An empirical study of the distribution of earthquakes with respect to rock type and depth

Whether fault slip occurs by earthquakes or by aseismic slip is thought to depend upon whether fault friction is velocity-weakening or velocity-strengthening. Because the dependence of friction behavior upon rock type and depth is not well constrained, we approach the problem empirically. We examine the distribution of earthquakes with respect to rock type and depth by comparing three-dimensional geologic models of the San Francisco Bay region and southern California to the three-dimensional seismicity distribution in these regions, which includes 99,158 earthquakes with magnitude greater than two. To account for the effects of differing volumes of rock types, the number of earthquakes within each rock type and depth interval are normalized by the corresponding volume to give a quantity we name 'earthquake density.' Depth is the primary parameter that determines the earthquake density, while whether the rock unit is sedimentary or basement rock has a secondary effect on earthquake density. This secondary effect is also depth dependent. At shallow depths, earthquake density is very small, and there is no difference between sedimentary and basement rocks. In the second depth range (3-7 km in southern California and 2-9 km in the San Francisco Bay region), the earthquake density of basement rocks is higher than that of sedimentary rocks. In the third depth range (7-9 km in southern California and 9-13 km in the San Francisco Bay region), the sedimentary rocks are more seismogenic. In the deepest depth range, the basement rocks are more seismogenic. A more detailed distribution with respect to rock type is achievable in the San Francisco Bay region, where we examine the distribution of 15,432 earthquakes with respect to the following rock units: Sedimentary, Granodiorite, Franciscan, and Gabbro. Surprisingly, we find that granodioritic rocks tend to experience fewer earthquakes than sedimentary rocks and that gabbro has the highest tendency to experience earthquakes.