Neotectonics of the San Cayetano fault, Transverse Ranges,

THOMAS ROCKWELL Department of Geological Sciences, San Diego State University, San Diego, California 92182

ABSTRACT INTRODUCTION used to assess the hazard associated with this fault. Study of late Quaternary alluvial deposits The San Cayetano fault is a major, north- cut by the San Cayetano reverse fault indi- dipping reverse fault on the north flank of Ven- GENERAL GEOLOGY AND cates that the dip-slip rate of faulting in- tura Basin, (Fig. 1), which STRATIGRAPHY creases from 1.05 ± 0.2 mm/yr at Sisar Creek displaces Tertiary and Quaternary rocks with as near the western end of the fault eastward to much as 9 km of stratigraphic separation. The The bedrock geology in the study region is 1.35 ± 0.4 mm/ yr at Bear Canyon and 2.35 ± fault has been extensively studied and drilled in fairly well known. The field work of many 0.55 to 4.15 ± 0.85 mm/yr at Mud Creek. At conjunction with exploration for petroleum theses has been completed in the vicinities of Timber Canyon near the central part of the (Cahill and others, 1976; Schlueter, 1976; Fillmore (Cemen, 1977; Eschner, 1957; John- fault, a minimum slip rate of 3.6 ± 0.4 mm/yr Cemen, 1977; Eldridge and Arnold, 1907; Fine, son, 1959; Robinson, 1955), Santa Paula is suggested by fanhead segmentation; how- 1954; Nagle and Parker, 1971), but this is the (Schlueter, 1976; Bertholf, 1967; McCullough, ever, a higher slip rate of 8.75 ± 1.95 mm/yr first study, other than the early general work on 1957; Loofbourow, 1941), Upper Ojai Valley is suggested by stratigraphic evidence. Strati- the fault by Putnam (1942), to specifically ad- (Schlueter, 1976; Bush, 1956), and along the graphic separation of 7.5 km in less than 1 dress its late Quaternary activity. Ventura Basin Ventura River (Putnam, 1942; Dembroff, 1983). m.y. across the fault between Fillmore and lies within the western Transverse Ranges prov- Oil company geologists have extensively Piru indicates a slip rate of at least 7.5 mm/yr ince of southern California, an east-west-trend- mapped the bedrock throughout the area, as for the eastern section of the fault. Recent ing zone of north-south-directed convergence. well. The above studies have generally ignored activity is shown by faulted Holocene or Many workers have attributed the north-south the varying nature of the surficial soils and left latest Pleistocene fan alluvium and colluvium convergence to compression generated by the the upper Quaternary deposits mostly undiffer- at Bear Canyon and the Silverthread oil field, "big bend" of the which lies entiated, however. respectively, and inferred at Timber Canyon directly to the north (Hamilton, 1961; Yeats, More than 12,400 m (40,000 ft) of sedimen- by fanhead deposition. 1981; Rodgers, 1975; Bird and Rosenstock, tary rocks of Eocene through Pleistocene age are Active folding of the footwall is demon- 1984). Most recently, Weldon and Humphreys exposed at the surface or penetrated by drill in strated by tilted and faulted Holocene and (1986) suggested that shortening across the Ventura Basin. The terminology for some of the Pleistocene alluvial fans: present tilt rates ap- western Transverse Ranges results from a major formations is confusing and varies among proach 1°/1Q ka yr, and associated bedding step in dextral motion from the Elsinore, Rose workers. In this paper, I use Sisquoc Formation plane faulting rates are about 0.5 to 1.5 Canyon, and southern California borderland instead of Santa Margarita Formation (after mm/yr. The hanging wall, however, does not faults south of the Transverse Ranges to the Thomas Dibblee, unpub. maps), San Fernando appear to be involved in active deformation, Hosgri and related coastal faults north of it. This Formation instead of Repetto, Pico, and Santa inasmuch as Pleistocene deposits as old as second model would place up to one-third of the Barbara Formations (after Jennings and Strand, several hundred thousand years are not tilted Pacific-North American plate motion on the 1969), and Saugus Formation for both marine or affected by bedding-plane faulting. The coastal system of faults and would result in up to and nonmarine units where many have used San only exception is in the Silverthread field 2 cm/yr of convergence across the western Pedro Formation. Because the primary purpose area, where hanging-wall deformation is di- Transverse Ranges. This value is in close agree- of this paper is to present evidence of late Qua- rectly relatable to fault geometry, not folding ment with those of Yeats (1983) and Rockwell ternary activity on the San Cayetano fault, the of the hanging-wall rocks. (1983) who reported 22 mm/yr and 17 ± 4 reader is referred to Rockwell (1983) for a com- mm/yr, respectively. The San Cayetano fault The lack of documented moderate- to plete discussion of the nomenclature. results from this north-south convergence and is large-magnitude historical on accommodating a significant part of the shorten- this fault is in strong contrast with its demon- CHRONOLOGY OF THE MIDDLE ing. This paper focuses on the late Quaternary strated late Quaternary activity. Based on its AND UPPER QUATERNARY style, rates, and recency of movement on the San length, slip rate, overall geometry, and seg- ALLUVIAL DEPOSITS Cayetano fault, as well as deformation asso- mentation into two parts, a recurrence inter- ciated with the hanging-wall and foot-wall val of between 200 and 600 yr for the western blocks of the fault. This information, in conjunc- The San Cayetano fault displaces alluvial segment and about 230 yr for the eastern tion with the historical record of seismicity, is deposits of varying age where older deposits, and segment is suggested. their corresponding surfaces, display greater dis-

Geological Society of America Bulletin, v. 100, p. 500-513,12 figs., 2 tables, April 1988.

500

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VENTURA RIVER OJAt / ' n v F,9- 2 SESPE CREEK ARROYO PARIDA SM*— j v

AYERS CREEK SYÑCLINE \ j £

RED

PACIFIC OCEAN STUDY AREA'

Figure 1. Major structural elements in central Ventura basin. Sections A-A', B-B', and C-C' are shown in Figures 5,9, and 11.

placements than do younger ones. The chronol- others, 1984; Hatch and Rockwell, 1986; fault were mapped in detail by age based upon ogy of the alluvial deposits is based on a dated Vaughan and Rockwell, 1986; Millman and the strength of the soil profile developed within soil chronosequence developed for and within Rockwell, 1986; Harden and others, 1986). The them, much like sedimentary rocks are mapped Ventura Basin (Keller and others, 1980; Rock- chronosequence presented in Table 1 and out- based on their similarity to type sections. Active well, 1983; Rockwell and others, 1985). Soil lined briefly below (discussed in depth in channel deposits with no soil development chronosequences, or groups of soils with mem- Rockwell and others, 1985) is utilized in this (Typic Xerofluvents) are designated Qi depos- bers that are differentiated by their properties as paper to assess rates of faulting along the San its. Flood-plain and recently inactive terrace a function of age, have been used to evaluate Cayetano fault. deposits with A/C soil profiles (Fluventic Hap- rates of faulting in several studies (Rockwell and The alluvial deposits along the San Cayetano loxerolls) are designated Q2 deposits. The Q3

