Earthquake Report: Chile!

There have been a number of earthquakes along the subduction zone offshore of Chile. These have happened near the boundary of two Great Earthquakes from 2010 and 2015. This region may be a segment boundary along the subduction zone, albeit possibly a non persistent one. The Juan Ferndandez ridge may control this segmentation. While this is in a region of low slip for the 2010 and 2015 earthquakes, due to the proximity of the Juan Fernandez fracture zone (which possibly promotes smaller earthquakes), there may not be a larger earthquake here. If there is, it might look something like the 1971 earthquake, with a magnitude of low 7 or so. (which could still be quite damaging).

The earthquakes from today and yesterday form a range of about 1 1/2 magnitudes (M 4.2- M 5.9). This may be considered a swarm (when there are a series of earthquakes along a fault with similar magnitudes), though there is an M 5.9 that could be considered the mainshock. But, I would not get hung up on terminology as that is not very important. However, there is a great page with a discussion about swarms, including some good examples.

Here are the USGS websites for these earthquakes

I took a look at the seismicity from the past century. Here are Google Earth kml files from the USGS website for earthquakes from 1917-2017 with magnitudes M ≥ 5.0, M ≥ 6.0, and M ≥ 7.0.

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 the USGS epicenters for earthquakes from 1917-2017 with magnitudes M ≥ 5.0. I outline the regions of the subduction zone that have participated in earthquake slip during the 21st century (in white dashed polygons). I include USGS moment tensors from the largest earthquakes. I plot the focal mechanism for the 1960 earthquake from Moreno et al. (2011). Note the gap in seismicity in the region of the 1960 M 9.5 earthquake, except for the 2016 M 7.6 earthquake. Also, note how the 1960 and 2010 earthquake slip patches overlap.

Much of the subduction zone has ruptured, except for some spots between the 2001 and 2015 earthquakes. In 2015, I speculated that the region north of the 2015 earthquakes constituted a seismic gap. This region may get filled by a Great subduction zone earthquake or may continue to slip in moderate sized earthquakes (or be aseismic). There was an earthquake in 1877 that spanned 19-23 degrees (overlapping with the 2014 earthquake). This is shown on the Schurr et al. (2014) figure below).

  • 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 figure from Lin et al. (2013) that shows the tectonic context of the 2010 Maule earthquake. On the map are plotted extents of historic earthquakes along this convergent plate margin. On the right is a large scale map showing the active magmatic arc volcanoes associated with this subduction zone. Finally, there is a cross section showing where the coseismic slip and postseismic slip occurred as part of the 2010 earthquake sequence. I placed a green triangle at the approximate location of this 2017 swarm.
  • In the lower right corner, I include a time-space diagram from Moernaut et al. (2010). There is also a map showing the fracture zones. I placed a green triangle at the approximate location of this 2017 swarm.
  • Above the Moernaut et al. (2010) figure, I present Figure 2 from Beck et al. (1998 ) on the map, the space-time plot of historic and prehistoric earthquakes associated with the Chile subduction zone. This space-time plot overlaps slightly with the Moernaut figure. I add a green line showing my interpretation for the strike length of the 2015 M 8.3 earthquake. Originally it appeared to match the 1943 and 1880 earthquakes, though it appears to extend further along strike. The 1922 and 1880 strike lengths are not well constrained, so this 2015 earthquake may indeed be slipping the same patch of this part of the subduction zone. Indeed, Juan Fernandez Ridge may be a structural boundary that may cause segmentation in this part of the subduction zone. If it does, it does not do so every time, as evidenced by the strike-length of the 1730 AD and 1647 AD earthquakes. I placed a green triangle at the approximate location of this 2017 swarm.
  • In the upper right corner is a space-time figure showing earthquakes for the past few centuries. This diagram does not overlap with the Beck figure. This figure shows the outline of some subduction zone earthquakes and shows how the 2014 earthquake is composed of two earthquakes (an M 8.1 and an M 7.6) that ruptured different but adjacent patches of the subduction zone.
  • In the upper left corner, I include a local map showing the MMI contours for the M 5.9 earthquake. I include the USGS moment tensors from most of the earthquakes in this swarm.

  • Here is the figure from Lin et al. (2013) that shows the tectonic context of the 2010 Maule earthquake. I include the figure captions as blockquote.

