Annual Report, 1997
Paleoseismic Investigation of the Northridge Hills fault, Northridge, California

Keith I. Kelson and John N. Baldwin
William Lettis & Associates, Inc.

Introduction

This report presents preliminary results of our pilot study on the Holocene behavior of the Northridge Hills fault. The Northridge Hills fault is a 15-km-long, north-dipping reverse fault that extends from Pacoima Wash in the central San Fernando Valley to the northwestern margin of the valley near Chatsworth (Figure 1). The fault exhibits strong geomorphic evidence of recent surface deformation, including south-facing topographic scarps and linear hill fronts, truncated late Quaternary fluvial terraces, uplifted and folded Plio-Pleistocene Saugus Formation and incised stream valleys in the hanging-wall block, and aligned alluvial-fan apices on the footwall block (Barnhart and Slosson, 1973; Saul, 1979; Hitchcock and Kelson, 1996). Syntheses of regional geologic date by Huftile and Yeats (1996), Davis and Namson (1994), and Greenwood (1995) suggest that the Northridge Hills fault lies within the hanging-wall block of the Northridge blind thrust fault. Despite this indirect evidence suggesting that the fault may be active, the recency of activity and interaction with other larger faults is unknown. This initial investigation of the Northridge Hills fault is designed to address the timing and style of Holocene surface deformation along the fault.

Evidence of possible Holocene surface deformation along the Northridge Hills fault raises two important questions: (1) Is the observed surface deformation produced by earthquakes on Northridge Hills fault, or is it a result of secondary movement caused by earthquakes on deeper, more continuous faults in the region (i.e., the Northridge or Santa Susana faults)?; and (2) If the Northridge Hills fault can independently produce surface-deforming earthquakes, what are the frequency and magnitude of such events? To help address these questions, we conducted a limited paleoseismic investigation adjacent to Northridge Community Park, located about 5 km north of the 1994 earthquake epicenter (Figure 1). The site has had only minor cultural mod)fication relative to other parts of the fault. Based on our previous mapping of surficial deposits and faults throughout the northern San Fernando Valley (Hitchcock and Kelson, 1996; Hitchcock and Wills, in prep.), this location is the most promising paleoseismic investigation site along the Northridge Hills fault. The site contains a subtle 1.5- to 2-m-high topographic scarp developed in Holocene (?) terrace deposits of Aliso Canyon Wash, and an exposure of north-dipping Saugus Formation located within the hanging-wall block of the fault (Figure 2) .

Paleoseismic Investigation

Our investigation at Northridge Park included: (1) drilling five 20-cm-diameter borings to depths as much as 25 m; (2) excavating one 40-m-long, approximately 4.5-m-deep trench; (3) excavating five test pits to depths as much as 4.5 m; and (4) preparing a detailed topographic site map. The five soil borings were completed in a transect across the topographic scarp to identify offset or folded stratigraphic markers and aid in the placement of the trench. The soil borings encountered moderately dense to dense fluvial deposits comprised of silty sand, sand and gravel overlying dense to very dense Plio-Pleistocene Saugus Formation deposits composed of silt, clay and sandy gravel with stage I to II calcium carbonate development. Subsurface borehole data suggest that the top of the Saugus Formation is warped into a southwest-facing monocline (Figure 3). Assuming that the contact between the fluvial deposits and the Saugus Formation north and south of the
surface scarp dips gently southwest at about 3 degrees (e.g., similar to the present-day surface), the apparent vertical separation of this contact across the Northridge Hills fault is 13 + 2 m.

The 40-m-long trench, which extends from the central scarp face to across the base of the scarp, exposes fairly distinct and continuous fluvial stratigraphy, including a distinct clayey gravel that continues the entire trench length. The basal contact of this gravel dips about 2 to 3 degrees in the

o northern part of the trench, and dips about 6 degrees where it lies beneath the base of the scarp. These data suggest that this contact is deformed near the base of the scarp and coincident with the underlying fold defined by the top of the Saugus Formation (Figure 3). Notably, there are no distinct colluvial deposits present at the base of the scarp, and there is no evidence of brittle faulting in the trench, showing that the Northridge Hills fault is blind.

Five test pits were excavated north and south of the trench to determine the elevation change of the base of the clayey gravel. Three test pits excavated north of the trench indicate that the base of the clayey gravel flattens northward. Two test pits south of the trench expose 4.5 m of younger fluvial deposits above the clayey gravel (Figure 3). Based on the test-pit and borehole data, we interpret that the scarp is tectonic in origin, and that the continuous clayey gravel basal contact has at least 6 + 2 m of vertical separation across the Northridge Hills fault (Figure 3). A large bone fragment sampled from directly below the clayey gravel, and one charcoal sample was recovered from soil boring B-3 within alluvium at about 7.5 m below the ground surface. These samples will provide a maximum limiting age for the deformation of the clayey gravel. The absence of substantial soil development in the clayey gravel suggests a Holocene age. The samples have been submitted for radiometric analysis; results are pending.

Implications for Seismic Source Characterization

Assessing whether the Northridge Hills fault is an independent or dependent seismic source is important for evaluating seismic hazards in the Los Angeles metropolitan region. One approach to making such an assessment is to obtain geologic evidence of distinct scarp-forrning events, such as large surface ruptures or folds, that may suggest the occurrence of large paleoearthquakes along the fault. Another approach is to obtain data on the amount and location of deformation during historical earthquakes on the fault or adjacent faults. We use both of these approaches to interpret whether deformation along the Northridge Hills fault is related to large or moderate earthquakes on the fault, or to deformation on other nearby structures. In addition, the amount of deformation recorded at the Northridge Park site helps estimate a range in recurrence interval for earthquakes that produce deformation along the Northridge Hills fault.

The data collected during this study suggest that surface deformation has occurred along the Northridge Hills fault during the Holocene, and perhaps during the past several thousand years. However, the trench and test-pits show no evidence of deformation large enough to produce a substantial surface scarp, including an absence of distinct scarp-derived colluvial deposits and secondary brittle fracturing or faulting. Thus, it seems more likely that the surface deformation at the Northridge Park site is related to either moderate-magnitude earthquakes on the Northridge Hills fault, or to secondary deformation produced by earthquakes on other faults. This interpretation is consistent with the probable maximum earthquake magnitude for the Northridge Hills fault estimated from fault length and depth. Using a fault length of 15 km (Jennings and Strand, 1969; Saul, 1979) and maximum down-dip width of 10 km (based on the geologic section by Huftile and Yeats, 1996), empirical regressions suggest a maximum earthquake magnitude of M6.2 (Wells and Coppersmith, 1994). Surface displacements associated with earthquakes of this size generally are 0.2 m or less. It is reasonable to assume that during a M6.2 earthquake, surface
folding above a blind thrust such as the Northridge Hills fault would be equal to or less than about 0.2 m.

Johnson et al. (1996) suggest that the 1994 Mw 6.7 Northridge earthquake may have caused coactive faulting along the Northridge Hills fault. Leveling surveys conducted before and after the earthquake indicate about 0.1 to 0.2 m of local vertical uplift coincident, in part, with the Northridge Hills fault (Johnson, et al., 1996). Thus, the Northridge Hills fault may experience displacement during movement on the Northridge blind thrust fault, and the surface folding at the Northridge Park site may be a result of secondary fold deformation.

The vertical separation of fluvial deposits exposed in the trench and test pits at the Northridge Park site may be used to estimate a recurrence interval for deformation events. Given an average uplift of 0.2 m per event, and a total uplift of 6 _ 2 m of the clayey gravel, there have been about 30 earthquakes similar to the 1994 earthquake. Assuming a broad age range for the gravel of 6 to 30 ka, these data suggest a range in recurrence interval of 200 to 1,000 years for surface-deforming events. This value is shorter than the 1,500 to 1,800 year recurrence interval based on a balanced cross section through the Northridge area (Davis and Namson, 1994). We acknowledge that these results and interpretations are preliminary, and are weakened by little constraint on the ages of surficial deposits and on the geometry of the southern limb of the monocline.

References Cited

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