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1 University of California,
Irvine
Department of Environmental Health, Science and Policy
Irvine,
California 92697-7070
(G.R.N., L.B.G.)
2 Arizona State
University
Department of Geological Sciences
Tempe, Arizona
85287-1404
(J R.A., J.J.Y.)
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Abstract |
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 Top Abstract IntroductionSite Location and HistoryMethodologyResultsDiscussionAcknowledgmentsReferences |
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A.D. 1160. Minimum and maximum slip rates ranging
from 29.3 to 35.6 mm/yr are derived from different assumptions about the timing
of channel incision and offset. The resulting slip rates at VMR agree
well with the late-Holocene slip rate of 33.9 ± 2.9 mm/yr at Wallace
Creek, approximately 18 km to the northwest, and imply that within measurement
uncertainty the 30–37 mm/yr velocity gradient across the SAF
from decadal time- scale geodetic measurements is accommodated across the
several-meter-wide SAF zone at VMR over the last
millennium. Online material: Supplemental unit descriptions, trench logs, and 14C data.
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Introduction |
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TopAbstractIntroductionSite Location and HistoryMethodologyResultsDiscussionAcknowledgmentsReferences |
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Slip rate is an essential input parameter for either time- dependent or
static calculations of seismic hazard on the SAF.
Argus and Gordon (2001) determined that 39 ± 2 mm/ yr of relative motion between the Pacific and
North American plates is accommodated along the SAF and central
California Coast Ranges. The difference between the SAF slip rate and
this regional slip budget will indicate the degree of localization of that
deformation and the potential for other active seismogenic structures in the
Coast Ranges.
Sieh and Jahns (1984) reported a
slip rate of 33.9 ± 2.9 mm/yr for the SAF in the Carrizo Plain
over the last 3700 years, in agreement with their late Pleistocene rate of
36 mm/yr. Our study provides a latest Holocene slip rate for comparison
with late-Pleistocene, mid-Holocene, and geodetic measurements of deformation.
The slip rate is derived from the measurement and dating of offset stream
channels. It also provides an opportunity to characterize evolution of the
offset stream channels, and thus develop understanding of their tectonic and
geomorphic development.
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Site Location and History |
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TopAbstractIntroductionSite Location and HistoryMethodologyResultsDiscussionAcknowledgmentsReferences |
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This study focuses on offset channel "gully #44" as reported by Sieh (1977, 1978, 1979; Figs. 3 and 5) and investigated by Sims et al. (1993). Sieh documented sites along the SAF where slip from the A.D. 1857 earthquake was well preserved, including VMR. He measured several offset channels numbered 44–50 and postulated that they were formed by 8 ± 0.5 m displacement in 1857, and two prior earthquakes with 7.5 ± 1 m and 10 ± 1 m slip (Fig. 3; Sieh, 1978, 1979).
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Methodology |
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TopAbstractIntroductionSite Location and HistoryMethodologyResultsDiscussionAcknowledgmentsReferences |
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Trenches were excavated with a backhoe and supported by hydraulic shoring. The trenches were logged at a scale of 1:25 and the thalwegs were surveyed for spatial control and precise offset measurement. Exposures of the trench walls were also recorded by mosaic photography.
Over 1400 surveyed points define the topography at the site (see Fig. 5). Channels once connected to the main channel (southeast) have been offset to the three beheaded channels to the northwest. Our excavations exposed offset channels in trenches T2, T3, T8, T6', and T7. The T7 channel has subsequently captured the small drainage immediately above it. Topographic survey data collected during this project were used to tie all the trenches into the same coordinate system. We also combined new data with pre- existing data (Sims et al., 1993) to produce the topographic map shown in Figure 5 and used them in our offset calculations (see following sections). In addition, we surveyed all major contacts exposed in the trench walls. Survey data were adjusted from the local datum to an absolute Universal Transverse Mercator (UTM) grid projection and the elevation fixed relative to sea level. Absolute location and elevation accuracy are only several meters (handheld GPS), but the internal precision of the survey is at the centimeter level.
