Subduction of an Oceanic Plateau Across Southcentral Alaska: High‐Resolution Seismicity

Abstract

AbstractIn southcentral Alaska, the Alaska‐Aleutian Wadati‐Benioff zone (WBZ) shows high seismicity rates west of 147°W. Further east, the Wrangell volcanic field (WVF) has some of the world's largest continental volcanoes but there is equivocal evidence for a WBZ. We deployed a dense seismometer array around the WVF between 2016 and 2018 and used the data to increase the number of detected earthquakes using an autodetection and location algorithm. One‐dimensional velocity inversion and double‐difference earthquake location further improve earthquake locations. Subcrustal earthquakes form a narrow band of dipping seismicity—a weak but clear WVF WBZ—which strikes parallel to the volcanic trend and dips highly oblique to plate motion. The WVF WBZ is continuous from the coast to a depth of 100 km beneath Mount Wrangell. Earthquakes shallower than 40 km are continuous between the two WBZs, indicating continuity of the subducting Yakutat terrane across the region. However, the earthquakes deeper than 40 km are offset by hundreds of kilometers, which may indicate a slab tear separating the Alaska–Aleutian WBZ from the WVF WBZ. Seismicity rates differ by over 2 orders of magnitude between the separate WBZs, despite the similar incoming plate, with the relatively seismically quiescent WBZ underlying the much more prolific WVF. Higher slab‐surface temperatures beneath the WVF, due to flow around the slab edges and the oblique geometry, may lead to low seismicity rates within subducting crust, as seen in other warm slabs, but abundant water is still transported to subarc depths within the mantle wedge.