Seaside, Oregon, Tsunami Pilot Study—Modernization of FEMA Flood Hazard Maps: GIS Data

Data Description

Historical Tsunami Events

In historic times, Seaside has experienced flooding from at least two major tsunamis—one 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.

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Tsunami Propagation and Inundation Modeling

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.

DEM Development. For this study, three nested grids of bathymetry and topography (otherwise known as digital elevation models or DEMs) were developed—a low-resolution (36-arcsecond) DEM of bathymetry to simulate tsunami generation and deep ocean propagation, a medium-resolution (6-arcsecond) DEM of bathymetry to calculate regional propagating waves, and high-resolution (1/3-arcsecond) DEM of bathymetry and topography to compute tsunami inundation within the study area (Tsunami Pilot Study Working Group, 2006).

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.

The DEMs are provided here in ASCII raster and xyz formats. Data sets associated with DEM development (such as data sources, grid development limits, and shoreline) are included as described in Venturato (2005). During the DEM development process, an analysis of historical shoreline was conducted due to significant variations between available shoreline data sources. Eleven data files of estimated shorelines based on aerial photography dating back to 1939 are provided in vector format.

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Source Development. The tsunami inundation model is part of the NOAA Facility for the Analysis and Comparison of Tsunami Simulations (FACTS) tool, which can run numerous simulations of tsunamigenic earthquakes from both far-field and near-field tsunami sources (Borrero and others, 2004). For this study, only interplate thrust fault tsunamigenic earthquakes within the Pacific Basin were considered that could potentially affect the aggregate tsunami probabilities according to the FEMA FIRM specifications. Distant or far-field sources for the probabilistic model calculations include large earthquakes along the Kuril-Kamchatka, Alaska-Aleutian, and southern Chile subduction zones. Local or near-field sources are based on several scenarios along the Cascadia subduction zone.

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).

Inundation Model Results. Of several parameters provided by the MOST model database (Borrero and others, 2004), the maximum wave height was selected for input to the probabilistic model. Maximum wave heights (meters referenced to Mean High Water) were determined for each of the 14 inundation models based on the far-field sources (10-m resolution) and the 13 models based on near-field sources (30-m resolution). Maximum velocity zones were also calculated for each source to develop tsunami impact metrics (Tsunami Pilot Study Working Group, 2006). Inundation model results are provided in ASCII raster grid and xyz formats.

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Probability Tsunami Hazard Analysis Surfaces

For each grid location x-y, the MOST inundation data have been combined with tidal data by probabilistic tsunami hazard analysis calculations to generate a hazard curve that describes the probability of recurrence at that location of a tsunami flood exceeding some wave height z (Mofjeld and others, 2007). The grid resolution is 36 arcseconds reflecting the coarser resolution of the near-field input data. For each wave height z from 0.5 m to 10.5 m at 0.5-m intervals, a grid of probabilities was generated and contoured at 0.002 (0.2 percent) intervals (Tsunami Pilot Study Working Group, 2006). To test the sensitivity of the model to input variables, additional PTHA model runs were conducted using (a) only Model 1 of Alaska-Aleutian segmentation, (b) a mean inter-event time less than 520 years for M~9 subduction zone earthquakes, and (c) a mean inter-event time greater than 520 years for M~9 subduction zone earthquakes. Including the unmodified run, these four sets of grids are provided in ASCII raster and xyz formats.

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100- and 500-Year Floods from Probabilistic Tsunami Hazard Analysis Model

Other GIS Data

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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Data Catalog

References