Ventura SFSA Field Trip Depart from Lobby (Box Lunches Provided)

Southern California Earthquake Center (SCEC) Ventura Special Fault Study Area Workshop August 15-16, 2013 Ventura, California Co-conveners: James Dolan (University of Southern California) Tom Rockwell (San Diego State University) John H. Shaw (Harvard University) Thursday 15 August 2013 07:00-08:00 Continental Breakfast La Playa Room 08:00-17:00 Ventura SFSA Field Trip Depart from Lobby (box lunches provided) 18:00-21:00 Working Dinner Puerto Escondido Room Participants present (in 2 slides, 2 minutes) their research results and/or expertise pertinent to future research in the Ventura SFSA. Friday 16 August 2013 07:00-08:00 Continental Breakfast Puerto Escondido Room 08:00-12:00 Meeting Session Puerto Escondido Room Develop a research strategy to better characterize these structures and the seismic hazards that they pose. The results of the workshop will be presented in a white paper to be finalized at the SCEC Annual Meeting. 12:00 Adjourn 1 SCEC Ventura Special Fault Study Area The Ventura fault underlies the Ventura Avenue anticline, which is one of the fastest uplifting structures in southern California, rising at a rate of ∼5 mm/yr [Rockwell et al., 1988]. Holocene terraces on the anticline suggest that it deforms in discrete events with 5-10 m of uplift, with the latest event occurring ~800 years ago (Rockwell, 2011). Moreover, recent excavations across the Ventura fault scarp, which runs through the city of Ventura, show evidence for large- displacement (> X m) paleoearthquakes in the Holocene (McAuliffe et al., 2011; 2013). The amount of uplift recorded by the terraces and in the trenches would require large earthquakes (M7.5-8.0), suggesting that the Ventura fault ruptures in conjunction with other faults in the Transverse Ranges. Subsurface studies suggest that the Ventura fault is linked at seismogeneic depths with the San Cayetano fault to the east (Hubbard et al., 2011; 2013) and the Pitas Point and Red Mountain fault to the west (Figs. 1 and 2) (Sarna-Wojcicki et al., 1976, 1982; Yerkes and Lee, 1987; Yerkes et al., 1987; Kamerling et al., 1999; 2003; Hubbard et al., 2011; 2013). Thus, the Ventura fault represents an important linkage between some of the largest, fastest- slipping reverse faults in the Western Transverse Ranges. Given the availability of geological, geophysical, seismological, and geodetic data in the region, the Ventura fault offers an excellent natural laboratory to investigate the potential for multi-segment thrust fault earthquakes and the hazards that they pose (Figs. 1 and 2). These hazards include the prospect of severe ground shaking due to the extreme depth of the Ventura basin (> 10 km), as well as the potential for strong regional tsunamis when the ruptures extend offshore. Goals for the Workshop The Ventura Special Fault Study Area was established to promote interdisciplinary science that seeks to better understand the prospects for large multi-segment earthquakes in southern California, and to assess and address the hazards that these potentially devastating earthquakes may pose. At this initial workshop, we seek to: 1) introduce scientists to the geological expression of the Ventura fault and Ventura Avenue anticline 2) discuss our current state of knowledge about the fault systems, its geometry, tectonic history, activity, slip rate, and paleoearthquake history 3) have participants share their insights about the fault system and expertise that might contribute to interdisciplinary studies 4) develop an initial workplan to promote collaborative research by SCEC and other agencies 2 Figure 1: View of the SCEC Community Fault Model (CFM 4.0) showing location of the Ventura fault (dark blue) in relation to other structures in the Transverse Ranges. The Pitas Point fault and the southern San Cayetano faults (light blue) lie west and east of the Ventura fault, respectively. Together these faults comprise the Ventura - Pitas Point fault system. Together with the Ventura-Pitas Point faults system, a series of east-west-striking reverse faults (shown in red) form a nearly continuous trend across the Transverse Ranges. 3 Figure 2: Map of the Ventura region. Red lines are mapped faults; teeth denote thrust fault hanging wall. The Ventura-Pitas Point fault is called the Ventura fault onshore and the Pitas Point fault offshore. Blue circles show the locations of wells with evidence of an intersection with the Ventura fault. Black lines show seismic reflection profiles crossing the Ventura fault (left is industry line VB1, center is Evergreen/Hall Canyon, and right is Brookshire profile). Box with hash marks shows the location of the 3D seismic reflection volume across the Dos Cuadras structure. The anticline in the hanging wall of the Ventura-Pitas Point fault is marked with a dashed line; this also represents a chain of oil fields, marked with yellow squares: VOF=Ventura Oil Field; ROF=Rincon Oil Field; COF=Carpineteria Oil Field; DCOF=Dos Cuadras Oil Field. North of the Ventura fault, this is called the Ventura Avenue anticline. (from Hubbard et al., 2013). 