Seismic waves trapped in a fault.

Just as physicians use sonar waves to image the inside of the human body, so seismologists send seismic waves through the earth's crust to learn about its structure. Now seismologists at the University of Southern California in Los Angeles have an idea for using certain seismic signals not just to detect the static structure of the crust but also for real-time monitoring of the changes in this structure along a fault, in the hope of catching the stresses that build up prior to earthquakes.

The key to this idea lies with trapped, or standing, waves--the kind of waves that are set up in organ pipes. Peter C. Leary and his colleagues have recently recorded such waves in a fault intercepted by a bore hole in Oroville in northern California. The researchers believe the seismic waves, generated at the surface and sensed in the hole, become trapped in the fault zone because it is weaker and has more cracks than the surrounding rocks. This difference in strength and structure means that waves, which travel more slowly in the fault zone, are reflected back into the zone when they try to escape.

Waves trapped inside the fault zone are more sensitive to changes in the fault rocks than are waves that travel once across the fault; hence, Leary thinks they would be more diagnostic of fault stresses leading to earthquakes. The big question is whether standing waves can be set up or detected in large faults with significant earthquake potential. In particular the researchers would like to test their idea on the San Andreas fault with a bore hole 4 kilometers away in the Cajon Pass. This drill hole has recently been made the "kickoff" hole of the U.S. Continental Scientific Drilling Program, which plans to begin extending the 2-km-deep hole to 5 km next August.