Active Faulting and Growth Folding in the Eastern Santa Barbara Channel, California

Active faulting and growth folding in the eastern Santa Barbara Channel, California

JOHN H. SHAW* 1 Department of Geological and Geophysical Sciences, GuyotHall, Princeton University, JOHN SUPPE J Princeton, New Jersey 08544-1003

ABSTRACT nitude to estimate the sizes of potential toric events, including the 1978 M,1 = 5.1 earthquakes. This analysis suggests that a Santa Barbara earthquake (Corbett and John- We develop new methods to identify ramp in the Channel Islands fault beneath son, 1982) (Fig. 1), indicate that a component blind-thrust fault systems, determine fault the Offshore Oak Ridge trend is capable of of shortening is being accommodated seismi-

slip rates, and estimate potential earth- rupturing in a Ms > 7.2 earthquake. Earth- cally. Therefore, the recognition and deter- quake magnitudes and recurrence intervals quakes of this magnitude may release ~2 m mination of slip rates on active faults in this in active fold-and-thrust belts. These meth- of slip, which, when combined with the es- area is vital to assess the seismic hazards ods are applied to compressive folds along timated slip rate (1.3 mm/yr), yields an posed to nearby regions of southern the Offshore Oak Ridge and Blue Bottle earthquake recurrence interval of —1500 yr California. trends, which overlie active blind-thrust for this Channel Islands fault ramp. In this study, we identify several active faults in the eastern Santa Barbara Chan- blind-thrust faults in the Santa Barbara basin, nel. These folds and their causative faults INTRODUCTION which lack surface breaks but deform Qua- are interpreted using fault-bend fold theory ternary sediments. As several recent earth- 2 and are represented in balanced models and The Santa Barbara basin and the Channel quakes (1982 New Idria, Mw = 5.4; 1983 cross sections that integrate surface and Islands are recognized as the southwestern Coalinga, Mw = 6.5; 1985 Kettleman Hills, subsurface data. The structures are mapped extension of the Transverse Ranges of M„ = 6.1; 1987 Whittier Narrows, Mw = 6.0) using a new technique of axial-surface map- south-central California (Reed and Hollis- have demonstrated, blind-thrust and reverse ping in seismic reflection grids, which deter, 1936; Vedder and others, 1969). The faults pose a significant seismic hazard (Davis fines three-dimensional structural geome- basin and islands mark the southern front of and others, 1989; Hauksson and Jones, 1989; tries and shows changes in slip and this active, east-west trending fold-and- Stein and Ekstrom, 1992). Folds in the Santa subsurface fault geometry along strike. thrust belt (Fig. 1) developed during Plio- Barbara basin associated with these faults are Analysis of syntectonic (growth) sediments cene through Quaternary contraction (Yeats, represented in balanced models and cross yields Pliocene and Quaternary fault slip 1983; Namson and Davis, 1988a). Geologic sections developed using the theory of fault- rates of 1.3 mm/yr on a deep thrust (>16 studies of folding and faulting (for example, bend folding (Suppe, 1983, 1985; Suppe and km) and 1.3 mm/yr on shallower faults (2-5 Namson and Davis, 1988a; Yeats and others, Medwedeff, 1990; Suppe and others, 1992). km). The combined 2.6 mm/yr slip rate 1988) and present-day stress measurements These cross sections and models integrate represents only part of the 6 mm/yr of short- (Mount and Suppe, 1992) indicate that the re- surface geology, seismic reflection profiles, ening measured by geodesy across the chan- gional contraction and compression are di- earthquake seismicity, and well-log data nel and estimated from relative Pacific- rected from north-south to northeast-south- (provided by industry) that confirm active North American plate motions across the west, subnormal to the San Andreas fault faulting and crustal shortening across the ba- Transverse Ranges. Additional shortening (Zoback and others, 1987; Hauksson, 1990). sin. Furthermore, analysis of syntectonic is probably accommodated on other active Shortening across this fold-and-thrust belt, (growth) sediments in these structures yields thrusts in the western Transverse Ranges which lies above a subducting mantle litho- estimates of long-term slip rates on the caus- and in the northern channel along the Santa spheric slab (Humphreys and Clayton, 1990; ative faults (Suppe and others, 1992) and de- Barbara coast. Humphreys and Hager, 1990), may resolve a fines the percentage of crustal shortening ac- Deformed seafloor sediments and a component of the discrepancy between the counted for by movement on recognized swarm of axial surface seismicity along the estimated relative Pacific-North American faults. fold trends indicate that the underlying plate motion and observed slip on the San In addition, we present a new method of thrusts are active and may pose significant Andreas fault (Minster and Jordan, 1978; mapping the axial surfaces of folds associated earthquake hazards to coastal southern Cal- Demets and others, 1987). with these blind faults. Axial-surface maps, ifornia. Unsegmented fault surfaces are Pliocene and Quaternary folding (Jackson constructed from dense grids of high-resolu- used through empirical relationships be- and Yeats, 1982; Namson and Davis, 1988a) tion seismic data, define three-dimensional tween fault surface area and rupture mag- and Global Positioning System (GPS) studies structural geometries and fault slip. Map pat- (Larson and Webb, 1992; Larsen and others, 1993) suggest several millimeters per year of * Present address: Texaco EPTD, 3901 Briar- northeast-southwest shortening across the ]M, = local Richter magnitude. park, Houston, Texas 77042. eastern Santa Barbara Channel. Several his- X, = moment magnitude (Kanamori, 1978).

Geological Society of America Bulletin, v. 106, p. 607-626, 22 figs., May 1994.

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120*W 119*W

Sania Ynez Fault TRANSVERSE 35*N Ranges Santa Barbara

Ventura . Basin 1 .Offshore

Santa Barbara Channel - s San Miguel arila Monica Mountains1 Island j Santa Rosa . Island ,

EXPLANATION [• • | = Plio-Quatemary (undivided) = Miocene and older Geology from: Jennings et al (1977) bathymétrie contour = 200 fathoms

Figure 1. Map of the western Transverse Ranges, Santa Barbara Channel, and Channel Islands group showing epicenters of historic earthquakes (M > 6.0) in the channel (Hamilton and others, 1969) and the 1978 Santa Barbara earthquake (Corfoett and Johnson, 1982). East-west-trending folds in the eastern Santa Barbara Channel (including the Offshore Oak Ridge and Blue Bottle trends) lie above active thrust faults that may pose seismic hazards. Cross section traces: X-X' (Fig. 7); X-Y (Fig. 11).

