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

Abstract

The 4 April 2010 moment magnitude (M w) 7.2 El Mayor-Cucapah earthquake revealed the existence of a previously unidentifi ed 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 Pacifi c-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 eastdipping 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 defi nes 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 Pacifi c 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.

A Crustal Velocity Model for the Southern Mexicali Valley, Baja California, Mexico

“…This photograph shows the clearest image of the rupture at the surface of the Indiviso fault. In agreement with this field observation, after modeling displacements derived from Global Positioning System and Interferometric Synthetic Aperture Radar observations, González-Ortega et al (2014) found a postseismic fault slip of ∼10-40 cm on the Indiviso fault. By analyzing the seismicity in the area of study (Fig.…”

Complex multifault rupture during the 2016 M w 7.8 Kaikōura earthquake, New Zealand

“…This gap coincides with a change from more reverse faulting in the south to predominantly strike slip in the north and is more than double the distance usually assumed as the limit for halting a fault rupture in standard seismic hazard models. The 2010 Mw 7.2 El Mayor-Cucapah in northern Mexico ruptured across a 10 km step-over in the surface faults, but gradients in optical pixel offsets and InSAR data indicated that slip continued at depth (7,34). During the Kaik ura earthquake, slip along the interface could also act as a linking structure at depth.…”

“…While numerical models and field observations suggests that fault step overs of more than 4-5 km can halt a ruptures' propagation (1,3,4), nearinstantaneous triggering over distances of more than 50 km has been documented (5,6). Furthermore, recent observations indicate that fault networks with both optimally oriented and misoriented faults can rupture during a single earthquake (7,8). Insights from complex ruptures involving multiple faults, including the 2010-2011 Christchurch earthquake sequence in New Zealand (9) and El Mayor-Cucupah in Mexico (7,8) are starting to feed into seismic hazard models relaxing some of the assumptions surrounding fault segmentation and multifault ruptures (10).…”

“…Furthermore, recent observations indicate that fault networks with both optimally oriented and misoriented faults can rupture during a single earthquake (7,8). Insights from complex ruptures involving multiple faults, including the 2010-2011 Christchurch earthquake sequence in New Zealand (9) and El Mayor-Cucupah in Mexico (7,8) are starting to feed into seismic hazard models relaxing some of the assumptions surrounding fault segmentation and multifault ruptures (10).…”

Implications from palaeoseismological investigations at the Markgrafneusiedl Fault (Vienna Basin, Austria) for seismic hazard assessment

“…However, there are not enough data so far to prove or disprove any of those scenarios, but recent earthquakes suggest that masked or blind faults may contribute to the development of multi-segment ruptures (e.g. M w = 7.2 2010 El Mayor-Cucapah Earthquake and M w = 7.8 2016 Kaikoura Earthquake; Fletcher et al, 2014;Clark et al, 2017).…”