Earthquake Report: Ecuador

Earlier today there was a moderate sized earthquake (M 6.0) along coast of Ecuador. This earthquake happened in the region of the 2016.04.16 M 7.8 subduction zone earthquake. Based upon the depth and our knowledge of this region, this earthquake may also be on the megathrust. However, the depth is poorly resolved (initially depth ~ 7 km, but now set at 10 km, the default depth).

Here is the USGS website for this earthquake.

Since I wrote this report, the depth has been updated to 13km. This places it about 7km shallower than the megathrust fault, based on the USGS Slab model. However, this slab model has some uncertainty. So, the earthquake [still] may or may not be on the megathrust interface.

Also, the original report included a link to the now defunct earthquakereport.com. That website no longer exists. But earthjay does!

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 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 contours plotted (Hayes et al., 2012), 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.

    I include some inset figures in the poster.

  • In the lower left corner I include a clipping of the map and cross section from the USGS Open File Report for the historic seismicity of this region (Rhea et al., 2010). I include the seismicity cross section in the upper left corner. This cross section shows earthquakes related to the downgoing Nazca plate.
  • Between the map and cross section, I include the MMI intensity maps for both the M 7.8 earthquake and this M 6.0 earthquake.
  • In the upper right corner, I include a map that shows the regional tectonics as published by Gutscher et al. (1999). These authors pose that the Carnegie Ridge exerts a control for the segmentation of the subduction zone.
  • In the lower right corner, I include a figure from Chlieh et al. (2014) that shows their coupling model. This model informs us about how strongly the subduction zone fault is seismogenically “locked” and how this varies spatially. They also plot historical earthquake locations and their “moment rate deficit” calculation (i.e. how much the plate motion rate has been accumulated as tectonic strain, which would presumably lead to earthquake slip). I include blue stars in the general location of these two earthquakes. The M 7.8 lies within the seismic gap hypothesized by Chlieh et al. (2014).


  • Here is the same poster but only with the seismicity from the past month and the MMI contours from the 2016 M 7.8 earthquake. (wait for now to get this done… need to restart software)


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  • Here are my two interpretive posters for the 2016 M 7.8 earthquake. Here is my initial report. Here is my update.
  • First is the initial interpretive poster.


  • Here is the updated interpretive poster.


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  • Below is the tectonic setting map from Gutscher et al. (1999). I include their figure caption as a blockquote.


    • 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.

  • Below is a low angle oblique view of the structures in the downgoing Nazca plate, from Gutscher et al. (1999). I include their figure caption as a blockquote.


    • 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.

  • The 2016 M 7.8 earthquake is near two historic earthquakes with similar magnitudes. Below I plot a map showing the seismicity from 1900-2016 for earthquakes with magnitudes greater than or equal to M 6.0. Here is the USGS query that I used to make this map.
    • 1906.01.31 M 8.3 occurred ~100 km to the northeast.
    • 1942.05.14 M 7.8 occurred <50 km to the southwest.

  • Here are a couple maps from Chlieh et al. (2014). I include their figure captions below. Chlieh et al. (2014) use GPS data to infer the spatial variation and degree to which the subduction zone megathrust is seismogenically coupled. They consider plate motion rates and estimate the moment (earthquake energy) deficit along this fault (how much strain that plate convergence has imparted upon the fault over time). Then they compare this moment deficit to regions of the fault that have slipped historically.
  • Tectonics and GPS motion rates.


    • 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 indicates the approximate rupture of the great 1906 Mw=8.8 Colombia–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. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

  • GPS velocities along with historic earthquake patches.


    • Interseismic GPS velocity field in the North Andean Sliver reference frame. The relative Nazca/NAS convergence rate is 46 mm/yr. The highest GPS velocity of 26 mm/yr is found on La Plata Island that is the closest point to the trench axis. The GPS network adequately covers the rupture areas of the 1998 Mw=7.1, 1942 Mw=7.8and 1958 Mw=7.7 earthquakes but only 1/4th of the 1979 Mw=8.2 and 2/3rd of the great 1906 Mw=8.8 rupture area. The black star is the epicenter of the great 1906 event and white stars are the epicenters of the Mw>7.01942–1998 seismic sequence. Grey shaded ellipses are the high slip region of the 1942, 1958, 1979 and 1998 seismic sources (Beck and Ruff, 1984;Segovia, 2001; Swenson and Beck, 1996). Red dashed contours are the relocated aftershocks areas of the 1942, 1958 and 1979 events (Mendoza and Dewey, 1984). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

  • Moment deficit along strike and historic earthquake locations (Chlieh et al., 2014). The 2016 M 7.8 earthquake may have occurred in the region marked “gap” in these figures.


    • (A) Along-strike variations of the annual moment deficit for all the interseismic models shown in Fig.5. (B)Maximum ISC model and (C)Minimum ISC model. (A)The blue, green and red lines correspond to the along-strike variation of the annual moment deficit rate respectively for models with smoothing coefficient λ1 =1.0, 0.25 and 0.1. (B) Smoother solution of Fig.5 ith a maximum moment deficit rate of 4.5 ×1018N m/yr. (C)Rougher solution of Fig.5 with a minimum moment deficit rate of 2.5 ×1018N m/yr. Yellow stars are the epicenters of subduction earthquakes with magnitude Mw>6.0 from the last 400 yr catalogue (Beauval et al., 2013). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

References

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