Introduction
This progress report presents preliminary results from our paleoseismic study of the Simi fault at Arroyo Simi within northwestern Simi Valley. The 28-km-long Simi fault is a major fault within the Simi-Santa Rosa fault system, located in SCEC's Simi-San Fernando seismic zone. Although the Simi fault likely is a significant regional seismic source, the earthquake potential of the fault is poorly constrained. The fault exhibits strong geomorphic evidence of Quaternary deformation, including a south-facing prominent linear escarpment along the northern margin of Simi Valley (Yeats, 1983; Hitchcock et al., 1996). Despite this geomorphic evidence, the recency of activity, slip rate and sense of fault movement is not well constrained. Our FY97 pilot study is an initial investigation of the Simi fault and is designed to address the timing and style of Holocene surface deformation along the fault. These data will provide important information for a comprehensive evaluation of probabilistic seismic hazards in the greater Los Angeles metropolitan region. To this end, we investigated the Simi fault at the western end of Simi Valley, where the fault crosses and appears to tectonically impound the valley (Figure 1). We cleaned and logged a natural streamcut exposure of the Simi fault, conducted detailed geologic and geomorphic mapping, and performed exploratory drilling on a late Pleistocene-early Holocene stream terrace across the Simi fault. We have documented Holocene activity and constrained the timing and sense of slip along a major fault strand, likely the main fault strand. Documented Holocene activity will have significant implications for zoning the fault under the State's Alquist-Priolo Earthquake Fault Zoning program. Additional focused investigations are proposed to refine the timing of the most recent event and assess slip rate on the fault.
Regional Setting
The Simi fault is a south-vergent reverse or oblique-slip fault of the Simi-Santa Rosa fault system, a series of north-dipping faults that trend southwest from the northeastern end of Simi Valley to the east edge of the Oxnard plain, within the hanging-wall of the Oak Ridge fault system (Dolan et al., 1995). CDMG (Petersen et al., 1996) and SCEC have used a slip rate of 0.7 mm/yr (± 0.3 mm/yr) obtained from the Springville fault (Gonzalez and Rockwell, 1991) to characterize the entire Simi-Santa Rosa fault system for regional seismic hazard studies. However, this slip rate, obtained from a fault splay at the western end of the fault system, may not be representative of the entire system.
The Simi fault, located at the eastern end of the Simi-Santa Rosa fault system, bounds the northern margins of Tierra Rejada Valley and Simi Valley. West of Simi Valley, the Simi fault has placed Miocene Conejo Volcanics over Plio-Pleistocene Saugus Formation rocks. The 15.5 ± 0.8 m.y.a. base of the Conejo Volcanics, identified in oil well logs, is inferred to have a dip-slip separation of about 425 to 550 m (Blake, 1991), suggesting a low long-term slip rate of about 0.03 mm/yr. However, substantial late Quaternary offset is suggested by the presence of more than 150 m of Pleistocene and younger alluvium that fills the east-west trending, down-dropped bedrock trough beneath western Simi Valley (Leighton and Associates, 1972; Yeats, 1983). In addition, trenching within faulted colluvial deposits in Tierra Rejada Valley has revealed evidence of multiple shears within Holocene (?) deposits (pers. observation, J. Treiman, 1996).
Site Description
Our study site is located at the northwestern end of Simi Valley where Arroyo Simi exits the valley (Figure 1). At the site, Arroyo Simi is confined to a narrow natural stream channel. Historic stream incision has produced a 5- to 8-m-high exposure of latest Pleistocene and younger deposits within the western stream bank. Late Pleistocene clay and silty clay deposits are exposed in fault contact with sandstone of the Oligocene Sespe Formation at the base of the exposure. The fault extends into overlying Holocene laminated clay, silty clay, and gravelly sand deposits. We interpret the laminated clay and silty clay deposits to be pond deposits formed due to blockage of Arroyo Simi from uplift along the fault. Downstream of the fault exposure, within the hanging-wall of the fault, Arroyo Simi is incised into the folded and faulted Sespe bedrock and overlying silt and clay deposits that contain Pleistocene or younger fresh-water fossils (pers. comm., L. Groves, LA County Museum, 1997). These deposits appear to be folded, consistent with late Quaternary uplift and folding of the hanging wall of the Simi fault.
