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Journey to the Center of the Quakes

The San Andreas Fault
Tucson - Apr 03, 2002
What causes earthquakes? Are they predictable? To answer these questions researchers from Stanford University and US Geological Survey will drill into the San Andreas Fault, one of the most famous earthquake-generating sites in the world.

The pilot hole they will bore this summer is the first step in creating SAFOD, an unprecedented subterranean natural laboratory to study earthquakes where they occur: directly in the fault zone.

For the past 10 years Mark Zoback, Stephen Hickman, and William Ellsworth have been working together to understand one of the most powerful natural forces- earthquakes. Cooperating with scientists around the world, they developed projects to drill holes in the Earth's crust to investigate the hidden workings of faults.

Hundreds of scientists have simulated earthquakes in the laboratory and proposed theoretical models to explain them. Still, the Earth holds many secrets about earthquakes. In particular, how faults work and whether it will ever be possible to accurately predict earthquakes.

Earthquake forecasting is a very important problem for both our society and science. Every year, on average 17,500 people die in earthquakes around the globe. These events also cause significant damage and economic loss.

Since a large number of "earthquake-prone" sites have been identified in densely populated areas, researchers are striving to understand them. Studies done in the early 1980s estimated the probability for devastating earthquakes in California, but researchers are still trying to improve those models.

"We want to be able to say how likely an earthquake is in the next 10, 30, or 50 years, so that we can guard ourselves against the damage they cause," says Bill Ellsworth, a geophysicist at the US Geological Survey in Menlo Park, CA, who specializes in earthquake seismology.

In recent years Ellsworth, Hickman, and Zoback focused their research on the area of Parkfield, a town in central California located on the infamous San Andreas Fault. This fault represents a boundary between two great plates on the Earth's surface.

The outer shell of our planet is a mosaic of about 20 rigid plates that slowly move with respect to one another. This motion is driven by the heat released as the Earth's interior continuously cools.

Plate motions cause many geologic events like mountain building, opening and closing of the ocean basins, and volcanic eruptions. The boundaries where these plates come together are where most earthquakes and volcanoes are located.

The San Andreas Fault separates the North American and the Pacific plates, which are sliding past each other at about 35 mm per year. This is about the same rate as your nails grow.

Earthquakes occur along the San Andreas Fault because the fault cannot continually slip; instead, the fault slips and then locks, causing strain or deformation of the surrounding rock to build up. When stress, or pressure, on the fault reaches a critical level, the fault slips, resulting in an earthquake.

The San Andreas Fault is the best understood fault in the world and Parkfield is the best known place on this fault. "But even at Parkfield we still have a number of major unanswered scientific questions," says Bill Ellsworth.

Parkfield lies at a juncture where the fault behavior changes dramatically. For 100 miles to the northwest, the fault releases its energy continuously in small earthquakes and creep.

"To the southeast the San Andreas Fault is very tightly locked and no sliding occurs on the fault. A great deal of strain energy is being stored for release in a future massive earthquake. We are quite sure because that's what happened in the past," says Ellsworth.

The last great earthquake (magnitude 8) on this locked segment of the fault occurred in 1857. During this event, called the Great Fort Tejon earthquake, the San Andreas Fault slipped as much as 8 meters. It takes a long time to build up the strain energy that produces such massive slip.

Historic records show that great earthquakes happen along this section of the San Andreas on the order of 300 years, so the next "Big One" is expected in about 150 years from now.

"Unfortunately, earthquakes are not like clock work. A major earthquake could occur today on that part of the fault and that would not surprise anyone," warns Bill Ellsworth.

The Parkfield segment is an important place for refining earthquake forecasting. Two moderate earthquakes (magnitude 6) happened there in 1966 and 1934. They were large enough to shake the ground, but caused only minor damage in this sparsely populated region.

"We have good evidence that similar size earthquakes occurred in 1922, 1901, 1881, and 1857. All of these earthquakes were separated by approximately 25 years. Since the last earthquake was in 1966, it has been longer than the average recurrence time. It is reasonable to expect a magnitude 6 or larger earthquake soon," says Bill Ellsworth.

Apart from being important, earthquake prediction is also very controversial because researchers are not sure if earthquakes are predictable.

