A comprehensive suite of geophysical logs was collected in the SAFOD Pilot Hole in Parkfield, CA. from a depth of 775 m to 2150 m in highly fractured Salinian granite. The Pilot Hole intersected numerous macroscopic fractures and faults with extremely varied orientations. Despite the highly variable orientation of the fractures and faults, the fast polarization direction of the shear waves is very consistent with the direction of maximum horizontal compression determined from wellbore breakouts and drilling induced tensile fractures. At least three major shear zones were intersected by the borehole that are characterized by anomalously low velocity and resistivity, anomalously high shear velocity anisotropy and an absence of stress-induced wellbore breakouts (which suggests anomalously low differential stress). We argue that the physical mechanism responsible for the seismic velocity anisotropy observed in the Pilot Hole is the preferential closure of fractures in response to an anisotropic stress state.
1. INTRODUCTION
The Parkfield region in central California has been the focus of intense investigation by the geophysical community because of its six historical magnitude six earthquakes [e.g., 1]. The Parkfield segment of the San Andreas Fault is generally accepted to be the transition zone between the 300 km-long locked portion of the fault to the southeast, that ruptured during the great earthquake of 1857, and the creeping section to the northwest.
In the summer of 2002 the Pilot Hole for the San Andreas Fault Observatory at Depth (SAFOD) was drilled to a depth of 2.15 km through 768 m of tertiary sediments and into Salinian granite, approximately 1.8 km southeast of the surface trace of the San Andreas Fault in Parkfield, CA (Figure 1). Data from the Pilot Hole provides a unique opportunity to measure the physical properties of the shallow crust and characterize the region adjacent to the San Andreas Fault zone prior to drilling of the main SAFOD borehole, which is expected to commence in June 2004.
In many geologic settings it is difficult to differentiate between stress-related anisotropy and structural anisotropy since major structural features are often sub-parallel to the maximum horizontal stress [3]. Parkfield is an ideal place to study shear wave anisotropy because the maximum horizontal stress is at a very high angle to the trend of the San Andreas Fault and secondary strike-slip and thrust faults (Figure 1) [4, 5]. Thus the ability to distinguish between stress-induced and structural anisotropy is increased. In this paper we characterize the physical properties of the crust in the Parkfield area using borehole data from the SAFOD Pilot Hole. We present observations of shear wave velocity anisotropy in the SAFOD Pilot Hole and correlate them with measurements of P- and S-wave velocity, resistivity, density and porosity, the distribution of faults and fractures intersecting the borehole and the state of stress inferred from borehole measurements.
2. PHYSICAL PROPERTIES
Based on studies of P-wave velocity determined from a seismic reflection/refraction profile [2] and interpretation of potential field data [R. Jachens, pers. comm.] it was anticipated that the Pilot Hole would encounter fractured granite beneath ~750 m of Tertiary and Quaternary sedimentary rocks; the fractured Salinian granite was encountered at 768 m.