Lawrence Livermore National Laboratory

Modeling an
Earth-Shaking Event

By Victor D. Chase

“Earthquakes don’t kill people,” seismologist Arthur Rodgers says.  “Buildings do.”

“There are casualties in earthquakes because buildings collapse, freeway sections collapse, and bridges go out,” says Rodgers, a member of an earthquake modeling team at the Department of Energy’s Lawrence Livermore National Laboratory (LLNL). In essence, “We are vulnerable to earthquake damage because we choose to build and live near places where earthquakes occur.”

To help understand and prevent such devastation, a team of computational scientists, applied mathematicians and seismologists at LLNL has created an earthquake simulation model as part of a larger Serpentine Wave Propagation (SWP) project. The project, headed by applied mathematician Anders Petersson, looks at the propagation of waves in nature.  Whether they’re seismic, electromagnetic, or sound waves, they’re all governed by essentially the same mathematical equations.

Propagation of shear waves through the 3D model of the San Franciso bay area Click image for larger version and more information

The team spent several years developing the advanced mathematics and algorithms necessary to run computer models of wave propagation. For its first practical application, the team used the software to model what happened during the most famous and damaging earthquake in U.S. history.

It began at 5:12 a.m. on April 18, 1906.  San Francisco Bay area residents were awakened when the San Andreas fault, a 296-mile fissure beneath the Pacific Ocean a few miles along the California coast and off shore, slipped. The displacement of a few meters along that fault line, where the Pacific and North American tectonic plates meet, was enough to set off one of the most monumental quakes in recorded history.  After a 20-second foreshock, the full power of the quake was felt for about one minute.  It would have measured 7.9 on the Richter scale — if the scale had existed then.

The shock was felt from Coos Bay, Oregon, to Los Angeles, and as far east as central Nevada.  In all, the area of devastation was about 400 miles long, and 30 miles on either side of the fault zone.  The quake and the resulting four-day San Francisco fire killed about 3,000 people, left 225,000 homeless, and destroyed about 28,000 buildings.

Poorly Understood

Earthquakes were poorly understood and little studied before the San Francisco disaster.  The 1906 event put an end to that and marked the beginning of the science of seismology in the U.S. and gave rise to a more quantitative approach, applying physics and mathematics to the problem.  Shortly after the quake, damage throughout the region was studied and quantified on the Modified Mercalli Intensity Scale.  Unlike the Richter scale, the Mercalli scale does not require instrumentation. It rates a witness’s impressions and physical damage to structures.  Scientists can use Mercalli scale information to backtrack and determine the kind of ground velocities corresponding to the reported destruction.

The report has proven invaluable to later earthquake investigators, including the SWP team, which developed its computer simulation quake to mark the quake’s 100th anniversary. Conducted under the leadership of the U.S. Geological Survey (USGS), the research also involved scientists from Stanford University, the University of California at Berkeley, and URS Corp., a worldwide engineering firm.  DOE’s Office of Advanced Scientific Computing Research supported the SWP group’s modeling work.

Each participating group created its own model of the quake using its own methods.  The results were quite consistent, proving the exercise’s value, Rodgers says.

The exercise took almost two years to complete because of the complexity involved in creating a computer simulation of an earthquake.  The rarity of major earthquakes in the 7.0 to 7.9 magnitude range, means there is less empirical data about them.

The findings of the centennial study were presented at the 2006 meeting of the Seismological Society of America, which was held in San Francisco to commemorate the100th anniversary of the famous quake — and of the society’s founding.

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