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The most recent large earthquake on the Rodgers Creek fault, San Francisco Bay area

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The Rodgers Creek fault (RCF) is a principal component of the San Andreas fault system north of San Francisco. No evidence appears in the historical record of a large earthquake on the RCF, implying that the most recent earthquake (MRE) occurred before 1824, when a Franciscan mission was built near the fault at Sonoma, and probably before 1776, when a mission and presidio were built in San Francisco. The first appearance of nonnative pollen in the stratigraphic record at the Triangle G Ranch study site on the south-central reach of the RCF confirms that the MRE occurred before local settlement and the beginning of livestock grazing. Chro-nological modeling of earthquake age using radiocarbon-dated charcoal from near the top of a faulted alluvial sequence at the site indicates that the MRE occurred no earlier than A.D. 1690 and most likely occurred after A.D. 1715. With these age constraints, we know that the elapsed time since the MRE on the RCF is more than 181 years and less than 315 years and is probably between 229 and 290 years. This elapsed time is similar to published recurrence-interval estimates of 131 to 370 years (preferred value of 230 years) and 136 to 345 years (mean of 205 years), calculated from geologic data and a regional earthquake model, respectively. Importantly, then, the elapsed time may have reached or exceeded the average recurrence time for the fault. The age of the MRE on the RCF is similar to the age of prehistoric surface rupture on the northern and southern sections of the Hayward fault to the south. This suggests possible rupture scenarios that involve simultaneous rupture of the Rodgers Creek and Hayward faults. A buried channel is offset 2.2 (1.2, 0.8) m along one side of a pressure ridge at the Triangle G Ranch site. This provides a minimum estimate of right-lateral slip during the MRE at this location. Total slip at the site may be similar to, but is probably greater than, the 2 (0.3, 0.2) m measured previously at the nearby Beebe Ranch site.
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844
Bulletin of the Seismological Society of America, Vol. 95, No. 3, pp. 844–860, June 2005, doi: 10.1785/0120040134
The Most Recent Large Earthquake on the Rodgers Creek Fault,
San Francisco Bay Area
by Suzanne Hecker, Daniela Pantosti, David P. Schwartz, John C. Hamilton,
Liam M. Reidy, and Thomas J. Powers
Abstract The Rodgers Creek fault (RCF) is a principal component of the San
Andreas fault system north of San Francisco. No evidence appears in the historical
record of a large earthquake on the RCF, implying that the most recent earthquake
(MRE) occurred before 1824, when a Franciscan mission was built near the fault at
Sonoma, and probably before 1776, when a mission and presidio were built in San
Francisco. The first appearance of nonnative pollen in the stratigraphic record at the
Triangle G Ranch study site on the south-central reach of the RCF confirms that the
MRE occurred before local settlement and the beginning of livestock grazing. Chro-
nological modeling of earthquake age using radiocarbon-dated charcoal from near
the top of a faulted alluvial sequence at the site indicates that the MRE occurred no
earlier than A.D. 1690 and most likely occurred after A.D. 1715. With these age
constraints, we know that the elapsed time since the MRE on the RCF is more than
181 years and less than 315 years and is probably between 229 and 290 years. This
elapsed time is similar to published recurrence-interval estimates of 131 to 370 years
(preferred value of 230 years) and 136 to 345 years (mean of 205 years), calculated
from geologic data and a regional earthquake model, respectively. Importantly, then,
the elapsed time may have reached or exceeded the average recurrence time for the
fault. The age of the MRE on the RCF is similar to the age of prehistoric surface
rupture on the northern and southern sections of the Hayward fault to the south. This
suggests possible rupture scenarios that involve simultaneous rupture of the Rodgers
Creek and Hayward faults.
A buried channel is offset 2.2 (1.2, 0.8) m along one side of a pressure ridge
at the Triangle G Ranch site. This provides a minimum estimate of right-lateral slip
during the MRE at this location. Total slip at the site may be similar to, but is probably
greater than, the 2 (0.3, 0.2) m measured previously at the nearby Beebe Ranch
site.
Introduction
The Rodgers Creek fault (RCF) is a major strand of the
right-lateral San Andreas fault system in the San Francisco
Bay area. The RCF is the northern continuation of the Hay-
ward fault across a right-releasing step beneath San Pablo
Bay (Fig. 1) and shows evidence of geologically recent ac-
tivity along at least 60 km of its length from the bay to near
the town of Healdsburg. The largest instrumentally recorded
earthquakes in the vicinity of the RCF are the 1969 M
L
5.6
and 5.7 Santa Rosa earthquakes (Fig. 2) (Bolt and Miller,
1975). Microearthquake activity, which has been monitored
since 1969, defines a steep northeast-dipping trend to a depth
of 10 km along the fault north of Santa Rosa (Fig. 2b).
South of Sonoma Mountain, the fault is seismically quies-
cent, suggesting the presence of a seismic gap (Fig. 2a)
(Budding et al., 1991; Wong, 1991).
The RCF is expressed at the ground surface by abundant
youthful geomorphology and has been long recognized as
an active fault (e.g., Willis and Wood, 1922; Weaver, 1949;
Brown, 1970; Huffman, 1971; Herd, 1978; Hart, 1982,
1992). Only since the 1990s, however, has enough infor-
mation on the RCF been developed to begin characterizing
its slip rate, recurrence, and earthquake potential (Budding
et al., 1991; Schwartz et al., 1992) and to include the fault
in assessments of the probability of large earthquakes in the
region (Working Group on California Earthquake Probabil-
ities, 1990, 1999, 2003). Paleoseismic evidence from studies
at the Beebe Ranch site (Fig. 1) on the south-central reach
of the fault has provided a slip-rate estimate of 6.4–10.4 mm/
yr, an average recurrence interval of 131–370 years
(Schwartz et al., 1992), and a surface-offset measurement
The Most Recent Large Earthquake on the Rodgers Creek Fault, San Francisco Bay Area 845
Figure 1. Shaded relief map of the San Francisco
Bay region showing major faults and locations men-
tioned in the text. Faults are Holocene age from Jen-
nings (1994), except for the trace of the Rodgers
Creek fault north of Windsor Creek (commonly
known as the Healdsburg fault), which is according
to Hart (1998) and is provisionally shown here as Ho-
locene. The Triangle G Ranch trench site is 1km
northwest of an earlier trench site at Beebe Ranch.
Other trench sites mentioned in the text are labeled as
follows: PR/FC, Panorama Reservoir and Foss Creek
Detention Basin (1.5 km apart); MV, Mira Vista;
MT, Montclair; TL, Tyson’s Lagoon. A triangle lo-
cates the center of seismic energy release for the 1898
“Mare Island” earthquake based on available intensity
data, and a dashed oval bounds the epicentral region
at a 50% level of confidence (Bakun, 1999). A star
locates approximate epicenter of the M6.8 1868
earthquake on the Hayward fault (Bakun, 1999);
dashed line shows minimum extent of 1868 rupture.
for the most recent earthquake (MRE) of 1.8–2.3 m (Budding
et al., 1991). Budding et al. (1991) estimated a minimum
elapsed time since the MRE of 182 years using historical
records. At the Triangle G Ranch site, 1 km north of the
Beebe Ranch site, Schwartz et al. (1992) identified possible
evidence of three events and concluded that the MRE oc-
curred after A.D. 1438–1654.
This article describes new data from the Triangle G
Ranch site, including new field observations, radiocarbon
dating, and pollen analysis, that, together with the record of
historical seismicity, provide significantly tighter constraints
on the timing of the MRE. The age of the MRE is an impor-
tant parameter for developing rupture scenarios and com-
puting probabilities of future major earthquakes. Constraints
on the geometry of a buried channel allow us to estimate a
minimum amount of slip during the MRE at the study site.
Setting
The Triangle G Ranch site lies in a valley flat along
Rodgers Creek, where the fault is expressed as a smallnorth-
west-trending pressure ridge within a floodplain (Fig. 3). The
pressure ridge, which is underlain by a fault-bounded slice
of Tertiary volcanic deposits, is 3 to 8 m wide and less than
1 m high. The distal portion of an alluvial slope impinges
on the site from the southeast (Fig. 3). Rodgers Creek, which
parallels the fault upstream of the site, makes a right bend
around the northern nose of the pressure ridge and diverges
from the fault downstream of the site.
Schwartz et al. (1992) excavated seven trenches parallel
to and across the RCF to expose stratigraphic and structural
relations at the site. In addition to these, we excavated two
trenches in 1998–1999 (trench 9 and trench 5b, Fig. 3). Our
interpretations here are based primarily on trenches 3 and 9,
which provided the best evidence of the MRE. Other trenches
at the site showed evidence of older faulting events, but the
ages of these earthquakes are poorly constrained.
Trench Stratigraphy
Stratigraphy at the site consists of fluvial and alluvial-
slope deposits that lie in fault and depositional contact with
colluvium underlying the pressure ridge. The main colluvial
deposit, unit 9, is a poorly sorted gravelly sand in a clayey
silt matrix (Figs. 4 and 5). The deposit is brown with yellow,
red, and pink variegations and appears to be derived from
the late Tertiary Sonoma Volcanics, the main bedrock unit
along the length of the RCF. A very dark grayish-brown A
soil horizon, about 30 cm thick, has formed on unit 9. A
dark brown-to-black deposit of possible colluvial origin
(unit 8) lies in fault and depositional contact with unit 9
along the southwest side of the pressure ridge beneath de-
posits of Rodgers Creek (Fig. 4b,c). Unit 8 comprises poorly
sorted gravel and sand in a clayey matrix and is unstratified.
Alluvial deposits exposed in trenches 3 and 9 that pre-
date the MRE are grouped as unit 4 and consist of gravelly
846 S. Hecker, D. Pantosti, D. P. Schwartz, J. C Hamilton, L. M. Reidy, and T. J. Powers
Figure 2. (a) Seismicity around Rodgers Creek fault (1969–2003) showing earth-
quakes greater than magnitude 2. Data source: Northern California Earthquake Data
Center; data contributed by Northern California Seismic Network, U.S. Geological
Survey, Menlo Park, and Berkeley Seismological Laboratory, University ofCalifornia,
Berkeley. Locations of the 1969 magnitude 5.6 and 5.7 Santa Rosa events, which are
highlighted, are from table 2 of Wong and Bott (1995). (b) Cross section shows seis-
micity within 5 km on either side of AA.
stream-channel and debris-flow deposits interbedded with
clayey silt overbank and sheet-flow sediments (Figs. 4 and
5). The channel deposits are typically sandy gravel in a
clayey silt matrix; the overbank and overland-flow deposits
are massive or weakly bedded and have scattered clasts of
gravel and very coarse-grained sand that increase in size and
abundance close to the pressure ridge. In general, the fine-
grained component of unit 4 is very dark gray to grayish
brown, although the lowest intervals of clayey deposits ex-
posed in trench 3 have a stronger chroma. Southwest of the
pressure ridge, some of the fine-grained deposits contain
enough organic matter to have a granular soil structure.
Northeast of the pressure ridge, a thin, white ash from an in-
place burn forms a distinctive horizon within the otherwise
dark, massive sediments. This ash marks the position of a
paleofloodplain near the top of unit 4 (Fig. 4). A gravelly
matrix-supported channel deposit exposed in both walls of
trench 9 (unit 4Y) parallels the pressure ridge and has a
gradient to the northwest toward Rodgers Creek. A similar
lenticular deposit exposed in trench 3 at a greater depth is
interpreted to be the same channeled debris flow (Figs. 4 and
5). The source of the deposit is a valley-side drainage that
feeds the alluvial slope southeast of the site (Fig. 3). The
unit 4Y deposit is cut partly into unit 9, suggesting that the
debris flow was diverted along and scoured into the northeast
side of the pressure ridge. The upper contact of unit 4, which
is formed by floodplain deposits, is flat-lying, in general, and
everywhere appears to be depositional in origin. Sharp in-
flections in the contact that coincide with mapped faults are
clearly tectonic (Fig. 4a, meter 6; Fig. 4b, meter 6.3). Gentler
rises in dip that correspond to similar dips in underlying de-
posits can be attributed to tilting within the zone of defor-
mation (Fig. 4a, meter 5–7; Figs. 4c and 5a, meter 6–8). Along
the southwest side of the pressure ridge, floodplain deposits
blanket pre-existing topography (Fig. 4b, meter 11–12).
