Earthquake Report: Ecuador

Well, as I was getting up this morning, my social media feed was abuzz about the intermediate depth earthquake in Peru. While it was quite deep (<130km), it was still widely felt and probably caused lots of damage. https://earthquake.usgs.gov/earthquakes/eventpage/us2000jlfv/executive

After just starting a new job and still in the midst of moving in, I am a little late getting this out. But there are some cool learning moments here…

This part of the world enjoys a variety of plate boundary fault systems and interesting plate tectonic interactions.

There are multiple spreading ridges, creating oceanic plates, and these ridges and plates interact in complicated ways. The results from these complicated relations (e.g. how a hotpot near a spreading ridge affects the thickness of the crust formed at that spreading ridge) can also impact the convergent plate boundaries as these plates subduct beneath the South America continental plate.

The subduction zone megathrust fault, formed where the Nazca plate dives under the South America plate, has an historic and prehistoric record of earthquakes. However, today’s MW=7.5 earthquake is not a megathrust event.
The earthquake is an extensional (normal) type of an earthquake. It probably occurred along a fault in the downgoing Nazca plate.

The plate here is undergoing extension either from some internal deformation within the plate, or due to what we call “slab pull” (the plate that is diving down and deep is pulling the plate that is less deep). So, the fault may be oriented perpendicular to the direction the plate is going down. However, sometimes there are pre-existing faults (like fracture zones, etc.) that may reactivate under different conditions from when they were formed.

On most subduction zones, these faults will, thus, be parallel to the subduction zone fault.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 6.5 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.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.li>

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly east-west trends of these red and blue stripes near the spreading ridge separating the Cocos and Nazca plates. These lines are parallel to the ocean spreading ridges from where they were formed. The Cocos plate is formed at two spreading ridges, so check out how the magnetic anomalies in the northern part of the Cocos plate are not parallel to the other anomalies. #Awesome

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

  • In the upper right corner is a section of the map from Rhea et al. (2010). This shows earthquakes and the major plate boundary faults. Note the profile A-A’ in green. The rectangle shows from where earthquakes are selected to plot in the cross section below. I placed a blue star in the general location of today’s M=7.5 quake.
  • In the lower right corner is the A-A’ cross section. Note the location of the earthquake (blue star). Why is is not “well behaved” (i.e. it does not look like it belongs in this data set)? My hypothesis is that the plate is misshapen here (as evidenced by the slab contours in the maps below). So, the cross section is not oriented perfectly for this part of the plate (in addition, the earthquake is also not within the rectangle of selected seismicity).
  • In the lower left corner is a great map detailing the plate tectonics of the Cocos and Nazca plates, where the spreading ridges are, fracture zones, and the main terrane boundaries in the South America plate (e.g. sliver or “block” boundaries, broken up parts of the South America plate). This is from Gutscher et al., 1999.
  • In hte upper left corner is a map from Rhea et al. (2010) that shows an estimate of the seismic hazard in this part of the world. This is largely based on seismicity rates. Some seismic hazard model maps also incorporate other measures of hazard like strain rates.
  • Here is the map with a month’s seismicity plotted.


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  • The first thing I noticed is that the orientation of the earthquake is not parallel to the deep sea trench formed by the subduction zone as mentioned above.
  • If we look at the shape of the Nazca plate beneath South America where this earthquake happened (shown as “slab contours” in these maps), we will see that the contours are not parallel to the subduction zone fault.
  • Where today’s M=7.5 earthquake happened, the contours are closer to east-west, which means the fault is dipping downward slightly more to the north, rather than to the east. This may explain why the moment tensor and the earthquake show extension in the northeast/southwest direction.
  • In the historic earthquake poster below, check out how there are analogical earthquakes to today’s quake, while further to the south, these extensional quakes are oriented closer to being perpendicular to the trench.
  • There are a few historic quakes on the map below that are thrust events (compressional), but they are much shallower in depth (about 17 and 24 km), compared to all other quakes are deeper than 70 km.
  • Here is the map with a century’s seismicity plotted.