  • (a) Regional tectonic map showing slab isodepth contours (blue lines) [Cahill and Isacks, 1992], M>=4 earthquakes from the National Earthquake Information Center catalog between 1976 and 2011 (yellow circles for depths less than 50 km, and blue circles for depths greater than 50 km), active volcanoes (red triangles), and the approximate extent of large megathrust earthquakes during the past hundred years (red ellipses) modified from Campos et al. [2002]. The large white vector represents the direction of Nazca Plate with respect to stable South America [Kendrick et al., 2003]. (b) Simplified seismo-tectonic map of the study area. Major Quaternary faults are modified after Melnick et al. [2009] (black lines). The Neogene Deformation Front is modified from Folguera et al. [2004]. The west-vergent thrust fault that bounds the west of the Andes between 32 and 38S is modified from Melnick et al. [2009]. (c) Schematic cross-section along line A–A0 (Figure 1b), modified from Folguera and Ramos [2009]. The upper bound of the coseismic slip coincides with the boundary between the frontal accretionary prism and the paleo-accretionary prism [Contreras-Reyes et al., 2010], whereas the contact between the coseismic and postseismic patch is from this study. The thick solid red line and dashed red line on top of the slab represent the approximate coseismic and postseismic plus interseismic slip section of the subduction interface. The thin red and grey lines within the overriding plate are active and inactive structures in the retroarc, modified from Folguera and Ramos [2009]. The red dashed line underneath the Andean Block represents the regional décollement. Background seismicity is from the TIPTEQ catalog, recorded between November 2004 and October 2005 [Rietbrock et al., 2005; Haberland et al., 2009].

  • Here is a cross section of the subduction zone just to the south of this Sept/Nov 2015 swarm (Melnick et al., 2006). Below I include the text from the Melnick et al. (2006) figure caption as block text.

  • (A) Seismotectonic segments, rupture zones of historical subduction earthquakes, and main tectonic features of the south-central Andean convergent margin. Earthquakes were compiled from Lomnitz (1970, 2004), Kelleher (1972), Comte et al. (1986), Cifuentes (1989), Beck et al. (1998), and Campos et al. (2002). Nazca plate and trench are from Bangs and Cande (1997) and Tebbens and Cande (1997). Maximum extension of glaciers is from Rabassa and Clapperton (1990). F.Z.—fracture zone. (B) Regional morphotectonic units, Quaternary faults, and location of the study area. Trench and slope have been interpreted from multibeam bathymetry and seismic-reflection profiles (Reichert et al., 2002). (C) Profile of the offshore Chile margin at ~37°S, indicated by thick stippled line on the map and based on seismic-reflection profiles SO161-24 and ENAP-017. Integrated Seismological experiment in the Southern Andes (ISSA) local network seismicity (Bohm et al., 2002) is shown by dots; focal mechanism is from Bruhn (2003). Updip limit of seismogenic coupling zone from heat-fl ow measurements (Grevemeyer et al., 2003). Basal accretion of trench sediments from sandbox models (Lohrmann, 2002; Glodny et al., 2005). Convergence parameters from Somoza (1998 ).

  • In March 2015, there was some seismicity in this September/November 2015 earthquake slip region. I put together an earthquake report about those earthquake of magnitudes M = 5.0-5.3. I speculate that the 1922 earthquake region is a seismic gap. Note that this September/November 2015 earthquake region is along the southern portion of the seismic gap that I labeled on the map below.
  • Here is a map that shows the recent swarm of ~M = 5 earthquakes. There are moment tensors for the earthquakes listed below, some recent historic subduction zone earthquakes. I placed the general along-strike distance for older historic earthquakes in green (and labeled their years). The largest earthquake ever recorded, the Mw = 9.5 Chile earthquake, had a slip patch that extends from the south of the map to just south of the 2010 earthquake swarm. The 2010 and 2014 earthquake swarm epicenters are plotted as colored circles, while most other historic earthquake epicenters are plotted as gray circles. Note how this March 2015 swarm is at the northern end of the 1922/11/11 M 8.3 earthquake. At the bottom of this page, I put a USGS graphic about what these moment tensor plots (beach balls) tell us about the earthquakes.

  • Here is the first space-time figure from Schurr et al., 2014. I include their caption as blockquote below.

  • Map of Northern Chile and Southern Peru showing historical earthquakes and instrumentally recorded megathrust ruptures. IPOC instruments used in the present study (BB, broadband; SM, strong motion) are shown as blue symbols. Left: historical1,2 and instrumental earthquake record. Centre: rupture length was calculated using the regression suggested in ref. 28, with grey lines for earthquakes M .7 and red lines for Mw .8. The slip distribution of the 2014 Iquique event and its largest aftershock derived in this study are colour coded, with contour intervals of 0.5 m. The green and black vectors are the observed and modelled horizontal surface displacements of the mainshock. The slip areas of the most recent other large ruptures4,5,7 are also plotted. Right: moment deficit per kilometre along strike left along the plate boundary after the Iquique event for moment accumulated since 1877, assuming current locking (Fig. 3a). The total accumulated moment since 1877 from 17u S to 25u S (red solid line) is 8.97; the remaining moment after subtracting all earthquake events with Mw .7 (grey dotted line) is 8.91 for the entire northern Chile–southern Peru seismic gap

  • Here is the Beck et al. (1998) space time diagram.

  • Finally, here is the southernmost space-time diagram from Moernaut et al. (2010). These data are largely derived from Melnick et al. (2009).

  • Setting and historical earthquakes in South-Central Chile. Data derived from Barrientos (2007); Campos et al. (2002); Melnick et al.(2009).

Here is an animation of seismicity from the 21st century

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