1

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Preliminary PotentialField Constraints on

the Geometry of the

San Fernando Basin,

Southern California

by V.E. Langenheim1, A. Griscom1, R.C. Jachens1, and T.G.

Hildenbrand1

Open-File Report 00-219

2000

This report is preliminary and has not been reviewed for

conformity with U.S. Geological Survey editorial standards or with

the North American Stratigraphic Code. Any use of trade, firm, or

product names is for descriptive purposes only and does not imply

endorsement by the U.S. Government.

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

1Menlo Park, California
2

TABLE OF CONTENTS

Abstract ..................................................................................................................2

Introduction .............................................................................................................2

Geologic Setting .....................................................................................................3

Gravity, Aeromagnetic, and Physical Property Data ...........................................4

Discussion of Potential-Field Anomalies and Modeling Results .......................7

 Modeling Results ................................................................................................8

 The Eastern Margin and the Verdugo Fault ..................................................8

 The Western Margin ........................................................................................9

 The Northern Margin .....................................................................................10

Discussion and Conclusions ................................................................................11

Acknowledgments ................................................................................................12

References ............................................................................................................13

TABLES

Table 1. Well Information ...................................................................................17

Table 2. Physical Property Data .........................................................................18

Table 3. Additional Well Information .................................................................19

FIGURES

Figure 1. Shaded-relief topographic map ...........................................................20

Figure 2. Simplified geologic map .......................................................................21

Figure 3. Isostatic gravity map ..........................................................................22

Figure 4. Locations of gravity stations ..............................................................23

Figure 5. Densities of core samples ..................................................................24

Figure 6. Density well log information ...............................................................25

Figure 7. Densities derived from sonic velocities .............................................26

Figure 8. Gravity and aeromagnetic maps with seismicity ..............................28

Figure 9. Models across the Verdugo Fault .....................................................29

Figure 10. Map of wells and model across the western margin .......................31

Figure 11. Model of Oakeshott geologic cross-section ....................................33

Figure 12. Models along profile A-A’ ................................................................34

Figure 13. Seismic tomography ..........................................................................35

Figure 14. Gravity models with seismicity ........................................................36

Figure 15. Structural models to explain Northridge earhquake ........................36
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ABSTRACT

Gravity and magnetic data provide new insights on the structural underpinnings of the San

Fernando Basin region, which may be important to ground motion models. Gravity data

indicate that a deep basin (>5 km) underlies the northern part of the San Fernando Valley;

this deep basin is required to explain the lowest gravity values over the Mission Hills thrust

fault. Gravity modeling, constrained by well data and density information, shows that the

basin may reach a thickness of 8 km, coinciding with the upper termination of the 1994

Northridge earthquake mainshock rupture. The basin is deeper than previous estimates by 2

to 4 km; this estimate is the result of high densities for the gravels of the Pliocene-Pleisocene

Saugus Formation. The geometry of the southern margin of the deep basin is not wellconstrained by the gravity data, but may dip to the south. Recently acquired seismic data

along the LARSE (Los Angeles Regional Seismic Experiment) II profile may provide

constraints to determine the location and attitude of the basin edge. Gravity and

aeromagnetic models across the eastern margin of the San Fernando Valley indicate that the

Verdugo fault may dip to the southwest along its southern extent and therefore have a normal

fault geometry whereas it clearly has a thrust fault geometry along its northern strand.

INTRODUCTION

The January 17, 1994 Northridge earthquake (Fig. 1) again highlighted the hazard of buried

thrust faults in California. It was not the first damaging thrust-mechanism earthquake in the

San Fernando Valley; it was preceded by the 1971 San Fernando earthquake. Despite the

flurry of research that resulted from the 1971 earthquake (e.g., U.S. Geological Survey, 1971;

Oakeshott, 1975a), the fault that produced the Northridge 1994 earthquake was not identified

as a near-term seismic hazard. One tangible hint that the area might be subject to another

damaging earthquake in the near future came from geodetic data, indicating relatively high

strain rates for the central Transverse Ranges (Donnellan and others, 1993). Given that the

region is riddled with exposed (and buried), potentially dangerous thrust faults, important

components for quantifying seismic hazard include geodetic data on strain rates and

information on where the greatest shaking will occur using various earthquake scenarios.

The geometry of the basin, including the geometry of the bounding faults, is needed in

modeling the ground motion caused by earthquakes. Wald and Graves (1998) compare the

predicted ground motions from three separate 3-dimensional velocity models for southern

California. The largest disparities in the response of these models are in San Fernando

Valley and the deepest part of the Los Angeles basin. According to their analysis, the

structure of the basin, in particular, its depth extent, is a major factor in producing the

discrepancies between the velocity models. Other factors such as fault geometry, geometry
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of Quaternary fill, and uncertainties in shear wave velocities also are important. The

disparity of predictions in the San Fernando Valley is not surprising, because the geometry of

the San Fernando Basin is not well known. Corbato (1963) documented a significant density

contrast between the basin fill and the basement rocks and thus used gravity data to infer a

basin 4.5 km deep in the central part of the valley, south of the Mission Hills thrust fault (Fig.

2) whereas a cross section by Oakeshott (1975b) shows that the deep basin is between the

San Gabriel Mountains and the Mission Hills thrust fault, with the deepest part (6 km) thrust

beneath the San Gabriel Mountains. We present new 2 1/2 dimensional* gravity and

magnetic models to determine the basin geometry across the San Fernando Valley, using

surficial geology and available density, seismic-reflection, and deep exploration well data as

constraints, and compare our model to seismicity and seismic tomography results.

GEOLOGIC SETTING

The San Fernando Valley is a triangular basin (Figs.1 and 2) within the Transverse

Ranges province, an area noted for its strong and relatively young deformation (Donnellan

and others, 1993; Yeats and others, 1994; Wright, 1990). The Transverse Ranges province is

a region of north-south shortening that lies at a high angle to the predominant northwest

trend of the San Andreas fault system. Its topographic features (Figure 1) and structures

trend east-west, most strikingly developed in the Ventura basin. The San Fernando Valley

lies at the extreme southeastern end of the Ventura basin. The southern margin of San

Fernando Valley, the Santa Monica Mountains, shares an east-west trend with the rest of

the Transverse Ranges. The southern margin of the Santa Monica Mountains is marked by a

system of north-dipping thrust faults, including the Malibu and Santa Monica thrust faults.

The northern corner of the San Fernando basin is marked by a steep topographic front (Fig. 1)

and by the north-dipping Santa Susana thrust fault (SST on Fig. 2). The Santa Susana thrust

fault is part of a discontinuous thrust belt extending from the San Cayetano fault on the west

to the Sierra Madre fault on the east.

Unlike the northern and southern boundaries of the San Fernando Valley, however, the

eastern margin of the valley trends northwest, parallel to the San Gabriel fault (SGF), and is

 

*

 2-1/2 dimensional modeling limits the lateral dimensions of the source bodies in and out of the plane of the

profile whereas 2-D models assume the source to extend infinitely in and out of the plane of the profile.
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bounded by the plutonic and metamorphic rocks of the Verdugo Mountains (Figure 2). The

western edge of the valley, defined by exposed Eocene and Cretaceous rocks of the

Chatsworth Hills, trends northeast.

