Reported October, 2007

BACKGROUND: Many earthquakes in the deep ocean are much smaller in magnitude than expected. Geophysicists from the Woods Hole Oceanographic Institute (WHOI) have found new evidence that the fragmented structure of seafloor faults, along with previously unrecognized volcanic activity, may be dampening the effects of these quakes.

ABOUT THE STUDY: The WHOI scientists examined data on ocean transform faults collected by ships and satellites from 19 locations in the Atlantic, Pacific and Indian oceans, augmented with bathymetry maps. Ocean transform faults are fractures in the rock of the ocean floor along which horizontal motion occurs. They cut across the mid-ocean ridge system, a 40,000 mile long mountainous seam in the Earth's crust that marks the edges of the planet's tectonic plates. The mid-ocean ridges are like the seams on a baseball, and the transform faults are like the red stitching, lying perpendicular to the ridge. These faults help accommodate the motion of the tectonic plates, cracking at the edges as the different pieces of rocky crust slip past each other. Along some plates, new crust is formed, while along others, old crust is driven back down into the earth.

WHAT CAUSES EARTHQUAKES? An earthquake is the result of a sudden release of stored energy in the Earth's crust triggered by shifting tectonic plates. The Earth's lithosphere is an elaborate network of interconnected plates that move constantly -- far too slow for us to be aware of them, but moving, nonetheless. Occasionally they lock up at the boundaries, and this creates frictional stress. When that gets to be too large a strain, the rocks give way and break and slide along fault lines. This can give rise to a violent displacement of the Earth's crust, which we feel as vibrations or tremors as the pent-up energy is released. However, only 10 percent or so of the total energy is released in the seismic waves. However, the rest is converted into heat, used to crush and deform rock, or released as friction.

The Incorporated Research Institutions for Seismology and the American Geophysical Union contributed to the information contained in the TV portion of this report.

If you would like more information, please contact:

Dr. Jian Lin, Senior Scientist
Woods Hole Oceanographic Institutionn
Woods Hole, MA 02543
(508) 289-2576
jlin@whoi.edu

Incorporated Research Institutions for Seismology
John Taber, Education & Outreach Program Manager
Washington, DC 20005
(202) 682-2220
taber@iris.edu

American Geophysical Union
Washington, DC 20009-1277
(800) 966-2481
http://www.agu.org

DID YOU KNOW?

Geologists no longer use the Richter scale to rate the severity of earthquakes. They now use the moment magnitude scale that relates the energy of the earthquake to pounds of high grade explosive. For example: a magnitude four earthquake is comparable to 120,000 pounds of C4, while a magnitude six earthquake is comparable to 120 million pounds of C4.

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