TABLE 1. RELATIVE AND ABSOLUTE AGE OF SOIL PROFILES

Geomorphic Brightest moist mixed Clayf Clay fTlm§ Estimated age in years surface color in B horizon XB/XA index before present and soil Color* Hue Chroma index

Ql AC profile, no B horizon noB 0 10-20 Q2 AC profile, no B horizon noB 0 <250 Qt3 10 YR 3 4 0.6 0 500-5,000»* Qt4 10 YR 4 5 1.3 3.0 8,000-12,000^ Qt5a 10 YR 4 5 ND 4.2 I5,000-20,000tt Qt5b 10 YR 4 5 ND 6 30,000tt Qt6a 7.5 YR 5 7 1.4 7.25 38,000§S Qt6b 7.5 YR 6 8 1.5 5.5 54,000*" ± 10,000 Qt6c 7.5 YR 7 9 1.6 7.0 92,000*** ± 13,000 Qt7 5 YR 6 9 ND 8.0 160,000-200,OOOttt

Note: data in this table from Rockwell and othere (1984,1985). *Color index is computed by adding chroma number to hue (of moist mixed sample), where 10 YR = 1,7.5 YR = 2,5 YR - 3. Indices from different profiles on same geomorphic surface are averaged. To determine color, a large air-dried bulk sample was passed through a 2~mm sieve, then fractionated in a mechanical splitter, moistened, hand homogenized to a putty consistency, and rolled to a sphere; then it was pulled into halves, and color was noted from one freshly broken surface. tRatio of the mean percentage of clay in 8 horizon to that in A horizon, computed from particle-size data. 'Index based on day film information contained in the profile descriptions and computed by adding the percent frequency of clay film occurrence to their thickness, as follows. Percent frequency: very few = 1; few = 2; common = 3; many = 4; continuous - 5. Thickness.- thin = 1; moderately thick ~ 2; ¿lick - 3. For example, if in the Bt horizon there are many to continuous (4.5) moderately thick and thick (2.5) clay films, the index is 7.0. **Age estimate collectively based upon tree rings of a number of mature oaks growing upon the surface, the degree of soil profile development, and a 1 date on charcoal collected from a presumed buried soil in Timber Canyon east of Oak View near Santa Paula. IlAge estimate based in part upon relative amount of displacement on flexural-slip faults between older surfaces in Orcutt and Timber Canyons near Santa Paula, and 14C dates on terraces near Ventura. §§Based on two14C dates on charcoal collected at the base of the Oak View Terrace. ***Age estimated based on rate of faulting for Arroyo Parida-Saota Ana fault. ' itAge estimate based on the rate of faulting for the Culbertson and Orcutt faults.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/4/500/3380245/i0016-7606-100-4-500.pdf by guest on 02 October 2021 Uoo Tsq Sisquoc Fm. • • • * Qal Active channel alluvium Tm Monterey Fm. (undif.) Q2-Q7 Quaternary deposits differentiated by Tml Monterey Fm. (lower) their strength of soil development Tr Rincón Fm. - refer to table I Tv Vaqueros Fm. San Cayetano fault: solid where delineated by scarps or outcrops, dashed Qls Landslide deposits Ts Sespe Fm. where inferred or poorly located , dotted where buried Qog Eroded gravel Tew Coldwater Fm. " geologic or geomorphic contact, dashed where inferred. Qs Saugus Fm. Ted Cozy Dell Fm. upright bedding Qtf Fernando Fm. Tmi Matilija Fm. -b- overturned bedding i i km Tj Juncal Fm. syncline and anticline

Figure 2. Geology of the western portion of the San Cayetano fault from Ojai Valley to Santa Paula Creek. Bedrock geology modified from unpublished maps of Thomas Dibblee.

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soils have strong Cox or cambic B horizons, and Sisar Creek Profiles the Q4 (Argixerolls and Haploxeralfe) through SOUTH Q7 (Palexerolls and Palexeralfs) soils have argil- 800 - Elevation lic horizons with increasingly stronger B horizon (meters) development with increasing numerical value 700 and age (Table 1). Some of the primary groups (Q5 for instance) are subdivided into a and b 600 degraded subgroups (for example, Qsb) based on more fault scarp subtle changes in the soil characteristics. In the 500 following text, displaced or deformed terrace and fan deposits will be referred to by age based upon their soil development referenced to Table 1, an implicit assumption being that deposits of equivalent age have soil profiles of approxi- Kilometers mately equivalent development in the study area San Cayetano Fault vert, exagg. x4 (Rockwell and others, 1985). The ages of the younger soils (Qi through Q6a) are constrained SOUTH NORTH by radiocarbon dates, the ages of the older soils B Bear Creek Profiles (Q6b through Q7) are inferred from rates of 600 r faulting along the Ventura River (Rockwell and others, 1984). Figure 3. Profiles of alluvial fan and stream terrace surfaces DESCRIPTION AND TIMING • at Sisar Creek and Bear Creek. OF DISPLACEMENTS E modern stream See Figure 2 for the explanation c channel ALONG THE FAULT o 500 of geological symbols. « > The San Cayetano fault extends about 40 km from Horn Canyon in Ojai Valley eastward to San Cayetano Fault Piru Creek (Fig. 1) and is divisible into two principal parts. The western segment from Sespe

