Skip Links

USGS - science for a changing world

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

>

Cascadia Subduction Zone

We considered two alternative scenarios for great earthquakes on the Cascadia subduction zone. For both scenarios we assumed that the recurrence time of rupture at any point along the subduction zone was 500 years. This time is in or near most of the average intervals estimated from coastal and offshore evidence (see Atwater and Hemphill-Haley, 1996; Geomatrix, 1995; B. Atwater, written comm., 1996). Individual intervals, however, range from a few hundred years to about 1000 years (Atwater et al., 1995).

The first scenario is for moment magnitude 8.3 earthquakes to fill the subduction zone every 500 years. Based on a rupture length of 250 km (see Geomatrix, 1995) for an M8.3 event and the 1100 km length of the entire subduction zone, this requires a repeat time of about 110 years for an M8.3 event. However, no such event has been observed in the historic record of about 150 years. This M8.3 scenario is similar to what was used in the 1994 edition of the USGS maps (see Leyendecker et al., 1995) and it is comparable to the highest weighted scenario in Geomatrix (1995). We floated a M8.3 rupture zone along strike of the subduction zone to calculate the hazard. We assign a weight of 0.67 for this scenario in the maps.

It was suggested (B. Atwater, written comm., 1996) that another plausible scenario was to have a succession of M8 earthquakes rupturing the entire subduction zone in a few decades. Of course, such a scenario is non-Poissonian. Implementing such a scenario in the probabilistic hazard maps would require judgments on where we were in the earthquake cycle. We felt this scenario needed more study and did not use it in the June maps.

The second scenario we used is for a moment magnitude 9.0 earthquake to rupture the entire Cascadia subduction zone every 500 years on average. We see no compelling reason to rule out such a scenario. This scenario would explain the lack of M8's in the historic record. It is also consistent with a recent interpretation of Japanese tsunami records by Satake et al. (1996). By ruling out alternative source regions, Satake et al. (1996) reported that a tsunami in 1700 could have been produced by a M9.0 earthquake along the Cascadia subduction zone. We assign a weight of 0.33 to the M9.0 scenario in the maps.

The subduction zone was specified as a dipping plane striking north-south from about Cape Mendocino to 50 degrees north (Figure 27). We assumed that the plane reached 20 km depth at a longitude of 123.8 degrees west, just east of the coastline. This corresponds roughly to the 20 km depth contour drawn by Hyndman and Wang (1995) and is consistent with the depth and location of the Petrolia earthquake in northern California. We assigned a dip of 10 degrees to the plane and a width of 90 km. We assumed the seismogenic portion of the plane extended to a depth of 20 km.

 

Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubsdata.usgs.gov/pubs/of/1996/532/Shearzone2.html
Page Contact Information: GS Pubs Web Contact
Page Last Modified: Wednesday, 07-Dec-2016 16:06:10 EST