2005 SCEC Progress Report

Fault Interaction within Major Strike-Slip Fault Restraining BendsSouthern California Region

Principal InvestigatorMark R. Legg

Legg Geophysical, Huntington Beach, California

IntroductionThis project examines the 3-D geometry and deformation history of a major restraining bend pop-up structurelocated offshore southern California S Santa Catalina Island. The Catalina fault links two major southernCalifornia right-slip fault systems, the Santa Cruz-Catalina Ridge and the San Diego Trough fault zones(Figure 1). The Catalina fault restraining bend is about 100 km in length, and considered capable of large

earthquakes, M=7.0-7.6 (Table1; Legg et al. 2004). The SanClemente fault zone alsomerges with the Catalina faultat the northwest corner ofCatalina Basin exhibitinggeometry resembling that ofthe San Jacinto fault zonewhere it merges with the SanAndreas fault near Cajon Pass. A high-resolution record ofthe long-term geologicalinteraction between faults atthe intersection of the SanClemente, Catalina, and SantaCruz-Catalina Ridge faultzones is preserved andrecorded in seismic reflectionprofiles. High-resolutionmultibeam swath bathymetryin this area allows accuratemapping of the faultmorphology. We use arelatively dense grid of public-domain seismic reflectionprofiles exists to study theupper crustal structure of theSanta Catalina Island pop-upand the complex deformationat the major fault intersection.

Figure 1. Map showing major segments of the San Clemente fault system offshore southern California andnorthern Baja California. See Table 1 for segment names and fault parameters.

Table 1. Major segments of the San Clemente Fault System.

No. Fault Name Length(km)

Magnitude(max)

Slip Rate(mm/yr)

1 Santa Cruz - Catalina Ridge 84.9 7.3 ?

2 Catalina Ridge 67.8 7.2 ?

3 Catalina Escarpment 107.7 7.4 ?

4 East San Clemente 49.8 7.1 ?

5 North San Clemente Ridge 35.0 6.9 ?

6 San Clemente Ridge 99.1 7.4 ?

7 San Clemente Island 138.2 7.6 5?

8 San Clemente (Navy Fan) 40.2 7.0 ?

9 Bend Region 130.9 7.5 5?

10 San Isidro 303.3 7.9 5?

11 San Diego Trough 241.6 7.8 ?

Figure 2. Stratigraphic sections in the vicinity of NW Catalina Basin.StratigraphyStratigraphic control (Figure 2) onmajor sedimentary sequencesimaged in the seismic reflectionprofiles is provided by bottomsamples of the U.S. GeologicalSurvey (Vedder et al, 1974; Vedder,1987) and a shallow borehole inSanta Monica Basin, ODP-1015,which provides control on latestPleistocene sediments (Lyle et al,1997; Normark and McGann, 2004;Normark et al, 2004). Well-beddedsedimentary sequences are imagedon seismic reflection profiles in thearea (Figure 4). These sequencesare inferred to represent Mioceneand younger turbidites andhemipelagic sediments correlativewith the Monterey, “Repetto”, and“Pico” facies of the Los Angeles,Santa Monica, and San Pedro basins(Figures 3 and 4). Acousticallytransparent units (few internalreflectors) are interpreted tor ep resen t more c l ay - r i chhemipelagic units, and well-bedded

units with long, continuous and coherent reflectors are considered turbidites. The shallowest unit in SantaMonica Basin consists of latest Pleistocene to Holocene turbidites from the Hueneme and other submarinefans along the northern and northeastern margins of the basin (Normark et al, 2004). The upper 150 m ofsediments cored in ODP-1015 is considered to be younger than about 60 ka (Lyle et al, 1997), so that thesehighly reflective turbidite units are latest Quaternary age. The less reflective hemipelagic units on thenortheast flank of the Santa Cruz-Catalina Ridge have been mapped as early Pliocene to late Miocene age,(Vedder et al, 1986). However, our high-resolution seismic profiles show that the shallowest sediments inthis area merge into or interfinger with late Quaternary to Holocene turbidites in Santa Monica Bay.

