Assembly of a large earthquake from a complex fault system: surface rupture kinematics of the April 4, 2010 El Mayor-Cucapah Mw7.2 earthquake

John M. Fletcher, Orlando J. Teran, Thomas K. Rockwell, Michael E. Oskin, Kenneth W. Hudnut, Karl J. Mueller, Ronald M. Spelz, Sinan O. Akciz, Eulalia Masana, Geoff Faneros, Joann M. Stock, Austin J. Elliott, Peter O. Gold, Jing Liu, Alejandro A. Gonzalez-Ortega, & David K. Lynch

Published June 24, 2014, SCEC Contribution #1697

The 4 of April, 2010 Mw 7.2 El Mayor-Cucapah earthquake revealed the existence of a previously unidentified fault system that extends ~120 km from the northern tip of the Gulf of California to the international border. The system is oriented ~NW-SE and is composed of at least six major faults linked by numerous smaller faults, which makes this one of the most complex surface rupture documented along the Pacific-North America plate boundary. Rupture initiated near the center of the fault system and propagated bilaterally into three distinct kinematic domains. Southeast of the epicenter, fault rupture is observed sporadically for ~53 km through the Colorado River delta and is characterized by oblique dextral southwest-down slip. Northeast of the epicenter, rupture propagated ~55 km through the Sierra Cucapah along multiple fault strands within an imbricate stack of east-dipping dextral-normal faults. The northern end of the rupture is located in the Yuha Desert and forms a separate domain characterized by minor offsets (<8cm) on several NNE-striking cross faults scattered throughout the uplifted and folded sediments of the Colorado River delta. One of the main characteristics of the rupture in the Sierra Cucapah is its structural complexity at the surface. Primary rupture occurred on four major faults separated by one fault branch and two accommodation zones. The accommodation zones are distributed in a left-stepping en echelon geometry, such that rupture passed systematically to structurally lower faults. The structurally lowest fault that ruptured in this event is inclined only ~20°, and presents a clear example of coseismic slip along a low-angle normal fault. Surface offsets in the Sierra Cucapah are, on average, ~200 cm with individual measurements commonly reaching 300-400 cm. Rupture kinematics varies greatly. In the southern Sierra Cucapah, surface offsets are dominated by strike-slip, whereas in the north, faults have highly varying ratios of normal slip to strike-slip. The zone of surface rupture is widest and most complex where it crosses the trace of the oppositely dipping Laguna Salada fault, which produced a similar sized earthquake in 1892. Paleoscarps associated with the 1892 event re-ruptured with minor vertical slip, and where both faults systems intersect in the shallow subsurface, the 1892 scarps re-ruptured with 2010 primary displacement. We find that instantaneous extension directions derived from synthesizing surface rupture measurements are consistently oriented 085°, which is consistent with geodetically derived extension directions for the adjacent Laguna Salada basin. Despite the complexity of its surface rupture, the dominant slip direction of the EMC earthquake is outwardly consistent with ongoing crustal deformation, here deviated 17° more westerly than the expected azimuth of relative plate motion as material begins to enter the transpressional regime surrounding the Big Bend of the San Andreas fault.

Fletcher, J. M., Teran, O. J., Rockwell, T. K., Oskin, M. E., Hudnut, K. W., Mueller, K. J., Spelz, R. M., Akciz, S. O., Masana, E., Faneros, G., Stock, J. M., Elliott, A. J., Gold, P. O., Liu, J., Gonzalez-Ortega, A. A., & Lynch, D. K. (2014). Assembly of a large earthquake from a complex fault system: surface rupture kinematics of the April 4, 2010 El Mayor-Cucapah Mw7.2 earthquake . Geosphere, 10(4), 797-827. doi: 10.1130/GES00933.1 .

Related Projects & Working Groups
Earthquake Geology