Constraints on Geometry of the San Joaquin Hills Blind Thrust Fault, Orange County California U.S.A, From Quaternary Geology and Recent Earthquakes
Description:
The landscape record of earthquakes on blind faults can be subtle. An active blind fault beneath Los Angeles (USA) was famously discovered on January 17, 1994 when it generated the Mw6.7 Northridge earthquake. In hindsight, geomorphic and subsurface evidence of the fault was recognized and geologists set about to find other potentially active blind faults in the region. The San Joaquin Hills in coastal Orange County, CA, are underlain by complexly faulted Miocene marine rocks that are uplifted and exposed at the southern margin of the Los Angeles basin. A suite of gently folded uplifted Quaternary marine terraces, an emergent Holocene shoreline and uplifted Holocene fluvial deposits have previously been mapped and interpreted as evidence of an active blind thrust fault dipping SW 20-30 degrees beneath an anticline subparallel to the Newport-Inglewood fault zone, with partitioned slip on the San Joaquin Hills thrust and strike-slip Newport-Inglewood faults. Published and unpublished models of the San Joaquin Hills blind fault differ in geometry, Quaternary uplift rate and fault slip rate. The Quaternary Fault and Fold Database of the USGS and the Community Fault Model of the Statewide California Earthquake Center include models of the San Joaquin Hills thrust. Better constraint on fault geometry is desirable for seismic hazard assessment. In June 2024 a sequence of M2.8, M3.6, M3.4 earthquakes occurred at a depth of 12-13km beneath Newport Mesa near where the axis of the anticline is exposed in upper Newport Bay. USGS focal mechanisms from all 3 events include nodal planes that dip 48-56 degrees SW with nearly pure thrust or right oblique thrust. Seismicity in combination with Quaternary geology of the San Joaquin Hills suggests that a moderately (~50 deg ) SW dipping fault with nearly pure thrust motion is the best model for the poorly constrained blind fault.
Session: The Landscape Record of Earthquakes and Faulting - II
Type: Oral
Date: 4/16/2025
Presentation Time: 11:00 AM (local time)
Presenting Author: Lisa
Student Presenter: No
Invited Presentation:
Poster Number:
Authors
Lisa Grant Ludwig Presenting Author Corresponding Author lgrant@uci.edu University of California, Irvine |
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Constraints on Geometry of the San Joaquin Hills Blind Thrust Fault, Orange County California U.S.A, From Quaternary Geology and Recent Earthquakes
Category
The Landscape Record of Earthquakes and Faulting