This project will create a deep observatory for seismic activity.

The first stage of the project involved drilling a pilot hole. Photo by Stephen Hickman, USGS.
An international team of scientists is embarking on a project to drill an angled hole through a seismically active portion of the San Andreas Fault to create the San Andreas Fault Observatory at Depth (SAFOD), a deep borehole observatory that will directly measure the physical conditions under which plate boundary earthquakes occur. Drilling of the borehole begins this month.

SAFOD is designed to directly sample fault zone materials (rock and fluids), measure a wide variety of fault zone properties and monitor a creeping and seismically active fault zone at depth. A 2.5-mile-deep hole will be drilled through the San Andreas fault zone close to the hypocenter of the 1966 Parkfield earthquake, where the San Andreas fault slips through a combination of small-to-moderate magnitude earthquakes and aseismic creep. The drill site is located sufficiently far from the San Andreas fault (as determined by geologic observations, microearthquake locations and geophysical imaging) to allow for drilling and coring deviated holes through the fault zone starting at a vertical depth of about 2 miles and continuing through the fault zone until relatively undisturbed country rock is reached on the other side.

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 will provide new insights into the composition and physical properties of fault zone materials at depth, and the constitutive laws governing fault behavior. It 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 over-pressured fluids, if they exist, is unknown. As a result, a myriad of untested and unconstrained 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 on the composition, physical state and mechanical behavior of a major active fault zone at hypocentral depths. In addition to retrieval of fault zone rock and fluids for laboratory analyses, intensive downhole geophysical measurements and long-term monitoring are planned both 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 and 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 opportunities to simulate earthquakes in the laboratory and on the computer using representative fault zone properties and physical conditions.