|
The San Andreas Fault is situated in California (USA) at the boundary between the Pacific Plate and the North American Plate. This strike-slip fault has a length of approximately 1300 km. The fault was identified and named by geology professor Andrew Larson from UC Berkeley in 1895. The two best known earthquakes taking place on the San Andreas Fault are the 1906 San Francisco Earthquake with a rupture length of 430 km and a magnitude of 7.8 as well as the 1989 Loma Prieta earthquake having a rupture length of 40 km and a magnitude of 7.1.
Arial view of San Andreas Fault in the Carrizo plain (picture: Ian Kluft)
The San Andreas Fault Observatory at Depth (SAFOD) is part of the Earthscope project investigating the structure and evolution of the North American continent especially physical processes controlling earthquakes and volcanic eruptions. It is funded by US NSF, USGS and ICDP. The drill site is situated at the Northern end of the rupture zone of the 1966 Parkfield earthquake (magnitude 6) and the borehole with a length of 3.2 kilometers reaching in the San Andreas fault at the location of a repeating microearthquake source is drilled in several stages (pilot hole in 2002, three stages of Main hole drilling from 2004 – 2007, see figure below).
Map of California showing the San Andreas Fault and indicating the surface traces of historic large earthquakes. The SAFOD drill hole is located close to the town of Parkfield, just at the SE end of the creeping and microseismically active segment of the San Andreas Fault (blue line). The inset shows a sketch of the SAFOD drilling plan of the pilot and main hole superimposed on electrical resistivity structure.
Source: http://www.nsf.gov
SAFOD provides a unique opportunity to bridge the gap between observations of large earthquakes and laboratory observations of acoustic emissions. In summer 2002, an array of 32 three-component geophones was installed at the Pilot Hole spanning a depth of 200 – 1400 m b.s.l. NORSAR has been involved in the SAFOD project since 2001 via a PhD student and work related to downhole seismic data. NORSAR's MIMO software has been installed at SAFOD in 2005 and has been applied to off-line data for microearthquake location using the downhole waveform data with full sampling rate (4000 Hz) and detection picks from the Parkfield High Resolution Seismic Network. A semi-automatic shear wave splitting analysis has been developed, providing insight on the anisotropy of the San Andreas Fault at small scales.
|
