logical and scientific challenges. The experience with recording large earthquakes is still fairly thin; with rare exceptions, areas of more moderate risk are currently serviced only by sparse seismographic networks with antiquated instrumentation and uneven capabilities for digital recording and processing. To remedy some of the deficiencies, the U.S. Geological Survey has begun to deploy the Advanced National Seismic System, which will improve regional networks, expand the distribution of strong-motion and building response sensors in the high-risk urban areas, and package and deliver earthquake information automatically (see Chapter 4). Research is needed in many aspects of post-event analysis, for example, assimilating data into strong-motion predictions, increasing the reliability of aftershock predictions, and identifying areas of enhanced short-term risk through the development of models of how earthquakes transfer stresses from one fault to another.

With appropriate technology, seismic information systems can be used as earthquake warning systems. Because electronic signals travel much faster than seismic disturbances, it is possible to notify regions away from the epicenter that an earthquake is in progress before any damaging waves arrive. Suboceanic earthquakes sometimes generate tsunamis (sea waves) that can inundate shorelines thousands of kilometers from the source. For example, the great 1964 Alaska earthquake generated tsunamis that killed 17 people along the Oregon-California coast, and tsunamis generated by the 1960 Chile earthquake killed 61 people in Hawaii and 122 people in Japan. These waves travel relatively slowly (500 to 700 kilometers per hour), so post-event predictions of tsunami arrival times and amplitudes can be used to warn coastal communities soon enough to allow for preparation and evacuation. Agencies such as the National Oceanic and Atmospheric Administration operate tsunami-warning networks that depend on precise seismic information to function properly (13). However, how tsunamis are generated by suboceanic earthquakes and how they run up along coastlines are still poorly understood.

Advance warnings of strong motions caused by the fast-moving (2 to 8 kilometers per second) ground waves are more problematic than for tsunamis, because there is so little time for an event to be evaluated and an advisory broadcast through civil defense or other warning systems. The damage zones of large earthquakes usually have radii of 200 kilometers or less, and it takes only 60 to 100 seconds for the most damaging waves (shear and surface waves) to propagate to this distance. Nevertheless, the time is adequate to issue electronic warnings that can initiate emergency shutdowns and other protective actions within power generation, transportation, and computer systems, provided that decisions can be automated reliably (see Chapter 3). The implementation of this capability will require seismic information systems that are robust with respect



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