A new paper from Dr. Tom Rockwell and others that discusses the complex rupture dynamics of the El Mayor-Cucapah earthquake on April 4th 2010. The primary multifault rupture occurred on four major faults separated by one fault branch and two accommodation zones.

Assembly of a large earthquake from a complex fault system: Surface rupture kinematics of the 4 April 2010 El Mayor–Cucapah (Mexico) Mw 7.2 earthquake

John M. Fletcher1, Orlando J. Teran1, Thomas K. Rockwell2, Michael E. Oskin3, Kenneth W. Hudnut4, Karl J. Mueller5, Ronald M. Spelz6, Sinan O. Akciz7, Eulalia Masana8, Geoff Faneros2, Eric J. Fielding9, Sébastien Leprince10, Alexander E. Morelan3, Joann Stock10, David K. Lynch4, Austin J. Elliott3, Peter Gold3, Jing Liu-Zeng11, Alejandro González-Ortega1, Alejandro Hinojosa-Corona1 and Javier González-García1

1 Departamento de Geologia, Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Carretera Tijuana-Ensenada No. 3918, Zona Playitas, Ensenada, Baja California, C.P. 22860, México  
2 Department of Geological Sciences, San Diego State University, San Diego, California 92182, USA
3 Department of Earth and Planetary Sciences, University of California Davis, One Shields Avenue, Davis, California 95616-8605, USA
4 U.S. Geological Survey, 525 & 535 S. Wilson Street, Pasadena, California 91106-3212, USA 
5 Department of Geological Sciences, University of Colorado Boulder, Boulder, Colorado 80309, USA
6 Universidad Autónoma de Baja California, Facultad de Ciencias Marinas, Carretera Tijuana-Ensenada No. 3917, Zona Playitas, Ensenada, Baja California, C.P. 22860, México
7 Department of Earth, Planetary and Space Sciences, University of California Los Angeles, 595 Charles Young Drive East, Los Angeles, California 90095, USA
8 Departament de Geodinàmica i Geofísica, Universitat de Barcelona, Zona Universitària de Pedralbes, Barcelona 08028, Spain
9 Jet Propulsion Laboratory, California Institute of Technology, M/S 300-233, 4800 Oak Grove Drive, Pasadena, California 91109, USA
10 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
11 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, A1# Huayanli, Dewai Avenue, Chaoyang District, P.O. Box 9803, Beijing 100029, China


The 4 April 2010 moment magnitude (Mw) 7.2 El Mayor–Cucapah earthquake revealed the existence of a previously unidentified fault system in Mexico that extends ∼120 km from the northern tip of the Gulf of California to the U.S.–Mexico border. The system strikes northwest and is composed of at least seven major faults linked by numerous smaller faults, making this one of the most complex surface ruptures ever documented along the Pacific–North America plate boundary. Rupture propagated bilaterally through three distinct kinematic and geomorphic domains. Southeast of the epicenter, a broad region of distributed fracturing, liquefaction, and discontinuous fault rupture was controlled by a buried, southwest-dipping, dextral-normal fault system that extends ∼53 km across the southern Colorado River delta. Northwest of the epicenter, the sense of vertical slip reverses as rupture propagated through multiple strands of an imbricate stack of east-dipping dextral-normal faults that extend ∼55 km through the Sierra Cucapah. However, some coseismic slip (10–30 cm) was partitioned onto the west-dipping Laguna Salada fault, which extends parallel to the main rupture and defines the western margin of the Sierra Cucapah. In the northernmost domain, rupture terminates on a series of several north-northeast–striking cross-faults with minor offset (<8 cm) that cut uplifted and folded sediments of the northern Colorado River delta in the Yuha Desert.

In the Sierra Cucapah, 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 as shallowly as ∼20°. Net surface offsets in the Sierra Cucapah average ∼200 cm, with some reaching 300–400 cm, and rupture kinematics vary greatly along strike. Nonetheless, instantaneous extension directions are consistently oriented ∼085° and the dominant slip direction is ∼310°, which is slightly (∼10°) more westerly than the expected azimuth of relative plate motion, but considerably more oblique to other nearby historical ruptures such as the 1992 Landers earthquake. Complex multifault ruptures are common in the central portion of the Pacific North American plate margin, which is affected by restraining bend tectonics, gravitational potential energy gradients, and the inherently three-dimensional strain of the transtensional and transpressional shear regimes that operate in this region. 

GeosphereJohn M. Fletcher, O.J. Teran, T.K. Rockwell, M.E. Oskin, K.W. Hudnut, K.J. Mueller, R.M. Spelz, S.O. Akciz, E. Masana, G. Faneros, E.J. Fielding, S. Leprince, A.E. Morelan, J. Stock, D.K. Lynch, A.J. Elliott, P. Gold, J. Liu-Zeng, A. González-Ortega, A. Hinojosa-Corona, J. González-García , Assembly of a large earthquake from a complex fault system: Surface rupture kinematics of the 4 April 2010 El Mayor–Cucapah (Mexico) Mw 7.2 earthquake, Geosphere, June 24, 2014, doi: 10.1130/GES00933.1