An unexplained tsunami: Was there megathrust slip during the 2020 Mw7.6 Sand Point, Alaska, earthquake?

Authors

  • Sean Santellanes University of Oregon
  • Dara Goldberg United States Geological Survey National Earthquake Information Center https://orcid.org/0000-0002-0923-3180
  • Pablo Koch Centro Sismológico Nacional de Chile
  • Diego Melgar University of Oregon Department of Earth Sciences
  • William Yeck United States Geological Survey National Earthquake Information Center
  • Brendan Crowell University of Washington Department of Earth and Space Sciences https://orcid.org/0000-0001-7096-601X
  • Jiun-Ting Lin Lawrence Livermore National Laboratory https://orcid.org/0000-0002-8134-9612

DOI:

https://doi.org/10.26443/seismica.v4i1.1336

Keywords:

tsunamigenic earthquakes, tsunamigenesis, tsunamis, Inversion

Abstract

On October 19, 2020, the Mw7.6 Sand Point earthquake struck south of the Shumagin Islands in Alaska. Moment tensors indicate the earthquake was primarily strike-slip, yet the event produced an enigmatic tsunami that was larger and more widespread than expected for an earthquake of that magnitude and mechanism. Using a suite of hydrodynamic, seismic, and geodetic modeling techniques, we explore plausible causes of the tsunami. We find that strike-slip models consistent with the moment tensor orientation cannot produce the observed tsunami. Hydrodynamic inversion of sea surface deformation from deep ocean and tide gauge data suggest seafloor deformation more closely matches a megathrust, rather than a strike-slip, source. Static slip inversions, using sea level and Global Navigation Satellite System data, allow for a portion of co-seismic megathrust slip that can explain tsunamigenesis. Combining all available geophysical datasets to model the kinematic rupture, we show that considerable, relatively slow, megathrust slip is allowable in the Shumagin segment, concurrent with strike-slip faulting. We hypothesize that the slow megathrust rupture does not contribute much seismic radiation allowing it to previously go unnoticed with traditional seismic monitoring.

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Published

2025-02-12

How to Cite

Santellanes, S., Goldberg, D., Koch, P., Melgar, D., Yeck, W., Crowell, B., & Lin, J.-T. (2025). An unexplained tsunami: Was there megathrust slip during the 2020 Mw7.6 Sand Point, Alaska, earthquake?. Seismica, 4(1). https://doi.org/10.26443/seismica.v4i1.1336

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Vol. 4 No. 1 (2025)

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Copyright (c) 2024 Sean Santellanes, Dara Goldberg, Pablo Koch, Diego Melgar, William Yeck, Brendan Crowell, Tim

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