Wrangell Benioff zone, southern Alaska

The first unequivocal evidence for the existence of a Benioff zone that may be related to the Quaternary Wrangell volcanoes comes from a set of 86 well-located hypocenters for earthquakes smaller than about magnitude 4 in the region between lat 61 and 62.5°N and between long 142 and 145.5°W. About half of the earthquakes occur at depths of 25 km or less. Below 40 km a clearly defined north-northeast dipping zone of seismicity, here termed the “Wrangell Benioff zone,” extends to a depth of about 85 km and continues for about 115 km along strike, subparallel to the volcanic trend. The western end of the zone may be offset from the northern end of the much more active Aleutian Benioff zone. Where the Benioff zone shoals to 30 to 40 km, it becomes nearly horizontal and cannot be clearly distinguished from upper-plate seismicity. It is uncertain whether the subducted plate segment that contains the Wrangell Benioff zone is structurally part of the Pacific plate or the Yakutat block.