ArticlePDF Available

Straightening of the Northern San Jacinto Fault, California, as Seen in the Fault-Structure Evolution of the San Jacinto Valley Stepover


We investigate a releasing step-over between the Casa Loma and Claremont strands of the northern San Jacinto fault zone to evaluate the Late Quaternary structural evolution of the fault zone, and to assess the likelihood of a rupture jumping across the step-over. Our new CPT and trench observations along the Claremont fault at Mystic Lake indicate that the main strand of the Claremont fault has jumped nearly a half kilometer westward into the San Jacinto releasing step-over during the late Quaternary. Multiple faults are inferred from the CPT data within a small sag at the NE side of the step-over that cuts through younger stratigraphy to the west of the basin bounding fault near Mystic Lake. Previous seismic reflection data also suggest the presence of a young fault that cuts basin strata beneath the middle of Mystic Lake farther west from our study area, and seismicity observations are consistent with the hypothesis that new fault strands have formed within the step-over zone. Numerous tectono-geomorphic features observed in satellite and LiDAR DEM imagery are interpreted to delineate the location of the currently active faults, as well as a zone of faults that cut across the basin from the northern end of the Casa Loma fault to the southern end of the active trace of the Claremont fault. Finally, new paleoseismic data from Mystic Lake site suggest that some late Holocene earthquakes may have jumped the step-over. All of these observations suggest that the San Jacinto step-over, which has been used as the primary basis for segmenting the northern San Jacinto fault zone, is being by-passed and that the fault zone may now be capable of larger earthquakes than previously expected.
Straightening of the Northern San Jacinto Fault, California as Seen in the
Fault-Structure Evolution of the San Jacinto Valley Stepover
by Gayatri Indah Marliyani,*Thomas K. Rockwell, Nathan W. Onderdonk, and Sally F. McGill
Abstract We investigate a releasing stepover between the Casa Loma and Clare-
mont strands of the northern San Jacinto fault zone to evaluate the Late Quaternary
structural evolution of the fault zone, and to assess the likelihood of a rupture jumping
across the stepover. Our new cone penetration test (CPT) and trench observations
along the Claremont fault at Mystic Lake indicate that the main strand of the Clare-
mont fault has jumped nearly a half kilometer westward into the San Jacinto releasing
stepover during the late Quaternary. Multiple faults are inferred from the CPT data
within a small sag at the northeast side of the stepover that cuts through younger
stratigraphy to the west of the basin-bounding fault near Mystic Lake. Previous
seismic-reflection data also suggest the presence of a young fault that cuts basin strata
beneath the middle of Mystic Lake farther west of our study area. Numerous tectono-
geomorphic features observed in satellite and LiDAR DEM imagery are interpreted to
delineate the location of the currently active faults, including a zone of faults that cut
across the basin from the northern end of the Casa Loma fault to the Claremont fault.
Seismicity observations suggest the presence of many faults within the stepover zone.
Finally, new paleoseismic data from the Mystic Lake site suggest that some late
Holocene earthquakes may have jumped the stepover. All of these observations sug-
gest that the San Jacinto stepover, which has been used as the primary basis for seg-
menting the northern San Jacinto fault zone, is being bypassed and that the fault zone
may now be capable of larger earthquakes than previously expected.
For many historical earthquakes, rupture terminations
commonly coincide with major steps, bends, or jogs in a
fault, which are then considered or interpreted as segment
boundaries (Knuepfer, 1989;Wesnousky, 2006;Sibson,
1986). These geometric discontinuities have been the subject
of intensive studies because they are one aspect of fault-zone
structure that is easily observable at the surface (Sanders and
Magistrale, 1997) and because whether or not a rupture
jumps across a stepover influences the ultimate size of the
earthquake (Wesnousky, 1994;Oglesby, 2008). The size
of a geometrical discontinuity with respect to the rupture
length may play an important role in controlling rupture ter-
mination, as shown by Wesnousky (2006) in a study of 22
historical earthquakes. In that study, Wesnousky (2006)
showed that ruptures jumped across stepover widths of less
than 34 km about 60% of the time, but did not propagate
across larger ones. In this paper we investigate a releasing
stepover between the Casa Loma and Claremont strands of
the northern San Jacinto fault (SJF) zone to evaluate the Late
Quaternary structural evolution of the fault zone and evaluate
the likelihood of a rupture jumping across the stepover.
The SJF zone is a major component of the southern San
Andreas fault (SAF) system in southern California (Fig. 1).
The cumulative movement along this fault has been of
oblique type with predominantly right-lateral motion accom-
panied by a subordinate vertical component (Sharp, 1967).
Large earthquakes are known to have occurred along this
fault, with at least 10 events of magnitude greater then
Mw6 recorded since 1890. The SJF straightness, continuity
throughout its known length, as well as its lateral strain rate,
suggest that it may be the most active member in the system
of faults in Southern California (Sharp, 1967;Sanders and
Kanamori, 1984;Ellsworth, 1990).
The SJF zone consists of several fault strands that are
separated by various discontinuities that may act as barriers
to lateral rupture propagation, and these have been interpreted
as segment boundaries (Sanders and Kanamori, 1984). Seven
segments have been identified, with segment lengths ranging
from 30 to 90 km (Working Group on California Earthquake
Probabilities, 1995,2007;Field et al.,2009). These include
(from north to south) the San Bernardino Valley, Claremont,
*Now at School of Earth and Space Exploration, Arizona State University,
ISTB4, Room 795, 781 Terrace Road, Tempe, Arizona 85287-6004.
Bulletin of the Seismological Society of America, Vol. 103, No. 3, pp. , June 2013, doi: 10.1785/0120120232
Casa Loma, Clark, Coyote Creek, Superstition Hills, and
Superstition Mountain segments (Fig. 1). At least four sig-
nificant historic earthquakes are inferred to have occurred
along the ClarkCasa Loma fault: the 1899 Christmas Day
MS6.4 (Mw7 inferred by Sanders and Kanamori, 1984) and
the 1918 Mw6.9 San Jacinto earthquakes along the northern
part of the zone (Rasmussen, 1981;Ellsworth, 1990;Doser,
1992), and the 1937 Mw5.6 Terwilliger Valley and the 1954
Mw6.3 Arroyo Salada earthquakes along the central and
southern sections of the fault, respectively (Sanders et al.,
1986;Doser, 1990). On the northernmost portion of the
SJF zone, at least two historical earthquakes were recorded:
the July 1899 and the July 1923 earthquakes. The July 1899
MI6.5 earthquake (Toppozada et al., 1981) has been sug-
gested by Thatcher et al. (1975) to have occurred on the San
Bernardino segment of the SJF near Cajon Pass. Meanwhile,
the location of the July 1923 Mw6.4 event is less certain.
Sanders and Kanamori (1984) suggest that the earthquake
occurred either on the San Bernardino segment of the SJF
near Loma Linda or on the San Bernardino Mountain seg-
ment of the SAF northeast of the San Bernardino Valley.
Active deformation on the ClarkCasa Loma fault steps
right to the Claremont fault to form the releasing San Jacinto
Valley stepover (Figs. 1and 2), which is a linear basin that
has been filled by as much as 2.5 km of Quaternary sedi-
ments (Christie-Blick and Biddle, 1985;Morton and Matti,
1993). Some workers (Sharp, 1972;Morton and Matti,
2001a,2001b) have mapped the stepover between the traces
of the Claremont and Casa Loma faults segments on the
northeast and southwest margins of the basin to be as wide
as 45 km (Fig. 2a), which according to numerical modeling
by Harris and Day (1993) and Harris et al. (1991) is large
enough to stop rupture propagation. However, other previous
studies (e.g., Park et al., 1995;Lee et al., 1996) suggest that
the stepover width is considerably less.
We present new interpretations of Light Detection and
Ranging Digital Elevation Model (LiDAR DEM) and satel-
lite imagery throughout the basin as well as new results from
cone penetration test (CPT) surveys, and analysis of seismic-
ity in the stepover zone. We first describe the current struc-
ture of the basin based on our analysis of LiDAR DEMs and
satellite imagery. We then provide a brief description of the
Mystic Lake site, which is located along the Claremont fault
on the northeastern side of the San Jacinto basin, followed by
presentation of the new CPT data collected at the Mystic
Lake site. We then combine these data with seismicity data
and previously published geological and geophysical data
(Park et al., 1995;Lee et al., 1996) to interpret the active
basin structure of the San Jacinto Valley stepover. We com-
pare our interpretation with paleoseismic data collected from
the Mystic Lake site (Onderdonk et al., 2013). Finally, we
compare our findings to models of strike-slip-basin forma-
tion to provide further insight into the structural evolution
of the San Jacinto basin and the current stage of basin devel-
opment through time.
We used a combination of field- and remotely based
mapping techniques to identify and map active faults in
the Mystic Lake site. We also evaluate the CPT and shallow
trenches data to further examine which fault strands are cur-
rently the most active, and which have mostly ceased their
Surficial Mapping from LiDAR and Air Photos
We examined the San Jacinto Valley stepover using
satellite imagery, historic aerial photography, and the high-
resolution B4 LiDAR dataset DEMs (Bevis et al., 2005),
available through to delineate surface
features that may be related to faulting. Lineaments that are
potentially fault-related were mapped directly on the imag-
ery to produce the map in Figure 2. Some of these linea-
ments can be identified as scarps, pressure ridges, deflected
streams, linear depressions, and sags that are clearly fault-
related. Others are of a more questionable origin. Because
the local surface gradient parallels the fault in much of this
Figure 1. Generalized map of the San Jacinto and southern San Andreas fault zones indicating the large scale segmentation of the fault
zones (modified from Sanders and Magistrale, 1997). Location of the paleoseismic sites referenced in the text is annotated as small squares.
Inset map shows the state of California as reference. The color version of this figure is available only in the electronic edition.