Creek west to the Ojai Valley is the part most 400 1— studied in this report. The eastern segment, ex- .5 I KM tending from Sespe Creek east to Piru, has flatter vert, exagg. XIO dips and is separated from the western segment by a lateral ramp similar to that documented by fine the slip to no more than about 1,000 m on The older surfaces depicted in the profile dia- aftershocks for the western end of the San Fer- the main strand. At Sisar Canyon, a recent well gram (Fig. 3) contain Q6 to Q7 soils with very nando fault (Cemen, 1977; Whitcomb and oth- drilled by Argo Petroleum (Newton -1) encoun- well developed argillic horizons. Long-term slip ers, 1973). The eastern segment is discussed tered the top and base of the fault zone at 700 m rates cannot be determined, because correspond- briefly to provide continuity, but no new work and 860 m, respectively, with Sespe and Cold- ing fan surfaces are buried at unknown depths in was done east of Fillmore for this investigation. water strata thrust over Rincon shale (Gary the Upper Ojai Valley beneath the present Sisar Most of the information on the eastern segment Nulty, 1980, oral commun.). At the mouth of fan. Based on the strength of soil development comes from subsurface petroleum exploration Sisar Creek, a 65-m scarp in alluvial-fan mate- alone, several hundred thousand years of fault- and previous surface mapping (Cemen, 1977; rial delineates the surface trace of the fault (Figs. ing may be represented by these old fan rem- Yeats, 1983). 2 and 3). Detailed mapping shows that there are nants. One major observation here is that all of The western end of the San Cayetano fault is at least fiveseparat e alluvial-fan and terrace lev- the surfaces are nearly parallel to the present in Ojai Valley at Horn Canyon fan where the els present on the hanging wall; the lowest and stream, indicating virtually no tilting or warping fault disappears within the Sespe Formation youngest terrace is 13 m above the present of the hanging wall over this long time period, (Fig. 2). Alluvial-fan gravels from Horn Canyon stream level at the mouth of the canyon and all deformation being accommodated by fault- with a Qy-strength soil cross the surface trace of contains a late Pleistocene Qsa soil with a weak, ing at this locality. the fault and are apparently uncut by it, suggest- relatively thin argillic horizon. Equivalently At Bear Canyon, slightly more than 1 km east ing little or no movement in at least 160 to 200 developed soils (Table 1) indicate an age of of Sisar Canyon, an alluvial fan with a Q4 soil ka. Bedrock vertical separation at this point is -15-20 ka for this terrace. The level of the Q5 crosses the fault and is vertically offset 9 ± 1 m small but indeterminate because of the similarity terrace above the present stream level is attrib- (Fig. 3), yielding a vertical slip rate of 0.95 ± 0.3 of the hanging-wall and foot-wall rocks and the uted to movement along the fault, the corre- mm/yr (1.35 ± 0.4 mm/yr on a 45° dipping lack of subsurface control. sponding fan on the foot wall being at or below fault, based on the structure contours of Vertical separation increases rapidly eastward the present Holocene fan surface level. Given Schlueter, 1976). This site is significant because with a thinned Sespe section faulted over Va- these assumptions, a minimum vertical slip rate the Qf4 fan is of latest Pleistocene or early Holo- queros Sandstone at Wilsie Canyon (Fig. 2). of 0.75 ± 0.15 mm/yr and a dip slip rate (45° cene age, indicating that some if not all of the Bush (1956) reported about 5,000 m of separa- north-dipping fault plane as determined by well slip occurred during Holocene time. control) of 1.05 ± 0.2 mm/yr is determined for tion at Wilsie Canyon, but this figure appears Terraces and fans with Q5) Q6, and Q7 soils too high; the Sespe-Vaqueros relationships con- the fault at the mouth of Sisar Creek. are preserved on the hanging wall of the fault at

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Figure 4. Surface profile of Upper Ojai Valley and w the Bear Canyon surface, providing an estimate of - Bear Canyon the slip on the San Cayetano fault since Q& time. 700 Surface (Qfgc) • 600 Sisar Bear fans i 500 Upper Ojai {QÍ3-C Valley Bear Canyon as well, but the corresponding fan X 400 levels on the foot wall are apparently buried by 300 the more recent Q4 fan alluvium. Any slip rate yielded by the older deposits will thus be a min- .5 1 2 San Cayetano imum. Bear Canyon surface, the large fan sur- Kilometer* fault face at the top of Koenigstein Road (Fig. 2), may be used to estimate a long-term slip rate if two assumptions are made. The first is that the fans. The gradient of the valley may be projected 80-100 ka (2.15 ± 0.3 mm/yr on a fault with a fan was graded to Upper Ojai Valley during under the more recent fan alluvium to the fault, 45° dip). deposition. Second, the corresponding level in establishing the approximate level of the buried These assumptions are reasonable, the latter Upper Ojai Valley is approximately the level of valley floor at the fault (Fig. 4). This yields a one being difficult to prove without substantiat- the main valley floor to the west of the younger vertical slip rate of 1.6 ± 0.2 mm/yr for the past ing data on soils on the foot wall of the fault.

EXPLANATION

Saugus Fm. Fernando Fm. Sisquoc Fm. Monterey Fm. (I : lower) Rincon Fm. Coldwater Fm. Cozy Dell Fm. Matilija Fm. Juncal Fm.

oil well

contact, dashed where inferred fault

.5 I mile

I .5 I Kilometer

Figure 5. North-south cross section (A-A' on Fig. 1) through the Silverthread Oil field showing the San Cayetano fault and related structures. Note the repeated Cozy Dell section due to movement on the northern branch (modified from Schlueter, 1976).

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ROADCUT EXPOSURE OF SAN CAYETANO FAULT IN SILVERTHREAD OIL FIELD NEAR SANTA PAULA

Figure 6. Log of expo- sure of the San Cayetano fault thrusting Miocene Monterey Formation over Colluvium derived entirely from .Fractured but well bedded. Tm of hanging wall. Sandstone dikes cut ~ Holocene colluvium in the obliquely across strata. Silverthread Oil field (lo- cation A on Figure 2). Gouge zone, phacoidal lensing « >« . - ' • \ ,« \ >S V\V Breccia^ . . x ,;v->Pico member contains conglomeratic \\\\ beds with well rounded clasts of ^ s Badly broken bedrock s ». a. t> r: v N64E Monterey shale and chert. \\\ 51N - \ V FERNANDO,^ ' ROAD FORMATION- ROAD

