]. (Base map modified from Bonis and others, 1970, and Plafker and others, this report.)

      • Here is their focal mechanism, showing a strike-slip mechanism with two possible fault planes (nodal planes): either a roughly east-west (~N70E striking) fault or a roughly north-south (~N20W striking). The faults are both nearly vertical.
      • Note how similar this is to the Kanamori focal mechanism above. The Kanamori mechanism used a little more data (compare the number of circles on each mechanism as each circle represents a datapoint/seismic record).
      • This focal mechanism is based on the first motions from seismometers. First motions are the direction that the ground first moves at each station.
      • Stations that go up first are plotted as black circles. These represent compression. Down first station represent extension.
      • Stations that are in a quadrant of the fault where the plate is moving towards them experience compression.
      • Thus, with this information, we can interpret that the fault either moves from right to left (looking across the fault) on the east-west fault (where the crust is moving from the white circles to the black circles). Or, the fault is moving from the left to the right (when looking across the fault) on the north-south fault.
      • With all the information we now have, we know that the earthquake slipped along the east-west fault.


        • Stereographic projection of P-wave first motions for the main event. Circles represent first motions especially measured for this study. Plus and minus represent first-motion data reported to the NEIS. The solution has been chosen to be consistent with all the readings made by us and, in addition, to be as consistent as possible with the first motions reported to the NEIS. The northeast-striking nodal plane corresponds in strike and sense of displacement to the Motagua fault.

      • This is a comprehensive map showing the geologic effects from this earthquake.
      • This looks much like the Plafker map (this was also prepared by Plafker) but there are different notations here, specific to the Espinosa report.


        • Relationship of the Motagua and Mixco faults to the main-event epicenter, the epicenters of large aftershocks, and major structural and volcanic features in northern Central America. Numerals along the Motagua fault refer to localities listed in table 7. Epicenters are from data of the NEIS and Person, Spence, and Dewey (this report); faults and volcanoes are modified from Dengo (1968) and Bonis, Bohnenberger, and Dengo (1970).

      • Here we see the offset of some crop rows offset about 1-meter.


        • View towards the north showing rows in a cultivated field west of El Progreso (station 11, table 7) that are offset 105 cm in a sinistral sense by the Motagua fault. See figure 23 for location of El Progreso.

      • Here is an aerial image showing ground rupture from the earthquake with about 0.9-meters of offset.
      • The patterns show that this is a left-lateral strike-slip fault.


        • Oblique aerial view of Motagua fault trace crossing a soccer field at Gualan (station 5, table 7). Note characteristic right-stepping en echelon fractures and sinistral offset (89 cm) of white sideline stripe at right. See figure 23 for location of Gualan.

      • This is an image looking along the fault offset. The hill on the right is the fault scarp, showing (apparent?) vertical offset.
      • Read that caption: a 5-meter diameter tree was split during the earthquake!


        • View looking east along fault trace at the most easterly locality visited on the ground (station 1, table 7). Fault trace trends along base of 5-m-high scarp in foreground and through the fallen tree in the distance, which has a base diameter of more than 5 m. The tree was split and toppled by fault movement of about 72-cm sinistral displacement and 37-cm displacement down to the north. The north-facing steep scarp was probably formed by many repeated earlier movements along this same trace.

      • This map shows the places of earthquake induced landslides, ground cracks, and liquefaction from this earthquake.


        • Areas of earthquake-induced landslides and of ground cracks probably related to liquefaction of unconsolidated deposits. Landslide distribution is from a preliminary study of post-earthquake aerial photographs by Edward Harp, Ray C. Wilson, and Gerry Wieczorek of the U.S. Geological Survey.

      • So you don’t need to scroll up, we can compare these two maps with the USGS models.


      • This is an aerial image showing landslides (the lighter areas on the hillslopes) from the earthquake.


        • Aerial view looking1 northeastward along Rio Pixcaya, due north of Chimaltenango, showing numerous landslides in pyroclastic deposits. The river was partially dammed by a major landslide, shown by arrow in the middle distance.

      • Here is a view looking along a railroad track that shows the tracks deformed by the earthquake.


        • Rails bent in Gualan, Department of Zacapa.

      • More bent tracks, this time with the fault trace in view.


        • Rails repaired between El Jicaro and Las Ovejas, Department of El Progreso. also shown in B is the surface faulting with an east-west trend. This photograph was taken from a helicopter in a eastward direction.

