On 4 and 6 July 2019, two large strike‐slip earthquakes with W‐phase moment magnitudes MWW 6.5 (foreshock) and MWW 7.1 (mainshock) struck the Eastern California Shear Zone, northeast of Ridgecrest. The faulting geometry and kinematic coseismic slip distribution of both events are determined by jointly inverting seismological and geodetic observations guided by aftershock and surface rupture locations. The foreshock ruptured two orthogonal faults with a prominent L‐shaped geometry with maximum slip of ~1.1 m on the NE‐SW segment. The mainshock faulting extended NW‐SE along several primary fault segments that straddle the foreshock slip. The surface rupture and slip model indicate mostly near‐horizontal strike‐slip motion with maximum slip of ~3.7 m, but there is a localized vertical dip‐slip motion. Both the foreshock and mainshock ruptures terminate in regions of complex surface offsets. High aftershock productivity and low rupture velocity may be the result of rupture of a relatively immature fault system.

The number of aftershocks increases with mainshock size following a well‐defined scaling law. However, excursions from the average behavior are common. This variability is particularly concerning for large earthquakes where the number of aftershocks varies by factors of 100 for mainshocks of comparable magnitude. Do observable factors lead to differences in aftershock behavior? We examine aftershock productivity relative to the global average for all mainshocks ( MW>6.5) from 1990 to 2019. A global map of earthquake productivity highlights the influence of tectonic regimes. Earthquake depth, lithosphere age, and plate boundary type correspond well with earthquake productivity. We investigate the role of mainshock attributes by compiling source dimensions, radiated seismic energy, stress drop, and a measure of slip heterogeneity based on finite‐fault source inversions for the largest earthquakes from 1990 to 2017. On an individual basis, stress drop, normalized rupture width, and aspect ratio most strongly correlate with aftershock productivity. A multivariate analysis shows that a particular set of parameters (dip, lithospheric age, and normalized rupture area) combines well to improve predictions of aftershock productivity on a cross‐validated data set. Our overall analysis is consistent with a model in which the volumetric abundance of nearby stressed faults controls the aftershock productivity rather than variations in source stress. Thus, we suggest a complementary approach to aftershock forecasts based on geological and rupture properties rather than local calibration alone.

Recent earthquakes have demonstrated that rupture may propagate through geometrically complex networks of faults. Ancient exhumed faults have the potential to reveal the details of complex rupture at seismogenic depths. We present a new set of field observational criteria for determining which of a population of pseudotachylyte fault veins formed in the same earthquake and apply it to map rupture networks representing single earthquakes. An exceptional exposure of an exhumed ancient strand of the Norumbega Shear Zone preserves evidence of multistranded earthquake rupture in the deep seismogenic zone of a continental transform fault. Individual fault strands slipped at least 2–18 cm, so significant slip is represented by each rupture network. Our data show that synchronously slipped faults intersect at angles of 0 to ∼55°, with the opening angles of fault intersections directed toward the dilational quadrants for dextral slip. Multistranded rupture on a fault network instead of rupture of a single fault may result in greater and/or more variable slip and cause slip rake to vary spatially and temporally. Slip on intersecting faults unequivocally means that there will be motion perpendicular to the average fault plane. Modern earthquakes displaying non‐double‐couple components to focal mechanism solutions and spatially varying rake, slip, and anomalous stress drop may be explained by rupture across fault networks that are too close (spatially and temporally) to be resolved seismically as separate events.

Abstract Recognizing earthquakes as foreshocks in real time would provide a valuable forecasting capability. In a recent study, Gulia and Wiemer (2019) proposed a traffic-light system that relies on abrupt changes in b-values relative to background values. The approach utilizes high-resolution earthquake catalogs to monitor localized regions around the largest events and distinguish foreshock sequences (reduced b-values) from aftershock sequences (increased b-values). The recent well-recorded earthquake foreshock sequences in Ridgecrest, California, and Maria Antonia, Puerto Rico, provide an opportunity to test the procedure. For Ridgecrest, our b-value time series indicates an elevated risk of a larger impending earthquake during the Mw 6.4 foreshock sequence and provides an ambiguous identification of the onset of the Mw 7.1 aftershock sequence. However, the exact result depends strongly on expert judgment. Monte Carlo sampling across a range of reasonable decisions most often results in ambiguous warning levels. In the case of the Puerto Rico sequence, we record significant drops in b-value prior to and following the largest event (Mw 6.4) in the sequence. The b-value has still not returned to background levels (12 February 2020). The Ridgecrest sequence roughly conforms to expectations; the Puerto Rico sequence will only do so if a larger event occurs in the future with an ensuing b-value increase. Any real-time implementation of this approach will require dense instrumentation, consistent (versioned) low completeness catalogs, well-calibrated maps of regionalized background b-values, systematic real-time catalog production, and robust decision making about the event source volumes to analyze.

