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Recurrence of Large Upper-Plate Earthquakes in the Salish Lowland, Washington State, USA

Description: 

Our paleoearthquake database consists of 32 earthquakes over the last 15,000 years on ten fault zones in the Salish lowland (Elk Lake fault, Boulder Creek fault, Bellingham coastal faults, Darrington-Devil’s Mountain-Leech River fault zone, North Olympics fault zone, Southern Whidbey Island fault zone, Seattle fault zone, Canyon River-Saddle Mountain fault zone, Tacoma fault zone, and Olympia fault zone). We take the OxCal modeled probability distributions for each earthquake age and calculate Highest Density Intervals (HDIs) for each earthquake. Subsequent Monte Carlo sampling used the HDIs to calculate the order of earthquakes and recurrence intervals. Recurrence calculations used earthquake pairs determined by the ordering analysis to sample recurrence intervals from the HDIs. The result is a large sample of recurrence intervals from the ordered earthquake pairs, from which we draw summary values, including average recurrence interval (ARI). The Seattle fault zone is the most active fault in the region (nine events) and has the smallest ARI (1400±260 years); the largest earthquake of this sequence is the 923CE earthquake. The next most active are the North Olympics fault zone (four, possibly five events, ARI=2285±1195 years), and the Darrington-Devils Mountain-Leech River fault zone (four events, ARI=3355±675). The ARI across the region for all 32 events is 820±534 years.

Earthquakes ages have less error after 4000 years BP but dating quality declines for earlier events. This may be a function of preservation (not as much material to sample) and possibly sample quality (delicate plant fossils vs. charcoal). We observe that overall earthquake rate apparently increases after 4000 years BP (10 events before, 22 events after). This may reflect preservation, but all of the fault zones have terrestrial stratigraphic records that extend to the late glacial, suggesting the post-glacial stratigraphic record along each fault zone is complete. Paleoseismic recurrence broadly agrees with recurrence calculated using geodetic data, while recurrence estimated from seismic catalogs is shorter.

Session: Cryptic Faults: Advances in Characterizing Low Strain Rate and Environmentally Obscured Faults - II

Type: Oral

Date: 5/1/2024

Presentation Time: 10:45 AM (local time)

Presenting Author: Brian

Student Presenter: No

Invited Presentation: 

Authors