A fellow of the American Geophysical Union and the Geological Society of America, Paul Silver also serves on many committees and boards. He is president-elect of the Seismology Section of the American Geophysical Union. Silver has also taken a lead role in the conception, development, and ultimate funding of the Plate Boundary Observatory (PBO), an initiative to monitor and understand the Earth’s movements and earthquakes between the North American and the Pacific plates.

By combining data on surface deformation and seismic anisotropy (see text at right), Silver is able to estimate the mantle flow field beneath western North America. Shown are the directions of fast shear wave polarization directions predicted from a best-fitting flow model at sites where shear wave splitting has been measured. White symbols denote good fits of predicted and observed directions; red shows sites where predicted directions are more than 14° clockwise than observed; blue indicates a similar counterclockwise misfit. In the best-fitting model the mantle moves eastward at 5.5 ± 1.5 centimeters per year, opposite to the motion of the North American plate. (Reprinted with permission from Science 295, 1055. Copyright 2002 AAAS.)

How are earthquakes triggered, and how do they interact with each other? Geophysicist Paul Silver believes that by observing the slow redistribution of stress and strain that accompanies earthquakes he can help answer these questions. Silver uses several approaches to detect and characterize slow deformation. He also studies seismic anisotropy — the fact that seismic wave velocity depends on the direction and polarization of the waves as they propagate through a material. Because seismic anisotropy is produced by deformation and flow in the Earth’s mantle, Silver uses the phenomenon to understand how the mantle is involved in mountain building, to map convection in the mantle, and to study how this flow interacts with the tectonic plates.

Silver and colleagues assembled an unprecedented 10- year data set on the strain behavior of a portion of the San Andreas Fault in central California — an area that is expected to experience a magnitude 6 earthquake soon. The strain records show that slow motion on the fault began to speed up in 1993, which started a complex multiyear slow earthquake involving the redistribution of stress along the fault and four moderate size earthquakes. It is the best-documented slow earthquake so far observed. Through especially sensitive seismic imaging of the crust, Silver and team also found that fluid in the fault zone was redistributed in association with the slow earthquake.

The magnitude 7.3 Landers, California, earthquake of 1992 triggered increased levels of seismic activity in regions as far as several hundred kilometers away. By investigating all of the earthquakes in these areas over time, Silver and collaborators found that an elevated state of earthquake activity persisted for five years. They also discovered that the triggered activity had an annual cycle: fall had the greatest number of earthquakes, spring the least. The scientists concluded that the annual variations resulted from changes in barometric pressure — less pressure yields reduced stress on the faults, which can therefore move more frequently.

Using several hundred global seismic stations, Silver has also mapped the deformation of the upper mantle using a manifestation of anisotropy known as shear-wave splitting, or birefringence. In addition to showing that the mantle plays a role in mountain building and continent formation, Silver used this technique to study the flow associated with mantle convection beneath surface plates. With collaborators, he developed the first method to measure this flow field directly and determined how and why the mantle is moving in various places, including western North America and Iceland.

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