The March 11 Anza earthquake, at magnitude 4.7, was classified as “moderate,” and was felt over a wide area as far as Los Angeles, San Diego and Orange County.
Kate Hutton, Caltech seismologist, said the granite composition around the Anza area epicenter slows down or attenuates seismic waves less than other materials, thereby enabling those waves to travel greater distances.
“Once the seismic waves leave the Peninsula Range [and the granite features characteristic of that range’s Palomar, Aguanga, Volcan and Cuyamaca mountains], the attenuation is more normal for the wider region,” she noted. Having said that, Hutton added, “Most quakes in this magnitude range are felt over approximately the same distance.”
For those in Idyllwild who experienced the quake as more rolling than the sharp single jolts that may be more familiar in Idyllwild as a result of San Jacinto Fault zone activity, Hudson noted distance is the key factor. “Whether a quake seems sharp or rolling depends more on where you are than any other factor,” she said. She explained the closer one is to the epicenter, the more likely the effect is a sharp jolt or jolts.
“Generally, if you are very close to the quake, it appears as one or two sharp jolts [the P, primary and S, secondary waves],” Hutton said. “At a greater distance, the waves have been dispersed and some of the high-frequency energy has been attenuated out, so the quake appears more rolling.”
Hutton noted the number of aftershocks from the Anza quake was greater than for other similar magnitude events, but the aftershocks registered as very small quakes and were not uncharacteristic of San Jacinto Fault zone activity.
She stated that a determination has not yet been made as to whether the quake occurred on the San Jacinto Fault or on one of the smaller lateral faults that cross the San Jacinto Fault and are part of its overall network.
As to future quakes and whether the magnitude-4.7 temblor indicates something larger in the near future, Hutton said one has to look at the zone’s history of large quakes. Large quakes occurred in 1899 and 1918, each magnitude 6.6, and caused major destruction in Hemet and San Jacinto. Additional large quakes followed in 1923, magnitude 6; 1937, magnitude 5.9; 1954, magnitude 6.2; 1968, magnitude 6.4; and 1987, magnitude 6.2 and 6.6, one 11 hours after the previous. Looking back over that history, the intervals between major quakes on the San Jacinto Fault zone, beginning in 1899, were 18, 5, 14, 17, 14 and 13 years. It has now been 26 years since the last major quake in the San Jacinto Fault zone, almost twice the average interval between major quakes over the past century.
Hutton also noted that the nascent earthquake early warning system currently under development in California did provide advance warning in Pasadena, by 40 seconds, of the effects of the Anza quake.
Even though the science of earthquake prediction is growing in sophistication, all experts stress preparation for a major quake before, not after, the quake occurs. See www.earthquake.usgs.gov/prepare for earthquake preparation tips.