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# Evidence for Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake, Northern San Jacinto Fault Zone, California

## Abstract and Figures

We present new data from the Mystic Lake paleoseismic site along the Claremont segment of the northern San Jacinto fault zone. The site is located within a sag formed between two fault strands that pass through the eastern side of Mystic Lake in the San Jacinto Valley. Trenches excavated across the sag exposed faulted and folded lacustrine and alluvial strata that record at least seven ground-rupturing earthquakes during the past 1600 yr. Evidence for past surface deformation includes upward terminating faults with associated fissure fills, folding, angular unconformities, and pinching of strata against a paleoscarp. All of the event horizons occur at the tops of paleosols and are overlain by massive lacustrine clay units. We interpret this pattern to represent development of soils at the surface between earthquakes that are buried when fault rupture causes subsidence and renewed filling of the depression with lacustrine sediments. The ages of the events are constrained by 50 radiocarbon dates determined from detrital charcoal. The recurrence interval for the past seven events ranges from 159 to 210 yr, and the most recent event occurred sometime between A.D. 1738 and 1850 based on radiocarbon ages trimmed by historical data. Some of the event ages at Mystic Lake overlap in time with events recorded at Hog Lake on the Clark strand of the San Jacinto fault zone to the south, suggesting that these events may have jumped the San Jacinto Valley releasing stepover, or that events on one fault triggered closely timed events on the adjacent fault.
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Evidence for Seven Surface Ruptures in the Past 1600 Years
on the Claremont Fault at Mystic Lake, Northern
San Jacinto Fault Zone, California
by Nathan W. Onderdonk, Thomas K. Rockwell, Sally F. McGill, and Gayatri Marliyani
Abstract We present new data from the Mystic Lake paleoseismic site along the
Claremont segment of the northern San Jacinto fault zone. The site is located within a
sag formed between two fault strands that pass through the eastern side of Mystic Lake
in the San Jacinto Valley. Trenches excavated across the sag exposed faulted and
folded lacustrine and alluvial strata that record at least seven ground-rupturing earth-
quakes during the past 1600 yr. Evidence for past surface deformation includes up-
ward terminating faults with associated fissure fills, folding, angular unconformities,
and pinching of strata against a paleoscarp. All of the event horizons occur at the tops
of paleosols and are overlain by massive lacustrine clay units. We interpret this pattern
to represent development of soils at the surface between earthquakes that are buried
when fault rupture causes subsidence and renewed filling of the depression with
lacustrine sediments. The ages of the events are constrained by 50 radiocarbon dates
determined from detrital charcoal. The recurrence interval for the past seven events
ranges from 159 to 210 yr, and the most recent event occurred sometime between
A.D. 1738 and 1850 based on radiocarbon ages trimmed by historical data. Some
of the event ages at Mystic Lake overlap in time with events recorded at Hog Lake
on the Clark strand of the San Jacinto fault zone to the south, suggesting that these
events may have jumped the San Jacinto Valley releasing stepover, or that events on
one fault triggered closely timed events on the adjacent fault.
Introduction
Documenting the timing of prehistoric large earthquakes
along a fault is necessary to estimate the hazard posed by the
fault for a potentially dangerous earthquake in the near fu-
ture. With knowledge of the timing of past ground-rupturing
earthquakes and a calculation of the average time interval
between such large earthquakes, we can estimate accumu-
lated strain along the fault and the likelihood of the next large
earthquake within a given period. Paleoseismic data can also
be used to interpret the mechanics of active faults though the
evaluation of seismic rupture patterns and the style of strain
distribution along a single fault or across a system of faults,
such as the southern San Andreas fault system. Ground-
rupturing earthquakes are often recorded by the disruption
of shallow stratigraphic units along a fault, the dating of
which provides a method of approximating the timing of
the events. Very specific geomorphic conditions are needed
to create an environment where evidence of paleoearth-
quakes is preserved. These conditions include a depositional
environment in which sediment is collected at a rate greater
than the interval between earthquakes, where abundant or-
ganic material is present for radiocarbon dating, and where
stratigraphy is not disturbed by surface erosion or burrowing
animals.
In Southern California, a growing paleoseismic database
from multiple sites is leading to an increased understanding
of how large, ground-rupturing earthquakes have been dis-
tributed across the San Andreas fault system both spatially
and temporally during the past 2000 yr (e.g., Seitz et al.,
1997;Fumal, Rymer, and Seitz, 2002;Fumal, Weldon, et al.,
2002;Rockwell et al., 2006;Biasi and Weldon, 2009;Akciz
et al., 2010;Scharer et al., 2010;Philibosian et al., 2011).
However, data gaps in some key locations, such as the
northern San Jacinto fault zone (Fig. 1), limit the extent to
which interpretations can be made, specifically in regard to
the existence of fault segment boundaries that may limit the
surface length of possible ruptures.
The northernmost strand of the San Jacinto fault zone,
the Claremont fault (Fig. 1), links to the San Andreas fault
zone via a zone of stepping splays in the northeastern San
Gabriel Mountains (Morton and Matti, 1993;Nourse, 2002).
Some plate boundary displacement must be transferred
across this juncture because the Late Quaternary rate of
1
Bulletin of the Seismological Society of America, Vol. 103, No. 1, pp. , February 2013, doi: 10.1785/0120120060
strike-slip displacement across the San Andreas fault zone
decreases sharply to the southeast in this vicinity. Slip rates
for the Mojave segment of the San Andreas fault zone are
36 !8mm=yr (e.g., Humphreys and Weldon, 1994) but
decrease to 25 mm=yr in the Cajon Pass (Weldon and Sieh,
1985) and to 13 mm=yr along the San Bernardino section
of the fault (McGill et al., 2010,2011). Slip rates in the San
Gorgonio Pass area may be even lower (Orozco, 2004). At
least part of the remaining slip is most likely transferred to
the San Jacinto fault. Published slip rates for the northern San
Jacinto fault zone range from 6 to 20 mm=yr (Prentice et al.,
1986;Morton and Matti, 1993;Kendrick et al., 2002) while
rates for that of the central range from 6 to 23 mm=yr, with a
preferred rate of 10 to 14 mm=yr (Sharp, 1981;Rockwell
et al., 1990;Blisniuk et al., 2010). Geodetic data support this
interpretation of fault slip on the Mojave segment of the San
Andreas being partitioned to the south and suggest that the
San Jacinto fault may be accommodating a similar amount
of plate boundary displacement as the southernmost San
Andreas fault (e.g., Savage and Prescott, 1976;Becker et al.,
2005;Fialko, 2006). It is unclear, however, how slip is trans-
ferred between the two fault zones.
The structural arrangement at the south end of the Clare-
mont fault presents a similar problem. There, the Claremont
Figure 1. Fault map of the San Jacinto Valley area showing the location of the Quincy, Mystic Lake, and Hog Lake paleoseismic sites.
Local city centers (black squares) and major highways (white lines) are shown for reference. Inset map shows the southern San Andreas fault
system with paleoseismic sites represented as black dots. PA, Pallet Creek; WT, Wrightwood; CC, Cajon Creek; PT, Pitman Canyon; PL,
Plunge Creek; BF, Burro Flats; TP, Thousand Palms; ML, Mystic Lake; HL, Hog Lake; CMT F, Claremont fault; CL F, Casa Loma fault; BR
F, Buck Ridge fault; C F, Clark fault; CC F, Coyote Creek fault; LA, Los Angeles; SA, Santa Ana; and R, Riverside.
2N. W. Onderdonk, T. K. Rockwell, S. F. McGill, and G. Marliyani
fault forms a 25 km-wide releasing stepover with the Casa
LomaClark segment of the San Jacinto fault zone, which
continues to the southeast (Fig. 1). However, as with the
junction between the northern end of the Claremont and the
San Andreas faults, it is unclear how slip is transferred be-
tween the faults. Is it possible for ruptures to jump these fault
junctures at the northern and southern ends of the Claremont
fault co-seismically? Or is slip transferred between the faults
in a transient manner in which an event on one fault loads the
other, leading to a rupture in a separate earthquake? The first
scenario would result in larger magnitude earthquakes based
on the correlation between surface rupture length and
magnitude observed in historic earthquakes with measured
surface ruptures (Wells and Coppersmith, 1994). It is there-
fore important to determine how slip is transferred between
these faults and a comparison of paleoseismic data from the
Claremont fault with previously published data from the San
Andreas fault zone in the area (e.g., Seitz et al., 1997;Fumal,
Weldon, et al., 2002; D. Yule, personal comm., 2010) and
from the Clark strand of the San Jacinto fault zone (Rockwell
et al., 2004,2006) is needed to evaluate this question.
In this paper, we describe a new paleoseismic site along
the northern San Jacinto fault zone, called the Mystic Lake
site, in which shallow stratigraphy record evidence of the
history of paleoearthquakes during the past 1600 yr on
the Claremont fault. The site is located at the northern end
of the ephemeral Mystic Lake, where excellent stratigraphy
and abundant detrital charcoal are present. We present evi-
dence for seven earthquakes within the past 1600 yr and use
this data to construct an earthquake history and calculate an
average recurrence interval. We then compare our new data
from the Mystic Lake site to the Hog Lake paleoseismic data
on the Clark strand of the San Jacinto fault zone to evaluate
how co-seismic strain release is distributed along the San
Jacinto fault zone, and we note possible correlations between
earthquake events. We also compare the Mystic Lake data to
paleoseismic data from the San Andreas fault in the area and
describe possible event correlations that are allowed by the
data, but not required.
Methods
We used a combination of field mapping, mapping
on digital elevation models derived from B4 light detection
and ranging (LiDAR) data (Bevis et al., 2006), and historic
aerial photos to identify and map the Mystic Lake site (Figs. 2
and 3). We chose the site based on the geomorphic and
hydrologic conditions that appeared favorable for the pres-
ervation of ground-rupturing events along the Claremont
fault. We also evaluated several other potential sites along
the Claremont fault from the Moreno to San Jacinto valleys
but found them to be inadequate for paleoseismic investi-
gations. Test trenches at these other sites revealed high bio-
turbation, poorly bedded stratigraphy, or frequent surface
erosion.
Figure 2. Faults (black) and deflected streams (white) in the vicinity of the Mystic Lake paleoseismic site mapped on B4 light detection and ranging (LiDAR) data.
Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake 3
We began trenching investigations at the Mystic Lake
site in fall 2009 with a 400-m-long locator trench (Trench 1)
that was excavated across the entire sag feature imaged in a
1940 aerial photo (Fig. 3). This trench was 1:8-m deep
along its entire length and was used to determine the location
of the primary fault zones at the site. We then excavated a
3-m-deep trench across an inferred fault trace near the
northeastern side of the sag structure (Trench 2), and two
additional 34-m-deep trenches across the prominent
degraded scarps that bounded the northeastern side of the sag
(Trenches 3 and 4). We constructed a 0.5-m-tall by 1-m-wide
string grid on all the trench walls for reference. Grid frames
for Trench 1 were photographed, digitally rectified, and
combined into composite images used as a base layer to
document stratigraphic and faulting relationships. Trenches
3 and 4 were logged by drawing sketches of the trench walls
and were not photologged due to time constraints. We
collected and documented the location of more than 200
samples of detrital charcoal in the four trenches. The trench
outlines, grids, and major fault zones were surveyed using a
Trimble R8 differential GPS that was established at a local
reference point with a relative horizontal accuracy of !2cm,
which allowed us to precisely relocate the faults and grid
panels the following summer during the second trenching
campaign.
In summer 2010, we revisited the site to focus on the
main southwestern fault zone exposed in Trench 1. We first
conducted cone penetrometer testing (CPT) across the sag to
locate the main fault zone farther to the southeast and to
investigate the deeper structure of the site (G. Marliyani,
unpublished manuscript, 2012). Using the CPT data as a
guide, we excavated a second, 300-m-long trench across the
southeastern extent of the sag (Trench 5) to determine how
stratigraphic resolution varied along strike of the fault zone.
Although the main fault zone was clearly exposed in Trench
5, the stratigraphy was sandier, more massive, and the units
were not as clearly defined as those in Trench 1, most likely
due to the fact that Trench 5 was located closer to the
southwestern edge of the sag (Fig. 3). We then excavated
Trenches 6 and 7 across the main fault zone on either side
of Trench 1 to further evaluate our interpretations of the fault-
ing relationships observed in fall 2009; all three trenches
were excavated to an approximate depth of 1.8 m. We con-
structed grids on the trench walls, photologged the relation-
ships, and collected more than 100 additional detrital char-
coal samples. The trench outlines, grids, and fault locations
were surveyed to allow us to precisely locate all features in
future excavations.
Site Description
Mystic Lake is an ephemeral lake that forms in the
lowest elevations of the San Jacinto Valley pull-apart basin
during extremely wet years. It is located at the northwestern
end of the stepover, where the surface expression of the Casa
Loma fault dies out (Fig. 1). The lake periodically fills due
Figure 3. Faults, streams, and Quaternary deposits mapped on a 1940 aerial photograph that shows water ponded behind a scarp on the
southwest fault strand. The trench locations are shown in black and cone penetrometer test (CPT) points are shown as white dots. Some
Quaternary deposits are outlined in dashed lines (Qoa, Quaternary older alluvium; Ht, Holocene terrace deposit). The star symbol on the Ht
deposit denotes location of dated optically stimulated luminescence (OSL) samples.
4N. W. Onderdonk, T. K. Rockwell, S. F. McGill, and G. Marliyani
to a combination of drainage from the elevated San
Timoteo Badlands on the northeastern side of the Claremont
fault and overflow from the San Jacinto River that passes
through the valley. Jackrabbit Canyon, one of the largest
canyons within the San Timoteo Badlands, drains directly
into Mystic Lake, and the alluvial fan at the mouth of this
canyon is prograding away from the canyon into the lake
basin (Fig. 2).
The Claremont fault passes through the northern shore-
line of Mystic Lake and comprises several fault strands in
this area (Figs. 2and 3). The northeastern fault strand,
Fault D, crosses the San Timoteo Badlands and is interpreted
to be an older, and most likely inactive, fault. The fault does
not deflect younger drainages, and outcrops in road cuts
0.5 km to the southeast of the site show the fault overlain by
unfaulted, older alluvial deposits. Fault D and the south-
eastern continuation of Faults B and C make a left-stepping
restraining bend that is most likely responsible for the
elevated topography near the mouth of Jackrabbit Canyon
(Fig. 2). Fault A is inferred to be the youngest strand and
has developed 200700 m outward from the mountain front
and bypasses this older restraining bend geometry. The local
geomorphology and trench exposures indicate that displace-
ment along the fault zone in this area is now transferred from
Fault C in the northwest to Fault A in the southeast across a
releasing stepover between these two faults, and a sag that is
0:5km wide and 1 km in length has formed between these
two faults. Aerial photographs from the 1930s to present day
show that the depression is occasionally filled with water.
This periodic inundation has resulted in the deposition of
well-defined and traceable units of fine-grained lake sedi-
ments that are interbedded with silt and sand derived from
the Jackrabbit Canyon fan and the smaller drainages directly
northeast of the sag. The stratigraphy is well preserved for
the most part because the generally wet subsurface environ-
ment has minimized bioturbation.
Figure 3shows the study site with faults and inactive
Quaternary deposits mapped onto a 1940 aerial photograph.
At the time the photograph was taken, water is visible in the
depression between the two fault strands and is ponded
against a northeast-facing scarp along Fault A, which bounds
the southwestern side of the sag. The trench exposures
(in subsequent sections of this paper) show that Fault A
has been the primary active strand for at least the past
1700 yr. There is no surface expression of this fault today,
as the sag observed in the photograph has been filled with
modern sediment that contains historical debris. However,
Faults B and C, which bound the northeast side of the re-
leasing stepover, are clearly defined by 45-m-high scarps.
These scarps mark the faulted edge of a Quaternary terrace
that was deposited on a strath cut across Plio-Pleistocene
sedimentary strata of the San Timoteo Formation. The
down-dropping and burial of this terrace on the southwestern
side of these faults attest to a component of vertical displace-
ment along the Claremont fault, as does older alluvial
material exposed higher up on the ridges to the east (Qoa
in Fig. 3). Trenches 3 and 4 (Fig. 4) were excavated across
the scarps to evaluate their origin. Both trenches exposed
southwest-dipping faults that maintained constant dip angles
through the full 3-m depth of the trenches and cut alternating
layers of clay, sand, and silt overlain by oxidized sandy silt
with scattered gravel (Fig. 4). In both trenches, the larger
southwest-dipping faults extend almost to the surface and
are overlain only by a 0.51-m thick layer of colluvium with
a weakly developed soil. None of the layers can be correlated
across these faults and indicate that dipslip separation must
be greater than 2.4 m in Trench 3 and more than 2 m in
Trench 4. These faults could not be traced upward through
the unstratified, recent colluvium that was present in the
upper 0.5 m of both trenches. This prevented us from making
a determination of the timing of the last rupture along these
faults.
The age of the Quaternary terrace material was deter-
mined by dating detrital charcoal at a 3-m depth in Trench
3, which yielded an age of 8645 !45 radiocarbon yr before
present (rcyBP). An optically stimulated luminescence (OSL)
sample taken from a soil pit at 1-m depth near the east end of
Trench 4 (Fig. 3) yielded an age of about 8300 yr before
present (yBP). These dates indicate that the terrace deposits
are early Holocene, and the faults that define the scarps may
be significantly older than Fault A on the southwestern side
of the stepover. We were unable to determine whether these
scarps are fault scarps or head scarps of shallow-seated land-
slides as suggested by previous mapping (D. Morton, J.
Matti; personal comm., 2011) due to the limited depth of ex-
posure in the trenches, and the fact that both the
geometry of the faults and the sense of offset along the faults
can fit with either interpretation. However, if these are land-
slide scarps, they are unusually linear and even slightly con-
vex in map view, and have formed along faults that can be
traced into the area from the northwest and southeast based
on geomorphic evidence.
The faults present at the southwest end of Trench 4 are
located 57 m southwest of the main scarp (Fig. 4) and may
be due to more recent activity. These faults dip steeply to the
northeast or are vertical, show a variable sense of offset, and
offset younger stratigraphy. A detrital charcoal sample col-
lected at an approximate 2-m depth in Trench 4 in sediments
cut by these faults returned an age of 1990 !20 yBP. This
indicates that Fault C has ruptured in the past 2000 yr and
may still be actively accommodating displacement through
the area. It is therefore possible that some events that rup-
tured through the site were not recorded on Fault A where
all of our paleoseismic data were collected (Fig. 3).
Sag Stratigraphy and Structure
Trench 1 was excavated across the entire sag pond
imaged on the 1940 aerial photo and up onto a small alluvial
fan on the northeastern side of the stepover. The trench
exposed numerous faults that offset stratigraphy composed
of sand, silt, and clay, with several paleosols. The stratigraphy
Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake 5
was laterally continuous enough to be traced along the full
400 m length of the trench, which allowed us to document
the structure of the depression, observe changes in lithology
and layer thickness across the depression, and correlate fault-
ing events by tracing event horizons from one fault zone to
another. The layers are generally more coarsely grained on
the northeastern side of the sag where sand and silty sand
dominate and increase in clay content to the southwest. Some
sandy layers pinch out to the southwest farther away from the
small fan. Radiocarbon dating shows that the upper 1.8 m of
strata exposed in the trench span the past 1700 yr. A tin can
found in one of the uppermost silt layers attests to the his-
torical age of the upper 2030 cm of sediment. Above this
silt layer is a 1020-cm-thick sand deposit that thickens to
the southwest in the sag and then thins abruptly across the
southwestern fault zone. We infer that these uppermost sand
and silt deposits largely filled in the topographic depression
observed in the 1940 aerial photograph. The larger structure
of the sag is a broad syncline that is abruptly truncated on the
southwest side by a fault zone (Fig. 5). This fault zone is
67 m wide and corresponds to the scarp observed in the
1940 aerial photograph that bounds the southwestern edge
of the ponded water (Fault A, Fig. 3). All but the youngest
strata (upper 0.5 m) are folded across this zone in a northeast-
facing monocline. Some layers thicken across the zone, some
layers pinch out against the raised southwest side, and others
are truncated by angular unconformities (Fig. 6).
A few faults are also present outside of this main zone
but are confined to within 30 m to the southwest or northeast
Figure 5. Simplified log of Trench 1 showing the structure of
the upper 1.8 m of the sag. Black lines indicate key horizons that
were traced across the width of the sag, and thicker black vertical
lines indicate the locations of faults. Fault offsets of the layers are
not shown in this figure.
meter
meter
Figure 4. Sketch logs of Trenches 3 and 4 excavated across the scarps along the northeastern side of the site.
6N. W. Onderdonk, T. K. Rockwell, S. F. McGill, and G. Marliyani
of the main fault zone. Most of these secondary faults are
present in the uplifted side of the main fault zone and termi-
nate upward at an unconformity below the historical layers.