The San Fernando Valley is underlain by Miocene to Holocene sedimentary rocks. The

Miocene section includes the sandstones and conglomerates of the Topanga Formation

(which contains locally basalt flows) and the marine sandstones and shales of the Modelo

and Towsley Formations. These rocks are overlain by the marine sandstones and mudstones

of the Pliocene Fernando Formation, which grades upward and laterally into the shallow

marine and nonmarine Saugus Formation. The Saugus Formation undergoes a clast change,

from basement lithologies presumably derived from the San Gabriel Mountains, to locally

derived clasts of Modelo and Towsley Formations (Treiman, 1986); this change in clast

composition has been used to date uplift on the Santa Susana thrust fault (Levi and Yeats,

1993). Cretaceous through Eocene rocks also underlie the western part of the basin (Yerkes

and Showalter, 1990), but wells in the eastern part of the valley (wells 5 and 6; Fig. 1; Table

1) show Miocene rocks on top of granite and quartz diorite. From these geologic relations,

Yerkes and others (1965) proposed that Cretaceous and Paleocene rocks thicken towards

the west in the Santa Monica Mountains, although Weber and others (1980) suggested that

the eastern limit of Cretaceous and Paleocene rocks under western San Fernando Valley may

be offset 7 to 10 km left laterally along concealed faults from their counterparts in the eastern

Santa Monica Mountains.

According to Weber and others (1980), basement rocks exposed in the Verdugo

Mountains appear to be equivalent to rocks in the San Gabriel Mountains south of the SGF.

The Verdugo Mountains rocks include gneissic quartz diorite and diorite and dioritic gneiss

intruded by granitic rocks and dikes. Gabbro and schist are also locally present. Basement

rocks of the Santa Monica Mountains consist of the Santa Monica slate which is intruded by

granitic rocks. A much smaller proportion of metamorphic inclusions occur there than in the

Verdugo Mountains. Because of this difference in lithology, a major fault may separate the

Santa Monica and San Gabriel Mountains terranes (Weber and others, 1980). Two possible

locations for such a fault have been proposed: Yerkes and others (1965) infer a fault 5 km

west of and paralleling the Verdugo fault (VF on Fig. 2) whereas Schnurr and Koch (1979)

show a high-angle fault separating Santa Monica slate from granitic rocks along the southern

edge of San Fernando Valley.
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GRAVITY, AEROMAGNETIC, AND PHYSICAL PROPERTY DATA

The isostatic gravity map of the study area (Fig. 3) is based on more than 5000 gravity

measurements (Fig. 4). The sources of these data include Oliver and others (1981),

Langenheim and Jachens (1996), and the Defense Mapping Agency. Most of the data west

of 118o30’ west longitude and between 34o and 34o30’ north latitude are from the Defense

Mapping Agency. Station spacing ranges widely in the region. Mountainous areas (e.g., the

San Gabriel Mountains) tend to have sparser gravity station coverage and may have only 1

station per 10 km2; basin areas generally have denser gravity station coverage, as many as

23 stations per km2 and averaging about 1 station per km2.

Gravity data were reduced using the Geodetic Reference System of 1967 (International

Union of Geodesy and Geophysics, 1971) and referenced to the International Gravity

Standardization Net 1971 gravity datum (Morelli, 1974, p. 18). Gravity data were reduced to

complete Bouguer and isostatic residual anomalies using a reduction density of 2.67 g/cm3.

These data have been corrected for earth-tide, instrument drift, and free-air, Bouguer, latitude,

curvature, and terrain effects. An isostatic correction using a sea-level crustal thickness of 25

km, a crustal density of 2.67 g/cm3, and a mantle-crust density contrast of 0.40 g/cm3 was

applied to the gravity data to remove long-wavelength crustal gravity effects due to

topographic loading (Jachens and Griscom, 1985). These parameters were used because (1)

they are consistent with model parameters used for isostatic corrections computed for

California (Roberts and others, 1990), and (2) changing the model parameters does not

significantly affect the resulting isostatic anomaly (Simpson and others, 1986).

Terrain corrections were computed to a radial distance of 167 km and involved a 3-part

process: (1) Hayford-Bowie zones A and B with an outer radius of 68 m were estimated in

the field with the aid of tables and charts, (2) Hayford-Bowie zones C and D with an outer

radius of 590 m were computed by hand, and (3) terrain corrections from a distance of 0.59 km

to 167 km were calculated using a digital elevation model and a procedure by Plouff (1977).

Total terrain corrections for the stations on Figure 4 ranged from 0.0 to 40.3 mGal (in the San

Gabriel Mountains), averaging 2.7 mGal. Uncertainties resulting from the terrain correction

range from less than 0.1 mGal in the relatively flat areas of the San Fernando Valley and the
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Los Angeles basin to as much as 4.0 mGal in the most rugged area of the San Gabriel

Mountains.

Density information is necessary for quantitative interpretation of gravity anomalies.

Corbato (1963) presented density information from four drillholes (wells 10, 14, 15, and 16;

Table 1; Figure 1) in San Fernando Valley. Measurements were made on core samples from

depths of 645 to 10,545 ft (197 to 3214 m). Most of the core samples are composed of the

gravels of the Pleistocene-Pliocene Saugus Formation. These densities (Figure 5) are high

for Pliocene sediments, especially compared to a density of 1.97 g/cm3 for a Pliocene siltstone

(McCulloh, 1957). Corbato also calculated the density of alluvium, based on porosity

measurements of Eckis (1934). He found that the density of the alluvium varied inversely

with distance from the San Gabriel and Santa Susana Mountains, from a maximum of 2.46 to

a minimum of 1.97 g/cm3 (average 2.26 g/cm3). His average density for Quaternary to

Miocene sedimentary rocks is 2.20 g/cm3, for mid-Miocene to Cretaceous rocks is 2.47 g/cm3

and for basement rocks is 2.67 g/cm3 based on measurements on hand samples (McCulloh,

1957).

Another source of density information comes from sonic and density logs (Brocher and

others, 1998). Brocher and others (1998) show sonic logs for wells 5, 6, 7 and 8 (Table 1)

and density logs for wells 7 and 8 (Figure 6). We correlated the sonic and density logs from

well 7 to determine the relationship between sonic velocity, v, in m/s, and density, , in g/cm3,

for calculating the density of the other wells. The linear regression is (where R is the

correlation coefficient):

=1.3002 + 0.00040811v R=0.643 (1)

We did not use data from well 8 because of the high degree of noise in the well log data. We

also determined a density-velocity relationship using well 7 and three other wells in the Los

Angeles Basin (Northam Station, Ferris, and Murphy Whittier; Brocher and others, 1998).

The resulting linear regression is:

 =1.7807 + 0.0001668v R=0.605 (2)

The latter regression gives a higher density at the surface, but increases more slowly with

velocity at depth. The densities based on equations 1 and 2 for wells 5 and 6 are shown in

Figure 7. Densities from equation (2) are almost the same as those calculated using the

velocity-density relationship of Gardner and others (1974); equation (1) predicts higher
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densities for velocities greater than 2 km/s. Regardless of the equation used and despite the

wide scatter in values, in general, the densities for the upper 1000 m are generally between

2.0 and 2.2 g/cm3. For depths greater than 1000 m, the average density approaches 2.4 g/cm3.

