Tectonic Implications of Small Earthquakes in the Central Transverse Ranges

Abstract

Fault-plane solutions for 22 small (local magnitude (M_L ≤ 4.6) earthquakes in the central Transverse Ranges were determined using an azimuthally varying crustal model. The dominant type of faulting observed is reverse faulting on east-striking planes, which suggests a regional stress field characterized by north-south compression. Some strike-slip faulting also occurs. There is some indication that strike-slip earthquakes may be more common than reverse-slip earthquakes during episodes of crustal dilatation. The rate of north-south crustal shortening attributable to small-0earthquake deformation during 1974-76 is two orders of magnitude smaller than the north-south contraction of 0.3 parts per million per year measured at the surface. The scatter in earthquake hypocenters and the general inconsistency of focal mechanisms with geologically determined motions on nearby major faults indicate that the small earthquakes in this region are not associated with large-scale block movements along major fault systems. Rather, they appear to represent fracturing along random minor zones of weakness in response to the regional stress field or, alternatively, small-scale block movements that are below the resolution of this study. Earthquakes in the San Gabriel Mountains north of the Santa Susana-Sierra Madre-Cucamonga frontal fault system tend to concentrate near the eastern and western ends of the range, where good evidence for late Quaternary movement along the frontal faults has been found. Seismicity is markedly lower north of the central section of the frontal fault system, where evidence for late Quaternary movement is lacking.