Preliminary Work on an Early Warning and Rapid Response Program for Moderate Earthquakes

Warning of imminent ground shaking due to a large earthquake would be useful to a variety of agencies. This kind of ground-motion prediction is possible in southern California for events with magnitude less than 6, where path effects dominate. The 28 June 1991 Sierra Madre earthquake is presented as a test case for this concept. A single-station inversion of the record from the Pasadena station 20 km SW of the epicenter produces reasonable source parameters for the event. With these source parameters and a library of Green's functions calculated for an average southern California crustal model, ground motions can be predicted throughout the region. In particular, since the peak displacement for the Sierra Madre event occurs at Pasadena before ground motion begins at a station near the San Andreas Fault in San Bernardino, ground motions near the San Andreas Fault can be calculated before the seismic energy has propagated into the area. Considering this scenario in the reverse direction, records from a station near an earthquake on the San Andreas Fault could be used to predict ground motions in the metropolitan Los Angeles area. Broadband, high--dynamic-range seismic instruments produce high-quality records for events over a wide magnitude range. Thus, the development of a warning system can be approached in stages, starting with small events. With path effects determined by modeling moderate-size events, work can begin on developing distributed fault models to predict ground motions of great earthquakes.