      • Here is the Espinosa et al. (1978) map showing shaking intensity (MMI contours). This map is also above in the intensity section but included here to compare with the next figure showing damage to adobe-type structures.


        • Modified Mercalli intensity distribution in Guatemala from the main event. Circle indicates epicenter location of the February 4 earthquake; dashed line indicates approximate isoseismal. (Base map modified from Guatemala Institute Geografico Nacional, 1974, 1:500,000.)

      • This map shows an overview of the damage to adobe-type structures.
      • As I drove around the region before PATA Days, I noticed that the dominant structure type is concrete block. Many appear to have steel reinforcement but I could not tell if they are properly engineered (based on the steel rebar sticking up above the walls, they don’t appear to be engineered properly).


        • Contour map showing damage to adobe-type structures in Guatemala owing to the February 4 earthquake. See figure 1 for Department names. (Base map modified from Guatemala Institute Geografico Nacional, 1974, 1:500,000.)

      • More train tracks, this time on a wharf.


        • Puerto Barrios wharf, Department of Izabal, destroyed by February 4 earthquake. Arrows show large warehouse partially submerged. See figure 40 for location of town.

      • Here is one of many photos showing damage to a building.


        • Collapse of the Hotel Terminal, caused by the failure of reinforced-concrete columns in its third story. This building is located in Guatemala City (Zone 4).

      • Here is a bridge that collapsed.


        • Collapse of three central spans of the Agua Caliente Bridge, Kilometre 36 on the road to the Atlantic Ocean. This bridge was constructed in 1959.

      • Obrist-Farmer et al. (2025) used sediment cores and seismic reflection profiles (ways to look at sedimentary layers in the subsurface) from lakes surrounding the epicentral region to document event deposits from the 1976 earthquake.
      • They also used these data to infer that the earthquake exerted ground motions that had strong evidence for directivity (the ground motions were higher in the direction of the fault movement).
      • They identified “event deposits” (sedimentary layers created by earthquakes) that were thicker in some lakes (e.g., in Lake Atitlan, near the earthquake fault rupture terminus) and thinner in other lakes (ones that were “off-axis” from the fault).
      • The map on the left shows the tectonic setting and outlines historical earthquakes.
      • The map on the right shows the shaking intensity from the 1976 earthquake, as well as the lakes used for this study.


        • (A) Tectonic map of western Central America and the Caribbean region showing the interaction between the North American, Caribbean, and Cocos tectonic plates. Red circles show epicentral location of large (>7) earthquakes along the western plate boundary since 1816. Contours are peak ground accelerations (PGA) larger than 50% g (red) and 20% g (yellow; from U.S. Geological Survey earthquake catalog). Teal contour shows area of maximum destruction during the 1816 Polochic earthquake (White, 1985). (B) Tectonic map of Guatemala showing the location of the four major faults of the Polochic-Motagua fault system (PMFS) and PGA contours from the 1976 Motagua earthquake (from U.S. Geological Survey earthquake catalog). The map also shows the lakes used in this study and discussed in the text. Lake sizes of Lachuá, Ayarza, and Chichoj are exaggerated.

      • This figure includes maps of Lakes Atitlán and Ayarza, as well as the sediment cores.
      • These sediment core data include color imagery scans and petrophysical data (density in red and magnetic susceptibility in blue).
      • These event deposits (turbidites, though they look like homogenites too) have higher density sediments at their base.
      • There exist prehistorical event deposits below the 1976 layer.
      • The lower panels show the seismic reflection profiles across these lakes. Note that the 1976 deposit is particularly thick in these lakes.


        • Bathymetric maps of lakes Atitlán (A) and Ayarza (C) showing cored locations (yellow dots). Correlation of three cores from Lake Atitlán (B) and Lake Ayarza (D) show the 1976 event deposit and correlation lines (black). Seismic reflection profiles on the western depocenter of Lake Atitlán (E) showing continuous high amplitude reflections. A low amplitude and discontinuous seismic reflection interval is interpreted as the 1976 event deposit, which thickens toward the central part of the depocenter (right side of figure), and (F) showing a low amplitude seismic package interpreted as the 1976 event deposit in the eastern depocenter of Lake Atitlán.

      • This figure is the same as above for lakes Chichoj, Lachuá, and Amatitlán.


