Evidence for possible horizontal faulting in southern California from ...
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.
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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. REFERENCES CITED Anderson, D. L., 1971, The San Andreas fault: Scientific American, v. 225, p. 53– 68. Bird, P., and Rosenstock, R. W., 1984, Kinematics of present crust and mantle flow in southern California: Geological Society of America Bulletin, v. 95, p. 946–957. Cheadle, M. J., and eight others, 1986, The deep crustal structure of the Mojave Desert, California, from COCORP seismic reflection data: Tectonics, v. 5, p. 292–320. Chen, W.-P., and Molnar, P., 1983, Focal depths of intracontinental and interplate earthquakes and their implications for the thermal mechan-
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ical properties of the lithosphere: Journal of Geophysical Research, v. 88, p. 4183– 4214. Crowell, J. C., 1974, Origin of late Cenozoic basins in southern California, in Dickinson, W. R., ed., Tectonics and sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 22, p. 190–204. Davis, T. L., and Namson, J. S., 1994, A balanced cross-section of the 1994 Northridge earthquake, southern California: Nature, v. 372, p. 167–169. Davis, T. L., Namson, J., and Yerks, R. F., 1989, A cross section of the Los Angeles area: Seismically active fold and thrust belt, the 1987 Whittier Narrows earthquake and earthquake hazard: Journal of Geophysical Research, v. 94, p. 9644–9664. Hadley, D. M., and Kanamori, H., 1977, Seismic structure of the Transverse Ranges, California: Geological Society of America Bulletin, v. 88, p. 1469–1478. Hadley, D. M., and Kanamori, H., 1978, Recent seismicity in the San Fernando region and tectonics in the west-central Transverse Ranges, California: Seismological Society of America Bulletin, v. 68, p. 1449–1457. Hauksson, E., and Jones, L. M., 1989, The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern California: Seismological and tectonic analysis: Journal of Geophysical Research, v. 94, p. 9569–9589. Hearn, T. M., and Clayton, R. W., 1986, Lateral velocity variations in southern California. II. Results for the lower crust from Pn waves: Seismological Society of America Bulletin, v. 76, p. 511–520. Henrys, S. A., Levander, A. R., and Meltzer, A. S., 1993, Crustal structure of the offshore southern Santa Maria basin and Transverse Ranges, southern California, from deep seismic reflection data: Journal of Geophysical Research, v. 98, p. 8335– 8348. Huang, W., 1995, Seismic strain rates and the state of tectonic stress in the southern California region [Ph.D. thesis]: Pasadena, California Institute of Technology, 197 p. Huang, W., Silver, L. T., and Kanamori, H., 1993, Non-SAF type focal mechanisms adjacent to the SAF, Mojave segment: Implications for blind thrust beneath the San Gabriel Mountains, southern California, in Continental earthquakes (Selected Papers of II International Conference on Continental Earthquakes): Beijing, Seismological Press, p. 405– 416. Humphreys, E. D., and Hager, B. H., 1990, A kinematics model for the late Cenozoic development of southern California crust and upper mantle: Journal of Geophysical Research, v. 95, p. 19,747–19,762. Jackson, J. A., 1987, Active normal faulting and crustal extension, in Coward, M. P., et al., eds., Continental extensional tectonics, Geological Society of London Special Publication 28, p. 3–17. Jennings, C. W., 1977, Geologic map of California: Sacramento California Division of Mines and Geology, scale 1:750 000. Li, Y.-G., Henyey, T., and Silver, T. L., 1992, Aspects of the crustal structure of the western Mojave Desert, California, from seismic reflection and gravity data: Journal of Geophysical Research, v. 97, p. 8805– 8816. Molnar, P., 1992, Brace-Goetze strength profiles, the partitioning of strikeslip and thrust faulting at zones of oblique convergence, and stress-heat flow paradox of the San Andreas fault, in Evans, B., and Wong, T.-F., eds., Fault mechanics and transport properties of rocks: San Diego, California, Academy Press Ltd., p. 435– 459. Sibson, R. H., 1984, Roughness at the base of the seismogenic zone: Contributing factors: Journal of Geophysical Research, v. 89, p. 5791–5799. Sibson, R. H., 1986, Rupture interaction with fault jogs, in Das, J., et al., eds., Earthquake source mechanics: American Geophysical Union Monograph 37, (Maurice Ewing Volume 6): p. 157–167. Silver, L. T., 1991, Are the Central Transverse Ranges undergoing active basement folding as well as faulting? [abs.]: Eos (Transactions, American Geophysical Union), v. 72, p. 121. Thatcher, W., 1992, Does the mid-crust flow?: Nature, v. 358, p. 454– 455. Webb, T. H., and Kanamori, H., 1985, Earthquake focal mechanisms in the eastern Transverse Ranges and San Emigdio Mountains, southern California and evidence for a regional decollement: Seismological Society of America Bulletin, v. 75, p. 737–757. Yeats, R. S., 1981, Quaternary flake tectonics of the California Transverse Ranges: Geology, v. 9, p. 16–20. Yeats, R. S., 1986, Faulting related to folding with examples from New Zealand: Royal Society of New Zealand Bulletin, v. 24, p. 273–292. Yeats, R. S., and Huftile, G. J., 1995, The Oak Ridge fault system and the 1994 Northridge earthquake: Nature, v. 373, p. 418– 420. Manuscript received May 12, 1995 Revised manuscript received October 10, 1995 Manuscript accepted October 23, 1995
Printed in U.S.A.
GEOLOGY, February 1996
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.
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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. REFERENCES CITED Anderson, D. L., 1971, The San Andreas fault: Scientific American, v. 225, p. 53– 68. Bird, P., and Rosenstock, R. W., 1984, Kinematics of present crust and mantle flow in southern California: Geological Society of America Bulletin, v. 95, p. 946–957. Cheadle, M. J., and eight others, 1986, The deep crustal structure of the Mojave Desert, California, from COCORP seismic reflection data: Tectonics, v. 5, p. 292–320. Chen, W.-P., and Molnar, P., 1983, Focal depths of intracontinental and interplate earthquakes and their implications for the thermal mechan-
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GEOLOGY, February 1996
