THE SURFACE RUPTURES ASSOCIATED WITH THE EL MAYOR-BORREGO EARTHQUAKE SEQUENCE

Surface rupture associated with the April 4, 2010 El Mayor-Borrego earthquake extends ~100 km from the northern tip of the Gulf of California to the international border and comprises two distinct geomorphologic and structural domains. The rupture is complex, with breaks along multiple fault strands, including minor re-rupture of the scarps associated with the 1892 Laguna Salada earthquake and several other older events. The southern part of the rupture consists of a zone of distributed fracturing and liquefaction that cuts across the Colorado River delta. Individual fractures vary widely in orientation and have relatively short strike lengths of the order of hundreds of meters. The zone itself may be related to faults that bound the eastern margin of the Sierra El Mayor, but field relationships are unclear, and the zone of more intense fracturing diverges significantly from the mountain front toward the south. The northern half of the rupture propagated 55 km through an imbricate stack of east-dipping faults in the Sierra Cucapah. In the southern Sierra Cucapah, rupture extends 20 km along the Laguna Salada and Pescadores faults and reached a maximum displacement of ~ 250 cm of right-lateral strike-slip. The amount of dip slip is variable and changes polarity along strike along the Laguna Salada Fault before becoming predominantly east-down with maximum offsets of 150 cm along the Pescadores Fault. This ruptured terminates in the high elevations of the sierra and jumps nearly 10 km north in a left stepover to the Borrego fault. Additionally the northern Laguna Salada fault (1892 segment) rebroke with minor (10-30 cm) dip slip along a segment that is adjacent to the primary Borrego rupture. Maximum measured displacement along the Borrego fault in Borrego Valley was about 3.1 m of strike slip and another 2 m of down-to-the-east dip slip on a nearly vertical fault, yielding oblique slip of nearly 4 m. A low-angle detachment intersects the footwall of the central portion of the Borrego fault at a segment boundary, and rupture bifurcates with a splay that follows the trace of the detachment in a more westerly direction. Over the next 6 km to the north, rupture steps left across a 2km wide zone before finally consolidating on a fault that we have named the Paso Superior Fault, extending an additional ~10 km farther north. The Paso Superior fault is well-exposed at Highway 2, where it is clearly involves a low-angle detachment. Scarps near the fault trace accommodate dip slip, and nearly twice as much strike-slip is spread across a 100-150 m wide zone of cracking and secondary faulting to the east.

Part of the complexity of the rupture can be attributed to interaction with detachment faults that allow the rupture to expand in the near surface. This rupture illustrates the complexity that can develop when a rupture propagates through a network of high- and-low angle faults that accommodate the three-dimensional strain of transtensional plate margin shearing.

Additional co-authors on this abstract: Ling-ho Chung; Sinan Akciz (Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139); John Galetzka (Tectonics Observatory, California Institute of Technology, CA 91125); Joann Stock (Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125); Kate Scharer