Geologic and structural controls on rupture zone fabric: A field-based study of the 2010 Mw 7.2 El Mayor–Cucapah earthquake surface rupture

Abstract:

We systematically mapped (scales >1:500) the surface rupture of the 4 April 2010 Mw 7.2 El Mayor–Cucapah earthquake through Sierra Cucapah to understand how faults with similar structural and lithologic characteristics control rupture zone fabric, which is here defined by the thickness, distribution and internal configuration of shearing in a rupture zone. Fault zone architecture and master fault dip showed the strongest controls on rupture zone fabric. Highly localized slip was observed along simple narrow fault cores (<20 m), whereas wide cores (>>50 m) composed of multiple zones of high shear strain had wider and more complex rupture zones that generally lacked principal-displacement scarps. Rupture zone thickness also increases systematically with decreasing fault zone dip. We observed that coseismic slip along faults that dip >40° was mostly confined to the fault core, whereas faults that dip as low as 20° had surface rupture entirely developed entirely outside of the fault zone. The lack of large off-fault strain along faults dipping >40° is contrary to predictions by dynamic stress modeling (e.g., Ma, 2009). We show that static tectonic loading, which varies significantly with fault orientation, has a significant effect not only on rupture zone fabric but also on the evolution of fault zone architecture in this transtensional setting.

Rupture zones in undeformed alluvium are dominated by secondary fractures associated with fault-tip propagation, and arrays of fault scarps become wider and more complex with oblique slip compared to pure normal dip-slip or pure strike-slip. Field relations show that as magnitude of coseismic slip increases from 0 to 60 cm, the linkages between kinematically distinct fracture sets increases systematically to the point of forming a through going principal scarp, which is contrary to many analogue models (e.g., Tchalencko, 1970; Naylor et al., 1995) Our data indicate that secondary faults and penetrative off-fault strain continue to accommodate the oblique kinematics of coseismic slip after the formation of a through going principal scarp.