Dr. Frank Vernon, research seismologist at Scripps Institution of Oceanography and University of California, San Diego, spoke in Anza about seismic monitoring in the Anza Valley.
Vernon spoke last Friday at Hamilton High School Library about field arrays that have been placed throughout the Anza Valley to monitor seismic activity on the San Jacinto Fault. “What we do is observe what is going on in real time [on the Anza broadband and real-time seismic network] and relay that information to scientists to analyze,” said Vernon. Since 1982, Vernon has been the principal investigator on the real-time network charting activity on one of the most active seismic zones in the U.S.
“We’re just in the process of compiling a large database,” said Vernon. With data and analysis, seismologists are able to forecast probable size of future quakes, although not necessarily when they will occur.
Vernon noted the Anza Valley is an excellent place to monitor seismic activity, situated as it is near the San Jacinto Fault and not far from the larger San Andreas Fault. “With more wilderness and less cultural noise, we can get more coverage and better readings,” said Vernon. “I’m in the field and observation side, installing the arrays and collecting data. We’re dealing in a statistical world where the more and better data we have the more likely we are to be able to forecast future quakes.”
Vernon noted that both the San Jacinto and San Andreas faults are locked, with similar slip rates. Although the San Jacinto is more active, with frequent smaller quakes, neither fault has had a huge quake for many years — 1700 for the southern San Andreas and 1800 for the San Jacinto. Vernon pointed out that the length of a fault is a factor in determining possible quake magnitudes. The San Andreas Fault zone is far longer than the San Jacinto and can therefore experience quakes in the 8 or greater magnitude range, with the San Jacinto topping out around 7 to 7.5 magnitude.
But even though both faults are locked and stress is building on each, predicting the timing of major quakes is difficult. “We cannot make a direct measurement of stress on faults,” Vernon explained. “What we can do is measure strain along faults using high-precision GPS measurements and/or shallow borehole strain meters. Using the history of earthquakes on a fault, both from observed earthquakes as well as results from paleoseismology, we can develop models of where future earthquakes may be likely to occur. The timing of the events is still unpredictable.”
Although California has a reputation for earthquake activity, one of the slides from Vernon’s presentation showed California does not historically have the greatest number of large quakes. That “honor,” according to Vernon, goes to the region around the New Madrid Fault system — with Illinois having 18, Missouri 16, Tennessee 15 and Arkansas 12 earthquake “events” in the last 30 years. California has had two major quakes since 1985.
Vernon pointed out the areas in the world that experience the largest quakes are those located along boundaries of tectonic plates, around the Pacific Rim specifically. The largest, according to a slide Vernon showed, was in 1960 in Chile, at magnitude 9.5. The next largest was the Alaska quake in 1964 at magnitude 9.2. By comparison, the destructive 1906 San Francisco quake measured approximately magnitude 7.8.