Preface

A project is underway to assess the tsunami hazards within Puget Sound communities and to provide information for tsunami planning and mitigation. It is one of the Tsunami Inundation Modeling Efforts within the National Tsunami Hazard Mitigation Program. It is recognized that the Seattle Fault zone. Using a finite difference model based on nonlinear shallow water wave theory and high-resolution digital elevation model, we simulate the generation, propagation, and inundation of tsunamis in Puget Sound. The tsunamis are generated as a result of possible earthquake scenarios for the Seattle Fault. The initial focus is on the major population centers and ports within the Main Basin, its bays, and side-inlets.

Background

It is recognized in western Washington state that significant seismic hazards are induced on the major populated regions in the Puget Lowland. An earthquake of Mw = 6.8 occurred at Nisqually, Olympia on February 28, 2001 and six earthquakes of M > 6 have occurred in western Washington State. These events were intraslab earthquakes, so called Benioff Zone earthquakes except for the 1872 event at the eastern side of Cascadia range. However, little seismicity in the shallower crust do not demonstrate that no concern is required for seismic hazards by shallow crustal earthquakes and also for generation of tsunamis.

Paleoseismic studies in the Puget Lowland of western Washington demonstrate that an earthquake of M > 7 occurred on the shallow crustal fault, which is called the Seattle Fault, 1100 years ago (Bucknam et al., 1992). It is believed that tsunami was accompanied by this earthquake. Atwater and Moore (1992) found the tsunami deposits at West Point and Cultus Bay which locate north of the Seattle Fault. Also, recent geophysical investigations suggest that the Seattle Fault is active and capable of generating a large earthquake of M > 7 (Pratt et al., 1997, Johnson et al., 1999, Brocher et al., 2000).

This project aims to simulate and map the tsunami hazards within Puget Sound communities which might be induced by the potential shallow crustal earthquakes, using a conventional finite difference model based on the nonlinear shallow water theory and high-resolution bathymetry/topography data of Puget Sound.

Contents

• Seattle Fault
• Numerical model
• Modeling results
• Tsunami model movies

List of Figures

• Figure 1 : Inferred structure of the Seattle Fault.
• Figure 2 : Computed seismic deformation due to the hypothetical earthquake on the Seattle Fault.
• Figure 3 : Computed tsunami waveforms at major ports and harbors in Puget Sound, due to the Seattle Fault earthquake.
• Figure 4 : Tsunami inundation map of the Seattle waterfront area.
• Figure 5 : Tsunami inundation map of Bremerton and Port Orchard area.

Considering the above interpretations, we assume a M7.2 earthquake on the Seattle Fault which covers an area of 60 km by 19 km as a possible tsunamigenic earthquake. We adopt the interpretation of Johnson et al. (1999) for the horizontal structure of the Seattle Fault. Also we adopt and slightly modify the thrust sheet hypothesis proposed by Pratt et al. (1997) for the vertical structure of the fault, to determine dip angles as 25 degree for the deeper fault plane (> 5.5 km) and 60 degree for shallower fault plane (< 5.5 km). Fig. 1 indicates the inferred horizontal structure of the Seattle Fault. We divide the fault plane into 12 segments to fit the inferred structure by Johnson et al. (1999). Strike angle of each segment is determined by the inferred structure shown in Fig. 1.

For the determination of the amount of fault displacement, we apply the empirical relationship between fault displacement and earthquake magnitude proposed by Wells and Coppersmith (1994). Table 1 indicates the determined fault parameters for estimating the seismic deformation by the fault movement. n is the number of segments of upper and lower fault plane, L and W are strike length and downdip width of each segment and H is the depth of top edge of each segment. Based on the parameters shown in Table 1 , we estimate the vertical seismic deformation of the land and sea bottom by using the theory of Okada (1985) to compute the static displacement due to inclined and tensile fault in a half space.

Fig. 2 shows the computed vertical seismic deformation of the land and sea bottom within Puget Lowland. The result shows 2.3 m of maximum uplift at the sea bottom between Bainbridge Island and Elliott Bay, Seattle.

Fig. 3 shows the computed tsunami waveforms at major ports and harbors in Puget Sound ; Mukilteo (MK), Kingston (KN), Edmonds (ED), Northern shore of Elliott Bay (NHB), Head of Elliott Bay (HEB), Bremerton (BR), Port Orchard (PO), Vashon (VS) and Tacoma (TC). Location of each port is shown in Fig. 1. The computation is performed under the assumption of mean tide during the event. The tsunami strikes Elliott Bay with more than 3 m of its water level at the northern shore and 1.5 m at the head of the bay right after the earthquake. This result suggests that we should expect inundation at the Seattle waterfront area. At Bremerton and Port Orchard, local seismic uplift generates 1.5 m tsunami at the moment of the earthquake, then the water is receded. At Tacoma, positive tsunami waves arrive within approximately 10 minutes and 1.5 m tsunami appears at 20 minutes. Tsunamis less than 1 m strike the ports and harbors other than those mentioned above. However, note that the tidal range within Puget Sound is approximately 3 m. If the tsunami strikes during high tide, we should expect more serious hazards which impacts local coastal communities.