Rigid-Block Translation terns show that slip varies along strike in these structures and locate changes in subsurface fault geometry that may act to segment the faults. Fault segment boundaries may limit rupture area and, therefore, affect the magnitude of single events as in the New Idria-Coalinga-Kettleman Hills earthquake sequence (Stein and Ekstrom, 1992). We use Figure 2. Purely rigid-block translation over overlap estimates of fault-segment areas to predict nonplanar fault surfaces would generate an the size of potential earthquakes, using rela- unreasonable "overlap" or void (crossed pat- tionships between fault-rupture area and tern) between fault blocks. However, folding of earthquake magnitude (Kanamori and Ander- the hanging-wall block localized along axial son, 1975). Finally, estimates of coseismic surfaces pinned to fault bends accommodates slip (based on slip in earthquakes of similar fault slip without generating an "overlap" or magnitude), combined with long-term fault void. slip rates, yield a recurrence interval of potential blind-thrust earthquakes in the Fault-Bend Folding channel. kink band ACTIVE FAULTING AND GROWTH FOLDING

The Santa Barbara basin is characterized by a number of east-west-trending folds that have been the sites of petroleum exploration

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Figure 3. Kinematic development of a growth-fault-bend fold after Suppe and others pra-growth (1992). (A) A thrust ramp in undeformed strata. (B) Fault slip causes folding of the hanging-wall block along active axial surfaces (A and B) that are pinned to fault bends. Inactive axial surfaces (A' and B') form at fault bends and are passively translated away from active axial surfaces by slip. Kink-band widths (A- A', B-B') measured along bedding equal slip on the underlying fault segment. The difference in kink-band widths reflects slip consumed in folding. (C) Progressive fault slip widens kink bands, which narrow upward in the growth section (growth triangles); surface strata are deformed at point P. Limb widths of growth horizons reflect the amount of fault slip growth since their deposition. The inflections in the in- trianol» active axial surfaces (A' and B') (G = growth- inactive axial surfaces) mark the transitions between p re-growth and growth (syntectonic) growth section.

lated away from the active axial surfaces during progressive fault slip. Therefore, kink- (A-B) = Active axial surfaces 1 band widths between sets of active and (A'-B ) = Inactive axial surfaces inactive axial surfaces are directly related to (G) = Growth (inactive) axial surfaces fault dip-slip. (P) = Surface deformation Balanced kinematic models developed by Medwedeff (1989) and Suppe and others (1992) demonstrate that sediments deposited and production (Fig. 1). Two of these struc- others, 1989). In addition, we incorporate atop and adjacent to active fault-related folds tures, the Offshore Oak Ridge and Blue Bot- into our cross sections analyses of the geom- (syntectonic or growth sediments) record tle trends in the eastern basin, are the focus of etry of syntectonic strata, which record the fold growth and fault slip. Active axial sur- this study. High-resolution seismic reflection kinematics of folding and faulting (Suppe and faces deform growth sediments as they are profiles presented here and additional profiles others, 1992). A brief description of growth folded above fault bends (Fig. 3). Growth presented by Luyendyk and others (1982) in- fault-bend fold theory follows, because it al- sediments deposited early in the slip history dicate that Quaternary sediments are de- lows quantitative evaluation of the history of of the underlying fault are translated away formed along these trends. We interpret this folding and fault slip. from active axial surfaces with time and, deformation as folding caused by active therefore, record a wider kink-band width thrust faulting at depth, which may pose a Growth Fault-Bend Folding than do growth sediments deposited later significant seismic hazard to the greater Santa (Fig. 3). As a result, growth strata form up- Barbara region. To identify the folding mech- Purely rigid-block translation of the hang- wardly narrowing kink bands, or growth tri- anisms and quantify fault slip across these ing wall over nonplanar thrust faults would angles, which have been recognized in the trends, the Blue Bottle and Offshore Oak generate voids or unreasonable "overlaps" Santa Barbara basin (Fig. 4) and throughout Ridge structures are depicted in balanced kin- between the fault blocks (Rich, 1934; Suppe, the world (Suppe and others, 1992). Growth ematic models and cross sections developed 1983) (Fig. 2). Folds of the hanging-wall triangles are generally bounded by an active using the theory of fault-bend folding (Suppe, block, however, may accommodate fault slip axial surface pinned to a fault bend and an 1983; Suppe and others, 1992). The develop- without producing unreasonable overlaps or inactive axial surface that moves with the ment of balanced cross sections is an ac- voids (Fig. 2) and are widely observed. Many over-riding block (Fig. 3). Inactive axial sur- cepted method of interpreting the structural upper crustal folds (for example, fault-bend faces in growth {growth axial surfaces) do not geometry of upper crustal deformation in folds, Suppe, 1983), grow by kink-band mi- bisect the angle between strata due to strati- fold-and-thrust belts (Dahlstrom, 1969,1970; gration with deformation of the over-riding graphic thickness changes across the surface. Suppe, 1983; Woodward and others, 1985), block localized along active axial surfaces These thickness changes record different and of assessing earthquake hazards (Nam- pinned to fault bends (Fig. 3). Inactive axial deposition rates caused by relative uplift son and Davis, 1988a, 1988b, 1990; Davis and surfaces form at fault bends and are trans- across the active axial surface.

Geological Society of America Bulletin, May 1994 609

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Offshore Oak Ridge Trend N

Figure 4. A growth triangle is imaged along the Offshore Oak Ridge trend in a north-south-trending migrated seismic reflection profile [(A) uninterpreted, (B) with axial surfaces]. Continuous and coherent seismic reflections across the trend preclude near-vertical, through-going faults. Note the thinning of strata across the southern growth inactive axial surface (G), consistent with the forward model prediction [inset in (B)]. In addition, the active axial surface (A) of the kink band folds near-seafloor sediments, indicating that the underlying fault remains active (see also Luyendyk and others, 1982). Seismic profile provided by Texaco USA. TWIT is two-way travel time.

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average fault slip rate = -=7=- — AT Agex- Age growth

Figure 5. If the ages of at least two growth horizons within a growth triangle are known, then the limb widening rate of the fold (AL/AT) can be determined. In the case of simple ramps from décollements, the rate of limb widening equals the fault slip rate. In cases where subsurface fault geometry is unknown, the limb widening rate provides a reasonable estimate (commonly within 20%) of the fault slip rate (Suppe and others, 1992).

Figure 6. A typical NNE- SSW-trending section from a 3-D migrated seismic reflection survey in the eastern Santa Barbara Channel that images narrowing upward kink bands, or growth triangles, along the Offshore Oak Ridge and Blue Bottle trends [(A) uninterpreted, (B) with selected axial surfaces and faults marked]. The kink- band geometries indicate that

Offshore northeast blind thrust faults lie from 1 Blue Bottle Trend Oak Ridge Trend to 3 sec two-way travel time beneath the Blue Bottle trend, and below the depth of section beneath the Offshore Oak Ridge trend. The north-dipping unconformity in the upper left is the extension of the Oak Ridge kink band beneath and south of the Blue Bottle trend (Fig. 7) (see text for discussion). TWIT is two-way travel time. Seismic profile provided by Texaco USA, BP Exploration, and Sun Operat- ing LP.

Axial Surfaces

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NE X' Blue Bottle Trend Oak Ridge Trend Oxnard Shelf Montalvo Trend Pitas Point Trend Conoco SBC-6 Sicnal Exxon Texaco 234

5km-

Figure 8. A north-dipping unconformity Offshore Oak Ridge Kink Band: above Miocene strata marks the southern boundary of the Offshore Oak Ridge kink Inactive Axial Surface (A') band (Fig. 7). Note that the inactive axial surface (A') does not extend to the seafloor. Pliocene and Quaternary strata lap onto dipping Miocene rocks creating the unconformity. Seismic profile provided by Texaco USA.