Unfortunately, a veneer of fine-grained fan allu- oil pumping-station platform cut (location A in surface indicates that movement is Holocene; no vium from Sulphur Mountain as well as the Fig. 2). The bench preserved the colluvium as it B horizon has developed, indicating a late Holo- coarse fan alluvium from Sisar and Bear Can- was eroded off the hanging wall during and after cene age for the uppermost part of the collu- yons covers the older fluvial and fan gravels in faulting (Fig. 6). The colluvium is comprised vium. The lack of recognizable stratigraphy or the Upper Ojai Valley, precluding direct soils almost entirely of Monterey clasts, indicating a buried soils indicates that the colluvial wedge correlation. It is known, however, that the very local hanging-wall source, because the only was accreted fairly rapidly. Upper Ojai Valley drained to the Ventura River Monterey Formation exposed along the fault is The northern strand of the San Cayetano during all of Qg time (Rockwell, 1983; Putnam, within the sliver in between the two southern fault (Bear Canyon Surface strand) (Fig. 2) is 1942) and that a Q6c stream level close to the strands. Nearly 15 m of thrust displacement is well exposed at two locations where Eocene present valley floor is consistent with the terrace recorded in the exposure, but only a few strati- rocks are thrust over Pleistocene alluvial fan levels along Lion Creek, which drained Upper graphic horizons were recognized, the remainder gravels. Both exposures are in road cuts with Ojai Valley. The slip rate is also compatible with of the deposit being massive colluvium. Thus, Cozy Dell shale thrust over gravel (fault loca- that determined using the offset younger fan displacement per event was not determined, al- tions B and C in Fig. 2). The total late Pleisto- slightly to the west. though it is probable that many events were re- cene displacement, as seen in one road cut, is Immediately east of Bear Creek, the San quired to produce this feature. It should also be estimated to be from 8 to 10 m (Fig. 7), but this Cayetano fault bifurcates; the frontal strand noted that this is only one of the two frontal or is tentative because brush covers the upper part wraps around the front of the hills through the southern strands exposed along this part of the of the section. A scarp is associated with the Silverthread oil field to Santa Paula Creek. The fault zone. surface trace, but the gravel surface is nearly southern segment of the San Cayetano fault The age of the base of the colluvium was not eroded away. No soil profile is exposed due to again bifurcates along this frontal section with a determined, and thus a slip rate has not been brush and colluvial cover, but because the de- sliver of Monterey shale occurring between Eo- determined, although the soil on the colluvial posit is topographically higher than the Qfc Bear cene and Pliocene rocks. At Santa Paula Creek, the fault branches join to become a single trace (Fig. 2). The structural relationships of the bedrock and various faults in the Silverthread area are complex but fairly well defined by subsurface drilling. Bush (1956) estimated 6,500 m of verti- cal separation on the San Cayetano fault with Coldwater sandstone thrust over a sliver of Mio- cene Monterey shale, which is in turn thrust over mid-Pliocene lower Fernando (Repettian) shale. Recent subsurface control indicates less slip, about 3,000 m, based on Figure 5 (from Schlueter, 1976), with Coldwater sandstone thrust over Monterey shale and Saugus gravel in the subsurface. A single exposure of bedrock thrust over col- luvium is the only evidence of late Pleistocene and Holocene activity in this area, due largely to the nearly complete lack of terrace or fan depos- its crossing the southern strands between Bear Canyon and Santa Paula Creek. The colluvium Figure 7. Cross section through the Bear Canyon surface (Q&.) showing relationships with is preserved on a bedrock bench exposed in an older (Q7?) fan deposits, overturned underlying Eocene strata, and the San Cayetano fault.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/4/500/3380245/i0016-7606-100-4-500.pdf by guest on 02 October 2021 Qal - Active channel alluvium Tr - Rincon Formation Q1-Q7 - Alluvial deposits differentiated Tv - Vaqueros Formation by soil development Ts - Sespe Formation Qs - Saugus Formation Tew - Coldwater Formation QTf - Fernando Formation Ted - Cozy Dell Formation Tsq - Sisquoc Formation Tma - Matilija Formation Tm - Monterey Formation

Figure 8. Map of the geology and Quaternary deposits along the San Cayetano fault between Santa Paula and Sespe Creeks. Late Quaternary deposits are mapped by relative age with Qi the youngest and Q7 the oldest (see text and Table 1) (bedrock geology modified from unpub. maps of T. Dibblee and F. H. Weber, 1973). (Note overlap in center.)

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Canyon surface, and other remnants of old sur- faces with soils above this surface correlate well with Q7, it is believed to be at least 160-200 ka old. Thus, this strand has a maximum long-term average slip rate of 0.05 ± .01 mm/yr. The fan gravel on the hanging wall corre- sponding to that in the road cut is back-tilted about 20° northward (Fig. 7). This gravel was deeply eroded and tilted prior to deposition of the Bear Canyon surface fan which itself is back- tilted only slightly near the fault. The main pe- riod of movement, then, for the northern strand must have been prior to deposition of the Bear Canyon surface fan. The northward tilting is at- tributed to the geometry of the fault plane in this area; the northern strand dips about 65°N and flattens with depth as it joins the main strand (Fig. 5). Thus, with movement on the northern strand, surficial gravels on the hanging wall are tilted northward; this is the only section of the San Cayetano where this type of hanging-wall deformation is documented. The northern strand offsets the Cozy Dell- Coldwater contact 1 km east of Bear Creek and explains the overthickened Cozy Dell section between Bear and Santa Paula Creeks. It appar- ently rejoins the main strand in the vicinity of Santa Paula Creek. The surface trace is inferred by mapping the hanging-wall deformation of gravel where the fault is not exposed. East of Santa Paula Creek, the San Cayetano fault cuts across the head of Anlauf Canyon to the head of Mud Creek and Orcutt Canyon (Fig. 8). In the area between Santa Paula and Sespe Creeks, very few alluvial deposits actually cross the fault; the alluvial fans mostly begin at the fault and appear to have been at least in part generated by it.

At Mud Creek, however, the fanhead of a Q4 to Q5a alluvial fan is apparently faulted apart about 32 ± 3 m vertically across the north strand (Fig. 8), yielding a minimum slip rate of 2.35 ± 0.55 mm/yr on a 53° north-dipping fault. The soil on this deposit yielded ambiguous data in that there were very few clay films, suggesting a Q4 age, but the soil color indicated a Q5 age. The color may have been partly inherited from the well-oxidized colluvium and talus feeding the Bedrock geology modified after unpublished maps by Schlueter (1976), fan head and thus may have biased the age as- Cemen (1977), and Thomas Dibblee. signment. If the alluvial fan is assumed to be of Q4 age, the dip-slip estimate increases to 4.15 ± 0.85 mm/yr. A Q3 fan deposit at the same local- ity is apparently not faulted enough to produce a Kilometers recognizable scarp. Intense grading, road con- struction, and road filling by oil companies, Figure 8. (Continued). however, may have obscured any scarp that may have originally been there. The next canyon to the east, Orcutt Canyon, contains a debris flow/fan deposit with a weak

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Q3 soil which crosses the fault and appears an undated paved road constructed for use by evergreen forest to chaparral (Heusser, 1978), uncut. Again, road building and platform clear- the oil industry is being actively buried. Radial sediment production probably increased, initiat- ing may have obscured a scarp up to a few profiles from the east and west fanhead sections ing deposition at the fanhead. A few false hem- metres high, but it may also indicate no move- (Fig. 10) show three distinct segments: the oldest lock still remain in this area, relics of the late ment in at least the last 0.5 ka. Older deposits corresponding to Q3 (0.5 to 5 ka; Table 1); the Pleistocene vegetation. are on the footwall of the fault but do not cross intermediate with Q2 (A/C soil); and the Tectonic uplift is also a prime factor in fan- it, thus precluding determination of a slip rate in youngest with Qj or active alluvium. Segmen- head deposition. Previous work by Bull (1977) this canyon. tation of alluvial fans and fanhead deposition suggests that fanhead deposition is associated At Timber Canyon, subsurface stratigraphie may be a result of tectonic uplift, climatic with high rates of uplift. As faulting takes place, relationships indicate about 4.8 to 5.0 km of strat- change, or vegetational change. The two most the uplifted hanging wall erodes quickly, pro- igraphie separation with Matilija sandstone probable causes in the past 5,000 yr are tectonic moting deposition just below or at the fault. The thrust over Sisquoc shale (Fig. 9). The Timber uplift and vegetational change. fan material regrades quickly and buries the Canyon section of the San Cayetano fault is Vegetational change is a reasonable explana- fault and scarp completely. Credence is given to completely buried by a very recent alluvial fan; tion, because as the vegetation changed from this alternative because the segmentation and

South North

Figure 9. North-south cross section (B-B' on Fig. 1) through the north flank of the Santa Clara trough and the San Cayetano fault at Timber Canyon showing the estimated amount of slip required to shed red-bed alluvium into Timber Canyon (after Schlueter, 1976).