4 Figure 3: Geologic map and cross section of the Ventura basin, including the Ventura Avenue Anticline, from Huftile and Yeats (1995). Note that the Ventura Avenue anticline is interpreted to be detached above a decollement in the Miocene section (Sisar decollement), with numerous thrust faults in the core of the fold that are constrained by wells. The Ventura fault is not shown, but outcrops near the base of the southern limb of the fold. 5 Figure 4: Schematic cross sections showing alternate models for the Ventura Avenue anticline and Ventura fault. (a) Cross section after Yeats (1982a) and Huftile and Yeats (1995). The Ventura Avenue anticline is a north-vergent detachment fold lifting off of the Sisar Decollement; secondary faulting in the interior of the anticline is well constrained by well data. The Ventura fault is a minor bending-moment fault in the syncline at the southern edge of the anticline. (b) Interpretation of Hubbard et al., (2011; 2013), modeled in part after Sarna-Wojcicki and Yerkes (1982). The Ventura Avenue anticline is produced as a consequence of shortening on the Ventura fault, which is a steeply dipping thrust fault rising from the Sisar Decollement. Slip on the blind thrust ramp to the north is partitioned between the Lion backthrust and the Ventura fault. See Sarna-Wojcicki et al., (1982) and Yeats (1982) (from Hubbard et al., 2013). 6 Figure 5: There are several observations that suggest that the Ventura fault extends to depth beneath the Ventura Avenue anticline, supporting model B in Figure 4. This cross section of the Ventura Avenue anticline shows the locations of fault cuts in wells, stratigraphic picks, and dipmeter data that are consistent with the projection to depth of the Ventura fault at a dip of ≈ 45-55°N. (from Hubbard et al., 2013). 7 Figure 6: Portion of industry seismic reflection profile VB1, which images the Ventura fault; (top) in two-way travel time and (bottom) depth-converted using the 1D velocity model of Brankman (2009). Arrows point to prominent reflector terminations; dashed line marks interpreted fault surface, with dipping reflectors in the hanging wall and nearly horizontal reflectors in the footwall. Line location shown in Figure 2. (from Hubbard et al., 2013). 8 Figure 7: Perspective view of the Ventura fault from the ENE, with data constraints shown. Note that all of the fault constraints are consistent with a planer, 45-55°N dipping fault surface. Red lines mark 1 km depth contours. The fault pick from industry line VB1 is shown in purple. Blue balls show fault picks from wells. Seismic profiles collected August 2010 (Brookshire and Evergreen/Hall Canyon) are shown. Red line at surface is mapped surface trace of fold scarp. Thin blue line marks the coastline. (from Hubbard et al., 2013). 9 Figure 9: Fault displacement on the Ventura Fault, as mapped on industry seismic line VB1. Displacement decreases towards the tip, and does not reach the surface. Note that in this image, line VB1 has been depth- converted and projected onto a line perpendicular to the structural trend, to correct for the fact that the line was originally acquired at a strong oblique angle. We next explore a constant-thickness fault-propagation fold model (Suppe and Medwedeff, 1990) to assess whether the known geometry and timing of uplift across the Ventura Avenue anticline are consistent with this interpretation (Shaw et al., 2005; Hughes, 2012). Fault propagation folds form as faults propagate to the surface over time, accommodating shortening by a combination of fault displacement and folding. Propagating faults typically show increasing offset with depth because the fold consumes slip on the ramp, producing zero offset at the fault tip and the greatest offset at the ramp base. The constant-thickness model for fault-propagation folds is a kinematic model that assumes angular fold hinges and conservation of bed length (Figure 11a; Suppe and Medwedeff, 1990). In such a model, the fault propagates upwards from a fault bend. An active syncline develops at the fault tip; as Figure 8: (top) Enlargedthe fault version propagates of fLineorward, VB it folds1 showing material offset up into along the forelimb. the Ventura As the fault.fault propagates (bottom), an Fault offset vs. depthanticline bsl develops,on the Ventura with material fault. moving Depth up is the measured fault into theon backlimb. the footwall, The bifurcation in meters point below sea level. Theof the upward backlimb decreasingis at the same amount stratigraphic of displacement level as the tip onof thethe propagating Ventura faulfaultt (Figure is consistent with development11a). The backlimb of the Venturadevelops likeAvenue a fault anticline-bend fold, as but a withfault-propagation a limb width that fold is greater (Shaw than et al., 2005), modified by thrust faulting in its core.

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