Evidence for possible horizontal faulting in southern California
from earthquake mechanisms
Weishi Huang*
L. T. Silver
H. Kanamori
Seismological Laboratory, 252-21, California Institute of Technology, Pasadena, California 91125
ABSTRACT
We find that 36 of the 505 fault-plane solutions (M > 3.0,
1981–1990) in southern California have a nodal plane dipping no
more than 30&. With the assumption of the low-angle nodal planes
being the fault planes, four cross sections are constructed to show
the possible horizontal faults in the middle and upper crust. More
than half of these low-angle faults are located within or adjacent to
the Transverse Ranges. The focal depths vary from 1 km in the
southern end of the Sierra Nevada and the southwestern Mojave
Desert to 20 km in the Transverse Ranges. The slip directions are
also diverse. In general, east-west extensional movements are dominant in the boundary between the southern Sierra Nevada extending to the San Emigdio Mountains and the western Mojave Desert,
whereas north-south compressional movements are dominant in
the Transverse Ranges. In the Peninsular Ranges and the Salton
Trough, both the slip directions and focal depths vary. These features suggest that seismically active low-angle faults in southern
California may exist at different depths and slip in various directions. Our data do not support the existence of a regional-scale
seismically active detachment in southern California. Only in the
western Transverse Ranges is there some suggestion of a large detachment surface at a depth of about 13 to 14 km.
Henrys et al., 1993); the nature and age of these features are still
speculative. In this paper, we focus on the possible evidence for
low-angle faulting in southern California from earthquake focal
mechanisms.
LOW-ANGLE FAULT-PLANE SOLUTIONS
We studied fault-plane solutions of 505 M $ 3.0 earthquakes
from 1981 to 1990 and found that 7% of these events have one plane
INTRODUCTION
Southern California contains a complex system of faults that
vary in orientation, sense of movement, and scale (Anderson, 1971;
Jennings, 1977). Among these are low-angle faults that commonly
do not have surface expressions and form the concealed faults as
exemplified by the 1987 M 5 5.9 Whittier Narrows and 1994 M 5
6.7 Northridge earthquakes. It is clear that such faults should be
taken into account for seismic hazard evaluation (Davis et al., 1989).
However, because of the limited data, detailed geometries of these
types of faults are still poorly known and different interpretations for
their tectonic significance are inevitable (see Davis and Namson,
1994; Yeats and Huftile, 1995). The central controversies lie in how
these low-angle faults extend below the focal depths. Do they maintain their dip throughout the upper crust, or rotate and become
more inclined toward the horizontal plane as they extend into a
deeper part of the crust, or are they very local and individual, each
truncated by larger low-angle faults? Although the relation between
the low-angle faults and the kinematic role they play is vigorously
debated, their existence in southern California is generally accepted
by geologists and seismologists. The seismological evidence for lowangle faulting in the crust includes (1) earthquakes with nearly flat
fault-plane solutions (Hadley and Kanamori, 1978; Webb and Kanamori, 1985; Huang et al., 1993); and (2) the existence of nearly
horizontal reflectors and discontinuities as revealed by COCORP
(Consortium for Continental Reflection Profiling), deep seismic reflection data, and gravity studies (Cheadle et al., 1986; Li et al., 1992;
*Present address: Department of Geology, Duke University,
Durham, North Carolina 27708.
Geology; February 1996; v. 24; no. 2; p. 123–126; 3 figures; 1 table.
123
Figure 1. Map showing focal mechanisms of earthquakes in southern California which have nodal plane dipping no more than 30°
and are possibly associated with horizontal faulting. Events shown are M > 3.0, 1981–1990. Of 36 events, 56% are located in shaded
region, Transverse Ranges (TR). Numbers in parentheses are focal depth in kilometres. Numbers outside parentheses are event
numbers keyed to Table 1. SJF is San Jacinto fault, SSN is southern Sierra Nevada.
dipping no more than 308 (Table 1). Most of these mechanism solutions were determined with more than 20 P-wave first-motion
data. Although the detailed geometry is subject to some uncertainty,
the overall geometry with the low-angle nodal plane is constrained
well with the data from the dense Southern California Seismic Network. Unfortunately, except for large earthquakes such as the 1987
Whittier Narrows earthquake (no. 27 in Table 1), we cannot determine which of the two nodal planes is the actual fault plane. In this
report, with the inevitable assumption that the low-angle nodal
planes are the fault planes, we present these mechanism solutions as
possible evidence for low-angle faulting in the crust.