2G. I. Marliyani, T. K. Rockwell, N. W. Onderdonk, and S. F. McGill
region, it can be difficult to distinguish fault-controlled rills
or channel walls from those that are not fault-related.
Cone Penetrometer Testing Data
CPT data are generated by a truck with internally
mounted equipment that inserts a cone-tipped probe down-
ward into the ground. As the probe penetrates, it continuously
records the sleeve friction and tip-penetration resistance of
the sediments, resulting in a vertical profile of these values
that is very sensitive to changes in lithology. CPT surveys can
be applied to relatively soft, fine-grained sediments. A group
of CPT signatures can be used to identify and correlate sedi-
mentary units between adjacent holes, and these can then be
used to create subsurface cross sections and maps (e.g., Grant
et al., 1997).
We conducted CPT work along two parallel transects
across a small sag feature near the northeastern margin of
Mystic Lake and used these data to produce cross sections
of the subsurface. The first CPT line (CPT line 1) was located
parallel to and just northwest of a 1.5-m-deep locator trench
(T1) that was excavated across the sag (Fig. 3). The second
CPT transect (CPT line 2) was located approximately 250 m
to the southeast of the first transect (Fig. 3). For our study, a
total of 38 CPT holes, extending to depths of up to 30 m, were
logged along the two transects (Fig. 4).
Faults are recognized in the CPT sections from two or
more types of evidence following methods described by
Grant et al. (1997); by abrupt drop of a vertical sequence
of otherwise laterally continuous units, by abrupt changes
in unit thicknesses, or by an abrupt change in lithology. Cau-
tion was used to differentiate between faulting and facies
changes. Upwardly decreasing apparent dips of stratified
units were interpreted as a result of faulting whereas dipping
units above flat-lying units were interpreted as facies changes.
Upward termination of a fault was assumed to be overlain by
Figure 2. Classical interpretation and reinterpretation of the San Jacinto Valley stepover. (a) The conventionally accepted configuration
of the San Jacinto Valley stepover that led to the segmentation model for the Casa Loma and Claremont faults (Working Group on California
Earthquake Probabilities, 1995,2007;Sanders and Magistrale, 1997;Field et al., 2009); the fault lines are adopted from USGS Quaternary
fault and fold database; (b) annotated geomorphic features that delineate the active fault strands (scarps, pressure ridges, and lineaments)
superposed on LiDAR and Google Earth images; (c) interpretation of the primary active faults within the San Jacinto Valley stepover zone:
bold lines, interpreted as the main fault zone; thinner lines, secondary fault strands; dashed lines, lineaments with no obvious vertical or
lateral offset. The transfer zone consists of several cross-basin faults that have been identified at the northern end of Casa Loma fault in Mystic
Lake. The color version of this figure is available only in the electronic edition.
Straightening of Northern SJF as Seen in Fault-Structure Evolution of San Jacinto Valley Stepover 3
Figure 3. Map of the Mystic Lake site on 1940 aerial photo and Google Earth imagery. Dotted lines marks the CPT transect, heavy solid
lines marks the trench location, solid lines represented the interpreted fault lines. The color version of this figure is available only in the
electronic edition.
Figure 4. Detailed map of the area of the two CPT lines and trenches. Dots mark the location of each fault strand identified in the trenches.
The color version of this figure is available only in the electronic edition.
4G. I. Marliyani, T. K. Rockwell, N. W. Onderdonk, and S. F. McGill
flat-lying units with uniform thickness. For every suspected
fault zone, we constructed plots of depth versus apparent ver-
tical displacement. An increasing vertical displacement with
depth confirms the fault existence and suggests a history of
multiple ruptures.
The fault identification cannot be solely based on strati-
graphic changes because some of the features might
be caused by a combination of folding and facies changes.
In this site, outside of the suspected fault zone, changes in
thickness and lithology of most units are gradual from the
hillslope to the center of the basin. The thickest units are lo-
cated closest to the hill-slope and thinning to the center of the
basin. Given the lateral continuity of the lithologic units
across the site, it is unlikely to interpret that the vertically
aligned, abrupt stratigraphic changes were caused solely
by depositional processes. Although apparent downdropping
of units could indicate either folding or faulting, the accom-
panying changes in thickness of units are more plausibly the
result of displacement and deposition over a scarp (as a result
of faulting) than folding.
Stratigraphic correlation shows the thinnest units (unit
A, cross section 1) have minimum thicknesses of approxi-
mately 25 cm. Therefore, the smallest recognizable vertical
offset of the thin-bedded units would be approximately
12 cm. Recognition of faulting in thicker units would require
larger vertical offsets.
The locations of the CPT holes were surveyed with a
Trimble R8 differential Global Positioning Systems (GPS)
system accurate to about 2cm vertically and horizontally.
This survey provided elevation control for constructing the
cross sections.
Shallow Trenches
Along with the collection of CPT data, shallow trenches
(mostly 1.5 m in depth) were excavated to identify the near
surface, active zone of faulting (Fig. 3). T1 was excavated
prior to the CPT survey as a locator trench. T5 was excavated
across the main fault zone identified on the second CPT
transect. T6 and T7 were excavated after the CPT survey to
further examine the main fault zone identified in T1 (see On-
deronk et al., 2013). T3 and T4 were excavated across scarps
that mark the eastern edge of the sag at the base of the San
Timoteo Badlands.
Seismicity Data
To further augment our study, we analyzed the seismic-
ity of the Mystic Lake stepover region for the period from
1980 to 2005, as listed in the LSH.2.1 catalog published
by Lin et al. (2007). We constructed a map of the distribu-
tion of seismicity, along with several cross sections, using
MATLAB software. Forty-nine cross sections were con-
structed perpendicular to the strike of the Claremont and
Casa Loma faults at intervals of 0.5 km along the entire
length of the San Jacinto basin; some of the most represen-
tative cross sections are shown in Figure 5. The cross sec-
tions include earthquakes that occurred within 750 m on
either side of the 15-km-long projection planes (Fig. 5).
Interpretation of LiDAR Data and Aerial Imagery
Geomorphic interpretation using aerial imagery is a criti-
cal element in identifying recently active fault traces (Wallace,
1990). We examined satellite imagery and the high-resolution
B-4 LiDAR dataset (Bevis et al., 2005), available through, of the San Jacinto Valley stepover to
delineate surface features (e.g., lineaments) that may be re-
lated to faulting (Fig. 2). We identified several prominent
lineaments (vegetation and tonal), pressure ridges, scarps,
and differentially incised rills (suggesting old scarps) in the
aerial photographs. We field-checked some of these features
to confirm that they are related to faulting. Many geomorphic
features along the active fault strands in the San Jacinto basin
have probably been obliterated by manmade developments
and seasonal lake erosion, but many features are still well
preserved and observable. This method of imagery interpre-
tation allowed us to document many recently active and sig-
nificant rupture traces (Fig. 2b,c).
A subsurface fault interpreted by Park et al. (1995) and
Lee et al. (1996) is located below the surface trace of the Casa
Loma fault. Prominent left-stepping fault patterns are ob-
served on the western side of the basin where the principal
trace of the Casa Loma fault forms several pressure ridges. An
obvious scarp observed along the Casa Loma fault on the
southwest side of Mystic Lake (labeled as fluvially modified
fault scarp on Fig. 2b) is a fluvial modification of a fault scarp
that may have originally been located approximately 100 m
basinward where more subtle, but younger-looking scarps are
also observed. Thus, we interpret these younger scarps as the
currently active fault (Fig. 2c). The overall surface trace of the
active Casa Loma fault zone exhibits a strike of approximately
N55°W in the south and N44°W in the north.
The Claremont fault passes through the northern shore-
line of Mystic Lake and comprises three main fault strands in
this area (Fig. 3). One of the fault strands cuts through the
San Timoteo badlands and is inferred to be older and most
likely inactive, based on the fact that it deflects larger streams
by 100120 m, whereas smaller streams that entrench youn-
ger Quaternary terraces are not deflected by the fault (Onder-
donk et al., 2013). Along the western side of the basin, the
northern Casa Loma fault bends toward the Claremont fault.
According to our mapping, the present-day step between the
Casa Loma and Claremont faults occurs at the northwest end
of the San Jacinto Valley. This interpretation is consistent
with the presence of Mystic Lake (the lowest point in the
basin) at the northwest end of the valley, rather than in
the center. Given this location for the step, and our new map-
ping of the Casa Loma fault, the width of the stepover is only
about 2.25 km (Fig. 2c), considering the strike of the Clare-
mont and Casa Loma faults to the northwest and southeast of
the step, respectively. In addition, this step is characterized
by multiple N25°W-striking faults that create a transfer zone
Straightening of Northern SJF as Seen in Fault-Structure Evolution of San Jacinto Valley Stepover 5
from the northern end of the Casa Loma fault to the southern
end of the active trace of the Claremont fault (Fig. 6). These
cross-basin faults are identified at the surface by the presence
of young scarps that would facilitate through-going rupture
across the stepover. The 2.25 km width of the stepover is also
observable from the U.S. Geological Survey and California
Geological Survey (2006; see Data and Resources) Quater-
nary fault and fold database. Our mapping corroborates this
width and adds greater detail on linkage structures between
the Casa Loma and Claremont faults.
Subsurface Structure of the Mystic Lake
Paleoseismic Site
Mystic Lake is an ephemeral lake that forms in the low-
est elevations of the San Jacinto Valley pull-apart basin. It is
filled with water during extremely wet winters, so most of
our field work was completed during the summer months.
Mystic Lake is located at the northwest end of the zone
of overlap between the Claremont and Casa Loma faults
(Figs. 2and 3).