In all of the trenches, the stratigraphic units were analyzed, identified, and logged. Sediment and soil analyses were based on unit color, rounding, sorting, clast size, matrix composition, and degree of bioturbation. Sediment analysis and interpretation was the basis for unit correlation and consequently for offset calculation. In each trench, units are numbered in hundreds corresponding to the trench number (e.g., 200 for trench T2). In trenches T2, T3, T6', and T8, correlative units have comparable numbers such that unit 660 in T6' correlates with unit 860 in T8, and soon. Numbers increase from oldest (e.g., unit 300) to youngest (e.g., unit 395) within a given trench. Correlations between trenches on the same side of the fault (i.e., T6' and T8) are for the same unit. Correlations of channel units across the fault zone represent the same stratigraphic position.
Radiocarbon analysis and sample pretreatment for this project were conducted at University of California, Irvine's (UCI’s) Keck carbon isotope Accelerator Mass Spectrometry Laboratory (AMS) under the supervision of J. Southon. Standards Ox-I, Ox-II, ANU (sucrose), and coal 50,000 years old were used to correct for natural graphitization and AMS fractionation. The dates extracted from the samples were calibrated using Calib 4.4 by Stuiver et al. (1998). Results are displayed in Table 1 and supplemental Table 1 (available in the electronic supplement to this article).
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Results |
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TopAbstractIntroductionSite Location and HistoryMethodologyResultsDiscussionAcknowledgmentsReferences |
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T2—Upstream Channel
T2 is a re-excavation of a trench from
Sims et al. (1993). The
trench is about 7 m long and 3 m wide and exposes relatively young channel
deposits on the upstream side of the fault
(Figs. 5 and
6a). The exposure geometry only
preserved short portions of the outer edge equivalents of the southwest wall.
Unit 200 is interpreted as local bedrock, mapped as Paso Robles Formation
(QTP) on regional maps by
Dibblee (1973). Along its upper
contact, unit 200 shows downslope warping of units, which probably results from
soil creep. Unit 200 was incised and then filled to an unknown level by channel
deposits and colluvium of unit 210, both of which are bioturbated and subject to
bioturbation and accumulation of pedogenic clay, carbonate, and gypsum. Unit 220
fills a channel cut into unit 200/210 with likely coeval colluvial packages of
units 240 and 230. (Both units 240 and 230 are burrowed and pedogenically
altered, but less so than unit 210.) Unit 250 fills a channel cut into units
240/220/230 with roughly coeval colluviation of unit 260 (and lower unit 270).
Unit 260 contains numerous clasts from regional unit QTP, presumably
derived from some local bedrock exposure upslope. Unit 270 at its base is
probably the same as unit 260, but then overlies all other units and is in
contact with the active soil horizon
(Fig. 6a; supplemental
Table 2, available in the electronic supplement to this article).
T3—Cross-Fault Trench
T3 is a re-excavation of a portion of a trench from
Sims et al. (1993)
(Fig. 6b; supplemental
Table 3, available in the electronic supplement to this article). This is the only trench
excavated across the SAF in 2004. Trench T3 clearly shows juxtaposed
stratigraphy with unit 300 correlative with the local bedrock unit 200 from
trench T2. Unit 300 is a well- lithified grayish unit with silts and thin sands
alternating with clast-supported pebbles dominated by Tertiary Monterey
formation–derived siltstone chips. Bedding dips to the northeast and the
unit is progressively sheared to the southwest until it is completely cut out.
