THE SAN ANDREAS FAULT OBSERVATORY AT DEPTH (SAFOD): TESTING FUNDAMENTAL THEORIES OF EARTHQUAKE MECHANICS

In the SAFOD experiment we propose to drill into the San Andreas fault at a site characterized by creep and regularly repeating small earthquakes near Parkfield, CA. The SAFOD experiment has two complementary sets of scientific objectives. First, we seek to constrain, through direct sampling and measurement, the hypotheses that currently exist about the composition, state (stress, pore pressure, temperature, etc) and on-going physical and chemical processes in an active, plate-bounding fault zone. The second set of scientific objectives are related to establishment of a long-term observatory directly within the active fault to study to study the processes associated with initiation, propagation and arrest of seismic (and aseismic) rupture.

In addition to drilling through the fault zone and making a broad suite of geophysical measurements, we will sample a continuous profile of fault-zone rocks and fluids and measure stress and pore pressure conditions within the SAFOD borehole. We will compare and contrast the state of stress and crustal strength of the currently active San Andreas fault zone with that of the adjacent crust. After the sampling and downhole measurements phases of the experiment, we will deploy a suite of seismic, strain, pore pressure and temperature monitoring equipment in the fault zone. We will use these instruments to monitor directly processes of strain accumulation and release during the cycle of repeating microearthquakes as well as monitor any changes in pore pressure and temperature that may occur through time. We will also have the capability to directly observe the earthquake nucleation process in the very near field and address a number of outstanding questions about the dynamics of earthquake rupture.

A great deal is known about the Parkfield area after almost two decades of study and monitoring. Several focused site characterization efforts have also been carried out to select the best drill site for SAFOD. Most recently, a 2-km deep pilot hole was drilled to test geologic models of the area and its suitability as a site for the SAFOD experiment. A vertical array of seismometers is being installed in the pilot hole to precisely locate the microearthquakes that will be targeted.