nature.net
October 2006
Ripping Earth
By Robert Anderson
Early on the morning of April 18, 1906, a section of the San Andreas Fault that was “locked,” or stuck in place by friction, suddenly gave way. The break began under the Pacific Ocean, two miles west of San Francisco, and raced outward in opposite directions: northwest 202 miles to Cape Mendocino; southeast 93 miles to San Juan Bautista. The resulting earthquake was the most destructive in U.S. history, and it left the thriving city of San Francisco a smoldering ruin. In the past year, as if to mark the hundredth anniversary of the disaster, the Earth has continued to wreak havoc: most notably, as of this writing, with a devastating quake in western Iran and several major quakes in Indonesia following the disastrous temblor of October 8, 2005, in Pakistan, which killed at least 80,000 people.
To recall the San Francisco earthquake and to remind the public of future threats, the U.S. Geological Survey has assembled an impressive collection of material on the Internet. Start with the “Virtual Tour” of that tumultuous 1906 event, which uses Google Earth software (available at earth.google.com) to explain its causes and effects. The section devoted to the tour, as well as other linked sections on the site, has video and animations worth viewing; to download them quickly you'll need high-speed Internet access. The San Diego Supercomputer Center's “TeacherTECH” program also has simulations of that infamous quake as it rippled outward from the fault.
A century after the calamity, seismologists still cannot pinpoint when a fault will finally release its pent-up stress. Computing power, however, has provided what are literally new views of the problem, leading to refined assessments of hazards. Seismologists begin by measuring where the Earth's crust is being pulled apart and where it is being squeezed. Go to the World Stress Map Project and, after you've entered the site, click on the “WSM poster” in the left menu to get a feel for the stress data (download the large pdf version)—depicted as swarms of vector arrows indicating where the rocks are likely to let go. You can also look at the maps generated by the Global Seismic Hazard Assessment Program to see where the danger lies.
In the past couple of decades, geologists have begun to model how earthquakes change the stresses in the upper part of the crust, to determine whether the new stress pattern makes temblors more or less likely on adjacent faults. The site posted by the USGS Earthquake and Volcano Deformation and Stress Triggering Group shows how their modeling works. Click on “Stress Triggering and Earthquake Probabilities” to get a brief explanation, then check out the animations page showing how quakes propagate over time along fault lines in several active regions.
Los Angelenos like me should be concerned by the complex network of faults beneath us. The University of Southern California has some remarkable movies showing the faults, which look like brightly colored ribbons dissecting the crust. The Puente Hills Fault, which extends directly under downtown L.A., looks particularly ominous. San Diego State's Education Center on Computational Science and Engineering has video clips that illustrate why Rayleigh waves (which move the ground up and down) are so destructive and why seismic waves of all kinds propagate at higher amplitudes in soft sediments. Search results for earthquakes at Carlton College Science Education Resource Center's Collections of Visualizations on Geoscience Topics will give you animations of plate tectonic movements and related events.
At the Southern California Earthquake Data Center, click on the last box, “Historic Earthquakes in Southern California,” to find a map of past events. Click on the large red dot at the upper left, and you'll learn about the 1857 Fort Tejon event, when the San Andreas Fault slipped along a 220-mile stretch. Far to the south, the Los Angeles River was reportedly “flung out of its bed.” Cracks in the ground appeared near San Bernadino and in the San Gabriel Valley.
Return to the San Diego Supercomputer Center's site, mentioned earlier, and select “Animation of a 7.7 Quake on the San Andreas Fault” to see the prolonged ground shaking that is likely in the L.A. basin if the San Andreas ruptures near Palm Springs. The simulation took four days to compute. I hope the people at FEMA—under orders from Congress to develop a detailed plan to reduce earthquake hazards—get a chance to see it.
Robert Anderson is a freelance science writer living in Los Angeles.
Copyright © Natural History Magazine, Inc., 2006