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Paleoseismic Investigation of the Simi fault at Arroyo Simi, Simi Valley, CA: Evidence for timing of Late Holocene earthquakes on the Simi-Santa Rosa fault zone

Christopher Hitchcock, Scott C. Lindvall, Jerome A. Treiman, Kristin D. Weaver, J. G. Helms, William Lettis, & G. D. Simpson

In Preparation 2003, SCEC Contribution #723

The Simi Fault is a left-oblique reverse fault within the Simi-Santa Rosa fault zone, a series of north-dipping faults that trend southwest from the northeastern end of Simi Valley to the Oxnard plain in southern California. Large historic earthquakes on adjacent reverse faults beneath San Fernando Valley to the east produced the 1971 San Fernando and 1994 Northridge earthquakes, yet the seismogenic potential of the Simi-Santa Rosa fault zone is not well constrained. Results from our paleoseismic investigations at Arroyo Simi within Simi Valley provide information on the recency of fault activity, sense of slip, amount of displacement per event on the Simi Fault, and allow us to estimate an average Holocene slip rate. We define the location of the main strand of the Simi Fault based on detailed mapping, shallow boreholes, and documentation of stream and trench exposures along Arroyo Simi at the northwestern end of Simi Valley. Stream bank and trench excavations reveal a narrow zone of brittle and ductile faulting, up to 3-m-wide, consisting of sub-vertical Holocene-active strands. Drag folds within clay layers and faulted stratigraphic horizons document a component of north-side-up reverse displacement across the fault. Reconstruction of a laterally offset late Holocene stream channel deposit, combined with well-developed slickensides and mullions on the main fault plane, indicate a significant component of left-lateral slip, suggesting that the fault has an overall left-lateral, reverse sense of slip. The most recent event may have occurred between about 1,350 BP (age of an offset stream channel deposit) and 1,205 BP (age of overlying unfaulted colluvium), or alternatively, based on a different stratigraphic interpretation, between 4,520 to 4,960 BP (the age of faulted clay deposits) and 2,060 to 2,360 BP (age of possibly unfaulted buried soils). Reconstruction of a faulted and warped clay marker bed and tilted buried soils suggest that the most recent event produced about 1 to 1.5 m of vertical separation, yielding 2 to 2.5 m of total oblique slip displacement. This amount of displacement per event is consistent with an Mw 7 earthquake. The available stratigraphic and age data at the site provide a broadly constrained estimate of Holocene slip rate of about 1 mm/yr.

Hitchcock, C., Lindvall, S. C., Treiman, J. A., Weaver, K. D., Helms, J., Lettis, W., & Simpson, G. (2003). Paleoseismic Investigation of the Simi fault at Arroyo Simi, Simi Valley, CA: Evidence for timing of Late Holocene earthquakes on the Simi-Santa Rosa fault zone. Bulletin of the Seismological Society of America, (in preparation).