DataHigh-resolution multibeam swath bathymetry compiled by Oregon State University ( C. Goldfinger & J.Chaytor, pers. commun., 2005) provide exceptional images of the seafloor morphology along the Catalinafault, Santa Cruz-Catalina Ridge and the northwest end of Catalina Basin (Figures 3 & 5). Lateral resolutionof seafloor features is approximately 50 m to 100 m depending on water depth, and vertical resolution is lessthan one meter. Data shown in this report have been gridded at a 100-m horizontal interval and mapped usinga Mercator projection. The bathymetry image was geo-registered into our Geographic Information System(GIS) so that shot point navigation, earthquake epicenters, and other spatial data may be overlaid to produceappropriate maps for this study (Figures 3 & 5). Lastly, the seafloor geomorphology was used in conjunctionwith the seismic reflection profiles to map the major fault traces and other geologic structure.

Multichannel seismic reflection profiles provide the primary subsurface images used to map faulting andshallow geologic structure. These data include moderate penetration profiles from the USGS (cruise L4-90-SC), some recently released Western GECO profiles (w1-70-sc), and older (1970s vintage) single channelanalog sparker profiles from the USGS archives. In addition, eight high-resolution, 24-channel, mini-sparkerprofiles along the southern Santa Cruz-Catalina Ridge (e.g., Figure 4) were provided by Oregon StateUniversity (C. Goldfinger, pers. commun., 2004). The latter profiles were processed through stack and post-stack migration at Legg Geophysical using the commercial software VISTA (GEDCO, Seismic ImagingSolutions). Depth converted profiles were loaded into the interpretation workstation, Geographix andSeisVision (Landmark). Velocity control for the depth conversion was based on the stacking velocityanalysis, wide-angle reflection and refraction seismic profiles obtained in the Borderland (Moore, 1969; Shoret al, 1976).

ResultsThe major faults of the San Clemente fault system, including the Santa Cruz-Catalina Ridge, Catalina, SanClemente, and San Diego Trough faults are near vertical right-slip faults (Figure 1 and Table 1). Non-verticaldip is interpreted for parts of the Catalina fault based upon elastic dislocation modeling of the Santa CatalinaIsland structural uplift (Legg et al (2004). The MCS profiles confirm the vertical dip of the major right-slipfaults, although complex “palm tree” structures exist where the main fault branches into multiple near surfacefaults. The overall length of the San Clemente fault system exceeds 600 km, and cascading ruptures alongmultiple fault segments may occur resulting in major (M>7.5) earthquakes. The well-defined fault characterexpressed in the seafloor geomorphology, with continuous fault scarps that extend for tens of kilometers withheights of hundreds of meters, demonstrates the late Quaternary fault activity. Youthful sediments partlyfilling elongate basins along the Santa Cruz-Catalina Ridge are displaced by the major fault segments.Abundant seismicity including the 1981 Santa Barbara Island and 1986 Offshore Oceanside earthquakesequences occurs along the major faults in this area (Figure 5), manifestation of the continued faultmovement. The gap in seismicity apparent between these two historical sequences may result from lockingof the Catalina fault restraining bend until future large earthquake ruptures between the San Diego Troughand Santa Cruz-Catalina Ridge occur. Such large earthquakes would likely cause severe damage to theadjacent coastal areas, especially if a local tsunami were generated (Legg et al, 2004).

Figure 3. High-resolution bathymetry of the intersection between the San Clemente fault and the Catalina fault. Red lineshows location of high-resolution seismic profile (mini-sparker source, 24-channel streamer).

Figure 4. High-resolution 24-channel seismic profile across the southeastern Santa Cruz-Catalina Ridge showing majorfaults and deformed sedimentary sequences.

