Crustal Structure and Earthquake Hazards of the Subduction Zone in Southwestern British Columbia and Western Washington

Multichannel seismic (MCS) reflection data collected across the continental margin in the Pacific Northwest are compiled into two regional cross sections through the crust. Seismic-reflection traveltime was converted to depth using primarily velocities derived from tomographic analysis of arrival times of both airgun shots and earthquakes. One cross section extends eastward from the Cascadia Trench, where the oceanic Juan de Fuca Plate dives beneath the continent, nearly to the eastern end of the Strait of Juan de Fuca. The other section stretches northward from the city of Tacoma to the international border with Canada in the southern part of Georgia Strait. The east-west section shows that reflective midcrustal rocks extend eastward from within 50 km of the trench to the eastern end of the Strait of Juan de Fuca, where the top of reflective rocks attains depths as great as 40 km. The thickness of this reflective section increases from 8 km to about 10 km with increasing eastward distance from the trench. Perhaps owing to weakened reflections, however, the reflective rock section appears to thin sharply at depths exceeding 25 km. Earthquakes appear to nucleate within the rock wedge that overlies the reflective rocks. The lower depth limit for earthquakes, however, is not controlled by rock structure but by rock temperature. The north-south regional cross section shows that reflections from midcrustal rocks die out southward into Puget Sound. This loss of reflectivity results, at least in part, from sound attenuation within the Cenozoic strata that locally thicken southward to more than 8 km below Puget Sound.

Three sources for convergent-margin earthquakes are: (1) faults in the upper part of the continental crust, (2) the subduction-zone interface, and (3) mineral phase changes in the deeply subducted oceanic crust. This study provides structural information about earthquakes that occur within continental crust, as well as information about earthquakes that nucleate along the shallow part of the interplate thrust. Faults in the upper part of the continental crust are revealed by prestack depth migrated and poststack migrated MCS data, which show what is probably a reflection from the plane of the Seattle Fault. This fault dips 40¼ south. Probable Eocene volcanic rocks in the hanging wall of this fault override the thick (~8 km) fill within the Seattle Basin. Another inferred fault-plane reflection is evident from within the Kingston Arch, which is a large, east-west-trending anticline that delimits the Seattle Basin on the north. Other major faults within the study area, such as the Southern Whidbey Island and Devils Mountain Faults, are thought to extend beneath northern Puget Sound and the eastern Strait of Juan de Fuca. However, MCS data presented here lack the resolution necessary to show recent offset along these faults, nor do these data reveal how the faults merge downward into midcrustal rocks, where earthquakes nucleate. The second source for subduction-zone earthquakes lies along the interplate contact. The structure of the accretionary wedge might indicate the extent of rupture for interplate quakes. Near the mouth of the Strait of Juan de Fuca, the vergence of thrust faults in the outer part of the wedge changes from landward to seaward and the landward dip of the downgoing plate increases. Where these changes occur might be a candidate for a rupture-zone boundary for earthquakes along the interplate thrust. A third source for earthquakes, involving the deeply subducted oceanic crust, cannot be studied with data presented here because reflections from deep rocks die out southward below Puget Sound, far north of where recent earthquakes of this type have struck.