We also measured the densities and magnetic susceptibilities from rock samples of

various basement lithologies (pre-Cenozoic metamorphic and igneous rocks) surrounding the

San Fernando Valley. These results are summarized in Table 2. The average density of 165

samples is 2.71 g/cm3. The Santa Monica slate is characterized by lower densities,

averaging 2.66 g/cm3 (N=9), and are essentially non-magnetic. Plutonic rocks that intrude

the slate are also essentially non-magnetic as is a sample from well 6 (Table 1). The

average density of the exposed plutonic rocks may be too low (2.59 g/cm3) because of

sampling bias of very weathered samples; the core sample from well 6 gives a higher density

of 2.65 g/cm3. Crystalline basement rocks exposed in the Verdugo and San Gabriel

Mountains are generally magnetic and have higher densities. The average density for rocks

in the Verdugo Mountains is 2.71 g/cm3 (N=32), for those rocks in the San Gabriel

Mountains south of the SGF, 2.70 g/cm3 (N=23) and rocks exposed north of the SGF, 2.72

g/cm3 (N=98).

High-resolution aeromagnetic data were collected over the region in 1995 (U.S. Geological

Survey, 1996) and added to existing data over the Ventura basin (U.S. Geological Survey,

1980). Most of the new data were flown along N-S trending flightlines spaced 800 m apart

with an average terrain clearance of 550 m.

DISCUSSION OF POTENTIAL-FIELD ANOMALIES AND MODELING

RESULTS

The San Fernando basin is at the eastern end of a large (~60 mGal), east-west-trending

gravity low over the Ventura basin (Fig. 3). The Ventura Basin gravity low curves to the

southeast without interruption into the San Fernando gravity low. Gravity modeling shows

that the Ventura gravity low is caused predominantly by Pliocene and younger deposits and

that the basin may be as deep as 15 km (Jachens and Griscom, 1985). The gravity data are

consistent with the cross sections of Nagle and Parker (1971) and Yeats and others (1994)
8

that indicate the Ventura basin is bounded by north- and south-dipping thrust faults on the

north and south sides, respectively.

The 40-mGal gravity low over the San Fernando Valley (Fig. 3) suggests that the

underlying basin is asymmetric. The lowest gravity values occur over the Mission Hills

thrust fault in the northern part of the valley (MHT on Fig. 2). A thrust fault bounding the

northern margin of the basin is clearly expressed by low gravity values (-20 mGal) over

exposed basement rocks (Fig. 3). This observation suggests that low-density material is

thrust beneath the crystalline basement rocks and is consistent with geologic mapping

(Oakeshott, 1975b). Another place where gravity data support a thrust fault geometry is

along the northern edge of the Los Angeles basin. Gravity data indicate a north-dipping

attitude for the Santa Monica thrust fault because the mapped trace of the fault lies at the

base of the gravity gradient (Fig. 3).

The gentle gravity gradient south of the Mission Hills thrust fault is interrupted abruptly at

its western end by a northeast-trending gravity gradient that corresponds in part to the

northeast-trending Chatsworth Reservoir fault (CRF on Fig. 2). The steepness of the gravity

gradient suggests that this northeast-trending boundary is steeply dipping. This fault may be

a high-angle tear or normal fault that juxtaposes the Ventura basin thrust fault system with

the thrust fault system underlying the San Fernando Valley. Shields (1978) shows a dip of

620 to the southeast for the Chatsworth Reservoir fault based on two oil exploration

boreholes. The gravity gradient deviates from the southern half of the fault trace (where the

mapped trace begins to strike more easterly), suggesting that the basin edge is

approximately linear with a strike of N450E.

The northeast-trending gravity gradient (Fig. 8a) corresponds to a northeast trend in the

1971 San Fernando aftershocks called the Chatsworth trend (Hanks and others, 1971) and

coincides approximately with the southeast edge of a magnetic block (“C” on Fig. 8b).

These aftershocks were southwest of the mainshock and were characterized by left-lateral

focal mechanisms. Whitcomb (1971) suggested that this seismicity represented motion of

the lower thrust block against a block to the northwest along a left-lateral strike-slip fault.

The aftershocks of the 1994 Northridge earthquake also appear to follow the northeasttrending gravity gradient as it bends into alignment with the east-west-trending gravity

gradients associated with the Ventura basin margins. However, Hauksson and others

(1995) did not find evidence of reactivation of the Chatsworth trend in the 1994 Northridge
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aftershocks. They instead suggested that the San Fernando lateral ramp in the Santa Susana

thrust fault influences the spatial distribution of aftershocks in the region. This lateral ramp is

on trend with the Chatsworth Reservoir fault and coincides with a change in trend from eastwest to northeast-southwest along the Santa Susana thrust fault (Fig. 2). The base of the

aftershock distribution west of the ramp is 3-5 km higher than that east of the ramp

(Hauksson and others, 1995).

The magnetic field over the San Fernando Valley is characterized by low-amplitude, shortand long-wavelength anomalies and suggests that the basement rocks of the San Fernando

Valley are weakly magnetic (Fig. 8b). Some of the short-wavelength anomalies have

sources within the basin fill, possibly related to the volcanic rocks in the Topanga Formation,

gravels of the Saugus Formation derived from the magnetic crystalline rocks exposed in the

San Gabriel Mountains, or an unknown source near the top of the Cretaceous sedimentary

sequence, which produces subtle, northeast-trending anomalies west of the Chatsworth

Reservoir fault (Fig. 8b).

The eastern margin of the San Fernando basin appears to be bounded by a high-angle fault

as indicated by steep gravity and aeromagnetic gradients. The basin is bounded on the east

by the Verdugo fault, which forms the southwestern margin of a highly-magnetic block. The

magnetic data confirm that Verdugo Mountain rocks extend east to the San Gabriel fault, as

suggested by Weber and others (1980). The magnetic data do not indicate a major fault

separating the Santa Monica and San Gabriel terranes along the southern edge of San

Fernando Valley as proposed by Schnurr and Koch (1979), but do suggest a major magnetic

terrane boundary at or near the Verdugo fault (see below).

Modeling Results

We created several gravity and aeromagnetic models across the San Fernando Valley

crossing the major structures bounding the basin. First we discuss the models across the

eastern and western margins of the basin, followed by models of the northern margin.

The Eastern Margin and the Verdugo Fault

Two models cross the eastern margin of the basin, where the major bounding structure is

the Verdugo fault. Both the gravity and magnetic data suggest that the Verdugo fault

changes character along strike. The northwest-trending gravity gradient associated with the
10

fault is interrupted by a north-south-striking gradient near point X (Fig. 8a). Northwest of

this point, the inferred fault trace lies near the bottom of the northwest-trending gradient;

implying a thrust geometry; southeast of X, the fault trace is near the top of the gradient,

suggesting a normal fault geometry. Southeast of point X, aeromagnetic data indicate a 5-kmwide gradient southwest of the fault; this gradient disappears northwest of point X.

The model along profile V-V’ addresses the geometry of the Verdugo fault southeast of

point X. Modeling across the mostly concealed Verdugo fault suggests that it dips steeply

(>600) to the southwest (Fig. 9a) and thus may not be a thrust fault as shown on a

schematic cross section by Weber and others (1980); their northeast-dipping attitude for the

Verdugo fault is based on foliation in the crystalline basement rocks. Weber and others

(1980) showed a fault with alluvium on crystalline bedrock with a southwest dip, but they

argued that it is a minor fault and not indicative of the geometry of the major basin-bounding

structure. An alternative explanation for the structure shown in V-V’ is that the apparent

normal geometry is the result of a stack of several thrust faults stepping southwest away

from the mountain front. Detailed gravity data could detect whether the gradient is the result

of one fault of normal geometry or several faults of thrust geometry.

