USGS, NOAA, FEMA
U.S. Geological Survey
Data Series 236
Data DescriptionProbabilistic tsunami hazard analysis (PTHA) is based on techniques developed in the related field of probabilistic seismic hazard analysis (PSHA) and attempts to model the magnitude of tsunami flooding from multiple sources for particular recurrence rates (Geist and Parsons, 2006; Tsunami Pilot Study Working Group, 2006). The probabilities of interest for the Flood Insurance Rate Maps are 1 percent and 0.2 percent per year or the 100-year and 500-year maps, respectively. These GIS data sets were developed with a combination of numerical models and geographic information systems (GIS) tools. All wave heights are relative to mean high water.
In historic times, Seaside has experienced flooding from at least two major tsunamisone from a Cascadia subduction zone earthquake in 1700 and another from the 1964 Alaskan earthquake (Priest, 1995). Mapping of tsunami deposits and, for the more recent event, eyewitness accounts, have defined minimum extents of the tsunami runup or flooding for each event (Horning, Apx. C of Tsunami Pilot Study Working Group, 2006); Priest and others, 1997; Fiedorowicz, 1997; Peters and others, 2003; Jaffe and others, 2004). The tsunami observations are included as point and line shapefiles.
The impact of tsunamis in the Seaside area was incorporated in the existing Flood Insurance Rate Maps (FEMA, 1981, 1999) from work by Houston and Garcia (1978) based on models of far-field tsunamis, tides, and local runup probabilities. They developed curves of coastal tsunami heights for 100-year and 500-year scenarios; in the Seaside/Gearhart area (latitude 45°58’ to 46°02’ N), their runup calculations are 2.3 m and 4.9 m (mean high water), respectively. For comparison, georeferenced copies of the FEMA maps are part of the data set.
The NOAA Method of Splitting Tsunami (MOST) is a numerical model that simulates tsunami generation, transoceanic propagation, and inundation on land (Titov and Synolakis, 1998; Titov and Gonzalez, 1997). The model requires two primary data inputs: nested grids of bathymetry and topography to simulate tsunami wave dynamics and tsunamigenic source parameters.
The MOST model also requires "bald earth" topography and assumes that tides interact linearly with a propagating tsunami wave. Therefore, the DEMs do not include manmade structures or vegetation and are based on the Mean High Water tidal datum. Mofjeld and others (2004) provided an analysis of background tides and sea level variation for the study region, and associated water-level probability distributions are provided in tabular format.
Far-field source specifications are provided as a series of unit sources within the FACTS database as defined by Titov and others (1999). Unit sources are provided as a polygonal shapefile with attributes defining the slip distribution for each of the fourteen different scenarios. Thirteen near-field sources are provided as ASCII raster grids or xyz files containing fault geometry based on Fluck and others (1997) and a heterogeneous slip distribution constrained by the results of Satake and others (2003).
The 0.010 contour is extracted from each of the probability grids into a map of wave heights with a probability of exceedance of 1 percent per year (that is, the 100-year tsunami map). Similarly, the 0.002 contours are collected to generate the 500-year tsunami map. The contour data extracted from each of the four sets of PTHA surfaces is included in the data set. A summary polygon layer (probzones) identifies the 100- and 500-year flood zones based on the results of the PTHA model.
Except for data sets strongly tied to the study, such as historic inundation lines and existing FEMA Flood Insurance Rate maps, this GIS report serves only data developed in the course of building the tsunami model. Sources for basemap layers (boundaries, elevation, orthophotos), thematic maps (geology), and other publicly available data sets are listed in the data catalog.
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