The precise age of these faults cannot be determined because
erosion of the elevated side of the fault removed at least
1.5 m of stratigraphy (1100 yr). Several faults are also
present to the northeast of the main fault zone within the
sag, and these helped define some of the younger events rec-
ognized in the trench as they terminate upward against the
younger stratigraphy. Most of the stratigraphy across the sag
is not faulted in the northeastern portion of Trench 1. Only
one small fault was recognized in that part of the trench and a
deeper trench (Trench 2, 3.5-m deep) was excavated adjacent
to this section of Trench 1 to search for evidence of older
movement along a fault here, but none was found.
Exposures in Trench 1 demonstrate that the locus of
deformation at the site is concentrated along Fault A. CPT
provided data to depths of 30 m and showed that the south-
west fault zone has been the main zone of deformation for at
least 8000 yr (G. Marliyani, unpublished manuscript, 2012).
Trenches 6 and 7, excavated across the southwestern fault
zone, showed the same folding and faulting relationships that
were observed in Trench 1. Trench 5 was excavated 250 m
to the southeast along this fault. At least three and possibly as
many as six faulting events were recognized in Trench 5 but
cannot be correlated to the events observed in the other three
trenches at this time. This is because we could not confi-
dently correlate units between the trenches due to the large
distance between Trench 5 and Trenches 1, 6, and 7 and the
lack of a connector trench. Consequently, the results from
Trench 5 are not addressed in this paper and require further
analysis.
Event Evidence
Trenches 1, 6, and 7
We document evidence for at least seven ground-
rupturing earthquakes in the upper 1.8 m of strata exposed
in the Mystic Lake trenches. Event evidence includes upward
terminations extending to the same stratigraphic level for
multiple fault strands, folding of strata across the south-
western fault zone, and associated onlap of stratigraphic
layers and truncations at angular unconformities. These lines
of event evidence were observed in both walls of multiple
trenches, and were continuous when trench walls were cut
back in increments of between 5 and 10 cm. Only the oldest
event (Event 7) was not observed among multiple trenches
because only Trench 1 was excavated deep enough to expose
Event 7. The event evidence is summarized in Table 1and
shown in Figures 617.
The youngest structures recognized in the trench
exposures were fractures and faults that cut up through the
entire stratigraphy into the uppermost layer, Unit 1, which is
composed of well-sorted, fine- to medium-grained sand. We
interpret this layer to have formed in the past 70 yr because it
has filled in the sag observed in the 1940 aerial photographs
and because it is above a silt layer (Unit 10) that contained a
tin can. This sand layer thins to the southwest across the
main fault zone where the scarp that bounds the southwest
side of the sag in the 1940 aerial photos was present. In
Trenches 1, 6, and 7, several faults and fractures present
in the main fault zone terminate upward at the base of Unit
1, and a few of the faults exhibit 13 cm of vertical separation
across them (Figs. 611). The underlying silt layer (Unit 10)
is warped down into the sag (most apparent in Trench 7),
suggesting that some of the surface displacement was asso-
ciated with these features; this deformation is denoted as E0
in Table 1.
The event horizon for Event E1 occurs at the base of
Unit 10, which is a tan, well-sorted silt deposit 10 cm thick.
Event E1 is recorded by numerous fractures and faults, some
of which are 12 cm wide and filled with dark clay, that
terminate at the base of Unit 10 (Figs. 8,9,12, and 13). Many
of the faults vertically separate the layers below by as much
as 10 cm, showing both west- and east-side-down motions.
Increased offset of deeper layers suggests that multiple
events may have occurred along these surfaces or that a com-
ponent of strike-slip is present. The tin can found within
Unit 10 indicates that this layer was deposited within the his-
toric period. Like Unit 1, this layer thickens to the northeast
across the fault zone, filling in the sag that developed during
Event E1. The unit below, Unit 100, is a 0.250.5 m, thick
gray, clay silt with two distinctive thin layers of white silt 8 in
the upper part. This unit has sustained more bioturbation than
the rest of the stratigraphy, making it difficult to trace indi-
vidual layers within the unit or to detect offsets along faults
that pass through it.
Evidence for Event E2 includes several faults in the
middle and eastern parts of the sag that terminate upward
against a clay layer 5 cm thick at the base of Unit 100
(Figs. 12,13, and 14). Evidence for E2 was not observed
in the main fault zone, possibly due to the fact that the youn-
ger units that contain the event horizon are difficult to define
in the main fault zone because of more extensive bioturbation
and thinning of the units over the main fault scarp. Because
Trenches 6 and 7 did not extend into the middle of the sag,
the only conclusive evidence for E2 is observed in Trench 1.
In Trench 6, however, several faults in the main fault zone die
out upward or are difficult to follow between Units 300 and
100 and may or may not terminate at the E2 event horizon at
the top of Unit 200.
Evidence for E3 was observed in Trenches 1 and 7. In
Trench 1, faults in a secondary fault zone within the sag
depression terminate upward against a clay layer 5 cm thick
(Unit 290). The layer directly below (Unit 300) is silt that has
a darker coloration due to the presence of organic material
and is interpreted to be a poorly-developed paleosol. In the
main fault zone, an angular unconformity is present at the
base of Unit 200 where Unit 300 and deeper are folded
across the main fault zone and truncated upward by the
E3 event horizon (Figs. 6and 7). This same relationship
Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake 7
Figure 7. 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 in red for clarity.
T1-300
T1-301
T1-226
600
590
590 590
590
350
370
340
400
390
300 300
400
500
500
100
10
600 600
590
450
570
450
580
570
580
570
560b
500
400
300
580
570
560b
560a
500
400?
270?
560a
560b 570
580
590
600a
600b
690
700
580
570
560b
500
T1-230
T1-227p
600
540
10
100
1
290
150- 280 (undifferentiated)
100
10
fault strike = 155° 155°
143° 159° 158°
54
11
3
34
4
5
45
6
6
7
5
6
Surface Surface
Base of Trench
Base of Trench
53
52
51
50
49
48
NE SW
Mystic Lake Trench 1 SE Wall
kk
k
k
k
Figure 6. Interpretation of the main southwestern fault zone in Trench 1 redrawn from photologs. Event horizons are marked as bold, black lines with stars denoting event number. Darker
gray shades indicate paleosol layers, and lighter shades indicate distinct clay layers that typically overlie event horizons. Faults are shown as black lines.
8N. W. Onderdonk, T. K. Rockwell, S. F. McGill, and G. Marliyani
was observed even more clearly in Trench 7 in which Unit
290 pinches out against a paleo-fold scarp and the units
above drape over the top (Figs. 10 and 11). Several faults that
terminate upward at the top of Unit 300 are also present in
Trench 7 and form a small graben. In Trench 6, excessive
bioturbation of Units 350 through 100 obscured faults
through this part of the stratigraphy.
Trenches 1 and 6 both exposed faults in the main fault
zone that terminate upward at the top of Unit 400, which
defines the E4 event horizon (Figs. 6,7,8,9, and 15). These
faults show both east- and west-side-down senses of sepa-
ration, and some have unit thickness changes across them,
suggesting a lateral component of displacement. Unit 400
is a silty clay that has a dark, organic stain representing soil
development when this layer was at the surface. Directly
above the event horizon is yellow-tan clay (Unit 390) that
thickens to the northeast into the sag. In Trench 1, the units
directly above this E4 horizon (Units 390 through 300) thin
or pinch out in the vicinity of the main fault zone, indicating
that a small scarp was present just after E4. In Trench 6, Unit
390 pinches out before reaching the main fault zone, and
Unit 370 (silty clay) directly overlies the event horizon.
Event 5 is represented by the upward termination of
faults in both Trenches 1 and 6, as well as by the thinning
of units above the E5 event horizon in Trench 1 (Figs. 6,7,8,
and 9). The event horizon is present at the top of Unit 500,
which is another organic-stained paleosol developed in a
silty clay layer. As with the other event horizons, it is
overlain by a clay layer (Unit 490). Units 490 through 400
decrease in thickness across the main fault zone, which sug-
gests that they were filling against a scarp that existed at
the time.
Evidence for E6 consists of upward terminations of
many faults at the top of Unit 570 in Trenches 1 and 6
(Figs. 6,7,8,9, and 16). Unit 570 is a dark, organically
stained, sandy silt layer 30 cm thick and is overlain by a
yellow-tan clay layer (Unit 560b), with the event horizon in
between. The faults show variable senses of vertical separation,
Table 1
Summary of Evidence for Earthquakes at Mystic Lake from T1, T6, and T7
Event Unit Exposure Evidence
E0 Top of 10 and
possibly higher
T1 M52 Thickening of the sand layer across the fracture zone into the sag and folding of Unit 10.
T7 M13 Faults offset Unit 10 with 13 cm down to the northeast separation and may extend up into Unit 1.
Upward termination of fractures.
Thickening of the sand layer across the fracture zone into the sag.
T6 M17 Faults offset Unit 10 with down to the southwest displacement.
E1 Top of 100 T1 M52,53 Upward termination of two clay seams with down to the southwest displacement.
T1 M17, 33 Faults cut Unit 200 and are lost in Unit 100.
T1 M76 Upward termination of fault.
T6 M14,15
nwM13,15
Upward termination of faults with down to the northeast displacement.
T7 M13,14,15 Upward termination of numerous fractures without resolvable displacement and one
or two faults that appear to disrupt two white silt layers within Units 100200.
E2 Top of 200 T1 M59 east Upward termination of a fault, fissure fill.
T1 M18 Upward termination of a fault. Southwest side down.
T1 M72 Upward termination of faults.
T1 M77 Upward termination of a fault.
T6 nwM16,17 Upward termination of faults.
E3 Top of 300 T1 M33 Upward termination of faults. Northeast side down.
T1 M45-53 Folding of Units 300 and below across the southwest fault zone.
T7 M12 Upward termination of faults that define a graben.
T7 M7-12 Folding of Units 300 and below truncated by an angular unconformity at the base
of Unit 250. Onlap and pinching of Units 290, 270 against a fold scarp.
E4 Top of 400 T1 M50,51 Upward termination of faults.
T1 M45-51 Thinning and pinching of Units 300400 against the E4 fold scarp.
T6 M14 Upward termination of faults.
E5 Top of 500 T1 M51 Upward termination of faults.
T1 M45-54 Thinning of units above the event horizon across the faults.
T6 M14 Upward termination of fault.
T6 M16, 17 Possible upward termination of faults.
E6 Top of 570 T1 M51,52,53 Upward termination of faults.
T1 M53 Possible fissure fill.
T6 M14 Warping of strata against the fault.
T6 M18 Upward termination of faults.
E7 Top of 600 T1 M53 Upward termination of fault.
T1 M54 Upward termination of fault.
T1 M73 Upward termination of faults.
Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake 9
Figure 9. Photomosaic of the main fault zone on the southeastern wall of Trench 6. Grid lines are spaced 1 m horizontally and 0.5 m vertically. Faults are traced in red for clarity.
Figure 8. Interpretation of the main fault zone in Trench 6 redrawn from photologs. Event horizons are marked as bold, black lines with stars denoting event number. Darker gray shades
indicate paleosol layers, and lighter shades indicate distinct clay layers that typically overlie event horizons. Faults are shown as black lines.
10 N. W. Onderdonk, T. K. Rockwell, S. F. McGill, and G. Marliyani
Figure 11. Photomosaic of the main fault zone on the southeastern wall of Trench 7. Grid lines are spaced 1 m horizontally and 0.5 m vertically. Faults are traced in red for clarity.
Figure 10. Interpretation of the main fault zone in Trench 7 redrawn from photologs. Event horizons are marked as bold, black lines with stars denoting event number. Darker gray shades
indicate paleosol layers, and lighter shades indicate distinct clay layers that typically overlie event horizons. Faults are shown as black lines.
Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake 11
and unit thickness changes across some of the faults, indicat-
ing a component of strike-slip displacement.
Event 7 is defined by a single fault observed in Trench 1
that terminates at the top of Unit 600. This fault was
observed in the main fault zone at the bottom of Trench 1,
which was excavated slightly deeper than were Trenches 6
and 7. Unit 600 is a silty clay that has a dark, organic stain
due to soil development and is overlain by a 20-cm-thick
Figure 12. Interpretation of panels 1618 on the southeastern wall of Trench 1. These panels are 30 m to the northeast of the main fault
zone (Fault A). Event horizons are marked as bold, sub-horizontal black lines with stars denoting event number. Darker gray shades indicate
paleosol layers, and lighter shades indicate distinct clay layers that typically overlie event horizons. Faults are shown as sub-vertical black lines.
Figure 13. Photomosaic of panels 1618 on the southeastern wall of Trench 1. Grid lines are spaced 1 m horizontally and 0.5 m ver-
tically. Faults are traced in red for clarity.
12 N. W. Onderdonk, T. K. Rockwell, S. F. McGill, and G. Marliyani
yellow-tan clay. Although a single fault relationship observed
in one trench would be considered weak evidence of a rupture
because the event horizon occurs at the boundary between a
paleosol and an overlying clay layer, as with the younger
event horizons, we suspect that more faults would have been
found that terminate at this stratigraphic level if this strati-
graphic depth had been exposed in the other trenches. This
hypothesis will be tested in future, deeper excavations.
The stratigraphy exposed in the trenches included sev-
eral very distinctive clay layers that allowed us to confidently
correlate units between the trenches without a connector
trench. Detrital charcoal dates (covered in subsequent discus-
Figure 14. Trench photo showing upward termination of two faults associated with Event 2. Key stratigraphic units are labeled. Grid
lines are spaced 1 m horizontally and 0.5 m vertically.
Figure 15. Trench photo showing upward termination offaults associated with Event 4. Key stratigraphic units are labeled. Grid lines are
spaced 1 m horizontally and 0.5 m vertically.
Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake 13
sion) from the three trenches support our correlations. The
stratigraphy also follows a cyclical pattern that provides
some insight into the depositional history of the lake as it
relates to faulting. The event horizons for Events E1 through
E7 occur at the top of paleosols developed in silty clay,
silt, or sandy silt. These units have a dark, organic stain or
coloration that we attribute to the surface accumulation of
organic humus and also contain abundant detrital charcoal.
Light-colored clay layers are deposited directly atop these
event horizons, and each of these defines an angular uncon-
formity. This repeated pattern of paleosols below the event
horizons overlain by clay (Fig. 17), along with the pinching
Figure 17. Oblique view looking south at the southeast wall of Trench 6. Paleosols are seen as layers with dark organic stains and are
overlain by lighter lacustrine clay units. Grid lines are spaced 1 m horizontally and 0.5 m vertically.
Figure 16. Trench photo showing upward termination of a fault associated with Event 6 and a clay-filled fracture associated with Event
0. Key stratigraphic units are labeled. Grid lines are spaced 1 m horizontally and 0.5 m vertically.
14 N. W. Onderdonk, T. K. Rockwell, S. F. McGill, and G. Marliyani
and thinning of units across the main fault zone, is inter-
preted to represent the repeated subsidence of the sag during
faulting events followed by flooding and filling of the de-
pression with lacustrine clay and other fine-grained, distal
alluvial fan sediments. The paleosols are interpreted to have
developed at the surface after the sag had filled and was dry,
followed by disruption of the stratigraphy during the next
earthquake, which again dropped the northeastern side of
the fault. The lack of seismites in the exposed section also
indicates that site is relatively dry most of the time and only
fills with water for brief periods following ground rupture,
which causes renewed depression of the sag. The base of
every distinctive clay unit, except Unit 540, corresponds with
an event horizon. Consequently, there may be another event
just below Unit 540 that was not recognized, which would
have occurred between Events E5 and E6. This hypothesis
will be tested in future excavations that will expose a greater
area of the fault zone at this stratigraphic level.
Event History
The timing of the events exposed in the trenches is con-
strained by radiocarbon dating of detrital charcoal recovered
from the stratigraphic units. More than 300 samples were
collected from Trenches 1, 6, and 7. We obtained radiometric
dates for 50 of these samples, which were processed at the
Keck Carbon Cycle Accelerator Mass Spectrometry Program
at the University of California, Irvine (Table 2). Multiple
samples were obtained from each stratigraphic layer to allow
us to evaluate the age range of charcoal samples within a
single layer. Because all of the samples were detrital char-
coal, there is an unknown amount of time between the
growth of the wood, formation of the charcoal during a brush
fire, and the deposition of the charcoal in the Mystic Lake
stratigraphy. Consequently, a layer is most likely younger
than the charcoal it contains, and the youngest charcoal age
from each layer is assumed to provide the closest approxi-
mation of the age of that layer. Because of this, we were
careful to avoid collecting samples that clearly had been
bioturbated. Using this logic, and the assumption that radio-
carbon samples with older ages than underlying units do not
represent the true age of the unit in which they exist, we elim-
inated about half of the dated samples from our stratigraphic
model (Table 2). A few samples that were significantly youn-
ger than numerous samples discussed previously were
assumed to be out of place due to bioturbation based on the
units in which they were collected and also were not used in
the analyses. All but two of these disregarded samples
returned modern ages, and the other two were collected from
massive, poorly stratified units that may have contained un-
recognizable burrows. The remaining samples were used to
determine an event history model (Fig. 18) using OxCal soft-
ware (Bronk Ramsey, 2009), which calculates the probability
density functions (PDFs) of sample ages based on the dendro-
chronologically calibrated radiocarbon curve of Reimer et al.
(2009). The PDFs of sample ages are trimmed using a
Baysian algorithm with the requirement that these samples
are in correct stratigraphic order. Event ages are given as
PDFs based on the ages of the samples that bracket the strati-
graphic event horizon. A PDF is also generated for the recur-
rence interval. In this model, the upper bound for Event E1
was set to 1850, after which a large earthquake on the
northern San Jacinto fault should be historically recorded.
This year also was chosen because the tin can found in Unit
10 above the event horizon limits the age of this unit to be no
older than 1850. Although the uppermost strata could be con-
siderably younger, historical records in the area extending
back to approximately this time do not include any large
earthquakes that could be attributed to the Claremont fault.
Event E0 was not included in the OxCal model because the
tin can constrained the age of Unit 10 to within the past
150 yr, and no dates were obtained from Unit 1 above the E0
event horizon. The timing of events from this model is shown
in Table 3.
Discussion
Interpretation of Deformation Events
Events E2 through E7 are interpreted to be the result of
ground-rupturing earthquakes based on the faulting and fold-
ing relationships observed in the trench exposures. All five of
these event horizons occur at the top of paleosols and are
overlain by clay or silty clay, which we interpret to represent
the progressive subsidence of the sag with each event. These
stratigraphic relationships clearly show discrete and sudden
subsidence events that are best interpreted to have occurred
as a result of large earthquakes, rather than by gradual sub-
sidence or fault creep.
The timing of the two youngest events requires that
interpretations other than co-seismic rupture be considered.
Event E0 is believed to have most likely occurred during the
past 150 yr or so, which is the well-recorded part of the his-
torical period. The lack of a large historical earthquake in the
area that could have produced surface rupture on the Clare-
mont fault indicates that this deformation is most likely not
due to co-seismic rupture. We instead interpret this deforma-
tion to be the result of either surface fractures and compac-
tion of the sag due to shaking from a nearby earthquake,
groundwater withdrawal in the area that has caused sub-
sidence and ground fissures in some parts of the San Jacinto
Valley (Morton, 1977), fault creep, or triggered slip. Several
large (M>6) earthquakes occurred in the vicinity of Mystic
Lake within the age range of this event, including the 1899
and 1918 earthquakes in the San Jacinto area to the southeast
and the 1923 earthquake in the San Bernardino area to the
northwest, all of which have been attributed to other parts of
the San Jacinto fault zone (Townley, 1918;Toppazada et al.,
2002). It is possible that fractures and small faults may have
formed at the Mystic Lake site in association with shaking
and/or localized liquefaction during these events or that these
events triggered minor amounts of tectonic slip at Mystic
Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake 15
Table 2
Mystic Lake, San Jacinto Fault. Dated C-14 Samples as of 10 February 2012
Events Unit Sample Number Trench Panel C-14 Age 1 Sigma Comments
Event 0 1
10 238 63 Modern
Event 1 Top of 100
100200 T7-13 18 105 15
100200 T6-64 17 120 15
100200 T7-44 18 130 15
100200 T7-43 5 620 15 Older than layers below.
100 T7-26 15 1740 90 Older than layers below.
190 T1-214 16 355 20 Out of stratigraphic sequence; probably too old.
Event 2 Top of 200
200 T1-219 33 40 20 Too youngbioturbated.
200 T1-7 E1 370 20 Older than layers below.
230 T1-210 16 195 20
240 T1-55 E20 300 20 Older than layers below.
250 T1-93p E57 195 20
270 T7-1 2 295 15
290 T1-220 34 670 90 Older than layers below.
290 T1-125 E67 345 20 Older than layers below, but error bars overlap.
290 T7-4 4 355 20 Older than layers below, but error bars overlap.
290 T1-65 E27 375 20 Older than layers below.
290 T1-208 16 625 20 Older than layers below.
Event 3 Top of 300
300 T1-5p E1 370 20 Older than layers below.
310 T1-221 34 820 20 Older than layers below.
320 T1-92p E56 325 20
320340 T7-9 9 325 15
340 T1-51 E20 390 20 Older than sample below, but source is uncertain.
360 T7-3 3 340 15
370 T1-212 17 1310 20 Older than other dates from same layer; probably too old.
370 T1-209 16 1095 20 Older than other dates from same layer; probably too old.
370 T1-226 49 865 20 Older than other dates from same layer
370 T6-49 12 440 15
370 T6-48 12 445 15
370 T6-52 14 1210 15 Older than other dates from same layer; probably too old.
390 T7-7 11 475 15
Event 4 Top of 400
400 T6-22 8 545 15 Older than sample from same layer.
400 T6-25 8 1120 15 Older than other dates from same layer; probably too old.
400 T6-46 11 480 30
430 T6-50 12 410 15 Younger than units abovebioturbated? In massive unit.
450 T6-42 11 1310 60 Older than other date from layer below; probably too old.
480 T6-45 11 785 15
400 T1-244 67 (890) 90 Bioturbated?
Event 5 Top of 500
500520 T1-302 50 740 15
500 T1-301 50 890 20
520 T6-61 16 1240 15 Older than units below.
530 T6-54 14 1045 15
540 T6-60 16 1120 15
560a T6-55 14 2160 80 Older than date from same layer and twice as old as nearby layers.
