Aftershock Summary for the July 28, 2021 M8.2 Chignik Earthquake
Abstract
The M8.2 Chignik Earthquake on July 28, 2021 was the largest earthquake to strike the U.S since the 1965 M8.7 Rat Islands earthquake. This latest large event occurred at a depth of approximately 35 km and ruptured the area between the Shumagin Islands to the southwest and Kodiak Island to the northeast. The most recent large earthquake to rupture this same area was a M8.3 in 1938. The Chignik earthquake appears to be related to another recent large earthquake, the M7.8 Simeonof earthquake of July 22, 2020--both occurred on adjacent patches of the Alaska-Aleutian megathrust. The largest aftershock to date is a M6.9 on August 14, 2021, and occurred at a depth of 30 km. There have been a total of 855 aftershocks recorded and manually reviewed by the Alaska Earthquake Center (AEC) on regional network seismic stations, between July 28 and August 19, 2021. This sequence appears to occupy a region NE of the previous years M7.8 Simeonof earthquake, and extends 220 km along-strike and 100 km along-dip of the megathrust. The Chignik aftershock sequence has a b-value of 0.6, and a magnitude of completeness (Mc) of 2.5. The a-value for this sequence is 4.3, compared to an annual a-value of 5.5 for the region. The aftershock sequence for the M7.8 Simeonof earthquake has a b-value of 0.7, a Mc= 1.9, and an a-value of 4.7. The a-value is a measure of the activity rate of the region. The Chignik sequence has a low a-value compared to the annual activity rate for the region, and compared to the Simeonof aftershock sequence. These values mean that the Chignik sequence is much less active than the Simeonof sequence, and our minimum magnitudes of detected earthquakes (Mc) are larger than those of Simeonof. The b-values for both the Chignik and Simeonof sequences are on the edge of the ranges for intraplate and interplate earthquakes (0.7-0.4 vs 1.0-0.7, respectively). The low b-values for both sequences indicate that both rupture patches have high stresses. This is contrary to prior stress models of this area, which suggest weak or pseudo-coupling of the region prior to the Chignik earthquake. We speculate that the combination of the M7.8 Simeonof and the M8.2 Chignik earthquakes may have changed the dynamics of the region from that of weak or pseudo-coupled to a higher degree of coupling and increased stress state.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.S54C..05M