SLIP RATE VARIATION OF THE CENTRAL SIERRA MADRE FAULT, SOUTHERN CALIFORNIA OVER THE PAST 200 KA

The Central Sierra Madre fault (CSMF) system uplifts the San Gabriel Mountains along the northern Los Angeles metropolitan area. A suite of late Quaternary alluvial fan surfaces are preserved near Arroyo Seco and offset across strands of the CSMF. We have focused on defining the slip rate of the CSMF averaged over periods represented by terraces with vertical separations across the fault zone of approximately 5-20 m and 260 m.

We estimate fault slip from the offset late Quaternary alluvial fan surfaces by analyzing swath profiles extracted from bare-earth lidar data and a range of fault dips from published maps and trench investigations. Uncertainties in slip are propagated from these geometric parameters. We have determined the surface ages of the offset fan terraces using both ¹⁰Be depth profile dating and IRSL dating. Modeled ages for two of the most prominent late Quaternary geomorphic surfaces along the CSMF are ~75 and ~40 ka.

Older offset across the CSMF is determined by offset of a thick, red argillic soil developed on the top of the prominent Gould Mesa terraces. The vertical separation of this soil across the fault is ~260 m, based on surface and subsurface well data. Two suites of quartz-rich cobbles were collected from ca. 8 m below the upper surface of Gould Mesa and analyzed for ¹⁰Be and ²⁶Al for isochron burial dating. Low nuclide concentrations precluded an accurate isochron age estimate; however, using the muon production dominant at the sampled depths and the lowest consistent ¹⁰Be concentrations from the two sites, we derive a mean exposure age estimate of ~170 ka.

From these sites, we calculate the CSMF slip rate is ~1 mm/yr since 75 ka and the time-averaged rate since ~170 ka is ~2 mm/yr, although the rates at the 95% confidence interval are considered. Uncertainty in the dip contributes most to the slip rate uncertainties. To make the slip rate constant would require that the fault dip has shallowed over time, for which we have no evidence. Alternatively, the apparent slowing of the CSMF slip rate may reflect recent earthquake recurrence behavior or secular transfer of strain rate southward to other structures in the Los Angeles basin.