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How Does the Earth "Relax" After an Earthquake?

Phil Resor
Geological and Environmental Sciences
Stanford University
June 2001

Since earthquakes have the potential to harm millions of people worldwide, continued research into these natural events is very important. To better assess earthquake hazard, we must first understand the earthquake process more fully. My research studies how forces caused by an earthquake dissipate, or “relax,” with time. These forces can cause subsequent earthquakes, and are therefore of great interest to Earth scientists.

As events of the last few years have shown, earthquakes can still damage buildings and highways and cause many injuries and deaths. We cannot directly predict individual earthquakes, the way we can the path of a hurricane, but we can make statistical estimates of the likelihood of a large, potentially damaging earthquake occurring in a particular area in the near future. My research will contribute toward improving these forecasts by increasing our understanding of the earthquake process.

During an earthquake, two blocks of the Earth's crust slide past one another along opposite sides of a fault. This sliding creates forces in the surrounding rocks that may trigger nearby earthquakes. Over time these forces dissipate or “relax” until the next earthquake occurs. Because we cannot directly observe the earthquake process (earthquakes originate 10-15 km (6-10 miles) below the Earth's surface), I plan to use several methods to study the relaxation of forces, including indirect observations of active faults by the examination of seismic waves created by recent earthquakes, the observation of ancient faults exposed at the surface, and computer modeling.

Earthquakes occur when the rock within a fault fails, suddenly allowing the two sides of the fault to slide past one another. The slip occurs over a limited area and the strength of the earthquake is a function of both the amount of slippage and the size of the area over which the slip occurs. For an earthquake to be felt by an ordinary person, a fault must slip about 10 cm (4 inches) over an area about the size of one city block (250 square meters). A slip of this size creates an earthquake of just over 3.0 on the moment magnitude scale (a logarithmic scale where an increase of 1 unit is equal to 10 times the energy), which is a more reliable measure of the size of an earthquake than the better-known Richter scale. For comparison, the great San Francisco earthquake of 1906 was 7.7 in magnitude, more than 10,000 times the smallest earthquake you can feel!

There are two main approaches to studying earthquakes. The first is to describe the pattern of earthquakes on a specific fault through time and define a recurrence interval that indicates when the next earthquake is likely. Unfortunately, because large earthquakes typically recur on the same fault on time scales of 100-1000's years, historic records are inadequate for capturing the pattern of repetition. Geologists try to expand the historic record by documenting prehistoric earthquakes that leave their trace in the rocks of an area. The most common approach is to dig a trench across a fault and look for rock or soil layers that have been offset by an ancient earthquake. If an offset layer contains organic matter, such as the remains of an old swamp, then carbon dating can be used to date the earthquake. This method has greatly increased our understanding of the earthquake record in many areas, but it is still limited to the recent past (in geologic terms, the last few thousand years) due to the shallow depths that can be reasonably excavated in trenches.

A second approach to studying earthquakes is to look at the physical processes that cause them: the forces in the earth and how rocks react to these forces. Because we cannot directly observe the entire earthquake process, we must use an indirect approach which may include the remote observation of the active earthquake process using geophysical methods, modeling the process using computer or analog models, and observing ancient faults that were once several kilometers deep but are now exposed at the surface and no longer active. I propose a combination of these three methods in order to understand how the forces that build up during an earthquake dissipate (relax) and how this may lead to subsequent earthquakes.

I will initiate my research with a study of aftershocks, small earthquakes that follow a major earthquake, from a 1995 earthquake in Greece. I plan to use new methods recently developed at Stanford University and the United States Geological Survey to locate aftershocks with higher precision than previous methods allowed. In California, this method has shown that aftershocks cluster along faults and thus give us a picture of the faults that slip during and immediately after an earthquake. By analyzing aftershock data from Greece, I will obtain a three-dimensional image of the main fault responsible for the 1995 earthquake and smaller faults whose slips were triggered by the main earthquake.

After mapping the faults using aftershocks, I will create computer models of the earthquake to determine if our current understanding of how the Earth deforms is sufficient to predict the distribution of aftershocks. One aspect of deformation after an earthquake that we clearly do not understand is the time delay between an earthquake and its aftershocks. Models used to describe the behavior of the shallow part of the Earth's crust predict that all aftershocks would occur instantly after a major earthquake; however, in reality the aftershocks are spread out over months. The cause of this delay is currently unknown. Thus, the final part of my research is to study ancient faults that are now visible at the surface, which should tell me how an active fault behaves deep within the Earth. I will then be able to answer to the question of how the force associated with one fault slipping spreads through the Earth and causes nearby faults to slip.

The forces that cause earthquakes and the details of how faults behave during an earthquake have been the focus of much previous research, but there has been less work on what happens after an earthquake. This period of "relaxation" is important for understanding both earthquake triggering and recurrence and, therefore, the entire earthquake cycle.