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A New Type of Paleoseismic Evidence From Lake Sediments

Description: 

Paleoseismic evidence resulting from past earthquakes can be used to infer shaking parameters using the relationships between PGA and the maximum distance of earthquake-triggered liquefaction and landslide evidence. Although this is useful information, probabilistic seismic hazard models need estimates of local shaking parameters where the potential for damage is the greatest. The most damaging earthquake ground motions are S-waves, and a new type of paleoseismic evidence has the potential to provide information about their frequency and duration. Preliminary results from a deposit sequence representing the 1873 CE Brookings earthquake sequence from lower Acorn Woman Lake suggests that, whereas the lower deposit is the result of a landslide likely formed in response to a crustal earthquake, the upper deposit was formed in response to shaking that lasted longer, supporting a published inference that the upper deposit was formed in response to a southern Cascadia subduction earthquake.

This new paleoseismic evidence is the presence of a long organic-rich deposit tail. Evidence suggests that organic tail deposits may form if liquefaction releases ground water and fine-grained sediment into the lake, which, during sustained shaking, results in flocculation and the rapid settling. Previous laboratory studies have suggested that flocculation can occur when stirring occurs at a frequency of ~4 Hz or less, for a long duration (a minute or longer). If shaking is shorter, flocs may not form, and if shaking is too strong, flocs may break apart. Because the upper deposit of the 1873 CE Brookings earthquake sequence has a tail, it supports the interpretation that this deposit was the result of a southern Cascadia subduction earthquake. These results are preliminary, and experiments are planned to quantify the range of S-wave frequencies and duration required to produce this type of paleoseismic evidence. This type of evidence could be extremely useful in regions where both crustal and subduction earthquakes occur, such as in Cascadia, where little information is known about the amount of shaking inland where most people live.

Session: Earthquake Shaking and the Geologic Record: Triggered Phenomena and Preserved Fragile Geologic Features [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Ann

Student Presenter: No

Invited Presentation: 

Poster Number: 25

Authors