560a T6-56 14 1185 15 Same age as date from unit below.
560 T1-227 50 960 25 Location uncertain.
Event 6 Top of 570
580 T1-268 79 1185 20
580 T1-232 60 modern Bioturbated?
Event 7 Top of 600
600 T1-254 68 1470 30
690 T1-230 55 1550 20
700 T1-234 60 1685 25
700 T1-235 62 1725 20 Older than other date from same layer, but error bars overlap.
(continued)
16 N. W. Onderdonk, T. K. Rockwell, S. F. McGill, and G. Marliyani
Lake. However, no historic observations of the Mystic Lake
area are available to either support or negate this hypothesis,
and we note that there was very little evidence of liquefaction
or seismites in the exposed stratigraphy.
Morton (1977) documented numerous faults and
fractures that developed in the northern San Jacinto Valley
between 1953 and 1974 that exhibited vertical offsets of a
few centimeters to almost a meter. These features were most
common on the southwestern side of the basin, coincident
and roughly parallel with the Casa Loma fault, and consis-
tently offset the ground surface in a basin-side-down sense of
displacement. Most of the features initially developed as lin-
ear depressions and small holes that later merged to form one
continuous fissure. Based on the morphology and geometry
of the fractures, and the history of water pumping and water
table draw-down in the basin since 1919, Morton (1977)
interpreted these features to be the result of compaction of
the basin due to groundwater withdrawal. This mechanism
may also be responsible for the fractures and sag subsidence
associated with Event E0 at the Mystic Lake site. Although
the relative vertical separation is opposite of that reported on
the other fractures in the valley, it is reasonable to assume
that subsidence of the sag on the northeast side of the fault
is due to a greater degree of compaction of accumulated sedi-
ment in the sag.
Fault creep is a third possible cause for E0. Right-lateral
creep has been documented within the San Jacinto Valley on
the Casa Loma fault at a rate of about 2mm=yr (Proctor,
1974;Morton, 1977). However, preliminary data from a new
creep meter installed across the main southwest fault zone at
the Mystic Lake site in August 2010 does not show conclu-
sive evidence of creep to date (R. Bilham; personal comm.,
2012). A longer monitoring time is needed to fully evaluate
possible fault creep at the site, but we consider this process
less likely to produce the subsidence of the sag that occurred
during E0. We also find it less likely that shaking from
nearby earthquakes could have caused the subsidence and,
therefore, consider groundwater withdrawal to be the most
feasible cause for Event E0.
The modeled time range for Event E1 overlaps with the
earliest historical records in Southern California and the PDF
for E1 is heavily skewed with most of the area under the
curve toward the younger ages between 1800 and 1850
(Fig. 19). Earthquakes were recorded in mission records as
early as 1769 in California (Ellsworth, 1990). However, this
record is most likely incomplete in the San Jacinto area prior
to 1850 due to the sparse population and lack of nearby
missions. It is therefore possible that a major earthquake
may have occurred during this time and was not recorded
(Toppazada et al., 2002). Consequently, we interpret this
event to be a ground-rupturing earthquake on the Claremont
fault and include it in our event history. An alternative ex-
planation is that the deformation associated with this event
is related to creep or minor surface faulting and fracturing
during shaking from a nearby earthquake. Both the 1812
Wrightwood earthquake on the San Andreas fault and the
1800 San Juan Capistrano earthquake, believed to be located
on the central San Jacinto fault (Salisbury et al., 2012), are
possible candidates for triggering this deformation. This
alternative seems unlikely, however, given the vertical sep-
arations of tens of centimeters on faults associated with E1,
which suggest that the Claremont fault sustained consider-
able displacement and substantial subsidence of the sag.
A third possibility is that one of these early historical earth-
quakes (1800 or 1812) ruptured a longer portion of fault than
is currently modeled or believed, and rupture may have
extended into the Claremont fault. This would imply that
either the rupture associated with the 22 November 1800
earthquake recognized at Hog Lake jumped the San Jacinto
stepover or that the 11 December 1812 earthquake recog-
nized at Pallet Creek and Wrightwood on the Mojave section
of the San Andreas fault also caused rupture on the northern
San Jacinto fault. These possible explanations (creep, trig-
gered slip, and co-seismic slip) will be tested in future
research by trying to resolve the lateral displacement for E1
at Mystic Lake and along the fault to the north and south
using measurements of offset features. At this time, we
consider the evidence to support a significant displacement
that is best explained by a large earthquake.
Recurrence Interval for the Mystic Lake Events
Events E1 through E7 were used to calculate a recur-
rence interval PDF with a 95.4% confidence interval that
ranges from 159 to 210 yr with a median of 184 yr (Fig. 20).
The timing of these events is fairly periodic with the excep-
tion of the large gap between Events E5 and E6 (Fig. 19).
This may represent a seismically inactive period along the
Claremont fault, similar to gaps observed in paleoseismic
records from other faults within the Southern California fault
system (i.e., Fumal, Weldon, et al., 2002;Rockwell et al.,
2004,2006;Dolan et al., 2007). However, this section of
stratigraphy contains an additional paleosol overlain by a
clay layer, mimicking the pattern that accompanies the
known faulting events. Based on this relationship, we suspect
Table 2 (Continued)
Events Unit Sample Number Trench Panel C-14 Age 1 Sigma Comments
750 T1-239 63 (1105) 35 Bioturbated?
800 T1-250 67 1705 25
Bold type indicates samples that were used in the OxCal model; italics indicate samples that were not used.
Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake 17
Figure 18. Modeled age of radiocarbon samples and events based on charcoal samples from units from Trenches 1, 6, and 7 produced
using OxCal software (Bronk Ramsey, 2009). R_Date denotes radiocarbon age range based on calibration with the IntCal09 calibration curve
(Reimer et al., 2009). E1 through E7 denote event ages based on the ages of units above and below event horizon. Darker probability density
function (PDF) shading indicates trimming of age ranges based on stratigraphic constraints.
18 N. W. Onderdonk, T. K. Rockwell, S. F. McGill, and G. Marliyani
that an additional earthquake may have occurred between
Events E5 and E6, triggering the deposition of the Unit
540 clay. If this unseen event is included in the event history,
the recurrence interval is most likely closer to the shorter end
of the range given above.
The most likely age range for the most recent ground-
rupturing earthquake at the Mystic Lake site is between 1800
and 1850 based on the PDF for E1 (Fig. 19). This means that
approximately 160210 yr have passed since the last major
event, roughly equal to the calculated recurrence interval.
This, in turn, suggests that the Claremont fault may be close
to the end of its recurrence cycle and, assuming continued
periodic behavior, may experience a major earthquake in
the near future.
The recurrence interval range at Mystic Lake is similar
to that calculated for the central San Jacinto fault at the Hog
Lake paleoseismic site. There, Rockwell et al. (2006) deter-
mined a recurrence interval of 210 !134 yr based on 18
events in the past 4000 yr. Data from these two sites show
that the Late Holocene recurrence interval for the San Jacinto
fault zone is roughly twice as long as that calculated along
the Mojave section of the San Andreas fault at Wrightwood
(93131 yr; Scharer et al., 2007) and roughly equal to that of
the southern San Andreas fault as determined from the
Thousand Palms Oasis (215 !25 yr; Fumal, Rymer, and
Seitz, 2002) and Burro Flats (215 yr; D. Yule, personal
comm., 2010) paleoseismic sites. These data indicate that
the seismic strain release along the San Andreas fault in
the Mojave area is partitioned between the southern San An-
dreas and San Jacinto faults south of their juncture in the
eastern San Gabriel Mountains, as also suggested by inter-
ferometric synthetic aperture radar (InSAR)(Fialko, 2006),
and is similar to the pattern of slip-rate distribution across
the southern San Andreas system (McGill et al., 2010,2011).
Comparison with Regional Paleoseismic Data
Here, we use new paleoseismic data from the Mystic
Lake site to evaluate the rupture patterns within the southern
San Andreas fault system by comparing the timing of events
with those documented farther south and to the north and east
along the San Jacinto and San Andreas fault zones, respec-
tively. Comparison of the Mystic Lake event history to the
Hog Lake site shows that there is no strong correlation of
events, suggesting that the San Jacinto Valley stepover may
act as a segment boundary much of the time (Fig. 21). Three
of the events at Mystic Lake (E2 older peak, E4, and E6) on
the Claremont fault appear to occur during gaps in activity at
Hog Lake on the Clark fault. This suggests that these events
did not rupture through the stepover and that the two fault
segments alternate stress release through time. However,
E5, E3, the younger peak in the PDF of E2, and E1 at Mystic
Lake overlap with events at Hog Lake, leaving open the pos-
sibility that some events may have jumped the San Jacinto
stepover, or that earthquakes on one fault triggered closely
timed events on the other. Wesnousky (2006) showed that
60% of historic earthquake ruptures jumped across fault
steps of 34 km in width or less, and that none propagated
across steps greater than this dimension. The San Jacinto
Valley stepover, between the Claremont and Casa Loma
faults, is 5km at its widest point, but only about 2 km wide
at Mystic Lake. If the San Jacinto stepover is not a segment
boundary and ruptures can propagate across it, this would
raise the maximum potential moment magnitude of earth-
quakes on this part of the fault zone from an Mw7.4 to an
Mw7.8 based on empirical relationships between surface
rupture length and earthquake magnitude (Wells and Copper-
Table 3
Event History from Mystic Lake
Event From (A.D.) To (A.D.) Median Age (A.D.)
