Determination of P-Wave Velocity Structures, Earthquake Hypocenters, and Focal Mechanisms in the Morgan Hill Region of Central California


P-wave velocity structure, earthquake hypocenters, and focal mechanisms are investigated by applying an analytic ray tracing technique to 2437 P-wave arrivals from 154 locally recorded earthquakes from the Morgan Hill region of central California Taking as our reference the results of a previous investigation by Michael (1988) on the effects of laterally varying structure on these parameters of hypocenters and focal mechanisms, we found that to the first order the principal structures are preserved, but that features associated with the fault zone tend to be better focused and of higher contrast when the analytic ray tracer is used. In particular, the low velocities associated with the Madrone Springs and Calaveras faults are confined to a narrow zone of about 5 km width. Contrasts in velocities associated with this zone are 2-3% larger at shallow depth. These contrasts gradually decrease with depth, but are evident to depths of at least 12 km. Hypocenters relocated in this structure shift systematically to the east by 1-2 km and their depths generally increase by as much as 5 km. The combined effect of the epicenter and depth shift is to decrease the apparent dip of the seismic zone by 4° and to move the surface intercept of the seismic zone to the east by 0.8 km. compared with those determined by approximate ray tracing, initial ray directions calculated with the analytic ray tracer are within ±5° in azimuth but can be as much as 30° different in take-off angle. The danger of attempting focal mechanism analysis with only a few arrivals is therefore evident. The effects of these differences on the deduced focal mechanisms themselves appear to be on the order of 10° in the strike and dip of the probable fault plane.

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