Abstract. Hypocenters of the Loma Prieta sequence 'form a dipping zone that rises from the mainshock hypocenter and is parallel to the mainshock nodal plane. Most aftershocks cluster around the perimeter of the zone, surrounding a relatively aseismic center which approximates the region of mainshock rapture. At its southeastern end, the dipping aftershock zone warps into a vertical surface that corresponds to the San Andreas fault. In the central and northwestern parts of the zone at depths above -10 kin, the aftershocks define numerous disjoint fault structures. The large component of reverse-slip observed in this event agrees with a simple model for slip on a dipping plane within a compressional fault bend. We do not believe that the Loma Prieta earthquake occurred on the Sargent fault. However, we are unable to conclude whether it ruptured the principal plate boundary fault or a less frequently active hult.

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Abstract. In 1997, the Federal Emergency Management Agency (FEMA), National Oceanic and Atmospheric Administration (NOAA), U.S. Geological Survey (USGS), and the five western States of Alaska, California, Hawaii, Oregon, and Washington joined in a partnership called the National Tsunami Hazard Mitigation Program (NTHMP) to enhance the quality and quantity of seismic data provided to the NOAA tsunami warning centers in Alaska and Hawaii. The NTHMP funded a seismic project that now provides the warning centers with real-time seismic data over dedicated communication links and the Internet from regional seismic networks monitoring earthquakes in the five western states, the U.S. National Seismic Network in Colorado, and from domestic and global seismic stations operated by other agencies. The goal of the project is to reduce the time needed to issue a tsunami warning by providing the warning centers with high-dynamic range, broadband waveforms in near real time. An additional goal is to reduce the likelihood of issuing false tsunami warnings by rapidly providing to the warning centers parametric information on earthquakes that could indicate their tsunamigenic potential, such as hypocenters, magnitudes, moment tensors, and shake distribution maps. New or upgraded field instrumentation was installed over a 5-year period at 53 seismic stations in the five western states. Data from these instruments has been integrated into the seismic network utilizing Earthworm software. This network has significantly reduced the time needed to respond to teleseismic and regional earthquakes. Notably, the West Coast/Alaska Tsunami Warning Center responded to the 28 February 2001 Mw 6.8 Nisqually earthquake beneath Olympia, Washington within 2 minutes compared to an average response time of over 10 minutes for the previous 18 years.

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