−117.15° −117.1° −117.05° −117° −116.95° −116.9° −116.85°
0 5 10 15 km
in km
Figure 5. Map of the distribution of seismicity (19812005; Lin et al., 2007) within the San Jacinto basin region. Irregular lines represent
fault locations as mapped during this study (see Fig. 2c). Depth profiles show earthquakes within 750 m on each side of the profile line
collapsed onto the section (see boxes). Depth sections show that much of the seismicity is diffuse and does not readily resolve onto discrete
surfaces, suggesting a complex geometry of fault at depth. The color version of this figure is available only in the electronic edition.
6G. I. Marliyani, T. K. Rockwell, N. W. Onderdonk, and S. F. McGill
A small releasing sag along the Claremont fault was
identified in 1940 aerial photographs along the northeastern
edge of the larger Mystic Lake (Figs. 3and 4). We excavated
a1:5m deep and 400 m long locator trench (T1) across
this sag (along with several shorter trenches) during our first
field season at Mystic Lake in 2009 to locate the faults within
the sag. In general, the layers are more coarsely grained on
the northeastern side of the sag where sand and silty sand
dominates and increases in clay content to the southwest.
The trench exposed a 50 m wide fault zone near the south-
west end. This fault zone consists of numerous faults that
offset stratigraphy composed of sand, silt, and clay, with sev-
eral paleosols (Fig. 7). One minor, recently active fault was
recognized near the northeast end of the trench. The stratig-
raphy was continuous enough to be traced along the full
length of T1 which allows documentation of the structure
of the basin and correlate faulting events from one fault zone
to another.
Two other mapped traces of the Claremont fault were
trenched (T3 and T4) and found to cut Early Holocene
sediments to produce substantial vertical separation (a few
meters), but the amount of lateral slip, if any, as well as the
presence of late Holocene activity remain unclear (see On-
derdonk et al., 2013).
During the second field season in 2010, we sited our first
CPT transect along the northern side of T1 and a second CPT
transect approximately 500 m to the southeast. Three addi-
tional shallow trenches (T5, T6, and T7) were also excavated
(Figs. 3and 4).
Two cross sections were constructed from the CPT data
(Figs. 8and 9), based on the correlation of distinctive units
identified from each CPT hole. The youngest stratum at the
top is labeled as unit A in cross section 1 and a in cross sec-
tion 2, whereas the oldest identified stratum is labeled as unit
Y in cross section 1 and unit u in cross section 2. We could
not confidently correlate units between the cross sections
due to the large distance between CPT line 1 and 2. At least
eleven and six faults were recognized in cross sections 1 and
2, respectively.
The shallow trench projection in CPT holes is indicated
on Figure 7. At the shallow depth of Trench 1, the CPT data
in line 1 correspond closely with trench observations to
within 10 cm resolution (Fig. 10). Specifically, unit J of the
CPT transect correlates to unit 800 (radiocarbon age:
1700 rcy B.P.) identified at the bottom of Trench 1 at a
depth of 1.4 m (Fig. 10). This correlation is based on both
depth and lithology. Based on the CPT correlations, unit J
reaches a maximum depth of 9m on the downthrown side
of the fault zone, yielding a late Holocene sedimentation
rate of more than 5mm=yr in the sag. This rate is similar
to the 35:6mm=yr sedimentation rate estimated for the
San Jacinto basin (Morton, 1977). If this rate is representa-
tive of the entire sampled section, the oldest units penetrated
by our CPT line are probably Early Holocene in age, on the
order of 7000 years.
The CPT correlations, along with direct observations
from the shallow trench exposure, demonstrate that the main,
currently active strand of the Claremont fault comprises
a zone of faulting that is tens of meters in width (Figs. 4
and 8). Most major faults identified at depth in the CPT lines
were also observed in the trenches, with the interpreted main
fault in Trench 1 being the same as Fault 1E in CPT profile 1
(Fig. 8). Away from the main zone of faulting, there are two
faults located between CPT 32 and 33 (faults 1A and 1B;
Fig. 8). These faults are located approximately 100 m to
the west of the older, inactive strand of the Claremont fault
identified in T3, and 300 m east of the currently active main
trace identified in T1 (Fig. 4). Fault 1A is apparently inactive
as it appears to only offset units M and deeper (Fig. 8), and a
fault was not observed at this location in Trench 1; Fault 1B
likely corresponds to a fault we observed in Trench 1, be-
tween holes CPT 34 and CPT 38.
We estimated the vertical separation of every distinctive
layer across each fault in the CPT profiles, as illustrated in
Figures 8and 9. The measurements confirm the general trend
of increasing displacement with depth for the main fault
zone, an observation that is consistent with multiple rupture
events. However, some faults to the west of the main fault
(e.g., faults 1F and 1I) have similar separation at depth from
unit M (about 10 m) downward, which suggests that they
initiated motion in the middle Holocene (about 4 ka, if the
sedimentation rate has been constant), and are therefore
younger than faults to the east. Thus, the CPT observations
Figure 6. Enlarged images from one part of the transfer zone,
northeast of Mystic Lake (location indicated in Fig. 2c). (a) Anno-
tated geomorphic features, including a field-checked fault scarp,
that delineate the active fault strands superposed on Google Earth
images; (b) our interpretation of the active faults. The color version
of this figure is available only in the electronic edition.
Straightening of Northern SJF as Seen in Fault-Structure Evolution of San Jacinto Valley Stepover 7
suggest that the faults along the northeast side of Mystic
Lake may decrease in age towards the center of the basin,
which would suggest that the width of the stepover is de-
creasing. The older trace of the Claremont fault, which has
sustained only minor Holocene lateral slip based on the geo-
morphology and trench evidence (T3 and T4; Onderdonk
et al., 2013), is nearly 0.5 km to the northeast of the currently
main strand (Fig. 3). These observations all indicate that the
primary activity of the Claremont fault has jumped towards
the basin in the late Quaternary.
Seismicity Observations
We found it difficult to correlate the surficial faults with
the subsurface alignments of microseismicity in this area.
Although some vertical cross sections show that the seismic-
ity delineates a near-vertical plane that extends to depths
of 1015 km (cross section E, Fig. 5), in most places the
distribution is fairly broad and diffuse (Fig. 5). In some
places there is better correlation between active surface struc-
ture and seismicity at depth, such as in cross section CC0,
where several fault strands at the surface coincide with the
surface-projection of several steeply dipping faults at around
1015 km depth. A cluster of seismicity appears to occur
southeast of the southern tip of the trace of Claremont fault.
However at depth, as observed from profile DD0(Fig. 5),
we cannot confidently determine the dip of the fault nor
unambiguously correlate the seismicity with surface faults.
Although it appears that there is a very weak northeast-dip-
ping alignment of seismicity that may project to the surface
at the Casa Loma fault (Fig. 5, cross section DD0), there is
also an indication that the seismic activity is occurring at
depth along the Claremont fault. Areas along the fault zone
with few earthquakes, such as the area between cross sec-
tions D and E, might suggest that the faults here are seismi-
cally locked or inactive, although a longer term of observation
is needed to prove it.
The field- and remotely-based mapping, CPT, and shal-
low trench data all indicate that the active deformation at the
San Jacinto basin is now closer to the center of the basin. The
data also show that the mapped fault traces at the southeast-
extension of the Claremont fault are currently inactive and
the primary activity of the Claremont fault has jumped
Figure 7. (a) Photomosaic of the main fault zone on the southeast wall of Trench 1. Grid lines are spaced 1 m horizontally and 0.5 m
vertically. Faults are traced for clarity. (b) Interpretation redrawn from photologs. Events horizons are marked as bold lines, lines with star
denoting event number. Darker gray shades indicate paleosol layers and lighter shades indicate distinct clay layers that typically overlie event
horizons (modified from Onderdonk et al., 2013). The color version of this figure is available only in the electronic edition.
8G. I. Marliyani, T. K. Rockwell, N. W. Onderdonk, and S. F. McGill
towards the basin by 0:5km. We compare our data with
seismicity, published subsurface and regional paleoseismic
data, and physical model of a pull-apart basin. We further
assess the implication of the current structure configuration
to the rupture pattern along the SJF.
Comparison of Surface Mapping with Seismicity
The seismicity data poorly correspond with the mapped
active traces at the surface (see Fig. 5, cross section DD0).
Disassociation between mapped surface traces of the fault
and the seismicity could be indicative of complex geometry
at depth. We interpret down-dip fault segmentation as pos-
sible explanation for complexity of seismicity at depth as de-
scribed by Nemser and Cowan (2009).Nemser and Cowan
(2009), based on analyses of cross sections of seismicity at
the southern SJF zone, reveal that the down-dip termination
of clusters of shallow earthquakes tends to roughly coincide
with the up-dip termination of deeper earthquakes. This may
indicate down-dip fault segmentation, which is also evident
in outcrop-scale observations of the southern SJF zone.
In the San Jacinto Valley stepover area, the microseismic-
ity is fairly diffuse. We interpret the diffuse microseismicity
distribution as evidence that the ClaremontCasa Loma fault
stepover comprises a complex broad band of right-lateral
shear, rather than a single fault at depth.
Although diffuse microseismicity can be due to error
in earthquake location, the seismicity used here has been
relocated (Lin et al., 2007), so its diffusivity is more likely
reflecting the presence of many faults within the stepover
zone. This is consistent with field observation, and with the
hypothesis that new intrabasin faults have formed that are
accommodating the straightening of the northern SJF.How-
ever, the relocated catalog of Lin et al. (2007) only includes
a few decades of data, making it inadequate for rigorous in-
Comparison of Surface Mapping with Published
Subsurface Data in the Basin
Park et al. (1995) proposed that the San Jacinto Valley
pull-apart basin is not a simple rhombochasm, based on its
unusual widthlength ratio and on geophysical studies that
suggest a more complex structure. During their study, three
high-resolution shallow seismic-reflection lines were acquired
in the northern part of the San Jacinto graben revealing struc-
tures in the upper (5001000 m) part of the basin. The deeper
structures were mapped using gravity data. They identified at
least one major intragraben structure, which they named the
Farm Road Strand (Figs. 3and 5). This fault is inferred to run
parallel to and lie in between the Casa Loma and Claremont
strands, and it approximately coincides with an alignment of
relocated earthquakes of Sanders and Magistrale (1997).
