HR: 1340h
AN: OS23D-1344    [Abstracts]
TI: Estimating Source Recurrence Rates for Probabilistic Tsunami Hazard Analysis (PTHA)
AU: * Geist, E L
EM: egeist@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 999, Menlo Park, CA 94025 United States
AU: Parsons, T
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 999, Menlo Park, CA 94025 United States
AB: A critical factor in probabilistic tsunami hazard analysis (PTHA) is estimating the average recurrence rate for tsunamigenic sources. Computational PTHA involves aggregating runup values derived from numerical simulations for many far-field and local sources, primarily earthquakes, each with a specified probability of occurrence. Computational PTHA is the primary method used in the ongoing FEMA pilot study at Seaside, Oregon. For a Poissonian arrival time model, the probability for a given source is dependent on a single parameter: the mean inter-event time of the source. In other probability models, parameters such as aperiodicity are also included. In this study, we focus on methods to determine the recurrence rates for large, shallow subduction zone earthquakes. For earthquakes below about M=8, recurrence rates can be obtained from modified Gutenberg-Richter distributions that are constrained by the tectonic moment rate for individual subduction zones. However, significant runup from far-field sources is commonly associated with the largest magnitude earthquakes, for which the recurrence rates are poorly constrained by the tail of empirical frequency-magnitude relationships. For these earthquakes, paleoseismic evidence of great earthquakes can be used to establish recurrence rates. Because the number of geologic horizons representing great earthquakes along a particular subduction zone is limited, special techniques are needed to account for open intervals before the first and after the last observed events. Uncertainty in age dates for the horizons also has to be included in estimating recurrence rates and aperiodicity. A Monte Carlo simulation is performed in which a random sample of earthquake times is drawn from a specified probability distribution with varying average recurrence rates and aperiodicities. A recurrence rate can be determined from the mean rate of all random samples that fit the observations, or a range of rates can be carried through the probability calculations enabling an estimate of parameter sensitivity. Similar techniques can also be used to directly estimate the recurrence rate of tsunamis exceeding a particular runup threshold at a given site. These empirical results can provide an important independent estimate for comparison to computational PTHA results.
UR: http://walrus.wr.usgs.gov/tsunami
DE: 7223 Seismic hazard assessment and prediction
DE: 7215 Earthquake parameters
DE: 4564 Tsunamis and storm surges
DE: 3210 Modeling
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting