Structural Controls on the Surface Rupture Associated with the Mw7.2 El Mayor-Cucapah Earthquake of 4 April 2010: A Comparative Analysis of Scarp Array Kinematics, Orientation, Lithology and Width
Abstract
The Mw 7.2 El Mayor-Cucapah earthquake revealed the existence of a previously unidentified plate-margin fault system that extends 120 km from the northern tip of the Gulf of California to the international border. The system is composed of at least 6 major faults linked by numerous smaller faults. We observed that localized kinematics on individual faults vary systematically with fault orientation. Faults with strikes ranging from N72W-N68W, N67W-N13W, N12W-N08W and N07W-N47E have kinematics dominated by dextral, oblique-dextral normal, normal, and oblique normal-sinistral shear, respectively. Stress inversion of the kinematic data yields high phi values (0.65-0.8) with sigma-1 close in magnitude to sigma-2 and sigma-3 oriented sub-horizontally with an azimuth of S80W. This demonstrates that kinematic variations of the 2010 rupture are most strongly controlled by regional stress related to transtensional shearing between the Pacific and North American plates. Detailed mapping of the surface rupture through the Sierra Cucapah demonstrates that scarp array width (or total local width of the rupture zone) ranges from 5 to 400 m and varies systematically with rock type, fault orientation, and fault kinematics. We classified seventy-one discrete fault segments according to the lithologies they juxtapose and in general scarp-array width increases in the following order: basement-on-basement, sediment-on-basement and sediment-on-sediment faults. Dips of the master faults vary from 30° to 85° and this parameter shows the strongest correlation with scarp array width, which increases dramatically with more shallowly dipping master faults. When faults are classified by both lithology and dip, interesting patterns emerge to show systematic variations in width with strike. Sediment-on-sediment faults that dip greater than 45° show a sinusoidal variation of strike versus width. Width is minimized with faults that strike ~N68W and ~N08W, which correspond well with the orientations of pure dextral and pure normal slip, respectively. Width is maximized with faults that strike ~N30W, which correspond well with the orientations of oblique dextral-normal slip. Therefore, scarp array width increases with the increasing kinematic complexity of oblique slip. Based on stress inversion, the oblique slip segments have higher shear stress, which we conclude is an important factor controlling scarp array width. The predictive power resulting from this parametric analysis may allow refined assessments of future fault surface rupture hazards for critical facilities and lifeline fault crossings.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T31B2348T
- Keywords:
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- 8100 TECTONOPHYSICS