Overlying unit 4 are floodplain sediments (unit 2/3) that
are lighter in color than those of unit 4. Unit 2/3 consists of
a brown silt to clayey silt with a few lenses of fine gravelly
silt and a few scattered cobbles. The unit is thickest on the
northeast, upstream side of the pressure ridge, where it in-
cludes an upper subunit with many strong brown mottles.
To the southwest of the ridge, the unit consists of a thin layer
of silt with modest components of clay and sand. These de-
posits extend up the flanks of the pressure ridge at the po-
sition of trench 9 and top the ridge at trench 3, where the
ridge is lower and closer to Rodgers Creek (Figs. 3 and 4).
Fault Structure and Evidence of the Most
Recent Event
Zones of faults bound the slice of unit 9 beneath the
pressure ridge (Fig. 4). Slip along the northeast zone occurs
The Most Recent Large Earthquake on the Rodgers Creek Fault, San Francisco Bay Area 847
Figure 3. Three-dimensional perspective model of
the Triangle G Ranch site looking southeast along the
trend of the Rodgers Creek fault (RCF). The general
path of the unit 4Y debris flow is inferred from trench
exposures and geomorphic setting. Path is dashed
where covered by floodplain deposits. The present-
day geometry of the 4Y deposit is used to constrain
slip along the northeast side of the pressure ridge in
the most recent event (see text for discussion).
Trenches 3 and 9, which are 19 and 16 m long, re-
spectively, are the principal exposures discussed in
this article. Trench 5b provides auxiliary information
on the geometry of unit 4Y.
on closely spaced reverse-oblique faults (zone 1) that dip
toward and appear to converge with generally higher-angle
faults on the opposite margin of the slice (zone 2).
Faulting within zone 1 extends to the top of unit 4. Some
fault strands that lie within unit 9 appear to terminate at the
base of channel deposits (Fig. 4a, meter 7); other strands cut
channel deposits (Fig. 4a and 4b, meter 6–7; Figs. 4c and
5a, meter 7.5–8.5). Fault traces clearly extend to the upper
contact of unit 4 at only two locations (Fig. 4b, meter 6.5;
Fig. 4c, meter 8–8.5). Identifying fault discontinuities in the
massive clayey silt overbank sediments is difficult, however,
and faults that appear to terminate at the top of channel de-
posits probably continue upward for another 10–20 cm to
the top of unit 4 (e.g., Figs. 4c and 5a, meter 7.5). Faults
inferred to extend to the top of unit 4 are dashed and queried
on the trench logs.
Deformation of the upper contact of unit 4 is interpreted
as having occurred at the ground surface and, thus, this con-
tact defines the event horizon for the MRE. This deformation
is expressed as small, 15- to 20-cm-high northeast-facing
buried scarplets and broader warping associated with fault
zone 1 (Fig. 4a and 4b, meter 5–7; Figs. 4c and 5a, meter
6–8.5). Unit 2/3 onlaps the fault zone and shows no evidence
of deformation. Warping of the top of unit 4 corresponds to
similar warping in underlying channel deposits (Fig. 4a, me-
ter 5–7; Figs. 4c and 5a, meter 6–8). A buried scarplet in
trench 3 (Figs. 4c and 5a, meter 6.5) lies slightly east offault
zone 1 and expresses fold-related deformation. Note that an
apparent scarp on the southeast wall of trench 9 at meter 7
is an artifact of a boulder mapped at the ground surface and
does not represent faulting in the MRE.
Although the earthquake event horizon is unambiguous
within the zone of deformation, the position of the horizon
is less clear northeast of fault zone 1. The burn ash near the
top of unit 4 rises slightly as it approaches the buried scarplet
in trench 3 (Figs. 4c and 5a, meter 6.2). The ash thins and
dies out on the margin of the deformation zone, but the ho-
rizon appears to project onto the scarp. This relation suggests
the possibility that the ash lies on or slightly above the event
horizon and that the ash and the 5 cm of overlying unit 4
floodplain deposits postdate faulting. This interpretation is
tentative both because of the discontinuous nature of the ash
near the scarplet and because the apparent inflection may be
one of many local irregularities in the horizon and unrelated
to deformation of the ground surface. The discontinuous na-
ture of the ash deposit near the scarplet is consistent with
evidence from trench 9 that the ash-forming burn was re-
stricted, at least locally, to the floodplain north of fault zone
1 (Fig. 4a,b). Note that if the ash had originally extended
across the scarplet at the position of trench 3 and was sub-
sequently eroded, one would expect to see reworked ash in
deposits above the in-place burn.
Fault zone 2 on the southwest side of the pressure ridge
varies from a narrow, well-defined steeply northeast-dipping
zone in trench 3 to a distributed, less distinct set of south-
west-dipping strands in trench 9. Two subparallel faults ex-
posed in trench 9 bound a slice of unit 8 between units 9
and 4 (Fig. 4b, meter 12–13). Neither of these faults could
be traced upward through the upper part of unit 4, which
laps onto the pressure ridge. However, a fault that splays
into unit 8 displaces the base of this onlapping section (Fig.
4b, meter 12.0–12.2). Because the unstratified, fine-grained
nature of these sediments makes detection of faulting diffi-
cult, each of these strands may in fact extend to the top of
unit 4. Fault zone 2 exposed in trench 3 juxtaposes units 8
and 9 and clearly offsets upper unit 4 channel deposits, but,
like elsewhere, was not recognized in the upper 20 cm of
massive silty overbank deposits (Fig. 4c, meter 12.8).
Slip during the Most Recent Event
We estimate the amount of right-lateral slip on fault
zone 1 during the MRE using the geometry of the fault-
parallel debris-flow unit 4Y (Fig. 3) and the overlying in-
terval of unit 4 floodplain deposits. The unit 4Y channel in
848
Figure 4. Logs of trench exposures showing evidence of the most recent surface-rupturing earthquake. Evidence includes faults
that extend to the upper contact of unit 4 and scarplets buried by unit 2/3. Trench-wall grids are registered across trench 9 (logs a and
b), but not between trench 9 and trench 3. Horizontal reference line at meter 2 of trench 9 highlights difference in vertical position of
channel deposit 4Y across the trench. The geometry of this channel constrains the amount of horizontal slip on fault zone 1 in the most
recent event (see Fig. 6 and discussion in text). See text for unit descriptions. Ages of charcoal samples are listed in Table 1. Pollen-
sampling profile (on log c) was taken in a pit adjacent to trench 3 after the trench was closed.
The Most Recent Large Earthquake on the Rodgers Creek Fault, San Francisco Bay Area 849
Figure 5. Detail of trench exposure on northeast side of the pressure ridge (trench
3, southeast wall). (a) Trench log enlarged from Fig. 4c. (b) Photograph showing char-
acter of deposits. Black lines are unit contacts; white lines are faults.
cross section is tilted by amounts comparable with the top-
of-unit-4 ground surface, indicating that the channel records
only one event (Fig. 4a, meter 5–7; Figs. 4c and 5a, meter
6.5–8). We primarily use the walls of trench 9 for slip re-
construction because exposures of unit 4Y can be confi-
dently correlated across a known distance. Note that,
whereas unit 4Y is present on the northeast side of the fault
zone on the southeast wall of trench 9 (Fig. 4a, meter 5–
6.5), it is absent 1 m away on the opposite wall (Fig. 4b).
This suggests that the channel is highly sinuous here and
therefore crosses the fault before reaching the opposite
trench wall. The channel crosses back to the northeast side
of the fault between trenches 9 and 3 (Fig. 4c).
Figure 6 illustrates our approach to estimating right-
lateral slip. Essentially, we determine the original fault-
parallel gradient of the channel (Fig. 6b) and then backslip
850 S. Hecker, D. Pantosti, D. P. Schwartz, J. C Hamilton, L. M. Reidy, and T. J. Powers
Figure 6. Perspective view toward south of the
walls of trench 9 as logged on the northeast side of
the pressure ridge (simplified from Fig. 4a, b; north
direction, approximate), depicting a geometric and
trigonometric approach to restoring offset of unit 4Y
during the most recent event. For clarity, trenchwidth
is exaggerated (1.5 ) and postfaulting deposits
(unit 2/3) are omitted. (a) Present configuration of
deposits. (b) Configuration of deposits with vertical
component of deformation removed. Note that the top
of unit 4 on the southeast wall is restored to horizontal
primarily by removing the tilt in the underlying unit
4Y channel. Restoration returns channel to its original
gradient. (c) Inferred preslip position of channel ex-
posures. Lateral slip (x) is restored by sliding the
northeast section of channel left-laterally until it
aligns with the gradient-projected position of the
channel on the other side of the fault (location marked
with a star). See text for definition of parameters and
discussion of channel-gradient and slip determina-
tions.
one side of the fault until the channel exposed in cross sec-
tion intersects the gradient-projected position of the channel
on the other side of the fault (Fig. 6c). This lateral translation
restores the channel to its prefaulting position.
Conceptually, the first step is to remove the small ver-
tical (pressure-ridge-forming) component of slip represented
by deformation of the originally flat-lying floodplain depos-
its that form the upper contact of unit 4 (Fig. 6a,b). The
vertical separation of channel 4Y across the fault zone is
partly the result of vertical slip and partly the result of jux-
taposition of deposits translated laterally along gradient. The
up-to-the-southwest sense of vertical separation that remains
after vertical slip is removed (Fig. 6b) is consistent with
right-lateral offset of a northwest-flowing channel (Fig. 3).
The vertical component of deformation is greater on the
southeast wall (35–40 cm) than on the northwest wall
(20–25 cm) of trench 9. This along-strike difference in
uplift, which acts to exaggerate the gradient of the channel,
is consistent with the cumulative pattern of uplift represented
by the pressure ridge (Fig. 3).
The gradient (
) of channel 4Y is estimated trigono-
metrically using the distance between trench walls (x) and
the difference between walls in the thickness of unit 4 above
the base of the channel on the southwest side of the fault
zone. That is,
arctan
[
(yy)/x
]
, (1)
ns
where y
n
and y
S
are the above-channel thicknesses of unit 4
on the downslope and upslope trench walls, respectively
(Fig. 6b). Our best estimate of
is 10NW (using x
1.05 m and y
n
y
s
0.19 m). Incorporating uncertainty
in xof 0.1 m and uncertainty in y
n
y
s
of 0.08 m
yields
that ranges from 6to 16NW.
The gradient along this 1-m reach of channel can be
compared with the gradient along the 4-m reach between
the southeast walls of trench 9 and trench 3 (Fig. 3). The
base of the channel section exposed in trench 3 is buried
0.62 m deeper below the top of unit 4 than the section on
the same (northeast) side of the fault in trench 9 (Fig. 4a,c).
These coordinates (x4 m and y
n
y
s
0.62 m)
yield
9, similar to the 10estimated between the walls
of trench 9.
The Most Recent Large Earthquake on the Rodgers Creek Fault, San Francisco Bay Area 851
Figure 7. Log of part of trench 5b showing an
exposure of channel unit 4Y that, as a supplement to
other exposures (Fig. 4), was used to estimate the gra-
dient of the channel to constrain the amount of slip
on fault zone 1 during the most recent event (see dis-
cussion in text).