At the base of the trench, unit 300 is juxtaposed against 310 across a zone of
inferred fissure fill (unit 380). Unit 310 is a tan-beige heterogeneous unit we
divided into three subunits. Unit 310A is heavily sheared pebbly silt with no
clear sedimentary fabric preserved. It grades laterally into unit 310B, which is
interbedded sand and silt. Units 310B and 310C are separated by a shear zone
unconformably overlain (probably erosionally) by fluvial sand: unit 320. Unit
310C has preserved vertical layering in silt, but it is largely sheared and
bioturbated so that the original sedimentary fabric has been obliterated. Unit
310 has some CaCO3 staining, further indicating its older age
relative to overlying units. Units 330, 340, and 360 represent progressively
younger bioturbated and pedogenically modified units. They are silty pebbly
sands that do not show any sedimentary structures. Units 350 and 370 on the
northeast side of the main fault zone and 375 on the southwest represent the
sedimentary fill sequences of the offset channel system that is the primary
focus of this study. They are predominantly matrix-supported pebble gravels
occasionally in upward fining sequences and plain and cross- laminated sands.
Unit 395 is the youngest, unfaulted colluvial and fluvial unit.
T3 exposes three main shear zones, which we interpret as evidence for a minimum of three displacement events. The oldest shear zone cuts through unit 300. The southwestern shear zone cuts unit 310 and is unconformably overlain by unit 320. Unit 330 may be cut by a strand of this zone. The most recently active shear zone is capped only by unit 395 and spectacularly disrupts units 375 and 370. Not only are the units sheared, but also one or two distinctly aged fissures are filled in this zone. We infer that the youngest fissure fill formed in the 1857 earthquake.
T6'—First Downstream Channel
T6' exposed offset channel deposits downstream of the fault
(Fig. 6c; supplemental
Table 4 and Fig. 1a, available in the electronic supplement to this article). The local
bedrock is a heavily bioturbated and pedogenically altered tan-beige set of
units (610, 620, 630) with occasional (unit 620) to no preserved sedimentary
fabric (units 610 and 630). Gravelly unit 640 is the first fill unit into the
offset channel. Its basal contact forms one of the thalwegs used to measure
channel offset (Fig. 5). Unit
640 is overlain by colluvial and/or pedogenically altered units 650 and 660,
which are in turn cut by the channel that was subsequently filled by the coarse,
angular, crudely bedded gravels in unit 670. In places, the texture of units 630
and 660 are indistinguishable, but the contact between them is well defined by a
color contrast, with darker unit 660 above lighter units 630 and 610. Several
14C samples were collected in trench T6'. However, only sample
T6'-4 from the top of unit 630 yielded a reliable date that gives a
maximum channel age (see Radiocarbon Dating section).
T8—First Downstream Channel
Coarse, angular, stratified gravels in units 840 and 870 with scour and fill
structures are interpreted as channel fill deposits in trench T8
(Fig. 6d; supplemental
Table 5 and Fig. 1b, available in the electronic supplement to this article). The base of
840 is the offset thalweg we matched with that exposed in T6' and T2. The
channels were cut into units 810, 820, and 830, which are interpreted as
colluvium. Unit 860 is a clayey silt deposit containing most of the radiocarbon-
dated samples near its top. It could be either colluvium, washed down from
adjacent hill slopes, or autochthonous alteration of channel deposits by
bioturbation and other pedogenic processes. Most of the samples collected for
dating were retrieved from this trench.
T7—Second Downstream Channel
Because of its older age, the stratigraphic relationships in trench T7
(Fig. 6e; supplemental
Table 6 and Fig. 1c, available in the electronic supplement to this article) are more
obscured by pedogenesis than in the younger trenches. Nonetheless, unit 730 has
eroded into the local bedrock (a Quaternary fan deposit) and its base represents
the thalweg that we correlate with that exposed in trenches T2, T6', and
T8. This channel thalweg is offset about 48.8 m. Unit 740 is heavily
pedogenically altered, but preserves a few channel gravel packages. Colluvial
unit 750 dominates the upper portion of the exposure. Unit 760 has occasional
disseminated lenses of coarse, angular-to-subangular sand and very fine angular
pebble gravel within a more massive clayey, sandy silt. This unit most likely
represents erosion and fill from the smaller watershed that the T7 channel has
recently captured (Fig. 5).
