Recurrence of Large Upper Plate Earthquakes in the Puget Lowland

Date: 4/24/2019

Time: 02:30 PM

Room: Vashon

Paleoseismic studies documented 27 paleoearthquakes from observations of postglacial deformation at 63 sites on 13 shallow fault zones in the northern Cascadia fore arc. These fault zones were created by northward fore arc block migration manifested as a series of bedrock uplifts and intervening structural basins in the Salish lowland between the 49^th parallel and Olympia, WA to the south, bounded on the east and west by the Cascade Mountains and Olympic Mountains. Estimates of paleoearthquake magnitude range from M~6.5 and M~7.5. For each paleoearthquake, we use published ages to calculate earthquake-timing probability density functions (PDFs); for some events broad PDFs reflect earthquakes constrained by only minimum or maximum limiting ages.

The earthquake record starts shortly after glacier retreat, which began ~16ka. Earthquakes prior to the mid-Holocene were apparently scarce, with only a handful of older earthquakes identified throughout the lowland. The paleoseismic record picks up in earnest ~4000 yrs BP, with 21 of the 27 paleoearthquakes on faults throughout the Salish lowland. A cluster of earthquakes started about 2500 yrs BP and lasted until about 900 yrs BP on faults located in the central and northern lowland.

A Monte Carlo approach was used to calculate the recurrence intervals and rates for earthquakes on individual fault zones as well as on the regional fault network as a whole. Thousands of samples were drawn from each earthquake age PDF, and these were sorted (following stratigraphic ordering where possible) and differenced, yielding distributions for inter-event times that reflect the uncertainty in the radiocarbon ages. For the Puget Lowland as a whole, the post-glacial mean recurrence interval is ~400 years, with a median of ~175 years and a mode of ~20 years (temporally proximal earthquakes are generally not on the same fault). These results are suggestive of earthquake clustering, and that a large earthquake may be followed soon after by additional large earthquakes on regional faults.

Presenting Author: Brian L. Sherrod

Authors