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fanhead deposition are spatially associated with rates are an average for a much longer period of Between San Cayetano Mountain and Sespe the main fault trace. If all of the segmentation time. These estimates do not necessarily conflict; Creek, two fault traces are mapped with a sliver and deposition were due to uplift along the San variable slip rates through time have been sug- of Miocene Monterey shale in between (Fig. 8, Cayetano fault, projection along the inferred 5 gested on other California faults (see Sharp, modified from Dibblee, unpub. maps, and ka (maximum age) surface indicates from 16 to 1981), and the Holocene rates are a minimum in Weber and others, 1973). The rocks on the 20 m of dip slip (slip measured along fault; see any case. hanging wall become progressively younger Fig. 10) and a minimum slip rate of 3.6 ± 0.4 All older fans between Timber Canyon and eastward from Matilija Formation at San Caye- mm/yr for this segment of the fault, consistent Sespe Creek are deeply entrenched; the present tano Mountain to Sespe Formation at Sespe with Mud Creek to the west. locus of deposition is about 1.5 km from the Creek. The footwall rocks also become progres- A higher slip rate is suggested by the presence fault. A few older fan remnants, however, are on sively younger eastward from Fernando mud- of red sandstone and shale clasts within the basal the hanging wall of the San Cayetano fault, and stone to Saugus gravel at Sespe Creek. Fine Q7 fan deposits in Timber Canyon (180 ± 20 one Q3 deposit which crosses the fault may be (1954) estimated about 9 km of stratigraphic ka; Table 1). The clasts resemble Sespe sand- cut by it. separation for the fault in this vicinity. Cemen stone but may have originated from red beds Between Timber Canyon and Boulder Creek, (1977) recalculated this value based on min- within the Coldwater Formation as well. The Q5 fan gravels are preserved on the hanging wall imum and maximum thicknesses of units, and clasts compose only about 5% of the total de- of the fault about 65 m above present stream determined it to be from 7.3 to 11 km, agreeing posit, indicating a minor source. The Q7 deposit level (Fig. 8). No corresponding surface is pres- with the previous estimate. is the only fan which contains the red clasts, all ent on the footwall near the fault, although Q5 At Sespe Creek, the Sespe Formation is thrust younger fans being composed entirely of clasts fan deposits occur about 1.5 km south near the over Pleistocene Saugus gravel. In the foreland of Eocene marine sandstone and shale. On the mouth of the canyon. The fault is well exposed on the west side of the creek, previous workers basis of the cross section in Figure 9, dip slip of at this locality with Eocene Matilija sandstone mapped the southern strand across a landslide 1,525 ± 150 m is required on the San Cayetano thrust over overturned Pliocene-Pleistocene area, assuming that landslide-disrupted blocks of fault in 160-200 ka if the clasts were derived Fernando mudstone. Sespe and Monterey Formation were in place. from the Coldwater red beds, yielding a slip rate Along Boulder Creek, Q3 terrace/fan depos- This interpretation is clearly wrong; Saugus is of 8.7 ±1.9 mm/yr. A higher slip rate of from its cross the surface trace of the fault. A 2.5-m- mappable on the west side of Sespe Creek be- 12 to 15 mm/yr is required if the clasts are high scarp is spatially associated with the neath landslide deposits northward to the fault. Sespe. A third possibility is that the clasts were bedrock fault, but it could not be determined if Topography suggestive of landsliding is also evi- reworked from older deposits, in which case the the scarp was definitely a result of fault dis- dent from study of aerial photography, as well as slip rates would be lower. The preservation of placement because heavy vegetation and collu- by ground observation. the shale clasts and angularity of some of the vium covers the canyon wall exposure. On the east side of Sespe Creek, Cemen sandstone clasts indicate, however, that rework- (1977) suggested that a 3-m-high scarp parallel ing is not likely. to Sespe Creek was possibly a fault scarp. The Slip rates for the central part of the San Caye- tano fault range from 2.35 ± 0.55 to 8.7 ± 1.9 TIMBER CANYON mm/yr. The lower rates are for upper Pleisto- Vertical Exaggeration X5 cene and Holocene deposits, whereas the higher 200-

160-

West Fork East Fork 120- m N 80-

40-

0 Figure 10. Radial fan profiles \ at Timber Canyon fan near the San Cayetano fault. Note fan Xs segmentation across the fault. s s

OI234 I I I I I Kilometers

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Geomorphic Displacement Tilting Estimated surface (yr) Thorpe Culbeitson Rudloph Present Degrees slope tilted

Qf3 4.5 2 6 6°

Note4, determinations made from USGS 7.S minute topographic maps with 40-ft (12-m) contour interval and by hand level and tape measurements. Measurements are believed to be good in all cases to ±3m for the vertical displacements and ±02° for the slope determination (from Rockwell and others, 1985).

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Figure 12. Topographic profiles of faulted and tilted alluvial fan deposits in Orcutt and Timber Canyons. Note that the older deposits are not preserved on the hanging wall of the San Cayetano fault.