Northwest of point X, along profile T-T’ (Fig. 2), Tsustumi and Yeats (1999) showed the

Verdugo fault as a northeast-dipping thrust fault. They furthermore suggested that the thrust

fault is a reactivation of a Miocene normal fault, based on a thicker Miocene section on the

upthrown block. We modeled their profile T-T’ (Fig. 9b), but as a straight section (see

dashed line in Fig. 2) because the gravity and aeromagnetic data suggest that the Verdugo

fault may not extend as far northwest as the bend in profile T-T’ (Fig. 8). We used the same

well constraints that Tsutsumi and Yeats show for their cross section. With these

constraints, a higher density is needed for the Saugus gravels on the northeastern end of the

model. We also model a magnetic source (volcanic rocks) within the Topanga Formation, in

the vicinity of well 2A (Tsutsumi and Yeats, 1999), to account for the 40-nT aeromagnetic

anomaly west of the Pacoima Hills (another possible source for this anomaly is a source

within the basement; the gradients of the anomaly point to a shallow source, but are

complicated by three-dimensional effects of the positive anomaly over the Pacoima Hills).

The model indicates a thrust geometry for the fault bounding the Pacoima Hills. This area is

very three-dimensional. The cross section of Tsutsumi and Yeats (1999) is not a straight

line, but jumps to northwest of the Pacoima Hills; thus, the fault that Tsutsumi and Yeats
11

(1999) call the Verdugo fault may actually be an east-west-trending thrust fault on strike

with and possibly related to the Mission Hills thrust fault.

The Western Margin

The western margin of the valley is not characterized by any outcrops of crystalline

basement. No wells penetrate crystalline basement; thus, its lithology and affinity are

unknown. However, the Simi Hills are composed of the oldest sedimentary rocks exposed in

the area, the late Cretaceous marine sandstones and conglomerates of the Chatsworth

Formation (Colburn and others, 1981). The exposed thickness of these rocks is about 1.4 km

(Sage, 1971); the base of the sequence is not exposed or reached by wells (Shields, 1977).

Although seismic-reflection profiles (Fig. 10a) do not clearly image the crystalline basement

in the valley (D. Okaya, oral commun., 1999), we used these sections to provide minimum

thicknesses of the Miocene/Eocene sedimentary sequence for profile C-C’ (Fig. 10b).

An interesting result of the modeling is the presence of a magnetic bed near the top of the

Cretaceous strata that produces a linear, 20-nT aeromagnetic anomaly over the Simi Hills.

The wavelength of the anomaly and the strike of the anomaly (parallel to the strike of the

strata) indicate that it should lie near or at the surface. These source rocks may be

responsible for short-wavelength anomalies within San Fernando Valley and thus could serve

as a marker for structures cutting the sedimentary section.

The Northern Margin

Figure 11 shows a gravity and magnetic model across the geologic cross section by

Oakeshott (1975b; O-O’ on Figure 2). His basin is asymmetric, with the deepest part of the

basin thrust beneath crystalline basement rocks northeast of the Santa Susana thrust fault.

The calculated gravity curve using Oakeshott’s geometry does not match the observed data

well southwest of the Santa Susana thrust fault. The basin must be deeper on the

southwestern part of the profile to account for the mismatch. Oakeshott constructed this

cross section before the discovery of the Pacoima oil field; borehole data (e.g., well 11)

indicate basin depths of 3 km and greater at the southwest end of O-O’. This model clearly

illustrates how, to a first order, geologic cross sections across the San Fernando Valley can

benefit from incorporating gravity data.
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Profile A-A’ bisects the San Fernando Valley along a north-south line (Fig. 1). Although

the orientation is not ideal for the northern end of the profile where it crosses the Soledad

basin, it maximizes the available deep borehole control and attempts to avoid edge effects

from the three-dimensional aspect of the San Fernando basin. The model is 2-1/2-

dimensional. The observed points are gravity stations within 1 km of the profile.

The gravity model A (Fig. 12) demonstrates that basin sediments may exceed 5 km in

thickness in the northern part of the basin to account for the gravity low. The inferred basin is

significantly thicker than that estimated by Corbato (1963) or Oakeshott (1975b). This

greater depth extent is forced by the use of higher densities in the Pliocene sedimentary rocks

as constrained by core density measurements presented in Corbato (1963; Fig. 5). Although

the core density measurements may be biased towards higher densities, densities are higher

closer to the range front (as high as 2.47 g/cm3) as indicated by porosity measurements of

alluvium (Corbato, 1963). The uniform decrease in gravity values from the Santa Monica

Mountains across the San Fernando basin cannot be modelled with a gently-north dipping

Miocene section unless one assumes that the Miocene sedimentary rocks have densities of

2.00 g/cm3 or less. As we have shown in Figure 7, a more representative density for the

Miocene sedimentary rocks is between 2.10 and 2.30 g/cm3.

Our models (Fig. 12) solve the Miocene density enigma by extending a lower-density

basement source to depths of 10 km. This lower-density basement could be composed

predominantly of Jurassic Santa Monica Slate which is exposed near profile A-A’ where it

crosses the Santa Monica Mountains. More likely, it could consist of granitic intrusions,

similar to the rocks found in drill holes 5 and 6 in the basin. Model B shows that the location

and attitude of the southern margin of the deep basin are not well constrained.

Although not the focus of the paper, geometries of faults and basins beyond San Fernando

Valley are shown in our models. At the southern end of profile A-A’, our models confirm a

north-dipping attitude for the Santa Monica fault. On the northern end of the profile, the

Soledad basin is nearly 4 km thick, consistent with estimates of the aggregate thickness of

the Tertiary section of 4300 m (14,000 ft; Oakeshott, 1975b).

DISCUSSION AND CONCLUSIONS

The gravity model along A-A’ is, to first order, consistent with the tomographic results of

Hauksson and Haase (1997; Fig. 13). Their results indicate velocities less than 6 km/s to
13

depths of 8 to 9 km in the area of our deep basin. Mori and others (1995) also show slower

velocities under the San Fernando Valley at depths of 5 and 10 km, although neither of these

models have particularly good resolution at these depths. The tomographic data (Hauksson

and Haase, 1997) hint of a southern dip for the southern boundary of the deep basin; however,

one must bear in mind that the tomography is based on a 10-km horizontal grid and will not

produce fine detail. For example, the sliver of basement rocks south of the San Gabriel fault

(between 40-45 km on Fig. 13) is not characterized by high velocities in the tomographic

model because it is too thin and/or because of fracturing. The gravity data (and model) do not

indicate a shallow, high-density body at about km 16, suggesting that the high-velocity

anomaly at 1 km depth is an artifact (Hauksson and Haase, 1997). Other possible artifacts

are caused by ray-tracing and are highlighted by arrows.

Figure 14 shows the aftershock seismicity from the 1994 and 1971 earthquakes plotted on

the gravity models (from within 2 km of the profile). The main south-dipping rupture defined

by seismicity does not coincide with the edge of the deep basin in either model A and B, but

this contact is not well constrained. It is possible that the edge of the basin does coincide

with the 1994 Northridge mainshock rupture. In both models, the bottom of the basin

coincides with the upper termination of the rupture at roughly 7-8 km determined by

Hauksson and others (1995), roughly the 6 km/s contour on Figure 13. Hauksson and others

(1995) interpret the diffuse cloud of seismicity at 7 km as deformation in an anticlinal fold;

however, another possible explanation is that the cloud of seismicity and the upper end of the

rupture are controlled by the base of the deep basin.