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  • Below I present a series of maps that are intended to address the excellent ‘new’ products included in the USGS earthquake pages: landslide probability and liquefaction susceptibility (a.k.a. the Ground Failure data products).
  • First I present the landslide probability model. This is a GIS data product that relates a variety of factors to the probability (the chance of) landslides as triggered by this earthquake. There are a number of assumptions that are made in order to be able to produce this model across such a large region, though this is still of great value (like other aspects from teh USGS, e.g. the PAGER alert). Learn more about all of these Ground Failure products here.
  • 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).
  • This model, like all landslide computer models, uses similar inputs. I review these here:
    1. Some information about ground shaking. Often, people use Peak Ground Acceleration, though in the past decade+, it has been recognized that the parameter “Arias Intensity” is a better measure of the energy imparted by the earthquake across the land and seascape. Instead of simply accounting for the peak accelerations, AI integrates the entire energy (duration) during the earthquake. That being said, PGA is a more common parameter that is available for people to use. For example, when I was modeling slope stability for the 2004 Sumatra-Andaman subduction zone earthquake, the only model that was calibrated to observational data were in units of PGA. The first order control to shaking intensity (energy observed at any particular location) is distance to the earthquake fault that slipped.
    2. Some information about the strength of the materials (e.g. angle of internal friction (the strength) and cohesion (the resistance).
    3. Information about the slope. Steeper slopes, with all other things being equal, are more likely to fail than are shallower slopes. Think about skiing. Beginners (like me) often choose shallower slopes to ski because they will go down the slope slower, while experts choose steeper slopes.
  • Areas that are red are more likely to experience landslides than areas that are colored blue. I include a coarse resolution topographic/bathymetric dataset to help us identify where the mountains are relative to the coastal plain and continental shelf (submarine). Note the blue line is the shoreline and that North is to the left. The M=7.5 epicenter is the green dot to the east of the mountains.


  • 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 the liquefaction susceptibility map. I discuss liquefaction more in my earthquake report on the 28 September 20018 Sulawesi, Indonesia earthquake, landslide, and tsunami here.


  • Here is a map that shows shaking intensity using the MMI scale (mentioned and plotted in the main earthquake poster maps). I present this here in the same format as the ground failure model maps so we can compare these other maps with the ground shaking model (which is a first order control on slope failure).
  • Let’s compare the MMI map below with the liquefaction susc. map. What might we conclude may be the largest factor for the landscape being susceptible to liquefaction?
  • Check out how the liquefaction map more directly resembles this MMI map, than the landslide map. In this case, my interpretation is that for the landslide model, slope is a larger controlling factor than ground shaking (though still a major factor).
  • And to answer my question, you were correct, liquefaction appears to be more highly controlled by ground shaking intensity.


  • Something else that is cool about the liquefaction map is we can see where the river valleys are. These regions have a higher liq. susc. because they are (1) closer to the earthquake and (2) they are composed of materials that are more susceptible to liquefaction (e.g. sediment rather than bedrock).

Other Report Pages

Some Relevant Discussion and Figures

  • Here is the Gutscher et al. (1999) map. Check out the complicated interaction between the structures in the Nazca and South America plates. The upper plate (South America) appears to have a major change in structures related to the Carnegie Ridge and the fracture zones (e.g. Grijalva FZ).


    • Tectonic setting of the study area showing major faults, relative plate motions according to GPS data [7] and the NUVEL-1 global kinematic model [8], magnetic anomalies [13] and active volcanoes [50]. Here and in Fig. 4, the locations of the 1906 (Mw D 8:8, very large open circle) and from south to north, the 1953, 1901, 1942, 1958 and 1979 (M  7:8, large open circles) earthquakes are shown. GG D Gulf of Guayaquil; DGM D Dolores–Guayaquil Megashear.

  • Here is another map from Gutscher et al. (1999) that shows several seismicity cross sections and we can see how the shape of the Nazca plate is different in different places. Today’s M=7.5 earthquake happened just south of the C’ label on the map (near the label “Region 3”).


