Virtual Earthquake Technique to Forecast Los Angeles Quake Risk

Stanford University scientists are using weak vibrations generated by the Earth's oceans to produce 'virtual earthquakes' that can be used to predict the ground movement and shaking hazard to buildings from real quakes. The technique was used to confirm a prediction that Los Angeles will experience stronger-than-expected ground movement if a major quake occurs south of the city. Stanford geophysicist Marine Denolle began by installing several seismometers along the San Andreas Fault to measure ambient seismic waves. Employing data from the seismometers, the researchers then used mathematical techniques they developed to make the waves appear as if they originated deep within the Earth.

Transcript

00:00:00 Stanford University.
>> This study was motivated by super computer simulations that showed that the, the structure of the Earth's crust in Southern California, would act to shape the basin stronger than we had previously anticipated. In order to test that simulation, what we did was develop a new approach, which we call the virtual earthquake approach. Now, the virtual earthquake approach uses what we call the ambient seismic field which are the waves that are present in the Earth at all times.

00:00:40 They are coursing under us right now. They're generated by the action of ocean waves.
>> And those waves, because they have different pressure then, they transfer to the sea bottom. And these get transmitted to the crust where we actually have instruments record it.
>> Those waves, even though they're billions of times weaker than earthquake waves. They interact with the complex geologic structure of the crust in just the same way.

00:01:08 In this instance, what it shows is that, seismic waves travel from the San Andreas Fault. Which is located to the east and to the north of Los Angeles. Through what we call a wave guide. So this is a, a series of shallow sedimentary basins.
>> Those waveguides and the basin have, they are basically surrounded by mountains, and mountains deposit a lot of sediments, with erosion. And those sediments are not compacted. It's like sand on the beach. So they are very compliant, so they can move more easily.

00:01:39
>> The waves travel through that corridor towards Los Angeles. And are essentially guided into the sedimentary basin that underlies Los Angeles. Once they're in that basin, they reverberate, they get amplified, and they cause stronger shaking than would otherwise occur. We have to do something about. [INAUDIBLE] It's a new way of testing these sort of simulations. And that's the information that we use to characterize. The hazard that say Los Angeles faces, and also, that's the information the engineers use to design structures.

00:02:11 There are other cities that, that lie on sedimentary basins, cities like Seattle, parts of the San Francisco Bay area, Tokyo Mexico City is a classic case, all of these cities are earthquake threatened, and all of them have this. Extra threat because of this space and amplification.
>> So, the, the sort of work that we've pioneered in Southern California, could be applied in many other parts of the world. [INAUDIBLE].
>> For more, please visit us at stanford.edu.