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- Title
Effects of Depth of Fault Slip and Continental Shelf Geometry on the Generation of Anomalously Long‐Period Tsunami by the July 2020 Mw 7.8 Shumagin (Alaska) Earthquake.
- Authors
Mulia, Iyan E.; Heidarzadeh, Mohammad; Satake, Kenji
- Abstract
The July 2020 Mw 7.8 Shumagin earthquake occurred in the seismic gap region along the Aleutian subduction zone. This interplate earthquake generated a small tsunami, but with unusual long‐period waves ranging between 40 and 90 min. We examined the cause of such an anomalous ocean wave through a source modeling inverted from tsunami and geodetic data. Our model indicates that the plate‐boundary rupture area was confined at depths of 20–40 km, although the slip resolvability decreases with depth. The coseismic seafloor displacement predominantly took place on the shallow continental shelf. Therefore, the initial water surface displacement at a mean water depth of ∼200 m is responsible for the long‐period waves, because tsunami period is inversely proportional to the square root of water depth. Furthermore, tsunami modeling implies that slip shallower than 20 km depth in the Aleutians would displace the seafloor beyond the continental shelf and generate shorter tsunami periods. Plain Language Summary: The tsunami generated by the July 2020 Mw 7.8 Shumagin earthquake exhibited extremely long‐period waves approximately ∼60 min on average, which is much longer compared with a larger event such as the 2011 Mw 9.0 Tohoku‐oki earthquake that produced tsunami wave periods of less than 30 min. We investigated the cause of such a rare occurrence by modeling the tsunami source using both tsunami and static Global Navigation Satellite System displacements data. Our source model shows that the slip on the plate interface was confined at depths of 20–40 km, resulting in coseismic displacement at shallow water depths on the broad continental shelf extending ∼150 km offshore, due to the unique continental shelf geometry of the region. Therefore, such a large ratio of long wavelength of displacement to small tsunami velocity determined by the shallow water depth would naturally induce long‐period waves. From examinations of other major subduction zones globally, we found that the long‐period tsunami is a unique signature attributed to the Aleutian subduction zone. Key Points: The July 2020 Mw 7.8 Shumagin (Alaska) earthquake generated anomalously long‐period tsunami waves relative to its magnitudeThe long‐period tsunami was caused by the initial displacement occurring on the broad and shallow continental shelfLong‐period tsunamis are rarely found and can be uniquely attributed to the geological setting of the Aleutian subduction zone
- Subjects
ALASKA; ALEUTIAN Islands (Alaska); TSUNAMIS; CONTINENTAL shelf; GLOBAL Positioning System; OCEAN waves; SUBDUCTION zones
- Publication
Geophysical Research Letters, 2022, Vol 49, Issue 3, p1
- ISSN
0094-8276
- Publication type
Article
- DOI
10.1029/2021GL094937