The majority of these low-angle events are located in, or adjacent to, the Transverse Ranges (shaded area in Fig. 1), where
north-south convergence is well recorded. A few are in the southern
Sierra Nevada–Tehachapi–San Emigdio Mountains region and the
western margin of the Mojave Desert. A very few are located in the
Peninsular Ranges. Two important features are displayed in Figure 1: (1) the nearly flat fault planes are distributed at different
depths and associated with diverse slip directions and (2) both reverse and normal fault events exist. There are at least three possibilities to explain these observations. First, they may reflect the
roughness of the presumably low-angle shear surfaces. The undulation of the shear surface can provide apparent thrust-fault mechanisms in some areas and normal-fault mechanisms in others. This
type of mechanism may explain the events with approximately the
same strike and at about the same depth. Possible events of this kind
124
are events 15, 24, and 34 at the depth of 6 –7 km, and events 3, 16,
and 30 at the depth of 13–14 km in the western Transverse Ranges
(Figs. 1 and 2). Second, the low-angle faulting may be associated
with folding in the basement above a basal decollement as inferred
in the central Transverse Ranges (e.g., Silver, 1991). Reverse slip
between or within layers is likely to occur at the flank of a syncline
where a local intensified compressional regime exists, whereas normal faulting is likely to occur at the hinge of an anticline where a
local enhanced extensional regime exists (Yeats, 1986). This mechanism may explain events with the same strike but at different
depths. Possible events of this kind are events 19 and 31 in the
central Transverse Ranges (Fig. 1). The third possibility is that the
low-angle faulting is associated with movement of major strike-slip
faults, being a normal fault at the divergent end and a reverse fault
at the convergent end. Normal faults have been observed in the
releasing bends (Crowell, 1974) or in the dilational jogs (Sibson,
1986), and reverse faults have been observed in the restraining
bends (Crowell, 1974) or in the antidilational jogs (Sibson, 1986) of
a strike-slip fault zone. This mechanism can explain events with
strike oblique to the general trend of a strike-slip fault. Examples of
this kind are in the San Jacinto fault zone, such as events 22, 33, and
36 (Fig. 1). In addition, in the area of complex fault geometry, both
normal and thrust faults can be caused by local block adjustments.
These possible low-angle faults are generally shallow (around
5 km) in the ‘‘tail’’ region of the Sierra Nevada and near the western
margin of the Mojave Desert (Fig. 2). The motion there is of priGEOLOGY, February 1996
Figure 2. Cross-section view of focal mechanisms and interpreted horizontal faults. Locations of these cross sections are shown
in Figure 1. They are across western (A1-A2), central (B1-B2), eastern (C1-C2), and southern Sierra Nevada, and western Mojave
desert (D1-D2). For simplicity, topography is omitted. APF—Arroyo Parida fault; BF—Banning fault; ORF—Oak Ridge fault; PMF—
Pine Mountain fault; PT—Pleito thrust; RF—Raymond fault; SAF—San Andreas fault; SCIF—Santa Cruz Island fault; SMF—Sierra
Madre fault; SYF—Santa Ynez fault; SJF is San Jacinto fault; SSNF is southern Sierra Nevada fault; NIF is Newport-Inglewood fault.
marily east-west extension (Fig. 1). In contrast, the low-angle faults
in the Transverse Ranges are deep (.10 km) (Fig. 2) and have slip
vectors indicative of a general north-south contraction (Fig. 1). The
movement directions from the fault-plane solutions in each of these
two regions are consistent with the surface geology. In the Peninsular Ranges and in the Salton Trough, complexity exists. The faultplane solutions there indicate that both slip directions and focal
depths vary. We prefer the low-angle fault planes to the high-angle
ones because the high-angle fault planes are inconsistent with the
observed surface faults, either in strike, or in slip direction, or both.