E0 1850 1940 1895
E1 1744 1853 1837
E2 1665 1820 1698
E3 1521 1616 1574
E4 1403 1445 1428
E5 1273 1419 1342
E6 807 961 887
E7 579 846 710
1400 1800
1000
600
200
Years A.D.
02012
1
2
3
4
5
6
7
No Data
Probability of earthquake occurrence during a calendar year
0.05
0.025
Figure 19. Normalized probability density functions (PDFs) of
event ages for the Mystic Lake site.
Figure 20. Probability density function (PDF) for the recur-
rence interval of events at Mystic Lake.
Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake 19
smith, 1994). It was not possible to test the feasibility of a
single rupture affecting both of these sites based on measure-
ments of displacement at a single point along a fault to infer
the rupture lengths (e.g., Biasi and Weldon, 2006) because
we were unable to determine slip vectors for any of the
events. Most of the events at the Mystic Lake site are rep-
resented by multiple faults with variable amounts and senses
of vertical displacement, and we have no constraints on the
amount of strike-slip displacement.
Preliminary paleoseismic data exist from two other sites
on the Claremont fault north of the Mystic Lake site, and
some interpretations can be made regarding the most recent
events on the Claremont fault by evaluating these data in
conjunction with the Mystic Lake record. In the northern
Moreno Valley, we conducted trenching along the Claremont
fault near the north end of Quincy road, 10 km northwest
of Mystic Lake (Quincy site, Fig. 1). The purpose of this
other trenching was to evaluate the slip rate (not presented
in this paper), but the trenches also exposed evidence for
the most recent two events along that section of the fault. The
age ranges for these two events are A.D. 16801850 (the be-
ginning of the historic record) for E1, and A.D. 15301675
for E2. These ages overlap with the last two events at Mystic
Lake and suggest that these earthquakes ruptured at least
10 km of the Claremont fault. In addition, if these are the
same events, combining the age ranges for the penultimate
event from both sites constrains the Mystic Lake Event E2 to
the earlier peak in the PDF, making it less likely that this
event correlates with E1 at Hog Lake (Fig. 21). Kendrick
and Fumal (2005) report two events in the past 200 rcyBP
(post-A.D. 1652) from a paleoseismic site in Colton,
30 km northwest along the Claremont fault. If these data
from separate sites record the same events, these observa-
tions suggest that the past two earthquakes ruptured at least
half the total length of the Claremont fault (40 km). Given the
poor historical record of earthquakes in the area until about
1890 (Toppazada et al., 2002), it is possible that one or both
of these events were not recorded. The alternative is that the
events at these sites do not correlate, which would require
maximum rupture lengths of less than 30 km (the distance
between Mystic Lake and Colton) and, consequently, smaller
earthquakes. The timing of the two most recent events at the
other sites also supports our interpretation that the ground
cracking associated with E0 at Mystic Lake is most likely
a local effect due to groundwater withdrawal.
Comparison of the Mystic Lake event history to
paleoseismic sites along the San Andreas fault in the San
Bernardino area presents some interesting observations. A
compilation of the available data shows that there have not
been any ground-rupturing earthquakes on these two major
faults south of the juncture since 1812 (Fig. 21). We are cur-
rently in a 200-yr interval of no ground-rupturing earth-
quakes, which is almost equal to the longest quiet period in
the last 1400 yr of available data for the San Andreas and San
Jacinto faults in the San Bernardino area, and is roughly equal
to the average recurrence intervals calculated for these faults.
This suggests that these faults may be approaching the ends of
their seismic cycles. The data also show several overlaps in
event ages between the Mystic Lake and Wrightwood sites.
E2 at Mystic Lake occurred at approximately the same time
Figure 21. Comparison of event histories recorded at paleoseismic sites in the vicinity of the junction between the northern San Jacinto
fault and the San Andreas fault. Wrightwood data was obtained from Fumal, Weldon, et al. (2002), Pitman Canyon data from Seitz et al.
(1997), Burro Flats data from D. Yule (personal comm., 2010), and Hog Lake data from Rockwell et al. (2004). Pitman Canyon, Burro Flats,
and Hog Lake probability density functions (PDFs) are normalized.
20 N. W. Onderdonk, T. K. Rockwell, S. F. McGill, and G. Marliyani
as Events 2 and 3 at Burro Flats and Wrightwood, respec-
tively. We find it unlikely that these three events are the same
earthquake as that would imply that a rupture on the Mojave
section of the San Andreas fault also ruptured both the
southern San Andreas and San Jacinto faults to the south.
However, the overlap in event ages may represent an earth-
quake on one fault triggering an earthquake close in time
on the other fault. Despite other overlaps in the event ages,
there is no clear evidence of ruptures jumping between the
San Andreas and San Jacinto faults, which we expect would
most likely be expressed as events that show overlap between
Mystic Lake and Wrightwood, but not between Wrightwood
and sites on the San Andreas fault south of the juncture be-
tween the two faults. We anticipate that additional trenching
will lead to a longer paleoseismic record at Mystic Lake that
can be used to further evaluate the seismic patterns in this part
of the fault system.
Possible Correlations of the Most Recent Events at
Mystic Lake with Historic Earthquakes
We consider two possible scenarios for the last two
events at Mystic Lake by assuming that any large ground-
rupturing earthquakes on the Claremont fault during the
post-1850 historical period would have been recorded. Event
E2 has a double peak in its PDF, which results from two
possible age ranges for the radiocarbon samples above and
below the event horizon because of variations in the tree-ring
calibration curve (Reimer et al., 2009). Taking the older age
ranges for these two samples would limit the age range of E2
to its older peak, which constrains the event to between A.D.
1665 and 1730 and predates the historical period. This would
also agree with the age of the penultimate event on the Clare-
mont fault dated at the Quincy site 10 km to the northwest.
This interpretation would allow E1 to have occurred between
1750 and 1850, with the age range overlapping the last event
at Hog Lake. A large earthquake occurred on 22 November
1800 that caused extensive damage at the San Juan Capi-
strano and San Diego missions (Toppazada et al., 1981).
Rockwell et al. (2004) and Salisbury et al. (2012) interpreted
this earthquake to have occurred on the Clark strand of the
San Jacinto fault zone and to be recorded as the most recent
event in the Hog Lake paleoseismic record. If this is also
correlative with E1 at Mystic Lake, then it is reasonable
to assume that the 1800 earthquake ruptured through the re-
leasing stepover between the Claremont and Casa Loma
faults. Geomorphic offsets along the Clark fault indicate a
minimum rupture length of 85 km for this earthquake, and
the addition of the Casa Loma and Claremont faults would
add another 6080 km. Therefore, this interpretation would
involve a total rupture length of at least 145165 km, which
corresponds to an Mw7.7 or larger earthquake (Wells and
Coppersmith, 1994). This agrees well with the estimated
magnitude based on damage at the San Diego and San
Juan Capistrano missions (Toppazada et al., 1981) and recent
estimates based on offset features along the Casa Loma
Clark fault to the northwest of Hog Lake (Salisbury et al.,
2012). We note, however, that this event was not recorded
at the San Gabriel mission, which may be due to a lack
of reporting, and may indicate that the rupture did not
propagate very far up the Claremont fault or may imply that
the earthquake ruptured from the northwest to the southeast,
producing directivity effects and thereby limiting the degree
of ground motion in the San Gabriel Valley 75 km to
the west.
An alternative interpretation can be reached if the youn-
ger ages in the radiocarbon calibration curve are used for
samples above and below E2 at Mystic Lake. In this case, E2
at Mystic Lake would not overlap with the penultimate event
at the Quincy site 10 km to the northwest and must have died
out between these two sites. Using this younger age for E2,
however, means it would overlap with the most recent
event at Hog Lake and may represent the 1800 San Juan
Capistrano earthquake. Event E1 at Mystic Lake would then
be interpreted as a younger historical event. The only strong
candidate for a younger large event in the area is the 1812
Wrightwood earthquake that caused severe damage in the
Los Angeles basin and is interpreted to have occurred on
the Mojave segment of the San Andreas fault (Jacoby et al.,
1988). Consensus has not been reached regarding the paleo-
seismic record of the 1812 event on the San Bernardino
strand of the San Andreas fault. Radiocarbon dates consistent
with an event at this time are present at the northwestern and
southeastern ends of this fault section (Seitz et al., 1997;
Yule and Seih, 2001), and the presence of European pollen
in layers faulted by the most recent event at Burro Flats
suggest it occurred after the mid- to late 1700s (D. Yule, per-
sonal comm., 2010). However, numerous radiocarbon dates
from the Plunge Creek site, in the middle of the San Bernar-
dino strand suggest the most recent event there occurred prior
to A.D. 1730 (McGill et al., 2002). Regardless of the uncer-
tainty in the southeastward extent of the 1812 earthquake
rupture on the San Andreas fault, the inferred isoseismals
constructed by Toppazada et al. (2002) do not fit with a
rupture along the northern San Jacinto fault zone. Because
of this, and the fact that using the younger ages precludes
the correlation of the penultimate events at the Mystic Lake
and Quincy sites, we consider this scenario unlikely and pre-
fer to interpret the most recent event at Mystic Lake to be
either the 1800 San Juan Capistrano earthquake or an unre-
corded earthquake during the early historical period.
Conclusions
A new paleoseismic site is explored along the Claremont
fault at the northern end of the ephemeral Mystic Lake in the
San Jacinto Valley. The site is a 500-m-wide sag between two
strands of the Claremont fault and contains strata favorable to
the preservation of prehistoric fault ruptures. The well-
preserved stratigraphy and abundant detrital charcoal indi-
cates that the site has the potential for a long paleoseismic
Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake 21
record similar to those developed at the Wrightwood and
Hog Lake paleoseismic sites.
There is evidence for seven earthquakes in the upper
1.8 m of strata that span the past 1700 yr. The recurrence
interval for these earthquakes is between 160 and 210 yr,
with the most recent earthquake occurring 200 yr ago. This
suggests that the Claremont fault may be close to failure.
Some of these earthquake events occurred during quiet peri-
ods at the Hog Lake site. However, at least three events are
similar in age between the two sites, which suggests that
some earthquakes may be capable of rupturing through the
San Jacinto releasing stepover between the Casa Loma and
Claremont fault segments. A comparison of event ages be-
tween the Mystic Lake site and paleoseismic sites along the
San Andreas fault north and south of the juncture between
these two faults does not show any strong evidence for events
jumping from one fault to the other. Overlaps in event ages,
however, may suggest that earthquakes on one fault trigger
closely timed earthquakes on the other.