Figure 8. (a) Cross section 1 constructed from compilation of CPT data along transect 1. Note that the vertical scale is exaggerated
(vertical exaggeration, 1.8), and the horizontal scale is compressed in the middle of the transect. (b) The measured values of vertical
separation of each distinctive unit across each of the identified faults. The color version of this figure is available only in the electronic
Straightening of Northern SJF as Seen in Fault-Structure Evolution of San Jacinto Valley Stepover 9
The gravity model constructed by Park et al. (1995)
shows that the Farm Road Strand, rather than the Casa Loma
strand, is the western boundary of the deepest part of the sedi-
mentary basin in the northern San Jacinto Valley (Fig. 11). The
basement forms a median step block bounding the San Jacinto
graben between the Casa Loma fault to the west and the Farm
Road fault. In the mid-basin area, the basement is shallow
(400800 m) to the west of the Farm Road fault and steps
down steeply to a depth of 2.5 km to the east of the Farm
Road fault.
These observations led Park et al. (1995) to conclude
that the San Jacinto graben is better represented as several
offset depocenters bounded by stepping faults, rather than
as a single basin with an unusually long overlap ratio. In their
model, the series of smaller basins are best represented with
average lengthwidth ratios (43). According to some pre-
vious models of pull-apart-basin evolution (Aydin and Nur,
1982;Mann et al., 1983;Hempton and Neher, 1986), this pat-
tern of offset sedimentary basins resembles the model of coa-
lescing en echelon basins (Fig. 12).
Seismic refraction and reflection studies by Lee et al.
(1996) in the central San Jacinto basin have identified at least
three faults within the basin in addition to the Casa Loma and
Claremont faults. One of the fault zones, Fault B (Fig. 5),
consists of several minor faults that occur only in the older
stratigraphy; they are spaced about 4080 m apart and have
no observed surface expression. Thus, these faults have never
been reported in the literature prior to the work of Lee et al.
(1996). A second fault zone was named Fault D and lies near
the presently mapped trace of the Casa Loma Fault. A third
Figure 9. (a) Cross section 2 constructed from correlation of distinctive units identified from each CPT log along CPT transect 2. As with
cross section 1, the vertical scale is exaggerated (vertical exaggeration, 3.8). (b) Measured values of vertical separation of each distinctive unit
across each of the identified faults in transect 2. The color version of this figure is available only in the electronic edition.
Figure 10. Enlargement of portions of CPT transect 1 (near the
CPT-4 hole) and Trench 1 showing that unit J on CPT transect 1
correlates to unit 800 identified at the bottom of Trench 1, at a depth
of 1.4 m. The color version of this figure is available only in the
electronic edition.
10 G. I. Marliyani, T. K. Rockwell, N. W. Onderdonk, and S. F. McGill
fault, Fault E, is located at the southern end of their seismic
lines, but according to Lee et al. (1996), its presence is
not certain because the reflector disruptions are possibly
due to interferences of basement relief, passing traffic, or
activity associated with a local store that may have affected
data quality.
The Farm Road fault is approximately 1 km from the
Casa Loma strand and 2 km from the Claremont strand
(Fig. 11) and was considered by Park et al. (1995) to be the
currently active basin-bounding fault on the west side of the
basin. However, based on the presence of scarps (Fig. 2b),
we interpret the currently most active strands to be within
the basin itself, crosscutting the earlier faults, which may
include the northernmost Casa Loma and Farm Road faults.
From our CPT study at Mystic Lake, the main currently
active fault on the northeast side of the basin is Fault 1E
(Fig. 8), which coincides with the main fault observed in
Trench 1 and is located 0.5 km to the west of the older strand
of the Claremont fault that marks the front of the Timoteo
Badlands (Fig. 3). These observations and interpretations
all suggest that the width of the fault step in this area is only
a couple of kilometers and that there are cross-basin linkage-
structures that have evolved to transfer slip. We must note
though that the fault arrangement indicated with bold lines
in Figure 2c is an interpretation of the main fault structure
and does not include all the active-appearing faults in
the area.
Comparison of Shallow Trenches with Regional
Paleoseismic Data
Recent paleoseismic work at Mystic Lake has docu-
mented evidence of seven events over the past 1600 years
in the uppermost 2.5 m of the section. The paleoseismolog-
ical aspect of the site was described in detail in Onderdonk
et al. (2013). We compared the paleoseismic record from
Mystic Lake with ruptures documented at the Hog Lake pa-
leoseismic site, which is located 50 km to the south along the
central SJF. These two sites are separated by the San Jacinto
Valley stepover, so if the documented ruptures on both faults
are similar in ages, this may indicate that some of the large
earthquakes have jumped across the stepover.
The age ranges for four of the past seven most recent
events at Mystic Lake (Onderdonk et al., 2013) overlap in
time with the possible ages of ruptures documented at the
Hog Lake paleoseismic site (Rockwell et al., 2000,2006).
This observation suggests that some large San Jacinto events
may jump across the San Jacinto Valley releasing stepover, or
that stress triggering along one segment causes the other to
fail in close succession (Onderdonk et al., 2013).
The strand of the Claremont fault that marks the front of
the badlands and exhibits the most dramatic scarps has sus-
tained only minor Holocene strike-slip activity, based on
trench evidence (T3 and T4; Onderdonk et al., 2013), and
is nearly 0.5 km to the northeast of the main strand identified
in trenches and CPT lines. These observations all indicate
that the primary activity of the Claremont fault has jumped
towards the basin by this amount in the late Quaternary.
Figure 11. Bouguer gravity anomaly contour map of the
northern part of the San Jacinto basin (after Park et al., 1995) with
the faults identified during this study superposed on the gravity
data. Note that the faults we identified from surface scarps crosscut
the deep basin interpreted from the gravity data, suggesting that the
deep basin is no longer the main depocenter. Location of the map is
shown in Figure 2c. Lines with number attached, Bouguer gravity
contour lines; lines with no number attached, fault lines; refer to
Figure 2caption for fault symbology. The color version of this fig-
ure is available only in the electronic edition.
Figure 12. Scale independent model of a pull-apart basin cre-
ated by coalescing of rhombohedral grabens associated with en ech-
elon dextral strike-slip faults (Aydin and Nur, 1982; cited in Park
et al., 1995).
Straightening of Northern SJF as Seen in Fault-Structure Evolution of San Jacinto Valley Stepover 11
Comparison to Models of the Evolution of
Extensional Jogs and Pull-Apart Basins and
Slip-Transfer Mechanisms
Physical (e.g., sandbox or clay) models of pull-apart
basins can provide insights into strike-slip basin evolution
by simplifying their geometry and rheology (e.g., Chinnery,
1966;Tchalenko, 1970;Rodgers, 1980;Segall and Pollard,
1980;Naylor et al., 1986;Zachariasen and Sieh, 1995;Rahe
et al., 1998).
In their sand analog model, Rahe et al. (1998) used un-
equal motion on crustal blocks on opposite sides of a strike-
slip fault using dry sand above a horizontal detachment
horizon. They described the structural evolution of a pull-
apart basin in terms of incipient, early, and mature develop-
mental stages (Fig. 13). The incipient pull-apart basin begins
by formation of steeply dipping normal faults approximately
parallel to the step-angle (in their model set as 40°) of the
pure strike-slip section of the horizontal detachment horizon
(Fig. 13a). This is followed by the development of Riedel
shears above the strike-slip regions (labeled as R in Fig. 13a).
Widening across the pull-apart basin is accommodated by
formation of additional normal faults parallel to the outer
boundary faults of the actively forming graben or half-
graben. The incipient stage of pull-apart basin development
ends as cross-basin strike-slip faults form diagonally across
the pull-apart basin (Fig. 13b). Progressive strike-slip even-
tually leads to the development of cross-basin faults that shift
deformation toward the center of the basin and result in the
linkage of strike-slip-fault segments to produce a through-
going fault (Fig. 13c). Finally, the mature stage of pull-apart
basin development is defined by the presence of a through-
going strike-slip fault, created by the coalescing of appropri-
ately oriented earlier-formed segments of the system (Fig. 13d).
The models presented by Rahe et al. (1998; Fig. 13d),
with their resulting deformation, most closely resemble the
configuration of the San Jacinto basin that we interpret from
the combination of geomorphic, trench, and CPT observa-
tions. The presence of a transfer zone consisting of several
cross-basin faults at the northern end of the Casa Loma fault,
as identified from the LiDAR DEMs and aerial imagery, in-
dicates that the basin is in such a mature stage of develop-
ment as modeled by Rahe et al. (1998). Strong similarities
with the San Jacinto basin include the presence of cross-
basin faults, and a mid-basin ridge (HG structure in the phys-
ical model, Fig. 13d), indicated by water ponding against the
scarp of the main fault in T1 as observed in the 1940 aerial
photographs (Fig. 3). As shown in the sandbox-analog mod-
els, during the final stage of pull-apart basin development,
Figure 13. Line drawings illustrating development of a pull-apart basin in a sand-analog model developed by Rahe et al. (1998). (a) For-
mation of normal faults bounding the pull-apart basin during the incipient stage of basin development, (b) development of relay ramps and a
cross-basin fault, (c) through-going cross-basin fault early in the mature stage of development, and (d) fully mature pull-apart basin with
complex array of normal, strike-slip, and oblique faults; gray shading, zone of subsidence. Note the presence of through-going cross-basin
faults that define the mature stage of basin development, similar to what we observe for the San Jacinto basin.
12 G. I. Marliyani, T. K. Rockwell, N. W. Onderdonk, and S. F. McGill
displacement on the normal bounding faults slows or stops.
