SCEC Award Number 20194 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title REVISED AND EXPANDED CHRONOLOGY OF WHITTER FAULT EARTHQUAKES THROUGH DATING ARCHIVED SAMPLES FROM THE OLINDA CREEK SITE, BREA, CALIFORNIA.
Investigator(s)
Name Organization
Sinan Akciz California State University, Fullerton
Other Participants
SCEC Priorities 1a, 5b, 5c SCEC Groups Geology, SAFS, Seismology
Report Due Date 03/15/2021 Date Report Submitted 11/12/2021
Project Abstract
 The Whittier Fault zone, the northern continuation of the NW-trending Elsinore Fault, extends from south of Santa Ana Canyon, where it splits off of the Chino Fault, to Whittier Narrows in Los Angeles County on the northwest.Paleoseismic data from the Fig Creek and Bee Canyon sites indicate that the most recent earthquake that ruptured the Whittier Fault occurred sometime between 1,400 and 2,200 years BP, and the penultimate earthquake occurred sometime between 3,000 and 3,400 years BP, suggesting a recurrence interval of about 1700 years. For this project, we dated six archived charcoal samples collected at Olinda Creek site along the Whittier Fault. While the stratigraphic evidence of the most recent faulting events was not preserved at this site due to continuing oil operations, evidence of four older earthquakes appear to be well-preserved in between distinct alluvial sediments which contain abundant detrital charcoal samples that were not possible to date at the time of their investigation. Older chronological data indicated two earthquakes between ~17,530 BC and 14,220 BC. New radiocarbon ages indicate that three of the oldest earthquakes preserved at the trench site occurred between ~17,300 BC and ~14,220 BC. While the earthquake record between ~1,500 and ~14,000 BC still remains undocumented, the incomplete record implies a recurrence interval of approximately 2000 yr.
Intellectual Merit The Whittier Fault zone, the northern continuation of the NW-trending Elsinore Fault, extends from south of Santa Ana Canyon where it splits off of the Chino Fault, to Whittier Narrows in Los Angeles County on the northwest. A M4.8 earthquake on September 3, 2002, a M5.4 earthquake on July 29, 2005, two M4.5 earthquakes on August 7 and 8 of 2012, and a recent M5.1 earthquake on March 29, 2014 as well as other smaller magnitude earthquakes along this fault zone indicate that Whittier Fault is a major seismic source within the Los Angeles Basin. Numerous geologic and geotechnical investigations have been conducted along the Whitter Fault as the cities of Brea and Yorba Linda has started to grow rapidly since the 1970s. If the available paleoseismic data is true, Whittier Fault has one of the highest slip rates of any fault in the Los Angeles Basin and it may have accumulated as much as 3 - 5 m of potential slip since the last large surface rupture, making it one of the most dangerous structures in the greater Los Angeles region.
Paleoseismic data regarding the date of the most recent surface rupturing earthquake and the recurrence interval between previous large magnitude earthquakes is still lacking. Determining the recurrence intervals between the most recent earthquakes will require developing a new paleoseismic site along the Whittier Fault. In this report, I summarize my findings of the new age constraints of five older and sequential earthquakes that ruptured the Whitter Fault. Older chronological data indicated two earthquakes between ~17,530 BC and 14,220 BC. New radiocarbon ages indicate that three of the oldest earthquakes preserved at the trench site occurred between ~17,300 BC and ~14,220 BC. While the earthquake record between ~1,500 and ~14,000 BC still remains undocumented, the incomplete record implies a recurrence interval of approximately 2000 yr.
Broader Impacts This project contributes directly to understanding the frequency of surface rupturing earthquakes along one of the fastest moving faults bounding the the densely populated LA Basin. The primary broader impact of this project is to make this paleoseismic data available to the SCEC community, as well as the community at large. The project provided an opportunity for PI’s Paleoseismology class to visit the site and discuss the implications of the new findings. No student was involved in the project due to Covid restrictions.
Exemplary Figure Figure 7. Results of Oxcal analyses (v.4.4.4) of radiocarbon dates.