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Detection Thresholds for Large to Great Subduction Earthquakes in South-Central Alaskan Marshes

Description: 

Coastal marshes provide records of the recurrence of great earthquakes in the Prince William Sound section of the Alaskan megathrust, with widespread evidence of surface deformation >1 m during seven great earthquakes in the last 4000 years. In the Kodiak section, there is paleoseismic evidence for four ruptures which occurred independent of the Prince William Sound section in the past 2000 years, each resulting in decimeter-scale subsidence. Based on records from subduction zones worldwide, it is suggested that subduction earthquakes ≤M_w8.5 displace the coast by <0.3 m and likely leave equivocal or no geologic evidence in coastal marshes. Recent studies suggest that in certain circumstances a detection limit of ~0.1 to 0.2 m may be possible. A better understanding of the lower limit of detection is essential for assessing seismic hazards of Alaskan earthquakes that are smaller than M_w9 but still pose significant risk.

We undertook fieldwork during September 2023 at Girdwood and Ocean View, Anchorage to test a methodology aimed at making marsh records of past great earthquakes in this region more complete. We focus efforts on the upper tidal marsh zone into the perimarine zone (where freshwater wetlands persist under the control of relative sea level). In these coastal peat forming environments, the most precise quantitative estimates of submergence can be reconstructed. At Ocean View, we traced the stratigraphic contact of the 785 ± 10 cal yr BP earthquake inland from the seaward extent of the marsh where it is expressed as a peat-to-silt contact until it changes to a peat-to-peat contact. This peat-to-peat contact, representing a known earthquake and associated submergence serves as an analogue to describe subsidence-derived changes in diatom assemblages from longer peat records. This allows us to assess whether, in perimarine to upper tidal marsh zones (where organic sedimentation dominates for the majority of an earthquake cycle), we can identify and quantify previously unknown episodes of decimeter-scale coseismic submergence representing earthquakes ≤M_w8.5 in this region over the last millennium.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments [Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Simon

Student Presenter: No

Invited Presentation: 

Authors