main trace of the fault coincides with this scarp, and a soil color change occurs across the scarp. A local rancher, augering for orchard planting in the fall of 1979, consistently encountered Sespe bedrock on the high side of the escarpment. Saugus Formation is clearly exposed within a few metres from the escarpment on the low side. The soil on the higher level is an A/C profile with carbonates leached to about 70 cm. A common red fired brick was found stratified within the C horizon, indicating a historical age for this deposit. Because no large historical earthquakes have been noted for this area since about 1782 (the year Mission San Buena Ven- tura was built), it is unlikely that the scarp is fault generated. More likely, Sespe Creek eroded to the approximate position of the fault, leaving a series of small teiraces as it migrated back toward the west and incised to a lower (present) remain overturned to Boulder Creek where the wall at Santa Paula Creek are elevated but also level. overturned strata of the upper plate are trun- show no significant tilting (Rockwell, 1983). From Fillmore eastward (Fig. 1), stratigraph- cated by an unnamed fault (Fig. 8). Between Between Santa Paula Creek and Sespe Creek, ic separation on the San Cayetano fault is 8,300 Boulder Creek and Sespe Creek, the hanging- there is a paucity of Quaternary alluvial deposits m at Fillmore, from 5,170 to 7,300 m 1.6 km wall strata are upright, but the foot-wall strata on the hanging wall, but where they are pre- west of the mouth of Hopper Canyon and from are overturned; the stratigraphic separation in- served, they do not appear to be significantly 4,600 to 5,300 m west of Edwards Canyon (see creases to its maximum at about Sespe Creek. tilted. Q4 through Qg deposits preserved on the Cemen, 1977). East from Edwards Canyon, Farther east, the upper-plate rocks are folded, hanging wall between Mud Creek and Boulder separation decreases sharply. Vertical separation but only in places are they overturned. The foot- Creek (Fig. 8) all slope between 4° and 6°, sim- on the Piru strand is only 230 m south of Piru wall rocks are also strongly deformed, being ilar to the present slopes for the active fans for (Cemen, 1977). Figure 11 shows a cross section overturned at the fault in the subsurface (Yeats, those drainages. through the San Cayetano fault with at least 7.5 1983). These data suggest that the hanging wall of km of dip separation in 1.0 m.y. (Yeats and Where Quaternary deposits are preserved on the San Cayetano fault, although strongly others, 1982; Yeats, 1983). Because the micro- the hanging wall of the fault, late Quaternary folded, has undergone little or no late Quater- fauna units indicated near the base of the Saugus deformation in the form of active folding ap- nary folding. The only noted exception is in the are not present on the hanging wall, this slip pears to strongly affect only the foot-wall rocks Silverthread field area, as noted above, where estimate is a minimum. The abundance of angu- along most of the length of the fault between the deformation is in the opposite sense from lar Monterey clasts within the lower Saugus ex- Ojai Valley and Sespe Creek. The only excep- that of the bedrock and probably results from posed along Sespe Creek indicates uplift on the tion is between Bear Canyon and Santa Paula fault geometry, not continued hanging-wall fold- north during deposition and may indicate fault- Creek, where upper-plate Quaternary deposits ing. Due to the general lack of well-preserved ing. A slip rate of 7.5 mm/yr is yielded if fault- are rotated northward; this rotation is in the op- Quaternary deposits along the central portion of ing was concurrent with deposition of the entire posite sense indicated by the bedrock and ap- the fault, some hanging-wall folding cannot be Saugus Formation; higher slip rates are attained parently results from movement on an imbricate precluded, however. if displacement began after deposition of part of strand of the fault (northern strand) in this area Between Santa Paula Creek and Sespe Creek, the Saugus. rather than further folding of the hanging-wall all Pleistocene fans overlying the foot-wall block strata. At Matilija Gorge, Rockwell and others display tilting with associated bedding-plane or DEFORMATION OF THE HANGING- (1984) showed that terraces of the Ventura flexural-slip faulting (Yeats, 1977; Rockwell and WALL AND FOOT-WALL BLOCKS River area are only being uplifted and not folded Keller, 1980; Rockwell, 1982,1983; Yeats and north of the Arroyo Parida fault. Similarly, Fig- others, 1981) (Fig. 8). Progressively older allu- The hanging-wall and foot-wall blocks of the ure 3 demonstrates that no folding of the hang- vial fans have progressively greater tilts (Fig. 12) fault have been strongly folded during Quater- ing wall at Sisar Creek has occurred for several and have progressively greater amounts of nary faulting and north-south shortening. The hundred thousand years. The Q7 fans in Ojai vertical displacement along bedding-plane faults fault dies out to the west in the Matilija overturn Valley that cross the projected trace of the fault which displace their surfaces (Table 2). The (Kerr and Schenck, 1928), where both the as well as the Matilija overturn (Fig. 2) also mid-Holocene Timber Canyon fan surface is upper and lower plate strata dip between 45°N display no tilting, suggesting that the Matilija also faulted and tilted and indicates that the foot- and 60°N and are overturned. Eastward from overturn has not undergone recent activity or wall deformation continues to the present Ojai Valley, both the hanging wall and foot wall folding. Remnant fluvial terraces on the hanging (Rockwell and Keller, 1980; Rockwell, 1983).