Two interpretations of the mainshock faulting have been suggested (Fig. 15). Namson

and Davis (1994) proposed a model with the mainshock fault as a south-dipping back-thrust

to the Elysian Park thrust fault beneath Los Angeles. Yeats and Huftile (1995),

alternatively, attribute the mainshock fault to an extension of the Oak Ridge fault (Fig. 2)

eastwards beneath San Fernando Valley. Yeats and Huftile (1995) use gravity data

(Langenheim and others, 1994) to support their interpretation; however, our modeled profile,

though provocative, does not prove their hypothesis. The gravity models support both

interpretations.

Our model of the Santa Susana fault is consistent with geologic cross sections by Huftile

and Yeats (1996) drawn at Aliso Canyon (5 km west of well 16; Fig. 1). The geologic cross

sections suggest highly constrained dips of about 530 to 600 NE for the Santa Susana fault,
14

with the leading edge flattening to 100 within 1 km of the surface. Our modeled geometry for

the Santa Susana fault shows that the fault may dip about 150 in upper 2 km, steepening to

700 as it intersects the San Gabriel fault. The deep basin projects beneath the San Gabriel

fault.

In conclusion, gravity data indicate that a deep basin underlies the northern part of the San

Fernando Valley. Gravity modeling, constrained by well data and density information, shows

the basin may be as deep as 8 km, coinciding with the upper termination of the 1994

Northridge mainshock rupture. The depth extent of the basin is highly dependent on the

assumption of high densities for the Pliocene gravels in the northern part of the valley. The

southern margin of the deep basin is not well constrained, but may dip to the south. Recently

acquired seismic data (LARSE II) may provide constraints that help determine the location

and attitude of the basin edge.

Gravity and magnetic models across the Verdugo fault show that the geometry of the fault

appears to vary along strike and that the fault along its southern extent may dip to the

southwest and therefore has a normal fault geometry, in contrast to the thrust fault geometry

clearly seen along its northern extent. An alternative explanation for the apparent normal

geometry may be a series of thrust faults that step out from the mountain front.

ACKNOWLEDGEMENTS

We would like to thank Tom Brocher and Bob Yerkes (U.S. Geological Survey, Menlo

Park, CA) for their reviews. We also thank Dave Okaya (University of Southern California)

for access to the seismic-reflection profiles in the San Fernando Valley and Tom Wright for

thoughtful discussions of the regional geology.

REFERENCES

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density logs from 12 oil test wells along LARSE lines 1 and 2, Los Angeles region,

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formation name for the Upper Cretaceous strata of the Simi Hills, California in Link, M.J.,

Squires, R.L., and Colburn, I.P. eds., Simi Hills Cretaceous Turbidites, Southern
15

California: Society of Economic Paleontologists and Mineralogists Field Trip Volume

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Corbato, C.E., 1963, Bouguer gravity anomalies of the San Fernando Valley, California:

University of California Special Publication 46, no. 1, p. 1-32.

Donnellan, A., Hager, B.H., King, R.W., and Herring, T.A., 1993, Geodetic measurement of

deformation in the Ventura basin, southern California: Journal of Geophysical Research,

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Eckis, R., 1934, South coast basin investigations--geology and groundwater storage capacity

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Fernando earthquake 2300 (GMT) February 10-1700 February 11, 1971 in San

Fernando, California Earthquake of February 9, 1971: U.S. Geological Survey

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Hauksson, E., Jones, L.M., and Hutton, K., 1995, The 1994 Northridge earthquake sequence

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Mw=6.7 aftershock zone) and Hopper Canyon segments of the western Transverse

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International Union of Geodesy and Geophysics, 1971, Geodetic Reference System 1967:

International Association of Geodesy Special Publication no. 3, 116 p.

Jachens, R.C., and Griscom, Andrew, 1985, An isostatic residual gravity map of California—A

residual map for interpretation of anomalies from intracrustal sources in Hinze, W.J., ed.,

The Utility of Regional Gravity and Magnetic Maps: Society of Exploration

Geophysicists, Tulsa, Oklahoma, p. 347-360.

Jennings, C.W., 1994, Fault activity map of California and adjacent areas: California Division

of Mines and Geology California Geologic Data Map 6, scale 1:750,000.

Langenheim, V.E., Griscom, Andrew, and Jachens, R.C., 1994, Gravity and magnetic data

help image the regional tectonic setting of the 1994 Northridge earthquake in

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Program for Northridge abstracts: Seismological Research Letters, v. 65, no. 3/4, p. 249.
16

Langenheim, V.E., and Jachens, R.C., 1996, Gravity data collected along the Los Angeles

Regional Experiment (LARSE) and preliminary model of regional density variation in

basement rocks, southern California: U.S. Geological Survey Open-File Report 96-682.

Levi, S., and Yeats, R.S., 1993, Paleomagnetic constraints on the initiation of uplift on the

Santa Susana fault, western Transverse Ranges, California: Tectonics, v. 12, p. 688-702.

McCulloh, T.H., 1957, Simple Bouguer gravity and generalized geologic map of the

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Morelli, C.(Ed.), 1974, The International Gravity Standardization Net, 1971: International

Association of Geodesy Special Publication no. 4, 194 p.

Mori, J., Wald, D.J., and Wesson, R.L., 1995, Overlapping fault planes of the 1971 San

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Nagle, H.E., and Parker, E.S.,1971, Future oil and gas potential of onshore Ventura basin,

California in Cram, I.H., editor, Future Petroleum Provinces of the United States--Their

Geology and Potential: American Association of Petroleum Geologists Memoir 15, p.

254-297.

Namson, T.L., and Davis, J.S., 1994, A balanced cross-section of the 1994 Northridge

earthquake, southern California: Nature, vol. 372, p. 167-169.

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Geology Bulletin 196, p. 19-30.

Oliver, H.W., Chapman, R.H., Biehler, S.H., Robbins, S.L., Hanna, W.F., Griscom, Andrew,

Beyer, L.A., and Silver, E.A., 1981, Gravity map of California and its continental margin:

California Division of Mines and Geology, Geologic Data Map No. 3, scale 1:750,000.

Plouff, Donald, 1977, Preliminary documentation for a FORTRAN program to compute gravity

terrain corrections based on topography digitized on a geographic grid: U.S. Geological

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California and offshore southern California: California Division of Mines and Geology,

Geologic Data Map No. 7, scale 1:750,000.

Sage, O.G., Jr., 1971, Geology of the eastern portion of the “Chico” Formation, Simi Hills,

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17

Shields, K.E., 1977, Structure of the northwestern margin of the San Fernando Valley, Los

Angeles County, California: Master’s Thesis, Ohio University, Athens.

Shields, K.E., 1978, Faults along the northwestern margin of the San Fernando Valley, Los

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histories Los Angeles Metropolitan area: Association of Engineering Geologists First

Annual California section conference, p. 44-52.

Schnurr, P.E., and Koch, C.E., 1979, Pacoima field in Pacific Section American Association of

Petroleum Geologists field summaries: Pacific Petroleum Geologists Newsletter Winter

1979-80, v. 34, no. 4, p. 4-5.

Simpson, R.W., Jachens, R.C., Blakely, R.J., and Saltus, R.W., 1986, A new isostatic gravity

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Treiman, J.H., 1986, Geological hazards in the west half of the Newhall quadrangle, Los

Angeles County, California: California Division of Mines and Geology Open-File Report

86-6.

Tsutumi, Hiroyuki, and Yeats, R.S., 1999, Tectonic setting of the 1971 and 1994 Northridge

earthquakes in the San Fernando Valley, California: Bulletin of the Seismological

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U.S. Geological Survey, 1971, The San Fernando, California earthquake of February 9, 1971:

U.S. Geological Survey Professional Paper 733, 254 p.

