California State University, Long Beach
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Geological Sciences Professor Receives Grant
from SCEC to Study San Jacinto Fault Zone

With the 75th anniversary of the magnitude 6.4 Long Beach earthquake in March and a new report indicating that Southern California has a 97 percent probability of experiencing a 6.7 magnitude quake during the next 30 years, understanding seismic risks is one of region's most important scientific research topics.

Nate Onderdonk, a CSULB assistant professor of geological sciences, is one of more than 600 researchers affiliated with the Southern California Earthquake Center (SCEC), based at University of Southern California and funded by the National Science Foundation and United States Geological Survey (USGS).

Onderdonk recently received a $25,000 grant from SCEC to continue his examination of the San Jacinto fault zone. The results of his project will be used to produce more precise California earthquake risk reports like the one released in April by the USGS, SCEC and the California Geological Survey.

"The San Jacinto fault splays off the San Andreas fault zone in the Cajon Pass area, runs southeast through the San Bernardino-Riverside area and then down along the southwest side of the San Jacinto Mountains," he said. "We know that the San Jacinto fault is the most active fault in Southern California -- it has the most small earthquakes -- and it produced several earthquakes of magnitude greater than 6 about 100 years ago.

"Some recent studies in the last 10 years suggest that the San Jacinto fault may also be slipping at a faster rate than the southern San Andreas fault on the other side of the mountains to the east," Onderdonk added,"but we know far less about the San Jacinto than the San Andreas."

With a previous SCEC grant, he began his work on two areas of study, the slip rate, or how fast the two sides of the fault are moving with respect to each other, and the timing of the most recent ground-breaking earthquakes. He noted that the magnitude 6 events that occurred 100 years ago did not appear to rupture the ground surface.

Image of Nate Onderdonk

Nate Onderdonk

"This information will help us estimate the probability of a big earthquake in the near future, as well as tell us how the San Jacinto fault compares to and interacts with the southern San Andreas fault nearby," he said.

The newest grant will enable Onderdonk to involve a graduate student as well as an undergraduate assistant in using a combination of field mapping, remote sensing and installing trenches across the fault to determine how frequently and recently major quakes occurred on the fault.

"This new data will give us a better understanding of the earthquake hazard for the Inland Empire and surrounding areas of Southern California," he said.