Aftershock Regions of Aleutian‐Alaska Megathrust Earthquakes, 1938–2021. Issue 7 (5th July 2022)

• Record Type: Journal Article
• Authors: Tape, Carl; Lomax, Anthony
• Is Part Of: Journal of geophysical research. Volume 127:Issue 7(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-07-05
• Subjects: Aleutian -- Alaska -- aftershock -- subduction zone -- earthquake relocation; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2022JB024336
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: With five earthquakes (1938, 1946, 1957, 1964, and 1965), the Aleutian‐Alaska subduction plate boundary ruptured a length of 3, 548 km. We revisit these five earthquakes—first studied in detail by Sykes (1971, https://doi.org/10.1029/JB076i032p08021 )—through probabilistic relocation of carefully selected mainshocks and aftershocks. Our final catalog of 324 events is established from a set of 12 mainshocks that includes all M w ≥ 7.7 megathrust earthquakes. Using the relocated catalog, we create revised aftershock regions delimited both parallel and normal to the trench. These aftershock regions exhibit significant differences from previous studies, with the following basic findings: the 10 November 1938 M w 8.3 earthquake extended further west, to the Shumagin Islands, and further east, into the Kodiak region, relative to the prevailing aftershock region established by McCann et al. (1979, https://doi.org/10.1007/978-3-0348-6430-5_2 ). The 01 April 1946 M w 8.6 sequence was anomalously concentrated near the trench, which implies near‐trench coseismic slip that contributed to the exceptionally large tsunami. The 09 March 1957 M w 8.6 aftershocks spanned a 1, 230 km length with numerous aftershocks within the outer‐rise region of the incoming Pacific plate. The 28 March 1964 M w 9.2 aftershocks extended east into the Pamplona thrust system (south of Icy Bay, Alaska), suggesting coseismic rupture into this region; this is consistent with coseismic static displacements, as well as current estimates of interseismic locking. The post‐1965 events we examine are all smaller than the earlier events, but since they occurred in an era of improved data collection (seismic, geodetic, and tsunami), they provide a better opportunity for assessing the link between the distribution of aftershocks and the occurrence of coseismic, postseismic, and interseismic slip. Plain Language Summary: The Aleutian‐Alaska subduction zone is one of the world's most active settings, having produced five large earthquakes (1938 M8.3, 1946 M8.6, 1957 M8.7, 1964 M9.2, and 1965 M8.6) along a length of 3, 500 km. Fundamental information about these earthquakes was previously determined from studying the spatial coverage of aftershocks following these five mainshocks. The aftershock regions established the approximate region of rupture during the mainshocks. Here, we revisit a study undertaken in 1971 by relocating 324 earthquakes using globally recorded arrival times. We use the earthquake relocations to determine revised aftershock regions. We apply this analysis to the five largest earthquakes, as well as to all other plate boundary earthquakes since 1965 with magnitude greater than 7.7. The resulting set of aftershock regions enables a better understanding of how slip occurs along the Aleutian‐Alaska subduction zone, both during earthquakes and between earthquakes. Key Points: A relocated catalog of 324 aftershocks from 12 earthquakes (1938–2021) along the Aleutian‐Alaska megathrust is presented The catalog establishes revised aftershock regions, with smaller events ( M w = 7.7–8.2) postdating five larger, earlier events (1938, 1946, 1957, 1964, and 1965) The aftershock regions provide better resolution of the trench‐normal extent of aftershocks, enabling a nuanced view of coseismic slip