Feature - Watch near real-time earthquake simulations

Feature - Watch near real-time earthquake simulations

(Click image for more information on the topographical map and to see cross-sectional diagrams of compressional wave speed.)

Every time an earthquake occurs in Southern California, thousands of seismograms are recorded at hundreds of seismic stations. The Caltech team collects these seismic recordings automatically from the Southern California Seismic Network (SCSN) through the internet. They subsequently simulate the seismic waves generated by the earthquake in a 3-D southern California seismic velocity model using the source parameters determined by the seismic network. At periods of about 4 seconds and longer, the resulting wave motion simulated at most stations closely match the gathered data recorded at these stations.

Image courtesy of Caltech; image previous page courtesy of Nate Brelsford, stock.xchng

When an earthquake rumbles through southern California, seismometers at almost 400 sites jump into action. They send measurements to the Southern California Seismic Network data center, where in short order researchers determine the magnitude and location of the quake. A seismic event pegged at a magnitude of 3.5 or greater triggers ShakeMovie - a system that produces a near real-time simulation of the event and posts it on an online Web portal for scientists and the general public to download and view.

The ShakeMovie portal is operated by the California Institute of Technology and Princeton University. It typically receives between 8,000 and 9,000 visits a month, and teachers are using it in classrooms to teach students about earthquakes and seismic waves. The simulations show the movement of the Earth's surface as the seismic waves propagate outward from the quake's epicenter.

ShakeMovie simulates two types of waves: fast-moving, compressional sound waves that produce movement in the direction of propagation, and slower-moving shear waves that produce movement perpendicular to the direction of travel. The simulations illustrate how the waves change speed as they travel through different media. Waves travel rapidly through hard rock, as they do in metal - the reason a distant train is audible with your ear to the track long before the sound waves in the air reach you. But in sedimentary basins, such as the one lying under Los Angeles, they slow down and the energy lost in speed produces a greater wave amplitude, causing greater motion - and more damage.

The ShakeMovie system uses the equivalent of 168 CPUs at each of two locations (for backup purposes) - at Caltech and the San Diego Supercomputing Center. It currently takes about 45 minutes to process a simulation and post it on the Web portal.

A visualization produced by Caltech showing the progress of the Chino Hills/Pomona earthquake in Los Angeles last August.

Video courtesy of Caltech.

Princeton will soon contribute a third ShakeMovie computing site when its new 3,584-core cluster with 10 terabytes of distributed memory comes online in the next few months. The new cluster's cores are twice as fast as the ones ShakeMovie currently uses.

A global version of the portal, also expected to launch in the coming months, will cover earthquakes occurring anywhere on Earth of magnitude 5.5 or greater. Portal visitors will be able to view the simulated event at a global level or zoom in to a particular area.

In addition to helping the public better understand earthquakes, the simulations have scientific value, said Jeroen Tromp, ShakeMovie team member and Princeton professor. "When the global ShakeMovie site comes online, seismologists will be able to access simulations from both the global and regional sites, compare them to recorded data, and use them to improve their earthquake models and three-dimensional models of the Earth's interior structure."

-Amelia Williamson, for iSGTW