Radiocarbon Dating
Carbonaceous samples collected from the excavations were large enough for
radiocarbon dating and sufficiently numerous to provide age control for the
units from which they were collected. Most of the samples dated were located in
trenches T6' and T8. Results of radiocarbon dating of samples most
relevant to determination of slip rate are presented in
Table 1. The median probability
calibrated dates of samples are also displayed in
Table 1 as a convenient summary
of each sample.
Results for all samples are in supplemental Table 1, (available in the electronic supplement to this article). Potential complications with age control in the Carrizo Plain include inherited age from detrital wood, the presence of shrubs that can live more than 800 years (Grant and Sieh, 1993) and anomalously old radiocarbon ages of terrestrial gastropods (Grant and Sieh, 1994). Three anomalously older dates were obtained from gastropod shells collected in trench T8 and from an undisturbed surface sample from outside the trenches.
The most relevant samples were collected from channel margin clayey silts (units 860 and 630) and channel fill deposits (unit 870) bounding the lower part of the channel margin in trenches T8 and T6'. None of the dated samples was indisputably collected from within the channel fill. Samples T8-8 and T8-11 were collected along a diffuse contact between channel fill (unit 870) and channel margin (unit 860). The older sample, T8-8 (supplemental Table 1, available in the electronic supplement to this article) has a median probability date up to two centuries older than other samples from unit 860 and therefore is presumed to be reworked.
Constraint on the date of channel cutting or maximum age of channel offset is obtained from analysis of the dates and stratigraphic positions of samples shown in Table 1 and output from the program OxCal v. 3.10 (Bronk Ramsay, 1995, 2001). The stratigraphically lowest samples are T6'- 3 (supplemental Table 1, available in the electronic supplement to this article) and T6'-4 (Table 1). Sample T6'-3 is a millennium older than other samples, is presumed to be reworked, and therefore is not included in the analysis. Sample T8-11 was collected from the outermost channel fill, or the channel margin. The remaining samples in Table 1 were collected from within unit 860. An OxCal model was constructed to sum the cluster of seven samples from T8 and place samples T8-11 and T6'-4 in stratigraphic order. The results (supplemental Table 8, available in the electronic supplement to this article) constrain the maximum age date of channel incision (A.D. 1160) to approximately the maximum age of sample T6'-4 (A.D. 1164; see Table 1).
Tectonic Geomorphology
The VMR site preserves a spectacular set of offset channels
(Figs. 3, 4, and
5).
They drew the attention of
Sieh (1977,
1978,
1979), who emphasized the study
of channels 47–48 because they preserved the record of the last offset (8
± 0.5 m). Channels 48, 46, and 45 were offset such that the
penultimate offset of 7.5 ± 1 m could be estimated. Channel 44 is offset
27 ± 2.7 m according to
Sieh (1977), and in combination
with the other offsets at the site yields an offset in the third event back of
about 10 ± 1 m. The offset history is interpretable because of the
excellent preservation of the channels and because of the clear record of
incision, offset, and abandonment
(Fig. 3). However, the number
of slip events that generated each offset cannot be determined directly from the
geomorphology. Evidence from trench T3 indicates a minimum of three slip events
contributed to the offset of Sieh’s channel 44 (our study site). As
described in the discussion, evidence from other studies suggests a maximum of
five or six slip events.
The pattern of geomorphology and stratigraphy at VMR implied the following model by Sims et al. (1993; see also Sims, 1994) of tectonic offset and subsequent channel response: (1) a new channel is cut or inherited; (2) after initial offset, the channel may continue to incise; (3) continued offset and channel lengthening decreases local along-fault channel gradient, causing sedimentation as shingled, nested cut-in-fill channel deposits; and (4) further offset along the fault promotes increased deposition and filling of the channel. The filled channel may be capped by fan deposits and allow incision of a new channel across the fault or be captured by a topographic low juxtaposed by recurrent offset. Channel 44 is in this latter stage in its current configuration. There is little room in the channel for further deposition and it is likely to be abandoned soon (and will probably occupy a channel initially formed by channel 45; Figs. 3, 4, and 5).
