The San Andreas Fault Observatory at Depth: Results from Site Characterization Studies and the 2.2-Km-Deep Pilot Hole
Abstract
The San Andreas Fault Observatory at Depth (SAFOD) is a comprehensive program to address fundamental questions about the physical and chemical processes controlling the initiation, propagation and arrest of earthquake ruptures within a major plate-boundary fault through a coordinated series of observations and experiments. The centerpiece of the observatory will be a deep drill hole that crosses the entire zone of active deformation, reaching a total depth of about 4 km. The drill hole will permit in situ measurement of the physical conditions (state of stress, pore fluid pressure, temperature, etc.) acting within the fault, recovery of fault zone samples for laboratory analysis of deformation mechanisms and frictional strength, and emplacement of instruments within the near-field environment of repeating (M 2) earthquake sources to monitor the evolution of physical conditions during multiple earthquake cycles. SAFOD is located just north of the rupture zone of the M 6 1966 Parkfield earthquake. This location was selected after extensive study and consultation to capitalize on the substantial knowledge base developed over the course of 15+ years of the Parkfield Earthquake Experiment. SAFOD's sister elements in EarthScope, USArray and PBO, are expected to further enhance the regional framework through their own instrumental deployments along this reach of the San Andreas Fault. Over the past several years, a wide variety of targeted geophysical investigations have been carried out near the SAFOD site. These studies include deep electromagnetic soundings, gravity and magnetic profiles, high-resolution seismic reflection and refraction profiles, velocity anisotropy analysis, and a 70 station microearthquake experiment - the Parkfield Area Seismic Observatory - a prototype for future USArray deployments. Earlier this summer, a 2.2 km deep, vertical "pilot hole" was drilled at the SAFOD well pad, located 1.8 km SW of the fault, as a collaborative effort between the International Continental Drilling Program, NSF and USGS. Analysis of the extensive suite of open-hole logs and cuttings from the pilot hole is now underway. A multi-level, 3-component, high-frequency seismic array was installed in the pilot hole at the conclusion of drilling and is being used for wide band observation of local microearthquakes. It will also record controlled source experiments scheduled for the fall of 2002. The stage is now set for the drilling of the main hole, from which we expect to learn the composition and mechanical properties of rocks in the fault zone, the nature of stresses responsible for earthquakes, the role of fluids in controlling faulting and earthquake recurrence, and the physics of earthquake initiation and rupture.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.T72E..02E
- Keywords:
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- 7209 Earthquake dynamics and mechanics;
- 8010 Fractures and faults;
- 8045 Role of fluids;
- 8164 Stresses: crust and lithosphere