Abstract

The Cascadia subduction zone (csz) can be divided into three distinct sections based on the characteristics of intraslab seismicity. Based on a 150-year historical record, no moderate-to-large intraslab earthquakes of moment magnitude (M) 5.5 or greater have occurred within the subducting Juan de Fuca plate of the central csz from south of the Puget Sound in northwestern Washington to the Oregon–California border. Also very few intraslab earthquakes as small as M 3 have been instrumentally located within the central csz since 1960, and a Wadati–Benioff zone is not apparent. In the southern csz beneath northwestern California, a Wadati–Benioff zone is present to a depth of about 40 km, but no large Gorda block earthquakes have been observed in the downgoing slab, although large events have occurred near the trench axis. In contrast, the Puget Sound region within the northern csz has been repeatedly shaken by large intraslab earthquakes of M ≥6.5 in the depth range of 40 to 60 km, such as the recent 2001 M 6.8 Nisqually event. A critical question addressed in this article is what is the potential for such large, shallow intraslab earthquakes in the central csz beneath western Oregon and southwestern Washington?

I have evaluated the available information on the thermal and physical properties, geometry, and historical and contemporary seismicity of the central csz, and performed thermal modeling. Based on these analyses and comparisons with other subduction zones worldwide, the lack of shallow intraslab earthquakes in the central csz is not unusual. The hot temperatures (>500°C) within the Juan de Fuca plate, particularly below a depth of 40 km where large events are expected, are not conducive to earthquake generation, resulting in either the complete absence of M ≥6.5 shallow intraslab earthquakes or long recurrence intervals (hundreds of years) between such events. Temperatures appear to be sufficiently high in the central csz so that no Wadati–Benioff zone can exist even at shallow depths (<40 km). The young plate age, slower convergence rate, and the insulating effect of the Siletz terrane above the plate are factors that probably lead to the hot temperatures in this portion of the slab. The variability in the maximum depth of the Wadati–Benioff zone along the csz, 60 km beneath the Puget Sound, 40 km within the subducting Gorda block, and essentially zero in the central csz, reflect the differing temperature conditions, that is, the cutoff temperature varies with depth and rock composition, and also the potential for large shallow intraslab earthquakes. In addition to the effects of temperature, the level of tectonic stresses, which vary along the length of the csz, must also be a factor in controlling the occurrence of large intraslab earthquakes. Large events can occur in the Puget Sound region, probably because of cooler intraslab temperatures and possibly because of a stress concentration or zone of weakness along the pronounced arch in the Juan de Fuca plate.

A previous study has suggested an intraslab subduction zone origin for a M 7.3 earthquake that occurred in 1873 near the town of Brookings, in southernmost Oregon. However, analysis of its seismotectonic setting and comparison with other historical earthquakes in northernmost California suggest that the event probably had a very shallow origin within the Gorda block (southern csz) and was not a deep intraslab earthquake in the central csz.

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