Based on interpretation of 1938 black-and-white stereo-pair photographs, and our field mapping, the Simi fault extends southwestward from the base of the linear range front bounding the northern margin of Simi Valley beneath paired and unpaired fluvial fill terraces at the Arroyo Simi site (Figure 1). The younger (Holocene?) of these terraces, located east of the arroyo (and the fault exposure), appears to be unfaulted and is inset into the older (late Pleistocene?) terrace remnants present on both sides of the arroyo. Several south-facing, east-west trending scarps are mapped within the older terrace based on air-photo interpretation and a pre-development contour map (2-foot contours, VCFD, 1967). These scarps are on trend with the fault exposed in the streambank, although the presence of a small knoll southwest of the exposure suggests that an additional fault strand may be present to the south of the documented fault.
Methods
We selected the Arroyo Simi site for its potential to yield information on the late Holocene behavior of the Simi fault. The fault is well exposed within bedrock and overlying late Pleistocene (?) and Holocene deposits along the western stream bank of Arroyo Simi. The deposits consist of fluvial, colluvial, and ponded sediments containing abundant charcoal. Tectonic geomorphology of the Simi fault in the vicinity of the Arroyo Simi site was analyzed through interpretation of aerial photography and detailed geologic and geomorphic mapping. Mapping confirmed that the fault within the stream exposure is a major fault strand of the Simi fault and documented the presence of deformed late Quaternary deposits within the hanging wall of the fault.
We cleaned and documented the stream-cut exposure of the Simi fault at the base of the southwest bank of Arroyo Simi. The exposure was cleaned and cut into roughly orthogonal sections for gridding and logging at a scale of 1 in = 0.25m (Figure 2b). The ages of deposits at the Arroyo Simi site are estimated based on six representative samples of charcoal submitted for radiocarbon analysis. We also conducted limited drilling to constrain the amount of vertical separation of late Pleistocene fluvial deposits across the Simi fault. Three 24-inch-diameter bucket auger holes were drilled, one within the hanging wall, downstream of the fault, and two within the foot wall, upstream of the fault (Figure 1). In-situ charcoal samples were obtained at various depths within the borings north and south of the fault, to provide age constraints on the offset deposits.
Site Stratigraphy
A sequence of fluvial, colluvial, and ponded (?) alluvial sediments are preserved across the Simi fault in a 50-m-long, 5- to 8-m-high, discontinuous exposure along the western stream bank of Arroyo Simi. At the base of the stream bank, ponded clay deposits are exposed in fault contact with sandstone of the Sespe Formation. The fault extends into overlying ponded deposits but is truncated locally by inset stream channel fluvial deposits. The upper part of the exposure is obscured by a thick sequence of colluvial deposits. We were unable to remove colluvial deposits in the upper part of the exposure because of incipient slope failure and logistical reasons. Stratigraphy above the fault exposure is inferred based on correlation of deposits exposed elsewhere along the stream bank and logged within the three auger holes (Figure 2a).
