Testing Fundamental Theories of Earthquake Mechanics:
The San Andreas Fault Observatory at Depth
The San Andreas Fault Observatory at Depth (SAFOD) is a component of EarthScope. EarthScope will investigate the structure and evolution of the North American continent and the physical processes controlling earthquakes and volcanic eruptions. EarthScope is funded by the National Science Foundation and conducted in partnership with the US Geological Survey.
SAFOD is motivated by the need to answer fundamental questions about the physical and chemical processes controlling faulting and earthquake generation within a major plate-bounding fault. SAFOD will drill and instrument an inclined borehole across the San Andreas Fault Zone to a depth of 3.2 km, targeting a repeating microearthquake source. The drill site is located west of the vertical San Andreas Fault on a segment of the fault that moves through a combination of aseismic creep and repeating microearthquakes (Figure 1). It lies at the extreme northern end of the rupture zone of the 1966, Magnitude 6 Parkfield earthquake, the most recent in a series of events that have ruptured the fault five times since 1857. The Parkfield region is the most comprehensively instrumented section of a fault anywhere in the world, and has been the focus of intensive study for the past two decades.
Figure 1: Shaded relief map of California showing the location of SAFOD. Major historical earthquakes along the San Andreas Fault are shown, with the creeping section of the fault in blue. (© USGS, click to enlarge)
In the summer of 2004 the borehole will enter the fault zone, and during the summer of 2005 the borehole will pass through the entire fault zone starting at a depth of about 3 km and continuing until relatively undisturbed country rock is reached on the other side (Figure 2). Rock and fluid samples recovered from the fault zone and country rock will be tested in the laboratory to determine their compositions, deformation mechanisms, frictional behavior and physical properties. Following an intensive series of downhole measurements, the hole will be instrumented as a long-term geophysical observatory to monitor earthquakes, deformation, fluid pressure and ephemeral properties of the fault zone through multiple earthquake cycles. Through sampling, downhole measurements and long-term monitoring directly within the San Andreas fault zone at seismogenic depths, we will learn the composition of fault zone materials and determine the constitutive laws that govern their behavior; measure the stresses that initiate earthquakes and control their propagation; test hypotheses on the roles of high pore fluid pressure and chemical reactions in controlling fault strength and earthquake recurrence; and observe the strain and radiated wave fields in the near field of microearthquakes.
Many scientific studies have already been carried out or are presently underway at the SAFOD site. These include a high-resolution seismic reflection profile, airborne and ground-based gravity and magnetic surveys, thermal and geochemical studies in shallow wells, magnetotelluric imaging, microearthquake relocations and seismic tomography.
EarthScope
The other elements of EarthScope include USArray, the Plate Boundary Observatory (PBO). USArray consists of a large transportable broadband seismic array, augmented by various smaller seismic arrays, and is being coordinated with the USGS Advanced National Seismic System. The PBO is a network of deformation sensors (Global Positioning System receivers, borehole strainmeters, etc.) for the western U.S.
Scientific studies using SAFOD and the other EarthScope facilities will be supported through programs within the NSF, USGS, and foreign countries.
The SAFOD Pilot Hole
In preparation of SAFOD, a 2.2km deep vertical pilot hole was drilled and instrumented at the SAFOD site in the summer of 2002. The pilot hole was a collaborative effort between the International Continental Scientific Drilling Program, NSF and the USGS, and it will guide subsequent SAFOD scientific investigations within the active fault zone.
Relevance to Earthquake Hazards
By drilling a hole into the hypocentral zone of an active fault and observing earthquakes in their near-field environment, SAFOD represents a major advance in pursuit of a rigorous scientific basis for earthquake hazard reduction. For example, directly evaluating the roles of fluid pressure, intrinsic rock friction, chemical reactions and other factors in controlling fault strength will allow scientists to simulate earthquakes in the laboratory and on the computer using representative fault-zone properties and physical conditions. This will lead to more realistic models for static stress transfer and earthquake triggering at a regional scale and between specific faults, as needed for intermediate-term seismic hazard forecasting following large earthquakes. Near field observations of the earthquake rupture process, including such hypothesized effects as fault-zone dilation and short-term changes in fluid pressure, will lead to improved predictions of strong ground motions and more reliable models for rupture propagation and arrest. These processes are believed to control earthquake size (i.e., whether a small earthquake will grow into a large one) and, hence, are crucial to long-term assessments of earthquake hazard. Finally, direct observation of the earthquake nucleation process will reveal whether earthquakes are preceded by accelerating fault slip or changes in fluid pressure, as has been proposed. This type of data is crucial for addressing the question of whether short-term earthquake prediction is possible or if there are longer-term variations in fault zone properties that might be related to the earthquake nucleation process.
EarthScope Facility Proposal | 1998 SAFOD Proposal to NSF
A detailed discussion of scientific rational and operation plan for SAFOD.
To find out more about SAFOD contact: Mark Zoback, Steve Hickman or Bill Ellsworth.
