Introduction to special section: Preparing for the San Andreas Fault Observatory at Depth

The San Andreas Fault Observatory at Depth (SAFOD) is a comprehensive project to drill into the hypocentral zone of repeating M ~ 2 earthquakes on the San Andreas Fault at a depth of about 3 km. The goals of SAFOD are to establish a multi-stage geophysical observatory in close proximity to these repeating earthquakes, to carry out a comprehensive suite of downhole measurements in order to study the physical and chemical conditions under which earthquakes occur and to exhume rock and fluid samples for extensive laboratory studies. In the vicinity of SAFOD, the San Andreas is moving through a combination of aseismic creep and repeating microearthquakes (Figure 1). SAFOD is one element of the National Science Foundation’s (NSF) new EarthScope initiative (see www. earthscope. org). Drilling, testing and instrumentation of SAFOD is beginning in the summer of 2004 and will be completed in 2007, with fault-zone monitoring activities scheduled to continue for at least 20 years. An overview of the generalized drilling, sampling, testing and monitoring plan for SAFOD can downloaded at http: //www. earthscope. org/assets/esₚartsI-IVₗo₁. 25. pdf. SAFOD will provide new insights into the composition and physical properties of fault zone materials at depth and the constitutive laws governing fault behavior. Even after decades of intensive research, numerous fundamental questions about the physical and chemical processes acting within the San Andreas and other major plate-bounding faults remain unanswered. SAFOD also will provide direct knowledge of the stress conditions under which earthquakes initiate and propagate. Although it is often proposed that high pore fluid pressure exists within the San Andreas Fault Zone at depth and that variations in pore pressure strongly affect fault behavior, these hypotheses are unproven and the origin of overpressured fluids, if they exist, is unknown. As a result, myriad untested laboratory and theoretical models related to the physics of faulting and earthquake generation fill the scientific literature. Drilling, sampling and downhole measurements directly within the San Andreas Fault Zone will substantially advance our understanding of earthquakes by providing direct observations of the composition, physical state and mechanical behavior of a major active fault zone at hypocentral depths. In addition to retrieval of fault zone rocks and fluids for laboratory analyses, intensive downhole geophysical measurements and long-term monitoring are planned within and adjacent to the active fault zone. Observatory-mode monitoring activities will include near-field, wide-dynamic-range seismological observations of earthquake nucleation and rupture as well as continuous monitoring of pore pressure, temperature and strain during the earthquake cycle. Directly evaluating the roles of fluid pressure, intrinsic rock friction, chemical reactions, in situ stress and other parameters in the earthquake process will provide the information needed to simulate earthquakes in the laboratory and on the computer using representative fault zone properties and physical conditions. In preparation for SAFOD, an extensive suite of geophysical site investigations has been conducted around the drill site and across the San Andreas Fault. In addition, a 2. 2-km-deep vertical pilot hole was drilled at the SAFOD site in the summer of 2002. This special section of Geophysical Research Letters, which is split into two separate issues of the printed journal, presents results from these site investigations together with geological and geophysical studies conducted in the SAFOD pilot hole. The first of these print issues presents nine papers discussing surfacebased and downhole investigations of earthquakes and large-scale crustal structure. The second print issue presents ten papers on the thermomechanical setting of the San Andreas Fault together with laboratory and in situ investigations of physical properties and mineralogy from the SAFOD pilot hole. In this introductory article, we present background material that will help place these papers in the broader perspective of the overall SAFOD project. (For the sake of brevity, no references are cited in this article, aside from a citation to the geophysical model shown in Figure 2. Instead, the relevant references are cited in papers contained within this special section).