We divide the deposits exposed in the stream bank into five units (Figure 2b): (1) massive sandstone and bedded conglomerate of the Oligocene Sespe formation, (2) highly sheared clay fault gouge, (3) silty clay Holocene pond deposits, (4) gravelly Holocene fluvial deposits, and (5) sandy Holocene colluvium (Figure 2b). The fault is expressed as a N70E striking zone of clay gouge bounded by a vertical bed (?) of cobbles within the Sespe Formation on the north and the ponded clay deposits to the south. Layers within the clay deposits containing varying silt and sand content exposed on the inferred downthrown fault block are warped upwards near the fault (Figure 2b). This apparent drag folding is consistent with reverse displacement on the fault. Future excavation of the fault gouge may expose slickensides that will confirm the presence or absence of lateral slip on the fault. The oldest fluvial deposits within the fault exposure are in unconformable contact with sandstone bedrock of the Sespe Formation, appearing to have been deposited within a channel incised into the bedrock. These coarse Holocene sediments contain shell remnants that likely are derived from reworking of material from early or middle Miocene (Vaqueros equivalent) and middle to late Pliocene (Las Posas Fm) deposits (pers. comm., L. Groves, 1997). The channel deposits are unconformably overlain by younger sandy fluvial deposits. The fluvial package is not obviously faulted and may postdate faulting of the clay deposits. However, the basal fluvial unconformity of the younger deposits dips approximately 5 to the north, suggesting possible tilting. The fluvial package, and adjacent faulted clay deposits, are overlain by three distinct colluvial deposits differentiated based on the presence of angular clasts, lack of alluvial texture, and slope-parallel dip. These colluvial deposits appear to be unfaulted.
Discussion
The timing of the most recent event on the Simi fault is constrained based on calibrated radiocarbon dates between 7666±50 years BP, the age of the faulted clay deposits, and 1205±80 years BP, the age of the overlying unfaulted colluvium. Bedding within the faulted clay deposits is warped upwards against the fault, consistent with down-to-the-south displacement across the fault, although at this time we can not rule out a component of strike-slip displacement. Multiple late Pleistocene and Holocene events on the Simi fault are strongly suggested by the cumulative deformation of the ponded deposits. In addition, the presence of ponded deposits to depths in excess of 50 feet (based on boring 2; Figure 2a), against elevated bedrock suggests repeated faulting has blocked stream flow and produced deposition of ponded sediment.
Conclusions
Preliminary results from our paleoseismic investigation at Arroyo Simi document Holocene activity and provide data on the sense of slip of the Simi fault. Based on documentation of warped bedding within faulted pond deposits, we have confirmed reverse displacement on the fault, although we can not rule out a lateral component of slip. We have placed broad constraints on the timing of the most recent earthquake based on the ages of the faulted pond deposits and overlying unfaulted fluvial and colluvial deposits.
References
Blake, T.F., 1991, Synopsis of the character and recency of faulting along the Simi-Santa Rosa fault system; in T F. Blake and R. A. Larson, eds., Engineering geology along the Simi-Santa Rosa fault system, p. 96-118.
Dolan, J.F., Sieh, K., Rockwell, T.K., Yeats, R.S., Shaw, J., Suppe, J., Huftile, G.J., and Gath, E.M., 1995, Prospects for larger or more frequent earthquakes in the LA Metro. Region: Science, v. 267, p. 199-204.
Gonzalez, T., and T.K. Rockwell, 1991, Holocene activity of the Springville fault in Camarillo; in T F. Blake and R. A. Larson, eds., Engineering geology along the Simi-Santa Rosa fault system, Ventura Co., CA, p. 369-383.
Hanson, D.W., 1981, Surface and subsurface geology of the Simi Valley area, Ventura County, California: unpublished M.S. Thesis, Oregon State University, 112 p.
Hitchcock , C.S., Loyd, R.C., and W.D. Haydon, 1996, Liquefaction susceptibility and hazard zone mapping, Simi Valley, California: [abs.], Eos, vol. 77, No. 46, p. F510. Leighton and Associates, 1972, Groundwater study (Phase II) of East and West Basin areas, City of Simi Valley, Ventura County, California: unpublished report, 18 p.
Petersen, M.D., Cramer, C., Bryant, W.A., Reichle, M.S., and Toppozada, T.R. ,1996, Preliminary Seismic Hazard assessment for Los Angeles, Ventura, and Orange Counties: BSSA, Vol. 86, No., Part B., p. S247-S261.
Yeats, R. S., 1983, Simi: A structural essay: in Cenozoic geology of the Simi Valley area, southern California: in Cenozoic Geology of the Simi Valley area, R. L. Squires and M. V. Filewicz eds., SEPM Field Trip Guide, p. 233-240.