In a trench 4 m southeast of trench 9 (trench 5b;
Fig. 3), a section of the channel that is caught up in the fault
zone is not buried by floodplain deposits (Fig. 7). If the base
of the channel at this location, with respect to the pressure-
ridge side of the fault, is level with the top of unit 4, then
the gradient between here and trench 9 is 5–6NW. This
is similar to the 6–7W slope of the active, unburied al-
luvial surface southeast of the pressure ridge (Fig. 3). The
steeper (6–16NW) slope of the buried channel between the
walls of trench 9 and trench 3 may reflect proximity to a
broad paleochannel of Rodgers Creek.
The amount of right-lateral slip (x) indicated by ex-
posures of channel 4Y in trench 9 can be calculated from
the channel gradient (
) and the difference across the fault
in the thickness of unit 4 that overlies the base of the channel.
Specifically,
x⬘⳱(y⬘ⳮy)/tan(
), (2)
ss
where and y
s
are the above-channel thicknesses of unit 4y
s
on the downthrown and upthrown sides of the fault, respec-
tively (Fig. 6c). Substituting equation (1) into equation (2)
gives
x⬘⳱
[
(y⬘ⳮy)/(yy)
]
x. (3)
ssns
Our best estimate of xis 2.2 m (using y⬘ⳮy0.39 m
ss
and the parameter values for a 10gradient). Incorporating
the ranges of uncertainty in our measurements (which in-
cludes 0.03 m for ) yields xthat ranges from 1.4y⬘ⳮy
ss
to 3.4 m.
This estimate of 2.2 (1.2, 0.8) m of right-lateral
slip along the northeast side of the pressure ridge is a min-
imum for the site. An unknown amount of additional slip
occurred on fault zone 2 flanking the southwest side of the
ridge. Total slip at the Triangle G Ranch site during the MRE
may be similar to, but is probably greater than, the 2 (0.3,
0.2) m of surface offset measured 0.7 km to the south at
the Beebe Ranch paleoseismic site (Budding et al., 1991).
Timing of the Most Recent Event
Spanish mission records indicate that no large-magni-
tude earthquakes have occurred on the RCF since at least the
early nineteenth century (Budding et al., 1991). Initial pa-
leoseismic evidence from the Triangle G Ranch site, in the
form of radiocarbon-dated detrital charcoal collected from
unit 4, indicates that surface rupture occurred on the fault
sometime after A.D. 1438–1654 (Schwartz et al., 1992).
Herein, we present new radiocarbon results that significantly
improve the maximum-age constraint and narrow the time
window for the MRE. In addition, we consider the robustness
of written records for constraining the minimum age of the
MRE and present data on the arrival of exotic pollen at the
site with respect to timing of the earthquake.
Historical Constraints
The Written Record. The recency of the MRE is con-
strained by the preinstrumental historical record. Franciscan
missions, located near the coast between San Diego and Son-
oma, kept detailed accounts of damaging earthquakes, until
about 1834 when the missions were secularized. Earth-
quakes that did not damage the missions were rarely reported
(Ellsworth, 1990). These written records, along with some
government correspondence, are the primary sources of in-
formation on earthquakes that occurred during the period
before the 1849 gold rush (Toppozada et al., 1981).
The oldest mission close enough to the RCF to be af-
fected by a major earthquake on the fault is Mission Dolores
in San Francisco, founded in 1776 (Fig. 1). The Presidio of
San Francisco was founded the same year, with the construc-
tion of a quadrangle and living quarters. The initial church
at Mission Dolores was built of wood; construction of the
adobe church that stands today was begun in 1782 and com-
pleted by 1791 (Engelhardt, 1924). This church was so well
built that it was not significantly damaged by the 1906 earth-
quake on the San Andreas fault, although it was damaged
by an earthquake in 1838 generally assumed to have been
on that fault (Toppozada and Borchardt, 1998). Whether
Mission Dolores would have been damaged by a large earth-
quake 50 km away on the RCF is unclear. The earliest
earthquakes reported at either the Mission or the Presidio is
a series of eighteen shocks over a period of a month in 1808
that damaged some adobe walls at the Presidio (Townley
and Allen, 1939; Toppozada et al., 1981). It is noteworthy
that the quality of construction at the Presidio during this
852 S. Hecker, D. Pantosti, D. P. Schwartz, J. C Hamilton, L. M. Reidy, and T. J. Powers
time was poor and buildings were in constant need of repair
(Langelier and Rosen, 1992). The 1808 earthquakes were
not reported as damaging at Mission Dolores (Toppozada
and Borchardt, 1998). Because the location of the 1808
earthquakes is unknown, Budding et al. (1991) allowed, but
did not conclude, that the source could have been the RCF,
thereby limiting the recency of the MRE to 1808. The ap-
parent absence of a dominant mainshock in the 1808 series
argues against the occurrence of a large, surface-rupturing
earthquake accompanied by an aftershock sequence at that
time. Toppozada and Borchardt (1998) similarly concluded
from the account of damage that the 1808 events did not
indicate a large earthquake, but instead may have been a
nearby moderate earthquake and its aftershocks. No other
damaging earthquakes are noted in the years before a mis-
sion was built in Sonoma, only 7 km east of the RCF,in
1824. The earliest earthquake mentioned in a chronicle of
the Sonoma Mission is a damaging event in 1868 (Smilie,
1975, in Budding et al., 1991), which was an estimated mo-
ment-magnitude (M)6.8 earthquake 70 km to the south-
east on the southern Hayward fault (Fig. 1) (Bakun, 1999).
An earthquake that occurred in 1898, the “Mare Island”
earthquake, caused substantial damage along the north mar-
gin of San Pablo Bay and to the northwest in communities
along the valleys of the Petaluma River and Sonoma Creek.
Bakun (1999) analyzed seismic intensity data and obtained
a moment magnitude of M6.3 and a source location cen-
tered in northeast San Pablo Bay (Fig. 1). Uncertainty in the
location allows that the earthquake could have occurred on
any of several faults, including the southern end of the RCF,
the West Napa fault, the Concord-Green Valley fault, the
north Hayward fault, or faults beneath fold structures eastof
Mare Island (Bakun, 1999). Toppozada et al. (1992) inter-
preted the intensity data as indicating that the epicenter was
probably on the southern RFC (southeast of the Triangle G
Ranch trench site) and the magnitude was about 6.7. Sub-
sequently, Toppozada revised the magnitude downward to
6.4 (Toppozada et al., 2000; Toppozada and Branum, 2002).
Wesson et al. (2003) used an approach that evaluates the
association of specific earthquakes with specific faults to de-
termine that the northern Hayward fault is more likely to be
the source of the 1898 earthquake than the RCF. Parsons et
al. (2003) cited accounts of a small tsunami as evidence of
coseismic vertical displacement of the bayfloor and, using
historical hydrographic surveys to identify bayfloor subsi-
dence and a hydrodynamic model to constrain rupture pa-
rameters, concluded that the earthquake occurred on a nor-
mal fault within the step-over between the RCF and Hayward
fault. Regardless of its location, the 1898 event appears to
be too small to be the MRE recognized at the Triangle G
Ranch and Beebe Ranch trench sites. Empirical relations in-
dicate that M6.5 earthquakes on strike-slip faults have
surface displacements that are generally less than a fewdeci-
meters (Wells and Coppersmith, 1994), considerably smaller
than the 2-m displacements observed at both trench sites.
Toppozada et al. (1981) consider that by 1800, the rec-
ord of earthquakes of magnitude 7 or greater is probably
complete within about 100 km of the coast between San
Diego and Sonoma counties. Almost certainly, the record of
large-magnitude earthquakes is complete for the RCF by
1824, when the mission at Sonoma was established. The
absence of damaging-earthquake accounts in documents
from Mission Dolores and the Presidio indicates that the
record for the RCF may be complete back to circa 1776.
Toppozada and Borchardt (1998) concluded that there is no
historical evidence for any major earthquakes in the San
Francisco Bay area before the 1838 earthquake on the San
Andreas fault, back to the founding of the Mission and the
Presidio in 1776.
The Pollen Record. The position of the MRE horizon rela-
tive to the earliest appearance of nonnative pollen in the
stratigraphic record at the Triangle G Ranch site provides an
additional constraint on the recency of the MRE. The arrival
of nonnative pollen in a region serves as a marker for the
beginning of the historical period and is playing an important
role in determining the timing of large prehistoric and early
historic earthquakes in California (e.g., Lienkaemper et al.,
2002; Stone et al., 2002). Erodium cicutarium, a weedy
Mediterranean annual, accompanied the first Spanish settle-
ments. In San Francisco, the first appearance of E. cicutar-
ium (in lake cores) is estimated to be 1800 (20 years)
(Reidy, 2001). In southern California, the plant’s arrival has
been precisely dated and determined to have occurred
slightly in advance of Spanish settlement (Mensing and
Byrne, 1998). E. cicutarium pollen identified in varved-
marine-sediment cores from the Santa Barbara Channel
dates from the period 1750–1765, which is at least two de-
cades before the founding of the Santa Barbara mission in
1786. Mensing and Byrne (1998) argue that E. cicutarium
dispersed northward through natural propagation from Baja
California, where it was probably introduced by livestock.
They estimate a migration rate of 30 or 50 km/yr, depend-
ing on when and where the plant was first established. If
migration continued northward at the same rate, E. cicutar-
ium would have reached our trench site on the RCF (490
km of Santa Barbara) in 1770–1775. However, the cooler,
moister climate and greater vegetation cover of northern
California may have slowed the advance of E. cicutarium.
In addition, San Francisco Bay may have been a barrier to
migration. Indeed, preliminary analysis of a core from Bol-
inas Lagoon, north of the San Francisco Peninsula, indicates
that E. cicutarium may have arrived in that area not long
before 1850 (Byrne, 2002; R. Byrne, personal comm., 2003).
We sampled for E. cicutarium pollen at the Triangle G
Ranch site in a pit adjacent to trench 3 along a vertical profile
east of fault zone 1 where unit 4 is warped (Fig. 4c). We
sampled at 1-cm intervals through the uppermost part of unit
4 and most of unit 2/3 to document the first appearance and
depth distribution of E. cicutarium pollen grains. Standard
pollen extraction and preparation procedures were followed
(Faegri and Iversen, 1975), and a minimum 800-mm
2
area
The Most Recent Large Earthquake on the Rodgers Creek Fault, San Francisco Bay Area 853
Figure 8. First arrival and abundance of E. cicu-
tarium pollen in the stratigraphic record relative to
the earthquake event horizon. Pollen samples were
taken at 1 to 5-cm intervals from the southeast wall
of a pit dug adjacent to trench 3 (see Fig. 4c for lo-
cation of samples). Slides made for pollen-grain
counts were carefully prepared to ensure consistency
in sample volumes. For each sample, a minimum area
of 800 mm
2
was scanned at 10magnification.
of each slide was scanned at 10magnification for the
large, easily identifiable E. cicutarium grains.
E. cicutarium pollen first appears in deposits that lie
above the earthquake horizon at the base of unit 2/3 (Fig. 8).
The shape of the pollen-depth curve in Figure 8 is similar to
curves from lake and marsh pollen studies (e.g., Cole and
Liu, 1994; Reidy, 2001), suggesting that the depositional
record is fairly continuous. The first appearance and gradual
increase in E. cicutarium pollen likely document the arrival
and expansion of E. cicutarium locally. The subsequent de-
cline and plateau in abundance likely reflect increasingcom-
petition from other nonnative weeds and grasses and then
quasistabilization of the population.
The first appearance of E. cicutarium lies 1 cm above a
prominent decrease in organic-matter content that defines the
contact between unit 4 and unit 2/3. We infer that this cor-
respondence reflects disturbance in the local environment
related to the introduction of livestock. In a study of envi-
ronmental changes on Santa Cruz Island in southern Cali-
fornia, Brumbaugh (1980) found similar evidence of two
distinct geomorphic regimes: one represented by fine-
grained alluvium containing charcoal fragments that reflects
a stable, vegetated landscape and the other represented by
coarser alluvium without charcoal that reflects increasedhill-
slope erosion due to livestock grazing. An increase in quartz
sand grains in the upper half of unit 2/3 (above a depth of
22 cm in our profile) may reflect increased hillslope erosion
at our site.
