Dr. Patrick Smith, professor of geology at Mt. San Jacinto College, is the next featured speaker for the Idyllwild Community Center Speaker Series. His talk, “Earthquakes Between Two Faults,” will discuss what Idyllwild and its environs may experience in the near future as a result of being situated between two earthquake fault zones — the San Andreas to the east and the San Jacinto to the west.
The San Andreas, famous for some major quakes on its northern sections, including the 1906 San Francisco quake, poses the greatest risk of having a devastating quake between 7.3 to 8.2 magnitudes in the very near future. “Based on new research by Dr. Kerry Sieh, former Cal Tech professor of geology and current director of the Nanyang Technological University’s Earth Observatory in China, we have new probabilities for predicting both the size and timing for earthquakes on the San Andreas.”
And, as Smith indicates, the probabilities are sobering. Using tectonic plate slip rates, dates of previous ruptures and carbon dating of soil in rupture areas, reasonably accurate predictions can ascertain likely locations on the fault as well as size of future quakes. With the long-dormant southern section of the San Andreas, a quake of 7.2- to 8.2-magnitude has a 50-percent probability within the next 25 years. “It is ready now,” said Smith.
The devastating San Francisco quake that destroyed 90 percent of the city and caused more than 3,000 deaths has been re-determined to have been between 7.7- to 8.3-magnitude, the very size that could occur on the San Andreas.
As if that were not sobering enough, to the west of Idyllwild is the San Jacinto Fault, called by Cal Tech’s Lucy Jones the most “active” fault in Southern California. Smith said probabilities on the San Jacinto are great for earthquakes in the 6.2- to 6.8-magnitude range. And whereas the last significant quake on the San Jacinto was on April 9, 1968, at 6.5 magnitude, and the one previous in 1918 at 6.8 magnitude (a 50-year interim), the southern San Andreas has not had a giant rupture in nearly 400 years. As Smith noted, it is ready.
And when asked about Idyllwild’s comparative risk of experiencing significant structural damage from a major rupture on either fault, Smith said our granite base may increase rather than decrease our risk. “Alluvium soil [at the mountain’s base] attenuates out high-frequency seismic waves,” said Smith. “The low waves that remain pose the greatest risk to high-rise buildings, since the low waves create harmonics that continue to resonate in the high-rise structure, causing significant damage.” Conversely, Smith noted high-frequency waves are very good at destroying smaller buildings, through strong jolts and shocks weaken structural components and cause building components to collapse.
Smith will explain earthquake probabilities of living between two major fault zones with a slide show and lecture. Although risks of earthquake damage are significant for our area, Smith noted it’s easy to forget about them. “It’s an easy thing not to keep in the forefront of our consciousness,” he said.
Smith is an engaging conversationalist who makes difficult geologic concepts easy to understand. His talk begins at 6 p.m. Thursday, March 19, at Silver Pines Lodge. There is a wine and cheese reception at 5:30. Both are free to the public.