Earthquakes on the Ventura Blind Thrust Fault: Implications for Multifault Ruptures in the Transverse Ranges of Southern California

This is a repository copy of Paleoseismologic evidence for large-magnitude (M-w 7.5-8.0) earthquakes on the Ventura blind thrust fault: Implications for multifault ruptures in the Transverse Ranges of southern California. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/108547/ Version: Accepted Version Article: McAuliffe, L. J., Dolan, J. F., Rhodes, E. J. et al. (3 more authors) (2015) Paleoseismologic evidence for large-magnitude (M-w 7.5-8.0) earthquakes on the Ventura blind thrust fault: Implications for multifault ruptures in the Transverse Ranges of southern California. Geosphere, 11 (5). pp. 1629-1650. ISSN 1553-040X https://doi.org/10.1130/GES01123.1 Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ 1 Paleoseismologic evidence for large-magnitude (Mw 7.5–8.0) 2 earthquakes on the Ventura blind thrust fault: Implications for 3 multifault ruptures in the Transverse Ranges of southern California 4 5 Lee J. McAuliffe1 6 James F. Dolan1 7 Ed Rhodes2,6 8 Judith Hubbard3,5 9 John H. Shaw3 10 Thomas L. Pratt4 11 12 1Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089 13 2Department of Earth, Planetary and Space Sciences, University of California Los Angeles, Los 14 Angeles, CA 90095 15 3Department of Earth & Planetary Sciences, Harvard University, Cambridge, MA 02138 16 4U.S. Geological Survey, Reston, VA 17 5Earth Observatory of Singapore, Nanyang Technical University, Singapore 18 6Department of Geography, University of Sheffield, Sheffield, S10 2TN, UK 19 20 Keywords: Paleoseismology, neotectonics, western Transverse Ranges, Ventura fault, San 21 Cayetano fault, blind thrust fault 22 23 24 Abstract 25 Detailed analysis of high-resolution seismic reflection data, continuously cored boreholes 26 and cone penetrometer tests (CPTs), and luminescence and 14C dates from Holocene strata folded 27 above the tip of the Ventura blind thrust fault constrain the ages and displacements in the two 28 most recent earthquakes. These two earthquakes are recorded by a prominent surface fold scarp 29 and a stratigraphic sequence that thickens across an older buried fold scarp, and occurred soon 30 after about 700-900 years ago (most recent event) and between 3-5 ka (penultimate event). 31 Minimum uplift in these two scarp-forming events was ~6 m for the most recent earthquake and 32 ~4.5 meters for the penultimate event. Individual uplifts of this amount require large magnitude 33 earthquakes, probably in excess of Mw7.5 and likely involving rupture of the Ventura fault 34 together with other Transverse Ranges faults to the east and west. The proximity of this large 35 reverse-fault system to major population centers, including the greater Los Angeles region, and 36 the potential for tsunami generation during ruptures extending offshore along the western parts 37 of the system, highlight the importance of understanding the complex behavior of these faults for 38 probabilistic seismic hazard assessment. 39 40 1 Introduction 41 1.1 Recognition of emerging thrust fault hazards 42 The recognition of the hazards posed by thrust fault earthquakes to urban centers around 43 the world has been highlighted by several recent events (e.g., 1994 Mw 6.7 Northridge, 1999 Mw 44 7.6 Chi-Chi, 2005 Mw 7.5 Kashmir, 2008 Mw 7.9 Wenchuan). These earthquakes demonstrate 45 the need to better understand the behavior of these faults and their associated folds, particularly 46 when these faults are “blind”, that is, the faults do not reach (or “see”) the surface. The 2008 Mw 47 7.9 Wenchuan earthquake further illustrated that thrust fault ruptures may link together adjacent 48 faults to generate extremely large-magnitude earthquakes. In southern California, the prospects 49 for large, multiple-segment thrust fault ruptures remain poorly understood. The numerous large 50 reverse and oblique-slip faults in the Transverse Ranges suggest that such large earthquakes are 51 possible, and would represent a serious hazard to property and life in the densely populated 52 southern California region. 53 Most seismic hazard assessments and models of reverse fault earthquakes in southern 54 California involve the rupture of individual faults in the Transverse Ranges in moderately large 55 magnitude (≥Mw 7) events (e.g., Sierra Madre fault, San Cayetano fault; WGCEP, 1995). 