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 1Departamento de Geologia, Centro de Investigacion Cientifi ca y de Educacion Superior de Ensenada, Carretera Tijuana-Ensenada No. 3918, Zona Playitas, Ensenada, Baja California, C.P. 22860, México 2Department of Geological Sciences, San Diego State University, San Diego, California 92182, USA 3Department of Earth and Planetary Sciences, University of California Davis, One Shields Avenue, Davis, California 95616-8605, USA 4U.S. Geological Survey, 525 & 535 S. Wilson Street, Pasadena, California 91106-3212, USA 5Department of Geological Sciences, University of Colorado Boulder, Boulder, Colorado 80309, USA 6Universidad Autónoma de Baja California, Facultad de Ciencias Marinas, Carretera Tijuana-Ensenada No. 3917, Zona Playitas, Ensenada, Baja California, C.P. 22860, México 7Department of Earth, Planetary and Space Sciences, University of California Los Angeles, 595 Charles Young Drive East, Los Angeles, California 90095, USA 8Departament de Geodinàmica i Geofísica, Universitat de Barcelona, Zona Universitària de Pedralbes, Barcelona 08028, Spain 9Jet Propulsion Laboratory, California Institute of Technology, M/S 300-233, 4800 Oak Grove Drive, Pasadena, California 91109, USA 10Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA 11State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, A1# Huayanli, Dewai Avenue, Chaoyang District, P.O. Box 9803, Beijing 100029, China ABSTRACT reverses as rupture propagated through mul- with some reaching 300–400 cm, and rup- tiple strands of an imbricate stack of east- ture kinematics vary greatly along strike. The 4 April 2010 moment magnitude (Mw) dipping dextral-normal faults that extend Nonetheless, instantaneous extension direc- 7.2 El Mayor–Cucapah earthquake revealed ~55 km through the Sierra Cucapah. How- tions are consistently oriented ~085° and the existence of a previously unidentifi ed fault ever, some coseismic slip (10–30 cm) was the dominant slip direction is ~310°, which system in Mexico that extends ~120 km from partitioned onto the west-dipping Laguna is slightly (~10°) more westerly than the the northern tip of the Gulf of California to Salada fault, which extends parallel to the expected azimuth of relative plate motion, the U.S.–Mexico border. The system strikes main rupture and defi nes the western margin but considerably more oblique to other northwest and is composed of at least seven of the Sierra Cucapah. In the northernmost nearby historical ruptures such as the 1992 major faults linked by numerous smaller domain, rupture terminates on a series of Landers earthquake. Complex multifault faults, making this one of the most complex several north-northeast–striking cross-faults ruptures are common in the central portion surface ruptures ever documented along the with minor offset (<8 cm) that cut uplifted of the Pacifi c North American plate margin, Pacifi c–North America plate boundary. Rup- and folded sediments of the northern Colo- which is affected by restraining bend tecton- ture propagated bilaterally through three rado River delta in the Yuha Desert. ics, gravitational potential energy gradients, distinct kinematic and geomorphic domains. In the Sierra Cucapah, primary rupture and the inherently three-dimensional strain Southeast of the epicenter, a broad region of occurred on four major faults separated by of the transtensional and transpressional distributed fracturing, liquefaction, and dis- one fault branch and two accommodation shear regimes that operate in this region. continuous fault rupture was controlled by a zones. The accommodation zones are distrib- buried, southwest-dipping, dextral-normal uted in a left-stepping en echelon geometry, INTRODUCTION fault system that extends ~53 km across the such that rupture passed systematically to southern Colorado River delta. Northwest structurally lower faults. The structurally Large surface ruptures that cascade slip of the epicenter, the sense of vertical slip lowest fault that ruptured in this event is along multiple faults offer an opportunity to inclined as shallowly as ~20°. Net surface off- learn about mechanical relationships between *Corresponding author: jfl etche@ cicese .mx sets in the Sierra Cucapah average ~200 cm, faults, along with factors that control the state Geosphere; August 2014; v. 10; no. 4; p. 797–827; doi:10.1130/GES00933.1; 25 fi gures; 6 supplemental fi les. Received 25 March 2013 ♦ Revision