Data and Resources
The event history was constructed using OxCal software
(Bronk Ramsey, 2009:http://c14.arch.ox.ac.uk/embed.php?
File=oxcal.html, last accessed February 2012), which calcu-
lates the PDFs of radiocarbon sample ages based on the
dendro-chronologically calibrated radiocarbon curve of Re-
imer et al. (2009). Creep data from the Mystic Lake site is
available at Roger Bilhams website at the University of
creepmeters.htm (last accessed March 2012).
Acknowledgments
We would like to thank a number of California State University (CSU)
students, Southern California Earthquake Center (SCEC) interns, and visiting
researchers that assisted with fieldwork during this study. These include Eu-
lalia Masana, Mark Swift, Brian Anderson, Karina Chung, John Duncan,
Rebecca Tsang, Nissa Morton, J. Barrett Salisbury, Mike Buga, Ramon Han-
cock, Scott Kenyon, and Blaise Delgado. We also thank Scott Sewell of the
San Jacinto Wildlife Area for his assistance with this project and allowing us
to access the Mystic Lake site. David Haddad and an anonymous reviewer
greatly improved this manuscript with 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 Contribution Number for
this paper is 1503.
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Department of Geological Sciences
California State University Long Beach
1250 Bellflower Boulevard
Long Beach, California 90840
nate.onderdonk@csulb.edu
(N.W.O.)
Department of Geological Sciences
San Diego State University
5500 Campanile Drive
San Diego, California 92182-1020
(T.K.R.)
Department of Geological Sciences
California State University San Bernardino
5500 University Parkway
San Bernardino, California 92407-2318
(S.F.M.)
School of Earth and Space Exploration
Arizona State University
Bateman Physical Sciences Center F-wing Room 686
Tempe, Arizona 85287-1404
(G.M.)
Seven Surface Ruptures in the Past 1600 Years on the Claremont Fault at Mystic Lake 23
... The most recent event recorded at Plunge Creek on the San Bernardino strand (site 3, Fig. 1) is dated to within the A.D. 1600s (McGill et al., 2002), but minor slip on secondary structures farther south along the San Bernardino strand, near Burro Flats (site 4, Fig. 1), dates to the early 1800s (Yule and Howland, 2001). Several paleoseismic sites along the northernmost strand of the San Jacinto fault record 1.8-3 m of slip during an early 1800s earthquake event (Kendrick and Fumal, 2005;Onderdonk et al., 2013Onderdonk et al., , 2015. This record supports the plausibility of the 1812 earthquake jumping the <2 km extensional stepover between the San Andreas and San Jacinto faults ( Fig. 1) and involving both faults (Onderdonk et al., 2013(Onderdonk et al., , 2015Rockwell et al., 2015). ...
... Several paleoseismic sites along the northernmost strand of the San Jacinto fault record 1.8-3 m of slip during an early 1800s earthquake event (Kendrick and Fumal, 2005;Onderdonk et al., 2013Onderdonk et al., , 2015. This record supports the plausibility of the 1812 earthquake jumping the <2 km extensional stepover between the San Andreas and San Jacinto faults ( Fig. 1) and involving both faults (Onderdonk et al., 2013(Onderdonk et al., , 2015Rockwell et al., 2015). Lozos (2016) used dynamic rupture models to investigate rupture scenarios that best fit the paleoseismic evidence and historical accounts of the Wrightwood earthquake. ...
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... Hauksson et al. 2012;Ross et al. 2017a;Ben-Zion & Zaliapin 2019), and is capable of producing large M w > 7.0 earthquakes (e.g. Peterson & Wesnousky 1994;Onderdonk et al. 2013;Rockwell et al. 2015). Several M w > 5 earthquakes occurred during the past decade in the complex trifurcation area of the SJFZ (Fig. 1a) associated with its three major fault branches, including the 2016 M w 5.2 Borrego Springs (BS) earthquake with a hypocentre at ∼12 km depth. ...
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We study temporal changes of seismic velocities associated with the 10 June 2016 Mw 5.2 Borrego Springs earthquake in the San Jacinto fault zone, using nine component Green's function estimates reconstructed from daily cross correlations of ambient noise. The analysed data are recorded by stations in two dense linear arrays, at Dry Wash (DW) and Jackass Flat (JF), crossing the fault surface trace ∼3 km northwest and southeast of the event epicentre. The two arrays have 9 and 12 stations each with instrument spacing of 25–100 m. Relative velocity changes (δv/v) are estimated from arrival time changes in the daily correlation coda waveforms compared to a reference stack. The obtained array-average δv/v time-series exhibit changes associated with the Borrego Springs event, superposed with seasonal variations. The earthquake-related changes are characterized by a rapid coseismic velocity drop followed by a gradual recovery. This is consistently observed at both arrays using time- and frequency-domain δv/v analyses with data from different components in various frequency bands. Larger changes at lower frequencies imply that the variations are not limited to the near surface material. A decreasing coseismic velocity reduction with coda wave lapse time indicates larger coseismic structural perturbations in the fault zone and near-fault environment compared to the surrounding rock. Observed larger changes at the DW array compared to the JF array possibly reflect the northwestward rupture directivity of the Borrego Springs earthquake.
... The SJFZ accommodates a large proportion of the plate motion with slip rates of up to 21 mm year -1 (Fialko 2006). Paleoseismic and historic records indicate that the SJFZ is capable of large (M W [ 7.0) earthquakes (e.g., Petersen and Wesnousky 1994;Rockwell et al. 2015), and has the potential to rupture the entire length of the fault zone in a single event (e.g., Salisbury et al. 2012;Onderdonk et al. 2013). The last through-going event probably occurred on 22 November 1800 (Salisbury et al. 2012), Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00024-018-2055-y) ...
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We analyze the background stress field around the San Jacinto fault zone (SJFZ) in Southern California with a refined inversion methodology using declustered focal mechanisms of background seismicity. Stress inversions applied to the entire fault zone, and three focus areas with high level of seismicity, provide three-dimensional distributions of the maximum horizontal compression direction (SHmax), principal stress plunges, and stress ratio $$R = (\sigma_{1} - \sigma_{2} )/(\sigma_{1} - \sigma_{3} )$$. The results are compared with coseismic strain parameters derived from direct summation of earthquake potencies and b values of frequency–size event statistics. The main stress regime of the SJFZ is strike-slip, although the northwest portion near Crafton Hills displays significant transtension. The SHmax orientation rotates clockwise with increasing depth, with the largest rotation (23°) observed near Crafton Hills. The principal stress plunges have large rotations below ~ 9 km, near the depth section with highest seismicity rates and inferred brittle–ductile transition zone. The rotations produce significant deviations from Andersonian strike-slip faulting, likely generating the observed increased dip-slip faulting of relatively deep small events. The stress ratio parameters and b value results are consistent with increasing number of dip-slip faulting below ~ 9 km. The derived coseismic strain parameters are in good agreement with the stress inversion results. No large-scale stress rotations are observed across the time of the 2010 Mw 7.2 El Mayor–Cucapah earthquake. The stress ratio near the Trifurcation area of the SJFZ changes after the El Mayor–Cucapah earthquake toward transpression, but this may have been produced by local M > 5.4 event or aseismic slip.
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Recurrence intervals for ground rupturing earthquakes are critical data for assessing seismic hazard. Recurrence intervals are presented here for 38 paleoseismic sites in California. Eleven of these include new or updated data; the remainder use data previously included in the Unified California Earthquake Rupture Forecast Version 3 (UCERF3). The methods and results are consistent with UCERF3. In addition, revised recurrence intervals are presented at every site. The revised recurrence intervals incorporate uncertainty in the interpretation of paleoseismic evidence, which is expressed as event likelihood. Event likelihood is the probability that the evidence has been correctly interpreted as a unique earthquake. Event likelihoods are estimated here for 85 inferred past earthquakes at eight paleoseismic sites in California, using a single, consistent methodology. The average event likelihood is 0.85. The revised recurrence intervals are 16% longer, on average, than conventional estimates, and their confidence intervals are disproportionately wider. These recurrence intervals are suitable for inclusion in a “grand inversion” rupture forecast, and they may be important for addressing a systematic misfit in the UCERF3 grand inversion. The revised recurrence intervals may also be important for assessing the unusually long earthquake hiatus in California. Other applications may not need to consider event likelihoods because the effects are small relative to typical uncertainties.
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We examine the stress field in Southern California with a focus on the region near the South Central Transverse Ranges (SCTR), using a refined stress inversion methodology. We independently invert declustered and aftershocks focal mechanisms from the years 1981 to 2017. Comparisons between the stress field orientations and the stress ratios from these inversions provide information on local sources of the stress field. Regionally, the SHmax tends to be toward NNE and the stress ratios vary from transtensional stress regime near the Eastern California Shear Zone (ECSZ), to strike‐slip faulting near the SCTR, and toward transpression near the Western Transverse Ranges. Detailed analysis of stress parameters near the SCTR indicates deviations from the regional strike‐slip faulting including the following: (1) The San Bernardino Mountain area shows SHmax direction toward NNW and transpressional stress components likely associated with the motions of the San Andreas Fault and ECSZ; (2) the Cajon Pass and San Gorgonio Pass show transpressional stress regime near the bottom of the seismogenic zone likely associated with the elevated topography; and (3) in Crafton Hills, rotation of the principal stress plunges and SHmax direction and transtensional stress regime below ~10 km, along with lower estimated apparent friction coefficient, suggest a weak fault possibly associated with deep creep. The performed multiscale analysis resolves effects of regional and local loadings. The average stress parameters in the analyzed 37 years do not show significant temporal variations of stress field near the SCTR region.
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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.
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We investigate the existence of bimaterial interfaces along the San Jacinto fault zone (SJFZ) and the San Andreas fault (SAF) around their intersection at Cajon Pass (CP) to clarify if large earthquakes can rupture simultaneously parts of both faults. Analysis of fault zone head waves reveals a bimaterial interface along the northern SJFZ with a >7.5% velocity contrast (slower northeast side) that extends continuously through and perhaps slightly beyond CP. Other seismological studies show an opposite sense of velocity contrast across the nearby SAF. These observations, combined with expected properties of bimaterial ruptures, suggest that large earthquakes on the northern SJFZ are likely to propagate to the southeast and hence unlikely to trigger large SAF events. However, large earthquakes propagating on the SAF near CP can trigger continuing rupture on the SJFZ to the southeast. This inference is consistent with ruptures of large paleoseismic earthquakes and directivities of smaller events.
