Bulk and shear moduli of near-surface geologic units near the San Andreas fault at Parkfield, California

Abstract

Shear and bulk moduli have been measured as a function of depth in three boreholesalong a 50 kilometer section of the San Andreas fault near Parkfield, California. The shear modulus values range from 1.45 * lO1 to 8.69 * 708 pascals at depths as great as 180 meters, and show a general increase with depth. The values of bulk modulus also increase with depth and range from 8.27 * 101 to 4.48 * 109 pascals to depths of 180 meters. These values are significantly lower than moduli values reported for unfractured crystalline rocks. Low rigidity and bulk modulus in fault zone materials could account for many of the apparent physical properties and behavior of this section of the San Andreas fault.These properties include low earthquake stress drops, low heat flow, "blocklike" fault slip behavior, and would suggest that strainfields and strain energy release are concentrated near the fault. Similar measurements at other borehole locations along the fault and at points off the fault are needed to fully quantify these effects.