U.S. Geological Survey, 1980, Aeromagnetic map of the Ventura Basin, Ventura and Los

Angeles Counties, California: U.S. Geological Survey Open-File Report 80-64, scale

1:62,500.

U.S. Geological Survey, 1996, Aeromagnetic map of parts of the Los Angeles, San

Bernardino, Bakersfield, Long Beach, and Santa Ana 1o by 2o quadrangles, California: U.S.

Geological Survey Open-File Report 96-550, scale 1:250,000.

Wald, D.J., and Graves, R.W., 1998, The seismic response of the Los Angeles basin,

California: Bulletin of the Seismological Society of America, v. 88, no. 2, p. 337-356.

Weber, F.H., Jr., Bennett, J.H., Chapman, R.H., Chase, G.W., and Saul, R.B., 1980,

Earthquake hazards associated with the Verdugo-Eagle Rock and Benedict Canyon fault

zones, Los Angeles county, California: U.S. Geological Survey Open-File Report 81-296,

116 p.

Whitcomb, J.H., 1971, Fault-plane solutions of the February 9, 1971, San Fernando

earthquake and some aftershocks: U.S. Geological Survey Professional Paper 733, p. 30-

32.

Wright, T.L., 1990, Structural geology and tectonic evolution of the Los Angeles basin,

California in Biddle, K.T., ed., Active Tectonic Basins: AAPG Memoir 52, p. 35-134.
18

Yeats, R.S., and Huftile, G.J., 1995, The Oak Ridge fault system and the 1994 Northridge

earthquake: Nature, v. 373, p. 418-420.

Yeats, R.S., Huftile, G.J., and Stitt, L.T., 1994, Late Cenozoic tectonics of the East Ventura

basin, Transverse Ranges, California: American Association of Petroleum Geologists

Bulletin, v. 78, p. 1040-1078.

Yerkes, R.F., and Showalter, P.K., 1990, Exploratory wells drilled in the Los Angeles

1:100,000-scale quadrangle, California: U.S. Geological Survey Open-File Report 90-627,

47 p., 1 plate.

Yerkes, R.F., McCulloh, T.H., Schoelhamer, J.E., and Vedder, J.G., 1965, Geology of Los

Angeles basin, California--an introduction: U.S. Geological Survey Professional Paper

420-A, 57 p.
Table 1. Selected wells in area of modeled transects.

Figure Well Total Lithology Township/ Well

ID Name Depth

 ( ft) at Bottom* Range/Section ID

1 Standard Oil Duff CoreHole 1 11209 Miocene 1S/15W/34 20551

2 Argo Petroleum Corp Argo- 12085 Santa Monica fault, 1S/15W/28 21927

Aladdin 179-1 7940; Upper Miocene

3 Hudson Gas & Oil Los Angeles 5515 Slate 4600 1S/15W/23 00990

Country Club 3

4 Hudson Gas & Oil Los Angeles 1990 Slate 1900 1S/15W/14 05451

Country Club Core Hole 1

5 Chevron Hazeltine 1 3823 granite 3785 1N/15W/14 05969

6 Chevron Leadwell 1 5066 quartz diorite 4840 1N/15W/03 05974

7 Chevron Frieda J. Clark 1 7323 Miocene 2N/16W/25 21018

8 Chevron Burnet Core Hole 1 10227 Miocene 2N/15W/16

9 Chevron University 1 5938 Miocene 3920 2N/15W/15 06001

10 Chevron Woo 1 9739 Middle Miocene 2N/15W/19 06006

11 Occidental Petroleum Pacoima 9291 Miocene 2N/15W/10 20519

EH 1

12 Chevron Rinaldi 1 4725 Middle Mohnian 2N/15W/4 06330

13 Shell Western Mission Core Hole 2 5687 Mohnian 2N/15W/4 05892

14 Gulf Oil Corp. Carey 1 10136 Upper Miocene 2N/15W/15

15 Sun Exploration 12027 Pliocene 3N/15W/21 06034

Stetson-Sombrero

16 ARCO T.I.&T. 1 8207 Pliocene 3N/15W/19 05180

17 Rothschild Oil Wickham- 1648 granite 1114 4N/15W/32 05814

Ferrier 1

18 Enterprise Oil Puckett 1 3992 Miocene 5N/15W/33 05382

19 C.R. Pohl & Associates 2331 schist 2298 5N/15W/29 05772

Pohl-Green 1

OTHER WELLS not shown in Figure 1.

Standard Oil Ferris 1 12182 granite 12073 1S/11W/21 05964

Chevron Murphy Whittier 304 14213 Sespe 14092 2S/11W/26 23652

Standard Oil Northam Station 11500 Topanga 11180 3S/11W/27 20435

 Core 1 

*Sources of stratigraphy are Brocher and others (1998) and Yerkes and Showalter (1990).
1

Table 2. Summary of physical properties of basement rocks

Crystalline rocks Density Magnetic susceptibility

 (g/cm 

3

 ) (10 

-3

 cgs units) 

All 2.71 +/-0.10 N=165 0.64 +/-1.01 N=77

Verdugo 2.71 +/-0.10 N= 32 0.81 +/-0.64 N=27

San Gabriel (South of SGF) 2.70 +/-0.08 N= 23 0.68 +/-1.02 N=15

San Gabriel (North of SGF) 2.72 +/-0.10 N= 98 0.72 +/-1.42 N=25

Santa Monica Slate 2.66 +/-0.06 N= 9 0.00 +/-0.01 N= 9

Santa Monica plutonic rocks 2.59* +/- 0.17 N= 3 0.01 +/-0.01 N= 3

Leadwell Well granodiorite? 2.65 N= 1 0.01 

*This value is likely underestimated because 2 of the 3 samples are very weathered.
Table 3. Additional wells shown in Figure 10b. 

Well Well Total Lithology Township/

ID Name Depth

 ( ft) at Bottom* Range/Section 

2-A Chevron Pacoima 2-A 10250 Modelo (directed)

05309 Barnes Core Drill Hole 1 2342 Miocene 1N/14W/16

05355 Doheny Eld-Reeves #1 4568 Upper Miocene 2N/15W/01

05522 Intex Oil Toon #1 4553 Middle Miocene 2N/15W/01

05598 H.E. Luttge No. 1 - 1700 Miocene 1N/14W/13

05714 Nadot Oil Nadot #1 3575 Pliocene (Saugus) 3N/14W/30

05841 San Val Co. Haggard #1 2100 Cretaceous 1N/16W/14

05897 Shell Oil Schonfeld #1 5162 Cretaceous 2N/16W/30

06210 Van Nuys Oil Comm. 1 3860 Miocene 1N/15W/16

06314 Shell Oil Davey Core Hole #1 1827 Cretaceous 2N/17W/25

*Sources of stratigraphy are Tsutsumi and Yeats (1999), Yerkes and Showalter (1990) and Shields (1978).
119° 118°45 ́ 118°30 ́ 118°15 ́ 1

0 10 20 30 40 50 KM

San

Fernando Valley

SAN

GABRIEL

MOUNTAINS Ventura Basin

Los 

 Angeles Basin

Verdugo

Mtns

Chatsworth

Hills

San

Gabriel

fault

Santa

Monica fault

PACIFIC OCEAN

San Andreas fault

SANTA

MONICA MTNS

Figure 1. Shaded-relief topographic map of the San Fernando Valley region. Solid circles are sTable 1). Stars highlight the 1971 and 1994 mainshock epicenters. Line A-A' is a modeled transFernando valley. Remaining black lines are faults from Jennings (1994). Teeth denote upthrowfault. Arrows indicate sense of strike-slip displacement.