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Paleoseismic investigations across the Mission Creek strand of the San Andreas fault at Thousand Palms Oasis indicate that four and probably five surface-rupturing earthquakes occurred during the past 1200 years. Calendar age estimates for these earthquakes are based on a chronological model that incorporates radio-carbon dates from 18 in situ burn layers and stratigraphic ordering constraints. These five earthquakes occurred in about A.D. 825 (770-890) (mean, 95% range), A.D. 982 (840-1150), A.D. 1231 (1170-1290), A.D. 1502 (1450-1555), and after a date in the range of A.D. 1520-1680. The most recent surface-rupturing earthquake at Thousand Palms is likely the same as the A.D. 1676 ± 35 event at Indio reported by Sieh and Williams (1990). Each of the past five earthquakes recorded on the San Andreas fault in the Coachella Valley strongly overlaps in time with an event at the Wrightwood paleoseismic site, about 120 km northwest of Thousand Palms Oasis. Correlation of events between these two sites suggests that at least the southernmost 200 km of the San Andreas fault zone may have ruptured in each earthquake. The average repeat time for surface-rupturing earthquakes on the San Andreas fault in the Coachella Valley is 215 ± 25 years, whereas the elapsed time since the most recent event is 326 ± 35 years. This suggests the southernmost San Andreas fault zone likely is very near failure. The Thousand Palms Oasis site is underlain by a series of six channels cut and filled since about A.D. 800 that cross the fault at high angles. A channel margin about 900 years old is offset right laterally 2.0 ± 0.5 m, indicating a slip rate of 4 ± 2 mm/yr. This slip rate is low relative to geodetic and other geologic slip rate estimates (26 ± 2 mm/yr and about 23-35 mm/yr, respectively) on the southernmost San Andreas fault zone, possibly because (1) the site is located in a small step-over in the fault trace and so the rate is not be representative of the Mission Creek fault, (2) slip is partitioned northward from the San Andreas fault and into the eastern California shear zone, and/or (3) slip is partitioned onto the Banning strand of the San Andreas fault zone.
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The San Timoteo badlands is an area of uplift and erosional dissection that has formed as a result of late Quaternary uplift along a restraining bend in the San Jacinto fault, of the San Andreas fault system in southern California. This bend currently is located in a region where late Quaternary deposits and associated surfaces have formed in lower San Timoteo Canyon. We have used morphometric analysis of these surfaces, in conjunction with computer modeling of deformational patterns along the San Jacinto fault, to reconstruct spatial and temporal variations in uplift along the bend. Morphometric techniques used include envelope/subenvelope mapping, a gradient-length index along channels, and denudation values. Age control is determined using a combination of thermoluminescence (TL) and near infrared optical simulation luminescence dating (IROSL) and correlation of soil-development indices. These approaches are combined with an elastic half-space model used to determine the deformation associated with the fault bend. The region of modeled uplift has a similar distribution as that determined by morphometric techniques. Luminescence dates and soil-correlation age estimates generally agree. Based on soil development, surfaces within the study area were stabilized at approximately 300-700 ka for Q3, 43-67 ka for Q2, and 27.5-67 ka for Q1. Luminescence ages (both TL and IROSL) for the formation of the younger two surfaces are 58 to 94 ka for Q2 and 37 to 62 ka for Q1 (ages reported to 1σ uncertainty). Periods of uplift were determined for the surfaces in the study area, resulting in approximate uplift rates of 0.34 to 0.84 m/ka for the past 100 ka and 0.13 to 1.00 m/ka for the past 66 ka. Comparison of these rates of uplift to those generated by the model support a higher rate of lateral slip along the San Jacinto fault than commonly assumed (greater than 20 mm/yr, as compared to 8-12 mm/yr commonly cited). This higher slip rate supports the proposal that a greater amount of slip has transferred from the San Andreas fault to the San Jacinto fault than generally held. The San Jacinto fault may have accommodated a significant portion of the plate boundary slip during the past 100 ka.
Chapter
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
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We have documented the stratigraphy and structure of several trenches across the San Bernardino strand of the San Andreas fault at the Plunge Creek site, near San Bernardino, southern California. The most recent faulting event exposed in the trenches (event W) appears to have occurred between about A.D. 1440 and A.D. 1660, if the radiocarbon dates are taken at face value. Two of the trenches reveal suggestive evidence for an older faulting event (event R), which postdates A.D. ∼1220. Because the age control at Plunge Creek is based on radiocarbon dating of detrital-charcoal samples, we must consider all of the radiocarbon ages as maximum estimates of the depositional ages for the layers from which the samples were collected. Thus, event W is not strictly constrained to predate A.D. 1660. We use ecological arguments to infer that the detrital-charcoal samples at the Plunge Creek site probably overestimate the depositional ages of the sedimentary layers by about 1 ± 1 fire-cycle (i.e., by about 70 ± 70 years). An independent estimate, based on extrapolation of sedimentation rates to the ground surface, suggests a similar value (0–95 years) for the lag time between the calibrated radiocarbon date of a sample and the depositional date of the layer from which it was collected. After applying an estimated correction (70 years) for the inherited ages of the detrital-charcoal samples, the date of event W is most likely between A.D. 1510 and A.D. 1730, with a preferred date of about A.D. 1630. The preferred date for event R is about A.D. 1450. Given the range of allowable dates, event W probably correlates with either the second-youngest earthquake (A.D. ∼1690) or the third youngest earthquake (A.D. ∼1600) documented at the nearest paleoseismic site to the northwest (Pitman Canyon). Comparison with the paleoseismic record at other sites along the southern San Andreas fault suggests a rupture length of 85–190 km for event W, implying a magnitude of M 7.3–7.7. Event R probably correlates with the fourth-youngest event at Pitman Canyon (∼A.D. 1450). Prehistoric earthquakes within several decades of this date have been documented all along the ∼450-km length of the southern San Andreas fault, suggesting (though by no means requiring) that event R could potentially have been a very large earthquake (M ∼8.2). If our interpretation is correct, surface rupture from the youngest earthquake at Pitman Canyon (A.D. 1812) apparently died out between Pitman Canyon and Plunge Creek. Previous estimates of the probability of future earthquakes have assumed that the entire length of the San Bernardino strand slipped during the 1812 earthquake. Our results suggest that the southeastern half of this section of the fault did not slip in 1812 and that strain has been accumulating on this portion of the fault for at least the past 3 centuries. Depending on the slip rate, accumulated strain sufficient to generate 4.2–7.5 m of right-lateral slip may currently be stored on the southeastern half of the San Bernardino strand of the San Andreas fault.
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We present structural and stratigraphic evidence from a paleoseismic site near Wrightwood, California, for 14 large earthquakes that occurred on the southern San Andreas fault during the past 1500 years. In a network of 38 trenches and creek-bank exposures, we have exposed a composite section of interbedded debris flow deposits and thin peat layers more than 24 m thick; fluvial deposits occur along the northern margin of the site. The site is a 150-m-wide zone of deformation bounded on the surface by a main fault zone along the northwest margin and a secondary fault zone to the southwest. Evidence for most of the 14 earthquakes occurs along structures within both zones. We identify paleoearthquake horizons using infilled fissures, scarps, multiple rupture terminations, and widespread folding and tilting of beds. Ages of stratigraphic units and earthquakes are constrained by historic data and 72 14C ages, mostly from samples of peat and some from plant fibers, wood, pine cones, and charcoal. Comparison of the long, well-resolved paleoseimic record at Wrightwood with records at other sites along the fault indicates that rupture lengths of past earthquakes were at least 100 km long. Paleoseismic records at sites in the Coachella Valley suggest that each of the past five large earthquakes recorded there ruptured the fault at least as far northwest as Wrightwood. Comparisons with event chronologies at Pallett Creek and sites to the northwest suggests that approximately the same part of the fault that ruptured in 1857 may also have failed in the early to mid-sixteenth century and several other times during the past 1200 years. Records at Pallett Creek and Pitman Canyon suggest that, in addition to the 14 earthquakes we document, one and possibly two other large earthquakes ruptured the part of the fault including Wrightwood since about A.D. 500. These observations and elapsed times that are significantly longer than mean recurrence intervals at Wrightwood and sites to the southeast suggest that at least the southermost 200 km of the San Andreas fault is near failure.
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Geodimeter measurements from 1969 to 1975 of a geodetic network (aperture 12 km) across the San Jacinto fault near Riverside, California, indicate simple right-lateral shear accumulation at the rate 0.4 +- 0.1 ..mu..strain/y (engineering shear) parallel to the fault. A 12-km level line extending across the fault shows no consistent pattern of elevation change. The measured shear accumulation is interpreted in terms of a conventional dislocation model for strike slip, i.e., continuous slip on the vertical fault beneath a surficial locked zone. The observation of strain accumulation constrains the model to cases in which the ratio of the slip rate to the depth of the locked zone is about 1.2 x 10â»â¶/y (e.g., 25 mm/y slip below 20 km).
Article
The San Timoteo badlands is an area of uplift and erosional dissection that has formed as a result of late Quaternary uplift along a restraining bend in the San Jacinto fault, of the San Andreas fault system in southern California. This bend currently is located in a region where late Quaternary deposits and associated surfaces have formed in lower San Timoteo Canyon. We have used morphometric analysis of these surfaces, in conjunction with computer modeling of deformational patterns along the San Jacinto fault, to reconstruct spatial and temporal variations in uplift along the bend. Morphometric techniques used include envelope/subenvelope mapping, a gradient-length index along channels, and denudation values. Age control is determined using a combination of thermoluminescence (TL) and near infrared optical simulation luminescence dating (IROSL) and correlation of soil-development indices. These approaches are combined with an elastic half-space model used to determine the deformation associated with the fault bend. The region of modeled uplift has a similar distribution as that determined by morphometric techniques. Luminescence dates and soil-correlation age estimates generally agree. Based on soil development, surfaces within the study area were stabilized at approximately 300–700 ka for Q3, 43–67 ka for Q2, and 27.5–67 ka for Q1. Luminescence ages (both TL and IROSL) for the formation of the younger two surfaces are 58 to 94 ka for Q2 and 37 to 62 ka for Q1 (ages reported to σ uncertainty). Periods of uplift were determined for the surfaces in the study area, resulting in approximate uplift rates of 0.34 to 0.84 m/ka for the past 100 ka and 0.13 to 1.00 m/ka for the past 66 ka. Comparison of these rates of uplift to those generated by the model support a higher rate of lateral slip along the San Jacinto fault than commonly assumed (greater than 20 mm/yr, as compared to 8–12 mm/yr commonly cited). This higher slip rate supports the proposal that a greater amount of slip has transferred from the San Andreas fault to the San Jacinto fault than generally held. The San Jacinto fault may have accommodated a significant portion of the plate boundary slip during the past 100 ka.