Insights into the 2020 Earthquake Swarm at Makushin Volcano, Alaska from Relative Relocations and Focal Mechanisms

Abstract:

On June 15^th, 2020, at 21:16 UTC, a locally-felt earthquake of magnitude 4.2 struck Unalaska Island, ~15 km west of Dutch Harbor. The event was followed by a M4.1 earthquake and several M3+ aftershocks, initiating a prolific swarm that is still active, with hundreds of earthquakes recorded into late July. The earthquakes all locate about 12 km southeast of the summit of Makushin Volcano at 8 to 10 km depth (BSL). To date, no eruptive activity or other surface changes have been observed at the volcano in satellite views, GPS, or webcam images. This swarm is the strongest seismic activity recorded at Makushin since its last minor eruption in 1995 and since instrumental monitoring began in 1996. Seismic swarms close to volcanoes are often associated with the onset of unrest that can lead to eruption. However, determining whether they reflect magmatic rather than tectonic stresses is challenging. To investigate triggering mechanisms of current Makushin seismicity, we integrate information from space-time patterns of the hypocenters of the swarm earthquakes with their fault-plane solutions. We relocate swarm events using absolute location techniques and a 3D velocity model (Lanza et al. 2020). We find that the events cluster into two perpendicular lineaments with NW-SE and SW-NE orientations, with no apparent migration in time towards a preferred alignment. In addition, we observe a consistent shift in depths with relocated events ~2 km shallower than the initial catalog locations. Fault-plane solutions (FPS) for all M3+ earthquakes have P-axis orientations consistent with the NW-SE regional stress field, while many of the lower-magnitude earthquakes have ‘90°-rotated’ P-axes perpendicular to the regional principal stress orientation. Usually, this diversity of FPS orientations is an indication of magmatic intrusion (e.g., Roman and Cashman 2006). Although the swarm and its characteristics do not necessarily signal an impending eruption, our analyses point to some degree of magmatic involvement at the origin of the swarm. Ongoing work includes double-difference relocations and Coulomb stress modeling to improve resolution of hypocenter locations and further assessment of their relationship to the role of magmatic intrusion to explain ongoing seismicity.