Earthquake Report: Chile Update #2

Today the swarm has reminded us to stay vigilant. This region of the Chile subduction zone is pretty active and adjacent to the most active part of the Chile subduction zone. Today there was a series of earthquakes with a maximum magnitude of M 5.9. Here is the USGS webpage for this M 5.9 earthquake.

• I have prepared several other reports for the recent seismicity here. More background information about the subduction zone history can be found there.
• 2017.04.23 M 5.9
• 2017.04.24 M 6.9
• 2017.04.24 M 6.9 Update #1

The current sequence is just to the south of the 1971 M 7.0 earthquake and has a similar along-strike distance. This may be all that we will see, but there is a small chance this will lead to an earthquake with a larger magnitude. I suspect this chance is not very high (low likelihood). If we consider the 1985 earthquake as an analog, there were only a few earthquakes prior to the mainshock. The 2017 swarm has had many earthquakes to date. I suspect that the M 6.9 is the mainshock for this series. Also, consider that the 1985 region overlaps slightly with the 2010 earthquake. While this region was an area of low slip in 2010, there might be a reason for this (e.g. the Juan Fernandez Ridge, JFR). The JFR may act as an asperity. Asperities have 2 main definitions: (1) region of largest slip during an earthquake (2) region of a fault across which strain is accumulated due to the material properties of the fault and crust. I use the second definition. Smooth subduction zone faults may be responsible for large magnitude earthquakes and rough subduction zone faults (with asperities like the JFR) may be responsible for smaller, more frequent, earthquakes. If we look at the Métois et al. (2016) figure below, this region of the subduction zone has a high rate of seismicity. If this seismicity is the result of a rough fault, it seems that this part of the megathrust may not store as much strain as other parts of the subduction zone. However, this part of the fault has slipped during Great earthquakes (M > 8.0) in the past. So, it is difficult to say. While I might be wrong, given what we know about this subduction zone, it seems like this swarm is not going to result in a Great earthquake this time. Others disagree with me and that is great! I am only looking at the seismicity from the past and others are actively testing the cycling of seismic strain (and coulomb stress) in this region.

Here is more about the Juan Fernandez Ridge:
• von Huene et al. (1997), Tectonic control of the subducting Juan Fernandez Ridge on the Andean margin near Valparaiso, Chile
• and Rodrigo and Lara (2014) Plate tectonics and the origin of the Juan Fernández Ridge: analysis of bathymetry and magnetic patterns.

I include the moment tensors from each of the Great Earthquakes, as well as the 2017 M 5.9, 6.9, and 5.9 earthquakes.

• In the interpretive poster below
• I outline the 1985 aftershock region in black dashed lines
• I outline the 2010 aftershock region in blue dashed lines
• I outline the 2015 aftershock region in white dashed lines
• I outline the 2017 aftershock region in red dashed lines

Below is my interpretive poster for this earthquake. Click on the map to enlarge.

• 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 one inset figure in the poster.

• In the lower right corner I present the space-time diagram from Métois et al. (2016). They plot seismicity vs. depth on the right. I placed a green bar with the approximate latitudinal range of the 2017 sequence.
• In the lower left corner I present the figure from Métois et al. (2016) that shows their estimates of seismic coupling (the proportion of the fault that is “locked” and accumulating strain). Regions of low coupling do not accumulate strain, do not resist earthquake rupture, and do not contribute to the release of energy during earthquakes.
  I place a green bar as for the space time figure on the right. Note the high rate of seismicity in this region. Also, note that the region of the fault that is participating in this 2017 sequence is a region of lower coupling between the 2015 and 2010 patches. One problem is that the 2015 patch did not release much strain in the higher coupled area (though this area has more earthquakes, so that might release strain, limiting the possibility of future larger earthquakes.
• In the upper right corner, I plot seismicity for the past 2.5 months along with the 30 day seismicity surrounding the 1985 earthquake. I also outline the 1985 and 2017 sequences.

• Here is the space time figure from Métois et al. (2016).




Left estimated extent of large historical or instrumental ruptures along the Chilean margin adapted from ME´ TOIS et al. (2012). Gray stars mark major intra-slab events. The recent Mw[8 earthquakes are indicated in red. Gray shaded areas correspond to LCZs defined in Fig. 3. Right seismicity recorded by the Centro Sismologico Nacional (CSN) during interseismic period, color-coded depending on the event’s depth. Three zones have been defined to avoid including aftershocks and preshocks associated with major events: (1) in North Chile, we plot the seismicity from 2008 to january 2014, i.e., between the Tocopilla and Iquique earthquakes; (2) in Central Chile, we plot the seismicity on the entire 2000–2014 period; (3) in South-Central Chile, we selected events that occurred between 2000 and 2010, i.e., before the Maule earthquake.

• Here is the seismic coupling figure from Métois et al. (2016).




a Histogram depicts the rate of Mw[3 earthquakes registered by the CSN catalog during the interseismic period defined for each zone (see Fig. 2) on the subduction interface, on 0.2 of latitude sliding windows. Stars are swarm-like sequences detected by HOLTKAMP et al. (2011) depending on their occurrence date. Swarms located in the Iquique LCZ and Camarones segment are from RUIZ et al. (2014). Empty squares are significant intraplate earthquakes. b Red curve variations of the average coupling coefficient on the first 60 km of depth calculated on 0.2 of latitude sliding windows for our best model including an Andean sliver motion. Dashed pink curves are alternative models with different smoothing options that fit the data with nRMS better than 2 (see supplementary figure 6): the pink shaded envelope around our best model stands for the variability of the coupling along strike. Green curves coseismic distribution for Maule (VIGNY et al. 2011), Iquique (LAY et al. 2014) and Illapel earthquakes (RUIZ et al. 2016). Gray shaded areas stand for the identified low coupling zones (LCZs). LCZs and high coupling segments are named on the left. The apparent decrease in the average coupling North of 30S is considered as an artifact of the Andean sliver motion (see Sect. 5.2). c Best coupling distribution obtained inverting for Andean sliver motion and coupling amount simultaneously. The rupture zones for the three major earthquakes are indicated as green ellipses. White shaded areas are zones where we lack resolution

• Here is a figure from Conteras-Reyes and Carrizo, 2011 that shows how the structure of the Nazca plate may exert heterogeneous forces along the subduction zone fault.




(upper) Structure and interpretation of the (A) Nazca Ridge, (B) easternmost portion of the Juan Fernández Ridge, and (C) Mocha FZ based on the 2D seismic velocity model of Hampel et al. (2004), Kopp et al. (2004), and Contreras-Reyes et al. (2008), respectively. Map locations of seismic profiles are shown in Fig. 1A. (below) Direct comparison of these HOF’s structure with typical Nazca oceanic crust (6.5km thick). The anomalous normal stress n (buoyancy force) depends on the thickness of the corresponding anomalous crustal thickness (Hc) and on thickness of the underplated magmatic material beneath the crust and/or thickness of the serpentinized mantle (Hm). n also depends on the mantle–crust density contrast (mc = 530 kg/m3), “normal” mantle–serpentinized mantle density contrast (m= 230 kg/m3) and “normal” crust-altered crust density contrast (c = 50 kg/m3). See further details in Table 1. Density values are taken from the density model of Tassara et al. (2006).

Some background about the heterogeneous megathrust in this region

• Here is the first of two figures from Moreno et al., 2010. Note that the M 6.9 is close in space to the 1985 earthquake. Also note the along strike heterogeneous seismogenic coupling. I include the figure caption below in blockquote.




Tectonic setting of the study area, data, observations and results. a, Shaded relief map of the Andean subduction zone in South- Central Chile. Earthquake segmentation along the margin is indicated by ellipses that enclose the approximate rupture areas of historic earthquakes (updated from refs 4–6). The inset shows the location of panel a (rectangle) relative to the South American continent. b, Compilation of GPS-observed surface velocities (1996–2008) with respect to stable South America before the 2010 Maule earthquake (for references see online-only Methods). Ellipses attached to the arrows represent 95% confidence limits. c, GPS 1 FEM modelled interface locking (fraction of plate convergence) distribution along the Andean subduction zone megathrust in the decade before the 2010 Maule earthquake. The epicentre (white star, USGS NEIC) and focal mechanism (beach ball, GCMT, http://www.globalcmt.org) of the 2010 Maule earthquake are shown in panels a and c.

• Here is the second of the two figures from Moreno et al. (2010).




Relationship between pre, co- and postseismic deformation patterns. a, Coseismic slip distribution during the 2010 (blue contours; USGS slip model26) and 1960 (green contours; from ref. 30) earthquakes overlain onto pre-seismic locking pattern (red shading $0.75), as well as early (during the first 48 h post-shock) M$5 aftershock locations (the grey circle sizes scale with magnitude; GEOFON data29). b, Histograms of early (first 48 h; total number of events, 80) and late (first 3 months; total number of events, 168) aftershock density along a north–south profile (GEOFON data29, M$5). c, Residual slip deficits since 1835 as observed after the 2010 earthquake along a north–south profile (left column, based on the USGS slip model26). The middle and right columns show the effects on slip deficit of overlapping twentieth-century earthquakes (the black lines are polynomial fits to the data). Coloured data points and dates indicate earthquakes by year of occurrence.