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DISCUSSION AND IMPLICATIONS Yerkes and Lee (1979) listed the epicenters of The Anacapa-Santa Monica fault zone may OF THE FAULT DATA AND 630 earthquakes that took place in the 6 yr pe- have been responsible for the 1973 M6.0 Point HISTORICAL SEISMICITY riod 1970 through 1975 in the western Trans- Mugu earthquake (Steirman and Ellsworth, verse Ranges. They reported that 97% of the 1976). The San Cayetano fault extends more than 40 epicenters are located south of the Santa Ynez Studies of recurrence intervals and slip rates km from the Ojai Valley to the Piru region. The fault, none of which is obviously associated with have been undertaken for the Cucamonga, San eastern segment, between Piru and Fillmore, is that fault. Many of the epicenters were related to Fernando, and Sierra Madre faults by Matti and separated from the segment west of Fillmore by specific structures such as the Red Mountain, others (1982), Bull (1978), and Crook and oth- a lateral ramp similar to that at the western end Pitas Point, Mid-Channel, and Anacapa faults. ers (1978), respectively. Matti and others sug- of the San Fernando fault rupture in 1971 The hypocenters generally range in depth gested a vertical slip rate of 3.0 mm/yr, reported (Whitcomb and others, 1973; Cemen, 1977). from 13 to 16 km, although hypocenters as deep as 4.5 ± 0.6 mm/yr dip slip on a 45° north- Together with a slightly different geometry (the as 19 km have been reported (Yerkes and Lee, dipping fault plane (Clark and others, 1984) eastern segment is more lobate and dips more 1979). Fault-plane solutions for many of the over the past 10,000 to 13,000 yr on the Cuca- gently), this suggests that the eastern and western events were determined in order to determine monga fault. They also suggested a fairly specu- segments may move independently of each fault dip at depth for many of the major lative 2.0-m (vertical) size event on the average other, thereby reducing the maximum rupture structures. of every 720 yr. Bull (1978) established a slip length to about 24 km for the western segment Of particular interest to this study are the rate of 2.0 mm/yr for the San Fernando fault and about 20 km for the eastern one. earthquakes associated with the San Cayetano but did not establish a recurrence interval. If all Dip-displacement rates along the western fault. Yerkes and Lee (1979) documented sev- other previous events were similar to the 1971 segment of the fault increase from 1.05 ±0.2 eral small earthquakes located north of the sur- event, a recurrence interval of a little more than mm/yr at Sisar Canyon near the western termi- face trace of the fault, and two of them have had 1,000 yr is reasonable. Crook and others (1978) num to 1.35 ± 0.4 mm/yr at Bear Canyon, 2.15 focal plane solutions determined. These indicate could not demonstrate late Holocene displace- ± 0.3 mm/yr for the upper Bear Canyon fan, a 45°-60° north-dipping fault plane to at least ment on the central part of the Sierra Madre 2.35 ± 0.55 to 4.15 ± 0.85 mm/yr at Mud 12-km depth with nearly pure thrust displace- fault and suggested a recurrence interval greater Creek, and 3.6 ± 0.4 to 8.75 ± 1.95 mm/yr for ment. This is consistent with the lack of any than 5,000 yr. This is somewhat anomalous in the Timber Canyon area. Total stratigraphic evidence for significant lateral faulting at the light of the demonstrated activity to the east and separation also increases accordingly and surface. west on similar and related faults but is ex- reaches a maximum of about 9 km immediately In evaluating the San Cayetano fault, it is plained by Weldon and Humphreys' (1986) west of Sespe Creek (Fine, 1954; Cemen, 1977). clear that the historic seismicity does not repre- model which suggests that there should be only minor recent convergence in that part of the Recency of faulting along the western seg- sent the long-term activity. Allen (1975) has Transverse Ranges. ment is established by an offset Holocene or shown that even in areas with long historic uppermost Pleistocene alluvial fan in the Upper records of seismicity, such as 2,000-3,000 years Late Quaternary displacement rates on the Ojai Valley, by displaced Holocene colluvium in in Japan, the Middle East, and China, earth- San Cayetano fault appear to be slightly higher the Silverthread oil field, by inferred Holocene quakes in these regions show surprisingly large than on the San Fernando or Cucamonga faults, displacement at Timber Canyon, and by a prob- long-term temporal and spatial variations. The suggesting either larger or more frequent events. able fault scarp in Q3 alluvium at Boulder very short historic record in the study area, then, Using Slemmon's (1982) rupture length versus Creek. Yeats (1983) documented at least 7.5 km is insufficient to establish the pattern of seismic- earthquake magnitude equation for reverse of reverse separation for the eastern segment be- ity for the major faults, and one must look at faults (M = 2.021 + 1.142 Log L), and assuming tween Piru and Fillmore, all post-1.0 m.y. B.P. both the historic earthquakes associated with a maximum rupture length of 24 km for the A minimum long-term average slip rate for the similar faults in the Transverse Ranges and the western section of the fault, an earthquake of eastern segment is therefore estimated at 7.5 late Quaternary record of faulting (paleoseismic- magnitude 7.0 is predicted. Using a similar equa- mm/yr. Recency of faulting based on landforms ity) on these and other similar zones of thrust tion for magnitude versus displacement per on the eastern segment has not been established faulting. event yields ~2 m of displacement per event. due to the lack of alluvial material on the hang- Of particular interest are the M6.6 1971 San Assuming that all displacement occurs as dip ing wall; the fault coincides fairly closely with Fernando earthquake along the San Fernando displacement during major earthquakes and the the north side of the Santa Clara River flood fault zone in the Los Angeles basin and the 1978 slip rate is between 3.6 ± 0.4 and 8.7 ± 1.9 plain, and recent activity may be obscured by M5.1 Santa Barbara earthquake in the Santa mm/yr for the western segment, a recurrence lateral erosion by the river. Barbara channel, both of which occurred on interval of between 200 and 600 yr is yielded. north-dipping fault surfaces with left oblique- Similarly, the eastern 18 km segment of the thrust displacement (Whitcomb, 1971; Lee and fault, with a slip rate of about 7.5 mm/yr, HISTORICAL SEISMICITY others, 1978), and the 1973 M6.0 Point Magu should generate a magnitude 6.9 earthquake earthquake, which had pure thrust displacement about every 230 yr. It should be emphasized, During the past 65 years, nearly 30 earth- on a 44° north-dipping fault surface (Steirman however, that these estimates are speculative be- quakes of local magnitude 6 or greater have oc- and Ellsworth, 1976). Two larger previous cause there are no historical earthquakes or pa- curred in southern California (Lee and others, events in the Santa Barbara area are inferred to leoseismic data on the San Cayetano fault. It is 1979). Five of these earthquakes occurred in the have occurred very near the 1978 earthquake; not known whether the eastern and western western Transverse Ranges, the most recent of the 1925 M6.3 and 1941 M6.0 earthquakes segments move independently or together, which was accompanied by rupture along the (Lee and others, 1978), both of which caused which will affect the size of the earthquake as San Fernando fault zone in 1971 (M6.6). significant damage to local cultural structures. well as the recurrence interval.

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CONCLUSIONS Cemen, I., 1977, Geology of the Sespe-Piru Creek area, Ventura County, Cayetano fault, western Transverse Ranges, California: Geological So- California [M.A. thesis]: Athens, Ohio, Ohio University, 68 p. ciety of America Abstracts with Programs, v. 14, p. 228. Clark, M. N., Lienkaemper, J. J., Harwood, D. S., Lajoie, K. R., Matti, J. C., 19S3, Soil chronology, geology, and neotectonics of the north central Perkins, J. A., Rymer, M. J., Sarna-Wojcicki, A. M., Sharp, R. V., Ventura Basin, California [Unpublished Ph.D. dissert.]: Santa Barbara, The San Cayetano fault displays significant Sims, J. D., Tinsley, J. C., HI, and Ztony, J. I., 1984, Preliminary California, University of California, Santa Barbara, 424 p. slip-rate table and map of late Quaternary faults of California: U.S. Rockwell, T., and Keller, E., 1980, Alluvial fan deformation along the San evidence of both late Pleistocene and Holocene Geological Survey Open-File Report 84-106. Cayetano fault, western Transverse Ranges, California: Geological So- activity with a maximum slip rate of 3.6 ± 0.4 to Crook, R„ Jr., Allen, C. R., Kamb, B., Payne, C. M., and Proctor, R. J., 1978, ciety of America Abstracts with Programs, v. 12, p. 140. Quaternary geology and seismic hazard of the Sierra Madre and asso- Rockwell, T. K., Keller, E. A., Clark, M. N., and Johnson, D. L., 1984, 8.75 ± 1.95 mm/yr along the western segment ciated faults, western San Gabriel Mountains, California: Final Techni- Chronology and rates of faulting of Ventura River terraces, California: cal Report for USGS No. 14-08-0001-15258, 117 p. Geological Society of America Bulletin, v. 95, p. 1466-1477. of the fault and about 7.5 mm/yr along the DembrofF, G., 1983, Tectonic geomorphology and soil chronology of the Ven- Rockwell, T. K., Johnson, D. L., Keller, E. A., and Dembroff, G. R., 1985, A eastern segment. In addition, deformation of the tura Avenue anticline, Ventura, California [M.S. thesis}: Santa Barbara, late Pleistocene-Holocene soil chronosequence in the central Ventura California, University of California, Santa Barbara, 122 p. Basin, southern California, U.S.A., in Richards, K., Amett, R,, and Ellis,

