Earthquake Report: Peru Update #1

Well, I missed looking further into a key update paper and used figures from an older paper on my interpretive poster yesterday. Thanks to Stéphane Baize for pointing this out! Turns out, after their new analyses, the M 7.1 earthquake was in a region of higher seismogenic coupling, rather than low coupling (as was presented in my first poster).
Also, Dr. Robin Lacassin noticed (as did I) the paucity of aftershocks from yesterday’s M 7.1. This was also the case for the carbon copy 2013 M 7.1 earthquake (there was 1 M 4.6 aftershock in the weeks following the M 7.1 earthquake on 2013.09.25; there were a dozen M 1-2 earthquakes in Nov. and Dec. of 2013, but I am not sure how related they are to the M 7.1 then). I present a poster below with this in mind. I also include below a comparison of the MMI modeled estimates. The 2013 seems to have possibly generated more widespread intensities, even though that was a deeper earthquake.

Below is my interpretive poster for this earthquake

I plot USGS seismicity from 2013.09.24 through 2014.01.26 (about 3 months), in addition to the 2018.01.14 M 7.1 earthquake.

  • 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 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. However, we must await slab v 2.0 to get a better view of these slab contours in this region.
  • I include some inset figures.

  • In the upper right corner is an updated time-space figure (showing along-strike lengths for historic earthquakes), along with slip patches for some of these earthquakes (Villegas-Lanza et al., 2016). I place a blue star in the general location of the 2018.01.14 M 7.1 earthquake.
  • This is the updated seismic coupling figure from Villegas-Lanza et al. (2016). I place a blue star in the general location of the 2018.01.14 M 7.1 earthquake. Note how this M 7.1 earthquake is in a region of higher coupling.

  • Here is a comparison of the intensity modeling for these comparable earthquakes. I present the intensity maps on top (with the moment tensors, labled with their strike, dip, and rake data; note how they are almost identical!), the attenuation relations in the middle (how intensity decays with distance from the earthquake), and the PAGER alerts at the bottom. More can be found out about PAGER alerts here.

  • Here is the Villageas-Lanza et al. (2016) figure 1, showing their time-space diagram, along with the historic earthquake limits and patches.

  • (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].

  • Here is a figure from Villegas-Lanza et al. (2016) that shows the along-strike variation in moment deficit. Moment deficit is the amount of energy absorbed into the tectonic system, from plate motions, that is stored as seismic strain to be released during earthquakes. Regions where the fault is slipping freely (aseismic), seismic moment does not accumulate, so there is no moment deficit there (e.g. along the subduction zone where the Nazca Ridge intersects the megathrust). The two panels on the right are their minimum and maximum seismogenic coupling maps (showing the end members of their models). I explain the coupling ratio (0-1, white to red in color) on my initial earthquake report.
  • The key update in this paper (an update to the Chlieh et al., 2011 results) is that these authors treated the accretionary part of the South America plate as an independent player, as a forearc sliver (sort of like a microplate)

  • (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. (see above, the time-space figure)

  • Here is the Chlieh et al. (2011) version of this figure for comparison.
  • This is the figure that adds moment deficit to the seismic moment plot and the coupling ratio to the slip patch map.

  • Comparison of interseismic coupling along the megathrust with ruptures of large megathrust earthquakes in central and southern Peru. (left) Interseismic coupling map for 3-plate model Short4; it indicates that where the Nazca ridge and the Nazca fracture zone subduct, the interseismic coupling is low. The largest earthquake there is the Mw8.1–8.2 earthquake of 1942. It is not clear whether the 1942 rupture propagated through the Nazca ridge or stopped south of it. High interseismic coupling patches correlate well with regions that experienced great megathrust earthquake Mw8.8 in 1868 and Mw8.6–8.8 in 1746. In the south, the presence of two wide asperities separated by a wide aseismic patch may explain partially the seismic behavior of this segment in the last centuries. Individual ruptures of these asperities would produce Mw8 events, as in 2001, but their simultaneous rupture could generate great Mw > 8.5 earthquakes as in 1604 or 1868. The along-strike coincidence of the high coupling areas (orange-red) with the region of high coseismic slip during the 2001 Arequipa and 2007 Pisco earthquakes suggests that strongly coupled patches during the interseismic period may indicate the location of future seismic asperities. (right) Moment deficit (dashed lines) since the last great earthquake of 1868, 1942 and 1746 compared with the seismic moment released during recent and historical earthquakes of Figure 8. The moment deficit is computed from the rate of moment deficit predicted by model Short4 considering a steady state interseismic process (Max) or 50% of it (Min) to account for time-variable interseismic process and transient events.

  • Here is an updated moment deficit figure for this part of the Peru-Chile trench (Villegas-Lanza et al., 2016). I include the Chlieh et al. (2011) figure for comparison.
  • Chlieh et al., 2011
  • This figure shows their results used to show how different parts of the subduction zone have higher or lower moment deficits. The Central Peru section shows that there is an unmet interseismic deficit, while the southern Peru profile shows that earthquakes have been keeping up with plate convergence here. The central Peru region is the region of the subduction zone shown in the 2 figures above this one (Arica, Peru is the southern boundary between the central and southern Peru regions of this subduction zone).

  • Cumulative deficit of moment and seismic moment released due to major subduction earthquakes since the 16th century (top) in central Peru and (bottom) in southern Peru. The cumulative deficit of moment is predicted from the rates of 3-plate models Short4 in central Peru and Short10 in southern Peru (Table 5 and Figure 4). The uncertainties of moment released by historical events lead to a minimum and maximum moment released (see Table S8 in the auxiliary material). The uncertainties on the cumulative deficit of moment allow that nonlinear interseismic and viscous processes could have released 50% of the accumulated moment deficit. The remaining fraction should reflect elastic strain available to drive future earthquakes unless it would have been totally released by anelastic deformation of the forearc.

  • Villegas-Lanza et al., 2016

  • Cumulative moment deficit corrected from large earthquakes moment released since 1746, computed using the maximum, mean, and minimum interseismic models presented in Figure 6 and Table S8.


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