Previous field observations suggest that fault slip surface roughness may decrease with slip. However, measurements have yet to confidently isolate the effect of slip from other possible controls, such as lithology or tectonic setting. We describe the evolution of slip surfaces in normal faults in SE Utah that cut well‐sorted, high‐porosity sandstones and accommodated regional extension at 1‐ to 4‐km depth. Tight controls on fault offset and uniform tectonic history allowed us to isolate the effect of slip during early stages of faulting (0‐ to 55‐m slip). Slip surfaces progress from rough joints and deformation bands toward smooth, continuous, and mirror‐like surfaces with increasing slip. We collected 123 scans of pristine slip surfaces, which measure surface geometry from micrometers to meters in length scale. Results indicate that slip surface roughness is systematically smoother than joint surfaces and deformation band edges. Over the best resolved length scales (0.1–10 mm), we observe roughness decreases with slip according to a power law with exponent −1 in the slip direction. Slip‐perpendicular profiles, though rougher, exhibit the same smoothing trend. The Hurst scaling exponent does not change with slip. These observations require wear to be multiscale. Boundary element method models suggest that mechanical wear of completely mated surfaces occurs by asperity failure and that the wear rate depends on the aspect ratio of asperities. These results indicate that at large slip, asperity failure at all length scales can cause slip surfaces to smooth while maintaining the fractal geometry characteristic of faults.

Co‐located and integrated observation of the surface and subsurface is necessary to characterize fault zone hydrogeology. The spectacular cliff‐face exposure of the Champlain Thrust fault at Lone Rock Point, Vermont, and a nearby well‐field site provides the opportunity for co‐located structural and hydrogeologic field observations. We mapped the prominent structural features of the Champlain Thrust fault and discrete groundwater seeps in outcrop, and also drilled through the fault near the outcrop and determined aquifer parameters from aquifer pumping tests. In outcrop, the fault core thickness varies on the meter scale, splays out into multiple strands, and is offset by a minor normal fault. Groundwater seeps are prevalent in the heavily fractured footwall, but limited in the fault core and hanging wall, suggesting that at the cliff face the water table is generally near the fault core and groundwater flow in the hanging wall is limited. Enrichment of more soluble minerals in cemented fault rock associated with older strands of the fault system may play an important role in localizing karst features in the hanging wall. At the well‐field site, the Champlain Thrust fault is offset significantly by a high‐angle normal fault, the water table is near the surface, and aquifer pumping tests reveal a complex hydrogeologic system, with karst and steep fractures as strong hydraulic conduits in the hanging wall and fault core. The most salient features of the fault zone hydrogeology in the surface and subsurface data are different, but can be integrated into a preliminary conceptual model. Together, the surface and subsurface methods underscore and emphasize the complexity and heterogeneity of the hydrogeology of this low‐angle sedimentary fault.

Gulia and Wiemer (2019; hereafter, GW2019) proposed a near-real-time monitoring system to discriminate between foreshocks and aftershocks. Our analysis (Dascher-Cousineau et al., 2020; hereinater, DC2020) tested the sensitivity of the proposed Foreshock Traffic-Light System output to parameter choices left to expert judgment for the 2019 Ridgecrest Mw 7.1 and 2020 Puerto Rico Mw 6.4 earthquake sequences. In the accompanying comment, Gulia and Wiemer (2021) suggest that at least six different methodological deviations lead to different pseudoprospective warning levels, particularly for the Ridgecrest aftershock sequence which they had separately evaluated. Here, we show that for four of the six claimed deviations, we conformed to the criteria outlined in GW2019. Two true deviations from the defined procedure are clarified and justified here. We conclude as we did originally, by emphasizing the influence of expert judgment on the outcome in the analysis.

Successive earthquakes can drive landscape evolution. However, the mechanism and pace with which landscapes respond remain poorly understood. Offset channels in the Carrizo Plain, California, capture the fluvial response to lateral slip on the San Andreas Fault on millennial time scales. We developed and tested a model that quantifies competition between fault slip, which elongates channels, and aggradation, which causes channel infilling and, ultimately, abandonment. Validation of this model supports a transport-limited fluvial response and implies that measurements derived from present-day channel geometry are sufficient to quantify the rate of bedload transport relative to slip rate. Extension of the model identifies the threshold for which persistent change in transport capacity, obliquity in slip, or advected topography results in reorganization of the drainage network.