Hosgri Fault Zone Research Articles

Hosgri Fault Transpressional Slip Rates Reproduce Observed Central California Coast Uplift Rates

Abstract Coastal uplift is a common feature of Pacific Rim tectonic deformation. The 1989 Mw 7.0 Loma Prieta earthquake illustrated that reverse-oblique slip on a transpressive segment of the predominantly horizontal slip San Andreas transform fault produces coastal uplift. In west-central California the 400 km long San Gregorio-Hosgri fault coastal fault system of near-vertical to steeply northeast-dipping, reverse-oblique, dextral strike-slip faults also contribute to coastal uplift. Published southern Hosgri fault dextral slip rates are ∼4× reverse rates. Viscoelastic deformation modeling using published transpressional slip rates along the southern Hosgri fault reproduces observed emergent marine terrace uplift rates, offshore Hosgri fault deformation morphology, and crustal structure from San Luis Obispo Bay to Estero Bay. Our coastal uplift rate modeling is constrained by a unique combination of measurements: (1) offshore reverse- and strike-slip Hosgri fault zone (HFZ) slip rates, and (2) ≤125 ka onshore emergent terrace uplift rates adjacent to the HFZ. The previously proposed conceptual rigid block models of the southern central California coast (CCC) invoke local onshore thrust or reverse faults to explain coastal uplift. Observed transpressional offshore Hosgri fault deformation rates alone rigorously reproduce observed coastal uplift rates and crustal structure along the southern CCC.

Seismic attribute detection of faults and fluid pathways within an active strike-slip shear zone: New insights from high-resolution 3D P-Cable™ seismic data along the Hosgri Fault, offshore California

Poststack data conditioning and neural-network seismic attribute workflows are used to detect and visualize faulting and fluid migration pathways within a [Formula: see text] 3D P-Cable™ seismic volume located along the Hosgri Fault Zone offshore central California. The high-resolution 3D volume used in this study was collected in 2012 as part of Pacific Gas and Electric’s Central California Seismic Imaging Project. Three-dimensional seismic reflection data were acquired using a triple-plate boomer source (1.75 kJ) and a short-offset, 14-streamer, P-Cable system. The high-resolution seismic data were processed into a prestack time-migrated 3D volume and publically released in 2014. Postprocessing, we employed dip-steering (dip and azimuth) and structural filtering to enhance laterally continuous events and remove random noise and acquisition artifacts. In addition, the structural filtering was used to enhance laterally continuous edges, such as faults. Following data conditioning, neural-network based meta-attribute workflows were used to detect and visualize faults and probable fluid-migration pathways within the 3D seismic volume. The workflow used in this study clearly illustrates the utility of advanced attribute analysis applied to high-resolution 3D P-Cable data. For example, results from the fault attribute workflow reveal a network of splayed and convergent fault strands within an approximately 1.3 km wide shear zone that is characterized by distinctive sections of transpressional and transtensional dominance. Neural-network chimney attribute calculations indicate that fluids are concentrated along discrete faults in the transtensional zones, but appear to be more broadly distributed amongst fault bounded anticlines and structurally controlled traps in the transpressional zones. These results provide high-resolution, 3D constraints on the relationships between strike-slip fault mechanics, substrate deformation, and fluid migration along an active fault system offshore central California.

Influence of fault trend, bends, and convergence on shallow structure and geomorphology of the Hosgri strike-slip fault, offshore central California