5

6

8 9 10 11

15

16

17

18

1

2 3

4

19

14

12 13

7

1971

1994

A

A'

20
0 10 20 30 40 50 KM

v

v

v

v

v v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v v v

v

v

v v

v

v

v

v

v

v

v v v

v

v v v

v

v

v

v v

v

v

v

v

v

v

v

v

119° 118°45 ́ 118°30 ́ 118°15 ́ 

Figure 2. Simplified geologic map of the San Fernando Valley region. Circles are selected wells (see FiguLines A-A', C-C', O-O', T-T', and V'-V' are profiles discussed in text. CRF, Chatsworth Reservoir fault; MHTNH, Northridge Hills fault; SST, Santa Susana thrust fault; VF, Verdugo fault. Unshaded areas are Quatern

MHT

NH

A

A'

San

Fernando Valley

SAN

GABRIEL

MOUNTAINS VENTURA BASIN

Los 

 Angeles Basin

Verdugo

Mtns

Simi

Hills

San

Gabriel

fault

Santa

Monica fault

PACIFIC OCEAN

San Andreas faul

SANTA

MONICA MTNS

San Cayetano fault

Oak

Ridge fault

SST

Paleocene and Miocene sedimentary rocksTertiary volcanic rocks

Mesozoic sedimentary rocks (including Santa Monica slate)

Plutonic and metamorphicbasement rocks

CRF VF

T

T'

V

V'

Sierra

O

O'

v

ELYSIAN PARK

THRUST SYSTEM

C

C'

21
0 10 20 30 40 50 KM

v

v

v

v v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v v v

v

v

v v

v

v

v

v

v

v

v v v

v

v v v

v

v

v

v v

v

v

v

v

v

v

v

v

119° 118°45 ́ 118°30 ́ 118°15 ́ 

Figure 3. Isostatic gravity map of the San Fernando Valley. See Figure 2 for explanation of simplifieContour interval, 2 mGal. Solid circles are wells; see Figure 1 and Table 1 for more information.

San

Fernando

Valley

22
0 10 20 30 40 50 KM

v

v

v

v v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v v

v

v

v

v v

v

v

v

v

v

v

v v

v

v

v v v

v

v

v

v v

v

v

v

v

v

v

v

v

119° 118°45 ́ 118°30 ́ 118°15 ́ 

Figure 4. Locations of gravity stations plotted on simplified geologic map (see Figure 2 foof geology).

23
2.1

2.2

2.3

2.4

2.5

2.6

2.7

0 2000 4000 6000 8000 10000 12000

density, in g/cm

3

depth, in feet

well 15 

well 14

well 16

well 10 south of Mission Hills thrust fault 

Figure 5. Densities of core samples from Corbato (1963).

All core samples are Tertiary or younger sedimentary rocks. 

Note that densities from the well south of the Mission Hills 

thrust fault are lower than those measured north of the Mission 

Hills thrust.

Saugus north of Mission Hills thrust fault

or

younger

older

than

Saugus

1 2 3 km

24
1.6

1.8

2.0

2.2

2.4

2.6

2.8

0 500 1000 1500 2000 2500 3000

well 8 (Chevron Burnet)

well 7 Chevron (Frieda Clark)

density, in g/cm

3

depth, in meters

Top Sunshine Ranch

(Saugus) 332 m

Top Pico (Fernando)

960 m

Top Towsley

1393 m

Top Modelo

1804 m

Top Topanga

2771 m

Top Sunshine 

Ranch

339 m

Top Pico

807 m

Top Towsley

1113 m

Top Modelo

1418 m 

Middle Miocene

Fig. 6. Density well log information from Brocher and others (1998).

Stratigraphic calls for well 7 are shown.

25
1.8

2

2.2

2.4

2.6

2.8

3

400 600 800 1000 1200

based on 4 wells

based on well 7

density, in g/cm

3

depth, in meters

basement

Figure 7a. Densities derived from sonic velocities for

well 5 (Chevron Hazeltine). Pico (Fernando) Towsley

Modelo

Topanga

26
2.0

3.0

0 200 400 600 800 1000 1200 1400 1

based on 4 wells

based on well 7

density, in g/cm

3

depth, in meters

basement

Fig. 7b. Densities derived from sonic velocities for

well 6 (Chevron Leadwell).

2.4

2.2

1.8

2.6

2.8

Sunshine Ranch

(Saugus)

Pico (Fernando)

Towsley

Modelo

27
-310

-275

-240

-205

-170

-135

-100

 -65

 -30

 5

 40

 75

 110

 145

 180

nT

119° 118°30 ́ 118°

34°

34°30 ́

118°45 ́ 118°15 ́

34°15 ́

0 5 10 15 20 25 KM

-72

-66

-60

-54

-48

-42

-36

-30

-24

-18

-12

 -6

 0

 6

 12

mGal

119° 118°45 ́ 118°30 ́ 118°15 ́ 118°

34°

34°15 ́

34°30 ́

0 5 10 15 20 25 KM

Figure 8. Gravity (A) and aeromagnetic (B) maps with 1994 (black circles) and 1971 (red circles) 

aftershock seismicity. Some aftershocks appear to be controlled by a northwest-trending structure 

coinciding with a gravity gradient and with the southeast edge of magnetic block C. The blue "X" 

marks where the geophysical character of the Verdugo fault changes along its strike. Faults are

shown in red, blue lines are profiles discussed in text (see Fig. 2).

high-resolution

aeromagnetic

survey

(B)

(A)

28

C

X

X
-18

-13

-8

-3

GRAVITY, IN MGALS

-500

0

500

MAGNETIC ANOMALY,

IN NT

0 10 20

DISTANCE, IN KILOMETERS

10

5

0

DEPTH, IN KILOMETERS

05309



2.71

0.000

2.71

0.001

2.69

0.0015 2.42; 0.000

V V'

SW NE

Verdugo Fault 

(magnetic terrane boundary)

2.69

.0027

2.69

0.0025

2.67

0.0025

2.66

0.0025

2.64

0.0025

2.66

0.0019

2.645

0.0013

2.745

0.001

2.07

0.000

2.17

0.000

San Gabriel Fault

calculated

Verdugo fault dipping

calculated

Verdugo fault dipping

observed

observed

calculated

Verdugo fault vertical

calculated

Verdugo fault vertical

Figure 9a. Gravity and magnetic model across the Verdugo fault (profile V-V' on Figure 2). 

Numbers are densities in g/cm3 and susceptibilities in cgs units. No vertical exaggeration. 

