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