Download Fault systems of the 1971 San Fernando and

Fault systems of the 1971 San Fernando and 1994 Northridge
earthquakes, southern California: Relocated aftershocks and
seismic images from LARSE II
Gary S. Fuis Earthquakes Hazards Team, U.S. Geological Survey, Menlo Park, California 94025, USA
Robert W. Clayton Seismological Laboratory, California Institute of Technology, Pasadena, California 91125, USA
Paul M. Davis Department of Earth and Space Sciences, University of California, Los Angeles, California 90024, USA
Trond Ryberg Division of Physics of the Earth and Disaster Research, GeoForschungsZentrum, Potsdam D-14473, Germany
William J. Lutter Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53711, USA
David A. Okaya Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA
Egill Hauksson Seismological Laboratory, California Institute of Technology, Pasadena, California 91125, USA
Claus Prodehl Geophysical Institute, University of Karlsruhe, Karlsruhe D-76187, Germany
Janice M. Murphy Earthquake Hazards Team, U.S. Geological Survey, Menlo Park, California 94025, USA
Mark L. Benthien Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA
Shirley A. Baher 
 Department of Earth and Space Sciences, University of California, Los Angeles, California 90024, USA
Monica D. Kohler 
Kristina Thygesen Geological Institute, University of Copenhagen, Copenhagen 1350, Denmark
Gerry Simila Department of Geological Sciences, California State University, Northridge, California 91330, USA
G. Randy Keller Department of Geological Sciences, University of Texas, El Paso, Texas 79968, USA
ABSTRACT
We have constructed a composite image of the fault systems
of the M 6.7 San Fernando (1971) and Northridge (1994), California, earthquakes, using industry reflection and oil test well data in
the upper few kilometers of the crust, relocated aftershocks in the
seismogenic crust, and LARSE II (Los Angeles Region Seismic Experiment, Phase II) reflection data in the middle and lower crust.
In this image, the San Fernando fault system appears to consist of
a decollement that extends 50 km northward at a dip of ;258 from
near the surface at the Northridge Hills fault, in the northern San
Fernando Valley, to the San Andreas fault in the middle to lower
crust. It follows a prominent aseismic reflective zone below and
northward of the main-shock hypocenter. Interpreted upward
splays off this decollement include the Mission Hills and San Gabriel faults and the two main rupture planes of the San Fernando
earthquake, which appear to divide the hanging wall into shingleor wedge-like blocks. In contrast, the fault system for the Northridge earthquake appears simple, at least east of the LARSE II
transect, consisting of a fault that extends 20 km southward at a
dip of ;338 from ;7 km depth beneath the Santa Susana Mountains, where it abuts the interpreted San Fernando decollement, to
;20 km depth beneath the Santa Monica Mountains. It follows a
weak aseismic reflective zone below and southward of the mainshock hypocenter. The middle crustal reflective zone along the interpreted San Fernando decollement appears similar to a reflective
zone imaged beneath the San Gabriel Mountains along the LARSE
I transect, to the east, in that it appears to connect major reverse
or thrust faults in the Los Angeles region to the San Andreas fault.
However, it differs in having a moderate versus a gentle dip and
in containing no mid-crustal bright reflections.
Keywords: crustal structure, tectonics, earthquakes, seismic imaging,
southern California.
INTRODUCTION
The San Fernando Valley region of southern California has undergone two moderate earthquakes since 1970, causing combined damages of tens of billions of dollars. Both earthquakes presented puzzles
to the earth science community. The 1971 M 6.7 San Fernando earth-
quake was complex, with a number of proposed models of the subsurface ruptures (Hanks, 1974; Allen et al., 1975; Langston, 1978;
Heaton, 1982). The 1994 M 6.7 Northridge earthquake could be fit
with simpler rupture models (e.g., Wald et al., 1996), but there is controversy over which fault was involved, because no surface rupture
occurred (cf. Davis and Namson, 1994, with Yeats and Huftile, 1995).
In addition, the causative fault dips south, in contrast to most exposed
faults, which dip north. In an effort to better understand the tectonics
of the San Fernando Valley region, a deep-seismic imaging profile, the
Los Angeles Region Seismic Experiment, Phase II (LARSE II, line 2),
was conducted in 1998–1999 (Fig. 1).