SSiiS SEE iitiii.mt»w>wmiiijii,i iff The geometry of these growth triangles records the kinematic history of the structure and slip rates of the underlying fault or faults. If the ages of at least two stratigraphie units within the growth triangles are known (by independent age-dating methods), then average rates of limb-widening or fold growth can be calculated (Fig. 5). In cases of simple ramps from décollements, the fault slip rate is equiv- alent to the change in limb width divided by the change in the ages of growth horizons. In cases where more complex subsurface fault geometries are defined by balanced cross sections, fault slip rates also can be determined. Even if fault geometry is poorly defined, however, fold limb width provides a reasonable estimate, commonly within 20%, of fault slip (Suppe and others, 1992). In cases of

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Figure 7. A balanced cross section across the and to the south by a growth axial surface (G) of growth strata near the fold (Fig. 9). During Offshore Oak Ridge and Blue Bottle trends (lo- (Fig. 4). As predicted in the forward model of the early slip history of the underlying Chan- cated in Fig. I0A) that integrates seismic re- Figure 3, thinning of strata is evident across nel Islands fault, sediments may have been flection and well-log data. A presumed fault the growth axial surface. In addition, de- confined to one of a number of pre-existing (the Channel Islands thrust) below the depth of formed near-seafloor sediments along the ac- Miocene rift basins (Fig. 9). Evidence of the section generates a kink band (A-A') and a tive axial surface (A) (see also Luyendyk and Miocene normal faulting includes offset mid- Pliocene-Quaternary growth triangle (A-G) others, 1982) indicate that the underlying dle Miocene and older strata and thickened along the Offshore Oak Ridge trend. Two shal- Channel Islands thrust is active (Fig. 4). Miocene units on the down-thrown fault lower faults form an imbricate fault-bend fold At depth, the Oak Ridge kink band be- block along the trend (Fig. 7). The early pe- along the Blue Bottle trend (see Fig. 12 and text tween axial surfaces A and A' dips to the riod of nondeposition near the fold is re- for discussion). Aside from Texaco #7, wells south beneath the Blue Bottle trend and off- corded by the unconformity that joins the are projected into the cross section plane an shore Oxnard shelf (Fig. 7). The inactive axial growth (G) and inactive (A') axial surfaces. average of 1.5 km along tie lines. Horizontal surface (A'), or southern boundary of the The depth to the fault beneath the Offshore equals vertical scale. kink-band structure, is marked on the shelf Oak Ridge trend is poorly defined by conven- by a folded Miocene unconformity (Fig. 8). ' tional subsurface data. The north-dipping North of the inactive axial surface (A'), the kink band beneath the growth structure ap- unconformity dips 16°-20° N, which is con- pears to extend below the resolution of re- sistent with the average dip of units within the gional seismic profiles, (5 sec two-way travel growth structure along the Offshore Oak time [TWTT]) (Fig. 6), which corresponds to Ridge trend (Fig. 7). Offset between the ter- about 8 km below sea level based on seismic oblique slip on the fault ramps, limb widths mination of the growth axial surface (G) at velocities obtained from sonic well log data record the rate of dip-slip motion. If the di- depth and the top of the pregrowth inactive and stacking velocities. Recent seismicity rection of the slip vector is known, then fault axial surface (A') suggests a period of slip on along the trend, however, provides additional slip rates can be calculated (Shaw and others, the underlying thrust fault prior to deposition subsurface constraints. The seismicity in- 1994a). Analytical techniques based on the theories of growth fault-related folding are applied in the following sections to structures Channel Islands Thrust in the Santa Barbara Channel. A ~r pre-growth Figure 9. A balanced, kine- The Offshore Oak Ridge Trend I matic model of the development of the Offshore Oak The Offshore Oak Ridge trend is a 40-km- Ridge trend. (A) Normal long, east-west-trending fold in the eastern faulting has offset pregrowth Santa Barbara Channel (Fig. 1). It has been strata (dark pattern) and the mapped as the western extension of the Oak No Deposition future positions of the active Ridge fault, which is defined in the onshore axial surface (A) and Channel Ventura basin (Yeats, 1988; Suppe and Med- Islands fault are shown. (B) basin wedeff, 1990). High-resolution seismic reflec- deposition Slip on the thrust ramp, tion profiles (Figs. 4 and 6) and well control which postdates normal fault- on both sides of the Oak Ridge trend (Fig. 7) ing, generates a kink band suggest that the structures of the onshore and B (A-A'); however, growth offshore trends are very different. Continu- (syntectonic) sediments are ous and coherent seismic reflections preclude confined to the pre-existing significant lateral or dip-slip offset across the rift basin. (C) Additional sed- offshore trend on a steeply dipping, through- iments fill the basin and lap / - --g-3 going fault. We suggest, therefore, that the / 1 / ^ onto the dipping part of the Offshore Oak Ridge trend and the onshore unconformity. This forms a Oak Ridge fault are separate structural narrowing-upward kink features. band, or growth triangle, as fault slip The narrowing-upward shape of the c / strata are folded through the imaged fold (kink band) along the Offshore active axial surface (A). The Oak Ridge trend is consistent with growth- \ period of non-deposition on the fold limb in stage B is refold patterns predicted in balanced-forward Unconformity models (Fig. 3). We interpret the Offshore flected by the dipping uncon- / Oak Ridge trend, therefore, as a growth tri- formity that separates the angle associated with a synclinal bend in an pregrowth (A') from growth underlying blind-thrust fault that we refer to (G) inactive axial surfaces in as the Channel Islands thrust. To the north, stage C. the kink band is bounded by an active axial surface (A) that deforms growth sediments

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119°30'

Figure 10A. Epicenters from an earthquake swarm in 1984 (Henyey and Teng, 1985) define the active axial surface (A) of the Offshore Oak Ridge trend. Single-event (C and D) and composite (E and F) focal mechanism solutions from the 1984 seismicity have gentle north dipping (C, D, and E) and horizontal (F) nodal planes (Henyey and Teng, 1985) consistent with folding through the active axial surfaces by bedding parallel slip (see Figure 10B). Cross section traces: X-X' (Fig. 7); X-Y (Fig. 11). SCIF = Santa Cruz Island fault. The dashed boxes outline earthquakes plotted on Figure 10B.

eludes a swarm of more than 400 earthquakes contain nodal planes (near horizontal and dip- shore trend (Fig. 11). Earthquake swarms (M, = 0.5-4.0) that occurred beneath the Off- ping -20° N) (Fig. 10A) that are consistent along the Offshore Oak Ridge trend prior to shore Oak Ridge trend in April 1984. The with folding by bedding plane slip through the the 1984 events (Sylvester and others, 1970; events were recorded by the University of active axial surface (A) (Fig. 10B). These fo- Lee and Vedder, 1973) may represent addi- Southern California network, which included cal-mechanism solutions and the spatial as- tional patches of folding seismicity and de- ocean-bottom seismometers. The hypocen- sociation of the swarm with the mapped axial formation along the active axial surface. tral locations define a steeply dipping plane surface suggest that the hypocenters outline The depth of the underlying fault and the (Henyey and Teng, 1985) beneath the active the extension to depth of the active axial sur- dip of strata within the growth sediments of axial surface (A) of the Offshore Oak Ridge face (A) of the Offshore Oak Ridge trend. The the Oak Ridge fold suggest that the Channel trend (Figs. 10A and 10B). Henyey and Teng depth of the seismicity along the axial surface Islands thrust rises toward the seafloor south (1985) determined four single-event and com- indicates that the Channel Islands fault ramps of Santa Cruz Island (Fig. 11). Seafloor ba- posite focal-mechanism solutions, which all upward from at least 16 km beneath the off- thymetry (Fig. 10A) and high-resolution seis-