Fig. 4 and Fig. 5 show the tsunami inundation maps of the Seattle waterfront area, and Bremerton and Port Orchard area, which are both developing populated waterfront communities. The figures describe the distribution of maximum inundation depths on the land, which is measured from the local ground to the surface of surging water. For these areas, we carried out the detailed modeling of tsunami inundation based on the nested 30 m grids. The inundation map of the Seattle waterfront area shows that the heavy inundation occurs at Pier 90, 91 with the flow depth of more than 2 m, and at Pier 55 to 77 and Pier 36 to 54 with approximately 1 m flow depth. Inundation map at Bremerton and Port Orchard area shows that the inundations mainly occur along the southern shore of Sinclair Inlet and the northern and southern shore of Dyes Inlet. The estimated flow depths are up to 2 m at the shore 1 km east of Port Orchard, 4 m at the northern shore of Dyes Inlet and 2 m at the southern shore of the inlet because of local seismic uplift and trapped tsunamis within the inlet.

Tsunami model movies

Central Puget Sound	Seattle Waterfront	Bremerton and Port Orchard

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References

1. Atwater, B. F. and A. L. Moore, A Tsunami about 1000 Years Ago in Puget Sound, Washington, Science, v.258, pp.1614-1617, 1992
2. Brocher, T. M., T. L. Pratt, K. C. Creager, R. S. Crosson, W. P. Steele, C. S. Weaver, A. D. Frankel, A. M. Tr\'{e}hu, C. M. Snelson, K. C. Miller, S. H. Harder and U. S. ten Brink, Urban Seismic Experiments Investigate Seattle Fault and Basin, EOS, Trans. AGU, v.81, pp.545, 551-552, 2000
3. Bucknam, R. C., E. Hemphill-Haley and E. B. Leopold, Abrupt Uplift within the Past 1700 Years at Southern Puget Sound, Washington, Science, v.258, pp.1611-1614 , 1992
4. Imamura, F., Review of Tsunami Simulation with a Finite Difference Method, Long-Wave Runup Models, World Scientific, pp.25-42, 1995
5. Johnson, S. Y., C. J. Potter and J. M. Armentrout, Origin and Evolution of the Seattle Fault and Seattle Basin, Washington, Geology, v.22, pp.71-74 , 1994
6. Johnson, S. Y., S. V. Dadisman, J. R. Childs and W. D. Stanley, Active Tectonics of the Seattle Fault and Central Puget Sound, Washington - Implications for Earthquake Hazards, GSA Bulletin, v.111, no.7, pp.1042-1053, 1999
7. Johnson, S. Y., C. J. Potter, J. M. Armentrout, J. J. Miller, C. Finn and C. S. Weaver, The Southern Whidbey Island Fault : An Active Structure in the Puget Lowland, Washington, GSA Bulletin, v.108, no.3, pp.334-354, 1996
8. Okada, Y., Surface Deformation due to Shear and Tensile Faults in a Half-space, Bulletin of the Seismological Society of America, Vol. 75, No.4, pp1135-1154 , 1985
9. Pratt, T. L., S. Johnson, C. Potter, W. Stephenson and C. Finn, Seismic Reflection Images beneath Puget Sound, Western Washington State : The Puget Lowland Thrust Sheet Hypothesis, Journal of Geophysical Research, Vol.102, pp.27469-27489 , 1997
10. Wells, D. L. and K. J. Coppersmith, New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement, Bulletin of the Seismological Society of America, Vol. 84, No.4, pp.974-1002 , 1994

Figure 1 : Inferred structure of the Seattle Fault. Abbreviations as follows : MK=Mukilteo, ED=Edmonds, KN=Kingston, NEB=Northern Elliott Bay, HEB=Head of Elliott Bay, BR=Bremerton, PO=Port Orchard, VS=Vashon, TC=Tacoma.

Figure 2 : Computed seismic deformation due to the hypothetical earthquake on the Seattle Fault.

Figure 3 : Computed tsunami waveforms at major ports and harbors in Puget Sound, due to the Seattle Fault earthquake.

Figure 4 : Tsunami inundation map of the Seattle waterfront area.

Figure 5 : Tsunami inundation map of Bremerton and Port Orchard area.

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