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 Motagua fault 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!

a

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 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 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.

  • Here is an educational animation from IRIS that helps us learn about how different earth materials can lead to different amounts of amplification of seismic waves. Recall that Mexico City is underlain by lake sediments with varying amounts of water (groundwater) in the sediments.

  • Here is an educational video from IRIS that helps us learn about resonant frequency and how buildings can be susceptible to ground motions with particular periodicity, relative to the building size.

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.


      • 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).


      • (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

    • Franco et al. (2012) used GPS observations to evaluate the kinematics (how the plates move and interact relative to each other) of this region. Below is a map that shows earthquake mechanisms that reveal the strike-slip faults as they converge. The forearc sliver (the block between the megathrust and the forearc sliver fault) is shaded gray.
    • These authors also use a model to estimate how much the megathrust is locked and accumulating elastic strain. They evaluate a range of possible physical properties of the find that the megathrust north of the forearc sliver is more highly locked (seismogenically coupled).


      • Proposed model of faults kinematics and coupling along the Cocos slab interface, revised from Lyon-Caen et al. (2006). Numbers are velocities relative to CA plate in mmyr−1. Focal mechanisms are for crustal earthquakes (depth ≤30 km) since 1976, from CMT Harvard catalogue.

    Seismic Hazard

      • Here is a map from Benz et al. (2011) that shows the seismic hazard for this region.


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


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


      • 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.


      • 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.

    The Earthquake

      • George Plafker sure did lots while at the USGS. A geology super hero!
      • Here are some figures from Plafker (1976).


        • 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 (4, 7, 35). Circled numerals along the Motagua fault indicate selected measured sinistral displacements in centimeters.


      • 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.


        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.


        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.


        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.


        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).


        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).


        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.


      • 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 °.

      • Obrist-Farner et al., 2025


      • (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.


      • 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.


      • 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).


      • (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.

      Social Media

      References:

      Basic & General References

    • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
    • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
    • Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
    • Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
    • Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
    • Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
    • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
    • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
    • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
    • Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
    • Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
    • Storchak, D. A., D. Di Giacomo, I. Bondár, E. R. Engdahl, J. Harris, W. H. K. Lee, A. Villaseñor, and P. Bormann (2013), Public release of the ISC-GEM global instrumental earthquake catalogue (1900–2009), Seismol. Res. Lett., 84(5), 810–815, doi:10.1785/0220130034.
    • Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198

