Tsunami Flooding Hazards Generated By Scenario Earthquakes in the Puget Sound, Washington.

April 21, 1999

ROBERT KARLIN¹ and GENE A. ICHINOSE^1,3

¹Department of Geological Sciences MS-172
University of Nevada
Reno, NV, 89557-0138 USA
email: karlin@mines.unr.edu

³Seismological Laboratory MS-174
University of Nevada
Reno, NV, 89557-0141 USA
email: ichinose@seismo.unr.edu

Abstract -

**Table 1.** Source parameters for two fault model scenarios. The dimensions of the fault are strike length L based on the number of 5 km strike length segments and down dip length W. The equation used for seismic moment is M_o=GLWu, where G is the shear rigidity equal to 3x10¹¹ dynes cm ^-2. Scenario A is an M_w=7.2 earthquake on the Seattle fault and scenario B is an M_w=7.2 on the Whidbey Island fault.
Scenario	Average Slip	Number of	L	W	M_o	M_w	Rake
.	u(m)	Segments	(km)	(km)	dyne*cm	.	°
A	3.11	9	45	18	7.6x10²⁶	7.2	90
B	3.01	11	55	18	8.9x10²⁶	7.2	135

Scenario A - Seattle fault

Fault Models. The contours of computed vertical component ground and lake bottom displacements for scenario "A". The bathymetry contours are 0, 10, 100, and 200 meters depth. Positive displacement is up. The focal mechanism shows the approximate geometry of faulting and slip direction [I need an arrow to point to actual fault plane].

The scenarios are earthquakes on thrust faults which dip at 33 ° S-SW that do not break the surface (Depth to top of fault is 1km). Scenario A is a pure thrust event with 4 meters maximum slip. Scenario B is an oblique thrust event with about 4 meters maximum displacement updip and 1 meter displacement along strike.

Scenario B - Whidbey Island fault

Fault Models. The contours of computed vertical component ground and lake bottom displacements for scenario "B". [same as above].

Click the icon to see animated GIF of tsunami propagation for scenario B. in netscape. You can also download and view using Xanim or GIF89a viewer. The animation has 72 frames at 25 seconds per frame (total simulation time 30min). The computation was done at 0.5 sec per iteration. The bathymetry grid spacing is 4 arc seconds or 111 meters over a 55 by 75 arc minute grid. We treat the tsunami wave propagation as an initial value problem and solve the linear Boussineq equations using the finite difference method. The constant slope boundary condition is used for the Puget sound entrance and inlets that go off the grid. The zero slope boundary condition is used for land water boundaries. Runup, advection and bottom friction was not considered in this simulation.

The first half hour of the simulation is apparently uneventful and shows no significant hazard except perhaps to marine shipping. An hour into the simulation, there are waves that get trapped in the sound and appear to shoal at shallow shorelines producing waves below 3 meters in scenario B. We do not see any significant free oscillations excited by scenario B.

Synthetic Tide Gauge Records computed at Everett, Seattle and Tacoma sites for scenario B [A is a typo in the figure]. The total run time was 2.5 hours. The peak wave amplitudes at these shoreline sites do not exceed 1.5 meters and appear to be associated with the direct arrival. The later reflected arrivals are significantly smaller in amplitude.