As a result, the cross-basin faults in the interior of the pull-
apart basin experience increasing rates and larger amounts of
normal slip. The faults merge and coalesce, and cross-basin
faults transect the full length of the pull-apart basin, linking
the two main regions of strike-slip displacement (Figs. 2c,3,
and 13d). The linkage of the main strike-slip displacement
zones in the mature stage would dramatically increase the
overall area of slippage in a single earthquake and thus the
potential for larger magnitude earthquakes.
The cross-basin faults could transfer slip through the San
Jacinto Valley stepover during large earthquakes, although slip
transfer across a stepover is not required for a step width of
only 2km (as modeled by Harris and Day, 1993 and Harris
et al.,1991).
Implications for Rupture Patterns along the SJF
Based on our interpretation of the current configura-
tion of the main structural elements of the SJF zone in the
vicinity of the San Jacinto Valley stepover, it appears that
the stepover width is now about 2.25 km. This is consider-
ably less than the 45 km proposed by Sanders and Magis-
trale (1997), who suggested that the stepover is large enough
to stop large earthquake ruptures; this was the primary basis
for segmenting the northern SJF into the Casa Loma and
Claremont faults (Sharp, 1967,1972,1975;Matti et al., 1985,
1992;Wesnousky, 1986;Sanders and Magistrale, 1997).
Farther south along the SJF, geological mapping by
Sharp (1967) characterized the Casa Loma and Clark faults
as separate segments. Other worker have considered these
two fault elements as one continuous segment (Wesnousky,
1986), as the two strands are essentially collinear. Moreover,
the presence of scarps in the LiDAR DEMs that delineate a
continuous zone of faulting support the interpretation that
these are essentially a single fault zone. Further, recent ob-
servations of slip in the most recent large earthquakes on
the Clark fault (Salisbury et al., 2012) also indicate that it
is likely that Clark fault ruptures may have extended consid-
erably farther north than Hemet onto the Casa Loma fault.
Substantial slip is documented for each of the past several
large earthquake ruptures on the south side of Hemet where
the fault passes into deep alluvium out of Blackburn Canyon
(with the exception of the inferred 1918 rupture, which was a
relatively small event; Salisbury et al., 2012).
From the above discussion, it appears plausible that rup-
ture of the entire ClarkCasa LomaClaremont fault zone, a
distance of nearly 200 km, could potentially occur and result
in an earthquake as large as Mw7.6, rivaling the size of
expected earthquakes on the SAF. If such an earthquake were
to occur, it would be sufficiently large to cause very strong
shaking, and potentially substantial damage to the Riverside
and San Bernardino areas in southern California. Further-
more, long ruptures may generate a substantial component
of their energy in the long-period spectrum, which may
excite the deep basins of southern California, including
the densely populated areas of Los Angeles and Orange
counties. Expected effects would include significant primary
surface faulting along the trace of the ClarkCasa Loma
Claremont fault, landslides within the Santa Rosa, San
Jacinto, and San Bernardino Mountain areas and other areas
with steep terrain, and possibly lateral spreading induced by
liquefaction in basins and river valleys where susceptible
conditions predominate.
We have presented new geological CPT data that show
that the primary active faults of the Mystic Lake stepover on
the northeastern side of the San Jacinto basin lie within Mys-
tic Lake itself, with the currently active fault located nearly
0.5 km to the west of the older strand of the Claremont fault
that marks the front of the Timoteo Badlands. The CPT ob-
servations also suggest that the faults within Mystic Lake are
apparently more active and are possibly younger towards the
center of the basin. The 1E fault, which accommodated the
most dip-slip separation, is currently the primary strand of
the Claremont fault. Its large vertical offset and its continuity
indicate that it is probably the most mature of the observed
intrabasin faults. By contrast, the eastern splays (fault 1A and
1B), although we have no quantitative measure of their age,
appear significantly less developed and fault 1A appears to
be inactive.
We have also presented new mapping of the location of
the currently active faults along the ClarkCasa Loma trend,
and within Mystic Lake itself, from the interpretation of sat-
ellite imagery and LiDAR DEMs. We identify a transfer zone
from the northern end of the Casa Loma fault to the southern
end of the active trace of the Claremont fault that appears to
crosscut the inferred location of the Farm Road fault. This
zone is characterized by multiple N25°W-striking strands in-
terpreted from the presence of scarps and other lineaments;
these appear to be the dominant faults in the late Quaternary
to have accommodated slip across the basin. Collectively,
these new observations, combined with previous work, all
argue that the San Jacinto basin stepover has evolved towards
a straighter trace and may now be small enough to allow rup-
tures to pass through, thereby dramatically increasing the
plausible size of earthquakes on the SJF.
Data and Resources
The B4 LiDAR dataset DEMs used in this research was
acquired in May 2005 as a pre-earthquake survey of the SAF
and SJF zones in southern California. For more information
about the B4 project, please visit http://www.earthsciences (last accessed January
2013). The B4 LiDAR dataset is available in many formats
via (last accessed January
2013) for neotectonic and paleoseismic research. The
Quaternary fault and fold database was obtained from the
U.S. Geological Survey and California Geological Survey
Straightening of Northern SJF as Seen in Fault-Structure Evolution of San Jacinto Valley Stepover 13
accessed through
(last accessed February 2013). The MATLAB software was
used to construct the seismicity distribution map and cross
We would like to thank a number of San Diego State University
(SDSU), California State University of San Bernardino (CSUSB) and Califor-
nia State University (CSU) students, Southern California Earthquake Center
(SCEC) interns, and visiting researchers that assisted with fieldwork during
this study and made this possible. These include Rebecca Tsang, Nissa Mor-
ton, Barrett Salisbury, Mike Buga, Katie Farrington, Eulalia Masana, Neta
Wechsler, Hurien Helmi, Mark Swift, Brian Anderson, Karina Chung, John
Duncan, Ramon Hancock, Scott Kenyon, and Blaise Delgado. We thank
Shuo Ma for discussion on the seismicity data analysis. We also thank Eliza
Nemser, Ivan Wong, and an anonymous reviewer for their excellent and
thorough reviews. This research was supported by funding from the SCEC
and a National Science Foundation (NSF) Grant (G00008274). SCEC is
funded by NSF Cooperative Agreement EAR-0106924 and U.S. Geological
Survey (USGS) Cooperative Agreement 02HQAG0008. The SCEC Contribu-
tion Number for this paper is 1628.
Aydin, A., and A. Nur (1982). Evolution of pull-apart basins and their scale
independence, Tectonics 1, 91105.
Bevis, M., K. Hudnut, R. Sanchez, C. Toth, D. Grejner-Brzezinska, E.
Kendrick, D. Caccamise, D. Raleigh, H. Zhou, S. Shan, W. Shindle,
A. Yong, J. Harvey, A. Borsa, F. Ayoub, R. Shrestha, B. Carter, M.
Sartori, D. Phillips, and F. Coloma (2005). The B4 Project: Scanning
the San Andreas and San Jacinto fault zones (abstract H34B-01), Abstr.
Programs AGU, H34B-01.
Chinnery, M. A. (1966). Secondary faulting, Can. J. Earth Sci. 3, 163190.
Christie-Blick, N., and K. Biddle (1985). Deformation and basin formation
along strike-slip faults, in Strike-Slip Deformation, Basin Formation,
and Sedimentation, N. Christie-Blick and K. Biddle (Editors), Vol. 37,
Special PublicationSociety of Economic Paleontologists and Min-
eralogists, Tulsa, OK, 134.
Doser, D. I. (1990). Source characteristics of earthquakes along the southern
San Jacinto and Imperial fault zones (1937 to 1954), Bull. Seismol.
Soc. Am. 80, 10991117.
Doser, D. I. (1992). Historic earthquakes (1918 to 1923) and an assessment
of source parameters along the San Jacinto fault system, Bull. Seismol.
Soc. Am. 70, 185201.
Ellsworth, W. L. (1990). Earthquake history, 17691989, in The San An-
dreas Fault System, California, R. E. Wallace (Editor), U.S. Geol. Surv.
Profess. Pap. 1515, 153187.
Field, E. H., T. E. Dawson, K. R. Felzer, A. D. Frankel, V. Gupta, T. H.
Jordan, T. Parsons, M. D. Petersen, R. S. Stein, R. J. Weldon II,
and C. J. Wills (2009). The uniform California earthquake rupture
forecast, version 2 (UCERF 2), Bull. Seismol. Soc. Am. 99, no. 4,
20532107, doi: 10.1785/0120080049.
Grant, L. B., J. T. Waggoner, T. K. Rockwell, and C. von Stein (1997).
Paleoseismicity of the north branch of the NewportInglewood fault
in Huntington Beach, California, Bull. Seismol. Soc. Am. 87, 277293.
Harris, R. A., and S. M. Day (1993). Dynamics of fault interaction: Parallel
strike-slip faults, J. Geophys. Res. 98, 44614472.
Harris, R. A., R. J. Archuleta, and S. M. Day (1991). Fault steps and the
dynamic rupture process: 2D numerical simulations of a spontaneously
propagating shear fracture, Geophys. Res. Lett. 18, 893896.
Hempton, M., and K. Neher (1986). Experimental fracture, strain and sub-
sidence patterns over en echelon strike-slip faults: Implications for the
structural evolution of pull-apart basins, J. Struct. Geol. 8, 597605.
Knuepfer, P. L. K. (1989). Implications of the characteristics of end-points of
historical surface fault ruptures for the nature of fault segmentation,
U.S. Geol. Surv. Open-File Rept. 89-315, 193228.
Lee, T. C., S. Biehler, S. K. Park, and W. J. Stephenson (1996). A seismic
refraction and reflection study across the central San Jacinto Basin,
southern California, Geophysics 61, 12581268.
