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Structural Controls on the Surface Rupture Associated with the Mw7.2 El Mayor-Cucapah Earthquake of 4 April 2010

Orlando J. Teran1, John M. Fletcher1, Thomas K. Rockwell2, Kenneth W. Hudnut3, and Michael E. Oskin4
1Geology, CICESE, Ensenada, BC, MEX, [email protected], [email protected]
2Geology, San Diego St. Univ., San Diego, CA, USA, [email protected]
3USGS, Pasadena, CA, USA, [email protected]
4Geology, UC Davis, Davis, CA, USA, [email protected]

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. Detailed mapping of the surface rupture through the Sierra Cucapah demonstrates that scarp array width (SAW) ranges from 5 to 400 m and varies systematically with rock type, fault orientation, and fault kinematics. We classified 71 discrete fault segments according to the rock types they juxtapose and in general SAW 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 SAW, 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 SAW with strike. Sediment-on-sediment faults that dip greater than 45° show a sinusoidal variation of strike versus SAW. SAW is minimized with faults that strike ~N68W and ~N08W, which correspond well with the orientations of pure dextral and pure normal slip, respectively. SAW is maximized with faults that strike ~N30W, which correspond well with the orientations of oblique dextral- normal slip. Therefore, SAW 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 SAW. The predictive power resulting from this parametric analysis may, for example, allow refined assessments in delimiting fractured reservoirs.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013