Core Penetrometer Tests, Continuous Cores and Paleoseismic Trenching Combined to Infer a Mid-Holocene Slip Rate for the Imperial Fault, California

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

We conducted a multi-disciplinary approach to study the paleoseismology on the Imperial fault along the section that ruptured during the Mw7 1940 earthquake. We investigated a small pull-apart basin in the Imperial Valley, located about 2 km north of the US/Mexico border. The Imperial fault is the main plate boundary fault between the North American and Pacific lithospheric plates and has been attributed with a slip rate up to 35-40 mm/yr. We excavated trenches across the fault to a depth of 4 m to expose the past 500 years of stratigraphy, and used cone-penetrometer (CPT) soundings and continuous coring to extend the record to the middle Holocene at 25 m depth.

Post-earthquake aerial photos show that the 1940 earthquake (event E1) produced 6 m of lateral slip through the sag depression, and >90 cm of subsidence within the sag. The penultimate earthquake (E2) in ca 1726 also produced ~1 m subsidence, whereas two other events (E3 and E4) produced only 0-6 cm of subsidence. We interpret these observations to imply that large 1940-type earthquakes activate the sag depression with about a meter of subsidence whereas smaller events, such as 1979, either do not reach as far south as the sag or produce only minor displacement.

Two cores and 26 CPTs were acquired across the sag that penetrated a succession of lake and deltaic deposits to ~25 m depth: the sediments west of the fault date to about 4500 years at the base of the core whereas in the sag itself, the sediments are about 2-2.2 ka at 25 m depth. Using a particularly distinctive CPT signature for the 2-2.2 ka lake deposit, the sag records ~17 m of vertical subsidence in the past 2.2 ka, with increasing subsidence with successively older lakes. If the ratio of lateral slip and subsidence has remained relatively constant, then these data imply that the Imperial fault carries most of the plate boundary slip across the international border.

Presenting Author: Thomas K. Rockwell

Authors