LEGG2005

The San Clemente fault merges with the Catalina and Santa Cruz-Catalina Ridge faults at the northwestcorner of Catalina Basin. High-resolution multibeam swath bathymetry provides a detailed map of the majorfaults at this intersection (Figure 3). The Catalina fault steps to the right near Farnsworth Bank and proceedsnorthwest along Catalina Ridge. An elongate, triangular-shaped block appears to be offset about 20 kmforming a pull-apart basin in its wake at this releasing step-over Where the San Clemente fault branches tothe northeast cutting diagonally across the northwest corner of Catalina Basin, another deep pull-apart basinoccurs. Farther north, where the East San Clemente fault nears the Catalina Ridge fault, narrow elongatebasins between these two major fault segments exist and are inferred to be transtensional sags or pull-apartbasins. Transfer of right-slip from the San Clemente fault zone to the Catalina Ridge fault is inferred to formthese structural depressions, similar to the Cajon Pass region where the San Jacinto fault zone merges withthe San Andreas fault. These basins have been uplifted by transpression along the Santa Catalina Islandrestraining bend and the Santa Cruz-Catalina Ridge.

Fault-normal shortening has uplifted Miocene and younger sedimentary sequences along the Santa Cruz-Catalina Ridge (Figure 3 and 4). To the southeast, where the San Clemente fault converges toward theCatalina Ridge fault, a complex set of northwest-trending folds and reverse faults occurs that likely representslocal termination of the San Clemente fault as well as overall transpression along the Santa Cruz-CatalinaRidge. Uplift of well-bedded turbidites along the northeast flank of the Santa Cruz-Catalina Ridge shows thattranspression initiated in Pliocene time, although the major uplift appears to be a Quaternary event. Thegreatest uplift is located along the Santa Cruz-Catalina Ridge fault zone where acoustic basement of eitherMiocene volcanic rocks or Catalina Schist crops out in fault slivers and the late Miocene to early Pleistoceneturbidites with moderate northeast dips (30 to 45 degrees) are truncated at the seafloor. On some high-resolution MSC profiles, well-bedded reflectors inferred to be late Pleistocene turbidites conformably overlaythe Tertiary sequences and are uplifted more than 400 meters. These data suggest that much of the upliftalong the Santa Cruz-Catalina Ridge occurred during late Quaternary time, possibly related to the Pasadenanorogeny. Subsidence of the northeast margin of Catalina Basin, with local uplift at the Catalina fault involvesthe youngest basin sediments and implies continuing uplift of the Santa Catalina Island restraining bend pop-up. Further research is needed to constrain more accurately the age of these late Quaternary sequencesuplifted along the Santa Catalina Island restraining bend and the Santa Cruz-Catalina Ridge fault zone. Suchconstraint will permit estimates of the slip rate along these important offshore faults.

ReferencesAstiz, L. and Shearer, P.M., 2000, Earthquake locations in the inner Continental Borderland offshore southern

California: Seismological Society of America Bulletin, v. 90, p. 425-449.Corbett, E. J., 1984, Seismicity and crustal structure studies of southern California -- Tectonic implications from

improved earthquake locations [unpublished Ph.D. dissertation]: California Institute of Technology, Pasadena,California, 231 p.

Hauksson, E., and Gross, S., 1991, Source parameters of the 1933 Long Beach earthquake: Seismological Society ofAmerica Bulleting, v. 81, p. 81-90.

Hauksson, E., and jones, L. M., 1988, The July 1986 Oceanside (ML=5.3) earthquake sequence in the continentalborderland, southern California: Seismological Society of America Bulletin, v. 78, p. 1885-1906.

Kennedy, M. P., Greene, H. G., and Clarke, S. H., 1987, Geology of the California continental margin: Explanation ofthe continental margin geologic map series: California Division of Mines and Geology Bulletin 207, 110 p.

Legg, M.R., Borrero, J.C., and Synolakis, C.E., 2004, Tsunami hazards associated with the Catalina fault in southernCalifornia: Earthquake Spectra, vol. 20, p. 917-950.

Legg, M.R., Nicholson, C., Goldfinger, C., Milstein, R., and Kamerling, M., 2004, Large enigmatic crater structuresoffshore southern California: Geophysical Journal International, vol. 158, p. 803-815.

