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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

Western United States

Note: the California portion of the hazard maps was produced jointly by us and Mark Petersen, Chris Cramer and Bill Bryant of the California Division of Mines and Geology (CDMG).

The scheme for mapping hazard in the WUS is shown in Figure 17. On the left side we consider hazard from earthquakes with magnitudes less than or equal to moment magnitude 7.0. For most of the WUS, we used two alternative models: 1) smoothed historical seismicity (weight of 0.67) and 2) large background zones (weight 0.33) based on broad geologic criteria and workshop input. Model 1 used a 0.1 degree source grid to count number of events. We changed the determination of a-value somewhat from the CEUS, to incorporate different completeness times for different magnitude ranges. The a-value for each grid cell was calculated from the maximum likelihood method of Weichert (1980), based on events with magnitudes of 4.0 and larger. We used M4.0 to 5.0 since 1963, M5.0 to 6.0 since 1930, and M6.0 and larger since 1850. For the first two categories, completeness time was derived from plots of cumulative number of events versus time. The catalogs are probably not complete for all magnitude 6 events since 1850, but we felt it was important to include these events in the a-value calculation. It is important to note that the calculation of a-value counts one magnitude 6 event the same as one magnitude 4 event (this is also true for the CEUS). We did not use M3 events in the WUS hazard calculations since they are only complete since about 1976 for most areas and may not even be complete after 1976 for some areas. For California we used M4.0 to M5.0 since 1933, M5.0 to 6.0 since 1900, and M6.0 and larger since 1850. The catalog for California is complete to earlier dates compared to the catalogs for the rest of the WUS (see below).

Another difference with the CEUS is that we did not use multiple models with different minimum magnitudes for the a-value estimates (such as models 1-3 for the CEUS). The use of such multiple models in the CEUS was partially motivated by the observation that some mb4 and mb5 events in the CEUS occurred in areas with few mb3 events since 1924 (e.g., Nemaha Ridge events and western Minnesota events). We wanted to be able to give such mb4 and mb5 events extra weight in the hazard calculation over what they would have in one run with a minimum magnitude of 3. In contrast it appears that virtually all M5 and M6 events in the WUS have occurred in areas with numerous M4 events since 1965. We also were reluctant to use a WUS model with a-values based on a minimum magnitude of 6.0, since this would tend to double count events that have occurred on mapped faults included in Figure 17 right.

For model 1, the gridded a-values were smoothed with a Gaussian with a correlation distance of 50 km, as in model 1 for the CEUS. The hazard calculation from the gridded a-values differed from that in the CEUS, because we considered fault finiteness in the WUS calculations. For each source grid cell, we used a fictitious fault for magnitudes of 6.0 and larger. The fault was centered on the center of the grid cell. The strike of the fault was random and was varied for each magnitude increment. The length of the fault was determined from the relations of Wells and Coppersmith (1994). The fictitious faults were taken to be vertical.

A maximum moment magnitude of 7.0 was used for models 1 and 2, except for four shear zones in northeastern California and western Nevada described below. Of course, larger moment magnitudes are included in the specific faults (Figure 17 right; see below). A minimum moment magnitude of 5.0 were used for models 1 and 2. For each WUS site, the hazard calculation was done for source-site distances of 200 km and less, except for the Cascadia subduction zone, where the maximum distance was 1000 km. The 200 km maximum distance may actually be too large in the sense that some of the attenuation relations used were based on data up to about 100 km (see below).

We did separate hazard calculations for deep events (> 35 km). These events were culled from the catalogs. Their a-values were calculated separately from the shallow events. Different attenuation relations were used (see below).

We calculated regional b-values based on the method of Weichert (1980), using events with magnitudes of 4 and larger and using varying completeness times for different magnitudes. We found a b-value of 0.80 ±0.03 for the western U.S. without California. Accordingly, a regional b-value of 0.8 was used in models 1 and 2 for the WUS runs based on shallow events. For the deep events (>35 km), we found an average b-value of 0.65. We used this low b-value in the hazard calculations for the deep events.

We used a b-value of 0.9 for most of California, except for the easternmost portion of California in our basin and range background zone (see below). This b-value was derived by CDMG.

 

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