The chief goal of LARSE is to produce seismic images of sedimentary basins and faults in the Los Angeles region to address earthquake hazards posed by these geologic features. A prior transect
(LARSE I, line 1) crossed the east-central Los Angeles region and the
San Gabriel Mountains (Fig. 1; Lutter et al., 1999). Along line 1, a
bright reflective zone was discovered in the middle crust (;20 km
depth) beneath the San Gabriel Mountains that appears to contain fluids
and to connect the San Andreas fault, by way of interpreted upward
fault splays, to compressional faults in the Los Angeles region to the
south (Ryberg and Fuis, 1998; Fuis et al., 2001b). In LARSE II, we
hoped to image the causative faults for the San Fernando and Northridge earthquakes but, instead, imaged their deep, aseismic extensions
into the middle crust. To complete an image of the upper crustal parts
of these fault systems we relocated aftershocks and used industry reflection and oil test-well data.
TECTONIC SETTING
Historic faulting in the San Fernando Valley, a Cenozoic sedimentary basin, has occurred along conjugate reverse fault systems, the
north-dipping San Fernando system (e.g., Allen et al., 1971, 1975; U.S.
Geological Survey Staff, 1971) and the south-dipping Northridge system (e.g., Hauksson et al., 1995; Wald et al., 1996) (Fig. 1). Mori et
al. (1995) suggested that the northeastern part of the Northridge aftershock zone is truncated by the southwestern part of the San Fernando
aftershock zone. Tsutsumi and Yeats (1999) suggested that the San
Fernando fault zone actually extends at depth southwestward of the
1971 surface breaks to the Northridge Hills fault, and they interpreted
the 1971 surface breaks and the Mission Hills fault as upward splays
q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; February 2003; v. 31; no. 2; p. 171–174; 5 figures, 1 insert.
171
appear best in unmigrated data. To create an image from this deeper
data, we first made an automatic line drawing using the method of
Alsdorf (1997), extended by Bauer (2001). These automatic picks were
checked by independent manual picking. Next, we migrated these lines
in a laterally varying velocity model using a method developed in this
study based on tracing rays with appropriate ray parameters (p 5 sin
u/v) for each reflector from the surface to the appropriate traveltimes
(u, dip of reflector; v, local P velocity).
The velocity model for line 2 (Fig. 3; see footnote one) was obtained from inversion of explosion traveltimes using the method of
Lutter et al. (1999). The darker colors in Figures 3 and 4 (see footnote
one) show the regions of the model with resolution values $0.4, a
threshold for acceptable resolution (Lutter et al., 1999).
To relocate the San Fernando and Northridge aftershocks, we used
P and S arrival-time data (1971 to present) from the Southern California Seismic Network (SCSN), arrival-time data from portable seismographs deployed for two months following the 1971 earthquake
(Mori et al., 1995), and P arrival-time data from 16 LARSE II shots
recorded by the SCSN. For starting models, we used Vp and Vp/Vs
models of Hauksson (2000). The arrival-time data and models were
inverted simultaneously using the method of Thurber (1993). Relocations of the LARSE II shots are within 0.3–1.0 km of actual locations.
To improve the relative spatial clustering of the aftershocks, we selected only events within 15 km of a station and relocated these using
the double-difference method of Waldhauser and Ellsworth (2000). We
obtained relative location accuracies on the order of hundreds of meters
for both the 1971 and 1994 sequences. For the epicenter of the 1971
mainshock, we chose that of Hadley and Kanamori (1978), who used
S-P travel times from strong-motion records to supplement SCSN data;
for the hypocentral depth we chose 13 km, determined by Langston
(1978) using teleseismic depth phases.
Figure 1. Shaded relief map of Los Angeles region, southern California, showing Quaternary faults (thin black lines, dotted where
buried), shotpoints (gray and orange filled circles), seismographs
(gray and orange lines), air-gun bursts (dashed yellow lines), and
epicenters of earthquakes .M 5.8 since 1933 (focal mechanisms
with attached magnitudes: 6.7a—Northridge [Hauksson et al., 1995],
6.7b—San Fernando [Heaton, 1982], 5.9—Whittier Narrows [Hauksson et al., 1988], 5.8—Sierra Madre [Hauksson, 1994], 6.3—Long
Beach [Hauksson, 1987]). Faults are labeled in red; abbreviations:
HF—Hollywood fault, MCF—Malibu Coast fault, MHF—Mission Hills
fault, NHF—Northridge Hills fault, RF—Raymond fault, SF—San Fernando surface breaks, SSF—Santa Susana fault, SMoF—Santa Monica fault, SMFZ—Sierra Madre fault zone, VF—Verdugo fault. NH is
Newhall.
from this southward extension. (For a more detailed summary of the
geology and tectonics along line 2, see Fuis et al., 2001a.)