56 Although the seismic threat posed by these individual faults is significant, as was demonstrated 57 by the 1994 Mw6.7 Northridge earthquake, which was the costliest natural disaster in US history 58 prior to Hurricane Katrina [Scientists of the U.S. Geological Survey and the Southern California 59 Earthquake Center, 1994], a larger threat presents itself if several of these faults rupture together. 60 The reverse and oblique-slip faults of the Transverse Ranges form an interconnected, >200-km- 61 long network of faults that could potentially rupture together to cause large-magnitude events. 62 While the potential for these faults to link and rupture together has recently been recognized 63 (e.g., Dolan et al., 1995; Hubbard et al., 2014), relatively little is known about the ages, repeat 64 times, and magnitudes of paleo-earthquakes generated by faults within the Transverse Ranges. 65 In this paper, we apply a multi-disciplinary approach utilizing continuously cored 66 borehole and cone penetrometer test (CPT) data, in conjunction with high-resolution and deeper- 67 penetration petroleum industry seismic reflection data, to document the structural evolution of 68 young folds formed by the Ventura fault, a major reverse fault in the western Transverse Ranges. 69 Together, these new data allow us to assess the geometry of buried fold scarps and identify 70 periods of stratigraphic growth that record discrete uplift events along the Ventura fault. We use 71 these data to determine the timing and displacements of Ventura fault paleo-earthquakes and 72 discuss these results in light of their implications for assessing the prospects for multi-segment 73 ruptures in the western Transverse Ranges, and more generally for seismic hazard in southern 74 California. 75 1.2 Regional Geology 76 The western Transverse Ranges are dominated by several major east-west faults and folds 77 that extend “transverse” to the general northwesterly structural grain of coastal California. These 78 structures are evidence of the north-south compressive forces that have been responsible for the 79 observed deformation since early Pliocene time (Luyendyk et al., 1985). The deformation of 80 Pleistocene and younger deposits along a similar structural grain, together with current geodetic 81 data, illustrate the ongoing style of deformation within this region. 82 Located about 75 km northwest of Los Angeles, the Ventura basin is a narrow, ~50-km- 83 long basin bounded on both the north and south by a complex network of E-W reverse and 84 oblique-left-lateral reverse faults (Figure 1). These structures include the Oak Ridge to the south, 85 and the faults responsible for uplift of the Ventura Avenue Anticline (VAA) and Topa Topa 86 mountains to the north. The Ventura basin is ~4 km across at its widest near the city of Ventura, 87 and narrows towards its eastern end where the northern basin-bounding San Cayetano fault 88 overrides the south-dipping Oak Ridge fault (Huftile and Yeats, 1995). 89 The Ventura basin, which is >10 km thick at its deepest point, is thought to have formed 90 during the mid-Miocene clockwise rotation of the western Transverse Ranges block to its current 91 east-west trend and experienced oblique, north-south shortening since the Pliocene (Hornafius et 92 al., 1986; Jackson and Molnar, 1990; Luyendyk, 1991). Geodetically determined north-south 93 convergence across the Ventura basin is as high as 7 to 10mm/yr (Donnellan et al., 1993a; 94 1993b; Hager et al., 1999; Marshall et al., 2008). The Ventura Avenue Anticline, located on the 95 north side of the basin (Figure 1), is rising at a rapid rate of ~5 mm/yr. This structure, which is 96 underlain by the Ventura fault, is thought to accommodate much of the north-south shortening 97 across the western basin (Rockwell et al., 1988; Stein and Yeats, 1989; Hubbard et al., 2014). 98 The blind Ventura fault is a ~12 km long, east-west-striking, north-dipping reverse fault 99 that is expressed at the surface by a monoclinal fold scarp extending across the city of Ventura 100 (Ogle and Hacker, 1969; Sarna-Wojcicki et al., 1976; Yeats, 1982; Perry and Bryant, 2002). 101 Some studies suggested that the Ventura fault is a shallow fault rooted in the syncline at the 102 southern end of the Ventura Avenue Anticline (e.g., Yeats, 1982; Huftile and Yeats, 1995), and 103 thus does not pose a major earthquake threat. Others, however, have suggested that is it a 104 seismogenic structure in its own right (Sarna-Wojcicki et al., 1976; Sarna-Wojcicki and Yerkes, 105 1982).

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