Abstract

Seismicity maps of the region are presented for the periods January 1900 to June 1973 and January 1961 to June 1973. Focal mechanism solutions for 24 earthquakes (mb ≧ 5.5) occurring between January 1963 to July 1973 have been determined. Four earthquakes which occurred in the vicinity of the Gorda ridge have a component of normal faulting, thus identifying it as a spreading ridge. However, departure of the trend of the axis of tension, determined in the focal mechanism solutions for these earthquakes, from being perpendicular to the trend of the ridge axis indicates that the tectonics of the Gorda basin is complex. The existence of the Blanco fracture zone and the Queen Charlotte Islands fault as dextral transform faults, east block moving south, is confirmed. Evidences derived from the focal mechanism solutions indicate that the Revere-Dellwood and Sovanco fracture zones are also right-lateral transform faults. The northern extent of the Queen Charlotte Islands fault probably terminates near 54.5°N, 135.2°W or 57.5°N, 136°W. Focal mechanism solutions for an earthquake occurring near Vancouver Islands and another near the Puget Sound region in Washington indicate normal faulting and suggest the presence of a downgoing slab of lithosphere in this region. The interpretation is that the former results from tension caused by the bending of the plate as it dips under the continent and the latter results from tension within the sinking slab. Two recent earthquakes which occurred on July 1 and July 3, 1973 at a latitude of about 58°N, have thrust fault solutions and together with the seismicity data suggest a zone of underthrusting of the lithospheric plate between latitudes 58°N to 59.5°N. The possibility of a third zone of subduction north of latitude 59.5°N between longitudes 139°W to 142°W is indicated.

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