For example, in the southern Sierra Nevada and the southwestern
margin of the Mojave Desert, the high-angle fault planes such as
events 7, 8, 14, 17, and 28 strike north-northeast to northeast, rather
than north-south, which is the general strike of the surface normal
faults.
On the basis of the above argument, we conclude that seismically active low-angle faults in southern California exist at different
depths and slip in various directions. They do not necessarily correspond to the surface traces of the known active faults. Thus, the
study of surface faults alone does not provide complete seismic
GEOLOGY, February 1996
hazard information. Our data do not support the existence of a
regional-scale seismically active detachment in southern California.
Only in the western Transverse Range, at a depth of about 13 to 14
km, is there some suggestion of a large detachment surface, as
shown by a dashed curve in Figure 2. However, we cannot exclude
the possibility that larger and deeper horizontal faults slip aseismically. Jackson (1987) found that earthquakes on normal faults have
dips between 308 and 608 and concluded that large earthquakes are
unlikely to occur on low-angle normal faults in the areas where
high-angle normal faults are seismically active.
KINEMATICS
It is known that the seismogenic zone in southern California is
generally above 15 km, but locally extends down to 20 km and
deeper in the Transverse Ranges, indicating that the brittle-ductile
transition is deepest beneath the Transverse Ranges (Fig. 3). The
kinematic role of the brittle-ductile transition zone is somewhat
speculative. It is argued that the middle crust can flow over a long
geological time scale (e.g., Thatcher, 1992). The lower crust may
behave plastically or ductilely because of high temperature (e.g.,
125
Figure 3. Schematic block diagram illustrating imbricate concealed
low-angle faults beneath Transverse Ranges and adjacent areas in
southern California. Large black arrows indicate regional principal
maximum compressional stress axis, orientation of which, determined
from inversion of stress tensors (Huang, 1995), is N6° 6 11°E. Dashed
line in middle crust indicates brittle-ductile transitional zone. EF—
Elsinore fault; NIF—Newport-Inglewood fault; SJF—San Jacinto fault.
Anderson, 1971; Hadley and Kanamori, 1977; Sibson, 1984; Hearn
and Clayton, 1986), and hence is relatively weak; this accounts for
the general absence of seismicity (Chen and Molnar, 1983). The
decoupling between the brittle and ductile layers of the crust may
have displaced the San Andreas fault (Hadley and Kanamori, 1977;
Yeats, 1981). However, the transitional zone may also be thick and
the upper and lower crust are coupled strongly enough to accommodate the plate motion (e.g., Bird and Rosenstock, 1984; Humphreys and Hager, 1990; Molnar, 1992). It remains to be resolved
how much strain is accommodated seismically and how much is
accommodated aseismically.
CONCLUSION
The available earthquake data indicate the possible existence of
active horizontal faults in the upper and middle crust throughout
southern California. It is evident that these low-angle faults may
generate moderate to large earthquakes and should be further evaluated for seismic hazard. The contemporary seismic motions in
southern California apparently involve movements on the nearly
horizontal planes both at the base of the seismogenic zone and at
different depths of the crust, and strain partitioning is carried out
both vertically and horizontally as the North American and Pacific
plates move against each other. The resulting block kinematics are
thus very complex.
ACKNOWLEDGMENTS
Supported by U.S. Geological Survey grant 14-08-0001-G1774. We
thank Tom Parsons and an anonymous reviewer for constructive reviews of
the manuscript, and Don Anderson and Joann Stock for helpful discussions.
Seismological Laboratory, Division of Geological and Planetary Sciences,
California Institute of Technology contribution 5542.
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Manuscript received May 12, 1995
Revised manuscript received October 10, 1995
Manuscript accepted October 23, 1995
Printed in U.S.A.
GEOLOGY, February 1996