      • Specific References

    • Benz, H.M., Dart, R.L., Villaseñor, Antonio, Hayes, G.P., Tarr, A.C., Furlong, K.P., and Rhea, Susan, 2011 a. Seismicity of the Earth 1900–2010 Mexico and vicinity: U.S. Geological Survey Open-File Report 2010–1083-F, scale 1:8,000,000.
    • Bruna, J.L.G., ten Brink, U.S., Munoz-Martin, A., Carbo-Gorosabel, A., and Estrada, P.L., 2015. Shallower structure and geomorphology of the southern Puerto Rico offshore margin in Marine and Petroleum Geology, v. 67, p. 30-56, http://dx.doi.org/10.1016/j.marpetgeo.2015.04.014
    • Calais, E., Symithe, S., de Lepinay, B.B., Prepetit, C., 2016. Plate boundary segmentation in the northeastern Caribbean from geodetic measurements and Neogene geological observations in Comptes Rendus Geoscience, v. 348, p. 42-51, http://dx.doi.org/10.1016/j.crte.2015.10.007
    • E. Calais(1), O.F. Gonzalez(2), E.D. Arango-Arias(2), B. Moreno(2), R. Palau(2), M. Cutie(2), E. Diez(2), C. 5 Montenegro(2), E. Rodriguez Roche(3), J. Garcia(3), E. Castellanos(4), S. Symithe(5,6, 2023. Current deformation along the northern Caribbean plate boundary from GNSS measurements in Cuba in Tectonophysics, v. 868, https://doi.org/10.1016/j.tecto.2023.230068
    • Franco, A., C. Lasserre H. Lyon-Caen V. Kostoglodov E. Molina M. Guzman-Speziale D. Monterosso V. Robles C. Figueroa W. Amaya E. Barrier L. Chiquin S. Moran O. Flores J. Romero J. A. Santiago M. Manea V. C. Manea, 2012. Fault kinematics in northern Central America and coupling along the subduction interface of the Cocos Plate, from GPS data in Chiapas (Mexico), Guatemala and El Salvador in Geophysical Journal International., v. 189, no. 3, p. 1223-1236. DOI: https://doi.org/10.1111/j.1365-246X.2012.05390.x
    • Franco, S.I., Kostoglodov, V., Larson, K.M., Manea, V.C>, Manea, M., and Santiago, J.A., 2005. Propagation of the 2001–2002 silent earthquake and interplate coupling in the Oaxaca subduction zone, Mexico in Earth Planets Space, v. 57., p. 973-985.
    • Funk, J., Mann, P., McIntosh, K., and Stephens, J., 2009. Cenozoic tectonics of the Nicaraguan depression, Nicaragua, and Median Trough, El Salvador, based on seismic-reflection profiling and remote-sensing data in GSA Bulletin, v. 121, no. 11/12, p. 1491-1521.
    • Garcia-Casco, A., Projenza, J.A., Iturralde-Vinent, M.A., 2011. Subduction Zones of the Caribbean: the sedimentary, magmatic, metamorphic and ore-deposit records UNESCO/iugs igcp Project 546 Subduction Zones of the Caribbean in Geologica Acta, v. 9, no., 3-4, p. 217-224
    • Kanamori, H. and Stewart, G. S., 1978. Seismological Aspects of the Guatemala Earthquake of February 4, 1976 in JGR, vol. 83, no. B7, p. 3427-3434
    • Leroy, S., et al., 2015, Segmentation and kinematics of the North America–Caribbean plate boundary offshore Hispaniola: Terra Nova, v. 27, p. 467–478, https://doi.org/10.1111/ter.12181.
    • Manaker, D.M., Calais, E., Freed, A.M., Ali, S.T., Przybylski, P., Mattioli, G., Jansma, O., Prepetit, C., de Chabalier, J.B., 2008. Interseismic Plate coupling and strain partitioning in the Northeastern Caribbean in GJI, v. 174, p. 889-903, doi: 10.1111/j.1365-246X.2008.03819.x
    • Manea, M., and Manea, V.C., 2014. On the origin of El Chichón volcano and subduction of Tehuantepec Ridge: A geodynamical perspective in JGVR, v. 175, p. 459-471.
    • Mann, P., 2007. Overview of the tectonic history of northern Central America, in Mann, P., ed., Geologic and tectonic development of the Caribbean plate boundary in northern Central America: Geological Society of America Special Paper 428, p. 1–19, doi: 10.1130/2007.2428(01). For
    • McCann, W.R., Nishenko S.P., Sykes, L.R., and Krause, J., 1979. Seismic Gaps and Plate Tectonics” Seismic Potential for Major Boundaries in Pageoph, v. 117
    • Obrist-Farner, J., et al., 2025, Paleoseismic evidence of directivity for the 1976 Mw 7.5 Motagua earthquake, Guatemala: Geology, v. 53, p. 971–976, https://doi.org/10.1130/G53449.1
    • Plafker, G., 1976. Tectonic Aspects of the Guatemala Earthquake of 4 February 1976 in Science, v. 193, no. 4259, p. 1201-1208
    • Porfido, S., Esposito, E., Spiga, E., Sacchi, M., Molisso, F., and mazzola, S., 2015. Impact of Ground Effects for an Appropriate Mitigation Strategy in Seismic Area: The Example of Guatemala 1976 Earthquake in G. Lollino et al. (eds.), Engineering Geology for Society and Territory – Volume 2, DOI: 10.1007/978-3-319-09057-3_117,
    • Symithe, S., E. Calais, J. B. de Chabalier, R. Robertson, and M. Higgins, 2015. Current block motions and strain accumulation on active faults in the Caribbean, J. Geophys. Res. Solid Earth, 120, 3748–3774, doi:10.1002/2014JB011779.
    • Young, C. J., Lay, T., and Lynnes, C., 1989. Rupture of the 4 February 1976 Guatemalan Earthquake, BSSA, v. 79, no. 3, p. 670-689.
    • Xu, X., Keller, G.R., and Guo, X., 2015. Dip variations of the North American and North Caribbean Plates dominate the tectonic activity of Puerto Rico–Virgin Islands and adjacent areas in Geological Journal, doi: 10.1002/gj.2708

    Return to the Earthquake Reports page.