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Expected Focal Mechanism indicate that slip on the Pitas Point thrust Solutions in Cross-Section was initiated in early Pleistocene time. Furthermore, active axial surfaces along the trend also may deform more recently deposited sediments, and the fold has a bathymetric expression (Fig. 6), suggest- Bedding Parallel ing that the underlying thrust faults are ac- SUP ^ tive and may pose a seismic risk. Shallow thrusting on the Pitas Point and Montalvo faults also folds strata to the north along the Offshore Oak Ridge trend. Slip on the shallow thrusts generates a steeply dipping kink band in the Offshore Oak Ridge growth structure that is equal in width to the north flank of the Blue Bottle trend (Figs. 6, SI S2 7, and 13). Flattening of the Montalvo thrust sea level south of the Oak Ridge growth triangle forms a gently south-dipping limb and offsets the growth axial surface (G) associated with the Channel Islands thrust (Figs. 6, 7, and 13). - 5km This shallow anticline within the Offshore Oak Ridge kink band forms the Montalvo trend of Yeats (1983) and others.

10km THREE-DIMENSIONAL FOLD AND FAULT GEOMETRY

Axial Surface Mapping -15km In the preceding sections, we have pre- 0= hypocenter sented two-dimensional balanced models and cross sections through the Offshore Oak Figure 10B. Seismicity from the 1984 earthquake swarm (Henyey and Teng, 1985) outlines the Ridge and Blue Bottle trends. Analysis of the dipping active axial surface (A) in cross section (hypocenters from the dashed boxes on Figure 10A three-dimensional nature of the trends, how- are plotted). The near-horizontal and low-angle nodal planes from focal mechanism solutions ever, is necessary to more completely define (Fig. 10A) are consistent with solutions expected from folding of rock through an active axial the structural geometry and fault slip and to surface by bedding parallel slip (left). The depth of seismicity along the trend suggests that the assess the hazards posed by the active, po- underlying Channel Islands thrust ramps from a depth of at least 16 km and nears the seafloor tentially seismogenic faults. Therefore, we south of Santa Cruz Island (see Fig. 11). apply a straightforward technique of mapping the axial surfaces (fold hinges) of these trends through grids of seismic reflection data. Axial surface mapping (Shaw and others 1992, 1994b) (1) defines the positions and lateral ex- mic profiles south of the island (unavailable in the eastern Santa Barbara Channel be- tent of fold trends and underlying faults (trend for publication in this report) suggest that the neath the asymmetric Blue Bottle anti- analysis); (2) records the magnitude and di- thrust flattens and wedges back to the north. cline, which trends sub-parallel to the Off- rection of slip on underlying faults; and (3) The thrust may surface as one or more of the shore Oak Ridge fold (Fig. 1). Based on defines fold and fault segment boundaries east-west-striking dip-slip faults offshore or seismic reflection profiles (Fig. 6) and well that may limit fault rupture areas and earth- mapped on the island by Weaver and others log data, we interpret the Blue Bottle trend quake magnitudes. A brief description of the (1969) and Dibblee (1982). Alternatively, as imbricated fault-bend folds developed method follows. oblique slip on the Channel Islands thrust by slip on two blind thrusts, the Pitas Point Axial surfaces bound kink bands that de- may be partitioned into more pure compo- and Montalvo faults (Figs. 7,12). The fold velop to accommodate slip through bends in nents of strike slip and dip slip (Mount and limbs (kink bands) of the Blue Bottle trend underlying faults (Figs. 2 and 3). Active axial Suppe, 1992) on several active faults on Santa extend downward only to the 1- to 2-sec surfaces are pinned to underlying fault bends; Cruz Island and offshore, including strike-slip level (TWTT) in seismic reflection profiles therefore, their positions in map view and motion on the Santa Cruz Island fault (Dib- (Fig. 6), indicating that the causative faults cross sections reflect fold as well as underly- blee, 1982). beneath the Blue Bottle trend occur at ing fault geometry. Along the Offshore Oak shallower depths than the Channel Islands Ridge and Blue Bottle trends, axial surfaces The Blue Bottle Trend thrust (Fig. 7). Distinctive growth triangles can be located readily on seismic reflection bound the northern and southern flanks of profiles (Figs. 4 and 6). Axial-surface maps Active blind-thrust faulting also occurs the Blue Bottle trend (Figs. 6 and 7) and are generated by projecting axial surfaces to

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sea floor A\X\kV\V\i

/j/itf.t,

: | - Plio-Quatemary axial surface I I = Miocene fault (dip-slip) = Oligocene fault (strike-slip) 15km Eocene ^W"***™ (A) away: (T) toward I I = Cretaceous (Paleocene?) fault, inactive in Plio-Quat&mary

Late Miocene - Early Pliocene

= axial surface

= fault (dip-slip)

= fault, active in Plio-Quatemary Channel Islands Thrust

Figure 11. A balanced geologic cross section across the eastern Santa Barbara Channel and Santa Cruz Island combines subsurface seismic reflection and well-log data (the section trace is in Figs. 1 and 10A). The Channel Islands thrust ramps beneath the Offshore Oak Ridge trend and approaches the surface south of Santa Cruz Island. The kink-band width (A-A') of the Offshore Oak Ridge trend represents dip slip on the underlying Channel Islands thrust. The shallow fold and fault geometry along the Offshore Oak Ridge and Blue Bottle trends is depicted in Figure 7. Strike-slip motion out of the section plane may occur on the Santa Cruz Island fault; however, moderate displacements on this fault should not significantly effect our area balance and restoration, because the strike-slip fault trace is perpendicular to the section plane (Fig. 10A). SCIF = Santa Cruz Island fault. Horizontal equals vertical scale.