The geomorphology and stratigraphy in the channels reveal a record of filling by fluvial sedimentation, lateral colluviation, and pedogenesis. Typically, the channel units (e.g., 220, 250, 640, 670, 840, 870, 730, and 740) cut into or are overlain by reworked fluvial units (210, 230, 240, 260, 270, 650, 660, 680, 860, 880, 700–720, and 750). The older reworked fluvial units usually contain CaCO3, which gives them a light color. In places, the basal boundary for the older colluvial units is diffuse (portions of T2) or sharp and scalloped (T9).
We have developed two end-member models for the development of the reworked fluvial units. In the first model the heavily bioturbated and pedogenically altered fluvial sediment no longer has sedimentary fabric preserved. This model is supported best by the relationships from T2 in which units 230 and 240 grade into the central channel unit 220 (Fig. 6a). In the second model colluvium is delivered to the channels during the relatively long periods of no fluvial transport in the channels. The colluvium shows interfingering relationships with the channel sediments. For example, T8 (Fig. 6d) has a wedge of colluvial material (unit 860) overlying the channel fill (unit 840), and is cut by unit 870.
The possibility of gullies, to the southeast of our study site, sourcing channels that would align with our gully is rejected by geomorphic and stratigraphic evidence. Strike- slip motion could have not have caused channel head 44 to line up with the older tails of channel 45 of our study and create an apparent offset. The displacement necessary to place the channel at this site location would be greater than the current acceptable displacement given the ages of the dated samples.
Offset Measurements
To measure the channel offsets, we surveyed the geomorphic trace of the
SAF at VMR (N56°W) and projected the trench survey
data that define the offset channel buried thalwegs to either be parallel or
perpendicular to the trace. The projection is also approximately parallel or
perpendicular to the orientation of the trench walls. The distance along the
fault versus elevation of the surveyed buried channel thalweg for the two walls
of each trench (and topographic profile along the SAF through the
trenches) is shown in the lower plot in
Figure 5. The offset is
measured as the relative distance between the lowest point in the T2 northeast
channel thalweg and the thalweg centers (as we estimated represented by a given
nail). We measured it for the pairs of buried offset channels in
T2–T6', T2–T8, T2–T7, and the associated geomorphic
offsets from the surveyed topography. We did not make such a determination for
T9 because evidence for an identifiable buried channel thalweg was ambiguous
(see supplemental Fig. 1d, available in the electronic supplement to this
article).
Results of offset measurements from surveying are displayed in Table 2. The buried thalweg of the currently active channel is offset 24.8 m, with a qualitatively defined conservative measurement uncertainty of ±1 m. The geomorphic offset is 27.6 m. The geomorphic offset has significantly higher uncertainty because the geomorphic thalweg is broader and not as well defined as the buried thalweg.
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The thalweg of the first beheaded channel that is exposed in T7 is offset 48.8 m with a geomorphic offset of 51.8 m. The geomorphic offset of the second beheaded channel that we tried to expose in T9 ranges from 71.9 to 79.0 m. There were no datable materials in trenches T7 and T9. As a result, only the downstream channel exposed by T6' and T8 is used for slip-rate measurement.
Slip Rate
Slip rate is obtained from channel offset divided by time interval. The
precision and accuracy of the slip rate depends on how accurately and precisely
the measured parameters represent the actual tectonic offset of the channel. We
measured offset of the buried thalweg, as exposed in the walls of trenches (24.8
m ± 1m), and as expressed by surficial geomorphology (27.6 m ±
1m). For slip-rate measurement, the buried thalweg is a better representation of
channel offset because it is better defined morphologically and because it was
formed at the time of initial channel incision. As described in the section on
radiocarbon dating, the maximum age of sample T6'-4 (approx.
A.D. 1160) provides a maximum age for incision of the channel, which
postdates deposition of the sample.
