S016-08
Triggered Seismicity, Deformation and Induced Velocity Changes Near Volcanoes Following the M7.8 Simeonof Island Earthquake
Tuesday, 8 December 2020: 18:00
Virtual
Matthew M Haney, U.S. Geological Survey Alaska Volcano Observatory, Anchorage, AK, United States, Scott Stihler, University of Alaska Fairbanks, Geophysical Institute, Fairbanks, AK, United States, John A Power, USGS, Alaska Volcano Observatory, Anchorage, AK, United States, Stephanie G Prejean, Alaska Volcano Observatory Anchorage, Anchorage, AK, United States, Ronni Grapenthin, University of Alaska Fairbanks, Geophysical Institute, Fairbanks, United States and David James Miller, University of Wisconsin, Madison, WI, United States
Abstract:
In addition to rupturing into the Shumagin Seismic Gap of the Alaska-Aleutian Megathrust, the 22 July 2020 M7.8 Simeonof Island Earthquake triggered seismicity at several locations along the volcanic arc including Mount Dutton, Mount Mageik, and Akutan Volcano. The observed onset of the triggered seismicity at each volcano varies from instantaneous (Mageik), to delays of several minutes (Akutan) to at most two hours (Dutton). The distance from the earthquake epicenter to the volcanoes is also variable, ranging from 250 km (Dutton) to between 400-500 km (Mageik, Akutan). Observed peak dynamic stresses of 0.2 MPa, 0.08 MPa, and 0.05 MPa at Dutton, Akutan, and Mageik, respectively, are within the range of values commonly associated with dynamic triggering in the crust and for previous instances of triggering at volcanoes in Alaska. This suggests that dynamic stresses initiated the triggered earthquakes at Mageik and Akutan; however, determining whether static or dynamic stresses dominated at Dutton is complicated by the fact that the earthquake ruptured toward the volcano.
Triggered permanent and transient deformation, observed on a local network of borehole tiltmeters, followed the earthquake at Akutan. The tilt signals exist on stations closest to the Hot Springs Bay geothermal field, which is in the same area that the triggered earthquakes at Akutan have been located using P- and S-wave phase picks. Such transients have not been observed on tiltmeters closer to the M7.8 on Unimak Island, indicating the deformation at Akutan, like the seismicity, was dynamically triggered. From long-term seismic records at the volcanoes, we quantify the amount of change in subsurface seismic velocity as a result of the M7.8 using ambient noise coda wave interferometry. Initial results show a relative velocity decrease of about 0.1% at Dutton, corresponding to a seismic velocity susceptibility (ratio of relative velocity change to dynamic stress) of 0.005 MPa-1.
