SEISMICITY AND LITHOSPHERIC STRUCTURE IN SOUTHERN CALIFORNIA

Southern California is a region undergoing continued deformation. Movement between the North American and Pacific plates manifests itself in high levels of seismicity. In this study we investigate the relationship between deeper structure and the seismicity of southern California.

This study endeavours to find a correlation between deep lithospheric structure and observed seismicity patterns. Our investigation is made possible by the release of the SCEC 3.0 Velocity Model, which incorporates the upper mantle. Primary consideration is given to the mantle downwelling centred above the Transverse Ranges. Relationships are sought with the depth, distribution and frequency of earthquakes. A wide variety of data sources are utilized and both existing and specially designed programs are used to manipulate the data, resulting in a synthesis of the regional seismicity and tectonics.

Deep seismicity is found to preferentially occur in regions overlying the high-velocity upper mantle “drip”. This observation is consistent with depressed isotherms. Large events appear to cluster in crust overlying high-velocity regions, however small events are almost uniformly distributed between the fast and slow regimes. This suggests no measurable variation in crustal strength, however this might be clarified with the removal of aftershock events. There is no evidence for subduction beneath the Transverse Ranges.