2019 M7.1 Ridgecrest earthquake slip distribution controlled by fault geometry inherited from Independence dike swarm.

Faults often form through reactivation of pre-existing structures, developing geometries and mechanical properties specific to the system's geologic inheritance. Competition between fault geometry and other factors (e.g., lithology) to control slip at Earth's surface is an open question that is central to our knowledge of fault processes and seismic hazards. Here we use remote sensing data and field observations to investigate the origin of the 2019 M7.1 Ridgecrest, California, earthquake rupture geometry and test its impact on the slip distribution observed at Earth's surface. Common geometries suggest the fault system evolved through reactivation of structures within the surrounding Independence dike swarm (IDS). Mechanical models testing a range of fault geometries and stress fields indicate that the inherited rupture geometry strongly controlled the M7.1 earthquake slip distribution. These results motivate revisiting the development of other large-magnitude earthquake ruptures (1992 M7.3 Landers, 1999 M7.1 Hector Mine) and tectonic provinces within the IDS.

Rapid shallow megathrust afterslip from the 2021 M8.2 Chignik, Alaska earthquake revealed by seafloor geodesy.

The shallower portions of subduction zone megathrust faults host Earth's most hazardous tsunamigenic earthquakes, yet understanding how and when they slip remains elusive because of challenges making seafloor observations. We performed Global Navigation Satellite System Acoustic seafloor geodetic surveys before and ~2.5 months after the 29 July 2021 Mw (moment magnitude) 8.2 Chignik, Alaska, earthquake and determine ~1.4 meters cumulative co- and post-seismic horizontal displacement ~60 kilometers from the megathrust front. Only for the 2011 Mw 9 Tohoku event have closer subduction zone earthquake displacements been observed. We estimate ~2 to 3 meters of megathrust afterslip shallower than 20 kilometers, a portion of the megathrust on which both inter- and co-seismic slip likely had occurred previously. Our analysis demonstrates that by 2.5 months, shallower and deeper moment had effectively equilibrated on the megathrust, suggesting that its tsunamigenic potential remains no more elevated than before the earthquake.

Mechanics of near-field deformation during co- and post-seismic shallow fault slip.

Poor knowledge of how faults slip and distribute deformation in the shallow crust hinders efforts to mitigate hazards where faults increasingly intersect with the expanding global population at Earth's surface. Here we analyze two study sites along the 2014 M 6.0 South Napa, California, earthquake rupture, each dominated by either co- or post-seismic shallow fault slip. We combine mobile laser scanning (MLS), active-source seismic tomography, and finite element modeling to investigate how deformation rate and mechanical properties of the shallow crust affect fault behavior. Despite four orders-of-magnitude difference in the rupture velocities, MLS-derived shear strain fields are remarkably similar at the two sites and suggest deceleration of the co-seismic rupture near Earth's surface. Constrained by the MLS and seismic data, finite element models indicate shallow faulting is more sensitive to lithologic layering and plastic yielding than to the presence of fault compliant zones (i.e., regions surrounding faults with reduced stiffness). Although both elastic and elastoplastic models can reproduce the observed surface displacement fields within the uncertainty of MLS data, elastoplastic models likely provide the most reliable representations of subsurface fault behavior, as they produce geologically reasonable stress states and are consistent with field, geodetic, and seismological observations.

Buried shallow fault slip from the South Napa earthquake revealed by near-field geodesy.

Earthquake-related fault slip in the upper hundreds of meters of Earth's surface has remained largely unstudied because of challenges measuring deformation in the near field of a fault rupture. We analyze centimeter-scale accuracy mobile laser scanning (MLS) data of deformed vine rows within ±300 m of the principal surface expression of the M (magnitude) 6.0 2014 South Napa earthquake. Rather than assuming surface displacement equivalence to fault slip, we invert the near-field data with a model that allows for, but does not require, the fault to be buried below the surface. The inversion maps the position on a preexisting fault plane of a slip front that terminates ~3 to 25 m below the surface coseismically and within a few hours postseismically. The lack of surface-breaching fault slip is verified by two trenches. We estimate near-surface slip ranging from ~0.5 to 1.25 m. Surface displacement can underestimate fault slip by as much as 30%. This implies that similar biases could be present in short-term geologic slip rates used in seismic hazard analyses. Along strike and downdip, we find deficits in slip: The along-strike deficit is erased after ~1 month by afterslip. We find no evidence of off-fault deformation and conclude that the downdip shallow slip deficit for this event is likely an artifact. As near-field geodetic data rapidly proliferate and will become commonplace, we suggest that analyses of near-surface fault rupture should also use more sophisticated mechanical models and subsurface geomechanical tests.