ANALYSIS
Reflection data (;15-fold maximum) was obtained from line 2, a
stationary array, with average shot and receiver spacing (in the southern 80 km) of 1200 and 100 m, respectively (see Fuis et al., 2001a;
Murphy et al., 2002). Shallow features in the data (,5 km depth) are
best observed in a migrated common-midpoint (CMP) stack (Fig. 21).
For this image, we used poststack Kirchhoff migration. Deeper features
1Loose insert: Figure 2, Reflection data for line 2 and corresponding migrated industry reflection data, Figure 3, Cross section of line 2 and velocity
models, and Figure 4, Similar to Figure 3, with expanded depth and distance
frame.
172
RESULTS
Reflections
High-amplitude bands of energy are present in our image beneath
the San Fernando and Santa Clarita Valleys (Fig. 2). These bands result
from stacking CMP sections out to 25 km offsets and, thus, wide angles. They do not represent true reflections but a combination of energy
from rays that graze the bottoms of basins, turning in high-velocity
gradients, and first arrivals. It is not possible with well data currently
available to interpret these energy bands in terms of actual basin
depths, but they probably convey first-order basin shape. These bands
are approximately between the 5.0 and 5.5 km/s velocity contours (Fig.
3); rocks below might be crystalline rocks or possibly Mesozoic or
older sedimentary rocks (basement rocks; see Lutter et al., 1999, for
velocity and rock interpretations on line 1).
These energy bands appear to be truncated in the San Fernando
Valley near the Northridge Hills fault and in the Santa Clarita Valley
near the San Gabriel fault (Fig. 2). Industry reflection data recorded
along and near line 2 document a moderate northward dip for the
Northridge Hills fault (Fig. 2), and we have interpreted a deep northward projection of this fault along the approximate base of San Fernando aftershocks (Fig. 3; see following). Well data in the Santa Clarita
Valley indicate a steep northward dip for the San Gabriel fault ranging
from 688 to 768 in the upper couple of kilometers, and truncation of
the basin-bottom energy at ;3.5 km depth is consistent with this dip
(Fig. 2). The projected Northridge Hills and San Gabriel faults intersect
near the projected hypocenter of the San Fernando earthquake.
A prominent zone of deep reflections extends downward and
northward from the hypocenter of the San Fernando earthquake (Figs.
3D, 3F, 4, and 5), with three separate north-dipping reflectors converging downward into a complex zone that broadens downward. Reflectors
at the top of this zone dip gently northward and terminate just south
of the San Andreas fault at ;21 km depth; reflectors at the bottom of
GEOLOGY, February 2003
h i
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FAULT (MHF) (SSF) Santa
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Along Line 2
(Zelzah Ave)
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Bottom panels are migrated industry reflection data (Chevron data
donated to Southern California Earthquake Center) for two profiles
parallel to or collinear with line 2. Panels were converted to depth
using velocity log from well a (Brocher et al.,1998). Northridge Hills
fault (NHF) and Mission Hills fault (MHF), interpreted in these panels
(magenta lines), are plotted (solid) or projected (dashed) onto upper
panel. Deep projection of NHF lies along base of 1971 San Fernando
aftershocks (see Fig. 3). San Gabriel fault (SGF) is projected to depth
from well o and other nearby wells. Both fault projections appear to
approximately truncate LARSE II basin-bottom energy bands and to
intersect near projection of 1971 main-shock hypocenter. Alternative
interpretation is that MHF truncates basin-bottom energy band in
northern San Fernando Valley. [Note that apparent extension of basinbottom energy band south of SGF (blue arrow) occurs because SGF is
not orthogonal to line 2 (see Fig. 1) and receivers on line 2 jog >1 km
east here (out of page).] Santa Susana fault (SSF; short-dashed blue
line) is taken from Tsutsumi and Yeats (1999); this fault (or possibly one
below it) was one locus of seismicity west of line 2 following Northridge
earthquake. S.L. is sea level.
