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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    • "For simplicity, we estimate the average horizontal and vertical displacement along a section of fault to calculate total oblique slip for the estimation of moment release. For this model, we use a range of locking depths of 12 2 km (10–14 km) based on the maximum depth of local seismicity (Hauksson et al., 2010; Wei et al., 2011) and apply the standard equation for moment M 0 , which is the product of rupture area, average displacement, and shear rigidity of the crust, assumed to be 3:1 × 10 11 dyn·cm 2 (Hanks and Kanamori, 1979). A 12 km locking depth on a 60°–90° dipping fault yields a fault width of about 12–14 km; and, using the Figure 9 "
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    ABSTRACT: We present the results of intensive field investigations of the scarp associated with the 23 February 1892 earthquake in northern Baja California. Newly recognized additional offsets suggest the rupture was about 58 km in length, twice as long as previous estimates. Slip produced in the 1892 event varied from purely dextral slip near the international border to roughly 1:1 oblique-normal slip farther south along the 2–4-km-deep portion of the Laguna Salada basin. The portion of the 1892 rupture with oblique-normal slip comprises a number of short, poorly organized, and discon-tinuous fault scarps with heights that vary in concert with their strike. Slip was linked farther south to a short, purely normal fault that forms a large releasing bend at the southern termination of the fault zone. Given the distribution of slip along the earthquake and a likely range of locking depths, we conclude the 1892 earthquake was between M w 7.1–7.3 in magnitude, consistent with previous estimates from macroseismic observations. The length of the Laguna Salada fault that ruptured in 1892 also accommodated minor normal sense displacement along much of its length in the recent 2010 M w 7.2 El Mayor–Cucapah earthquake, which guided the remapping effort.
    Bulletin of the Seismological Society of America 10/2015; DOI:10.1785/0120140274 · 2.32 Impact Factor
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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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