Earthquake focal mechanisms in the Eastern Transverse Ranges and San Emigdio Mountains, Southern California and evidence for a regional decollement

Earthquake focal mechanisms obtained from P-wave first motions are presented for the Eastern Transverse Ranges and the San Emigdio Mountains in Southern California. The former region shows a predominance of strike-slip faulting whereas Quaternary faults in the region show thrust motion. We suggest that the observed strike-slip mode of deformation cannot continue indefinitely without the occurrence of more thrust faulting. Fault deformation in the San Emigdio Mountains inferred from focal mechanisms is in accord with displacements across Quaternary faults in the area. This study and a search of the literature has yielded 19 mechanisms with shallow-dipping nodal planes. Previous workers have interpreted such mechanisms as evidence for a regional decollement. If such a regional decollement exists, these data give some indication of its regional extent. Slip directions inferred from the focal mechanisms with shallow-dipping nodal planes show some regional consistency, but this pattern cannot be entirely explained with current tectonic models. A comparison of the stress drop of an event having a shallow-dipping nodal plane with an event with steeper planes gave inconclusive results.