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Santa
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Depth (km)
Projected along strike
onto Line 2 from 2 km
east (Balboa Ave)
FIGURE 2. Upper panel is reflection data (~15-fold maximum) for part
of line 2 showing energy bands interpreted to mimic shapes of
sedimentary basin bottoms in San Fernando and Santa Clarita Valleys
(see text). ("Santa Clarita Valley" is informal name for topographic
basin along Santa Clara River in area of Newhall, Calif.; Fig. 1.) Data
have been migrated by using velocity model for line 2 (see Fig. 3).
Thick yellow-orange line segments—base of Cenozoic sedimentary
rocks based on outcrops or well data; thin yellow-orange
lines—possible base of Cenozoic sedimentary rocks based on energy
bands (see above; lines drawn at tops of bands). Wells shown are
from Stitt (1986) and Tsutsumi and Yeats (1999): (a) Chevron Frieda J.
Clark 1, (b) Chevron Woo 1, (c—e) Porter Sesnon Greenman, Orchard,
and Quinby, (f ) Union Edwards 1, (g) Chevron Mission 5-1, 5-2, 5-3, (h)
UMC O'Melveny Park 5, (i) Mobil Macson Mission 1, (j) R.S. Rocco D & C
1, (k) Mobil Circle J 1, (l) Murray Teague Thompson 1, (m) Mobil H & M
1, (n) Mobil Circle J 2, (o) Union Bermite 1, (p) Lago Vista Roland 1, (q)
E. Landgarten Lucky Lucky 4, (r) R.W. Young Walker 1. Wells bottomed
in crystalline rocks (black), Cretaceous sedimentary rocks (green),
lower Tertiary sedimentary rocks (orange), upper Tertiary
sedimentary rocks (pale orange), and Pliocene-Pleistocene
sedimentary rocks (yellow). Wells were projected onto line 2 from
less than 2.5 km distance, except well b (3.2 km).
SANTA
MONICA
FAULT
SAN GABRIEL
FAULT (SGF)
3
3
4
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N
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Time (s)
Time (s)
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M 6.7 (projected)
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cted f
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MISSION SUSANA
NORTHRIDGE
HILLS FAULT Santa
HILLS
Santa
FAULT
FAULT
Susana
Monica
San Fernando Valley
Mts
Mts
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S
NHF MHF
SGF
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K
Depth (km)
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Figure 3
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Figure 4. Similar to Fig. 3, with expanded depth and distance frame.
See caption for Fig. 3 for definition of red, magneta, and blue lines;
orange line—interpreted San Andreas fault (SAF); yellow
lines—south-dipping reflectors of Mojave Desert and northern
Transverse Ranges; "K" —reflection of Cheadle et al. (1986), which is
out of plane of this section. SAF is not imaged directly;
interpretation is based on approximate northward termination of
upper reflections (best constrained) in San Fernando reflective zone
(magenta lines). (See similar interpretation for SAF on line 1—Fig. 5.)
Wells shown in Mojave Desert are (s) H&K Exploration Co., (t) Meridian
Oil Co. (Dibblee, 1967). For well color key, see caption for Fig. 3. Thin,
dashed yellow-orange line—estimated base of Cenozoic
sedimentary rocks in Mojave Desert based on velocity. Darker,
multicolored region (above region of light violet) represents part of
velocity model where resolution ≥ 0.4 (see color bar).
FIGURE 3. Cross section along part of line 2 with superposition of
various data layers. A: Tomographic velocity model plus line drawing
extracted from reflection data (see text); heavier black lines represent
better-correlated or higher-amplitude phases. B: Velocity model
plus relocated aftershocks of 1971 San Fernando and 1994
Northridge earthquakes (brown and blue dots, respectively); main
shock focal mechanisms (far hemispheres) are red (San Fernando;
Heaton, 1982) and blue (Northridge; Hauksson et al., 1995).