Grazing near the trench site probably began about 1830,
when the first land grants were established in the region (G.
Burch, California State Parks, personal comm., 1998). Thus,
the evidence provided by E. cicutarium pollen reinforces our
conclusion that the earthquake did not occur after the nearby
Mission Sonoma was founded in 1824. If the arrival of E.
cicutarium actually predates local grazing (our inferences
concerning geomorphic changes notwithstanding), and E. ci-
cutarium dispersed through natural propagation from south-
ern California, the timing constraint may be similar to that
provided by the founding of Mission Dolores and the Pre-
sidio in 1776.
Radiocarbon Constraints and Chronological Modeling
Dating of detrital charcoal from the upper 0.5 m of
unit 4 (Table 1; Figs. 4 and 9) provides us with the oppor-
tunity to place well-constrained limits on the maximum age
of the MRE. Charcoal samples were collected from overbank
and channel deposits on both sides of the pressure ridge; 20
samples were submitted for radiocarbon analysis. Most of
the samples analyzed were near faults and were clearly
within the upper part of the faulted section. Four of the sam-
ples are in situ charcoal recovered from the burn-ash hori-
zon. All the samples collected appeared to be part of the
original deposit and not introduced by postdepositional pro-
cesses. We took care in the field to avoid sampling charcoal
that may have been part of a root, was located in any feature
resembling a burrow, or showed signs of having been bio-
turbated into place.
Although the radiocarbon ages of the samples analyzed
range up to 2000
14
C years B.P., the ten youngest dates
cluster between 110 and 230 (40)
14
C years B.P. (Fig. 9).
We regard this group of young dates, which span most of
the sampled interval, as a reliable constraint on the age of
the stratigraphy and the older dates as the ages of wood from
long-lived trees or charcoal that was reworked and is sig-
nificantly older than the surrounding deposit.
One surprising result is that the burn-ash horizon, which
lies just a few centimeters below the top of unit 4, yielded
some of the oldest dates (Fig. 9). Because charcoal from an
in situ burn has not been subject to reworking and redepos-
854 S. Hecker, D. Pantosti, D. P. Schwartz, J. C Hamilton, L. M. Reidy, and T. J. Powers
Table 1
Radiocarbon and Calendar Dates of Charcoal Samples from Upper Part of Unit 4
Sample
Name*
Lab
No.
Grid Position:
vert; horiz (m)
Depth Below Top
of Unit 4 (m)
Radiocarbon Age, yr B.P.
(1rlab error)
Calibrated Ages
(95.4% Probability Level)
§
Charcoal
Type
3S-3 CAMS-51766 1.84; 12.78 0.36 110 40 1670 AD–1780 AD (33.1%) Detrital
1800 AD–1960 AD (62.3%)
3S-54 CAMS-51769 2.13; 6.74 0.16 120 40 1670 AD–1780 AD (36.2%) Detrital
1800 AD–1960 AD (59.2%)
5bN-3 CAMS-58487 2.19; 2.93 0.06 140 50 1660 AD–1960 AD (95.4%) in situ
3N-46 CAMS-51772 2.11; 7.07 0.18 170 40 1650 AD–1890 AD (79.2%) Detrital
1910 AD–1960 AD (16.2%)
3N-47 CAMS-51773 2.18; 7.96 0.28 170 40 1650 AD–1890 AD (79.2%) Detrital
1910 AD–1960 AD (16.2%)
3S-57 CAMS-51770 1.88; 7.60 0.5 180 40 1640 AD–1710 AD (20.1%) Detrital
1720 AD–1890 AD (59.5%)
1910 AD–1960 AD (15.9%)
3N-48 CAMS-51774 2.29; 7.21 0.04 190 50 1640 AD–1890 AD (80.7%) Detrital
1910 AD–1960 AD (14.7%)
9N-4 CAMS-51763 1.66; 12.87 0.12 200 40 1640 AD–1700 AD (25.3%) Detrital
1720 AD–1820 AD (50.8%)
1830 AD–1880 AD (4.9%)
1910 AD–1960 AD (14.4%)
3S-4 CAMS-51767 1.84; 12.54 0.38 230 40 1520 AD–1580 AD (7.3%) Detrital
1620 AD–1690 AD (38.4%)
1720 AD–1820 AD (40.3%)
1920 AD–1950 AD (9.4%)
9N-35 CAMS-51765 1.85; 4.41 0.24 230 50 1510 AD–1600 AD (13.2%) Detrital
1610 AD–1700 AD (32.5%)
1720 AD–1820 AD (36.7%)
1830 AD–1880 AD (3.2%)
1910 AD–1960 AD (9.8%)
3N-43 TO-2927 1.54; 12.95 0.62 340 60 1440 AD–1660 AD (95.4%) Detrital
9N-26 CAMS-50077 2.02; 2.25 0.04 350 40 1450 AD–1640 AD (95.4%) in situ
3S-20 CAMS-51768 1.86; 7.35 0.52 380 40 1440 AD–1530 AD (55.1%) Detrital
1540 AD–1640 AD (40.3%)
3N-61 CAMS-51776 1.52; 13.30 0.66 400 40 1430 AD–1530 AD (67.3%) Detrital
1550 AD–1640 AD (28.1%)
3N-10 CAMS-51771 1.60; 12.74 0.56 440 50 1400 AD–1530 AD (81.0%) Detrital
1560 AD–1630 AD (14.4%)
9S-34 CAMS-51762 2.01; 4.44 0.07 480 50 1320 AD–1360 AD (6.2%) in situ
1380 AD–1520 AD (87.4%)
1590 AD–1620 AD (1.8%)
3S-18 TO-2767 2.04; 6.95 0.28 640 60 1270 AD–1420 AD (95.4%) Detrital
9N-25 CAMS-50576 1.98; 2.11 0.1 710 90 1150 AD–1430 AD (95.4%) in situ
3N-59 CAMS-51775 1.70; 13.48 0.46 970 40 990 AD–1170 AD (95.4%) Detrital
3S-16 TO-2766 2.06; 6.90 0.26 1960 70 160 BC–130 BC (1.4%) Detrital
120 BC–230 AD (94.0%)
*Includes trench number (north or south wall), and sample no. See Figure 4 for locations (except for sample 5bN-3, not shown).
Ages from the Center for Accelerator Mass Spectrometry (CAMS), Lawrence Livermore National Laboratory, are by AMS analysis; ages fromIsoTrace
Radiocarbon Laboratory at the University of Toronto (TO) are conventional.
Ages corrected for d
13
C (assumed value of 25, appropriate to type of material) (Stuiver and Pollach, 1977). Ages in bold form young cluster shown
in Figure 9.
§
Calibrated with OxCal Program, ver. 3.8 (Bronk Ramsey, 2002). Calibration curve from Stuiver et al. (1998).
ition, we expected samples from the horizon to be among
the youngest and closest in age to the surrounding deposit.
One possibility is that much of the wood that produced the
charcoal was detrital from old trees, possibly redwoods, and
was considerably older than the ground surface on which it
was lying when it burned. Radiocarbon dating determines
the age of the wood itself, not when the wood burned and
became part of the deposit. The general antiquity and range
of ages obtained from the young burn illustrates how sig-
nificant this distinction can be. One of the burn-horizon sam-
ples (5bN-3) has a young radiocarbon age (140 50 years
B.P.) similar to the youngest detrital charcoal ages. This sug-
gests that some of the wood that burned to produce the ash
was not much older than the underlying ground surface.
The ages of the youngest charcoal samples, when con-
verted from radiocarbon to calendar ages, have distributions
The Most Recent Large Earthquake on the Rodgers Creek Fault, San Francisco Bay Area 855
Figure 9. Depth versus age distribution of charcoal samples from unit 4. Bars rep-
resent one standard deviation confidence in the analytical ages. Arrows adjust depths
of samples taken from an inset channel-fill sequence (southwest side of pressure ridge,
trench 3) to better reflect the true stratigraphic position of the host deposits. Dashed
line demarcates group of young dates (to the left of line) used to constrain timing of
MRE (see text for discussion). Note break in timescale between 1000 and 2000 years.
that are broad and multimodal (Fig. 10, distribution out-
lines). Because the distributions overlap, incorporating
stratigraphic and other chronological ordering constraints
can reduce the likely range of dates (Biasi and Weldon,
1994). The MRE can be treated as an undated event in the
sequence and thus the age of the earthquake can be more
precisely and probabilistically determined. We accomplish
this using the radiocarbon-calibration program OxCal,
which uses Bayesian statistics to analyze and incorporate
stratigraphic information (Bronk Ramsey, 1995, 2001, 2002).
This approach has been applied in developing detailedpaleo-
earthquake chronologies elsewhere along the San Andreas
fault system (e.g., Fumal et al., 2002; Lienkaemper et al.,
2002; Lindvall et al., 2002).
At the Triangle G Ranch site, uncertainties in determin-
ing relative stratigraphic position between trench walls,
across faults, and across the pressure ridge preclude placing
the entire set of samples in stratigraphic order. Even a se-
quence of only a few dates, however, tightens the constraint
on the age of the earthquake. Historical limits on the recency
of the earthquake can be included to further narrow and more
accurately define a probability distribution for the date of the
event.
The stratigraphic sequence of charcoal samples is inter-
preted by using cross-cutting relations between channel and
overbank deposits, evidence of layering within the overbank
deposits, and, where layering is not apparent, depth below
the top of unit 4. Individual overbank deposits are probably
thin, such that charcoal samples vertically separated bymore
than a few centimeters are likely to be from different-age
deposits. The 5-cm thickness of unit 4 sediments above
the paleo floodplain demarcated by the burn ash is consistent
with this concept.
We identify a stratigraphically ordered sequence of
three samples collected from the northeast side of the pres-
sure ridge on the southeast wall of trench 3 (samples 3S-20,
54, and 57) and the uppermost sample collected on the op-
posite wall (sample 3N-48; Figs. 4c and 5a). The latter sam-
ple is from 4 cm below the top of unit 4 and is the highest
in the faulted section. Two of the samples (3S-57 and 3S-
20) were located only 2 cm apart, but separated by a change
in deposit color and grain size that likely represents a layer
boundary. The lowest sample (3S-20) is not from the group
of young dates, but is included to determine whether older
dates may have an effect on the model.
The results of the chronological model for this sequence
of samples (Fig. 10a) indicate that the MRE likely occurred
no earlier than A.D. 1715 (at the 95-percentile level). The
model uses A.D. 1776, the year that Mission Dolores and the
Presidio were founded in San Francisco, as the beginning of
the historical earthquake record. This single-year constraint
produces an age distribution for the event that is highly
asymmetric, with the probability weighted toward the his-
torical period (such that the age range at 68 percentile begins
at 1750). If a younger date is substituted for the historical
cutoff, the age distribution broadens and shifts to younger
856 S. Hecker, D. Pantosti, D. P. Schwartz, J. C Hamilton, L. M. Reidy, and T. J. Powers
Figure 10. Results of chronological modeling of
constraints on the timing of the MRE (using the pro-
gram OxCal, v. 3.8, Bronk Ramsey, 2002). Open
curves show probability distributions of calibrated
radiocarbon ages prior to running the model. Shaded
portions of curves are the model distributions that re-
flect limits imposed by other age constraints, includ-
ing the start of the historical period. Solid black
curves are model distributions for the date of the MRE
(lower limiting age at 95% is shown). We made al-
ternative runs to address the uncertainty in construct-
ing a chronological model from our knowledge of the
stratigraphy: (a) Model that we prefer, because it in-
corporates the largest number of radiocarbon-dated
samples that can be confidently placed in stratigraphic
order and uses the likely historical constraint of 1776.
