Geologic data suggest that the Coachella Valley segment of the southern San Andreas fault (southern California, USA) is past its average recurrence time period. At its northern edge, this right-lateral fault segment branches into the Mission Creek and Banning strands of the San Andreas fault. Depending on how rupture propagates through this region, there is the possibility of a throughgoing rupture that could lead to the channeling of damaging seismic energy into the Los Angeles Basin. The fault structures and potential rupture scenarios on these two strands differ significantly, which highlights the need to determine which strand provides a more likely rupture path and the circumstances that control this rupture path. In this study, we examine the effect of different assumptions about fault geometry and initial stress pattern on the dynamic rupture process to test multiple rupture scenarios and thus investigate the most likely path(s) of a rupture that starts on the Coachella Valley segment. We consider three types of fault geometry based on the Southern California Earthquake Center Community Fault Model, and we create a three-dimensional finite-element mesh for each of them. These three meshes are then incorporated into the finite-element method code FaultMod to compute a physical model for the rupture dynamics. We use a slip-weakening friction law, and consider different assumptions of background stress, such as constant tractions and regional stress regimes with different orientations. Both the constant and regional stress distributions show that rupture from the Coachella Valley segment is more likely to branch to the Mission Creek than to the Banning fault strand. The fault connectivity at this branch system seems to have a significant impact on the likelihood of a throughgoing rupture, with potentially significant impacts for ground motion and seismic hazard both locally and in the greater Los Angeles metropolitan area.

Oblique convergent margins host slip-partitioned faults with simultaneously active strike-slip and reverse faults. Such systems defy energetic considerations that a single oblique-slip fault accommodates deformation more efficiently than multiple faults. To investigate the development of slip partitioning, we record deformation throughout scaled experiments of wet kaolin over a low-convergence (<30°), obliquely slipping basal dislocation. The presence of a precut vertical weakness in the wet kaolin impacts the morphology of faults but is not required for slip partitioning. The experiments reveal three styles of slip partitioning development delineated by the order of faulting and the extent of slip partitioning. Low-convergence angle experiments (5°) produce strike-slip faults prior to reverse faults. In moderate-convergence experiments (10°–25°), the reverse fault forms prior to the strike-slip fault. Strike-slip faults develop either along existing weaknesses (precut or previous reverse-slip faults) or through the coalescence of new echelon cracks. The third style of local slip partitioning along two simultaneously active dipping faults is transient while global slip partitioning persists. The development of two active fault surfaces arises from changes in off-fault strain pattern after development of the first fault. With early strike-slip faults, off-fault contraction accumulates to produce a new reverse fault. Systems with early lobate reverse faults accommodate limited strike-slip and produce extension in the hanging wall, thereby promoting strike-slip faulting. The observation of persistent slip partitioning under a wide range of experimental conditions demonstrates why such systems are frequently observed in oblique convergence crustal margins around the world.

Several reports suggest that extracellular electron shuttles influence fermentative metabolism in a beneficial manner for bioremediation and biotechnology strategies. The focus of this research was to characterize the effects of reduced electron shuttling molecules on fermentative H(2) production. Reduced electron shuttles may provide reducing equivalents to generate H(2), which influences alternate cellular processes. Electron shuttling compounds cycle between reduced-oxidized states and influence fermentative physiology. Clostridium beijerinckii fermentation was altered using a physiological approach that resulted in H(2) production with the reduced extracellular electron shuttle anthrahydroquinone-2,6,-disulfonate (AH(2)QDS) and biologically reduced humic substances as the primary electron donors. Cells were suspended in a buffer with an excess of the biological electron transfer molecule NAD(+), with AH(2)QDS (100-1000 microM) or biologically reduced humic substances (0.01-0.025 g/L) as the sole electron source. Increasing concentrations of AH(2)QDS and reduced humics increased H(2) production, while H(2) production was suppressed by Fe(III) hydroxides, which outcompeted the cells for electrons from the reduced shuttles, suggesting that the shuttles are in fact electron donors for H(2) production. Oxidized AQDS/humics did not increase H(2) production. Organic acid production shifted toward butyric acid in the presence of reduced electron shuttles, particularly with growing cells. Growth and hydrogen production rates in growing cells were initially faster in the presence of the reduced electron shuttles; however, the final biomass yield was inversely proportional to the starting AH(2)QDS concentration, which suggests that reduced shuttles may compete with anabolic cell processes for available energetic resources or that the shift to excess butyrate becomes toxic to the cells.

Earthquake hazard assessments rely on observations from the field and geophysical data that provide fault slip rate estimates at specific sites and inform the geometry of active faults; however, uncertainty remains for both slip rate and geometry. Furthermore, incompatibilities between inferred fault geometry and geologic slip rates arise within crustal deformation models where model and geologic slip rates disagree. The impact of these incompatibilities may be local to sites or have wider effect on the fault system deformation. Here, we investigate the roles of structural position of sites and uncertainty of slip rates using three-dimensional mechanical models that simulate deformation across many earthquake cycles along southern San Andreas fault near the San Gorgonio Pass in California. Within the models, the impact of strike-slip rate sites on the fault system depends on their structural positions. Slip rates at sites along short and segmented faults has lesser impact on the slip along the fault system than either slip rates at sites along longer faults or at sites within fault branches. Consequently, inaccuracies in the slip rate estimates used for seismic hazard assessment may have differing impacts on the fault system depending on location and structural position of the slip rates. Fault branches along strike-slip faults warrant detailed investigation not only because these areas have high spatial variability of slip rate and accrue nearby off-fault deformation but also because changes in slip rates along branches has larger impact on deformation along fault system than other sites. Lack of data or large uncertainty in slip rate data from fault branches can affect our ability to accurately assess seismic hazard of the region.

Present-day shear tractions along faults of the San Gorgonio Pass region (southern California, USA) can be estimated from stressing rates provided by three-dimensional forward crustal deformation models. Due to fault interaction within the model, dextral shear stressing rates on the San Andreas and San Jacinto faults differ from rates resolved from the regional loading. In particular, fault patches with similar orientations and depths on the two faults show different stressing rates. We estimate the present-day, evolved fault tractions along faults of the San Gorgonio Pass region using the time since last earthquake, fault stressing rates (which account for fault interaction), and coseismic models of the impact of recent nearby earthquakes. The evolved tractions differ significantly from the resolved regional tractions, with the largest dextral traction located within the restraining bend comprising the pass, which has not had recent earthquakes, rather than outside of the bend, which is more preferentially oriented under tectonic loading. Evolved fault tractions can provide more accurate initial conditions for dynamic rupture models within regions of complex fault geometry, such as the San Gorgonio Pass region. An analysis of the time needed to accumulate shear tractions that exceed typical earthquake stress drops shows that present-day tractions already exceed 3 MPa along portions of the Banning, Garnet Hill, and Mission Creek strands of the San Andreas fault. This result highlights areas that may be near failure if accumulated tractions equivalent to typical earthquake stress drops precipitate failure.