    • Shaded hill relief and seismicity of the study area. Bathymetry and topography from Smith and Sandwell’s TOPEX database [51] with active volcanoes (red triangles) [50]. Seismicity (1964–1995), 1230 events Mb > 4:0, from Engdahl et al.’s global hypocenter relocation [18] scaled by depth and magnitude, omitting upper plate seismicity (<70 km depth >200 km east of the trench) in map. Oceanic plateaus defined by 2500 m contour. Location and sampling boxes of seismological sections indicated. Depth contours to the Wadati–Benioff zone indicated as dotted lines. Seismotectonic Regions 1–4 (see text) also shown. Section A–A0, Region 1, steep ESE-dipping subduction, narrow volcanic arc. Section B–B0, Region 2, intermediate-depth seismic gap, subduction of Carnegie Ridge with inferred flat slab shown, broad volcanic arc is spread out over 150 km. Section C–C0 , Region 3, narrow volcanic arc, steep NE-dipping slab. Section D–D0 , Region 4, Peru flat slab segment, no volcanic arc. Section F–F0, Andes-parallel profile illustrating the intermediate depth seismic gap and inferred Carnegie ‘flat slab’ in Region 2. Note the tear north of the steep NE-dipping slab in Region 3.

  • Here is a great visualization from Gutscher also. This shows how the structures in the Nazca plate may be controlling the tectonics here. Note their interpretation of several tears in the plate, and how the plate on the left shows that it is dipping to the northeast (consistent with the northeast extension from today’s temblor).


    • 3-D view of the two-tear model for the Carnegie Ridge collision featuring: a steep ESE-dipping slab beneath central Colombia; a steep NE-dipping slab from 1ºS to 2ºS; the Peru flat slab segment south of 2ºS; a northern tear along the prolongation of the Malpelo fossil spreading center; a southern tear along the Grijalva FZ; a proposed Carnegie flat slab segment (C.F.S.) supported by the prolongation of Carnegie Ridge.

  • This is a fantastic map from Chlieh et al. (2014) which shows how the earth moves based on GPS rate data. We can see how the major South America fault systems (e.g. GG and DGFZ) are accumulating tectonic strain (as evidenced by the shange in plate rate / vecor lenght at GPS sites on either sides of this fault system.


    • Seismotectonic setting of the oceanic Nazca plate, South America Craton (SoAm) and two slivers: the North Andean Sliver (NAS) and the Inca Sliver (IS). The relative Nazca/SoAm plate convergence rate in Ecuador is about 55mm/yr (Kendrick et al., 2003). Black arrows indicate the diverging forearc slivers motions relative to stable SoAm are computed from the pole solutions of Nocquet et al.(2014). The NAS indicates a northeastward long-term rigid motion of about 8.5 ±1mm/yr. The ellipse indicatesthe approximate rupture of the great 1906 Mw=8.8Colombia–Ecuador megathrust earthquake. The Carnegie Ridge intersects the trench in central Ecuador and coincides with the southern limit of the great 1906 event. Plate limits (thick red lines) are from Bird(2003). DGFZ =Dolores–Guayaquil Fault Zone; GG =Gulf of Guayaquil; GR =Grijalva Ridge; AR =Alvarado Ridge; SR =Sarmiento Ridge.

  • Since we are talking about the subduction zone megathrust, we can take a look at the history of subduction zone earthquakes here. This map is from Villegas-Lanza et al. (2016). We can use the slab contours to help us navigate today’s earthquake location relative to this map. Ecuador is unlabeled on the map, but is located between Colombia and Peru.