Sources extend to 10 km. The calculated gravity curve matches the observed gravity data if 

the Verdugo fault dips to the southwest. A dipping fault also matches the gentle decrease in 

magnetic values southwest of the fault better than if the Verdugo fault is vertical. Geometries

of the San Gabriel and Sierra Madre faults are poorly constrained by the gravity data because 

of sparse gravity coverage along the profile (Fig. 4). vertical Verdugo fault

Sierra Madre fault

29
-50

-40

-30

-20

-10

Gravity,

in mGals

-200

-150

-100

-50

0

50

100

150

200

250

300

350

400

Magnetics, in

nT

-4 0 4 8 12 16

Distance, in kilometers

10

5

0

Depth, in kilometers

    



 



   

            



6 05522

2-A

(directed

06210 hole) 05355

05714

1971 San

Fernando rupture

D=2.00

 S=0

D=2.25

S=0

D=2.20

S=0

V.E.=1

last

gravity

station

Pacoima

Hills

SW NE

Saugus

gravels

Figure 9b. Gravity and aeromagnetic model across T-T' (straight line). Data points are gridded 

values and are poorly constrained at the far northeastern end. Numbers above wells refer to

wells in Tables 1 and 3.

observed

calculated

calculated

observed

D=2.25

S=0.002

D=2.69

S=0.0001

D=2.67

S=0.001

D=2.68

S=.00011

D=2.67

S=0.0018

D=2.30

S=0

30
118 45' 118 30' 118 15'

34¡30«

34 15'

34¡

0 10 20 KM

v

v v

v

v

v

v

v v v

v

v

v

v v

v

v

v

v

v

v

v

v

v

v

v v v v

2A

05841

2893-Q

2893-O

2893-P

2893-AC

2893-AD

C

C'

V

T

T'

05309

05598

05897

06314

6

5

4

3 2

05714

05355 05522

06210

7

10

8 9

12 11

13

14

16

17

15

Profile A-A'

85-9

85-11

85-8

85-12

84-202

84-205

Figure 10a. Map showing locations of additional wells and seismic-reflection profiles. Solid black

circles are wells listed in Table 1; gray circles are listed in Table 3. Dashed lines are seismicreflection profiles.

31
-40

-35

-30

-25

-20

-15

-10

-160

-140

-120

-100

-80

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Distance (km) 

10

9

8

7

6

5

4

3

2

1

0

-1

-2

    

Figure 10b. Gravity and aeromagnetic model across the northwestern margin of 

San Fernando Valley. Numbers on bodies refer to densities (D) in g/cm3 and 

magnetic susceptibilities (S) in cgs units. No vertical exaggeration.

D=2.60 D=2.30

D=2.40

Gravity (mGals) Magnetics (gammas) Depth (km) 

05897

06314

2893-P

2893-AC

D=2.00

05841

D=2.60

NW SE 2893-AD 2893-O

observed

observed

calculated

calculated

M=0.002

D=2.67

M=0.0003

D=2.40

?

?

? ?

?

?

?

?

?

Simi Hills San Fernando Valley

Cretaceous

strata

Cretaceous

strata

Santa Monica Slate/intrusions

D=2.67

D=2.60

M=.0005

32
-55

-35

-15

5

Gravity,

in milligals

-300

-200

-100

0

100

200

300

Magnetics,

in nanoteslas 

Calculated Observed

0 10 20

Distance, in kilometers

10

6

2

-2

Depth,

in kilometers

Sedimentary rocks

D=2.67

S=0.000

D=2.67

S=0.0004

D=2.67

S=0.0012 D=2.27

S=0

D=2.47

S=0

D=2.17

 S=0

D=2.70

S=0.0005

D=2.70

S=0.0005

D=2.70

S=0.005

V.E.=1

SW NE

Calculated

Observed

Figure 11. Gravity and magnetic model of Oakeshott (1975b) cross-section O-O'. Densities 

are in g/cm3;susceptibilities are in cgs units. Note mismatch between the calculated and observed 

gravity values at the southwest end of the model. Thus, the basin must be deeper to account for

the gravity low on the southwestern part of the profile. Well 11, which bottomed in Miocene, 

(Table 1) confirms this conclusion.

11

Santa Susana

thrust fault

San Gabriel fault

San Fernando

thrust

Pre-Cenozoic

crystalline

basement

O O'

San Fernando

Valley

33
7



DEPTH,

in kilometers

10

5

0 2.30 2.40

2.45

2.20

2.40 2.45

2.55

2.20

2.00 2.10

2.40

2.00

2.10

2.71

B

-60

-40

-20

0

ISOSTATIC GRAVITY,

in milligals

10 

5

0

DEPTH,

in kilometers

DISTANCE, in kilometers

1 23 4 5 6 9 11

12,13

15 17 18 19 SGF

14

     2.30 2.40

2.45

2.20

2.40 2.45

2.55

2.20

2.00 2.10

2.40

2.00

0 5 10 15 20 25 30 35 40 45 50

2.10

Soledad

San Fernando Valley Basin

Santa Monica

Mountains

Los Angeles

Basin

San Gabriel

Mountains

34

Model A without

lower-density

basement

Model A with

lower-density

basement

S N

Figure 12. Gravity models across the San Fernando basin along profile A-A'. Small crosses

on models are gravity station locations. Numbers in model are densities in g/cm3. Numbers 

above the model panel A refer to wells shown in Figure 1 and described in Table 1. MHT, 

Mission Hills thrust fault; SGF, San Gabriel fault; SMT, Santa Monica thrust fault; SST, 

Santa Susana thrust fault.

SST

SMT

MHT

A

Model B

  

2.71 2.65

(low-density basement)

2.65

2.71

2.71

SGF SGF

SST

SST

SMT

SMT
6

6.4

5.6

5.2

4.8 4.4 4.0

3.6 km/s

6.8

6

Figure 13. (A) P-velocity across A-A' from Hauksson and Haase (1997). Units are in km/s. 

Small crosses, 1994 and 1971 aftershock seismicity. All aftershocks within 2 km of A-A' were 

projected onto profile. Red arrows point to possible artifacts caused by single wavepath 

smearing. (B) P-velocity with superimposed gravity model.

1994

mainshock

S N

35

ELEVATION

in kilometers

ELEVATION,

in kilometers

DISTANCE, in kilometers

(B)

(A)
20

15

10

5

0

DEPTH,

in kilometers

DISTANCE, in kilometers

1 23 4 5 6 9 11

12,13

14

SGF

18 19

2.30 2.40

2.45

2.20

2.40 2.45

2.55

2.65

2.20

2.00 2.10

2.40

2.00

0 5 10 15 20 25 30 35 40 45 50

2.10

2.71

2.71

SST

SMT

MHT

1994 mainshock

Figure 14. Gravity model with 1971 and 1994 seismicity (small crosses) projected 

onto profile A-A'. Gray outline shows basement geometry from model B.

S N

20

15

10

5

0

DEPTH,

in kilometers

1 23 4 5 6 9 11

12,13

14

SGF

18 19

2.30 2.40

2.45

2.20

2.40 2.45

2.55

2.65

2.20

2.00 2.10

2.40

2.00

DISTANCE, in kilometers

0 5 10 15 20 25 30 35 40 45 50

2.10

2.71

2.71

SST

SMT

MHT

1994 mainshock

S N

20

15

10

5

0

DEPTH,

in kilometers

DISTANCE, in kilometers

1 23 4 5 6 9 11

12,13

14

SGF

18 19

2.30 2.40

2.45

2.20

2.40 2.45

2.55

2.65

2.20

2.00 2.10

2.40

2.00

0 5 10 15 20 25 30 35 40 45 50

2.10

2.71

2.71

SST

SMT

MHT

1994 mainshock

S N

Figure 15. Structural models for

the Northridge earthquake superimposed on gravity model.

(A) Model taken from Yeats and

Huftile (1995) where the Northridge

thrust occurred along the continuation

of the Oak Ridge thrust fault. 

(B) Model from Namson and Davis

(1994) where the Northridge earthquake occurred along a back-thrust

of the Elysian Park thrust fault. SST,

Santa Susana thrust fault. The gravity

models can be used to support either 

interpretation.

Oak Ridge blind thrust

Elysian Park thrust

SST

SST

A

B

36