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Peers Honor Quake Scientist

The American Geophysical Union recognizes Charles Sammis

By Kirsten Holguin
May 1, 2007

Peers Honor Quake Scientist

Charles Sammis, professor of earth sciences in USC College, has devoted almost four decades to studying earthquakes. His contributions to the field of geophysical sciences recently led to his being named a Fellow of the American Geophysical Union (AGU).

The designation is conferred upon no more than 0.1 percent of all AGU members in any given year. A committee of fellows selects new fellows, who are chosen in recognition of their acknowledged prominence in one of the four fundamental areas of the geophysical sciences: atmospheric and ocean sciences; solid-Earth sciences; hydrologic sciences; and space sciences.

"I am very proud to have been named a Fellow of the AGU. USC has been very supportive with the [USC] Associates’ Award in both teaching and research, but this is my first serious national and international recognition,” said Sammis. “The esteem of one’s colleagues is the ultimate academic reward."

Sammis’ current research examines how the structure of an earthquake fault affects the generation of seismic waves, which, in turn, produce the shaking and damage on Earth’s surface. This involves examining ancient fault zones that have been exposed by uplift and erosion of the crust, creating a fault zone in the laboratory to capture a simulated earthquake with high-speed digital photography, and developing theoretical computer models.

These tests have shown that the short branching cracks and pulverized rock that border large fault planes can have a strong effect on the rupture velocity during an earthquake. He’s also revealed how fluctuations in these fault zone characteristics can affect the frequency of the resultant seismic waves, and hence their destructive capability.

In addition, Sammis has a research contract with the U.S. Air Force to monitor countries that are conducting underground nuclear tests. He is devising new ways to distinguish seismic waves generated by an underground nuclear explosion from those generated by a quake. Sammis also is exploring ways to determine the size of a nuclear bomb test. This is a complicated issue because many variables affect this estimate. For instance, nuclear yield is estimated from the amplitude and frequency of the seismic waves, but these are very sensitive to the type of rock and soil that surround the buried nuclear bomb at the testing site.

The importance of understanding the rock composition can best be illustrated by mistakes made in analyzing nuclear tests conducted by the Soviet Union, Sammis said. The Soviets were testing in permafrost north of the Arctic Circle. Americans monitoring these tests did not appreciate that ice in the cracks in the rock could affect the generation of seismic waves. As a consequence, their calculations of the strength of the Soviet nuclear devices were significantly low.