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SUMMARY
An incoming tsunami may be anticipated in many ways, from direct human recognition of cues such as earthquake shaking or an initial recession of the sea, to technological warnings based on environmental sensors and data processing. This chapter reviews and evaluates the technological detection and forecasting capabilities of the U.S. tsunami warning centers (TWCs) paying specific attention to the infrastructure of the earth and ocean observation networks and to the data processing and tsunami modeling that occur at the TWCs. The next chapter discusses the centers’ operations, their human resources, and the infrastructure for their warning functions.
The initial decisions by the TWCs to issue an initial tsunami advisory, watch, or warning after an earthquake are based on analyses of data from a global seismic detection network, in conjunction with the historical record of tsunami production, if any, at the different seismic zones (see Weinstein, 2008; Whitmore et al., 2008 for greater detail on the steps taken). Although adequate for most medium-sized earthquakes, in the case of very large earthquakes or tsunami earthquakes1 the initial seismological assessment can underestimate the earthquake magnitude and lead to errors in assessing the tsunami potential (Appendix G). Far from the tsunami source, data from sea level networks provide the only rapid means to verify the existence of a tsunami and to calibrate numerical models that forecast the subsequent evolution of the tsunami. Near the source, a tsunami can come ashore before its existence is detected by the sparse sea level observation network.
Two separate U.S. TWCs monitor seismic activity and sea levels in order to detect tsunamis and warn of their presence. Based on their own data analysis, the TWCs independently decide whether to issue alerts to the emergency managers in their respective and complementary areas of responsibility (AORs). The TWCs must not only provide timely warnings of destructive tsunamis, but also must obviate needless evacuations that can cost money and even lives. An ideal warning would provide emergency managers with the necessary information to call for an evacuation in a timely fashion at any particular location in the projected tsunami path. The ideal product would also be clearly worded so that the general public easily understands the threat and who is affected by the threat. This information includes predictions of the time of arrival of the ocean waves, the duration of the occurrence of damaging waves, when the larg-
1
An earthquake that produces an unusually large tsunami relative to the earthquake’s magnitude