Superficial simplicity of the 2010 El Mayor-Cucapah earthquake of Baja California in Mexico

Nature Geoscience (Impact Factor: 11.74). 07/2011; 4:615-618. DOI: 10.1038/ngeo1213


The geometry of faults is usually thought to be more complicated at the surface than at depth and to control the initiation, propagation and arrest of seismic ruptures(1-6). The fault system that runs from southern California into Mexico is a simple strike-slip boundary: the west side of California and Mexico moves northwards with respect to the east. However, the M(w) 7.2 2010 El Mayor-Cucapah earthquake on this fault system produced a pattern of seismic waves that indicates a far more complex source than slip on a planar strike-slip fault(7). Here we use geodetic, remote-sensing and seismological data to reconstruct the fault geometry and history of slip during this earthquake. We find that the earthquake produced a straight 120-km-long fault trace that cut through the Cucapah mountain range and across the Colorado River delta. However, at depth, the fault is made up of two different segments connected by a small extensional fault. Both segments strike N130 degrees E, but dip in opposite directions. The earthquake was initiated on the connecting extensional fault and 15 s later ruptured the two main segments with dominantly strike-slip motion. We show that complexities in the fault geometry at depth explain well the complex pattern of radiated seismic waves. We conclude that the location and detailed characteristics of the earthquake could not have been anticipated on the basis of observations of surface geology alone.

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Available from: Eric J Fielding, Oct 02, 2015
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    • "The northern Baja California peninsula has active normal and strike-slip faults originating from the transtensional limit between the Pacific and North America plates (Stock et al., 1991). In this limit, separation of the Baja California peninsula from the continental North America plate occurs and continues to the northwest through the right-lateral movement (Fig. 1 inset ) of the San Andreas fault system (Axen and Fletcher, 1998; Suárez-Vidal et al., 2007; Plattner et al., 2007; Suárez-Vidal et al., 2008; Armstrong, 2010; Wei et al., 2011; Oskin et al., 2012). Within our study area (Fig. 1), south of Mexicali Valley, the Cucapah and El Mayor mountain ranges are located northwest of the Gulf Extensional Province (Suárez-Vidal, et al. 2008). "
    Seismological Research Letters 01/2015; 86(1). DOI:10.1785/0220140007 · 2.16 Impact Factor
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    • "But for earthquakes in the Imperial Valley, the depths appear different because of the deep sediment basin. The southern San Andreas fault system has a long history of significant earthquakes including the recent 2010 M w 7.2 El Mayor–Cucapah event (Wei et al., 2011). The most recent burst of earthquakes occurred on 26 August 2012 with the largest one having M w 5.4. "
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    ABSTRACT: Resolving earthquake parameters, especially depth, is difficult for events occurring within basins because of issues involved with separating source properties from propagational path effects. Here, we demonstrate some advantages of using a combination of teleseismic and regional waveform data to improve resolution following a bootstrapping approach. Local SS-S differential arrivals from a foreshock are used to determine a local layered model which can then be used to model teleseismic depth phases: pP, sP, and sS. Using the cut-and-paste (CAP) method for which all strike (theta), dip (delta), rake (lambda), and depth variations are sampled for several crustal models. We find that regional data prove the most reliable at fixing the strike, whereas the depth is better constrained by teleseismic data. Weighted solutions indicate a nearly pure strike-slip mechanism (theta = 59 degrees +/- 1 degrees) with a centroid depth of about 4.0 km and an M-w of 5.4 for the mainshock of the 2012 Brawley earthquake.
    Bulletin of the Seismological Society of America 04/2013; 103(2A):1141-1147. DOI:10.1785/0120120324 · 2.32 Impact Factor
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    • "e synthetics are computed using the three velocity models in Figure 2a . Both data and synthetics are filtered to 0 . 02 ~ 3 . 0 Hz . The peak amplitudes of data ( first ) and synthetic ( second ) are shown . data prove particularly useful in defining the slip distribution for large complex ruptures such as the M w 7 . 2 El Mayor - Cucapah event [ Wei et al . , 2011 ] . Unfortunately , use of the static GPS data alone for the present analysis is problematic due to the accumulated deformation of the swarm activity , and the relatively small magnitudes of the swarm events . How - ever , combining the geodetic and seismic data in the analysis provides a powerful tool for examining the relative contrib"
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    ABSTRACT: We investigate the finite rupture processes of two M > 5 earthquakes in the 2012 Brawley swarm by joint inversion of nearby strong motion and high-rate GPS data. Waveform inversions up to 3 Hz were made possible by using a small event (Mw3.9) for path calibration of the velocity structure. Our results indicate that the first (Mw5.3) event ruptured a strong, concentrated asperity with offsets of ~20 cm centered at a depth of 5 km. The subsequent Mw5.4 event occurred 1.5 h later with a shallower slip distribution that surrounds and is complementary to that of the earlier event. The second event has a longer rise time and weaker high-frequency energy release compared to the Mw5.3 event. Both events display strong rupture directivity toward the southwest and lack of very shallow (<2 km) coseismic slip. The hypocenters for these events appear to be near or in the bedrock, but most of the slip is distributed at shallower depths (<6 km) and can explain a large part of the GPS offsets for the swarm. The complementary slip distributions of the two events suggest a triggering relationship between them with no significant creep needed to explain the various data sets.
    Geophysical Research Letters 03/2013; 40(5). DOI:10.1002/grl.50259 · 4.20 Impact Factor
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