Last month’s magnitude 5.1 earthquake in Southern California got people talking. What about “The Big One?” When will it hit, and how bad could it be?
Maybe not as bad as some previously thought, according to a new study. Seismic models may overestimate ground shaking in Los Angeles during a major earthquake by as much as 70 percent, new research says, due to assumptions about the rocks in the region.
Seismic models try to predict the ground shaking that would result from a major earthquake. In heavily populated areas, governments use the maps generated by these models to estimate what damage an earthquake could inflict, and how best to prepare.
Geologists believe that the San Andreas Fault of Southern California could unleash a magnitude 7.8 or greater earthquake. Simulations of such an earthquake prepared in connection with earthquake drills showed strong ground shaking in Los Angeles, due to a series of valleys amplifying the seismic waves emanating from the earthquake’s epicenter.
But researchers noticed that these models assumed that seismic waves moved through rigid rock. In a large earthquake, rocks can only take so much shaking before they break, absorbing some of the earthquake’s energy.
Also, the properties of the rock between the epicenter and Los Angeles determine how much earthquake energy reaches Tinseltown. Less seismic energy travels through a less-cohesive material such as loose sand than through a highly-cohesive rock such as solid granite.
Taking these factors into account, the team re-modeled a large earthquake in Southern California, using realistic ranges for rock cohesion in the region. Their results, published in Geophysical Research Letters, show that the sediments in the seismic-wave-amplifying valleys absorbed some of the earthquake’s energy.
If true, that could mean much less shaking for the 18 million residents of the Los Angeles area.
Photo: A magnitude 7.2 earthquake struck this embankment in Calexico, Calif. on Easter Sunday 2010 fracturing the hillside. Credit: Adam DuBrowa/FEMA