Lin, G., P. M. Shearer, and E. Hauksson (2007). Applying a three-
dimensional velocity model, waveform cross correlation, and cluster
analysis to locate southern California seismicity from 1981 to 2005,
J. Geophys. Res. 112, no. B12309, doi: 10.1029/2007JB004986.
Mann, P., M. Hempton, D. Bradley, and K. Burke (1983). Development of
pull-apart basins, J. Geol. 91, 529554.
Matti, J. C., D. M. Morton, and B. F. Cox (1985). Distribution and geologic
relations of fault systems in the vicinity of the Central Transverse
Ranges, southern California, U.S. Geol. Surv. Open-File Rept. 85-365.
Matti, J. C., D. M. Morton, and B. F. Cox (1992). The San Andreas fault
system in the vicinity of the central Transverse Ranges province,
southern California, U.S. Geol. Surv. Open-File Rept. 92-354, 40 pp.,
scale 1:250,000.
Morton, D. M. (1977). Surface deformation in part of the San Jacinto valley,
southern California, J. Res. U.S. Geol. Surv. 5, 117124.
Morton, D. M., and J. C. Matti (1993). Extension and contraction within an
evolving divergent strike-slip fault complex: The San Andreas and
San Jacinto fault zones at their convergence in southern California,
in The San Andreas Fault System: Displacement, Palinspastic
Reconstruction, and Geologic Evolution, R. E. Powell, R. J. Weldon,
and J. C. Matti (Editors), Geol. Soc. Am. Memoir, Vol. 178, 217230.
Morton, D. M., and J. C. Matti (2001a). Geologic Map of the Lakeview 7:50
Quadrangle, Riverside County, California, U.S. Geol. Surv. Open-File
Rept. 01-174, scale 1:24,000,
Morton, D. M., and J. C. Matti (2001b). Geologic map of the Sunnymead
7:50quadrangle, Riverside County, California, U.S. Geol. Surv. Open-
File Rept. 01-450.
Naylor, M. A., G. Mandl, and C. H. K. Sijpesteijn (1986). Fault geometries
in basement induced wrench faulting under different initial stress
states, J. Struct. Geol. 8, 737752.
Nemser, E. S., and D. S. Cowan (2009). Downdip segmentation of strike-slip
fault zones in the brittle crust, Geology 37, no. 5, 419422, doi:
Oglesby, D. (2008). Rupture termination and jump on parallel offset faults,
Bull. Seismol. Soc. Am. 98, no. 1, 440447, doi: 10.1785/0120070163.
Onderdonk, N. W., T. K. Rockwell, S. F. McGill, and G. I. Marliyani (2013).
Evidence for seven surface ruptures in the past 1600 years on the
Claremont fault at Mystic Lake, northern San Jacinto fault zone, Cal-
ifornia, Bull. Seismol. Soc. Am. 103, no. 1, 519541, doi: 10.1785/
Park, S., D. Pendergraft, W. Stephenson, K. Shedlock, and T. Lee (1995).
Delineation of intrabasin structure in a dilational jog of the
San Jacintofault zone, southern California, J. Geophys. Res.100, no. B1,
Rahe, B., D. A. Ferrill, and A. P. Morris (1998). Physical analog modeling of
pull-apart basin evolution, Tectonophysics 285, 2140, doi: 10.1016/
Rasmussen, G. S. (1981). Nature of surface rupture and recurrence interval,
Casa Loma fault, in Geology of the San Jacinto Mountains: South
Coast Geological Society, A. R. Brown and R. W. Ruff (Editors), An-
nual Field Trip Guidebook, Vol. 9, 4854.
Rockwell, T. K., J. A. Dewhurst, C. W. Walls, W. J. Pollard, A. Orgil, G.
Faneros, and T. E. Dawson (2000). High-resolution paleoseismology
in southern California: Investigation of segment controls on the rupture
history of the southern San Jacinto fault, in Active Fault Research for
the New Millenium, Proceedings of the Hokudan International Sym-
posium and School on Active Faulting, K. Okumura, K. Takada, and H.
Goto (Editors), Letter Press Ltd., Hiroshima, Japan, 413419.
Rockwell, T. K., G. Seitz, T. Dawson, and J. Young (2006). The long record
of San Jacinto fault paleoearthquakes at Hog Lake: Implications for
14 G. I. Marliyani, T. K. Rockwell, N. W. Onderdonk, and S. F. McGill
regional patterns of strain release in the southern San Andreas fault
system, Seismol. Res. Lett. 77, 270.
Rodgers, D. A. (1980). Analysis of pull-apart basin development produced
by en echelon strike-slip faults, in Sedimentation in Oblique-Slip
Mobile Zones, P. F. Ballance and H. G. Reading (Editors),
Special Publication International Association of Sedimentologists,
4, 2741.
Salisbury, J. B., T. K. Rockwell, T. J. Middleton, and K. W. Hudnut (2012).
LiDAR and field observations of slip distribution for the most recent
surface ruptures along the Central San Jacinto fault, Bull. Seismol. Soc.
Am. 102, 598619.
Sanders, C. O., and H. Kanamori (1984). A seismotectonic analysis of the
Anza Seismic Gap, San Jacinto fault zone, Southern California, J. Geo-
phys. Res. 89, 58735890.
Sanders, C., and H. Magistrale (1997). Segmentation of the northern
San Jacinto fault zone, southern California, J. Geophys. Res. 102,
Sanders, C. O., H. Magistrale, and H. Kanamori (1986). Rupture patterns
and preshocks of large earthquakes in the southern San Jacinto fault
zone, Bull. Seismol. Soc. Am. 76, 11871206.
Segall, P., and D. D. Pollard (1980). Mechanics of discontinuous faults, J.
Geophys. Res. 85, 43374350.
Sharp, R. V. (1967). San Jacinto fault zone in the Peninsular ranges of
southern California, Geol. Soc. Am. Bull. 78, 705730.
Sharp, R. V. (1972). Map showing recently active breaks along the
San Jacinto fault zone between the San Bernardino area and Borrego
Valley, California, scale 1:24,000, U.S. Geol. Surv. Misc. Geologic
Investigations Map, I-675.
Sharp, R. V. (1975). En echelon patterns of the San Jacinto
Fault Zone, Special Report California Division of Mines Geology
118, 147152.
Sibson, R. H. (1986). Rupture interaction with fault jogs, in Earthquake
Source Mechanics, S. Das, J. Boatwright, and C. H. Scholz (Editors),
American Geophysical Monograph, 37, 157167.
Tchalenko, J. (1970). Similarities between shear zones of different
magnitudes, Geol. Soc. Am. Bull. 81, 16251640.
Thatcher, W., J. A. Hileman, and T. C. Hanks (1975). Seismic slip
distribution along the San Jacinto fault zone, southern California,
and its implications, Geol. Soc. Am. Bull. 86, 11401146.
Toppozada, T. R., C. B. Real, and D. L. Parke (1981). Preparation
of isoseismal maps and summaries of reported effects for pre-1900
California earthquakes, Calif. Div. Mines and Geol. Open-File Rept.
Wallace, R. E. (1990). Geomorphic expression, in The San Andreas Fault
System, California, R. E. Wallace (Editor), U.S. Geol. Surv. Profess.
Pap. 1515,1458.
Wesnousky, S. G. (1986). Earthquakes, Quatemary faults, and seismic haz-
ard in California, J. Geophys. Res. 91, 12,58712,631.
Wesnousky, S. (1994). The GutenbergRichter or characteristic earthquake
distribution, Which is it? Bull. Seismol. Soc. Am. 84, no. 6, 19401959.
Wesnousky, S. (2006). Predicting the endpoints of earthquake ruptures,
Nature 444, 358360.
Working Group on California Earthquake Probabilities (WGCEP) (1995).
Seismic hazards in southern California: Probable earthquakes,
19942024, Bull. Seismol. Soc. Am. 85, 379439.
Working Group on California Earthquake Probabilities (WGCEP) (2007).
The Uniform California Earthquake Rupture Forecast, Version 2
(UCERF 2), U.S. Geol. Surv. Open-File Rept. 2007-1437, and Califor-
nia Geol. Surv. Special Rept. 203.
Zachariasen, J., and K. Sieh (1995). The transfer of slip between two en
echelon strike-slip faults: A case study from the 1992 Landers earth-
quake, southern California, J. Geophys. Res. 100, 15,28115,302.
Department of Geological Sciences
San Diego State University
5500 Campanile Drive
San Diego, California 92182
(G.I.M., T.K.R.)
Department of Geological Sciences
California State University Long Beach
1250 Bellflower Boulevard
Long Beach, California 90840-3902
Department of Geological Sciences
California State University, San Bernardino
5500 University Parkway
San Bernardino, California 92407-2318
Manuscript received 14 July 2012
Straightening of Northern SJF as Seen in Fault-Structure Evolution of San Jacinto Valley Stepover 15
... At greater depths (e.g., below 10 km; Figure 5d and Figures S7c-S7d in Supporting Information S1), a sharp velocity contrast from west to east in the Peninsular Ranges is observed, which is related to the Hemet stepover (Marliyani et al., 2013). Clearer velocity contrasts across major fault systems, such as Elsinore Fault (EF), SJF and SAF are depicted in the map views of the final Vs model (Figure 5d and Figures S7c-S7d in Supporting Information S1), suggesting the derived Vs model yields higher resolutions compared to the CVM-H. ...
Full-text available
Machine learning algorithm has been applied to shear wave velocity (Vs) inversion in surface wave tomography, where a set of starting 1‐D Vs profiles and their corresponding synthetic dispersion curves are used in network training. Previous studies showed that the performance of such trained network is dependent on the diversity of the training data set, which limits its application to previously poorly understood regions. Here, we present an improved semi‐supervised algorithm‐based network that takes both model‐generated and observed surface wave dispersion data in the training process. The algorithm is termed Wasserstein cycle‐consistent generative adversarial networks (Wasserstein Cycle‐GAN [Wcycle‐GAN]). Different from conventional supervised approaches, the GAN architecture enables the inclusion of unlabeled data (the observed surface wave dispersion) in the training process that can complement the model‐generated data set. The cycle‐consistency and Wasserstein metric significantly improve the training stability of the proposed algorithm. We benchmark the Wcycle‐GAN method using 4,076 pairs of fundamental mode Rayleigh wave phase and group velocity dispersion curves derived in periods from 3 to 16 s in Southern California. The final 3‐D Vs model given by the best trained network shows large‐scale features consistent with the surface geology. The resulting Vs model has reasonable data misfits and provides sharper images of structures near faults in the top 15 km compared with those from conventional machine learning methods.