Aftershocks are projected onto line 2 from up to 10 km east. C: Same
as (A) plus Northridge aftershocks and main shock. D: Same as (A)
plus San Fernando aftershocks and main shock. E: Same as (C) plus
interpreted faults (blue lines). Short-dashed blue line is Santa Susana
fault (or possibly fault beneath it) interpreted from aftershock cluster
west of line 2 (not shown). Long-dashed blue line is uninterpreted
reflection. F: Same as (D) plus interpreted faults and (or) reflectors
(solid magenta lines); short-dashed magenta line is possible out-ofplane reflection. Dashed red lines are modeled rupture planes of
main shock from Heaton (1982). In panels A—F: thick yellow-orange
line segments—base of Cenozoic sedimentary rocks based on
outcrops or well data; thin yellow-orange lines—possible base of
Cenozoic sedimentary rocks based on interpreted basin-bottom
energy bands (Fig. 2). Darker, multicolored region (above region of
light violet) represents part of velocity model where resolution ≥ 0.4
(see color bar).
In both (E) and (F), faults are interpreted as follows: below
mainshocks, from line 2 reflective zones; above mainshocks, from
aftershocks; near surface, from industry reflection and well data (see
also Fig. 2). Line 2 reflective zones project upward along bases of
both aftershock zones. For San Fernando earthquake, deep reflective
zone, base of aftershocks, and Northridge Hills fault (NHF) appear
coplanar; this plane is interpreted as decollement. Modeled rupture
planes of Heaton (1982), San Gabriel fault, and Mission Hills fault
appear to define shingle- or wedge-shaped bodies of rock that are
interpreted to slide on this decollement.
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Distance (km)
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Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California:
Relocated aftershocks and seismic images from LARSE II
Fuis et al.
Figures 2–4
Supplement to Geology, v. 31, no. 2 (February 2003)
50
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Depth (km)
5
Figure 5. Comparison of velocity models and reflectivity of LARSE lines 1 and 2, aligned along surface trace
of San Andreas fault (SAF; see Fig. 1 for locations). Line 2: Gray area—refraction coverage. Thin black lines—
velocity contours labeled in km/s; interval—0.25 km/s. Short blue lines—vertical-incidence reflections. Orange
lines—interpreted active faults and/or reflective zones (pink for Northridge faults). Yellow lines—reflections in
Mojave Desert. Line 1 (see Fuis et al., 2001b): gray area—refraction coverage. Thin black lines—velocity contours or boundaries; contour interval 0.5 km/s to 5.5 km/s and arbitrary above 5.5 km/s. Blue lines—wide-angle
reflections south of SAF. Yellow lines—wide-angle reflections in Mojave Desert. Orange lines—interpreted active faults. On both lines 1 and 2, large numbers on either side of SAF are average basement velocities (in km/
s).
this zone dip moderately and terminate just north of the San Andreas
fault at depths of 27–30 km (Figs. 4 and 5). The main zone of reflections dips ;258.
This zone of reflections is overprinted by a south-dipping zone of
reflections extending from the central Mojave Desert to just south of
the San Andreas fault (Figs. 4 and 5). One of the reflectors in this zone
correlates with reflector K of the COCORP data set, for which Cheadle
et al. (1986) determined a dip of ;258 west-southwest; this reflector
is not orthogonal to line 2 and would produce out-of-plane reflections.
We have examined particle motions for out-of-plane components to all
line 2 reflections, but the results were indeterminate.
A thin but perceptible zone of reflections extends downward and
southward from the Northridge hypocenter to ;20 km depth beneath
the Santa Monica Mountains, and an uninterpreted subparallel zone of
reflections is seen ;5 km above this zone (Figs. 3C, 3E).
Aftershocks
Aftershocks of the San Fernando earthquake are projected onto
line 2 from a zone extending 10 km east of the line (Figs. 3B, 3D).
This wide zone of projection is required, given the sparseness of welldefined hypocenters, to see the full structure of the San Fernando event.
The interpreted downward projection of the Northridge Hills fault coincides with an upward projection of the deep reflective zone, along
the approximate base of the aftershocks. Interpretation of a throughgoing decollement along these coplanar projections is supported by the
fact that thrust-fault mechanisms predominate near the base of the aftershocks in the results of Hanks (1974) and Allen et al. (1975). Note,
however, that these authors preferred a stepped rupture plane that coincides in the west with our interpretation but steps up to the east. Our
interpreted decollement is similar to the interpretation of Tsutsumi and
GEOLOGY, February 2003
Yeats (1999). The deep projection of the San Gabriel fault to the 1971
hypocenter makes it a likely structural component in the San Fernando
earthquake. When the two modeled rupture planes of Heaton (1982)
are superposed on our interpretation (Fig. 3F), they, along with the San
Gabriel fault and Mission Hills fault, appear to define shingle- or
wedge-like blocks in the hanging wall of the interpreted decollement.
