Tectonic Implications of Small Earthquakes in the Central Transverse Ranges

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