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the horizontal datum(s) of the reflection profiles and plotting their locations on shot point Blue Bottle Trend maps (Fig. 14). Here we apply the parallel projection method (PPM), where projections are made along the trace of the axial surfaces in section to the horizontal datum (Fig. 14). These projections along the trace of the axial surfaces in section can be made simply and Figure 12. A balanced, provide direct information on fold limb width kinematic model of the de- and fault slip. Furthermore, in most cases of velopment of the Blue Bot- simple ramps from décollements, limb widths tle trend. (A) Normal along the horizontal datum and in map view faults that offiset Miocene are equal to fault dip slip (Fig. 15). and older strata lie within The intersections of the projected axial the north-dipping kink surfaces and the datum from each profile in band of the Offshore Oak the seismic reflection grid are plotted on base Ridge trend (see Fig. 9). Growth maps. Axial surface maps are constructed by (B) The Pitas Point thrust connecting the projection points of individual ramps at the normal fault axial surfaces. Here, control points of active scarps and forms a axial surfaces (that is, those pinned to fault growth-fault-bend fold bends) are denoted by solid symbols, and in- (Rg. 3). Slip on this thrust active axial surfaces by open symbols. Axial decapitates a piece of an surfaces map patterns, when compared with old scarp and translates it those predicted in balanced three-dimen- south along the uncon- sional forward models (Figs. 14 and 16), pro- Pitas Point formity. (C) The Montalvo vide direct information on the subsurface Thrust thrust also ramps in the vi- structure and slip on the underlying fault(s). cinity of the normal fault Map-view kink-band widths are related di- scarps; slip on this thrust rectly to fault slip magnitude (Fig. 15). There- generates a steep kink fore, straight and parallel inactive and active band and additional sec- axial surface pairs in map view suggest con- ondary folds on the south stant slip and constant subsurface fault ge- side of the trend. Units ometry along strike (Fig. 14). Alternatively, Growth are thinned where axial converging axial-surface pairs in map view surfaces do not bisect suggest a decrease in slip along strike on the bedding. underlying fault (Figs. 16A and 16B). Sharp bends and offsets of axial surfaces also help to identify subsurface tear faults or other discontinuities that mark dramatic changes in subsurface fault geometry (Figs. 16C and 16D).

The Offshore Oak Ridge Trend in Map View

An axial-surface map of the Offshore Oak Ridge trend was generated using dense grids channel. The axial surface is mapped from (A') axial surfaces of the Offshore Oak Ridge of more than 75 high-resolution, two-dimen- southwest of the city of Santa Barbara east to trend bend sharply to trend west-northwest sional seismic reflection profiles and two the onshore Ventura basin. The inactive axial (Fig. 17). In addition, the limb width of the three-dimensional surveys that cover the surface (A'), which bounds the southern fold decreases rapidly to 5 km across the tran- eastern Santa Barbara Channel (Fig. 17). The flank of the fold, runs nearly parallel to the sition. These combined changes in the trend Offshore Oak Ridge trend consists of a north- active axial surface (A) along the offshore and limb width of the Offshore Oak Ridge dipping kink band bounded by active (A) and Oxnard shelf (Fig. 17). The limb width of the fold suggest a change in both fault strike and inactive (A') axial surfaces, and the kink- fold (L), which can be measured directly slip (Fig. 16) across this transition. A series of band width reflects slip on the underlying from the map, is roughly constant at 10 km steeply dipping tear faults that trend north- Channel Islands thrust (Figs. 11 and 15). The along the center of the trend. This value rep- east-southwest (Fig. 17) and show offset to active axial surface, which bounds the north- resents a minimum estimate of the dip-slip on the surface (Fig. 18) probably accommodates ern flank of the Oak Ridge trend (axial sur- the underlying Channel Islands thrust this strain transition in the over-riding fault face [A], Fig. 17), extends east-west contin- (Fig. 11). block. These and other mapped tears uously for more than 40 km in the eastern To the west, the active (A) and inactive (Fig. 17) strike parallel to faults imaged off-

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Montalvo Trend Pitas Point Trend Figure 13. Slip on the Montalvo and Pitas Texaco 234 Point thrusts generates additional kink bands (B) along the Offshore Oak Ridge trend, which are imaged in seismic reflection profiles (Fig. 6). Flattening of the Montalvo thrust also generates a gentle south-dipping limb that forms the offshore Montalvo anticline of Yeats (1983). The limb widths of dated horizons 1,2, and 3 between axial surfaces A and G (bold, dashed) minus the limb width B (associated with shallow faulting) are used in Figure 21 to determine the slip rate on the Channel Islands thrust (Figs. 9 and 11). The limb width of horizon 4 along the Blue Bottle trend (not shown) is used in Figure 21 to determine the slip rate on the shallow thrusts and is approximately Montalvo equal to limb width B. Horizons 1 and 3 were Thrust picked in well Texaco 234-#7; horizons 2 and 4 were tied through seismic lines from well Sig- nal I-S (Yeats, 1983). See Figure 21 for stratigraphic information. Horizontal equals vertical scale. 'ft//,

to Channel Islands Thrust

Map View

Figure 14. (A) Maps of axial surfaces are generated by projecting axial surfaces in section (or in seismic reflection profiles) to a horizontal datum, and plotting their locations on maps. The parallel projection method used here projects axial surfaces parallel to their trace in section. (B) A balanced 3-D fault-bend fold, with constant slip and unchanging fault geometry along strike, yields straight and parallel axial surfaces in map view. Straight and parallel axial surfaces in map view may also exist in cases where fault slip is greater than fault ramp width (see Shaw and others, 1994b); however, this special case requires active folding along anticlinal axial surfaces (A') that is not evident along the Offshore Oak Ridge (Fig. 8) and Blue Bottle trends (Fig. 6).

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shore on the Channel Islands shelf (Luy- Isoscetes triangle (Lq = Lmj endyk and others, 1982, 1983) and exposed on the north side of Santa Cruz Island (Wea- ver and others, 1969). The combined changes in fault geometry and total slip across this transition, defined by the map pattern and tear faults, form a distinct segment boundary of the Channel Islands thrust ramp. Although the thrust does extend west of this transition, we speculate that this segment boundary is a likely candidate for the termination of individual ruptures on the fault ramp. North of the Anacapa Islands, the kink- band width of the fold decreases more gradually, because the inactive axial surface (A') Figure 15. The parallel projection method (Fig. 14) generates kink-band widths in map view is located more to the north (Fig. 17). North- (Lm) equal to fold limb widths (L„) in section when axial surfaces bisect fold limbs, conserving east-southwest-striking minor tear faults also layer thickness, and kink bands are bounded by horizontal strata. For simple ramps in décol- mark the front of this gradual transition. The lements, these limb widths equal dip slip on the underlying fault. gradual change in kink-band width, without dramatic bend or offset of the active axial surface (A), suggests that slip on the underlying thrust gradually decreases to the east without a significant change in the subsurface fault geometry (Fig. 16). This area of relatively constant subsurface fault geometry is, therefore, not likely to limit the area of earthquake rupture on the underlying thrust ramp. The eastern termination of this ramp in the Channel Islands thrust, however, is less defined. The axial surfaces (A and A') are mapped from seismic sections onshore along the coast in the Ventura basin (Fig. 17); however, the eastern extent of the fold and underlying fault cannot be determined with available seismic reflection data. Additional evidence, however, suggests that a major segment boundary exists through the Ven- tura basin and Oxnard plains (Fig. 19). The proposed segment boundary is marked by the steep bathymétrie slope that bends sharply east of the Anacapa Islands to trend north-

Figure 16. Balanced 3-D forward models of fault-bend folds produce axial-surface map patterns that reflect subsurface fault geometry and slip. (A) A plunging fault-bend fold in which slip on the underling thrust decreases linearly from left to right. (B) The axial surface map pattern of the fold in model (A) has con- vergent axial surfaces that reflect the gradual change in slip on the underlying thrust. (C) A fault-bend fold with constant slip and a fault ramp offset by a lateral ramp. (D) Axial surfaces are offset in map view above the tear fault. In this model, the orientation of the line that offsets axial surfaces in map view is the slip direction.