Note that the ordering constraint has no effect on the
model distribution of the oldest sample. (b) Model
with the same suite of ordered samples, but with an
historical constraint of 1824. (c) Model with all the
young charcoal samples (see Fig. 9 and Table 1) con-
sidered to be a coherent group older than the earth-
quake and whose relative-age sequence is unknown.
For completeness, we include sample 5bN-3 from the
burn ash, whose stratigraphic position relative to the
event horizon is uncertain (see text discussion). Omit-
ting this date from the group has no effect on the
model. (d) Model with only the youngest radiocarbon
date used to constrain the timing of the earthquake.
dates. For the certain constraint of 1824 (when the nearby
Sonoma Mission was founded), 95% of most likely results
are no older than A.D. 1740 (Fig. 10b).
The accuracy of the model depends on whether the
stratigraphic sequence of the charcoal samples reflects their
chronological order. The prior calibrated-age distributions
for samples in the young group (Fig. 10c, distribution out-
lines) are so similar that they mask any differences in actual
ages. Because of this and because detrital charcoal can be
older than the host deposit, individual samples in the model
may in fact be out of chronological order. However, if all
the young samples in the section lacked stratigraphic order,
we would not expect their ages to form such a cluster. We
infer that the concentration of radiocarbon ages within a
100-year period (Fig. 9) is evidence of an age progression
that is simply unresolvable at the scale of radiocarbondating.
To test the sensitivity of the model to the number and
choice of input dates, we used alternative suites of samples
that appear to be in stratigraphic order (samples 3N-48, 9N-
4, 3S-3, and 3S-4, altogether and with 3N-48 or 9N-4 omit-
ted) and obtained minimum-limiting ages for the MRE rang-
ing from A.D. 1710 to A.D. 1736 (at the 95- percentile level).
The modeling results indicate that if as few as three of the
young samples from unit 4 are in chronologically correct
order, we can conclude that the earthquake likely occurred
after the beginning of the eighteenth century. We have great-
est confidence in the stratigraphic order of samples shown
in Fig. 10a and thus prefer an event-age distribution that
begins at A.D. 1715 (at the 95-percentile level).
The Most Recent Large Earthquake on the Rodgers Creek Fault, San Francisco Bay Area 857
To avoid inferring the relative ages of samples, we can
model the group of young dates as a “phase” within an over-
all sequence (Bronk Ramsey, 1995, 2002). This structure
produces a likely earliest date for the earthquake of A.D.
1690 (Fig. 10c). A model can also be run that includes the
age of only a single charcoal sample. Selecting the sample
with the youngest radiocarbon age (110 40
14
C years B.P.,
sample 3S-3) provides the best constraint. This simple model
yields a likely earliest date for the earthquake of A.D. 1695
(Fig. 10d), similar to the group result.
In summary, we conclude that the MRE on the RCF oc-
curred no earlier than A.D. 1690 and probably no earlier than
A.D. 1715. The earthquake may have occurred shortlybefore
the beginning of the historical period, inasmuch as the age
distribution for the event is skewed toward young dates. For
the preferred model, the range for 68% of most likely results
begins at A.D. 1750. Also, if the group of young radiocarbon
ages is more chronologically ordered than represented byour
models, then the corresponding event age would verge even
more closely on the beginning of the historical period.
Discussion
The timing of paleoearthquakes is fundamental to un-
derstanding the space-time behavior of faults and provides
critical input for seismic hazard estimates. We have de-
scribed field relations, radiocarbon dating, pollen evidence,
and the record of historical seismicity that allow us to con-
strain the age of the most recent large surface rupture on the
RCF to after A.D. 1715 and before 1776, with some confi-
dence, and to after A.D. 1690 and before 1824, with cer-
tainty. The age of this event can be compared with that of
recent earthquakes on the Hayward fault to better understand
possible fault-rupture patterns. In addition, the elapsed time
since the MRE can be compared with the average recurrence
interval for the RCF to gauge the imminence of a future
earthquake.
Rupture Scenarios and Earthquake Size
The timing of the MRE on the RCF has been used to-
gether with similar information for the Hayward fault to de-
velop segmentation models and potential rupture scenarios
for the Hayward–Rodgers Creek fault system (Working
Group on California Earthquake Probabilities, 1999, 2003;
hereafter referred to as WG02). Segmentation is an impor-
tant concept for characterizing the spatial distribution of
earthquake sources and the magnitudes of expected events.
WG02 divides the Hayward–Rodgers Creek into three dis-
tinct fault segments: the Rodgers Creek, north Hayward, and
south Hayward segments. The northern extent of rupture in
the 1868 earthquake (Fig. 1) defines the boundary between
the south Hayward and north Hayward segments. The 6-km
releasing step-over from the north Hayward to the RCF be-
neath San Pablo Bay forms a likely structural and geometric
barrier to rupture and thereby defines another segment
boundary. However, the timing of prehistoric earthquakes
suggests that these segments may not always rupture inde-
pendently.
The MRE on the north Hayward fault, documented at
the Mira Vista site in El Cerrito (Fig. 1), occurred after A.D.
1640 (Hayward Fault Paleoearthquake Group, 1999). A
study of historical documents by Toppozada and Borchardt
(1998) indicates that an earthquake in 1836, commonly be-
lieved to be on the north Hayward fault, actually occurred
south of the San Francisco Bay area and that the most recent
large earthquake on the north Hayward fault occurred before
the beginning of the written record in 1776.
The south Hayward fault ruptured in 1868 and in late
prehistoric time as well. In a study at Tyson’s Lagoon in
Fremont (Fig. 1), Lienkaemper et al. (2002) used chrono-
logical modeling to estimate a maximum age for the pen-
ultimate earthquake of A.D. 1650. Like the MRE on the RCF
at the Triangle G Ranch site, the penultimate event on the
south Hayward fault at Tyson’s Lagoon lies just below the
appearance in the stratigraphic record of the nonnative pol-
len, E. cicutarium. In addition, Lienkaemper and Williams
(1999) postulate that the penultimate earthquake recorded at
the Montclair site near the north end of the 1868 rupture in
north Oakland (Fig. 1) corresponds to the 1640–1776 event
documented farther north at the Mira Vista site.
The general correspondence in timing of surface rupture
on the RCF and the north and south Hayward faults (after
A.D. 1640 and before circa 1776, Fig. 11a) suggests that
segments of the fault system may have failed together in a
single event or in separate events that were closely timed.
WG02 considered this information to define four possible
rupture scenarios (Fig. 11b). In developing models of fault-
rupture behavior, WG02 regarded the San Pablo Bay step-
over as a strong segmentation point and assigned a low
weight (⬍⬃10%) to simultaneous rupture of the RCF and
the north (or entire) Hayward fault. Evidence that the 1995
M6.9 Kobe, Japan, earthquake nucleated in a 4-km-wide
extensional step and ruptured bilaterally (e.g., Wald, 1996)
supports these low-probability scenarios and suggests the
possibility that San Pablo Bay may be a nucleation point for
bilateral rupture on the RCF and Hayward fault (WG02).
Where rupture terminates to the north on the RCF is
uncertain. Budding, et al. (1991) and Wong and Bott (1995)
identified an area of complex faulting near Santa Rosa where
the fault takes a 1-km right step as a possible barrier to
rupture. Two damaging earthquakes (M
L
5.6 and 5.7) oc-
curred on 1 October 1969 in the step-over area (Wong and
Bott, 1995) (Fig. 2) and a M4.3 event occurred there on 25
May 2003 (Northern California Earthquake Data Center). To
the north of Santa Rosa, the RCF is known also as the
Healdsburg fault. WG02 considers the north end of potential
rupture on the RCF to be the north end of geomorphologi-
cally recognized Holocene faulting at Windsor Creek (Bry-
ant, 1982; Hart, 1992) (Fig. 1). Exploratory trenches at two
sites on fault strands north of Windsor Creek in Healdsburg
(the Panorama Reservoir and Foss Creek Detention Basin
858 S. Hecker, D. Pantosti, D. P. Schwartz, J. C Hamilton, L. M. Reidy, and T. J. Powers
Figure 11. Earthquake ages and rupture scenarios for the Hayward–Rodgers Creek
fault system. (a) Shaded rectangles represent the 95-percentile range and darker ellipses
represent the 68-percentile range for preferred ages of the most recent event on the
Rodgers Creek and north Hayward faults and the penultimate event on the south Hay-
ward fault. Note that we use 1776 as a nominal cutoff for the age of paleoearthquakes
along the fault zone, although Lienkaemper et al. (2002) incorporate the uncertainty in
this historical constraint to obtain a modeled age for the event on the south Hayward
fault that extends to 1790 (at 95-percentile confidence). The bar indicates the 1868
earthquake on the south Hayward fault. Not shown is the prepenultimate event on the
south Hayward fault, which has a modeled age of A.D. 1530–1740. (b) Rupture sce-
narios proposed by W02. Small solid rectangles represent segment boundaries, or rup-
ture end points, between the Rodgers Creek (RC), north Hayward (NH), and south
Hayward (SH) faults (modified from Working Group on California Earthquake Prob-
abilities, 2003, figure 3.2). The correspondence in earthquake ages shown in (a) sup-
ports the possibility of simultaneous rupture of the RC and NH (scenario 2), the NH
and SH (scenario 3), or the RC, NH, and SH (scenario 4).
sites; Fig. 1) exposed evidence suggestive of Holocene dis-
placement (Harlan Tait Associates, 1996; S. Hecker, field
observations, 1996; Kleinfelder, Inc., 1996; Hart, 1998).
Perhaps faulting north of Windsor Creek occurs less fre-
quently or as discontinuous, distributed slip as part of the
right step-over to the Maacama fault.
The size of an earthquake on the RCF depends on
whether the Hayward fault also ruptures. WG02 calculated
earthquake magnitudes for alternative rupture scenarios us-
ing a weighted combination of several area-magnitude re-
lations wherein fault-plane areas are reduced to account for
the effect of creep (an issue for the Hayward fault). Given
rupture that is restricted to the RCF (with the north end at
Windsor Creek and the south end in the center of San Pablo
Bay), WG02 calculations provide a magnitude of M6.8–
7.1. If rupture occurs on the RCF and the north Hayward
fault, the expected magnitude is M6.9–7.3. In the event that
rupture involves the south Hayward fault as well, the esti-
mate increases to M7.1–7.4.
We can calculate Mdirectly from seismic moment (M
o
):
M(2/3 log M
o
)10.7 (Hanks and Kanamori, 1979),
where M
o
luA, and where lis the shear modulus of
elasticity (taken to be 3 10
11
dyne/cm
2
) and uis the av-
erage displacement over the ruptured fault plane with area
A. We do this for the case where earthquake rupture is re-
stricted to the RCF by assuming that the 2 (0.3, 0.2) m
of surface slip observed at Beebe Ranch approximates the
average slip on the fault in a typical event. Using the length
(53–73 km) and width (10–14 km) adopted by WG02 for
the RCF, we calculate M7.0–7.2.
Earthquake Recurrence and Elapsed Time
Timing constraints discussed in this article define the
elapsed time since the MRE on the RCF. Given a date for the
MRE of between A.D. 1715 and 1776, the elapsed time (as
of 2005) is between 229 and 290 years. Considering the most
conservative age interval for the MRE (no earlier than A.D.
1690 and no later than 1824), the elapsed time is certainly
no longer than 315 years and no shorter than 181 years. The
elapsed time, together with the average recurrence interval,
is critical for calculating conditional probabilities of the next
large earthquake on the fault.
