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Open-File Report 96-532

National Seismic Hazard Maps: Documentation June 1996

By Arthur Frankel, Charles Mueller, Theodore Barnhard, David Perkins, E.V. Leyendecker, Nancy Dickman, Stanley Hanson, and Margaret Hopper

Special Cases

There are a number of special cases which need to be described.

1. Blind thrusts in the Los Angeles area. Following Petersen et al (1996) and as discussed at the Pasadena workshop, we assigned 0.5 weight to blind thrusts in the L.A. region, because of the uncertainty in their slip rates and in whether they were indeed seismically active. These faults are the Elysian Park thrust, the Compton thrust. The Santa Barbara Channel thrust (Shaw and Suppe, 1994) also has partial weight, based on the weighting scheme developed by CDMG.

2. Offshore faults in Oregon. We assigned 0.05 weight to three offshore faults in Oregon identified by Goldfinger et al. (in press) and tabulated by Geomatrix (1995): the Wecoma, Daisy Bank and Alvin Canyon faults. We felt the uncertainty in the seismic activity of these faults warranted a low weight, and we used the 0.05 probability of activity decided in Geomatrix (1995). We assigned a 0.5 weight to the Cape Blanco blind thrust.

3. Lost River, Lemhi and Beaverhead faults in Idaho. Here we assumed that the magnitude of the Borah Peak event (M7.0) represented a maximum magnitude for these faults. As with (3), the characteristic model floated a M7.0 along each fault. The G-R model considered magnitudes between 6.5 and 7.0. Note that using a larger maximum magnitude would lower the probabilistic ground motions, because it would increase the recurrence time.

4. Hurricane and Sevier-Torroweap Faults in Utah and Arizona. The long lengths of these faults (about 250 km) implied a maximum magnitude too large compared to historical events in the region. Therefore we chose a maximum magnitude of M7.5. The characteristic and G-R models were implemented as in case (3). Other faults (outside of California) where the Mmax was determined to be greater than 7.5 based on the fault length were assigned a maximum magnitude of 7.5.

5. Wasatch Fault in Utah. We did not use slip rate to determine recurrence rates. We used recurrence times derived from dates of paleoearthquakes by Black et al. (1995) and the compilation of McCalpin and Nishenko (1996).

6. Hebgen Lake Fault in Montana. We used a characteristic moment magnitude of 7.3 based on the 1959 event (Doser, 1985).

7. All short faults with characteristic magnitudes of less than 6.5 were treated with the characteristic recurrence model only (weight=1). No G-R relation was used. If a fault had a characteristic magnitude less than 6.0, it was not used.

8. For the Seattle Fault, we fixed the characteristic recurrence time at 5000 years, which is the minimum recurrence time apparent from paleoseismology (R. Bucknam, pers. comm., 1996). Using the characteristic magnitude of 7.1 derived from the length and a 0.5 mm/yr slip rate yielded a characteristic recurrence time of about 3000 years.

9. For the Eglington fault near Las Vegas, we fixed the recurrence time at 14,000 years, similar to the recurrence noted in Wyman et al. (1993).

10. For a few faults, we determined the maximum magnitude from the magnitude of historic events (see fault table).

 

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