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3D Subsurface Structural Imaging of the Irish Hills, California, Using Active Seismic, Gravity, and Magnetic Data

​22 million first-breaks from 2011-2012 Irish Hills active seismic data were jointly inverted with gravity data to build 60-m cells of 3D P-wave velocity (Vp) and density (ρ) to 3 km depth and 15x3m Vp cells <2.5 km from the Diablo Canyon Power Plant (DCPP) to 0.3 km depth

23 Apr 2015
Daniel O’Connell, Jamey Turner, Josh Goodman, Lia Lajoie, Janet Sowers, Michael Angell, Stuart Nishenko, Dengguo Zhou and Weizhong Wang


Surface mapping, 3D Vp and ρ, and magnetic attributes delineate a shallow (<1 km) network of saucer-shaped diabase sills in a 7x4 km region containing the DCPP. The Miocene diabase source conduit location inferred from the highest observed Vp (6.4 km/s) and ρ (2.8 g/cc) at 2.5 km depth is 2.5 km SE of Pt. Buchon. Moduli ratios (Vp*Vp*ρ) to ~3 km depth show the south flank of the inverted Pismo syncline basin is 2-4 times stiffer than the north side of the basin. The stiffness contrast between north and south basin sides corresponds to high-Vp and -ρ mafic Franciscan rocks and Miocene diabase intrusions in the south, whereas the north flank is comprised of low-Vp Franciscan clastic rocks to the NW, and low-Vp zones of Franciscan mélange to the N-NE providing compliant pathways for thin-skinned footwall breakout faulting involving Mesozoic basement during inversion, as observed in depth-migrated 2D/3D reflection data. Transpressional basin inversion produced shallow (< 3 km) asymmetric north-flank thin-skinned deformation similar to deformation observed in a basin-inversion sandbox experiment with a stiff basin flank and a shallow compliant boundary on the opposite side. Prominent vertical faults cross-cut transpressional folds north and south of the basin in 2D and 3D depth-migrated data, similar to 3D offshore faulting in the Pismo Syncline observed in 3D reflection and bathymetry. Vertical faults coincide with lineaments of near-vertical seismicity (4-12 km depths) including the inferred SE extension of the San Simeon fault joining the Los Osos fault at the NE side of the Irish Hills. The Shoreline fault may left step onshore onto the steeply-north-dipping Irish Canyon fault west of Pt. San Luis.


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