Recurrence intervals estimated for the RCF encompass
the elapsed time since the MRE. Schwartz et al. (1992) de-
rived an average recurrence interval of 131 to 370 years
(with a best estimate of 230 years) using a slip-rate estimate
of 6.4–10.4 mm/yr for the past 750 years and event-slip in-
formation from the Beebe Ranch site. The earthquake model
of WG02 computes a recurrence interval for the RCF of 136
to 345 years (with a mean of 205 years), similar to the value
determined from geologic data. Because the elapsed time on
the RCF appears to have reached, or to be approaching, the
average repeat time for large earthquakes on the fault, a large
event may be likely in the near future. WG02 estimated the
mean probability of a M6.7 event on the RCF in the next
The Most Recent Large Earthquake on the Rodgers Creek Fault, San Francisco Bay Area 859
30 years at 17%, greater than that on any other fault segment
in the San Francisco Bay region.
Conclusions
The timing of the most recent surface-rupturing earth-
quake on the south-central RCF is well determined from
chronological modeling using radiocarbon and historical
constraints to be no earlier than A.D. 1690 and probably no
earlier than A.D. 1715. Overlap in radiocarbon dates, com-
bined with information on stratigraphic order, of abundant
charcoal from the top of the faulted section at the Triangle
G Ranch site allows good resolution of the maximum earth-
quake age. The presence of nonnative pollen associated with
livestock grazing above the faulted section indicates a pre-
settlement date (before circa 1830) for the most recent rup-
ture. The lack of evidence in the written record of a large
earthquake on the RCF implies that the surface-rupturing
event occurred before 1824, when a mission was built near
the fault, and likely before 1776, when the written record
began regionally in San Francisco. The timing of rupture on
the RCF is similar to prehistoric rupture on the north and
south segments of the Hayward fault (A.D. 1640–1776). This
permits the possibility that an earthquake ruptured both the
RCF and the Hayward fault across the San Pablo Bay step-
over. The elapsed time since the MRE on the RCF is probably
between 229 and 290 years and certainly between 181 and
315 years, which is comparable with a calculated average
recurrence interval for the fault of 131 to 370 years (best
estimate, 230 years). Slip in the MRE is estimated to be at
least 2.2 (1.2, 0.8) m at the Triangle G Ranch site, simi-
lar to or greater than the 2 (0.3, 0.2) m of total slip
measured at the nearby Beebe Ranch site.
Acknowledgments
We thank the Triangle G Ranch for graciously granting us permission
over the years to trench on their property. We thank Bill Bryant, Tom
Fumal, Jim Lienkaemper, and Pat Williams for their careful reviews and
suggestions that improved the manuscript. We also thank Bob Simpson for
so agreeably providing illustrations of the region’s seismicity and Pauline
Curiel for formatting the references and Table 1. We wish to acknowledge
Koji Okumura and Francesca Cinti, who contributed their insights as part
of the first field crew at the Triangle G Ranch site in the early 1990s.
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U.S. Geological Survey
345 Middlefield Rd., M.S. 977
Menlo Park, California 94025
shecker@usgs.gov
(S.H., D.P.S., J.C.H., T.J.P.)
Istituto Nazionale di Geofisica e Vulcanologia
Sezione Sismologia e Tettonofisica
Via di Vigna Murata 605
I-00143 Roma, Italy
(D.P)
Department of Geography
507 McCone Hall, #4740
University of California
Berkeley, California 94720
(L.M.R.)
Manuscript received 9 July 2004.
... Paleoseismic data currently considered in the UCERF 3.0 model for the Rodgers Creek fault (RCF) are sparse and based primarily on two sites along the central part of the RCF (Figure 1; Budding et al., 1991;Schwartz et al., 1992;Hecker et al., 2005). Our recent research conducted at two sites located along the southern RCF in Sonoma County, California-Tolay Marsh (Rams Gate Winery) and Cline Vineyard Sites-focused on obtaining paleoseismic data to improve existing forecasting models. ...
... The RCF represents the central part of a 275-km-long fault system that includes the Hayward, Rodgers Creek-Healdsburg, and Maacama faults ( Figure 1; McLaughlin et al., 2012). The 60-km-long RCF has a late Holocene dextral slip rate of 6.4 to 10.4 mm/yr (Budding et al., 1991;Schwartz et al., 1992;Hecker et al., 2005;Blisniuk and Walker, 2018). The central and southern sections of the RCF are divided by a possible 1km-wide step over, and the southern section is separated from the Hayward fault across a 4-km-wide right bend through San Pablo Bay (Figure 1; Watt et al., 2016). ...
... The RCF has had several historic moderate-sized earthquakes including the 1969 M5.6 and M5.7 earthquakes near Santa Rosa (Wong, 1991), and the poorly located 1898 M6.2 to M6.7 Mare Island events ( Figure 2; Toppozada et al., 1992). Hecker et al., 2018) and paleoseismic sites along the fault (Buntz et al. 1991;Schwartz et al., 1992;Randolph-Loar, 2002;2003;Hecker et al., 2005;Givler et al., 2010;. Blue dashed line presents the historical margin of tidal marsh. ...
Conference Paper
Full-text available
The most recent Uniform California Earthquake Rupture Forecast (UCERF 3.0) (Field et al., 2015) assigns a probability of 32% that the Hayward-Rodgers Creek fault will produce an earthquake of M≥6.7 in the next 30 years, the highest probability for any San Francisco Bay Region fault other than the San Andreas fault (Figure 1). As compared to previous models, the UCERF 3.0 model also increased the number of multi-fault ruptures and a doubling of the probability of events larger than M7.2 from earlier UCERF models (Field et al., 2015). The age of the most recent event (MRE) and earthquake recurrence are important parameters (especially for time-dependent models like UCERF 3.0) for developing rupture scenarios as well as computing probabilities of future major earthquakes along the Hayward-Rodgers Creek fault (HRCF). Data currently considered in the UCERF 3.0 model for the Rodgers Creek fault (RCF) are sparse and based on primarily two sites along the central part of the RCF (Figure 1; Budding et al., 1991; Schwartz et al., 1992; Hecker et al., 2005). Our recent research conducted at two sites – Tolay Marsh (Rams Gate Winery) and Cline Vineyard Sites - located in Sonoma County, California, and along the RCF focused on obtaining paleoseismic data to improve existing forecasting models. As we summarize here available paleoseismic data constrains only two to three events in the last one to two thousand years along the faults at any one site. Given the generally broad age constraints it is possible to correlate events between paleoseismic sites, but further work is required at high-sedimentation sites to further constrain event timing and recurrence intervals along the Rodgers Creek fault. For further details regarding this research see Givler et al. (2010; 2016; 2018).
... At Arano Flat (AF), 1.5 km southeast of MC ( Fig. 1), 113 detrital charcoal samples were analyzed (Fumal et al., 2003), and 31% were in stratigraphic order (see the supplemental material to this article for site-specific information). By comparison, charcoal samples submitted for analysis and deemed reliable at other paleoseismic sites in California ranges from 20% to 67% (see Table S4 for sites, number of samples dated and retained in OxCal models; Schwartz et al., 1998;Fumal et al., 2002;Lienkaemper et al., 2002;Hecker et al., 2005;Kelson et al., 2006;Lienkaemper and Williams, 2007;Scharer et al., 2007). Although detrital charcoal samples analyzed and retained in age models yield stratigraphically consistent results, resulting layer age results do not account for inbuilt detrital charcoal age or well estimate the true age of the deposit. ...
... Decisions to include or exclude information on the first occurrence of pollen from nonnative species in statistical age models should include uncertainty such that site age models are not overconstrained. E. cicutarium is the most common exotic pollen source used as a relative age indicator by paleoseismologists in California (e.g., Lienkaemper et al., 2002;Fumal et al., 2003;Hecker et al., 2005;Lienkaemper and Williams, 2007;Fumal, 2012;Schwartz et al., 2014). Erodium was introduced by the Spanish and first identified in cores from the Santa Barbara basin dated around 1750-1765, preceding significant Spanish settlement in California (Mensing and Byrne, 1998). ...
... A paleoseismic study on the Rodgers Creek fault at Triangle G Ranch, in north San Francisco Bay, finds the first occurrence of E. cicutarium above a prominent stratigraphic marker that reflects land-use disturbance. This first occurrence of Erodium probably coincides with the establishment of the first land grants and the local onset of grazing (Hecker et al., 2005). This is reported to have begun around 1830, and the most recent event (MRE) at this site occurred just before that ( Fig. 1; Hecker et al., 2005). ...
Article
Dendrochronological age constraints at the Hazel Dell (HD) paleoseismic site provide a means to explore the potential for systematic bias in age estimates of earthquakes in forested settings including the Santa Cruz Mountains section of the San Andreas fault, California. Age constraints developed from detrital charcoal are compared with absolute dates from dendrochronology. We develop a new method for estimating inbuilt layer ages that makes use of more than just the youngest radiocarbon dates in a given layer. The improved age model relates likely layer deposition dates with observed C14 ages. We find that for HD, the most likely charcoal sample (mean) is ∼322 yr older than the actual age of the deposit that contains it. With this correction, two historical Bay area earthquakes are confirmed to have ruptured the surface in 1838 and 1890. Earlier work based on similar charcoal dates proposed these ruptures occurred in the mid-1700s or earlier, but historical ages of these events are unequivocal because they rupture layers containing wood chips and a redwood stump from European logging at the site. This charcoal correction method also shifts the fourth earthquake, which is constrained only by detrital charcoal, ∼300 yr younger to A.D. ∼1266. Considering the full site record at HD, recurrence of ground rupture has averaged ∼150 yr for the last four events, not ∼250 yr as inferred from unadjusted C14. Our research shows that reasonable estimates of inbuilt age can be made for charcoal-dominated paleoseismic sites and that correcting for the inbuilt age can make a significant difference for earthquake ages and potential correlations with other sites on the fault. We also examine other common inputs to Bayesian age models and find when adequate uncertainty is not incorporated, modeled ages may exclude the actual age of the event of interest.
... This is the highest probability for any San Francisco Bay area fault other than the San Andreas fault. It is important to note that the seismic hazard models (especially time-dependent probabilistic models) are based primarily on slip rate and earthquake timing information derived from two paleoseismic sites on the central part of the RCF (Schwartz et al., 1992;Hecker et al. 2005). The timing of the most recent event (MRE) remains poorly constrained for the southern Rodgers Creek fault (SCRF), which is important to addressing the probability of a linked fault rupture between the HF-RCF. ...
... Although the event timing constraints are broad for the Cline Vineyard Site, the ages of these events correlate with other paleoseismic events identified on the RCF to the north (Figure 1). For example, Event E1 from trench T-3 (<680-760 cal yr B.P.) may represent the MRE (234 to 315 cal yr B.P.) identified at Triangle G Ranch by Hecker et al. (2005). Alternatively, Event E1 at the Cline Vineyard Site may have occurred between 680-720 cal yr B.P. and 800-920 cal yr B.P. The timing of the alternative interpretation of the MRE and Event E2 (>800-920 cal yr B.P.,) at the Cline Vineyard Site correlate with the penultimate event (>962-762 cal yr B.P.) identified at Triangle G Ranch (Hecker et al., 2005). ...
... For example, Event E1 from trench T-3 (<680-760 cal yr B.P.) may represent the MRE (234 to 315 cal yr B.P.) identified at Triangle G Ranch by Hecker et al. (2005). Alternatively, Event E1 at the Cline Vineyard Site may have occurred between 680-720 cal yr B.P. and 800-920 cal yr B.P. The timing of the alternative interpretation of the MRE and Event E2 (>800-920 cal yr B.P.,) at the Cline Vineyard Site correlate with the penultimate event (>962-762 cal yr B.P.) identified at Triangle G Ranch (Hecker et al., 2005). Thus, the results from this study support co-seismic surface rupture of the SRCF at our site with the RCF to the north (and possibly the northern Hayward fault to the south) in a single rupture. ...
