Aftershock Regions of Aleutian‐Alaska Megathrust Earthquakes, 1938–2021

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 Mw ≥ 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 Mw 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 Mw 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 Mw 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 Mw 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.

An Alaska-Aleutian subduction zone interface earthquake recurrence model from geology and geodesy

“…Hutchinson & Crowell (2007) examined regional archeological and paleoseismic ages and deduced that the Kenai section ruptures together with the adjacent PWS section but independently of the Kodiak section. Tape and Lomax (2022) show that the Kenai and neighboring Barren Islands section correspond to a local minimum in the spatial distribution of aftershocks of the 1964 rupture.…”

“…Study area along the Alaska-Aleutian subduction zone (AASZ) showing the locations of fault sections and approximate historical rupture areas. For older events, rupture areas are inferred from aftershock zones (Tape and Lomax, 2022), and recent events are taken from fault rupture models (Freymueller et al, 2021;Tape & Lomax, 2022;Ye et al, 2022). Slab interface contours are from Hayes et al, 2018.…”

Surface Wave Isotropic and Azimuthally Anisotropic Dispersion Across Alaska and the Alaska‐Aleutian Subduction Zone

“…The white arrow indicates the direction of motion of the Pacific Plate. The yellow polygons show the rupture zones of previous megathrust earthquakes (Tape & Lomax, 2022). The green dashed line shows the plate boundary between the Pacific and North American Plates.…”

Variations in Reflection Signature and Slip Behavior of the Subduction Interface Offshore Alaska Peninsula From 152º to 161º W

“…ALEUT study area seem to support the seismologically determined landward end of rupture for the 1964 Good Friday earthquake. Rupture areas from past earthquakes based on aftershock locations carried significant uncertainties and a more recent re-evaluation of large earthquake rupture areas byTape and Lomax (2022) leads, for instance, to a better correlation with our megathrust slip nature for the 1964 earthquake. Moreover, the landward end of the rupture zone within the Kodiak Asperity based on our reflection imaging study is located between the landward rupture ends determined for the 1964 Good Friday and 1938 Semidi Segment earthquakes based on seismological and tsunami data, respectively.…”