... The faults can be separated from one another by the occurrence of fault steps, bends, and/or abrupt changes in the fault's general strike. In general, two faults can be separated into different segments if the separation between the segments is wider than 4 km (Wesnousky, 2006;Duman and Emre, 2013;Marliyani et al., 2013). This study uses the term "fault sub-segment" or "section" without implying any term particular to their geological or seismological significance. ...
Full-text available
Sumatran fault in western Indonesia is one of the largest strike-slip fault in the world. The fault was formed as a result of the slip partitioning of the oblique convergence between the Indo-Australian and Eurasian plate along the Sunda trench. The right-lateral movement of the fault is accomodated by 19 fault segments that dissects the entire Sumatra island. We study the Aceh fault segment, which is located at the northernmost parts of the fault. The Aceh fault segment spans 250 km long passing through three districts: West Aceh, Pidie Jaya, and Aceh Besar and is affecting a total of ~546.143 population in the area. The current segmentation model assumes that Aceh fault segment acts as a single fault segment, which would generate closer to a M8 earthquake. This estimation is inconsistent with the ~M6-7 historical earthquake data. We conduct a detailed active fault mapping using the ~8 m resolution digital elevation model of DEMNAS and the sub-m DEM data from UAV-based photogrammetry to resolve the segmentation model of this fault. Our study indicate that the Aceh fault segment can be divided into 8 subsegments: Beutong, Kuala Tripa, Geumpang, Mane, Tangse, Jantho, Indrapuri, and Pulo Aceh. The fault kinematics identified in the field is consistent with right-lateral faulting. We measured cumulative displacement of geomorphic features (channels and ridges) ranging from 12.7 to 1931 m at some area. Findings of our study provide better estimation of the fault geometry and the maximum magnitude of potential earthquake along the Aceh fault segment as well as recommendation of prospective sliprate study sites. These informations are important for the development of seismic hazard analysis of the area.
... (8) Different from the initial model, a clear shift in the velocity contrast interface location is observed by comparing Vs at 10 and 15 km for our final Vs model at the south SAF (Figure 17), indicating a northeast dipping fault plane. (9) The highest velocities are observed in the Peninsular Ranges, and a sharp velocity contrast from west to east at greater depth (7-15 km; white vertical line in Figure 17) that corresponds to the Hemet stepover (Marliyani et al., 2013) is observed much clearly in the final model. (10) Velocity contrasts across major faults (e.g., SAF and SJF) previously imaged in other tomography (e.g., Fang et al., 2016;Share et al., 2019) and fault zone head wave studies (Share & Ben-Zion, 2016 are observed clearly in the final Vs model. ...
Full-text available
We use Eikonal tomography to derive phase and group velocities of surface waves for the plate boundary region in southern California. Seismic noise data in the period range 2 and 20 s recorded in year 2014 by 346 stations with ~1-30 km station spacing are analyzed. Rayleigh and Love wave phase travel times are measured using vertical-vertical and transverse-transverse noise cross-correlations, and group travel times are derived from the phase measurements. Using the Eikonal equation for each location and period, isotropic phase and group velocities and 2-psi azimuthal anisotropy are determined statistically with measurements from different virtual sources. Starting with the SCEC Community Velocity Model, the observed 2.5-16 s isotropic phase and group dispersion curves are jointly inverted on a 0.05°×0.05° grid to obtain local 1D piecewise shear wave velocity (Vs) models. Compared to the starting model, the final results have generally lower Vs in the shallow crust (top 3-10 km), particularly in areas such as basins and fault zones. The results also show clear velocity contrasts across the San Andreas, San Jacinto, Elsinore and Garlock faults, and suggest that the San Andreas fault southeast of San Gorgonio Pass is dipping to the northeast. Investigation of the non-uniqueness of the 1D Vs inversion suggests that imaging the top 3 km Vs structure require either shorter period (≤ 2s) surface wave dispersion measurements or other types of dataset such as Rayleigh wave ellipticity.
... At greater depth, the velocity contrast across the northern SJFZ becomes confined to where rocks of the western PRB and SGMB lower plate meet. 9. Rocks of the western PRB, which border the SJB at shallow depth (3), intersect the SJFZ at greater depth ([ 7 km) and occupy the region known as the Hemet step-over (Marliyani et al. 2013). 10. ...
Full-text available
We derive high-resolution P and S seismic velocities (VP and VS) within the South-Central Transverse Ranges section of the San Andreas Fault (SAF), using a new double-difference tomography algorithm incorporating both event-pair and station-pair differential times. The addition of station-pair data allows for better absolute event locations and higher model resolution at shallow depths. Velocities within a 222 km × 164 km region are inverted using > 1,000,000 P and S arrival (picked with an automatic detection algorithm) and differential times from > 10,000 local events recorded by > 250 stations. Similarly large P and S datasets lead to high-quality VP/VS estimates of the region. The resulting models include low velocities along major fault segments and across-fault velocity contrasts. They also show very high VP/VS anomalies near shallow damaged rock, whereas fault zones exhibit either low (< 1.73) or high (> 1.73) VP/VS characteristics at greater depth. The variations in amplitude of these anomalies along the SAF through San Gorgonio Pass (SGP) suggest abrupt west-to-east changes in elastic properties. Moreover, their geometries imply near-vertical SAF segments northwest of SGP and northeast dipping faults southeast of that area. The SAF near Coachella Valley is estimated to dip by 57°. Regional-scale low and high VP/VS values are related to relative abundances of crystalline or metamorphic rocks. Near-fault VP/VS anomalies at depth are likely associated with changes in wet crack geometries. The obtained results can improve future calculations of seismic motion from large earthquakes in the area and related seismic hazard estimates.
... Cone penetrometer testing (CPT) is a nondestructive subsurface mapping method that can be implemented in soft, ne-grained sediment (e.g., clay, silt, sand, and ne gravel), that has proven to be effective for paleoseismology studies (e.g., Grant et al., 1997;Marliyani et al., 2013a). CPT recognizes the differing resistance of varied lithology to applied pressure, as recorded by a freely moving sleeve attached to a cone-tipped probe pushed into the ground. ...
Trenching is the primary investigation method used onshore to collect paleoseismic data from active fault zones for research and engineering applications. Excavations provide near-surface exposures of fault zones that can be analyzed to determine the style of faulting, width of the fault zone, recency of rupture, rupture history, and slip rate. The most important factor in determining the success of a trench investigation is identifying a site that has suitable geology for exposing the fault zone and revealing its rupture characteristics. This is largely indicated by the geology and geomorphology around the site. In the last decade, the methods of trench excavation and safety procedures have changed little, but the methods of collecting and analyzing trench data have changed significantly. Development of new imaging technologies, such as LiDAR and Structure from Motion (SfM), and improvements in digital photography and portable computing, allow rapid collection of data such as digital images of the trench walls. Trench logging, which was previously done by hand on a blank grid, can now be overlain on images of the trench walls, and exported for publication or review by others after the trench has been closed. Significant advances in geochronologic methods for dating Quaternary samples, and development of Bayesian analysis software tools, provide new opportunities for understanding fault rupture histories. Application of trenching methods varies by style of faulting and site geology. To illustrate methods, we show examples from end member cases encountered by practicing geologists in California. A case study of the San Andreas fault in the Carrizo Plain provides an example of trenching a strike-slip fault in the central Coast Range. A case study of the Pasuran fault in Indonesia is a proxy example of normal faulting in volcanic areas of eastern California.
... Intersegments are regions of distributed and pervasive cracking and faulting [e.g., King, 1983;Manighetti et al., 2004;De Joussineau and Aydin, 2009;Allam and Ben-Zion, 2012;Allam et al., 2014] (inset Figure 7) that accommodate off-fault deformation at the expense of on-fault slip [e.g., Dawers and Anders, 1995;Manighetti et al., 2001a;Davis et al., 2005]. However, natural fault data show that as a fault accumulates more displacement, its discrete segments increasingly coalesce so as to form a throughgoing fault, whereas on-fault slip deficit at the intersegments is smoothed off [e.g., Wesnousky, 1988;Peacock, 1991;Stirling et al., 1996;Rahe et al., 1998;Walsh et al., 1999;Ferrill et al., 1999;Manighetti et al., 2001aManighetti et al., , 2009Manighetti et al., , 2015Soliva and Benedicto, 2004;Cembrano et al., 2005;De Joussineau and Aydin, 2009;Aydin and Berryman, 2010;Marliyani et al., 2013] (Figure 7). This is commonly described as the accumulation of slip leading to a geometrically simpler fault [e.g., Wesnousky, 1988;Ben-Zion and Sammis, 2003;King and Wesnousky, 2007;Wechsler et al., 2010]. ...
Full-text available
Earthquake slip distributions are asymmetric along strike, but the reasons for the asymmetry are unknown. We address this question by establishing empirical relations between earthquake slip profiles and fault properties. We analyze the slip distributions of 27 large continental earthquakes in the context of available information on their causative faults, in particular on the directions of their long-term lengthening. We find that the largest slips during each earthquake systematically occurred on that half of the ruptured fault sections most distant from the long-term fault propagating tips, i.e., on the most mature half of the broken fault sections. Meanwhile, slip decreased linearly over most of the rupture length in the direction of long-term fault propagation, i.e., of decreasing structural maturity along-strike. We suggest that this earthquake slip asymmetry is governed by along-strike changes in fault properties, including fault zone compliance and fault strength, induced by the evolution of off-fault damage, fault segmentation and fault planarity with increasing structural maturity. We also find higher rupture speeds in more mature rupture sections, consistent with predicted effects of low velocity damage zones on rupture dynamics. Since the direction(s) of long-term fault propagation can be determined from geological evidence, it might be possible to anticipate in which direction earthquake slip, once nucleated, may increase, accelerate and possibly lead to a large earthquake. Our results could thus contribute to earthquake hazard assessment and Earthquake Early Warning.