We mapped an ∼94-km-long portion of the right-lateral Hosgri fault zone in offshore central California using a dense network of high-resolution seismic reflection profiles, marine magnetic data, and multibeam bathymetry. These data document the location, length, and continuity of multiple fault strands, highlight fault-zone heterogeneity, and demonstrate the importance of fault trend, fault bends, and fault convergence in the development of shallow structure and tectonic geomorphology along strike-slip faults. Eight sections (A through H) of the Hosgri fault are mapped. The fault trends ∼335° to 341° in the southern ∼40 km of the study area (sections A through C) where shallow deformation is primarily dilational. The absence of tectonic uplift in this area has contributed to localization of the Santa Maria River and delta and, as a result, Holocene sediments cover the fault zone. The Hosgri fault generally trends 329° to 337° in the central ∼24 km of the study area (sections D through F), which coincides with oblique convergence of the Hosgri and the more northwest-trending Los Osos and Shoreline faults. This convergence has resulted in local restraining and releasing fault bends, transpressive uplifts, and extensional basins of varying size and morphology. Notably, development of a paired fault bend is linked to indenting and bulging of the Hosgri fault by a strong crustal block translated to the northwest along the Shoreline fault. Two diverging Hosgri fault strands bounding a central uplifted block characterize the northern ∼30 km of the Hosgri fault (sections G and H) in this area. The eastern Hosgri passes through significant releasing (329° to 335°) and restraining (335° to 328°) bends before passing onland at San Simeon; the releasing bend is the primary control on development of an elongate, asymmetric, 15-km-long × 300- to 2400-m-wide, “Lazy Z” sedimentary basin. The western strand of the Hosgri fault passes through a significant restraining bend (329° to 316°) and continues northward until slip is transferred to faults underlying the Piedras Blancas fold belt. Earthquake hazard assessments should incorporate a minimum rupture length of 110 km based on continuity of the Hosgri fault zone through this area. Lateral slip rates may vary along the fault (both to the north and south) as different structures converge and diverge but are probably in the geodetically estimated range of 2–4 mm/yr.

Seismicity of South-Central Coastal California: October 1987 through January 1997

Earthquakes recorded along south-central coastal California from October 1987 through January 1997 by the Pacific Gas and Electric Company9s 20-station Central Coast Seismic Network and the U.S. Geological Survey9s Northern and Southern California Networks are used to study their relation to previous seismicity, mapped Quaternary faults, and regional tectonic features. We developed P - and S -wave velocity models, station corrections, and duration magnitude calibrations to process 1184 reliable earthquake locations and 212 well-constrained focal mechanisms. Seismicity generally coincides with mapped Quaternary faults and areas of tectonic uplift, including the southern offshore Santa Maria Basin and areas to the west, and the uplifted blocks within the Los Osos tectonic domain. Comparison of our data to previous microearthquake locations (1969–1987) and larger ( M ≥5) historical earthquakes shows that these patterns have been persistent. Seismically quiescent areas have few or no mapped Quaternary faults and typically are structural lows. The base of the seismogenic zone is about 12 km in depth, above the relic oceanic crust. Microearthquake locations and strike-slip focal mechanisms document the strike-slip nature of the Hosgri fault zone, and its step-over to the San Simeon fault, along the western boundary of the Los Osos domain. Uplifting blocks within the Los Osos domain are most seismically active toward their western ends along the Hosgri fault zone, and they exhibit reverse and oblique faulting consistent with block uplift and interaction with the Hosgri fault zone. The observed patterns and character of microseismicity and macroseismicity clearly show transpressional deformation associated with the Hosgri fault zone overprinted by rotation and westward migration of the Western Transverse Ranges. Within this tectonic setting, clockwise rotation of the Western Transverse Ranges causes internal deformation of the Los Osos domain, and westward migration of the ranges appears to deflect the Hosgri fault zone westward and causes east-west-directed crustal shortening in the southern offshore Santa Maria Basin.

Thermal-maturity trends within Franciscan rocks near Big Sur, California: Implications for offset along the San Gregorio–San Simeon–Hosgri fault zone

Conventional neotectonic interpretations place the Lucia and Point Sur subterranes of the Franciscan subduction complex on opposite sides of the San Gregorio–San Simeon–Hosgri dextral fault system and connect that system through the Sur fault zone. Our reconstructed paleotemperature contours, however, are not offset across the San Simeon segment, so differential displacement between the subterranes after peak heating appears to have been negligible. One explanation is that dextral slip on the faults has totaled only 5–10 km. A second possibility is that a discrete Hosgri–San Simeon segment extends offshore of the amalgamated Point Sur and Lucia subterranes and that an en echelon stepover transfers dextral slip eastward to the San Gregorio–Palo Colorado segment. In either case, the Sur fault zone appears to play a relatively insignificant role in the late Cenozoic tectonic evolution of central California.

