Rupture of the 2020 M_W 7.8 Earthquake in the Shumagin Gap Inferred From Seismic and Geodetic Observations

Abstract

The eastern portion of the Shumagin gap along the Alaska Peninsula ruptured in an M W 7.8 thrust earthquake on 22 July 2020. The megathrust fault space-time slip history is determined by joint inversion of regional and teleseismic waveform data along with co-seismic static Global Navigation Satellite System (GNSS) displacements. The rupture expanded westward and along-dip from the hypocenter, located adjacent to the 1938 M W 8.2 Alaska earthquake, with slip and aftershocks extending into the gap about 180 to 205 km, respectively, at depths from 15 to 40 km. The deeper half of~75% of the Shumagin gap experienced faulting. However, the patchy slip is significantly less than possible accumulated slip since the region's last major rupture in 1917, compatible with geodetic seismic-coupling estimates of 10-40% beneath the Shumagin Islands. The rupture terminated in the western region of very low seismic coupling. There was a regional decade-scale decrease in b-value prior to the 2020 event. Plain Language Summary A large M W 7.8 underthrusting earthquake ruptured on the plate boundary between the Pacific and North American Plates along the Alaska Peninsula on 22 July 2020. The fault slipped in the region between large plate boundary earthquakes in 1938 and 1946 that has been called the Shumagin seismic gap. The last major earthquake on this part of the plate boundary occurred in 1917, with both that event and the 2020 earthquake rupturing the eastern half of the Shumagin gap. The slip was concentrated in several patches, with the largest slip patch located directly below the Shumagin Islands. Global Positioning Satellite (GPS) stations on the islands recorded the ground displacement clearly. Using GPS displacements along with seismic wave ground motions recorded by regional strong motion and global broadband stations, the slip space-time history during the faulting is determined. The coseismic slip is significantly less than the amount that could have accumulated from relative plate motions since 1917, which is consistent with geodetic estimates of low seismic coupling on this portion of the plate boundary. There was a regional decrease in the slope (b-value) of the earthquake magnitude distribution over the decade before the event.

Dynamic Sea Level Variation From GNSS: 2020 Shumagin Earthquake Tsunami Resonance and Hurricane Laura

“…displays the maximum computed sea-surface amplitude from the tsunami model in the vicinity of the Shumagin Islands. A very similar tsunami model is obtained using the slip inversion ofLiu et al (2020), with the configuration of the Shumagin islands producing strong resonance within the ring of islands that is locally not sensitive to details of the slip distribution.…”

“…The same GNSS stations, along with regional strong‐motion and broadband seismic stations and the global seismic network, recorded the ground motions produced by the 2020 earthquake, and these data have been used to invert for time‐varying slip distributions on the fault (e.g. USGS‐NEIC; Crowell & Melgar, 2020; Liu et al., 2020). Our own inversion for the slip distribution based on global seismic body waves and regional GNSS displacements with fault placements guided by the Slab2 model (Hayes et al., 2018; Figure ) is used to compute the time‐varying sea level generated by the seafloor deformation.…”

Triggering an unexpected earthquake in an uncoupled subduction zone

“…This sequence highlights the issue of which type of seismic gap is the Shumagin Gap-uncoupled and creeping, or coupled and (akin to Cascadia, perhaps) generally aseismic between great earthquakes. The initial earthquake in the 2020 sequence was a subduction interface earthquake (14,15). It occurred within the inferred rupture zone of the 1938 great earthquake and so was not unexpected.…”

Amphibious Shear Wave Structure Beneath the Alaska‐Aleutian Subduction Zone From Ambient Noise Tomography

“…Between 1938 and 1965, a group of large megathrust earthquakes ruptured almost the entire Alaska‐Aleutian subduction zone, with one notable exceptional patch located near the Shumagin island. The patch is known as the “Shumagin gap” and researchers have devoted great effort (e.g., Liu et al., 2020; Shillington et al., 2015) trying to understand the physical process at this location. The Shumagin gap (Figure 1b) has not ruptured great megathrust earthquakes for at least ∼150 years until recently (July 22, 2020, Liu et al., 2020), however, it exhibits abundant small interpolate and intraplate earthquakes at a variety of depths.…”

“…The patch is known as the “Shumagin gap” and researchers have devoted great effort (e.g., Liu et al., 2020; Shillington et al., 2015) trying to understand the physical process at this location. The Shumagin gap (Figure 1b) has not ruptured great megathrust earthquakes for at least ∼150 years until recently (July 22, 2020, Liu et al., 2020), however, it exhibits abundant small interpolate and intraplate earthquakes at a variety of depths. In contrast, the Semidi segment (Figure 1b) exhibits much sparser seismicity although great megathrust earthquakes historically occurred here (Shillington et al., 2015).…”