In Earthquake Science, Future Shock Lotta Shakin’ Goin’ On

Los Angeles has its faults--just ask geophysicist Gary Fuis of the U.S. Geological Survey, co-investigator of the Los Angeles Region Seismic Experiment, a project begun in 1993 to map quake faults in the L.A. basin. LARSE, a massive effort by federal, state, city and foreign government agencies along with local universities and colleges, has detonated and measured 93 explosions deep underground at L.A.-area sites. News bulletin: The San Gabriel Valley basin is 50% deeper than originally believed, a potentially greater earthquake hazard. We asked Fuis about future shocks.

Why is this study important?

For 30 years, since the San Fernando earthquake, we’ve never really understood what happened or what seismic structures were involved. We needed an image of the subsurface structure.

Was it hard to get cooperation for the tests?

Not really. The public in L.A. recognizes the seismic risk down there and that something has to be done. The only problem we had was with the land developers. Most were afraid we’d gather shallow ground information that would jeopardize their investments.

Give us the nutshell version of your results.

We found the deep parts of the Northridge and the San Fernando faults and what we think is the deep part of the Whittier Narrows Fault, to the points where they connect with the San Andreas Fault. The major thrust faults that are along the south side of the San Gabriel Mountains and the Santa Susanna Mountains are connected northward to the San Andreas Fault at great depth, of 12 to 15 miles. We didn’t know that before. We were surprised at the connection.

What happens to these findings?

The most immediate use is our imaging of the sedimentary basins. We’ve imaged one part of the L.A. Basin and we’ve got a detailed view of the San Fernando Valley, Santa Clarita Valley and San Gabriel Valley. We know the shapes and depths of each of those basins at least at one crossing. We can make a better estimate of the basin, and make a model. We look at past earthquakes to see what happened and what might happen in the future. In other words, a blueprint for future ground shakings.

You had to fund this study from several sources, including the German government.

This kind of work has always been funded poorly [in proportion to] its impact. Spending a few million bucks to mitigate disasters that run into the hundreds of billions of dollars makes good sense to me. We hope Congress will fund a new initiative called Earth Scope, which would fund studies [to] find the kind of things we’re finding with LARSE.

What does the LARSE data imply for city planning and emergency preparedness?

The city of L.A. called me last January and asked if the building codes should be looked at. What we’ve done is research. The next step is an engineering/scientific study where we take a model of L.A. and shake it and see how [these events] modify the old knowledge.

You’ve said you’re “worried” about faults near downtown L.A.

There’s an inferred blind thrust fault under the Elysian Park hills northeast of L.A. that is growing, and we need to find out why and what it means for the city.

What precautions does the study call for?

I think earthquake preparedness should be of the highest concern in L.A. Future damaging earthquakes will happen.

How big is the threat?

No earthquakes in the U.S. have [centered in] cities since San Francisco in 1906. The chance of getting killed has been--so far--relatively low. Things could change significantly if we have a San Fernando- or Northridge-type earthquake near any of the urban parts of L.A.

Will earthquakes ever be predictable?

It’s like predicting the weather: We can do better now than we did in the past.

Could Southern California fall into the Pacific Ocean? No, L.A. is moving up toward San Francisco. Seismologists always refer to the ground underwater as the “continental shelf,” but it’s not a shelf hanging on the wall that can fall off. It’s a bad choice of words by geologists.