Lyle, M. Koizumi, L., Richter, C., et al., 1997, Proceedings of the Ocean Drilling Program, Initial Reports, vol. 167,Ch. 9. Site 1015, p. 223-237.

Moore, D. G., 1969, Reflection profiling studies of the California Continental Borderland--Structure and Quaternaryturbidite basins: Geological Society of America Special Paper 107, 142 p.

Santa CatalinaIsland

1933

19861986

Catalina Escarpment

Catalina Escarpment

San Clemente Island

San Clemente Island

Catalina Ridge

Catalina Ridge

EmeryKnoll

Offset RimEmery Knoll

Crater

Santa Cruz -

Santa Cruz -

Catalina Ridge

Catalina Ridge1981

Palos Verdes

Palos Verdes

San Pedro Basin

San Pedro Basin

Los Angeles

Coronado Bank

Coronado Bank

San Diego Trough

San Diego Trough

Figure 5. Map showing Santa Catalina Island and vicinity with major faults and earthquake epicenters. Solid circles are relocated epicentersfor the 1981 and 1986 earthquake sequences (Astiz & Shearer, 2000). Small stars are SCSN locations of M4 or larger events (1932-1998), andlarge stars show locations of moderate earthquakes (M~6) based on special studies (Corbett, 1984; Hauksson & Gross, 1991; Hauksson & Jones, 1988). Offset rim of Emery Knoll Crater shows 60 km of post-Middle Miocene right-slip along the San Clemente fault (Legg et al, 2004).

Normark, W.R., Baher, S., and Sliter, R., 2004, Late Quaternary sedimentation and deformation in Santa Monica andCatalina Basins, offshore southern California: in Legg, M., Davis, P., and Gath, E., eds., Geology and Tectonicsof Santa Catalina Island and the California Continental Borderland, South Coast Geological Survey Annual FieldTrip Guidebook No. 32, p. 291-317.

Normark, W.R., and McGann, M., 2004, Late Quaternary deposition in the inner basins of the California ContinentalBorderland--Part A. Santa Monica Basin: U.S. Geological Survey, Scientific Investigations Report 2004-5183,http://pubs.usgs.gov/sir/2004/5183.

Shor, G. G., Jr., Raitt, R. W., and McGowan, D. D., 1976, Seismic refraction studies in the southern Californiaborderland, 1949-1974: Scripps Institute of Oceanography Reference 76-13, 70 p.

Vedder, J. G., 1987, Regional geology and petroleum potential of the southern California borderland: in Scholl, D. W.,Grantz, A., and Vedder, J. G., eds., Geology and resource potential of the continental margin of western NorthAmerica and adjacent ocean basins, Beaufort Sea to Baja California: Circum-Pacific Council for Energy andMineral Resources, Houston, Texas, Earth Science Series, volume 6, p. 403-447.

Vedder, J. G., Beyer, L. A., Junger, A., Moore, G. W., Roberts, A. E., Taylor, J. C., and Wagner, H. C., 1974,Preliminary report on the geology of the Continental Borderland of southern California: U.S. Geological Survey,Miscellaneous Field Studies Map MF-624, Scale 1:500,000.

Vedder, J. G., Greene, H. G., Clarke, S. H., and Kennedy, M. P., 1986, Geologic map of the mid-southern Californiacontinental margin: California Division of Mines and Geology, California Continental Margin Geologic MapSeries, Area 2 of 7, sheet 1 of 4, scale 1:250,000.

Publications(Presented September 28, 2005, Geological Society of London International Conference: Tectonics of Strike-Slip Restraining & Releasing Bends in Continental & Oceanic Settings, Cunningham, D., and Mann, P., eds.;Manuscripts submitted for publication in monograph December 31, 2005; anticipated publication date:December 2006)Legg, M.R., and Kamerling, M.J., 2006, Strike-slip restraining and releasing bends in the evolving Pacific-

North America transform in southern California.Legg, M.R., Goldfinger, C., Einstein, D.E., Chaytor, J., and Wang, H.D., 2006, Geomorphology and shallow

crustal structure of restraining bends: California Continental Borderland examples.