"I'm undecided. Today we don't have any reason to suspect that we can predict them, but we also don't have a sufficient understanding of the problem, so we certainly can't say that it is impossible," argues Mark Zoback, Professor of Geophysics at Stanford University.

The San Andreas Fault itself is hard to understand. Though earthquakes happen at Parkfield and there are occasional great earthquakes to the southeast, the fault segment extending about 100 miles north of Parkfield does not seem to produce any big events.

The northern region seems to be continuously sliding or creeping, so the strain caused by plate motions is released without allowing a lot of the energy to accumulate.

However, this creeping zone of the fault does induce an abundance of micro earthquakes (magnitude 2 and smaller), so Mark, Bill, and Steve find it ideal for running their experiments.

"Everything we know about earthquakes has been learned by indirect studies. We have good models to describe them, we simulate the process in the laboratory, exhume rocks from ancient faults, and we have samples of recent fault breaks at shallow depths from various places in the world. But we never really got down to where the energy is being stored and released during an earthquake" explains Bill Ellsworth. The details of the process are still open questions.

"Although we've been studying faults and earthquakes for many years, we are still uncertain whether our models of earthquake processes, developed on the basis of field studies, theoretical models or laboratory simulations are correct. This question is absolutely imperative to address if we want to make substantial progress in understanding earthquakes," argues Mark Zoback.

This is why they designed SAFOD-the San Andreas Fault Observatory at Depth. The experiment is part of EarthScope, an ambitious project to study the structure and evolution of the North American continent.

SAFOD is an attempt to drill into the active zone of the San Andreas Fault and explore the micro earthquakes. Smaller earthquakes tend to recur more frequently, so by sampling a piece of a fault that has a magnitude 2 earthquake every year or two, during the 20-year life span of SAFOD we are able to actually sample multiple seismic cycles.

"If we were trying to drill through the piece of the fault that is locked in northern California and only ruptures in a magnitude 7 earthquake, we have to wait 100-150 years between earthquakes to capture the complete seismic cycle. That is not realistic," points out Steve Hickman, a research geophysicist at the USGS in Menlo Park and the chief of the project investigating the physics of earthquakes in laboratory and the field.

For Bill, Mark, Steve, and many other scientists SAFOD is a journey of discovery, an opportunity to test long-debated laboratory results and models. They will drill a hole down 3 km and across the San Andreas Fault.

An intact rock core will be retrieved for laboratory analyses and various instruments will be placed in this hole to investigate the hidden part of the fault. Some of the instruments will continue observations for two decades.

"The first task is to answer very basic science questions about the mechanics of earthquakes," explains Bill Ellsworth. Drilling through the San Andreas Fault allows measuring its physical and mechanical properties as well as pressures of fluids in the fault zone.

"We know that the amount of stress needed to cause an earthquake can be greatly modulated by the pressure of fluids in the fault zone. If the fluid pressure is high, it takes less stress to slide the fault. One of the main questions about the San Andreas Fault is how much stress is needed to move it," he adds. This experiment will help determine that.

Earthquake scientists are particularly interested in understanding how the energy released in an earthquake translates into earthquake damage.

"If you think of Earth as a giant spring, the San Andreas Fault takes the energy stored in that spring and converts it into different forms of energy. The one we care about most as far as hazards are concerned, is the radiated energy. It is very important to know how efficient faults are in converting that stored energy into earthquake shaking," explains Steve Hickman.

A milestone for the project will be drilling of the pilot hole that starts this July. "We'll go straight down for 2 kilometers," says Hickman. "Though the pilot hole will not penetrate the fault, it will follow the initial vertical segment of the SAFOD hole to be drilled in the future.

"It will let us immediately get information about rock types and physical properties adjacent to the San Andreas Fault. We'll be able to refine our models of how the fault works and learn about the stress and temperature conditions at depth," explains Hickman.

Most importantly, it will allow scientists to more precisely locate the micro earthquakes. This information is crucial to maximize research results in the following years. As for Bill, Mark, and Steve-they believe that drilling the pilot hole in July brings them closer to uncovering some of Earth's biggest secrets.

Related Links
SAFOD: A Deep Borehole Observatory
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