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Open-File Report O-19-04, Comparison of Oregon tsunami hazard scenarios to a probabilistic tsunami hazard analysis (PTHA), by George R. Priest and Jonathan C. Allan; 94 p. report.

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EXECUTIVE SUMMARY

The Oregon Department of Geology and Mineral Industries (DOGAMI) published in 2013 coast-wide velocity, flow depth, and momentum flux data for five Cascadia subduction zone (CSZ) tsunami inundation scenarios (categorized in “T-shirt” sizes of SM1, M1, L1, XL1, and XXL1) (Priest and others, 2013; Witter and others, 2013). This paper is primarily aimed at evaluating which scenario best approximates a 1,000-yr exceedance event based on comparison of results generated from a 2019 probabilistic tsunami hazard analysis (PTHA) undertaken by AECOM Technology Corporation (Thio, 2019). We also compare DOGAMI and AECOM wave amplitudes at 100 m depth for 2,475-yr (Thio, 2017) and 1,000-yr exceedances (Thio, 2019). Based on a quasi-probabilistic PTHA considering only CSZ earthquake source parameters, the DOGAMI M1 scenario should approximate a conservative 1,000-yr exceedance event. The next larger DOGAMI scenario, L1, would be a highly conservative choice for a 2,475-yr exceedance. However, AECOM PTHAs do not generally match these estimates. In general, the 1,000-yr and 2,475-yr tsunami amplitudes at 100 m depth offshore resemble DOGAMI scenarios L1 and XL1, respectively. Tsunami flow depths, velocities, and momentum flux data at open coastal bridge sites in northern and central Oregon generally followed the same pattern. Open coastal sites on the south coast had AECOM tsunami flow depths and inundation resembling or slightly smaller than the SM1 scenario. Understanding the reasons for the mismatch with the quasi-probabilistic PTHA was hampered by lack of access to digital point data on slip and vertical deformation of AECOM CSZ sources, recurrences of sources, and details of the PTHA computations. Some potential reasons may be AECOM’s use of (1) much larger coseismic slips than deemed reasonable by DOGAMI from consideration of slip balance on the CSZ for the last ~10,000 yrs of paleoseismic record, (2) much heavier emphasis than DOGAMI on partial CSZ fault ruptures known to have occurred over the last 10,000 yrs on the southern CSZ, and (3) global seismic data not representative of the CSZ to estimate coseismic slip from rupture area.. Comparison of AECOM to DOGAMI data is difficult at inland sites owing to some mismatches of digital elevation models (DEMs) used in the respective studies and possible differences in numerical models, as well as adoption of zero bottom friction by DOGAMI versus Thio’s Manning friction coefficient of 0.025.

If any DOGAMI tsunami scenarios are to be used for bridge design, we recommend that the scenarios be conservatively large to reflect the relatively large uncertainties revealed by comparison of the DOGAMI and AECOM approaches. For example, the tsunami amplitudes of AECOM at 100 m depth approximate the DOGAMI scenario L1 at 1,000-yr exceedance and XL1 at 2,475-yr exceedance so use of these two DOGAMI scenarios for these exceedances should encompass hazard estimates of both approaches. Any new simulations of these or other source scenarios should be run with improved inputs, including bottom friction and computational grid spacing small enough to simulate key elements of each site. Time histories of current forces and direction should be extracted to evaluate how long peak forces persist. These time histories are not routinely provided by a PTHA unless obtained by disaggregation of the simulations and collecting time history data during each computer run.

References in Executive Summary

Priest, G. R., Witter, R. C., Zhang, Y. J., Wang, K., Goldfinger, C., Stimely, L. L., English, J. T., Pickner, S. G., Hughes, K. L. B., Willie, T. E., and Smith, R. L., 2013, Tsunami inundation scenarios for Oregon: Oregon Department of Geology and Mineral Industries Open-File Report O-13-19.

Thio, H. K., 2017, Probabilistic tsunami hazard maps for the State of California (phase 2): draft report (in review) by AECOM to the California Geological Survey.

Thio, H. K., 2019, Notes on the production of the 1,000 year probabilistic tsunami hazard maps for the Western United States: draft report (in review) by AECOM to the California Geological Survey.

Witter, R. C., Zhang, Y. J., Wang, K., Priest, G. R., Goldfinger, C., Stimely, L., English J. T., and Ferro P. A., 2013, Simulated tsunami inundation for a range of Cascadia megathrust earthquake scenarios at Bandon, Oregon, USA: Geosphere, v. 9, no. 6, 1783–1803. https://doi.org/10.1130/GES00899.1