Group , Poster #082, Earthquake Geology

New age data constrain the recurrence interval on the Whittier fault

Thomas K. Rockwell, Sinan O. Akciz, & Eldon M. Gath
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Poster Presentation

2021 SCEC Annual Meeting, Poster #082, SCEC Contribution #11655 VIEW PDF
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. Based on geological mapping and analysis of its geomorphological expression, the Whittier Fault Zone is interpreted to be an active, steeply north-dipping, left-stepping transpressional structure that has experienced predominantly right-slip since the late Quaternary. 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 e...arthquake 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 ~18,875 BC and 14,220 BC. While the earthquake record between ~1,500 and ~14,000 BC still remain undocumented, the incomplete record implies a recurrence interval of roughly 1800 yr. Along with the existing Holocene to present slip rate of at least 1 to 1.5 mm/yr on the SW Olinda site strand, with additional and possibly similar amount of displacement on a parallel strand, our new chronological data indicate that the Whittier Fault has accumulated as much as 1.7 – 2.4 m of potential slip, and possibly as much as twice that value, since the last surface rupture that occurred sometime between 1,400 and 2,200 years BP.
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