Research
The Cline Vineyard paleoseismic study compliments our earlier pilot study at the same site (Givler et al., 2016). The previous pilot investigation consisted of two trenches (T1 and T2) that identified two strands of the SRCF that displace Holocene alluvium. The SRCF is mapped as a series of discrete NW-striking fault strands marked by topographic scarps, shutter ridges, deflected drainages, sag ponds, and aligned springs. As part of the latest paleoseismic study, seven trenches (T-3 to T-8) and one test pit were excavated across the eastern and western SCRF strands at the site. Trenches (T-3, T-4, T-5, and T-6) intersect the western strand, which bounds the margin of a Holocene stream terrace (Qt1) marked by a prominent east-facing scarp and spring. Trenches T-5, T-7, and T-8 cross the eastern strand that bisects the Qt1 terrace. Overall, the data collected at the Cline Vineyard Site suggest that the SRCF forms a series of northweststriking, en echelon fault strands that record recent surface ruptures. Trenches T-3, T-5, T-7, and T-8 exposed evidence for late Holocene deformation that includes a combination of co-seismic folding and faulting that is distributed across the eastern and western strands of the SRCF. Based on 14C AMS data and trench exposures, the Qt1 terrace surface is composed of early to late Holocene fluvial deposits separated by as many as five buried soils capping fining upward sequences. The 14C results from trench T-3 indicate the latest deposition on the Qt1 surface was likely younger than 680-760 cal yr B.P., which predate the deep late Holocene incision of the adjacent creek. The paleoseismic trench results from this study indicate the following event chronology on the SRCF at the Cline Vineyard Site: (1) Event E1 occurred less than 680-760 cal yr B.P. (e.g., from trench T-3); (2) Events E2 and E3 occurred prior to 800-920 cal yr B.P. (e.g., from Trench T-3); (3) Event E4 includes a faultedgravel deposit exposed in trenches T-5, T-7, and T-8 and occurred less than approximately 3,000 cal yr B.P. (this event may correlate with events E1, E2, or E3 based on the limited deposition on the Qt1 surface in the late Holocene); and (4) several earlier events >4,000 cal yr B.P. are inferred from available data (e.g., in trenches T-5, T-7 and T-8, but these events remain poorly constrained). Although the event timing constraints are broad for the Cline Vineyard Site, the ages of these events correlate with other paleoseismic events identified on the RCF to the north (Figure 1). For example, Event E1 from trench T-3 (<680-760 cal yr B.P.) may represent the MRE (234 to 315 cal yr B.P.) identified at Triangle G Ranch by Hecker et al. (2005). Alternatively, Event E1 at the Cline Vineyard Site may have occurred between 680-720 cal yr B.P. and 800-920 cal yr B.P. The timing of the alternative interpretation of the MRE and Event E2 (>800-920 cal yr B.P.,) at the Cline Vineyard Site correlate with the penultimate event (>962- 762 cal yr B.P.) identified at Triangle G Ranch (Hecker et al., 2005). Thus, the results from this study support co-seismic surface rupture of the SRCF at our site with the RCF to the north (and possibly the northern Hayward fault to the south) in a single rupture.
... The most recent Uniform California Earthquake Rupture Forecast (UCERF 3.0) (Field et al., 2015) assigns a probability of 32% that the HF-RCF will produce an earthquake M≥6.7 in the next 30 years; the highest probability for any San Francisco Bay area fault besides for the San Andreas fault. It is important to note that the probabilistic hazard models (especially time-dependent probabilistic models) are based primarily on slip rate and earthquake timing information derived from just two paleoseismic sites on the central part of the RCF (Schwartz et al., 1992;Hecker et al. 2005). The most recent event (MRE) remains poorly constrained for the southern Rodgers Creek fault (SCRF), which is key to addressing the question regarding the probability of a linked fault rupture between the HF-RSV, as well as the timing of past late Holocene events. ...
... However, data used to develop the UCERF 3.0 probabilities for the RCF are based on findings from only two sites along the central part of the RCF (Figure 2; Budding et al., 1991;Schwartz et al., 1992;Hecker et al., 2005). Obtaining additional data from the southern part of the fault is critical to gaining a better understanding of: (1) the timing and recurrence of large paleoearthquakes, (2) potential for multi-segment ruptures with the northern HF (Parsons et al., 2003), and (3) seismic hazard and probabilities of large earthquakes in the rapidly expanding urban area of the northern SFBR. ...
... The RCF includes the central part of an extensive 275-km-long fault system that comprises the Hayward, Rodgers Creek-Healdsburg, and Maacama faults (Figure 1; McLaughlin et al., 2012). The 60-km-long RCF, located between San Pablo Bay and Santa Rosa, California, strikes approximately N35°W, and is characterized by a late Holocene right-lateral slip rate of 6.4 to 10.4 mm/yr (Budding et al., 1991;Schwartz et al., 1992;Hecker et al., 2005). ...
Technical Report
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The timing and recurrence of earthquakes along the Rodgers Creek fault (RCF), including consideration of fault segmentation with the Hayward fault (HF), and are among the most critical issues for reducing uncertainties in probabilistic analyses of seismic hazard in Northern California. The most recent Uniform California Earthquake Rupture Forecast (UCERF 3.0) (Field et al., 2015) assigns a probability of 32% that the HF-RCF will produce an earthquake M≥6.7 in the next 30 years; the highest probability for any San Francisco Bay area fault besides for the San Andreas fault. It is important to note that the probabilistic hazard models (especially time-dependent probabilistic models) are based primarily on slip rate and earthquake timing information derived from just two paleoseismic sites on the central part of the RCF (Schwartz et al., 1992; Hecker et al. 2005). The most recent event (MRE) remains poorly constrained for the southern Rodgers Creek fault (SCRF), which is key to addressing the question regarding the probability of a linked fault rupture between the HF-RSV, as well as the timing of past late Holocene events. Our pilot paleoseismic study at the Cline Vineyard Site was designed to test whether the stratigraphy at the site records evidence of late Holocene surface ruptures along the SRCF. This site was chosen because of the apparent high sedimentation rate and presence of prominent tectonic geomorphology. The SRCF is mapped as a series of discrete NW-striking fault traces marked by topographic scarps, shutter ridges, deflected drainages, sag ponds, and aligned springs. Two paleoseismic trenches (T-1 and T-2) were excavated across the SCRF. Trench T-1 was excavated across a 1.5-m-high west-facing scarp and northwest-trending swale on the margin of a Holocene terrace (Qt1). The trench exposed folded and tilted late Pleistocene to early Holocene fluvial deposits unconformable overlain by middle to late Holocene colluvium and fine-grained organic-rich overbank deposits. The results from trench T-1 indicate: northwest-striking faults displace the late Pleistocene to late Holocene stratigraphy with a west-side-up dextral displacement and likely form a series of en echelon fault strands that step across a late Holocene terrace. We interpret a minimum of three late Holocene events in T-1, although the timing of these events are unconstrained based on a lack of datable material in this trench. Trench T-2 was excavated across a northwest-trending east-facing scarp that separates Sonoma Volcanics on the west from the Qt1 terrace on the east. T-2 exposed Sonoma Volcanics bedrock overlain by a series of hillslope and scarp-derived (?) colluvial deposits that interfinger with stratified Qt1 fluvial terrace deposits (960 to 805 years BP). Exposures in T-2 suggest that the fault here exhibits primarily strike-slip displacement with a vertical component of east-side-down separation. Multiple scarp-derived colluvial wedge-like deposits are interpreted to record multiple surface rupturing events within the late Holocene (minimum of two events <1,000 years BP). Based on the initial results of this pilot study, we believe further paleoseismic investigations at the Cline Vineyard Site will yield additional constraints on the paleoseismic history of the SRCF. This study establishes the location of Holocene strands of the SRCF crossing late Holocene fluvial terraces (<1,000 years BP) and results from T-2 exposures suggests a minimum of two surface rupturing events in the last 1,000 yrs. We believe this site has a high potential for success due to the very young site stratigraphy that overlies the active fault zone, abundant possibilities for assessing earthquake timing information and possibly slip-per-event and/or potentially slip rate.
... In this study, we use lidar, seismic reflection, gravity, magnetic, and geotechnical data to identify surface and subsurface faulting within the urban area of Santa Rosa, located atop a 1-km-wide, 4-km-long releasing bend between the Healdsburg and Rodgers Creek (sensu stricto) sections of the right-lateral RCF (Fig. 1 ). The RCF has not had a surfacerupturing earthquake since an event sometime in the eighteenth century documented from trenching studies south of Santa Rosa (Hecker et al., 2005), despite having a robust late Holocene slip rate (∼6–10 mm=yr right lateral; Schwartz et al., 1992). This discordance suggests that accumulated slip may have reached or exceeded the amount released in the prior event (∼2 m along the southern portion of the RCF; Budding et al., 1991; Hecker et al., 2005). ...
... The RCF has not had a surfacerupturing earthquake since an event sometime in the eighteenth century documented from trenching studies south of Santa Rosa (Hecker et al., 2005), despite having a robust late Holocene slip rate (∼6–10 mm=yr right lateral; Schwartz et al., 1992). This discordance suggests that accumulated slip may have reached or exceeded the amount released in the prior event (∼2 m along the southern portion of the RCF; Budding et al., 1991; Hecker et al., 2005). The 2007 and 2014 Working Groups on California Earthquake Probabilities considered the mean 30-yr probability of an M w ≥ 6:7 earthquake on the RCF, in combination with the Hayward fault to the south, (a value of 32%) to be as high or higher than for any other fault in the SFBA (Field et al., 2009Field et al., , 2015). ...
Article
Airborne light detection and ranging (lidar) topography reveals for the first time the trace of the Rodgers Creek fault (RCF) through the center of Santa Rosa, the largest city in the northern San Francisco Bay area. Vertical deformation of the Santa Rosa Creek floodplain expresses a composite pull-apart basin beneath the urban cover that is part of a broader 1-km-wide right-releasing bend in the fault. High-resolution geophysical data illuminate subsurface conditions that may be responsible for the complex pattern of surface faulting, as well as for the distribution of seismicity and possibly for creep behavior.We identify a dense, magnetic basement body bounded by the RCF beneath Santa Rosa that we interpret as a strong asperity, likely part of a larger locked patch of the fault to the south. A local increase in frictional resistance associated with the basement body appears to explain (1) distributed fault-normal extension above where the RCF intersects the body; (2) earthquake activity around the northern end of the body, notably the 1969 ML 5.6 and 5.7 events and aftershocks; and (3) creep rates on the RCF that are higher to the north of Santa Rosa than to the south. There is a significant probability of a major earthquake on the RCF in the coming decades, and earthquakes associated with the proposed asperity have the potential to release seismic energy into the Cotati basin beneath Santa Rosa, already known from damaging historical earthquakes to produce amplified ground shaking.
... This rate is the highest probability of occurrence among the seven major fault systems in the San Francisco Bay area. The largest earthquake on the Hayward Fault in recorded history occurred in 1868, with an estimated magnitude of M 6.8 [11] was known as the "Great San Francisco Earthquake" for the damage it caused to the major population center of San Francisco. According to the San Francisco Bulletin, the total loss of property varied from $300,000 to $5,000,000 (RMS, 2013 [12]). ...