• Here are the major earthquakes from the 21st century.
• 1960.005.22 M 9.5
• 2001.06.23 M 8.4
• 2006.04.30 M 6.7
• 2007.11.14 M 7.7
• 2010.02.27 M 8.8
• 2014.04.01 M 8.2
• 2014.04.03 M 7.7
• 2015.19.16 M 8.3

References:

• Beck, S, Barientos, S., Kausel, E., and Reyes, M., 1998. Source Characteristics of Historic Earthquakes along the Central Chile Subduction Zone in Journal of South American Earth Sciences, v. 11, no. 2., p. 115-129.
• Contreras-Reyes, E. and Carrizo, D., 2011. Control of high oceanic features and subduction channel on earthquake ruptures along the Chile–Peru subduction zone in Physics of the Earth and Planetary Interiors, v. 186, p. 49-58 doi:10.1016/j.pepi.2011.03.002
• Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
• Hayes, G.P., Smoczyk, G.M., Benz, H.M., Villaseñor, Antonio, and Furlong, K.P., 2015. Seismicity of the Earth 1900–2013, Seismotectonics of South America (Nazca Plate Region): U.S. Geological Survey Open-File Report 2015–1031–E, 1 sheet, scale 1:14,000,000, http://dx.doi.org/10.3133/ofr20151031E.
• Lin, Y.N., Slkaden, A., Ortega-Culaciati, F., Simons, M., Avouac, J-P., Fielding, E.J., Brooks,, B.A., Bevis, M., Genrich, J., Rietbrock, A., Vigny, C., Smalley, R., and Socquet, A., 2013. Coseismic and postseismic slip associated with the 2010 Maule Earthquake, Chile: Characterizing the Arauco Peninsula barrier effect in JGR, v. 118, p. 1-18, doi:10.1002/jgrb.50207, 2013
• Melnick, D., Bookhagen, B., Echtler, H.P., and Strecker, M.R., 2006. Coastal deformation and great subduction earthquakes, Isla Santa María, Chile (37°S) in GSA Bulletin, v. 118, no. 11/12, p. 1463-1480.
• Melnick, D., 2009. Journal of Geophysical Research 114, B01407
• Métois, M., Vigny, C., and Socquet, A., 2016. Interseismic Coupling, Megathrust Earthquakes and Seismic Swarms Along the Chilean Subduction Zone (38°–18°S) in Pure and Applied Geophysics, DOI 10.1007/s00024-016-1280-5
• Moernaut, J., Batist, M., Haeirman, K., Van Daele, M., Brümmer, R., Urrutia, R., Wolff, C., Brauer, A., Roberts, S., Kilian, R., Pino, M., 2010. Recurrence of 1960-like earthquake shaking in South-Central Chile revealed by lacustrine sedimentary records in proceedings Chapman Conference on Giant Earthquakes and Their Tsunamis Valparaíso, Viña del Mar, and Valdivia, Chile 16–24 May 2010.
• Moreno, M., Rosenau, M., and Oncken, O., 2010. 2010 Maule earthquake slip correlates with pre-seismic locking of Andean subduction zone in Nature, v. 467, p. 198-202
• Moreno, M., Melnick, D., Rosenau, M., Bolte, J., Klotz, J., Echtler, H., Basez, J., Bataille, K., Chen, J., Bevis, M., Hase, H., Oncken, O., 2011. Heterogeneous plate locking in the South–Central Chile subduction zone: Building up the next great earthquake in Earth and Planetary Science Letters, v. 305, p. 413-424.
• Rhea, Susan, Hayes, Gavin, Villaseñor, Antonio, Furlong, K.P., Tarr, A.C., and Benz, H.M., Seismicity of the earth 1900–2007, Nazca Plate and South America: U.S. Geological Survey Open-File Report 2010–1083-E, 1 sheet, scale 1:12,000,000.
• Rodrigo, C. and Lara, L.E., 2014. Plate tectonics and the origin of the Juan Fernández Ridge: analysis of bathymetry and magnetic patterns in Lat. Am. J. Aquat. Res, v. 42, no. 4, p. 907-917
• Schurr, B., Asch, G., Hainzl, S., Bedford, J., Hoechner, A., Palo, M., Wang, R., Moreno, M., Bartsch, M., Zhang, Y., Oncken, O., Tilmann, F., Dahm, T., Victor, P., Barrientos, S., and Villotte, J-P., 2014. Gradual unlocking of plate boundary controlled initiation of the 2014 Iquique earthquake in Nature, v. 512, p. 299-3202, doi:10.1038/nature13681
• von Huene, R. et al., 1997. Tectonic control of the subducting Juan Fernandez Ridge on the Andean margin near Valparaiso, Chile in Tectonics, v. 16, no. 3, p. 474-488.