Seattle Fault Parameter
for Simulation of Seattle Inundation
by the Method of Splitting Tsunami (MOST) Model
In this MOST model study, the tsunami is generated by a Seattle fault deformation model that simulates the 1100 bp event as a credible worst-case scenario of magnitude 7.3 (see Table 1, below, and the figure entitled "Geometry of Seattle Fault Source "). The magnitude was chosen to be consistent with the 2002 USGS update of the National Seismic Hazard Maps (Craig Weaver, personal communication). Parameter values are based on the paper of Brocher et al. (2001) and consistent with Calvert & Fisher (2001). The U.T. ten Brink et al. (2002) manuscript and discussions with T. Pratt, B. Sherrod and others were also used in developing the model. Most studies suggest that the general fault mechanism involved is one of steeply dipping thrust. The Brocher et al. interpretation suggests dips of ~60 to 80 degrees and fault widths of ~30km (these values were inferred from a figure -- values were not mentioned in the text). Calvert & Fisher (2001) suggested dip angles of 40 to 80 degrees (60 ± 20 deg). There also seems to be a general consensus that slip was concentrated under Puget Sound, about midway along the fault (T. Pratt, personal communication).
The slip distribution was constrained, through trial-and-error, to match available field estimates of vertical displacement at three sites - Alki Point, Restoration Point and West Point (see Table 2, below, and the figure entitled "Geometry of Seattle Fault Source ").
It is worth noting that a M7.6 magnitude event was also modeled, and that the tsunami inundation values and patterns were essentially the same as for the M7.3 event. This is no doubt due to the fact that the deformation patterns and values were very similar for both events, since they were each constrained by field estimates at the three sites. Also, the smaller ground displacement zone of the M7.3 event forms a more concentrated tsunami source that compensates for its smaller overall displacement.
Table 1. Sub-fault parameters for a magnitude 7.3 Seattle Fault earthquake model.
| Sub-fault (East to West) |
Depth (km) |
Length (km) |
Width (km) |
Strike (deg) |
Dip (deg) |
Slip (m) |
| 1 | 0.5 | 15.2 | 20.0 | 87.9 | 60.0 | 1.0 |
| 2 | 0.5 | 6.3 | 20.0 | 86.6 | 60.0 | 1.0 |
| 3 | 0.5 | 8.9 | 20.0 | 96.0 | 60.0 | 12.0 |
| 4 | 0.5 | 3.2 | 20.0 | 128.8 | 60.0 | 11.0 |
| 5 | 0.5 | 11.5 | 20.0 | 99.3 | 60.0 | 4.0 |
| 6 | 0.5 | 14.9 | 20.0 | 81.0 | 60.0 | 1.0 |
Table 2. Vertical deformation values for Field estimates and model.
| Location | Field Estimate (m) |
Model (m) |
| Alki Point | 4 | 3.9 |
| Restoration Point | 7 | 7.1 |
| West Point | -1 +/- 0.5 | -1.2 |
References
M. Brocher, T. Parsons, R.J. Blakely, N.I. Christensen, M.A. Fisher, R.E. Wells and the SHIPS Working Group (2001), Upper crustal structure in the Puget Lowland, Washington: Results from the 1998 Seismic Hazard Investigation in Puget Sound, J. of Geophys. Res.,106(B7), 13541-13564.
A.J. Calvert & M.A. Fisher (2001), Imaging of the Seattle Fault Zone with high resolution seismic tomography, Geoph. Res. Lett., 28(12), 2337-2340.
U. T. ten Brink, P. C. Molzer, M. A. Fisher, R. J. Blakely, R. C. Bucknam , T. Parsons, R. S. Crosson, and K. C. Creager (2002), Subsurface geometry and evolution of the Seattle Fault Zone and the Seattle Basin, Washington, BSSA (in preparation).