Conference Paper
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History of earthquake’s damages have illustrated the high vulnerability and risks associated with failure of water transfer and distribution systems. Adequate mitigation plans to reduce such seismic risks are required for sustainable development. The first step in developing a mitigation plan is prioritizing the limited available budget to address the most critical mitigation measures. This paper presents an optimization model that can be utilized for financial resource allocation towards earthquake risk mitigation measures for water pipelines. It presents a framework that can be used by decision-makers (authorities, stockholders, owners and contractors) to structure budget allocation strategy for seismic risk mitigation measures such as repair, retrofit, and/or replacement of steel and concrete pipelines. A stochastic model is presented to establish optimal mitigation measures based on minimizing repair and retrofit costs, post-earthquake replacement costs, and especially earthquake-induced large losses. To consider the earthquake induced loss on pipelines, the indirect loss due to water shortage and business interruption in the industries which needs water is also considered. The model is applied to a pilot area to demonstrate the practical application aspects of the proposed model. Pipeline exposure database, built environment occupancy type, pipeline vulnerability functions, and regional seismic hazard characteristics are used to calculate a probabilistic seismic risk for the pilot area. The Global Earthquake Model’s (GEM) OpenQuake software is used to run various seismic risk analysis. Event-based seismic hazard and risk analyses are used to develop the hazard curves and maps in terms of peak ground velocity (PGV) for the study area. The results of this study show the variation of seismic losses and mitigation costs for pipelines located within the study area based on their location and the types of repair. Performing seismic risk analysis analyses using the proposed model provides a valuable tool for determining the risk associated with a network of pipelines in a region, and the costs of repair based on acceptable risk level. It can be used for decision making and to establish type and budgets for most critical repairs for a specific region.
... In evaluating the potential for cascading rupture along the linked Hayward-Rodgers Creek fault, we must take into consideration that the path an earthquake rupture takes depends not only on fault geometry and connectivity but also on other factors, including earthquake history, stress imparted by past events, and the amount and distribution of creep occurring along the faults (10,12,17,32). Geodetic and paleoseismic evidence (17,28,33,34) indicates that the Hayward and Rodgers Creek faults have each accumulated enough stress to produce a large earthquake. In addition, stress along the southern Rodgers Creek and northern Hayward faults may be elevated as a result of static Coulomb stress changes resulting from the nearby 2014 M6 Napa earthquake ( Fig. 1) (35). ...
Article
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The next major earthquake to strike the ~7 million residents of the San Francisco Bay Area will most likely result from rupture of the Hayward or Rodgers Creek faults. Until now, the relationship between these two faults beneath San Pablo Bay has been a mystery. Detailed subsurface imaging provides definitive evidence of active faulting along the Hayward fault as it traverses San Pablo Bay and bends ~10° to the right toward the Rodgers Creek fault. Integrated geophysical interpretation and kinematic modeling show that the Hayward and Rodgers Creek faults are directly connected at the surface—a geometric relationship that has significant implications for earthquake dynamics and seismic hazard. A direct link enables simultaneous rupture of the Hayward and Rodgers Creek faults, a scenario that could result in a major earthquake (M = 7.4) that would cause extensive damage and loss of life with global economic impact.
Article
Full-text available
The Hayward fault in California's San Francisco Bay area produces large earthquakes, with the last occurring in 1868. We examine how physics‐based dynamic rupture modeling can be used to numerically simulate large earthquakes on not only the Hayward fault, but also its connected companions to the north and south, the Rodgers Creek and Calaveras faults. Equipped with a wealth of images of this fault system, including those of its 3D geology and 3D geometry, in addition to inferences about its interseismic creep‐rate pattern and rock‐friction behavior, we use a finite‐element computer code to perform 3D dynamic earthquake rupture simulations. We find that the rock properties affect the locations and amount of slip produced in our simulated large earthquakes. Crucial factors that control rupture behavior in our modeling are the earthquake nucleation locations, the fault geometry, and the data that reveal where the fault system is creeping or locked. Our findings suggest that large Rodgers Creek‐Hayward‐Calaveras‐Northern Calaveras (RC‐H‐C‐NC) fault‐system earthquakes may result from dynamic rupture that starts in a locked part of the fault system, but is then stopped by the creeping parts, leading to high‐magnitude‐6 earthquakes; or, from dynamic rupture that starts in a locked part of the fault system, then cascades through some of the creeping parts, leading to magnitude‐7 earthquakes.
Article
Recent geophysical imaging indicates that the Hayward Fault hard links to the Rodgers Creek Fault at 5 m depth within the San Pablo Bay, CA, suggesting that earthquakes may be able to rupture continuously through the fault network. To investigate fault propagation, interaction, and linkage in segmented fault networks, including those within the San Pablo Bay, we simulate the development of two idealized, underlapping faults within an extensional step over at seismogenic depths using work optimization. We test the sensitivity of fault growth to strength anisotropy, material heterogeneities, and initial fault geometry. The optimal faults propagate toward each other until linking with the other fault at its tip and form a single hard-linked transverse fault. These faults propagate with relatively high propagation power or rate of efficiency gain. Less efficient faults form wider basins and develop with reduced propagation power. Models with initial fault geometries that more closely match the shallowly imaged Hayward and Rodgers Creek faults suggest that the faults link at seismogenic depths if a mapped segment of the Rodgers Creek that extends into the San Pablo Bay is currently inactive. Predictions of average slip rate, slip per earthquake, and earthquake magnitude from these models closely match paleoseismic estimates. The hard linkage of the Hayward and Rodgers Creek faults imaged in the near-surface, and predicted by these models, increases local seismic hazard by increasing the upper limit of throughgoing earthquakes to M 7.6.
Article
Surface creep rate, observed along five branches of the dextral San Andreas fault system in northern California, varies considerably from one section to the next, indicating that so too may the depth at which the faults are locked. We model locking on 29 fault sections using each section’s mean long-term creep rate and the consensus values of fault width and geologic slip rate. Surface creep rate observations from 111 short-range alignment and trilateration arrays and 48 near-fault, Global Positioning System station pairs are used to estimate depth of creep, assuming an elastic half-space model and adjusting depth of creep iteratively by trial and error to match the creep observations along fault sections. Fault sections are delineated either by geometric discontinuities between them or by distinctly different creeping behaviors. We remove transient rate changes associated with five large (M ≥5:5) regional earthquakes. Estimates of fraction locked, the ratio of moment accumulation rate to loading rate, on each section of the fault system provide a uniform means to inform source parameters relevant to seismic-hazard assessment. From its mean creep rates, we infer the main branch (the San Andreas fault) ranges from only 20% ± 10% locked on its central creeping section to 99%–100% on the north coast. From mean accumulation rates, we infer that four urban faults appear to have accumulated enough seismic moment to produce major earthquakes: the northern Calaveras (M 6.8), Hayward (M 6.8), Rodgers Creek (M 7.1), and Green Valley (M 7.1). The latter three faults are nearing or past their mean recurrence interval. © 2014, Bulletin of the Seismological Society of America. All rights reserved.
Article
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Count rates, representing the rate of 14 C decay, are the basic data obtained in a 14 C laboratory. The conversion of this information into an age or geochemical parameters appears a simple matter at first. However, the path between counting and suitable 14 C data reporting (table 1) causes headaches to many. Minor deflections in pathway, depending on personal interpretations, are possible and give end results that are not always useful for inter-laboratory comparisons. This discussion is an attempt to identify some of these problems and to recommend certain procedures by which reporting ambiguities can be avoided.
Article
Summarizes available information pertaining to large earthquakes in the San Francisco Bay region. By using the available information, reasonable assumptions, and simple models, it strives to make projections about the locations, sizes, and time of future earthquakes. The types of basic information used in this report include the following: 1) Identification of fault segments judged to be capable of producing large earthquakes in the future. 2) Estimates of the amount of displacement and thus the magnitude of future large earthquakes likely to occur during rupture of these fault segments. 3) Estimates of the median recurrence intervals (and their uncertainties) for major earthquakes on the fault segments. 4) Estimates of the date of the most recent large earthquake for each of the fault segments. -from Authors
Article
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.
Article
The moment magnitude M 7.8 earthquake in 1906 profoundly changed the rate of seismic activity over much of northern California. The low rate of seismic activity in the San Francisco Bay region (SFBR) since 1906, relative to that of the preceding 55 yr, is often explained as a stress-shadow effect of the 1906 earthquake. However, existing elastic and visco-elastic models of stress change fail to fully account for the duration of the lowered rate of earthquake activity. We use variations in the rate of earthquakes as a basis for a simple empirical model for estimating the probability of M ≥6.7 earthquakes in the SFBR. The model preserves the relative magnitude distribution of sources predicted by the Working Group on California Earthquake Probabilities' (WGCEP, 1999; WGCEP, 2002) model of characterized ruptures on SFBR faults and is consistent with the occurrence of the four M ≥6.7 earthquakes in the region since 1838. When the empirical model is extrapolated 30 yr forward from 2002, it gives a probability of 0.42 for one or more M ≥6.7 in the SFBR. This result is lower than the probability of 0.5 estimated by WGCEP (1988), lower than the 30-yr Poisson probability of 0.60 obtained by WGCEP (1999) and WGCEP (2002), and lower than the 30-yr time-dependent probabilities of 0.67, 0.70, and 0.63 obtained by WGCEP (1990), WGCEP (1999), and WGCEP (2002), respectively, for the occurrence of one or more large earthquakes. This lower probability is consistent with the lack of adequate accounting for the 1906 stress-shadow in these earlier reports. The empirical model represents one possible approach toward accounting for the stress-shadow effect of the 1906 earthquake. However, the discrepancy between our result and those obtained with other modeling methods underscores the fact that the physics controlling the timing of earthquakes is not well understood. Hence, we advise against using the empirical model alone (or any other single probability model) for estimating the earthquake hazard and endorse the use of all credible earthquake probability models for the region, including the empirical model, with appropriate weighting, as was done in WGCEP (2002).
Article
This paper highlights some of the main developments to the radiocarbon calibration program, OxCal. In addition to many cosmetic changes, the latest version of OxCal uses some different algorithms for the treatment of multiple phases. The theoretical framework behind these is discussed and some model calculations demonstrated. Significant changes have also been made to the sampling algorithms used which improve the convergence of the Bayesian analysis. The convergence itself is also reported in a more comprehensive way so that problems can be traced to specific parts of the model. The use of convergence data, and other techniques for testing the implications of particular models, are described. © 2001 by the Arizona Board of Regents on behalf of the University of Arizona.
Article
Moment magnitude M with objective confidence-level uncertainties are estimated for felt San Francisco Bay region earthquakes using Bakun and Wentworth's (1997) analysis strategy for seismic intensity observations. The frequency-magnitude distribution is well described for M ≥5.5 events since 1850 by a Gutenberg-Richter relation with a b-value of 0.90. The seismic moment rate ΣM0/yr since 1836 is 2.68 X 1018 N-m/yr (95% confidence range = 1.29 X 1018 N-m/yr to 4.07 X 1018 N-m/yr); the seismic moment rate since 1850 is nearly the same. ΣM0/yr in the 56 years before 1906 is about 10 times that in the 70 years after 1906. In contrast, ΣM0/yr since 1977 is about equal that in the 56 years before 1906. 80% (1σ = 14%) of the plate-motion moment accumulation rate is available for release in earthquakes. The historical ΣM0/yr and the portion of the plate-motion moment accumulation rate available for release in earthquakes are used in a seismic cycle model to estimate the rate of seismic activity in the twenty-first century. High and low rates of future seismic activity are both permissible given the range of possible seismic-cycle recurrence times T and the uncertainties in the historical ΣM0 and in the percentage of plate motion available for release in earthquakes. If the historical seismic moment rate is not greater than the estimated 2.68 X 1018 N-m/yr and the percentage of the plate-motion moment accumulation available for release in earthquakes is not less than the estimated 80%, then for all T, the rate of seismic moment release from now until the next 1906-sized shock will be comparable to the rate from 1836 to 1905 when M 6 1/2 shocks occurred every 15 to 20 years.
Article
Near-source ground motions, teleseismic body waveforms, and geodetic displacements produced by the 1995 Kobe, Japan, earthquake have been used to determine the spatial and temporal dislocation pattern on the faulting surfaces. Analysis of the slip model indicates that the ground motions recorded within the severely damaged region of Kobe originated from the region of relatively low slip (about 1 m) deep beneath Kobe and not from the shallow, higher slip regions (about 3 m) beneath Aqaji Island. Although the slip was relatively low beneath Kobe, the combined effects of source rupture directivity, a short slip duration, and site amplification conspired to generate very damaging ground motions within the city.