received 18 March 2014 ♦ Accepted 25 April 2014 ♦ Published online 24 June 2014 For permission to copy, contact [email protected] 797 © 2014 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/4/797/3334074/797.pdf by guest on 29 September 2021 Fletcher et al. of stress in the seismogenic crust. The 4 April 2010, moment magnitude (Mw) 7.2 El Mayor– Cucapah (EMC) earthquake activated slip on at least 7 different faults; many were unnamed and some were not even known to exist prior to the earthquake. Most of the faults have rela- tively short total strike lengths (<30 km), none of them control topography, and Holocene paleoscarps are rare. Individually each fault could have been thought to present a low seis- mic risk, and no one suspected that they had the physical links required for them to behave as a single fault system that was capable of produc- ing a major earthquake. The fault system has strands that dip at very shallow angles (as low as 20°) and are likely to be severely misoriented relative to the inferred regional stress. Along its entire length, the complex fault system is located within 1–3 km of faults that can be considered the dominant strands of the plate margin in this area. These dominant faults include the Laguna Salada and Cerro Prieto faults, which have pro- duced many major historical earthquakes and together accommodate to 90% of relative plate motion (Anderson and Bodin, 1987; Mueller and Rockwell, 1991; Bennett et al., 1996; Gen- rich et al., 1997). Therefore, the EMC rupture signifi cantly tests our understanding of rock mechanics and failure criteria to explain why Figure 1. Tectonic map of western North America showing the main shear zones (purple plate-margin shearing bypassed optimally ori- shading) that compose the Pacifi c North American plate margin. Simplifi ed fault traces ented faults to become partitioned onto an adja- include Quaternary faults (black; simplifi ed from U.S. Geological Survey, 2006; and cent complex fault network. regional mapping in western Mexico by us, i.e., Fletcher and others) and inactive Neo- The EMC surface rupture is arguably the gene faults (gray; simplifi ed from Muehlberger, 1996). Maximum horizontal compressive most complex ever documented on the Pacifi c– stress shown by thick horizontal lines defi nes regions of transtension (white), transpression North American plate margin (Fig. 1). However, (yellow ), and extension (green) (simplifi ed from World Stress Map database; Heidbach multi fault ruptures that have complex kine matics et al., 2008). In this study the plate margin is divided into three main domains: Gulf of and occur off of the main strands of the plate California, Big Bend, and San Andreas. Transtensive relative plate motion is accommo- margin are not uncommon in the central portion dated in a narrow belt in the Gulf of California with minor slip (~5 mm/yr) occurring in of the plate margin, as is well demonstrated by the continental shelf west of Baja California (Dixon et al., 2000). In the Big Bend domain, the three most recent earthquakes in this region, plate margin shearing changes from transtensive to transpressive and bifurcates into the including the M 7.2 EMC in 2010, M 7.1 Hec- north-northwest–trending eastern California shear zone (EC) and a network of west- tor Mine in 1999, and M 7.3 Landers in 1992 northwest–striking faults that includes the Big Bend segment of the San Andreas fault. (Fig. 2). We argue that the central domain of the In the San Andreas domain, relative plate motion is partitioned into the transpressive plate margin is affected by several geodynamic San Andreas fault system (SA), transtensive Walker lane shear zone (WL), and extension processes, including regional redistribution of in the Great Basin. Minor extension is also partitioned into the Rio Grande Rift (RGR), plate margin shearing, gravitational potential which forms an isolated belt of shearing that overlaps with portions of all three domains. energy gradients emanating from the Great Black star marks epicenter of the 4 April 2010 Mw 7.2 El Mayor–Cucapah earthquake Basin, and the inherently three-dimensional located within the Big Bend domain. A—Agua Blanca fault, G—Garlock fault. strain of transtensional and transpressional shearing. We propose that these factors help explain the surprising and somewhat unpredict- when viewed from the far fi eld (S. Wei et al., surface rupture and its kinematic behavior, the able nature of