Ripping Earth

By Anderson, Robert | Natural History, October 2006 | Go to article overview

Ripping Earth


Anderson, Robert, Natural History


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 (earth quake.usgs.gov/regional/nca/1906). 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 (educa tion.sdsc.edu/teachertech/videos.html) 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 (world-stress-map.org) and, after you've entered the site, click on the "WSM poster" to get a feel for the stress data-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 (www.seismo.ethz.ch/GSHAP) 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. …

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