    • (a) Seismotectonic setting of the South American subduction zone. The red ellipses indicate the approximate rupture areas of large subduction earthquakes (M≥ 7.5) between 1868 and 2015 [Silgado, 1978; Beck and Ruff, 1989; Dorbath et al., 1990; Beck et al., 1998]. The blue ellipses indicate the locations of moderate tsunami-earthquakes [Pelayo and Wiens, 1990; Ihmle et al., 1998]. The bathymetry from GEBCO30s highlights the main tectonic structures of the subducting Nazca Plate, which are from north to south: Carnegie Ridge (CR), Grijalva Ridge (GR), Alvarado Ridge (AR), Sarmiento Ridge (SR), Virú Fracture Zone (VFZ), Mendaña Fracture Zone (MFZ), Nazca Ridge (NR), Nazca Fracture Zone (NFZ), Iquique Ridge, Juan Fernandez Ridge, Challenger Fracture Zone (CFZ), and Mocha Fracture Zone (MCFZ). The white arrow indicates the convergence of the Nazca Plate relative to the stable South America (SSA) reference frame [Kendrick et al., 2003]. The slab geometry isodepth contours are reported every 50 km (solid lines) and 10 km (dashed lines), based on the Slab1.0 model [Hayes et al., 2012]. The dashed rectangle corresponds to Figures 1b and 1c. The N.A.S. and C.A.S. labels indicate the North Andean and the Central Andes Slivers [Bird, 2003], respectively. (b) Temporal and spatial distributions of large subduction earthquakes with Mw ≥ 7.5 that occurred in Peru since the sixteenth century. The rupture extent values (in km) of historical (gray) and recent (red) megathrust earthquakes along the Peruvian margin are shown as a function of time (in years). A triangle indicates if a tsunami was associated with the event. The orange bands denote the entrance of the NR and the MFZ delimiting the northern, central, and southern Peru subduction segments. The rupture lengths were taken from its corresponding published slip models [Silgado, 1978; Beck and Ruff, 1989; Dorbath et al., 1990; Pelayo and Wiens, 1990; Ihmle et al., 1998; Giovanni et al., 2002; Salichon et al., 2003; Pritchard et al., 2007; Bilek, 2010; Delouis et al., 2010; Moreno et al., 2010; Schurr et al., 2014], and for historical earthquakes, we estimated its approximated lengths using scaling law relationships [Wells and Coppersmith, 1994]. (c) A map of the rupture areas of large subduction earthquakes that occurred in the twentieth century [Silgado, 1978; Beck and Ruff, 1989; Dorbath et al., 1990; Ihmle et al., 1998; Giovanni et al., 2002; Sladen et al., 2010; Chlieh et al., 2011], with their associated gCMT focal mechanisms. In northern Peru, the 1960 (Mw = 7.6) Piura earthquake and the 1996 (Mw = 7.5) Chimbote earthquake, which are shown by cyan-colored polygons, were identified as tsunami-earthquake events [Pelayo and Wiens, 1990; Ihmle et al., 1998; Bilek, 2010].

  • Part of looking at the above map is that we can look at the same article to find this great figure, that we can compare with the Gutscher and Chlieh maps.


    • Schematic description of the principal continental slivers contributing to the deformation partitioning of the Peruvian margin: North Andean Sliver (NAS; yellow) Peruvian Sliver (PS; in red), and Eastern Cordillera–Subandean regions (in green), which are separated by the limit between Western Cordillera and Eastern Cordillera. All of the motions are in reference to SSA and are expressed in millimeters per year (mm/yr). The inset shows the kinematic triangles and obliquity partitioning vectors for Ecuador (latitude 1°N), the Guayaquil Bend (latitude 5°S), and the Arica Bend (latitude 18°S). The lines with triangle symbols indicate the local trench axis. The green and purple lines are, respectively, the along- and normal trench components of Nazca/SSA convergence vector. The blue arrows indicate the Nazca/NAS and Nazca/PS convergence vectors, and the red arrows are the NAS/SSA and PS/SSA convergence vectors.

  • Finally, lets look here to see how Chlieh et al. (2014) use their modeling of the megathrust to infer the percent that the megathrust fault is accumulating tectonic strain (fault coupling). Warmer colors = higher coupling (like the fault has brand new vecro in red areas and worn out velcro in white areas).
  • See how there is low coupling in the area of the fault that where there are fracture zones or oceanic ridges in the Nazca plate (e.g. the Nazca Ridge and Mendaña fracture zone. Something that they did not label is the Carnegie Ridge, which is the area of low coupling near distance 1800-2000, near Plura, Ecuador and the North arrow.