Aftershocks of the Northridge earthquake are also projected onto
line 2 from 10 km east (Figs. 3B, 3C). The base of these aftershocks,
dipping ;338 south, appears coplanar with an upward projection of a
weak aseismic reflective zone below the main-shock hypocenter. One
might interpret a fault here, similar to the decollement interpreted in
the San Fernando fault system, that terminates upward at the San Fernando decollement in a fashion similar to that in the interpretation of
Mori et al. (1995). West of line 2, where the San Fernando and Northridge aftershocks do not overlap, this simple picture becomes more
complex.
DISCUSSION
Reflectivity along line 2 (and also along line 1; Fuis et al., 2001b)
is similar to reflectivity along many deep continental profiles, in that
most reflections are confined to the middle and lower crust (see Matthews and Smith, 1987). The origin of this deep reflectivity has been
debated, but most evidence favors the presence of lithostatically pressured fluid in cracks and pores (grain boundaries), under conditions of
greenschist metamorphism, in the ductile part of the crust (e.g., Hyndman and Shearer, 1989). Anisotropic mylonites, which are also produced at these conditions, are another possible origin. On line 1, fluids
appear to be responsible for the brightest reflections (Ryberg and Fuis,
1998). On line 2, we see that reflectivity along interpreted fault zones
terminates upward near the hypocenters of the 1971 and 1994 earth173
quakes (Fig. 3). One interpretation of this pattern is that fluid-induced
impedance contrasts (and hence strong reflectivity) persist upward in
fault zones to the points where these fluids are periodically released in
large earthquakes, as in the fault valve model of Sibson (1992).
Comparison to Line 1
Comparing structures interpreted on LARSE lines 1 and 2, one
sees similarities and differences (Fig. 5). On both lines, the San Andreas fault forms a rough axis of symmetry for reflective zones. Although we do not image the San Andreas fault directly, we infer its
deep projection based on truncation of deep reflections (see Fuis et al.,
2001b). On line 1 these deep reflections dip gently, whereas on line 2
they dip moderately. On both lines 1 and 2 the reflective zones that
originate at or near the San Andreas fault and extend southward are
interpreted to connect to surface reverse fault zones. On both lines, the
reflective zones north of the San Andreas fault are not clearly connected to any surface tectonic features. Thus, in our interpretation,
earthquake tectonics in the Los Angeles region involves a fundamental
link between the San Andreas fault and compressional faults to the
south.
ACKNOWLEDGMENTS
We are indebted to many government agencies, organizations, companies, and private
individuals who granted permission and, in many cases, vital assistance to LARSE II (see
Table 3 in Fuis et al., 2001a). This research was supported by the U.S. Geological Survey
(USGS Cooperative Agreement 00HQGR0076 and internal funds), the National Science
Foundation (NSF Cooperative Agreement EAR-97-25413), the Southern California Earthquake Center (SCEC, which is funded by NSF Cooperative Agreements EAR-8920136 and
USGS Cooperative Agreements 14-08-0001-A0899 and 1434-HQ-97AG01718), the Deutsche Forschungsgemeinschaft, and the GeoForschungsZentrum, Potsdam, Germany. Instruments were supplied by IRIS/ PASSCAL, University of Texas, El Paso, Geophysical Instrument Pool, Potsdam, Canadian Geological Survey, Copenhagen University, SCEC, and
USGS. This is SCEC contribution 665. Reviews by Bill Ellsworth, Rufus Catchings, and
Dave Wald substantially improved this paper, as did discussions with Jim Mechie, Tom
Hanks, and Keith Richards-Dinger.
REFERENCES CITED
Allen, C.R., Engen, G.R., Hanks, T.C., Nordquist, J.M., and Thatcher, W.R., 1971, Main
shock and larger aftershocks of the San Fernando earthquake, February 9 through
March 1, 1971, in Pecora, W.T., The San Fernando, California, earthquake of February 9, 1971: U.S. Geological Survey Professional Paper 733, p. 17–20.
Allen, C.R., Hanks, T.C., and Whitcomb, J.H., 1975, Seismological studies of the San
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Manuscript received 25 March 2002
Revised manuscript received 2 October 2002
Manuscript accepted 3 October 2002
Printed in USA
GEOLOGY, February 2003