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119°30'

Figure 17. An axial-surface map of the Offshore Oak Ridge kink band (A-A') (Figs. 7 and 11), generated by the parallel projection method (Fig. 14). Sharp bends in the axial surfaces south of Santa Barbara suggest changes in subsurface geometry and slip (Fig. 16D) on the underlying Channel Islands thrust (Fig. 11). A gradual decrease of the kink-band width to the east without offset of the active axial surface (A) suggests a gradual change in slip (Fig. 16B), and not fault geometry, as the thrust approaches Ventura. Oblique, left-lateral thrusting (total slip S — 11 km) on the underlying Channel Islands fault, suggested by the axial-surface map pattern, is consistent with present-day stress directions, tear-fault orientations, earthquake P axes, and the measured geodetic shortening direction. Fold limb width (L) equals minimum fault dip slip. tr1H = estimate of the trace of the maximum compressive stress that is perpendicular to borehole breakout directions. Epicenter (1), 1973, M, = 5.0 Anacapa earthquake, Gawthrop (1975); (2), 1978, M = 5.1 Santa Barbara earthquake, Corbett and Johnson (1982); Yeats and Olson (1984). Shortening (1), Larson and Webb, 1992; (2), Larsen and others, 1993.

east-southwest through Hueneme canyon. The slip direction on the Channel Islands dition, the shape of the Santa Cruz and Ana- Continued onshore along this trend lies a dis- fault is reflected in the shape of the active (A) capa Islands shelf, which is uplifted by placement transfer zone recognized by Huf- and inactive (A') axial surface traces in map movement on the Channel Islands thrust tile and Yeats (1994). We speculate that this view, which suggests oblique, left-lateral (Fig. 11), matches the slip distribution pattern trend line, marked by offshore bathymetry, thrusting. This inferred slip direction is con- of the fault (kink band A-A') along the in- onshore displacement transfer, and converg- sistent with the shape and lateral onset of ferred northeast-southwest slip direction ing fold and fault trends (Fig. 19), may form trend changes of the active (A) and inactive (Fig. 17). This general slip direction also is an eastern segment boundary of the underly- (A') axial surfaces along the general north- consistent with the azimuth of the horizontal ing Channel Islands fault ramp. east-southwest trend of the tear faults. In ad- component of the maximum compressive

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stress determined from bore-hole breakouts (Mount, 1989) and recent earthquake P-axes, and the 016° ± 3° (Larson and Webb, 1992) and 025° (Larsen and others, 1993) shortening directions measured by geodesy across the eastern channel (Fig. 17). The inferred slip direction is also compatible with estimated convergence across the Transverse Ranges described by the vector difference between relative North American-Pacific Plate motions and strike-slip along the San Andreas fault (Demets and others, 1987).

The Blue Bottle Trend in Map View

Traces of selected axial surfaces of the Blue Bottle fold also are mapped using the seismic reflection grids (Fig. 20). In general, the limb widths of the fold and fault slip on S DIU N the Pitas Point and Montalvo thrusts remain constant across the eastern channel. The pronounced widening of the Blue Bottle fold, south of Santa Barbara, is a result of refolding of the shallow structure in the deeper Offshore Oak Ridge trend. The lateral termination of the Blue Bottle fold in the eastern channel (Fig. 20) indicates that the causative thrusts do not ramp east of this termination. The faults may continue east across this transition, however, as bedding-parallel detachments. The shallow depth of faulting beneath the Blue Bottle trend and the multiple ramps and flats in the fault surfaces (Fig. 7) suggest that the thrusts are active but do not pose a significant seismic threat in the immediate region. The potentially dangerous seismogenic segments of these faults should lie at greater depth to the north, because the faults step down in a series of folds (Dos Cuadras, Sum- Figure 18. A N-S-trending migrated seismic reflection profile images stratigraphic offset to the merland Offshore trends; Fig. 1) beneath the seafloor across a NE-SW-striking vertical tear fault mapped in Fig. 17. Uplift in the center of Santa Barbara coastline. In future work, the these left-stepping tears is consistent with greater slip on the eastern segment of the underlying magnitudes and rates of slip on these faults Channel Islands thrust, denoted by the kink-band width (A-A'), which generates a small re- along the Blue Bottle trend should help to straining bend within the tears. Seismic profile provided by Texaco USA. define the behavior of the deeper faults segments, allowing us to assess their potential hazards. others, 1969; Ingle, 1980; Yeats, 1983; Lagoe (Lagoe and Thompson, 1988) allow us to in- and Thompson, 1988). It is important to note corporate more accurate stratigraphic ages FAULT SLIP that slip-rate estimates based on this tech- that yield consistent results (see Fig. 13). nique depend strictly on the accuracy of these Movement began on the Channel Islands Fault Slip Rates stratigraphic ages determined by independ- fault below the Offshore Oak Ridge trend ent methods. Difficulties exist in developing (Figs. 7 and 11) and between the end of dep- Sediments deposited during the active slip a detailed chronostratigraphic framework osition of the syntectonic Pliocene Repetto history of underlying faults quantitatively within the California borderland basins due to Formation and the middle Miocene Mon- record fold growth and fault slip (Fig. 5) the stratigraphic complexities, highly varia- terey Formation. The beginning of faulting is (Suppe and others, 1992). Ages of selected ble sedimentation rates, and changes in local marked by the different inclinations of the in- growth horizons in the Offshore Oak Ridge paleo-environments (Lagoe and Thompson, active axial surface (A') (Fig. 7) in growth and and Blue Bottle trends are estimated by cor- 1988). Fortunately, stratigraphic and micro- pregrowth strata (Fig. 3). Assuming that the relating well logs and seismic data from ad- faunal horizons near dated ash horizons in Channel Islands fault ramps from a near-hor- jacent regions where ages of the strata have the Ventura basin (Yeats, 1983) and cali- izontal detachment at depth, the 10-km limb been published (Dibblee, 1966; Weaver and brated against paleomagnetic stratigraphies width of the fold (Fig. 17) represents the dip-

