Revealing Geometry and Fault Interaction on a Complex Structural System Based on 3D P-Cable Data: The San Mateo and San Onofre Trends, Offshore Southern California

Deformation observed along the San Mateo (SMT) and San Onofre trends (SOT) in southern California has been explained by two opposing structural models, which have very different hazard implications for the coastal region. One model predicts that the deformation is transpressional in a predominantly right lateral fault system with left lateral step-overs. Conversely in the alternative model, the deformation is predicted to be compressional associated with a regional blind thrust that reactivated detachment faults along the continental margin. State-of-the-art 3D P-Cable seismic data were acquired to characterize the geometry and linkage of faults in the SMT and SOT. The new observations provide evidence that deformation along the slope is more consistent with step-over geometry than a regional blind thrust model. For example, regions in the SOT exhibit small scale compressional structures that deflect canyons along jogs in the fault segments across the slope. The deformation observed in the SMT along northwesterly trending faults has a mounded, bulbous character in the swath bathymetry data with steep slopes ( ∼ 25°) separating the toe of the slope and the basin floor. The faulting and folding in the SMT are very localized and occur where the faults trend more northwesterly (average trend ∼ 285°) with the deformation dying away both towards the north and east. In comparison, the SOT faults trend more northerly (average trend ∼ 345°). The boundary between these fault systems is abrupt and characterized by shorter faults that appear to be recording right lateral displacement and possibly accommodating the deformation between the two larger fault systems. Onlapping undeformed turbidite layers reveal that the deformation associated with both major fault systems may be inactive and radiocarbon dating suggests deformation ceased in the middle to late Pleistocene (between 184 and 368 kyr). In summary, our preferred conceptual model for tectonic deformation along the SMT and SOT is best explained by left lateral step-overs along the predominantly right lateral strike-slip fault systems.

Towards understanding earthquake triggering due to enhanced geothermal systems

The effect of fluid pulse driven fractures (FPDF) propagating in poroelastic media on fault slip in the presence of natural fractures is a complicated interplay between fracture propagation, fracture-fracture interaction, fracture-fault interaction, friction model governing fault slip and wave propagation associated with pulsing injection. Furthermore, the problem is stochastic due to the uncertainty associated with the existing fracture-fault topology.

High-Resolution 3D Seismic Imaging of Fault Interaction and Deformation Offshore San Onofre, California

The Inner California Borderland (ICB) records a middle Oligocene transition from subduction to microplate capture along the southern California and Baja coast. The closest nearshore fault system, the Newport-Inglewood/Rose Canyon (NIRC) fault complex is a dextral strike-slip system that extends primarily offshore approximately 120 km from San Diego to Newport Beach, California. Holocene slip rates along the NIRC are 1.5–2.0 mm/year in the south and 0.5 mm/year along its northern extent based on trenching and well data. High-resolution 3D seismic surveys of the NIRC fault system offshore of San Onofre were acquired to define fault interaction across a prominent strike-slip step-over. The step-over deformation results in transpression that structurally controls the width of the continental shelf in this region. Shallow coring on the shelf yields a range of sedimentation rates from 0.27–0.28 mm/year. Additionally, a series of smaller anticlines and synclines record subtle changes in fault trends along with small step-overs and secondary splay faults. Finally, sedimentary units onlapping and dammed by the anticline, place constraints on the onset of deformation of this section of the NIRC fault system. Thickness estimates and radiocarbon dating yield ages of 560,000 to 575,000 years before present for the onset of deformation.

Dynamic Fault Interaction during a Fluid-Injection-Induced Earthquake: The 2017 Mw 5.5 Pohang Event

ABSTRACT The 15 November 2017 Mw 5.5 Pohang, South Korea, earthquake has been linked to hydraulic stimulation and fluid injections, making it the largest induced seismic event associated with an enhanced geothermal system. To understand its source dynamics and fault interactions, we conduct the first 3D high-resolution spontaneous dynamic rupture simulations of an induced earthquake. We account for topography, off-fault plastic deformation under depth-dependent bulk cohesion, rapid velocity weakening friction, and 1D subsurface structure. A guided fault reconstruction approach that clusters spatiotemporal aftershock locations (including their uncertainties) is used to identify a main and a secondary fault plane that intersect under a shallow angle of 15°. Based on simple Mohr–Coulomb failure analysis and 180 dynamic rupture experiments in which we vary local stress loading conditions, fluid pressure, and relative fault strength, we identify a preferred two-fault-plane scenario that well reproduces observations. We find that the regional far-field tectonic stress regime promotes pure strike-slip faulting, whereas local stress conditions constrained by borehole logging generate the observed thrust-faulting component. Our preferred model is characterized by overpressurized pore fluids, nonoptimally oriented but dynamically weak faults and a close-to-critical local stress state. In our model, earthquake rupture “jumps” to the secondary fault by dynamic triggering, generating a measurable non-double-couple component. Our simulations suggest that complex dynamic fault interaction may occur during fluid-injection-induced earthquakes and that local stress perturbations dominate over regional stress conditions. Therefore, our findings have important implications for seismic hazard in active georeservoir.