SCEC Award Number 14044 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Testing Recurrence Models for a Simple Plate Boundary Fault: Paleoseismic Study of the Imperial Fault in the Region of Large 1940 Displacement
Investigator(s)
Name Organization
Thomas Rockwell San Diego State University Yann Klinger Institut de Physique du Globe
Other Participants Andy Jerrett - graduate student, undergraduate student to be named
SCEC Priorities 2a, 1a, 4a SCEC Groups EFP, CSEP, SoSAFE
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 The goal of this work is to test basic recurrence models for the Imperial fault. We excavated trenches across a small sag pond evident in aerial photography that was flown after the 1940 Imperial Valley earthquake. Six meters of displacement passed through the sag, and offset crop rows allowed us to measure the transfer of slip through this step. The trenches, one of which is still open as the study is ongoing, revealed evidence for several past faulting events. The vertical component of slip is accommodated by warping or folding of the sediments down into the sag, as observed in the 1940 rupture and in the trenches. Brittle faults are also present, and together record evidence for at least 4 surface ruptures. The timing of these prehistorical events has not yet been resolved, as we are now preparing charcoal samples to be dated. Preliminary interpretation of the earthquake event horizons, along with preliminary correlation of the lake deposits in the trench to the regional lake chronology, suggests that the border site exhibits a longer recurrence interval than the Dogwood site. If our preliminary correlations of lake units is correct, there have been only 4 large surface-rupturing earthquakes in the past 700-800 years, suggesting a RI of about 200 years, which is more than twice the RI determined at the Dogwood site along the northern Imperial fault. This observation, if verified by the radiocarbon dating, suggests that the region of large 1940 displacement in the border area is a resilient asperity.
Intellectual Merit This project advances our understanding of short- and long-term behavior of a major plate boundary fault. Specifically, the 1940 earthquake produced anomalously large displacement in the border region (up to 7 m) for a 60 km rupture, and one goal of this work is to test whether this asperity is long-lived, which would be suggested by a long recurrence interval, or whether the 1940 earthquake itself was anomalous in its slip distribution. Preliminary analysis supports a long-lived asperity.
Broader Impacts This project has funded two graduate (MS) thesis projects (one male, one female graduate student). The trench exercise was also used as a teaching laboratory for Geol 550 (Engineering Geology) during the Spring 2015 academic semester.
Exemplary Figure Figure 6. Log of Border Trench 2, south wall, detail in fault zone. Note the evidence for surface ruptures at different stratigraphic levels. The age of the basal clay lake deposits is inferred to be about 800 years, which will be confirmed with dating of detrital charcoal (pending).