Location, Extent, and Creep Rate of a Newly Identified Active Strand of the Major Concord Fault in the Northeast Bay Area, CA

Austin J. Elliott, Danielle V. Madugo, Jessie Vermeer, & Eric J. Fielding

Published September 8, 2024, SCEC Contribution #13863, 2024 SCEC Annual Meeting Poster #082

The Concord fault is a major branch of the Pacific-North America plate boundary in northern California, comprising the 20-km-long most highly urbanized portion of the 400-km-long Greenville-Concord-Green Valley-Bartlett Springs fault system. Like many of the faults in coastal northern CA its deformation is partially accommodated by aseismic slip (creep). While creep has been recognized and monitored on the northern half of the fault since 1979, the southern portion of the fault has remained enigmatic, as how slip transfers between the Concord and Calaveras or Greenville faults remains an outstanding question. New observations presented here indicate that the active trace of the fault in the suburbs south of downtown Concord is not where previously reported and is indeed actively creeping.
We report observations of shallow aseismic tectonic slip continuing 7 km farther south along the Concord fault than previously reported, along a fault strand not previously recognized. Concrete street curbs and sidewalk slabs on both sides of every street that crosses the fault in southeast Concord and northeast Walnut Creek exhibit right-lateral offsets. We document the magnitude and location of these offsets using precision surveying by Total Station. Offsets range in magnitude from 4 – 20 cm, over widths that vary considerably, from narrow breaks along concrete joints, to ~10-m-wide zones of deflection. The magnitude of accumulated offsets appear to correlate with feature age estimated from municipal parcel data, yielding decadal creep rates of ~3 mm/yr, consistent with the rates measured on the northern half of the fault from alignment arrays.
Significantly, this active trace is 100-450 m west of where the Quaternary active trace has conventionally been mapped, placing it within—rather than bounding—the built area of suburban Concord. Slip along the fault has already been responsible for infrastructure damage in recent years.
Road and sidewalk damage observed in the field aligns precisely with a discontinuity in InSAR line-of-sight displacement fields spanning the fault zone, corroborating its origin as tectonic slip. Prior analysis of these geodetic data indicated that an accelerated creep event occurred in 2018, consistent with periodic ~1 cm creep events identified every 3-4 years in alignment array measurements. These results highlight spatial and temporal variability in fault creep that reveal fault zone processes and may modulate seismic hazard.

Key Words
aseismic slip, fault mapping, northern california, concord fault

Citation
Elliott, A. J., Madugo, D. V., Vermeer, J., & Fielding, E. J. (2024, 09). Location, Extent, and Creep Rate of a Newly Identified Active Strand of the Major Concord Fault in the Northeast Bay Area, CA. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy