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Bulletin of the Seismological Society of America, 90, 3, pp. 629–642, June 2000

Late Quaternary Fold Deformation along the Northridge Hills Fault, Northridge, California: Deformation Coincident with Past Northridge Blind-Thrust Earthquakes and Other Nearby Structures? by John N. Baldwin, Keith I. Kelson, and Carolyn E. Randolph

Abstract Paleoseismic investigation of the Northridge Hills fault in the northern San Fernando Valley, California, helps assess the timing and style of near-surface late Quaternary deformation in the epicentral area of the 1994 Northridge earthquake. The Northridge Hills fault, a 15-km-long, north-dipping reverse fault, exhibits geomorphic evidence of late Quaternary surface deformation, including topographic scarps across late Quaternary fluvial terraces and aligned alluvial-fan apices on the footwall block. We excavated one 40-m-long trench and six test pits, and drilled nine boreholes across a 2-m-high scarp developed on a probable Holocene fluvial terrace adjacent to Aliso Canyon Wash. A continuous clayey gravel identified in the trench, test pits, and boreholes defines a south-facing monocline with 6 Ⳳ 1 m of vertical separation across the Northridge Hills fault. Based on pedochronology, the clayey gravel ranges in age from 6 to 30 ka. The borehole data also suggest that an unconformity developed on the Plio-Pleistocene Saugus Formation is warped into a monocline that has 13 Ⳳ 2 m of vertical separation across the fault. These preliminary data yield a dip-slip rate of 1.0 Ⳳ 0.7 mm/yr for the Northridge Hills fault. The absence of distinct scarp-derived colluvium in trench exposures at the base of the scarp and secondary brittle fracturing or faulting suggests that the monocline is related to folding during small, incremental uplifts rather than large uplifts that generate distinct scarp relief. We postulate that such uplift could be produced via moderatemagnitude earthquakes (MW 61⁄4) on the Northridge Hills fault, or secondary deformation induced by earthquakes on other faults. For instance, evidence of surface uplift near the trench site during or following the 1994 earthquake suggests that all or part of the observed deformation is a result of secondary slip on the Northridge Hills fault produced by movement on the underlying Northridge blind reverse fault or other nearby large structures. Based on our geologic investigations, the distribution of aftershocks following the 1994 earthquake, and pre- and post-1994 leveling and geodetic surveys, we interpret that the Northridge Hills fault underwent triggered slip during the 1994 earthquake. Introduction Based on detailed Quaternary geologic and geomorphic mapping of the San Fernando Valley (Hitchcock and Kelson, 1996), we identified a site on the Northridge Hills fault with a reasonable likelihood of providing late Holocene paleoseismic data. We ask the following questions: (1) Is the Northridge Hills fault active? (2) Is the Northridge Hills fault an independent source of large-magnitude earthquakes? and (3) If not, is the observed surface deformation produced by secondary movement caused by earthquakes on deeper, more continuous faults in the region (i.e., the Northridge blind thrust or Santa Susana fault)? To address these ques-

The Northridge Hills fault is a poorly characterized, 15km-long reverse fault, that extends northwest-southeast across the densely populated northern San Fernando Valley (Fig. 1). The Northridge Hills fault generally is considered a potentially active fault based on prominent geomorphology, the presence of a groundwater barrier, and deformed late Quaternary deposits. However, little information is available on the timing of paleoearthquakes, seismogenic behavior, or slip rate of the fault. Therefore, this study was intended to address the seismogenic potential and estimate the earthquake recurrence of the Northridge Hills fault. 629

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J. N. Baldwin, K. I. Kelson, and C. E. Randolph

Figure 1.

Regional fault map of the northern San Fernando valley (white), and Santa Susana and San Gabriel Mountains (shading), showing zones of ground deformation produced during the 1994 Northridge earthquake (stipple pattern). Modified from EERI (1994).

tions, we conducted a paleoseismic investigation adjacent to Northridge Community Park, in San Fernando Valley, California, located about 5 km northeast of the 1994 Northridge earthquake epicenter (Fig. 1). The 1994 (MW 6.7) Northridge earthquake was produced by rupture along a south-dipping blind-thrust fault (the “Northridge blind thrust”) near a depth of about 18 km, and propagated northward to within 5 km of the surface (Hauksson et al., 1995; Wald et al., 1996). Most of the slip, which averaged between 1.0 to 1.3 m (Hauksson et al., 1995; Wald et al., 1996), occurred between the depths of 6 and 13 km, and little or no slip was recorded in the upper 5 km of the crust (Hudnut et al., 1996). No primary surface rupture was associated with the Northridge earthquake (United States Geological Survey and Southern California Earthquake Center, 1994; Hart et al., 1995). However, several lines of evidence suggest that a significant amount of both coseismic and postseismic deformation occurred in the shallow crust from the Northridge earthquake. This evidence includes patterns of regional deformation assessed by levelline surveys (Johnson et al., 1996a), surficial mapping (Johnson et al., 1996b), global positioning system (GPS) geodesy

studies (Shen et al., 1996; Donnellan et al., 1998; Donnellan and Lyzenga, 1998), and kinematic analysis of aftershocks (Unruh et al., 1997). Some of this regional deformation may have occurred along the Northridge Hills fault. Our paleoseismic investigation of the Northridge Hills fault incorporates the findings of these regional studies of the Northridge earthquake to help assess the seismogenic behavior of the Northridge Hills fault.

The Northridge Hills Fault San Fernando Valley lies north of Los Angeles and is bounded on the north by the San Gabriel and Santa Susana Mountains, on the east by the Verdugo Mountains, and on the west by the Simi Hills (Fig. 1). The San Gabriel and Santa Susana Mountains consist primarily of granitic and metamorphic rocks of Mosozoic and older age. Miocene to Pleistocene sedimentary rocks comprise the Simi Hills, and Precambrian to Mesozoic granitic and metamorphic rocks comprise much of the Verdugo Hills. The Northridge Hills are in the northern part of San Fernando Valley, located directly west of the Santa Susana Mountains, and are com-

Late Quaternary Fold Deformation along the Northridge Hills Fault, Northridge, California

posed predominantly of folded Plio-Pleistocene deposits. Much of San Fernando Valley consists of Quaternary alluvial, fluvial and basin deposits laid down by rivers flowing southward out of the Santa Susana and San Gabriel Mountains. The northwest-striking Northridge Hills fault is a 15km-long, north-dipping reverse fault that extends from near Pacoima Wash in the central San Fernando Valley to the northwestern margin of the valley near Chatsworth, and is closely associated with the origin of the Northridge Hills (Fig. 1). The fault is within the epicentral region of the Northridge earthquake. The fault exhibits strong geomorphic evidence of late Quaternary surface deformation, including south-facing topographic scarps and linear hill fronts, truncated late Quaternary fluvial terraces, uplifted and folded Plio-Pleistocene Saugus Formation, incised stream valleys in the hanging-wall block, and aligned alluvial-fan apices on the footwall block (Barnhart and Slosson, 1973; Saul, 1979; Dibblee, 1992; Hitchcock and Kelson, 1996). The Northridge Hills fault is mapped as a 600-m-wide complex fault zone that is considered potentially active by Wentworth and Yerkes (1971) and Barnhart and Slosson (1973). However, the California Division of Mines and Geology concluded that the Northridge Hills fault “does not appear to be sufficiently active” to require zonation by the State of California (Smith, 1977). Syntheses of regional geologic data by Davis and Namson (1994), Greenwood (1995), Huftile and Yeats (1996), and Tsutsumi and Yeats (1999) suggest that the Northridge Hills fault lies within the hanging-wall block of the Northridge blind thrust. Focal mechanisms and aftershock data of the 1994 earthquake suggest that the Northridge blind thrust dips about 40⬚ southwest beneath the southern San Fernando Valley and underlies a series of southwest-vergent contractional structures including the Northridge Hills, Mission Hills, and Santa Susana faults (Fig. 2; see also figure 4 of Huftile and Yeats, 1996). The north-dipping Northridge Hills fault includes at least two northwest-striking fault strands in Chatsworth, and a single-fault strand in Northridge (Fig. 1). Recent subsurface mapping using industry oil-well and geophysical survey data collected across the Northridge Hills fault (Fig. 1; Hartzell et al., 1997; Tsutsumi and Yeats, 1999) shows a fault-propagation fold above an underlying blind thrust fault dipping northward at about 45⬚ (Figs. 2 and 3). This fold is expressed at the surface of the Northridge Hills as a series of en echelon, south-vergent anticlines with generally steep southwest-facing forelimbs and shallow northeast-facing backlimbs (Saul, 1979; Dibblee, 1992). The Northridge Hills fault has been interpreted either as a backthrust within the hanging-wall block of the Northridge blind thrust (Wald et al., 1996), or as a younger valley-ward extension of the north-dipping Santa Susana fault system (Huftile and Yeats, 1996; Tsutsumi and Yeats, 1999). Both of these interpretations seem plausible based on surface and subsurface structural data (Fig. 2).

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From southeast to northwest, the Northridge Hills increase systematically in elevation. For instance, the southeastern Northridge Hills between Reseda Boulevard and Sepulveda Boulevard are as much as 20-m high, whereas near Chatsworth the Northridge Hills exhibit about 50 m of topographic relief. In the southeast, the lesser relief probably reflects smaller amplitude folding beneath the hills as shown by the shallower bedding dips in the Plio-Pleistocene Saugus Formation (Tsutsumi and Yeats, 1999; Dibblee, 1992). In addition, the hills in this area appear to be partially buried by deposits from Pacoima and Tujunga Washes. The northwestern part of the Northridge Hills, west of Reseda Boulevard, exhibit more steeply dipping forelimbs and backlimbs (Dibblee, 1992), and brittle bedrock faulting (Saul, 1979). This is consistent with reconstructed cross sections across the northwestern part of the fault, which show faulted PlioPleistocene Saugus Formation at depth (Tsutsumi and Yeats, 1999). It is likely that the decrease in bedding dip and drop in elevation of the Northridge Hills from northwest to southeast, respectively, reflect a steepening fault dip and a fault tip shallowing along the northwestern part of the Northridge Hills fault (Tsutsumi and Yeats, 1999). Our research was designed to obtain paleoseismic data on the style and timing of surficial deformation along the Northridge Hills fault, through the investigation of a topographic scarp adjacent to Aliso Canyon Wash (Figs. 1 and 4). This site was selected on the basis of detailed geologic and geomorphic mapping of surficial deposits in the northern San Fernando Valley (Hitchcock and Kelson, 1996; Wills and Hitchcock, 1999). We chose to investigate a site at Northridge Community Park based on the presence of a relatively undisturbed, 2-m-high topographic scarp developed on Holocene terrace deposits flanking Aliso Canyon Wash. This is one of the few undeveloped localities in the San Fernando Valley where the fault is overlain by culturally undisturbed Holocene alluvium (Fig. 4). The site is directly south of an exposure of moderately north-dipping (10–15⬚) Saugus Formation located in the hanging-wall block of the Northridge Hills fault. The topographic scarp at the site is aligned with south-facing linear hill fronts along the Northridge Hills fault, suggesting that the scarp may be tectonic in origin (Fig. 4).

Methods of Investigation To assess the style and timing of near-surface deformation along the Northridge Hills fault, we (1) constructed a detailed topographic site map and topographic profile (Figs. 5 and 6); (2) drilled nine 20-cm-diameter borings to depths as much as 25 m; (3) excavated one 40-m-long, approximately 4.5-m-deep trench; and (4) excavated six test pits to depths as much as 4.5 m. Our intent was to document the presence or absence of deformation of alluvial and/or colluvial deposits beneath the site, and evaluate the age of deformed and undeformed deposits. The nine boreholes were drilled, using a standard hollow-stem auger, along a

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Figure 2.

Balanced geologic cross-section across San Fernando Valley showing down-dip projection of the Northridge Hills fault (modified from Tsutsumi and Yeats, 1999). See Figure 1 for location. Numbers refer to well designations. Tm, Miocene Modelo Formation; Ttw, Miocene Towsley Formation; Tf, Pliocene Fernando Formation; Qts, Pliocene-Quaternary Saugus Formation.

Figure 3.

Fault propagation fold above the Northridge Hills fault. Seismic reflection line along Balboa Blvd. (modified from Hartzell et al., 1997-Chevron U.S.A. line). See Figure 1 for location.


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Figure 4.

Map of surficial deposits and the Northridge Hills fault near Northridge Park, California. Topographic base from City of Los Angeles, dated 1970 (contour interval: 5 ft). With the exception of the 70-meter-wide undeveloped corridor directly west of the Aliso Canyon channel, all areas shown on this map are culturally modified. Geologic units modified from Wills and Hitchcock (1999).

northeast-southwest transect across the south-facing scarp (Fig. 5). The purpose of the boreholes was to identify fluvial stratigraphy beneath the site that might help constrain the location and relative amount of faulting and folding, as well as target a specific location for exploratory trenching. Based on borehole data, one trench was excavated orthogonal to the inferred surface projection of the Northridge Hills fault. The 40-m-long trench was placed orthogonal (N32⬚E) to the central scarp face and extended to just south of the base of the scarp (Fig. 5). The trench was limited in length and configuration due to permit constraints and the presence of highvoltage electric towers at the south end of the trench. Trench walls were cleaned, surveyed, and logged at a scale of 1 in. to 0.5 m (about 1:20-scale). In addition, six test pits were excavated in a northeast-southwest transect across the scarp beyond the limits of the trench to characterize the lateral continuity of the near-surface stratigraphy across the site. We also prepared a site topographic map with a 0.2-m-contour interval to document the locations of the boreholes, pits,

and trench, as well as to constrain the amount of surface deformation across the Northridge Hills fault (Fig. 5). Samples of charcoal collected from the test pits and trench were submitted for radiocarbon analysis by accelerator mass spectrometry. The radiometric dates were dendrochronologically corrected to calibrated years according to the procedure of Stuiver and Reimer (1993). Furthermore, we submitted a bone fragment to the Research and Collections department of the Natural History Museum of Los Angeles County, where it was identified by Dr. Jay Stewart as a late Pleistocene to Holocene mammal bone.

Near-Surface Stratigraphy at the Northridge Park Site The boreholes, test-pits, and trench exposed several fairly distinct Holocene to late Pleistocene alluvial deposits overlying Plio-Pleistocene Saugus Formation. The late Pleistocene to Holocene deposits generally consist of alternating packages of fining upward sequences of silty sand

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Figure 5.

Topographic map of the Northridge Park site, showing locations of boreholes trenches and test pits. A-A⬘ shows the location of geologic cross section shown in Figure 6.

and coarse sandy gravel. These flood-plain and channel deposits were laid down by Aliso Canyon Wash, and were derived from the Santa Susana Mountains. The Saugus Formation consists of heterogeneous alluvial-fan and debrisflow deposits derived from the Santa Susana Mountains, and overall, is a nonmarine conglomerate with abundant angular to blocky porcelaneous shale fragments, and a subordinate amount of igneous and metamorphic clasts supported by a silty, carbonate-rich matrix (Saul, 1979; Dibblee, 1992). On the basis of magnetostratigraphy (Levi and Yeats, 1993), the Saugus Formation ranges in age from 2.3 to 0.5 Ma. The Pliocene to Pleistocene Saugus Formation is mapped at the site as the Upper Member of the Saugus Formation by Dibblee (1992). The Upper Member is exposed in a roadcut along Aliso Canyon Wash north of the site and within the hanging-wall block of the Northridge Hills fault (Fig. 4). It consists of massive to poorly bedded porcelaneous gravel and silt interbeds, dipping northeast at about 22⬚. Our boreholes at the Northridge Park site encountered dense to very dense Saugus Formation composed of silt, clay, and sandy gravel with several beds containing stage I to II calcium carbonate development (Birkeland, 1984). From the borehole data alone, the Saugus Formation is

difficult to differentiate from younger alluvial deposits. To assist in differentiating Saugus Formation from younger alluvial material we used (1) high blow counts (standard penetration tests) suggesting greater consolidation, and thus, greater age; (2) differences in degrees of calcium carbonate accumulation; and (3) a distinct contrast in color between the Saugus Formation and overlying younger deposits. The Saugus Formation is red in color as exposed along Aliso Canyon Wash north of the site (Fig. 4). Deposits overlying the Saugus Formation commonly have lower blow counts, little or no accumulation of calcium carbonate, and are dark yellowish-brown in color. Directly overlying the Saugus Formation is a less consolidated alluvial deposit consisting of interbedded silty sand and gravel (Unit Qfu, Fig. 6). Borehole data show that this deposit is as much as 5-m thick north of the topographic scarp, and as much as 10-m thick south of the scarp. We tentatively correlate the deposit with map unit Qf1 identified by Wills and Hitchcock (1999). The uppermost part of this deposit was encountered at the base of the trench, and consists of slightly mottled, dark yellowish brown (10YR 4/4d) to yellowish-brown (10YR 5/4d) fine-grained fluvial deposits. This unit consists of fining-upward sequences of massive silt, silty clay, silty sand, fine-grained sand and lenses of


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Figure 6.

Cross-section A-A⬘ showing warping of basal contact of PleistoceneHolocene terrace deposits (Qt4) and undifferentiated Quaternary fluvial deposits (Qfu) at Northridge Park. See Figure 5 for cross section location.

fine-grained sand with gravel. The fine-grained deposits interfinger in the southern part of the trench with planar laminated fine- to medium-grained sand and fine gravel. Individual beds within this deposit are difficult to differentiate due to subtle changes in texture, color, and stratigraphic relations (i.e., interfingering), yet the strata are distinct enough to be traced nearly continuously across the length of the trench. Generally, these strata have a southerly dip between 1⬚ to 3⬚ in the northern part of the trench and steepen to about a 4⬚ to 6⬚ southerly dip in the middle and southern parts of the trench. Individual coarse-grained strata in the southernmost portion of the trench dip southerly up to 10⬚ to 18⬚. Overall, unit Qfu lacks soil structure, bioturbation, and sedimentary structure, and is unconformably overlain by a distinct, moderately consolidated, brown clayey gravel (Unit Qt4; Fig. 6). A 3-cm-wide bone collected near the top of unit Qfu was sent to the Natural History Museum of Los Angeles County for identification, where Dr. Jay D. Stewart of Research and Collections identified it as a thoracic vertebra from a modern mule deer (Order Artiodactyla). Based on the weathering of the bone and presence of manganese and iron oxide, Stewart (personal communication, 1998) estimated that the bone is at least early Holocene to late Pleistocene.

Unfortunately, the bone contained insufficient collagen to be dated by radiocarbon techniques. Unconformably overlying unit Qfu is a distinct clay-rich gravel (Unit Qt4) that is continuous across the entire trench length (Fig. 6). This distinct clay-rich gravel varies in color from dark-brown (10YR 4/3) to dark yellowish-brown (10YR 4/4) and consists of massive to poorly bedded, sandy gravel to gravelly sand that is partially supported by a clayey sand matrix. Clast lithologies include primarily angular to subrounded, laminated porcelinite, graywacke sandstone, iron-oxide-weathered siltstone, and a trace of volcanic and granitic clasts. The majority of the clasts are less than 4 cm in diameter, with about 10–20% present as cobble-sized gravel. Generally, the deposit is moderately sorted, yet commonly exhibits normal grading and imbrication. Clasts within Qt4 probably were derived from the Northridge Hills, as well as north of the site from the Santa Susana Mountains. Based on the imbricated clasts and the presence of a channel backedge, the flow direction of the former Aliso Canyon Wash channel was about S15⬚E, which is oblique to the strike of the Northridge Hills fault. The basal contact of Qt4 dips southerly about 2⬚ to 3⬚ in the northern part of the trench, and dips southerly about 4⬚ to 6⬚ at the southern end of the trench, where it lies beneath the base of the scarp (Fig.

636 6). The southern projection of the basal contact steepens between boreholes B-3 and B-2, then shallows again between boreholes B-2 to B-9 in a southerly direction (Fig. 6). The absence of datable material within these deposits limits our ability to estimate the age of the Qt4 deposit; therefore, we described two, 2-m-deep soil profiles from the trench, and used limited pedochronological methods to estimate broadly the age of unit Qt4. The profiles showed a 50-cm-thick A horizon, consisting of a massive, dark grayish brown (10YR 3/2m) to dark brown (10YR 4/3m) loamy sand. The upper 17 cm of the A horizon consisted of a plowed (Ap) or disturbed zone. The A horizon is underlain by a 60- to 80-cm thick, 2Bw soil horizon, which consists of a yellowish-brown (10YR 5/4d) to dark reddish-brown (10YR 4/3m to 7.5YR 4/6m) loam to clayey loam. The 2Bw horizon is characterized by moderate medium subangular blocky soil structure, with few thin clay films along pores. The 2Bw is underlain by a 2C horizon, characterized by yellowish brown (10YR 5/6d) loamy sand to clayey loam. It has weak to moderate angular to subangular blocky structure, with few discontinuous silt films. These soil horizons are continuous across the trench, and are consistent with the degree of soil development observed in the test pits northeast of the trench. Southwest of the trench, young fluvial deposits (see description below) are incised into the Qt4 deposits, and have removed much of the soil horizons exposed in the trench. Based on relative soil development, correlation to soils and deposits with known or estimated ages characterized previously in San Fernando Valley and the nearby region (Wills and Hitchcock, 1999; Spellman et al., 1984; McFadden, 1982), we broadly estimate the age of these soils between 6 and 30 ka. Further, we believe the broad age estimate of 6 to 30 ka for Qt4 provides enough conservatism for the purpose of assessing the age of deformation at the Northridge Park site. In the southernmost part of the trench and in the two southern test pits (TP-4 and TP-5), unit Qt4 is overlain by two young, poorly consolidated alluvial deposits (units Qal1 and Qal2). The lower of these (Qal1) consists of poorly to moderately consolidated silty sand that appears to onlap unit Qt4 in the southern part of the trench (Fig. 6). Unit Qal1 generally consists of a massive, brown, silty fine-grained sand with 5 to 10% fine gravel. Near the base of Qal1, there is an increase in sandy and clayey gravel interbeds. Based on borehole data and test-pit exposures, the unit thickens to at least 2.5 m to the south. This unit is moderately bioturbated and contains rare charcoal. The basal contact with Qt4 is clear in the test pits, but less certain at the south end of the trench. One small charcoal fragment from the top of Unit Qal1 yielded a calibrated age of 615 Ⳳ 75 yr BP. Overlying unit Qal1 is a poorly consolidated, loose sand and gravel deposit (unit Qal2). Unit Qal2 consists of laminated, lightbrown to tan, fine- to coarse-grained sand and gravel, and is as much as 2-m thick. It contains a distinct, 1-m-wide channel that trends approximately east-west across TP-5, and contains rare charcoal. One small charcoal fragment col-


lected from this deposit yielded a calibrated age of 250 Ⳳ 50 yr B.P. Lastly, unit Qal2 is overlain by sandy artificial fill that is as much as 0.5-m thick. These stratigraphic relations and very young ages for Qal1 and Qal2 support our interpretation that the underlying Qt4 deposits may be at least Holocene in age (between 6 and 30 ka).

Near-Surface Structure Structural relations inferred from the trench, test pits, and boreholes provide information on the style and amount of surficial deformation across the central part of the Northridge Hills fault (Fig. 6). No direct evidence of brittle faulting or distinct fault-derived colluvial wedges at the base of the topographic scarp were present in the trench exposure. However, there is direct evidence to indicate that the alluvial units are warped into a south-facing monocline. For instance, as shown in Figure 6, the unconformity between Qfu and Qt4 steepens considerably in the vicinity of the topographic surface scarp. Furthermore, subsurface borehole data show that the top of the Saugus Formation is warped into a southwest-facing monocline (Fig. 6). Assuming that the contact between the fluvial deposits (unit Qfu) and the Saugus Formation north and south of the topographic scarp dips gently southwest at about 3⬚ (e.g., similar to the present-day surface), the apparent vertical separation of this contact across the monocline is 13 Ⳳ 2 m. This also is consistent with the warping of the base of Qt4 north and south of the escarpment. For instance, the four test-pits excavated north of the trench (TP-1, TP-2, TP-3 and TP-6) suggest that the base of unit Qt4 flattens northward; test-pits TP-4 and TP-5, and boring B-2 suggest that the contact remains moderately to steeply south-dipping south of the trench. In addition, borings B-7 to B-9 suggest a shallowing of the Qt4 basal contact south of the surface scarp (Fig. 6). The contact between Qt4 and unit Qfu suggests a south-facing monocline at the site. This contact is nearly flat (1⬚ to 3⬚) in the northern portion of the trench but steepens considerably (4⬚ to 6⬚) in the middle to southern part of the trench, similar to the dips of individual discontinuous beds within unit Qfu. On the basis of these relations exposed in the trench, test pits, and boreholes, we interpret at least 6 Ⳳ 1 m of vertical separation of unit Qt4 across the south-facing monocline (Fig. 6).

Discussion The subsurface investigation at Northridge Park suggests that the Northridge Hills fault is associated with nearsurface folding and development of a south-facing monocline. There has been approximately twice as much vertical separation of the base of unit Qfu than of the overlying, unconformable unit Qt4 (Fig. 6). Unit Qfu is thicker on the south side of the monocline. In addition, late Holocene alluvium onlaps unit Qt4 on the southern side of the monocline and is interpreted to have been deposited on the downthrown side of the fold following a surface-deforming earthquake.


The bone sample from unit Qfu suggests a late Pleistocene to early Holocene age for the top of this unit, and thus a possible early Holocene age for the overlying unit Qt4. Based on two soil profile descriptions we broadly estimate the age of Qt4 between 6 and 30 ka. Charcoal sampled from unit Qal1 suggests a late Holocene age for deposits overlying Qt4. Thus, with broad age ranges for the warped deposits exposed at Northridge Park, we estimate the activity of the Northridge Hills fault. Is the Northridge Hills Fault Active? Based on the broad estimated age of Qt4 (6 to 30 ka), we conclude that some of the surface deformation above the Northridge fault Hills Hills fault likely is a product of Holocene fault movement. Furthermore, based on the findings of our subsurface investigation, we infer that the warping of young deposits at the Northridge Park site is a result of triggered incremental growth of a fault-propagation fold caused by earthquakes rupturing the Northridge Hills fault on or a nearby structure (see following sections). If this is true, then by using the vertical separation between the warped fluvial deposits encountered at the site and the broad age range for Qt4, we estimate a late Holocene shortening rate and a dipslip displacement rate for the Northridge Hills fault. To estimate a late Holocene shortening rate, we use the change in line length of the basal contact of Qt4 across the blind Northridge Hills fault (Fig. 6). We estimate between 12 to 15 m of shortening of unit Qt4. Using this estimate of shortening and an age of 6 to 30 ka for Qt4, the average shortening rate is about 1.5 Ⳳ 1 mm/yr. As a comparison, this range is compatible with Huftile and Yeats’ (1996) horizontal shortening rate of 1.2 mm/yr and Davis and Namson’s (1994) rate of 1.4 to 1.7 mm/yr for the Northridge blind thrust. The shortening rates calculated by Huftile and Yeats’ (1996) and Davis and Namson (1994) are based on balanced cross-sections and the amount of deformation interpreted for the Plio-Pleistocene Saugus Formation. The average dip-slip displacement along the Northridge Hills fault since deposition of unit Qt4 also can be estimated based on the cumulative vertical surface separation and an assumed fault dip. We assume an age of 6 to 30 ka for unit Qt4, and a vertical separation of 6 Ⳳ 1 m, to estimate a Holocene uplift rate of 0.7 Ⳳ 0.5 mm/yr. This rate is consistent with the uplift rate of 1.0 Ⳳ 0.3 mm/yr estimated from longitudinal stream terrace profiles constructed across the Northridge Hills fault (Hitchcock and Kelson, 1996). In addition, assuming a fault dip of about 45⬚ (Tsutsumi and Yeats, 1999), these data yield an average dip-slip displacement rate of 1.0 Ⳳ 0.7 mm/yr. The lower-end of this rate is consistent with the minimum dip-slip rate of 0.35 mm/yr from a balanced cross-section by Tsustumi and Yeats (1999) for the Northridge Hills fault, and the age of the Plio-Pleistocene Saugus Formation. Huftile and Yeats (1996) calculated a long-term slip rate of 1.7 mm/yr for the past 2.3 Ma for the Northridge blind thrust, which is at the high end of

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our 1.0 Ⳳ 0.7 mm/yr Holocene slip rate for the Northridge Hills fault. In summary, the Northridge Hills fault is probably a Holocene active fault based on warped early Holocene to late Pleistocene alluvial deposits at the Northridge Park site. Furthermore, we use the estimated age of the warped deposits and the amount of vertical separation across the monocline to yield an early to middle Holocene rate of deformation for the Northridge Hills fault. Is the Northridge Hills Fault an Independent Seismic Source of Large Magnitude Earthquakes? One approach to answering this question is to obtain geologic evidence of discrete large surface ruptures or folds produced by distinct scarp-forming events, which 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 earthquakes on other nearby structures. In addition, if the amount of deformation recorded at the Northridge Park site can be attributed to individual events, we can estimate a range in earthquake recurrence for the Northridge Hills fault. To assess whether the Northridge Hills fault is an independent seismogenic source capable of producing a moderate to large magnitude earthquake, we first estimate the maximum earthquake likely to be produced by the fault. Using empirical relations between rupture area and earthquake magnitude (Wells and Coppersmith, 1994), we estimate the maximum earthquake. For calculating rupture area, we conservatively assume that the maximum rupture length of the blind-thrust fault corresponds to the maximum length of the associated anticline mapped at the surface; and that the fault at depth is continuous and uninterrupted by a lateral ramp, tear fault or other transverse structure that might reduce the length of rupture. For this rupture scenario, we assume a rupture length of 15 km based on the mapped length of the Northridge Hills anticline (Jennings and Strand, 1969; Saul, 1979). There are several apparent steps along the surface trace of the fold, but these are less than 1-km wide and are not likely to arrest rupture propagation (Barka and Kadinsky-Cade, 1989). We estimate a rupture width of 10 km by assuming a 45⬚-north-dipping fault plane that extends to at least 7 km in depth (Huftile and Yeats, 1996; Tsutsumi and Yeats, 1999). The exact depth at which the fault terminates is unknown, however subsurface control from oil-well and seismic data suggest that the fault extends to at least 7 km in depth (Tsutsumi and Yeats, 1999). We also assume that on the basis of: (1) an absence of surface faulting and brittle fracturing in the trench and test pits exposures at Northridge Park; (2) Northridge aftershock data (see Section 3.0a); and (3) leveling surveys across the Northridge Hills fault (see

638 Section 3.0b), that the Northridge Hills fault is a backthrust within the Northridge blind thrust similar to the model proposed by Wald et al. (1996). These assumptions and geometric constraints yield a rupture area of about 150 km2 and a maximum earthquake of about MW 61⁄4 for the Northridge Hills fault. As a comparison, this scenario suggests a maximum earthquake on the Northridge Hills fault of nearly similar magnitude to the 1983 MW 6.4 Coalinga earthquake (Rymer and Ellsworth, 1990), but smaller than the 1994 MW 6.7 Northridge earthquake (Wald et al., 1996). Alternatively, it is possible that the Northridge Hills fault extends to near the focus of the 1971 San Fernando earthquake (about 12 km in depth; Fig. 2), thus based on rupture area and earthquake magnitude produces a larger magnitude earthquake than estimated from the backthrust model. We argue that the trench and test-pit exposures at Northridge Park, coupled with the absence of surface faulting or significant surface deformation along the Northridge Hills fault during the 1971 San Fernando earthquake (Bonilla et al., 1971; Weber, 1974) suggest that this earthquake scenario is unlikely, yet we can not preclude it. Average surface displacements associated with earthquakes (normal, reverse, and strike-slip) of about MW 61⁄4 generally are 0.2 m or less (Wells and Coppersmith, 1994). The estimated average surface displacement for a MW 61⁄4 earthquake is consistent with our findings at Northridge Park. For instance, the absence of distinct brittle fracturing and scarp-derived colluvial deposits, coupled with the presence of uniform warping of sediments, suggests that if the Northridge Hills fault is seismogenic, it is capable of producing only moderate-magnitude earthquakes with little or no surface rupture. Because surface rupture and brittle fracturing are absent at the site, it is reasonable to assume that during a MW 61⁄4 earthquake surface folding above a blind thrust, such as the Northridge Hills fault, would be equal to or less than about 0.2 m. Assuming that the cumulative deformation observed on the basal unconformity of Qt4 represents only coseismic deformation, and the average surface displacement from a MW 61⁄4 earthquake is about 0.2 m or less, we estimate at least 25 moderate-magnitude earthquakes could have occurred on the Northridge Hills fault within the past 6 to 30 ka. This yields a poorly constrained recurrence interval of 170 to 1200 years for moderate magnitude earthquakes on the Northridge Hills fault. We acknowledge that this is a very broad range. Unfortunately, there is no direct or indirect evidence from the subsurface investigation to evaluate whether the observed deformation is or is not a product of primary deformation from earthquakes occurring on the Northridge Hills fault. In contrast, there is abundant indirect evidence to suggest that historical secondary deformation has occurred along the Northridge Hills fault from earthquakes nucleating on other nearby faults. We hypothesize that secondary deformation features along the Northridge Hills fault suggest that fault movement may be dependent on other seismogenic sources.


Is the Observed Surface Deformation Produced by Secondary Movement on the Northridge Hills Fault from Earthquakes on Other Nearby Faults? We utilize data collected from regional seismicity and local deformation studies within the 1994 Northridge epicentral area, coupled with our findings at the Northridge Park site, to assess the origin of the Northridge Hills monocline. Studies of the 1994 Northridge earthquake aftershock distribution, historical land-level changes within the San Fernando Valley, and geodetic studies within the 1994 Northridge epicentral region support the hypothesis that movement on the Northridge Hills fault commonly is related to movement on other seismogenic sources. A balanced geologic section across the Northridge Hills shows that the Northridge Hills fault may be a shallow structure that extends to at least 7 km in depth (Fig. 2). The cross section permits the interpretation that the Northridge Hills fault is either a backthrust to the underlying Northridge blind thrust (Wald et al., 1996), or a south-vergent thrust fault within the Santa Susana fault system (Tsutsumi and Yeats, 1999). The Northridge blind thrust and the Santa Susana fault system may represent the independent earthquake sources causing the observed deformation at the Northridge Park site. We discuss the available indirect evidence for this scenario subsequently. 1994 Northridge Earthquake Aftershock Distribution. Aftershocks associated with the Northridge earthquake delineate the mainshock rupture zone between depths of approximately 7 and 17 km (Fig. 2). Above the tip of the rupture zone, the aftershocks are distributed throughout the hangingwall block of the Northridge blind thrust (Hauksson et al., 1995; Unruh et al., 1997), part of which may contain the Northridge Hills fault (Tsutsumi and Yeats, 1999). The proximity of the aftershocks to the Northridge Hills fault suggests that the fault may deform coseismically when larger magnitude earthquakes occur on other nearby structures, such as the Northridge blind thrust (see figure 3 of Unruh et al., 1997; Mori et al., 1995). This pattern of aftershock activity is not unique to the Northridge earthquake. Similar microseismicity patterns were noted, for example, after the 1971 San Fernando earthquake within the hanging-wall block of the San Fernando fault and near the Northridge Hills fault (see figure 3 of Mori et al., 1995). Similar microseismicity patterns also were noted after the 1985 MW 6.1 Kettleman Hills earthquake (Ekstrom et al., 1992) and the 1989 MW 6.9 Loma Prieta earthquake (Langenheim et al., 1997). Shallow microseismicity patterns following the mainshock of the 1985 Kettleman Hills earthquake were interpreted by Ekstrom (1985) as small adjustments within the core of the hanging-wall block and overlying anticline. Similarly, after the 1989 Loma Prieta earthquake, shallow microseismicity occurring along the eastern margin of the Santa Cruz Mountains was attributed to reactivation of range front thrust faults (Hitchcock and Kelson, 1999; Langenheim et al., 1997). In addition, the


1983 MW 6.4 Coalinga earthquake had aftershocks extending into the core of an anticline overlying the mainshock, further suggesting triggered deformation of the hanging-wall block either through folding or faulting (Eberhart-Phillips, 1989). Thus, it is permissible that some of the deformation associated with the Northridge earthquake aftershocks was accommodated through secondary deformation on the Northridge Hills fault. Hauksson et al. (1995) also identified several shallow thrust faulting events located west of the 1994 Northridge mainshock epicenter and within the hanging-wall block of the Northridge blind thrust. Hauksson et al. (1995) attributes these thrust events to secondary faults associated with the formation of an overlying fold, similar to the observations made after the Kettleman Hills, Coalinga, and Loma Prieta earthquakes. A review of the Northridge earthquake aftershocks indicates that a ML 5.9 earthquake occurred at a depth of about 5 to 6 km (Hauksson et al., 1995), and near the subsurface down-dip projection of the Northridge Hills fault (Huftile and Yeats, 1996). The proximity of this aftershock to the subsurface projection of the Northridge Hills fault provides further indirect evidence that adjustments likely were being accommodated through secondary faulting in the hanging-wall block of the Northridge blind thrust. Lastly, based on the aftershock distribution from the 1994 Northridge earthquake and similar deformation patterns observed during and after other historical reverse earthquakes in California, we interpret that the Northridge Hills fault may experience secondary rupture during large earthquakes on the Northridge blind thrust, in order to accommodate coseismic deformation within the hanging-wall block of the Northridge blind thrust. It is also possible that the Northridge Hills fault ruptures sympathetically with earthquakes on the Santa Susana fault system. For instance, the aftershock distribution of the 1994 Northridge earthquake and the 1971 San Fernando earthquake overlap along strike of the two rupture areas, whereas several MW 3 aftershocks of the San Fernando earthquake are associated closely with the Northridge Hills fault (Barnhart and Slossen, 1973, Ziony and Yerkes, 1985). Aftershocks of the Northridge earthquake, however, end abruptly at about 5 to 8 km in depth near the 1971 aftershock zone (see Figure 2; see also figure 3 of Mori et al., 1995), whereas aftershocks from the San Fernando earthquake extend south into the San Fernando Valley, presumably along structures such as the Northridge Hills fault (Mori et al., 1995). Tsutsumi and Yeats (1999) speculate that the Northridge Hills fault may be the updip extension of the 1971 fault plane and the youngest north-dipping thrust fault in the San Fernando Valley. The 1971 aftershock data appear to support the hypothesis that the north-dipping Northridge Hills fault, apart from being a backthrust within the Northridge blind thrust, may be a north-dipping blind thrust fault that merges at depth with the larger Santa Susana fault system (Huftile and Yeats, 1995) and that it may deform secondarily during events similar to the 1971 San Fernando earthquake. Under

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this scenario it also implies that the Northridge Hills fault may be capable of earthquakes greater than the MW 61⁄4 magnitude estimated using relations between rupture area and earthquake magnitude. The findings from the trench and testpit exposures at Northridge Park, coupled with the absence of surface rupture on the Northridge Hills fault during the 1971 San Fernando earthquake, suggest that this scenario is unlikely. Leveling Surveys. Measurements of normalized length changes of streets in the San Fernando Valley collected between 1980 and before and after the 1994 Northridge earthquake exhibit a distinctive strain pattern that appears to support an interpretation of sympathetic faulting within the hanging-wall block of the Northridge blind thrust (Fig. 7; Johnson et al., 1996a; Cruikshank et al., 1996). Leveling

Figure 7.

Differential vertical displacements between 1980 and 1994 Northridge earthquake along a north-south transect across the Northridge Hill fault zone (from Johnson et al., 1996a).

640 surveys conducted between 1980 and 1994 indicate about 0.1 m of local vertical uplift in the region of Northridge Park (Fig. 8). The elevation data of Johnson et al. (1996a) also show that the greatest elevation change in this part of the San Fernando Valley roughly coincides with the Northridge Hills fault (Fig. 8). Secondary deformation features mapped in the Northridge Park area by Johnson et al. (1996a) also are coincident with regions of uplift along the Northridge Hills fault (Fig. 8). Assuming that the uplift is entirely coseismic in origin, this local uplift of about 0.1 m is only part of about 0.4 to 0.5 m of regional uplift and growth of the Santa Susana anticlinorium from the Northridge earthquake (Hudnut et al., 1996). Presumably the 0.4 to 0.5 m of regional uplift is accommodated partly by reverse faults, such as the Northridge Hills and Mission Hills faults, and bedding plane faults within the anticlinorium (Johnson et al., 1996b). This secondary deformation, coupled with the aftershock distribution of the 1994 Northridge earthquake, provides additional indirect evidence that deformation may occur along the Northridge Hills fault during earthquakes on the underlying Northridge blind thrust and possibly during episodes of growth of the Santa Susana anticlinorium. Level-line data and gravity studies following the 1971 San Fernando earthquake indicate at least 2 m of coseismic vertical uplift of the Santa Susana and San Gabriel Mountains (Oliver et al., 1974; Savage, 1971). The coseismic vertical uplift is significantly greater than the 0.4 to 0.5 m of uplift estimated from the Northridge earthquake (Hudnut et al., 1996). Gravity data for the 1971 earthquake suggest that a very small change in gravity occurred several kilometers from the southern margin of the Santa Susana Mountains near Pacoima, indicating possible subtle elevation changes on structures south of the mountain front (Oliver et al.,


1974). We speculate that the structures located south of the Santa Susana Mountains, such as the southern part of the Northridge Hills, if surveyed following the 1971 earthquake likely would have shown components of vertical uplift from sympathetic rupture of the Northridge Hills fault. No direct evidence of surface rupture was observed to suggest that the Northridge Hills fault ruptured sympathetically during the 1971 San Fernando earthquake (Bonilla et al., 1971; Weber, 1974). As discussed earlier, the aftershock pattern (Mori et al., 1995) from the San Fernando earthquake shows that it is permissible to interpret that secondary deformation may have occurred on Northridge Hills fault to accommodate deformation occurring in the hanging-wall block of the 1971 seismogenic fault and within the Santa Susana anticlinorium. Geodesy Studies. GPS modeling provides indirect evidence that also supports the occurrence of secondary deformation on the Northridge Hills fault during the 1994 earthquake. For instance, Shen et al. (1996) and Donnellan and Lyzenga (1998) use GPS data to characterize the rupture mechanisms of the Northridge earthquake. They find that in order to fit the geodetic data with a fault model whose primary fault patch is confined to a plane through the aftershocks, a secondary fault within the shallow upper crust is required. Both studies propose that a shallow fault is located above the main rupture plane, but Shen et al. (1996) propose a north-dipping fault, and Donnellan and Lyzenga (1998) propose a shallow south-dipping fault. Regardless of the fault dip direction, Donnellan and Lyzenga (1998) further suggest that if deformation were to occur on the near-surface secondary fault, it would likely result in deformation of the upper crust through folding and slip on bedding plane faults. These inferences of secondary deformation are consistent

Figure 8.

Differential vertical displacements along a north-south transect along Reseda Boulevard, between 1980 and 1994 (from Johnson et al., 1996a). See Figure 7 for profile location.

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with the observed monoclinal warping of young sediments in the Northridge Park trench, and the coseismic and aseismic uplift documented near the Northridge Park site from the Northridge earthquake. We speculate that the GPS models provide further support for the hypothesis that the Northridge Hills fault ruptures sympathetically during earthquakes on nearby faults.

Conclusions The paleoseismic study at Northridge Park provides direct evidence of relatively young shallow, surficial deposits warped into a south-facing monocline across the Northridge Hills fault. On the basis of stratigraphic relations exposed in the trench, test pits, and boreholes, we interpret at least 6 Ⳳ 1 m of vertical separation since the early Holocene to late Pleistocene. The borehole data also suggest that an unconformity developed on the Plio-Pleistocene Saugus Formation is warped into a south-facing monocline that has 13 Ⳳ 2 m of vertical separation. Based on an absence of scarp-derived colluvial deposits and brittle faulting in the trench and test pits, we interpret that the Northridge Hills fault does not produce large magnitude earthquakes. This is in contrast to earlier studies that suggested the fault was capable of producing seismic events in excess of MW 61⁄2 with an upper limit of MW 71⁄2 (Barnhart and Slosson, 1973). We estimate the Northridge Hills fault may produce a maximum earthquake of about MW 61⁄4 that deforms surficial deposits into a south-facing monocline but does not have distinct surface fault rupture. This is consistent with a very limited, worldwide database for moderate- to large-magnitude reverse earthquakes (MW 5.4 to 8.7) that suggests the occurrence of Quaternary folds without surface faults appears to correspond to moderate-magnitude earthquakes of MW 6.0 to 6.4 (Lettis et al., 1997). We tentatively present a poorly constrained recurrence interval of 170 to 1200 years for moderate magnitude earthquakes on the Northridge Hills fault. There is no direct evidence to support or refute a model in which the Northridge Hills fault behaves as an independent seismogenic source; however, studies on the northwestern part of this fault, where the fault tip nears the ground surface, could help establish the presence or absence of surface fault rupture. We also speculate observations and findings from recent seismicity studies, leveling surveys, and geodetic modeling that the Northridge Hills fault undergoes secondary deformation during and/or following large magnitude earthquakes on nearby structures, such as the Northridge blind thrust or the Santa Susana fault system. Despite the various methods used to characterize the behavior of the fault, we cannot determine unequivocally if the Northridge Hills fault behaves as a dependent or independent seismogenic source. Yet this study demonstrates that the Northridge Hills fault in part behaves dependently, and deforms during rupture on nearby larger seismic sources.

Acknowledgments We acknowledge that mapping performed by Christopher Hitchcock provided the basis for this work and would like to thank him and Chris Wills of the California Division of Mines and Geology for their efforts and guidance. We also thank Scott Lindvall and Jeffrey Unruh of William Lettis & Associates, Inc. who provided useful comments. Drs. Hiroyuke Tsutsumi and William Lettis provided reviews of this article. Mr. Ron Tognazzini and Jeff Owen of Los Angeles Department of Water and Power allowed access to the site. This research was supported by Southern California Earthquake Center Program, under University of Southern California Purchase Order No. 019430 and Award Number 1434-HQ-97-AG-01718 of U.S. Geological Survey (USGS). The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied of SCEC or the USGS.

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