quake recurrence intervals. This includes the location and dip of those faults reaching the
surface and blind faults that are expressed at the surface by folding or elevated topography.
Slip rate determinations are based on several timescales. The tectonic regime of the
Miocene was generally extensional, and the north-south contractional regime came into being in
the early Pliocene with the deposition of the Fernando Formation (Wright, 1991; Yeats and
Beall, 1991; Crouch and Suppe, 1993). The longest timescale for slip-rate estimates, then, is the
time of imposition of the north-south contractional regime, the past 5 x 10
6
years. Another
timescale is the early and middle Quaternary (~ 2 x 10
6
years), the time of deposition of the
upper Pico member of the Fernando Formation plus the shallow-marine to nonmarine San Pedro
Formation. Information for the first two timescales is derived from the subsurface using oil-well
and water-well logs, multichannel seismic profiles, and surface geology. A third timescale is the
late Quaternary (10
2
-10
5
years), information for which is obtained through trench excavations,
boreholes, and high-resolution seismic profiles and ground-penetrating radar augmented by the
232-year-long record of historical seismicity in the Los Angeles area. The shortest timescale (10
yrs) is that afforded by repeated GPS observations.
The late Quaternary rate is the most representative long-term rate in forecasting future
behavior because it provides a geologically- and statistically-significant averaging time but is
unlikely to be contaminated by Pliocene and early Pleistocene geologic processes no longer
active today. Two examples illustrate this problem. (1) The post-Miocene slip rate on the Las
Cienegas blind fault was estimated as 2.1-2.3 mm/yr by Schneider et al. (1996) based on
Fernando and San Pedro growth strata, but only as 0.09-0.13 mm/yr by Ponti et al. (1996) based
on thickness changes of late Quaternary strata between the upthrown and downthrown blocks of
the Las Cienegas fault. (2) The late Quaternary displacement on the Whittier fault is almost
purely by strike slip (Rockwell et al., 1992), yet the total lateral displacement is too small to be 2
expressed in offset facies changes of members of the Miocene Puente Formation (Bjorklund and
Burke, in review).
The late Quaternary rate may be different from the rate based on GPS observations. For
example, the GPS rate across the Eastern California Shear Zone (Sauber et al., 1994; Thatcher et
al., 1999; Miller et al., 2001; Dixon et al., 2000) is considerably higher than the late Quaternary
geologic estimates. In California, similar differences between GPS and geology may occur on
the Garlock fault. In this instances, the GPS rate may not be steady state but may represent a
short-term strain transient.
This report summarizes the evidence for slip rates across faults of the Los Angeles
metropolitan region and calculates the north-south component of shortening to compare with the
convergence rates of about 4.4 mm/yr between downtown Los Angeles and the San Gabriel
Mountains based on GPS (Bawden et al., 2001). The references are largely those that summarize
recent SCEC-supported work, and they should be consulted for earlier references such as Hoots
(1931), Yerkes et al. (1965), Ziony (1985), and Wright (1991) that made important contributions
to an understanding of active faulting in Los Angeles.
Transverse Ranges Southern Boundary Fault System
Santa Monica fault
The Santa Monica fault is part of the Transverse Ranges Southern Boundary fault system,
a west-trending system of reverse, oblique-slip, and strike-slip faults that extends for more than
200 km along the southern edge of the Transverse Ranges (Dolan et al., 1997, 2000a). Other
faults in this system, included in this review, are the Hollywood and Raymond faults. The
Anacapa-Dume, Malibu Coast, Santa Cruz Island, and Santa Rosa Island faults to the west are
also part of this system, but are not included in this report.
The Santa Monica fault extends east from the coastline in Pacific Palisades through Santa
Monica and West Los Angeles and merges with the Hollywood fault at the West Beverly Hills
Lineament in Beverly Hills, west of the crossing of Santa Monica Boulevard and Wilshire
Boulevard, where its strike is northeast. The surface expression of the fault is a series of left-
stepping en échelon, south-facing scarps with an overall southward-convex map pattern.
Onshore, the fault offsets the surface 2-3.5 km south of the Santa Monica Mountains range front;
the range front itself is marked by the inner edge of the Stage 5e marine terrace (Dolan et al.,
2000a). Accordingly, the fault traverses alluvium that allows the Quaternary history of the fault
to be characterized based on geomorphology, stratigraphy, and seismic reflection characteristics
(Dolan and Pratt, 1997; Dolan et al., 2000a).
Uplift of an alluvial-fan surface north of the fault requires a reverse-slip rate of ~0.5
mm/yr (Dolan and Pratt, 1997). The inner-edge altitude of the Stage 5e marine terrace at Potrero 3
Canyon in Pacific Palisades requires an overall uplift rate of 0.6-0.7 mm/yr and a reverse-slip
rate on the fault of about 0.6 mm/yr (McGill, 1989; Dolan et al., 2000a).
