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Persistent Late Holocene Megathrust Creep Probably Accompanied M7–8 Seismicity in the Region of the 2020 m7.8 Simeonof Island, Alaska, Earthquake

Session: The 2020 Simeonof Island, Alaska, Earthquake: Observations, Modeling and Tectonic Insights

Type: Oral

Date: 4/21/2021

Presentation Time: 05:00 PM Pacific

Description: 

Coastal investigations on three Shumagin Islands, in Alaska, consistently revealed no geological evidence for giant (M≥9) subduction earthquakes or high tsunami in the late Holocene. Rather, our observations along the Shumagin section of the Aleutian megathrust, a well-known seismic gap, imply a paleoseismic history of persistent aseismic slip and large (M7–8) subduction earthquakes, like the 2020 M7.8 Simeonof Island earthquake. Work on Simeonof Island found no evidence of uplifted marine terraces or subsided shorelines recording past earthquakes. Instead, freshwater peat draping coastal lowlands above present high tide implies long-term tectonic stability and gradual relative sea level (RSL) rise (<0.2 m/ka) since 3.4 ka. At Unga Island we examined coastal deposits at Delarof Harbor, a site impacted by an earthquake and tsunami in 1788 per Russian written accounts. Terrestrial peat and tephra exposed along the bay margin indicate that past RSL has been near or at its present position since ~6.0 ka. We find no evidence for earthquake-related land-level change. Beach ridges facing the Aleutian trench contained no evidence of earthquake or tsunami impacts in the past 1.3 ka. On Nagai Island we examined marsh and dune deposits in coastal lowlands of trench-facing Larsen Bay. Drift logs and thin (<10 cm), discontinuous sand sheets deposited ≤3m above highest tide probably record storms and/or tsunami generated by M7–8 events. Saltmarshes contained no stratigraphic evidence for sudden coseismic vertical movements caused by past earthquakes. M7–8 earthquakes probably cause <0.3 m of coseismic land-level change, which is less than our geological studies can detect. Our observations provide context for megathrust rupture models used in seismic and tsunami hazard assessments. Our findings suggest that M≥9 ruptures are rare, or nonexistent, in the Shumagin gap and emphasize the persistent role of creep and large (M7–8) earthquakes as fundamental strain release mechanisms on the Shumagin megathrust in the late Holocene.

Presenting Author: Robert Witter

Student Presenter: No

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