    • (left) Along-trench variations of moment deficit rate for (middle) minimum and (right) maximum interseismic coupling models. Even though the interseismic pattern might vary significantly between models, the locations of the peaks and valleys in the rate of moment deficit are very persistent characteristics that highlight the locations of the principal
    asperities (peaks) and creeping barriers (valleys). The dashed ellipse contours in the middle map show the approximate rupture area of large earthquakes, as described in Figure 1.

  • Here are a couple figures I just came across (h/t Pablo, see tweet below). This first one shows an excellent visualization of the Nazca plate (Yepes et al., 2016).


    • Slab bending depicted as a hypothetical contorted surface. The drawings represent the subduction and bending of Farallon and Nazca plates from three different perspectives. The margin convexity (concavity from the perspective of the continental plate) forces the slab to flex and shorten at depth which accumulates stresses in most strained areas. Present-day position of the Grijalva rifted margin at the trench coincides with a noticeable inflection point of the trench axis (in red). A horizontal grid has been added to help visualize the plates dipping angles. A transparent 100 km thick volume has been added below the contorted surface to simulate the plate, but at intermediate depths the depicted surface should be representing the plate inner section. (a) South to north perspective showing the different dipping angles of Farallon and Nazca plates. The slab depth color scale is valid for the three drawings. (b) West to east oblique perspective at approximately the same angle as Nazca plate’s dip. The contortion of the Farallon plate at depth south of the Grijalva rifted margin is clearly noticeable from this perspective. (c) East to west perspective. Intermediate depth seismicity (50–300 km) from the instrumental catalog [Beauval et al., 2013] is drawn at the reported hypocentral depth. Two areas ofmaximum strain in the Farallon plate are shown (hachured): the El Puyo seismic cluster (SC) and the 100–130 km depth stretch of high moment release seismicity related to a potential hinge in the subducting plate. Lack of seismicity in the Nazca plate is explained due to the fact that this young plate, even though it is also strained, is too hot for brittle rupture.

  • This figure shows how convergence rate (through obliquity) varies along strike (Yepes et al., 2016).


    • Convergence obliquity. Circles represent obliquity values calculated at each epicentral latitude for Mw ≥6 earthquakes. Plus signs represent ruptured fault plane strikes obtained from focal mechanisms assuming the most probable fault plane candidate. We have used the Harvard focal mechanism catalog (http://www.globalcmt.org) from 1976 to 2013. Colors indicate focal mechanism. Tsunami events are highlighted, but they also correspond to thrust events. Light gray lines are the trench azimuth (crosses) ± 10% error. Notice the good agreement between fault strike and trench azimuth for thrust interface events. Some Mw <6 earthquakes have been included to show lack of agreement for noninterface events such as the Yaquina graben normal and the Grijalva rifted margin strike-slip events.

  • This figure shows their interpretation about how the Nazca plate (Nazca vs. Farallon slab) changes the subduction zone along strike (Yepes et al., 2016).


    • Inslab seismogenic source zones. Seven inslab SSZs have been defined along with three complementary sources (see the text for explanation). SSZs are colored according to their Mo release density (MoRD). Farallon slab zones are overlapped as seen in Figure 9a. Only colors in the “visible” part of the plate in plan view are shown as original. They are identified by three labels: source zone name and depth range, western boundary (W), and eastern boundary (E). Nazca plate SSZs are not overlapped. Complementary zones below the interface SSZs are hachured. Yellow triangles are active late Holocene volcanoes. Circles and stars correspond to seismicity presented in Figure 2b. (a) A-B 150 km wide cross section in Farallon domain to show how plates overlap. (b) Normal faulting, shallow (Z ≤ 50 km) focal mechanisms related to the Yaquina graben extracted from the Harvard global centroid moment tensor catalog [Dziewonski et al., 1981] from 1976 to 2013. Focal mechanisms correspond mainly to internal tearing. Outer trench bending is ruled out as the cause for normal faulting since dilatational focal mechanisms are obviously present east of the trench, where the slab is already plunging with normal angles and is not subjected to bending forces.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).


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  • Here is another way to look at these beach balls.

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.

    • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.


    • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.


    • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

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