Berkeley Seismologists Tackle Volcanic Seismology
by Aaron Pierce
Most residents of the Bay Area are aware that California is a hotbed of seismic activity. It is perhaps less well known that California also undergoes a substantial amount of volcanic activity. Berkeley seismologists Dr. Douglas Dreger and Hrvoje Tkalcic have recently studied a collapsed volcano located near Mammoth Lakes, known as the Long Valley Caldera. Their study, which appeared in Science, has led to a new understanding of the origins of seismic activity associated with the center of the volcano.
The Long Valley Caldera was formed by a massive eruption approximately 730,000 years ago. In that explosion, over 600 cubic kilometers of rock were expelled. If assembled into a cube, this amount of rock would measure more than eight thousand meters on a side, roughly the height of Mt. Everest. The area surrounding the Caldera is seismically quite active and even during periods of minimal activity may have ten or more earthquakes of magnitude 3.0 or smaller per day. Occasionally the amount of seismic activity increases dramatically beyond this impressive background level. In 1997, the Caldera displayed just such a surge in seismic activity. Dreger, Tkalcic, and their colleague Malcolm Johnston of the U.S. Geological Survey examined data from the set of earthquakes that occurred in the region around the Caldera in that year. Utilizing a technique known as waveform analysis, they were able to discern that an anomalous type of vibration was present in these earthquakes.
Waveform analysis begins with an examination of the wave shapes seen on seismograms following an earthquake. After the waveforms are identified, the next step in the process is to understand how the seismic waves recorded on seismograms have propagated through the earth. Armed with this understanding, scientists can then trace the waves back to their origin. In fact, using this technique, it is possible not only to pinpoint the origin of an earthquake, but also to glean information about the properties of the source itself. In this case, Dreger and his colleagues were able to trace the waveforms from the 1997 regional earthquakes back to the Long Valley Caldera, and then propose a mechanism for the earthquake.
The Berkeley team’s examination of the Long Valley Caldera netted some fascinating results. The group analyzed the waveforms seen in six 1997 earthquakes of magnitude 4 or greater, and noticed that the waveforms had anomalous shapes. Normal seismic activity, of the sort seen along the Hayward or San Andreas faults, takes the form of tension-releasing “strike-slip” events, which result in what is known as “double-couple” radiation, a pattern of radiation that results from the release of tension along a fault plane. Interestingly, the earthquakes at the Caldera contained a statistically significant presence of seismic radiation that was not of the double-couple form. Instead, the researchers found a substantial amount of isotropic radiation, a type of shaking associated with a change in volume at the source. Isotropic radiation would be seen if the shaking were caused, for example, by an explosion, in which case it would propagate outward in a purely radial fashion. In fact, searching for isotropic radiation is one means by which it is possible to verify adherence to nuclear test ban treaties.
From their discovery of the isotropic component in the seismograms, Tkalcic and his colleagues were able to conclude that the seismic activity associated with the Caldera is not limited to the usual strike-slip events that are prevalent along the San Andreas Fault. The team of seismologists suspect that the earthquakes associated with the Caldera may instead be caused by high-pressure fluid rushing through small crevices, thereby opening cracks in the rock. For example, a dike of magma might rapidly heat water, thus bringing it to supercritical state; this heated water could be subsequently injected through a small opening, forcing the opening to expand, thus causing a change in volume, which would register as isotropic radiation.
According to Tkalcic, previous authors had suggested that such a mechanism might be at work, but this study was the first to conclusively show that a statistically significant component of isotropic radiation was indeed present in the waveforms.
Tkalcic is careful to emphasize that all of this work relies crucially on prior knowledge of how seismic waves propagate within the earth. According to Tkalcic, our understanding of the mechanics of terrestrial wave motion has been laboriously pieced together from thousands of seismic events over the course of many years, resulting in detailed computer models of wave propagation. It is these models that allow seismologists to trace the waveforms seen on a seismograph back to an earthquake’s source.
Tkalcic also notes that seismology has now developed to the point that scientists are able to use seismographs “much in the same way that doctors can use a CAT scan” instead of a surgical biopsy. In this non-invasive fashion, seismologists are able to utilize the elastic waves that are generated and propagated inside the Earth to learn about the Earth itself.
Lava flows of the Mono-Inyo Craters volcanic chain in California’s Long Valley Caldera. The most recent eruptions from along this chain occurred between about 250 and 600 years ago. Berkeley seismologists Douglas Dreger and Hrvoje Tkalcic are studying the unique signature of this volcano’s seismic waves (USGS Long Valley Observatory).