A trench excavation on the grounds of the Veteran's Administration hospital at Sawtelle
(here called the VA trench), west of I-405, supplemented by a high-resolution seismic profile
(Dolan and Pratt, 1997), provided evidence for at least six surface ruptures in the past 50 ky, and
at least two and probably three events after the burial of a prominent paleosol dated as 16-17 ka
(Dolan et al., 2000a). According to these authors, a well-documented surface rupture occurred
between10 and 17 ka, although a more recent earthquake probably occurred in the vicinity of the
trench 1-3 ka. This leads to an average earthquake recurrence interval of 7-8 ky, which is much
longer than the ~1.9-3.3 ky recurrence interval for earthquakes of M
w
6.9-7.0 that would be
expected if the entire Santa Monica fault ruptured at once. The longer recurrence interval may
be explained by the Santa Monica fault rupturing along with other faults to the west (Anacapa-
Dume fault) or east (Hollywood fault), resulting in greater slip per event.
In the subsurface, the active Santa Monica fault is shown to be the youngest of several
faults, the oldest of which sustained major left-lateral strike-slip of basement rocks and Eocene
strata prior to the deposition of alluvial strata south of the range front (Yeats, 1968; Tsutsumi,
1996; Tsutsumi et al., 2001). The South strand of the Santa Monica fault underwent normal
separation in the late Miocene as documented by a thick sequence of Mohnian strata north of the
fault relative to a thinner sequence to the south. The separation changed to south side down in
the Delmontian and continued through the deposition of the Fernando Formation. The South
strand cuts strata as young as the Middle Pico Member of the Fernando Formation. Thickness
differences in the Upper Pico Member indicate that the South strand continued to be active as a
blind fault throughout the deposition of the Upper Pico (age 2.5-0.9 Ma, Tsutsumi et al., 2001).
The Quaternary San Pedro Formation shows no variation in thickness across the upward
projection of the South strand, evidence that it post-dates this strand.
The out-of-sequence North strand of the Santa Monica fault underwent all of its dip
separation of 180-200 m during and after deposition of the San Pedro Formation, or in the last ~1
my (D, Ponti in Hummon et al., 1994). If the 0.6 mm/yr dip separation rate characterizes the
entire history of the fault, then the North strand of the fault became active at about 300 ka (Dolan
et al., 2000a).
The Santa Monica fault has not yielded direct evidence for its strike-slip rate. Evidence
for left-lateral strike slip includes the left-stepping pattern of en-échelon faulting, numerous
small strike-slip faults in the VA trench (Dolan et al., 2000a), and left-lateral stream offsets on
the Malibu Coast fault north of Point Dume (Drumm, 1992; Treiman, 1994). The abrupt changes
of dip with depth: steep close to the surface, low-angle at depth (Tsutsumi et al., 2001), suggest
a major component of strike slip, possibly a flower structure, with the high-angle strike-slip fault 4
beneath the range front at depth. Treiman (1994) estimated that the strike-slip rate north of Point
Dume is currently < 0.5 mm/yr, diminished from a longer-term Quaternary rate of up to 2
mm/yr.
Santa Monica Mountains blind thrust
Davis and Namson (1994) suggested on the basis of a balanced cross section that the
Santa Monica Mountains are uplifted along a north-dipping blind thrust with a slip rate of 3.9-5.9
mm/yr over the past 2-3 my. However, Johnson et al. (1996) indicated that this blind fault has a
slip rate < 1 mm/yr based on the uplift of marine terraces along the Malibu coast. The 120-ka
terrace at Point Dume and Pacific Palisades is being uplifted at a rate of 0.1-0.2 mm/yr (Dolan et
al., 2000a). Uplift of the footwall block of the Santa Monica fault at Potrero Canyon (McGill,
1989) is taking place at a rate of < 0.2 mm/yr along the coast (Dolan et al., 2000a). Meigs et al.
(1999) show that the south flank of the Santa Monica Mountains has been uplifted over the past
several million years at an average rate of 0.5 +/- 0.4 mm/yr, and the north flank has been
uplifted at a rate of 0.24 +/- 0.1 mm/yr.
It is unclear if the Santa Monica fault and the blind thrust are the same fault, or if the two
faults represent strain partitioning. If the 0.6 mm/yr dip-slip rate is the same as that on the blind
thrust, then north-south shortening on the entire structure is 0.4 mm/yr (Dolan et al., 2000a).
Hollywood fault
The Hollywood fault extends ENE for