        • Bathymetric maps of lakes Chichoj (A), Lachuá (C), and Amatitlán (E) showing cored locations (yellow dots). Yellow line shows the cross section of the cores shown on (B) and (D). Correlation of two cores from Lake Chichoj (B), Lake Lachuá (D) and one of the cores from Lake Amatitlán showing the 1976 event deposit and correlation lines (black).

      • Here are some ground shaking data. The upper left panel is the result of a quasi-dynamic rupture model (color represents ground shaking, peak ground acceleration).
      • The upper right panel is a directivity plot using data from that rupture model. The fault is oriented like the line of numbers in the center of the plot and note how the ground shaking (higher shaking in darker red) is in the direction that the fault points towards.
      • The lower map compares the MMI contours (shaking intensity) with their rupture model results (pga). Fault displacement measurements are plotted in a gray scale along the fault (in little squares). Landslides were mapped in the ares shown in gray. They plot the event deposit thicknesses as vertical black bars (Lake Atitlán shows the thickest deposit).


        • (A) Peak ground acceleration (PGA) from a quasi-dynamic rupture model generated using the Crust 2.0 layered Earth model (see Methods file in the Supplemental Material; see text footnote 1). Black line is the fault trace, and the star is the location of the modeled rupture. (B) Directivity plot of the radial acceleration for the quasi-dynamic rupture at 100 km distance for frequencies of 0.01–0.1 Hz, showing increased accelerations along the rupture path. (C) Modeled PGA from the quasi-dynamic model compared to the Modified Mercalli Intensity (MMI) contours, highlighting areas of destruction in Guatemala (Espinosa et al., 1976). The plot also shows areas of landslides surveyed by Harp et al. (1981) and displacement measurements along the Motagua fault (Plafker, 1976). Vertical black bars show the normalized cumulative event thickness for the studied lakes. Star is the location of the 1976 earthquake epicenter. Modeled PGA values are based on results from the model with a straight fault. The color bar of (C) is the same as (A). NCET—normalized cumulative event thickness.

    Seismic Hazard

      • Here is a map from Benz et al. (2011) that shows the seismic hazard for this region.
      • Seismic hazard is represented by color. Darker red shows areas that have a probability for shaking with higher intensity (or, acceleration, as in
        peak ground acceleration, pgs).
      • Note how, for Guatemala, the seismic hazard in this map is dominated by the subduction zone.
      • What allows me to write this? The primary control for ground motions from an earthquake are the distance from the earthquake fault. There are other factors but distance has the most influence. Well, distance and earthquake magnitude.
      • One may observe how the hazard is highest at the coast, nearest the subduction zone. and the hazard tapers to the northeast, as the distance to the megathrust increases.


      • Here are some maps from Caccavale et al. (9) that show the seismic hazard for this region.
      • These authors sought to generate an updated seismic hazard model.
      • They considered both the subduction zone and the Motagua/Polochic fault system.
      • The hazard curves plot allow one to estimate what ground shaking would be for a given magnitude earthquake. Hazard curves are calculated for specific locations, so distance is embedded in these plots.
      • The vertical axis represents the annual probability of exceedance (the chance that, on a given year, the ground shaking would exceed the amount shown on the plot.
      • One may also view this plot as how the hazard changes with time. So, the lower part of the plot stands for a longer time period. In this way, we can see that the longer the time period, the higher the ground shaking.
      • These first maps are for rock sites (site type is a secondary factor that controls ground shaking).


        • Evaluation of SZ contribution to the hazard. (A) Hazard curves for the 24 target sites. (B) Hazard map for TR = 300 years at rock ground considering only the “D3_G6” SZ; (C) considering all the SZ; (D) considering the crustal SZ. The stars represent the target sites and the red line the Motagua fault.

      • Here are some maps from Caccavale et al. (3029) that show the seismic hazard for this region.
      • These maps are also for sites with bedrock.


        • Hazard maps for rock ground and 300 years return period for different configurations: (A) Using crustal SZ; (B) using only the SZ “D3_G6”; (C) deterministic PGA distribution calculated from Motagua fault (red line) for M = 7.5. The black stars represent the target sites.

      • Here are some maps from Caccavale et al. (3029) that show the seismic hazard for this region.
      • These maps are for sites that are constructed on soil, not bedrock like the previous map sets.


        • Hazard maps for soil ground and 300 years return period for different configurations: (A) Using crustal SZ; (B) using only the SZ “D3_G6”; (C) deterministic PGA distribution calculated from the Motagua fault (red line) for M=7.5. The black stars represent the target sites.

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