A study of earthquake focal mechanisms in the hollister area, southern Coast Ranges, California

Abstract

The study is based on some 1200 fault-plane solutions derived from local, low-magnitude earthquakes which occurred during 1969–1971 in the Hollister area (Fig. 1). The procedure of analysis was as follows: (1) to use a solution with two orthogonal vertical nodal planes (VNP solution) i.e. a pure strike-slip solution, if it would satisfy the distribution of compressions and dilatations; (2) if not, to use a more general solution with inclined nodal planes (INP solution). Emphasis is given to the orientation of P-axes. According to the orientation of P-axes the events are classified into nine classes (Fig. 2). The general reliability of the solutions is demonstrated independently in events associated directly with the Calaveras and the San Andreas fault zones (Fig. 3). Some remarkable regularities in the regional orientation of P-axes are observed: NNE—SSW to NE—SW orientation of P-axes is predominant in the Diablo block, east of the Calaveras—Paicines fault zone, as well as in the San Benito wedge, located between the San Andreas and Paicines fault zones. NE—SW orientation of P-axes is observed within the Gabilan block SW of the San Andreas fault zone. On the other hand, a regime of N—S orientation of P-axes characterizes the Sargent wedge, located between the Calaveras and San Andreas fault zones.