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Seismicity Relocations Between 2016 and 2019 Near the 1872 Entiat Earthquake in Central Washington

Description: 

The 14 December 1872 Entiat earthquake of estimated M 6.8 is the largest known crustal earthquake to have occurred in the Pacific Northwest. The event has been an enigma for decades and has motivated numerous investigations to identify its source as part of siting studies for critical facilities in Washington. The Entiat area has been a persistent area of concentrated seismicity and it has long been suspected to be the epicentral area of the 1872 earthquake. Based on LiDar data, Sherrod et al. (2015) was the first to suggest that the Spencer Canyon fault (SCF) was the source of 1872 and this was confirmed by Sherrod et al. (2021) based on a paleoseismologic investigation of the fault. Brocher et al. (2017) analyzed the Entiat seismicity and concluded that the events were aftershocks of the 1872 earthquake. To better characterize the seismicity, the Washington Geological Survey (WGS) deployed a dense local portable PASSCAL array of 13 three-component short-period seismometers around the Entiat seismicity from 2016 to 2019. The array provided an ideal dataset to calculate accurate locations and improve depth estimates with the intent of characterizing the relationship with and geometry of the SCF and/or other possible structures in this area. In this analysis, we combined data from the WGS array with regional observations from the Pacific Northwest Seismic Network (PNSN) to develop an enhanced earthquake catalog with high-precision hypocenter locations. Using a refined velocity model and a combination of PhaseNet-derived automatic picks and cross-correlation differential travel times, we relocated 203 events out of the 266 events with magnitudes ranging from M_L -0.4 to 3.4 from the PNSN catalog using the double-difference algorithm. Our results reveal several distinct, near-vertical clusters in the upper crust (<10 km depth), with no clear association with the northwest-dipping SCF. This suggests that these small earthquakes may be occurring on secondary structures within the broader deformation field of the 1872 rupture, potentially representing triggered aftershocks accommodating ongoing strain release.

Session: Compiling Active Faults for Improved Hazard Modeling from Cascadia to Alaska - I

Type: Oral

Date: 4/17/2025

Presentation Time: 05:30 PM (local time)

Presenting Author: Qimin

Student Presenter: No

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