A minimum slip rate, 29.3 mm/yr, is derived from offset of the buried thalweg (24.8 m) divided by the maximum time interval (A.D. 2005–A.D. 1160). This scenario assumes that the channel was offset immediately following incision and uses the maximum possible date of incision as defined by sample T6'-4 and results of the OxCal model.
It is likely that some time passed between channel incision and offset. A faster slip rate can be derived based on the assumption that the channel was incised circa A.D. 1160 and offset by surface-rupturing "Event E" of Grant and Sieh (1994) dated A.D. 1218–1276. The shortest time interval of measurement would be the time between the youngest date of this event (A.D. 1276) and the 1857 earthquake. The resulting slip rate is 34.0 mm/yr. This slip rate is in excellent agreement with the late-Holocene slip rate of 33.9 ± 2.9 mm/yr measured at Wallace Creek (Sieh and Jahns, 1984).
A maximum slip rate can be derived from a shorter time interval. For example,
the date of the most recent slip event, the 1857 earthquake, could be used as
the end point of the time interval to obtain a slip rate of 35.6 mm/yr (i.e.,
24.8 m/[A.D. 1857–A.D. 1160]). This
slip rate also agrees well with
Sieh and Jahns’s (1984) late-Pleistocene measurement of 36 mm/yr at Wallace Creek.
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Discussion |
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TopAbstractIntroductionSite Location and HistoryMethodologyResultsDiscussionAcknowledgmentsReferences |
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Figure 2 shows the
SCEC Crustal Motion Model 3
(Shen et al., 2003)
from which we selected those velocities in a 100-km-wide swath
perpendicular to the SAF centered on the Carrizo Plain. We projected
them to the N40°W direction to determine a velocity parallel to the average
trace of the SAF northwest of the Transverse Ranges (note that
projecting them to N56°W—local VMR SAF
trace—only changes the gradient by 1–2 mm/yr). The N40°W
velocity profile shows a steep gradient of about 30 mm/yr across the
SAF at the Carrizo Plain. Assuming that these data represent steady
interseismic motion, these data can be interpreted by calculating the
surface-displacement profile from strike slip from the brittle–ductile
transition zone (here assumed to be 12 km) to great depth (e.g., along an
infinitely long vertical screw dislocation;
Thatcher, 1990). Two curves for
30 and 37 mm/yr of steady deep slip approximately bound the observed velocities
in Figure 2. Further detailed
analysis of these data is beyond the scope of this article, but clearly they are
consistent with the slip rate we have determined. Importantly, the slip at the
VMR site indicates that all of the regional shear of the
SAF system in the area of the Carrizo Plain has been accommodated
across a narrow, few-meter- wide fault zone for the last millennium.
If the deformation is so localized at the VMR site, an interesting question is how many surface ruptures generated the measured offset. Sieh (1978) proposed that the last three ruptures at VMR involved 8 ± 0.5 m, 7.5 ± 1 m, and 10 ± 1 m slip respectively, for a total offset of approximately 25 m. Detailed logging of trench T3 revealed evidence of at least three ruptures, but the total number of events could not be discriminated due to laterally discontinuous cut-and-fill channel stratigraphy. From extrapolation of findings at Wallace Creek (Liu et al., 2004), the number of earthquakes could range from three to six. At Wallace Creek, displacement from the last five ruptures totaled 22.1 m. Therefore, at VMR the 24.8 ± 1 m offset of the buried thalweg could have been produced by five or six comparable ruptures, with an average dextral slip of 4 to 5 m each.
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TopAbstractIntroductionSite Location and HistoryMethodologyResultsDiscussionAcknowledgmentsReferences |
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Manuscript received May 12, 2005
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  J. J. Lienkaemper and C. B. Ramsey OxCal: Versatile Tool for Developing Paleoearthquake Chronologies--A Primer Seismological Research Letters, May 1, 2009; 80(3): 431 - 434. [Full Text] [PDF]
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