foot wall of the fault continues into the Holo- Eldridge, G. H.t and Arnold, R., 1907, The Santa Clara Valley, Puente Hills, S., eds., Geomorphology and soils: London, England, George Allen and and Los Angeles oil districts, southern California: U.S. Geological Sur- Unwin, p. 309-327. cene with both tilting and faulting of alluvial vey Bulletin 309,266 p. Rodgers, D. A., 1975, Deformation, stress accumulation, and secondary fault- fans. These data contrast markedly with the lack Eschner, S., 1957, Geology of the central part of the Fillmore quadrangle, ing in the vicinity of the Transverse Ranges of Southern California Ventura County, California [M.A. thesis]: Los Angeles, California, Uni- [Ph.D. thesis]: Providence, Rhode Island, Brown University, 181 p. of moderate or large-magnitude historical earth- versity of California, Los Angeles, 57 p. Sarna-Wojcicki, A. M., Bowman, H. R., Meyer, C. E., Russel, P, C., Asaro, F., Fairbanks, H. W., 1904, Description of the San Luis quadrangle, California: Michael, H., Rowe, J. J., Baedecker, P. A., and McCoy, G., 1980, quakes associated with this fault and suggest that U.S. Geological Survey Atlas San Luis Folio, No. 101. Chemical analyses, correlations and ages of late Cenozoic tephra units the historical record is too short to characterize Fine, S. F., 1954, Geology and occurrences of oil in the Ojai-Santa Paula area, of east-central and southern California: U.S. Geological Survey Open- Ventura County, California: California Division of Mines and Geology File Report 80-231,53 p, 1 pi. its seismicity. Bulletin 170, Map sheet 28. Schlueter, J., 1976, Structure of the Sisar, Big Canyon, and western San Caye- Hamilton, W., 1961, Origin of the Gulf of California: Geological Society of tano faults, California [M.A. thesis]: Athens, Ohio, Ohio University, America Bulletin, v. 72, p. 1307-1318. 64 p. Harden, J. W., Matti, J. C., and Terhune, C., 1976, Late Quaternary slip rates Sharp, R. V., 1981, Variable rates of late Quaternary strike slip on the San ACKNOWLEDGMENTS along the San Andreas fault near Vucaipa, California, derived from soil Jacinto fault zone, southern California: Journal of Geophysical Re- development on fluvial terraces: Geological Society of America Ab- search, v. 86, no. B3, p. 1754-1762. stracts with Programs, v. 18, p. 113. Slemmons, D, B., 1982, Determination of design earthquake magnitudes for This study was supported by U.S. Geological Hatch, M. E., and Rockwell, T. K., 1986, Neotectonics of the Agua Bianca microzonation: Third International Earthquake Microzonation Confer- fault, Agua Bianca Valley, northern Baja California, Mexico: Geological ence, Proceedings, Volume I of III, p. 119-130. Survey contracts 14-08-0001-17678 and 14-08- Society of America Abstracts with Programs, v. 18, p. 114. Steirman, D. J., and Ellsworth, W. L, 1976, Aftershocks of the February 21, 0001-19781 through Edward Keller at the Uni- Huesser, L. E., 1978, Pollen in the Santa Barbara Basin, California: A 12,000 1973, Point Magu, California earthquake: Seismological Society of year record: Geological Society of America Bulletin, v. 89, p. 673-678. America Bulletin, v. 66, no. 6, p. 1931-1952. versity of California at Santa Barbara and was Jennings, C. W., and Strand, R. G., 1969, Geologic map of California, Los Vaughan, P., and Rockwell, T. K,, 1986, Alluvial stratigraphy and neotectonics Angeles sheet California Division of Mines and Geology, scale of the at Agua Tibia Mountain, southern California, completed as part of my Ph.D. thesis work. I 1:250,000. in Neotectonics and faulting in Southern California: Geological Society thank Edward Keller, Arthur Sylvester, Don Johnson, R. L., 1959, The geology of the northeastern quarter of Fillmore of America, Cordilleran Section, Annual Meeting Field Trip Guidebook, quadrangle, Ventura County, California [M.A. thesis): Los Angeles, p. 177-191. Johnson, and all the others for their support, California, University of California, Los Angeles, 55 p. Weber, F. H., Cleveland, G. B., Kahle, J. E., Kiessling, E. F., Miller, R. V.,

Keller, E. A., Johnson, D. L., Clark, M. N., and Rockwell, T. K., 1980, Mills, M. F.v and Morton, D. M., 1973, Geology and mineral resources assistance, and guidance during my stay at Tectonic geomorphology and earthquake hazard, north flank, central study of southern Ventura County, California: California Division of UCSB and in this study. I am indebted to Ventura Basin, California: U.S. Geological Survey Open-File Report Mines and Geology, Preliminary Report 14,102 p. 81-376,167 p. Weldon, R., and Humphreys, E., 1986, A kinematic model of southern Cali- Thomas Dibblee for generously providing me Kerr, P. F., and Schenck, H. G., 1928, Significance of the Matilija overturn fornia: Tectonics, v. 5, p. 33-48. (Santa Ynez Mountains, California): Geological Society of America Whitcomb, J. H., 1971, Fault plane solutions on the February 9, 1971, San his unpublished maps, and to the oil companies, Bulletin, v. 39, p, 1087-1102. Fernando earthquake and some aftershocks: U.S. Geological Survey especially Argo Petroleum, for their support Lajoie, K. R., Sarna-Wojcicki, A. M., and Yerkes, R. F., 1982, Quaternary Professional Paper 733, p. 30-32. chronology and rates of crustal deformation in the Ventura area, Cali- Whitcomb, J. H., Allen, C. R„ Garmany, J. D., and Hileman, J. A., 1973, The with subsurface data. I also thank R. S. Yeats, fornia, in Neotectonics of the Ventura Basin: Geological Society of 1971 San Fernando earthquake series: Focal mechanisms and tectonics: America, Cordilleran Section Field Trip No. 3 Guidebook, p. 43-52. Reviews in Geophysics and Space Physics, v. II, p. 693-730, G. Huftile, and D. B. Slemmons for their Lee, W.H.K., Johnson, C. E., Henyey, T. L„ and Yerkes, R. L., 1978, A Yeats, R. S., 1977, Evaluation of fault hazard: Surface rupture vs. earthquake thoughtful, critical reviews which led to im- preliminary study of the Santa Barbara, California, earthquake of Au- potential: Geological Society of America Abstracts with Programs, v. 9, gust 13,1978 and its major aftershocks: U.S. Geological Survey Circu- p. 529. provement of this manuscript. lar 797, 11 p. 1981, Quaternary flake tectonics of the California Transverse Ranges: Lee, W.H.K., Yerkes, R. F., and Simirenko, M„ 1979, Earthquake activity and Geology, v. 9, p. 16-20. focal mechanisms, western Transverse Ranges: U.S. Geological Survey 1983, Large-scale Quaternary detachments in Ventura basin, southern Circular 799-A. California: Journal of Geophysical Research, v. 88,no.Bl,p. 569-583. Loofbourow, J. S., Jr., 1941, Geology of a portion of the Santa Paula quadran- Yeats, R. S., Clark, M. 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