... These measurements are electronically processed, and the separate measurements and ratios are compared to derive a stratigraphic profile, classify soil types, and their distribution [e.g., Robertson, 1990], and quantify soil strength. CPTs are increasingly being used to investigate active faults [e.g., Grant et al., 1997;Marliyani et al., 2013] and are especially useful when paleoseismic trenches are not practical. We undertook 10 individual CPT soundings at locations spaced along a transect striking perpendicular to the scarp on the terrace surface 6-10 m back from the top of the river exposure using a 1.2 ton CPT rig with a 15 cm 2 cone (Figure 2: see supporting information). ...
The 300-km-long South Westland Fault Zone (SWFZ) is within the footwall of the Central Alpine Fault (<20 km away), has 3500 m of dip-slip displacement, but it has been unknown if the fault is active. Here the first evidence for SWFZ thrust faulting in the “stable” Australian Plate is shown with cumulative dip-slip displacements up to 5.9 m (with 3 m throw) on Pleistocene and Holocene sediments and gentle hanging wall anticlinal folding. Cone penetration test (CPT) stratigraphy shows repeated sequences within the fault scarp (consistent with thrusting). Optically stimulated luminescence (OSL) dating constrains the most recent rupture post-12.1 ± 1.7 ka with evidence for 3-4 events during earthquakes of at least Mw 6.8. This study shows significant deformation is accommodated on poorly-characterized Australian Plate structures northwest of the Alpine Fault and demonstrates that major active and seismogenic structures remain uncharacterized in densely forested regions on Earth.
... On the basis of Eq. 1, we calculated that rupture of individual fault segments can produce M w of 6.8 for the Claremont Fault, 6.5 for the Casa Loma Fault, 6.7 for the northern Clark Fault from Park Hill to Hog Lake, and 7.2 for the entire Clark Fault. Alternatively, using the rupture distribution for the 1918 and 1800 earthquakes determined by use of offset geomorphic features (SALISBURY et al. 2012), and the depth of seismicity along the Clark Fault (extends to as deep as 20 km in the Anza area, with 15-18 km depths to the NW and SE; SANDERS and MAGISTRALE 1997;MARLIYANI et al. 2013) to estimate fault width, we calculate the moments in these earthquakes to be approximately 1.5 9 10 26 and 1.3 9 10 27 dyne-cm, respectively, or moment magnitudes of 6.75 and 7.3, respectively, similar to the magnitude estimates based on rupture area. Estimating moment release for the Casa Loma Fault assuming 1 m of displacement [similar to 25 December 1899) and the Claremont Fault, assuming 2.25 m of displacement [based on the recurrence interval of 185 ± 25 years from ONDER- DONK et al. (2013)] and a 12-13 mm/year slip rate (BLISNIUK et al. 2013) yields values of 1.1 9 10 26 and 4.6 9 10 26 dyne-cm, respectively, which equate to M w 6.7 and 7.1 earthquakes, respectively. ...
Full-text available
Application of cone penetrometer testing (CPT) is a promising method for studying subsurface fault zones in stratified, unconsolidated sediment where trenching is not feasible. Analysis of data from 72 CPTs, spaced 7.5 to 30.0 m apart, and 9 borings indicates that the North Branch fault, the active strand of the Newport-Inglewood fault zone (NIFZ) in Huntington Beach, has generated at least three and most likely five recognizable surface ruptures in the past 11.7 ± 0.7 ka. Additional smaller earthquakes similar to the M w 6.4 1933 Long Beach earthquake may also have occurred but would not be recognizable with this method. The minimum right-lateral Holocene slip rate of the NIFZ in the study area is estimated to be 0.34 to 0.55 mm/yr. The actual slip rate may be significantly higher.
Full-text available
Propagation of moderate to large earthquake ruptures within major transcurrent fault systems is affected by their large-scale brittle infrastructure, comprising echelon segmentation and curvature of principal slip surfaces (PSS) within typically ˜1 km wide main fault zones. These PSS irregularities are classified into dilational and antidilational fault jogs depending on the tendency for areal increase or reduction, respectively, across the jog structures. High precision microearthquake studies show that the jogs often extend throughout the seismogenic regime to depths of around 10 km. On geomorphic evidence, the larger jogs may persist for periods >105 years. While antidilational jogs form obstacles to both short- and long-term displacements, dilational jogs appear to act as kinetic barriers capable of perturbing or arresting earthquake ruptures, but allowing time-dependent slip transfer. In the case of antidilational jogs slip transfer is accommodated by widespread subsidiary faulting, but for dilational jogs it additionally involves extensional fracture opening localized in the echelon stepover. In fluid-saturated crust, the rapid opening of linking extensional fracture systems to allow passage of earthquake ruptures is opposed by induced suctions which scale with the width of the jog. Rupture arrest at dilational jogs may then be followed by delayed slip transfer as fluid pressures reequilibrate by diffusion. Aftershock distributions associated with the different fault jogs reflect these contrasts in their internal structure and mechanical response.
A variety of extensional and contractional structures is produced by strike slip faulting. The variety and extent of the structures are directly related to the kind and extent of geometric complexities of the fault zone or system. The San Jacinto fault zone formed in response to a structural knot in San Gorgonio Pass probably within the past 1.5 Ma. In the area of their convergence it is proposed that slip is transferred both east and west from the San Jacinto fault zone northward to the San Andreas fault zone over a 60 to 70 km band that extends northwestward from the south end of the San Bernardino basin to the east end of the San Gabriel Mountains. Several structural adjustments are proposed as a consequence of onset or acceleration of lateral movement on the San Jacinto fault zone. The uplift and compression in San Gorgonio Pass resulted from two formerly disparate structural blocks - the eastern San Bernardino and San Jacinto blocks - becoming a relatively coherent block, and the San Gorgonio Pass area constituting a left step between the San Andreas fault zone in the Coachella Valley area and the San Jacinto fault zone in the San Jacinto Valley area. In partitioning slip between the San Andreas and San Jacinto fault zones, consideration should be given to the bandwidth over which horizontal strain has accumulated. -from Authors
Body waveform inversion techniques are used to study the source parameters of four earthquakes occurring between 1937 and 1954 along the southern San Jacinto and Imperial faults (1937 Buck Ridge, 1940 Imperial Valley, 1942 Borrego Mountain, and 1954 Salada Wash events). All earthquakes had simple rupture histories with the exception of the 1940 Imperial Valley main shock, which consisted of at least four subevents whose relative locations indicate unilateral rupture toward the southeast. Earthquakes in regions of high heat flow (>80 mW/m2) had focal depths near the base of the seismogenic zone (8 to 10 km). The 1937 Buck Ridge earthquake, located in a region of lower heat flow, however, appears to have occurred at a shallow (3 ± 2 km) depth. The location, mechanism, and aftershock distribution for the 1942 Borrego Mountain earthquake suggest it could have occurred along the Split Mountain fault, a recently identified northeast-trending cross fault located between the Elsinore and Coyote Creek faults or along an unnamed fault that parallels the trend of the Coyote Creek fault. Moment and rupture length estimates obtained from this study agree well with estimates obtained in previous studies that used different data sets.
We relocated the large 1937, 1942, and 1954 earthquakes in the San Jacinto fault zone. The epicenters of the main shocks, aftershocks, and some preshocks were determined using empirical station corrections from recent small events in the study areas. The 1937 (ML 5.9) earthquake has an epicenter between the surface traces of the San Jacinto and Buck Ridge faults, and aftershocks suggest about 7 km of rupture predominantly to the northwest. A significant increase in small earthquake activity occurred about Formula yr before this event. The 1954 (M_L 6.2) earthquake is located at the southeast end of the mapped trace of the San Jacinto fault, and aftershocks suggest about 15 km of rupture further southeast into an area of folded young sediments with no surface fault trace. This event was preceded by a cluster of small earthquakes which occurred within an 8-hr period 10 weeks before the main event and in the eventual rupture zone. The 1942 (M_L 6.3) earthquake is located southwest of the southeast end of the Coyote Creek fault. Large aftershocks of this event are spread over a 15 by 18 km area southwest of the Coyote Creek fault and are not associated with any one fault. The relation of the 1942 event to the San Jacinto fault zone is not simple.
Many of the geological terms having to do with strike-slip deformation, basin formation, and sedimentation are used in a variety of ways by different authors (eg pull-apart basin), or they are synonymous with other words (eg left-lateral, sinistal). Rather than enforcing a rigorously uniform terminology in this book, we decided to set down our preferred definitions in a glossary, and where appropriate to indicate alternative usage. Some words (eg cycle) have additional meanings in the geological sciences not included here, and this glossary should therefore be used in the context of strike-slip basins.-from Authors
Basins associated with strike-slip deformation are filled with wide range of sedimentary facies, deposited in nonmarine to deep marine environments. The principal controls on sedimentation are crustal type and thickness, plate-tectonic setting, amounts and rates of subsidence, relative sea level, topographic relief, and climate. Abrupt facies changes and discontinuities are relatively common. Subsidence rates are generally high, but there is significant variation within individual basins and from one basin to another. The type and degree of associated volcanic activity at any locality are related to tectonic setting and the amount of lithospheric extension. Heat-flow history is extremely variable, even within a single basin; consequently, the level of maturation of petroleum source rocks is notoriously difficult to predict in these types of basins.