Tracing the subducted oceanic crust beneath the Central California Continental Margin: Results from ocean bottom seismometers deployed during the 1986 Pacific Gas and Electric EDGE Experiment

Large aperture seismic data were collected on several ocean bottom seismometers (OBS) deployed along a deep crustal seismic profile that was shot across the central California continental margin. The line of shots extends from the oceanic crust seaward of the Santa Lucia Escarpment to the California coast near Morro Bay and crosses the Santa Lucia Basin, Santa Lucia Bank, Santa Maria Basin, and Hosgri fault zone. The OBS data permit us to trace the subducted oceanic crust from seaward of the Santa Lucia Escarpment to beneath the central part of the Santa Maria Basin. A complementary study using onshore recordings of the same shots extends the crustal model onshore. Just seaward of the Santa Lucia Escarpment, the oceanic crust is subhorizontal and covered by a thin layer of low‐velocity sediment. The velocity (4.5 km/s) and gradient (1.20–1.25 km/s/km) of the upper oceanic crust in this region are well determined and agree with earlier determinations of the crustal structure of the eastern Pacific. Beneath the Santa Lucia Escarpment and Santa Lucia Basin, the oceanic crust dips approximately 16° to the east. It is overlain by material with a velocity that increases from 4.8 to 6.4 km/s at a depth of 1.7–5.5 km below the seafloor beneath the Santa Lucia Basin. A low‐velocity zone may be sandwiched between the subducted crust and this shallow high‐velocity material, which we interpret to represent obducted oceanic crustal material. Beneath the eastern edge of Santa Lucia Basin, the dip of the subducted oceanic crust decreases to less than 2°. The configuration of the subducted crust in this region is consistent with imbrication of the subducted crust, as suggested by others based on common midpoint reflection data. Beneath the central Santa Maria Basin, the top of the subducted oceanic crust is at a depth of about 14–16 km and the Moho is at 19–21 km.

Deep structure and structural inversion along the central California Continental Margin from edge seismic profile RU‐3

Deep‐penetration seismic reflection profile RU‐3 crosses the central California continental margin revealing a subducted oceanic plate, a modified accretionary prism, and complex structures of the overlying sedimentary basins. This structural framework was established by subduction processes during Paleogene and earlier time and subsequently was modified by Neogene transform motion combined with apparent components of extension and compression. Subducted rocks are indicated by deep, gently dipping reflectors that extend beneath the continental margin for at least 38 km at a depth of about 15 km. We interpret the subducted crust as either a part of the Pacific plate or, more likely, a subducted fragment derived from the Farallon plate. A set of more steeply dipping, deep events may indicate faulting within the subducted plate or its boundary with a no‐slab zone. The overlying, largely nonreflective layer of accreted material rapidly reaches 10 km in thickness landward of the paleotrench and increases to 15 km in thickness near the coast. The Santa Lucia Basin, landward of the steep continental slope, originated as a slope basin during Paleogene subduction. The lower strata of this basin were deposited onto and partially incorporated into the accretionary complex. The offshore Santa Maria Basin exhibits a variety of compressional structures that formed in the last 3–5 m.y. and whose locations correspond to an earlier framework of extensional faults. Structural inversion has occurred in Miocene depocenters adjacent to the Santa Lucia Bank fault and at the Queenie structure. Miocene and lower Pliocene strata also thicken toward the Hosgri fault zone where subsequent compression is characterized by low‐angle thrusts and folding.

Structural Character of Hosgri Fault Zone and Adjacent Areas in Offshore Central California: ABSTRACT

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