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and ramps upward from a décollement. Evi- dence of active faulting includes deformed near-seafloor sediments (Fig. 4) (Luyendyk and others, 1982) and recent seismicity along the fold trend (Fig. 10) (Henyey and Teng, 1985). The minimum of 4 km of slip recorded since the end of deposition of the Pliocene Repetto Formation suggests that —7 km of the total fault slip (11 km) occurred prior to 3 Ma. If the 1.3 mm/yr slip rate has been constant since the beginning of fault slip, the faulting must have begun at ca. 8.5 Ma to generate the total slip of 11 km. This beginning of fault slip (8.5 Ma) occurs during the deposition of the Monterey Formation in the Ven- tura basin area (Ingle, 1980), which is thought to have been deposited during Miocene rift- ing and extension of the California borderland (Crouch and Suppe, 1993). More likely, however, is that fault motion began in late Miocene or early Pliocene time. If thrusting began at 5.5 Ma, then the fault must have slipped at an increased rate of about 2.8 mm/yr (7 km/2.5 m.y.) dining early Pliocene time. This increased slip rate and associated uplift may have been much greater than the local deposition rate, producing the unconformity that offsets the growth from the pregrowth inactive axial surface along the Off- shore Oak Ridge trend (Fig. 9). Using the same technique, a horizon near the transition between growth and pregrowth strata and deformed near seafloor sediments along the Blue Bottle trend indicates a combined average 1.3 mm/yr slip rate on the underlying Pitas Point and Montalvo faults since early Pleistocene time (Fig. 21). This 1.3 mm/yr estimate is a minimum rate, Figure 19. The steep bathymétrie slope south of the Channel Islands, developed by thrusting because we lack a detailed chronostratigraph- on the Channel Islands fault, bends sharply east of the Anacapa Islands to trend NE-SW toward ic framework in the shallow (0-1 km depth) Hueneme canyon. Continued onshore along this trend into the Ventura basin, the Ventura Blue Bottle section to provide absolute ages Avenue anticline and South Mountain fold are offset along a displacement transfer zone recog- of when each fault became active. This slip nized by Huftile and Yeats (1994). In addition, this NE-SW trend marks the western convergence rate should increase as the faults ramp down- of the Oak Ridge axial surface (A) and the Ventura syncline near the Montalvo anticlines. We ward to the north, because slip is consumed speculate that this trend line, marked by offshore bathymetry, onshore displacement transfer, in folding and back thrusting in the northern and converging fold and fault trends, may be a segment boundary of the underlying Channel channel along the Pitas Point, Dos Cuadras, Islands fault ramp. Bathymétrie contour intervals equal 10 m to depth of 50 m, and 100 m to and Summerland Offshore folds and the on- maximum depth. MAT = Montalvo anticlines; ORF = Oak Ridge fault. shore Transverse Ranges (Fig. 1). The combined Pliocene and Quaternary fault slip rates calculated for the channel slip magnitude. The limb width corresponds fore, a plot of the limb widths versus the age faults (2.6 mm/yr) may account only for a to an ~ll-km minimum oblique slip estimate of selected growth horizons reveals the rate fraction of the northeast-southwest shorten- whose trace is oriented at —020°, consistent of fault slip (Fig. 21). ing across the channel measured geodetically with axial-surface maps (Fig. 17), regional Since the end of the deposition of the (6 ± 1 mm/yr, Larson and Webb, 1992; 6.4 ± stress directions (Mount, 1989), and the geo- Pliocene Repetto Formation (ca. 3 Ma; 0.9 mm/yr, Larsen and others, 1993) (as detic shortening direction (Larson and Webb, Yeats, 1983), the Channel Islands fault has aseismic slip on décollement fault segments 1992; Larsen and others, 1993). In the growth slipped at an average rate of —1.3 mm/yr or stored as elastic strain between large earth- section, the limb widths of selected dated ho- (Fig. 21). This rate was calculated using es- quakes). This combined slip also may accom- rizons reveal the amount of dip slip on the timated ages of three distinct growth horizons modate only about one quarter of the regional underlying fault after their deposition. There- and assuming that the fault remains active shortening directed normal to the San An-

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119°30'

bathymétrie contour interval (*) faults mapped by Luyendyk et al., (1982) and Luyendyk et al., (1983). (for additional symbols, see Figs. 1, 10A)

Figure 20. A map of selected axial surfaces along the Blue Bottle trend (Fig. 7) generated by the parallel projection method (Fig. 14). Roughly constant kink-band width along strike of the trend suggests constant subsurface geomeby and slip (—13 km) on the underlying Pitas Point and Montalvo faults (Fig. 14B). The pronounced widening of the trend south of Santa Barbara is caused by refolding of the shallow Blue Bottle trend in the underlying Oak Ridge kink band.

dreas fault (Demets and others, 1987). This including the 1978 M, = 5.1 Santa Barbara sections can be combined to yield estimates shortening resolves the normal component of earthquake (Corbett and Johnson, 1982; of the size and recurrence intervals of poten- the vector that describes the discrepancy Yeats and Olson, 1984) (Fig. 17), suggest that tially dangerous earthquakes on these active between the calculated relative Pacific-North most of the additional shortening may be ac- blind faults. The area of slip in these potential American plate motion and estimates of slip commodated by thrusting and reverse fault- seismic ruptures can be estimated from the rates on the San Andreas fault. Combined, ing along the Santa Barbara coastline. size of unsegmented fault surfaces located on these geodetic and relative plate motion rates axial-surface maps (Fig. 17). As in the New suggest that other faults in the western Trans- EARTHQUAKE MAGNITUDES AND Idria-Coalinga-Kettleman Hills earthquake verse Ranges probably accommodate addi- RECURRENCE INTERVALS sequence (Stein and Ekstrom, 1992), fault tional shortening. High present-day conver- tears or cross structures that segment faults gence rates across the eastern channel Method may limit the area ruptured in single seismic (Larson and Webb, 1992; Larsen and others, events. Using relations between fault surface 1993), abundant tight folds (Jackson and The detailed structural analysis and fault rupture area and earthquake magnitude Yeats, 1982) (Fig. 1), and recent seismicity, slip information presented in the preceding (Utsu and Seki, 1954; Kanamori and Ander-

