THE SAN ANDREAS FAULT OBSERVATORY AT DEPTH: STATUS REPORT AND KEY OPPORTUNITIES FOR GEOLOGIC AND GEOPHYSICAL STUDIES OF THE MECHANICS OF FAULTING

The San Andreas Fault Observatory at Depth (SAFOD) component of EarthScope is designed to directly measure the physical and chemical conditions under which earthquakes occur. SAFOD is located along a segment of the San Andreas Fault that moves through a combination of aseismic creep and repeating microearthquakes, just north of the rupture zone of the 2004 M6 Parkfield earthquake. By drilling, sampling testing and instrumenting a 3-km-deep inclined borehole across the San Andreas Fault Zone where repeating (M~2) earthquakes occur, SAFOD will make it possible to study the physics of earthquake nucleation and rupture and determine the composition, physical properties and mechanical behavior of an active, plate-bounding fault at depth.

At the surface, the SAFOD drill site is located 1.8 km southwest of the vertical San Andreas Fault such that the inclined borehole passes through the entire fault zone at seismogenic depths. Phase 1 of the project, rotary drilling to 2.5 km vertical depth and to within 600 m of the San Andreas, was carried out during the summer of 2004. Phase 2 of the project is being conducted during the summer of 2005 and involves rotary drilling through the San Andreas while collecting nearly continuous cuttings and gas samples, several “spot” cores (up to 10 m in length) and appreciable geophysical logging data. These data include continuous profiles of stress, heat flow and other physical properties across the fault zone. Rock and fluid samples recovered from the fault zone and country rock will be tested in the laboratory to determine their composition, deformation mechanisms, frictional behavior and physical properties. We will use the locations of microearthquakes and ongoing fault creep (detected through repeat measurements of casing shear) to select intervals in which to obtain continuous core from sidetracks to the main SAFOD borehole in the summer of 2007, during Phase 3 of the project. This will permit scientists to compare the mineralogy, physical properties and deformational behavior of fault rocks that fail primarily through creep against those that fail during earthquakes. After drilling, the borehole will be instrumented as a long-term observatory to monitor earthquakes, deformation, fluid pressure and temperature within and adjacent to the fault zone through multiple earthquake cycles.