Over 100 well-recorded microearthquakes (local magnitude less than 3), which occurred between January and September 1980, have been relocated, and focal mechanisms have been estimated for six regions of recent seismic activity for the Livermore Valley. Each of these microearthquakes had a minimum of 10 stations distributed in at least three quadrants around the epicenter. Data from these microearthquakes were combined with data from three USGS refraction experiments and used to develop a velocity model for Livermore Valley. Using this model, together with source region and station corrections to compensate for lateral velocity variations in the upper crust, we recalculated the locations and focal mechanisms.
Comparing the results from these regions, we found differences in focal depths, patterns of epicenter locations, and focal mechanisms. Focal depths in the northern regions were usually between 5 and 11 km. These were slightly greater than focal depths (2 to 8 km) in the southern regions. The pattern of strike-slip focal mechanisms with P axes trending NNE and the linear trend of epicenters along the right-lateral strike-slip Greenville fault system in the northern regions is in contrast with the pattern of a mixture of focal mechanisms in southern regions (which includes about one-third with thrust-type mechanisms where the T axis is nearer vertical than horizontal). In the southern regions, there is some indication of short offset (en echelon) segments and an absence of the extended linear trend found in the northern regions. We speculate that this more diffuse pattern of locations and focal mechanisms in the southern regions of the valley results from general north-south compressional tectonics with both strikeslip and thrust faulting occurring in a localized zone of deformation between the Livermore Valley block and the Diablo Range block to the south.