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through the following relationship of Kan- Santa Cruz Island (Fig. 11). This ramp prob- amori and Anderson (1975)3: ably does not contain additional bends north of Santa Cruz Island, because they would ne- log S = Ms - 4.0 cessitate active folding within the offshore Oxnard shelf, which is not recognized in nu- In addition, the amount of slip during these merous regional seismic profiles. The area of potential seismic events may be estimated by the central segment of the Channel Islands comparing them to modern earthquakes of ramp (1,900 km2) can therefore be measured similar size in the region. Earthquake recur- using the cross section (Fig. 11) and axial- rence intervals may then be estimated by di- surface map (Fig. 22). If, however, a single viding the amount of slip released in an event rupture of the fault ramp extended beyond by the long-term slip rate on the causative our proposed segment boundaries, then the fault. rupture area would increase. The present lim- In the following analysis of potential earth- its of our data suggest a minimum total area of 2,200 km2 for the Channel Islands fault ramp B Montalvo and Pitas Point Thrusts quake magnitude and recurrence interval for the Channel Islands thrust, we discuss seis- (Fig. 22). mic rupture of only the fault ramp, and not Using the relationship of Kanamori and the associated near-horizontal décollement, Anderson (1975), a seismic rupture on this based on the record of major thrust earth- 1,900 km2 fault ramp should generate a Ms = quakes in southern California. Most recent 7.2 to 7.3 earthquake. Similarly, an earth- major earthquakes in southern California quake that would rupture the entire mapped fold-and-thrust belts, including the Santa extent of the fault ramp (2,200 km2) yields a Barbara Channel (1978 Santa Barbara, M, = similar magnitude prediction (Ms = 7.3-7.4). 5.1 [Corbett and Johnson, 1982]; 1973 Ana- The amount of slip released in such an earth- capa, M, = 5.0 [Gawthrop, 1975]) and the quake can be estimated by comparing it to greater Los Angeles area (including the 1989 recent earthquakes of similar magnitude for Age (Ma) Malibu, M, = 5.0; 1987 Whittier Narrows, M, which the slip amounts are known. As an ex- = 5.9; 1971 San Fernando, M, = 6.4; and oth- ample, the 1989 Loma Prieta event (Ms = 7.1) Figure 21. A plot of fault slip (S*) since the ers [see Hauksson, 1990]) suggest dipping- in central California released ~2 m of slip deposition of growth horizons versus the ages preferred nodal planes consistent with rup- (Lisowski and others, 1990; Marshall and of the horizons (see Fig. 5). Fault slip was de- tured fault ramps. This general lack of others, 1991). Similarly, the 1968 Iran (Ms = termined from the limb widths (L) of folded horizontal-preferred nodal planes for large 7.3), 1967 Turkey (Ms = 7.1), 1966 Aleutian horizons in growth triangles (Fig. 13). To yield earthquakes suggests that near-horizontal dé- (Ms = 7.2), and 1954 Fairview (Ms = 7.1) true fault slip, limb widths have been corrected collement fault segments may, in general, slip earthquakes all had between 1.6 and 2.1 m of for the obliquity of the long-term slip vectors aseismically or through combined small seis- coseismic displacement (Geller, 1976). As- on the faults (L X 1.1 = S*; Figure 17). (A) mic events. Therefore, fault ramp segments suming that the Channel Islands fault ramp The slope of the best-fit line through the data likely pose the greatest risk of generating dan- periodically ruptures in a Ms —7.2 event that 4 suggests a 1.3 mm/yr Pliocene-Quaternary slip gerous earthquakes. releases 2 m of slip , then the earthquakes rate on the Channel Islands thrust beneath the must recur roughly every 1,500 yr to accom- Offshore Oak Ridge trend. (B) Two horizons The Channel Islands Thrust modate a long term slip rate of 1.3 mm/yr. along the Blue Bottle trend also suggest a 1-3 Alternatively, if less slip is released in these mm/yr (minimum) Quaternary slip rate on the The axial-surface map of the Offshore Oak periodic events, then the recurrence interval shallower Montalvo and Pitas Point faults Ridge trend (Fig. 22) reveals the unseg- would decrease proportionally (an increase in combined. Horizons: 1 = top of the Repetto mented area of the underlying Channel Is- recurrence rate). Fin., age from Yeats (1983); 2 = biostrati- lands thrust ramp. In our previous discussion Although this estimated recurrence rate is graphic horizon (1) of Yeats (1983); 3 = top of of the structural geometry of the Offshore reassuring, an event of this magnitude still the lower Pico Fin., age from top of the Ven- Oak Ridge trend, we defined two segment poses a significant hazard to adjacent popu- turian stage of Lagoe and Thompson (1988); boundaries of the fault ramp, one in the chan- lated regions. The recurrence rate does not 4 = biostratigraphic horizon (5) of Yeats nel south of Santa Barbara (Fig. 17) and the provide a specific prediction of the time of the (1983); S = seafloor. other onshore in the Ventura basin (Fig. 19). next earthquake on this fault segment be- Furthermore, the depth of the fault bend and cause, at present, we do not know the last the dip of overlying kink band suggest that time the fault segment ruptured. Moreover, the fault must approach the seafloor south of this result represents the potential of only one

son, 1975), estimates of the size of unseg- 3This relation was derived for circular ruptures 4A 2-m slip event on the 1,900 km2 thrust ramp mented fault surfaces yield predictions of the assuming complete stress drop and "is essentially would generate a M„ = 7.3 event using the mo- the Utsu and Seki (1954)" empirical relation that is ment magnitude scale of Kanamori (1978) and seis- size of potential seismic fault ruptures. Un- based on both dip-slip and strike-slip earthquakes mic moment of Aki (1966), assuming a rigidity con- segmented fault surface areas may be used to (Kanamori and Anderson, 1975). (S is the rupture stant of 3 x 1011 dyne/cm2. See Namson and Davis 2 estimate potential earthquake magnitudes area in km ; Ms is surface wave magnitude). (1990) for an additional application.

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T7TT7TT7TT7r7T?TrrTT119°30' W ,L-^HBK' ' " * 'V.1.1 rrr> Santa Barbara • y~iüi-c< .• : I I , * S y jtf ****************** _ _ hj rt \ ' ' i ' .Iti //J VV. .... S^U^t*********)*********** > ./I• /f".

f****//////////A1 Fault Ramp Area (1+2) - 2200km

Ventura

Basin J

Fault Ramp Area (1) = 1900km Santa Cruz Island

/ Ana capa Islands

• • = active a*ia] surface O • = inactive axial surface (symbols mark, control pninr.t)

fault ramp area 1 |

fault ramp area 2 bathymetrig contour interval = 100m (for additional symbols, see Figs. UOA)

Figure 22. The axial-surface map of the Oak Ridge kink band (see Fig. 17) outlines the area of the underlying Channel Islands thrust ramp that approaches the seafloor south of Santa Cruz Island (Fig. 11). The sharp change in trend and width of the kink band (A-A') south of Santa Barbara overlies a change in fault geometry that may terminate a seismic rupture of the thrust ramp. The fault ramp area east of this transition (fault area 1 = 1,900 km2) and the entire mapped fault ramp area (fault area 1 + 2 = 2,200 km2) are used to predict the magnitude of earthquakes that may occur along this blind thrust.

segment of the Channel Islands thrust, and trends that deform Quaternary sediments, lower depths (2-5 km) along the Blue Bottle surely the adjacent segments of this fault, and the Offshore Oak Ridge and Blue Bottle trend. These faults ramp downward in the other blind-thrust faults near the Santa Bar- trends in the eastern Santa Barbara Channel, northern channel and along the Santa Bar- bara coastline (including the Pitas Point and indicate the presence of underlying, active bara coastline, where they may pose a signif- Montalvo faults at depth) also pose similar thrusts. The structural trends are represented icant seismic hazard. hazards. in balanced models and cross sections that Syntectonic sediments deposited on these integrate subsurface seismic reflection pro- structures produce distinctive growth trian- SUMMARY AND CONCLUSIONS files, well log data, and regional seismicity. gles predicted by growth-fault-bend fold the- The Offshore Oak Ridge trend is generated ory (Suppe and others, 1992). Independent We developed and applied new methods of by slip on an oblique, left-lateral thrust that age estimates of growth horizons in these tri- two- and three-dimensional structural analy- ramps upward from a depth of at least 16 km angles yield a Pliocene and Quaternary slip sis for the assessment of earthquake hazards and uplifts the Santa Cruz and Anacapa Is- rate of 1.3 mm/yr on the Channel Islands in active fold-and-thrust belts. Two fold lands. Active faults are also present at shal- thrust beneath the Offshore Oak Ridge trend.

Geological Society of America Bulletin, May 1994 625

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