The San Andreas Fault Observatory At Depth

T h e S a n A n d re a s F a u l t O b s e r vat o r y at Depth

The SAFOD drill rig illuminates the night.

Digging Deeper into the Mysteries of an by Active Fault Julia Guth

I

n a project fraught with terrifying moments and gratifying highs, scientists from around the world are collaborating on the San Andreas Fault Observatory at Depth (SAFOD). As the first observatory located within an active fault zone, SAFOD is a drilling project into the San Andreas fault. The data extracted and the knowledge gained from the observatory may enable scientists to answer long-standing questions pertaining to the origin of earthquakes and the properties of the fault at depth - specifically unusual thermal signatures and chemical compositions within.

Lab Underground

“This is a project whose fundamental aim is to study the physical and chemical processes associated with earthquake nucleation in situ – in place – where it is actually happening,” explains Dr. Mark Zoback, Professor of Geophysics at Stanford University. Zoback has led the SAFOD project since 1982, along with Stephen Hickman and William Ellsworth of the United States Geological Survey (USGS). Previously, scientists have attempted to simulate earthquakes in the laboratory or using computer models. Without physical data, however, a great deal of speculation was required. The SAFOD data contains clues to better understand how earthquakes result from complex geophysical phenomena.

An Ideal Site The site for SAFOD was specifically chosen so that data could be obtained from a dynamic region of the fault. Geologically, California can be delineated into two areas: locked and creeping. Northern and Southern California, both known as locked areas, produce intermediate earthquakes every several decades. The region between the two locked areas – the creeping region – produces small or micro-earthquakes frequently. For this reason, Parkfield, California was chosen as the ideal site for the SAFOD project. Located halfway between Los Angeles and San Francisco, Parkfield effectively straddles two tectonic plates grinding in opposite directions: the North American plate and the Pacific plate.

Drilling Two Miles Down A vertical hole drilled in 2002 provided initial data and allowed for the transport of instruments later used in SAFOD.

36 stanford scientific

http://quake.wr.usgs.gov/research/parkfield/safod_pbo.html

In this cross section of SAFOD, red dots in drill holes represent locations of monitoring instruments, while white dots depict areas of persistent minor seismic activity.

http://www.nsf.gov/news/news_images.jsp?cntn_id=104300&org=NSF

earthquake

Feature: Earthquake

Drilling for SAFOD’s main hole began in 2004 and finally punctured through the fault at a depth of almost 2 miles in 2005, initiating a series of scientific studies. The researchers of SAFOD attempt to answer questions about the mechanisms of earthquakes, the physics of the fault zone and the chemical composition of the fault at depth. Various instruments were installed at different depths to monitor what happens before, during, and after small earthquakes. From this underground observatory, scientists measure forces in the earth, gauge the pressure of fluids within the fault and obtain core samples for laboratory study to answer pressing questions about earthquakes on the San Andreas fault.

The Stress/Heat-Flow Paradox A long standing paradox in earthquake studies has been the stress/heat flow paradox. The mystery surfaced in the 1960s, when graduate student Tom Henyey of the California Institute of Technology lowered electronic thermometers 2,000 feet down into drill holes near the San Andreas fault. Previously, a well-established geophysical theory, based upon lab experiments with many different types of rock, dictated the expected temperatures at different depths in the fault. The observed heat, however, was only two to five times cooler than predicted. The Pacific plate is moving past the North American plate at a rate of about 3 centimeters/year, a relatively quick rate in geological terms. “If the forces The SAFOD rig is the first to drill were as high as we thought they through a section of an active fault. were, based on measurements in the laboratory and in deep holes around the world, there should be strong thermal signature to the fault,” says Zoback. This thermal signature is analogous to the sensation one feels when quickly rubbing one’s hands together to generate heat. Oddly, SAFOD proved that no heat is produced at the fault’s depths. Due to the constant motion of the plate and yet lack of heat, scientists infer that there is little frictional resistance, though the causes remain unknown.

In addition to discerning the fault’s thermal signature, SAFOD scientists have measured the physical properties of the rock at depth and witnessed how these properties have been altered by earthquakes and millions of years of geological transformations. From their observations, they uncovered the mechanism by which the fault slips at depth and found that there are multiple branches of active fault lines. Some branches, they determined, creep at a stable rate due to an unusual chemical composition in the fault. “The mineral that is thought to be responsible for fault creep – [the slow movement of faults] -- is serpentinite, which forms due to An aerial view of the San the chemical alteration of the ocean’s Andreas Fault in the Carrizo crust,” explains Zoback. “These rocks Plain, Central California. were abducted up onto the continent as California was assembled through a variety of plate tectonics processes.” Further analysis of the chemical processes responsible for fault creep may provide more information as to the origins of earthquakes and the possibility of predicting the next major earthquake. Scientists of the project are currently attempting to discern a link between the chemical composition of the rocks at depth and the occurrence of the numerous micro-earthquakes in Parkfield, hoping to eventually predict the occurrence of larger-scale tremors. http://pubs.usgs.gov/gip/earthq1/how.html

SAFOD Project The SAFOD project is part of the National Earthquake Hazards Reduction Program, enacted in 1977 by Congress. The program focuses on the prediction of earthquakes and the mitigation of risk, and aims to protect people’s lives and property from the earthquakes that are inevitable in California and the entire United States.

Feature: Earthquake

Data from the Depths

http://quake.wr.usgs.gov/research/parkfield/safod_pbo.html

SAFOD has established an observatory within an active fault zone, which has never been achieved before.

layout design:

layout design : Pam Bhattacharya

The Future of SAFOD Even with the project’s discoveries, scientists and researchers have yet to tap SAFOD’s full potential. According to Zoback, there are plans to return to the site in 2007 and extract core samples. Because the drilling process reduced many rocks to small particles -- making it impossible to extract whole samples -- the researchers plan to cut windows into the steel encasing the drilled hole and extract large core samples for more thorough study. In 2008, Zoback intends to deploy monitoring instruments, wrapping up the current stage of the project. He has no plans to desert the observatory, however. He and his colleagues plan to monitor many earthquakes within SAFOD, using the information toward the goal of predicting when, where, and how hard future quakes will hit. Reflecting upon his experience with SAFOD, Zoback remarks, “The exciting thing is that it’s a type of beginning, which is a nice feeling after all these years.” S JUliA GUTh is a freshman majoring in Earth Systems, with a minor in Economics. She enjoys biking in the rain, reading German literature, and discussing the role of corporations in the U.S. government.

volume iv