HR: 16:20h
AN: S44A-02    [Abstracts]
TI: The Challenge of Tsunami Warning in the Near Field
AU: * Fryer, G J
EM: gerard.fryer@noaa.gov
AF: Pacific Tsunami Warning Center, 91-270 Fort Weaver Rd, Ewa Beach, HI 96706-2928, United States
AB: "If the ground shakes, get away from the ocean." That oft-repeated advice is soon ignored where earthquakes are common. Add the proviso "if shaking is so severe that you cannot stand," and people ignore slow earthquakes, if they are even aware of them. Tragedies such as the West Java tsunami of 2006 can only be avoided with a local warning system, but by the nature of the warning problem such systems cannot be as reliable and foolproof as those for tele\-tsunamis. Where local warning systems are well developed, such as Japan, the West Coast of North America, and Hawaii, initial warning messages are routinely issued within five minutes of an earthquake. Such rapid messages are necessarily based solely on seismic data, usually just epicentral location and earthquake magnitude. Since magnitude is not directly correlated with tsunami size, there is inevitably a high false-alarm rate. Direct measurement of the tsunami at sea would avoid false alarms, but the the density of sensors required is almost prohibitive. The first positive wave of a tsunami generated by a subduction earthquake will typically reach the adjacent shoreline in 35--45 minutes. If coastal residents are to be provided with twenty minutes of warning, the tsunami must reach the nearest sensor in no more than ten minutes (the additional time being consumed by the finite duration of the earthquake and the need to see several minutes of data on a gauge before a dangerous wave can be confirmed). The ten-minute restriction means that deep-ocean sensors can be no more than 20\thinspace km from the trench axis and must be spaced no more than 120\thinspace km apart along strike. The 1000-km-long Cascadia Subduction Zone thus requires a minimum of eight sensors. At present, it has four {\sc dart}s, but all are too far off shore to record the tsunami quickly enough for effective local warning. Hawaii faces an even more severe challenge: the first positive wave of a tsunami from a basal-slip earthquake will reach the adjacent shore in as little as six minutes. Until an inexpensive and robust technique can be devised for direct measurement of a tsunami very close to its source, local warning will have to depend on shore-based measurements. Dense GPS networks are promising, but only for earthquakes larger than about magnitude 7.6. For smaller earthquakes, coseismic deformation is too small to measure rapidly on land, even though a damaging local tsunami may have been generated. We are left with seismometers. M_WP has been used successfully at close range in Hawaii, but for large or slow earthquakes it will saturate. To render the current rapid tsunami warnings more reliable, the performance of other P-wave-based estimates (such as m_BC) in the near field must be explored. A rapid near-field measure of the energy- to-moment ratio, Θ, would be especially valuable.
DE: 4564 Tsunamis and storm surges
DE: 6309 Decision making under uncertainty
DE: 7215 Earthquake source observations (1240)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: 2007 Fall Meeting