“The springs on the San Andreas system have been wound very, very tight. And the southern San Andreas fault, in particular, looks like it’s locked, loaded and ready to go,” Jordan said in the opening keynote talk.
Other sections of the San Andreas fault also are far overdue for a big quake. Further southeast of the Cajon Pass, such as in San Bernardino County, the fault has not moved substantially since an earthquake in 1812, and further southeast toward the Salton Sea, it has been relatively quiet since about 1680 to 1690.
Here’s the problem: Scientists have observed that based on the movement of tectonic plates, with the Pacific plate moving northwest of the North American plate, earthquakes should be relieving about 16 feet of accumulated plate movement every 100 years. Yet the San Andreas has not relieved stress that has been building up for more than a century.
Jordan said it’s important that California focus on becoming resilient to a potential huge earthquake, one as strong as a magnitude 8. He praised Los Angeles’ plan to require earthquake retrofits on apartment and concrete buildings, pushed into law by Mayor Eric Garcetti.
“It’s remarkable that this happened,” Jordan said. “We know politically how difficult it is to make these kinds of changes.”
Using the world's largest supercomputer at the time, the Southern California Earthquake Center in 2010 unveiled a simulated magnitude-8 earthquake that begins in Monterey County, like in 1857, but travels even farther south, heading toward the Mexican border. The L.A. Basin and the San Fernando Valley will be hit hard because the shaking will be trapped by soft soils in the valley and basin.
"You can see that this area of influence by the shaking has now expanded out to huge proportions," Jordan said. "You see that big directivity pulse out in front, as that energy is being shoved down that fault, that directivity pulse leads energy into seismic waves that excite the sedimentary basins, like the San Fernando Valley and the Los Angeles basin," and through San Bernardino, Jordan said.
"You'll notice large shaking in the Los Angeles region persisting for long periods of time," Jordan said ( via latimes.com ).