Coseismic surface deformation in large earthquakes is typically measured using field mapping and with a range of geodetic methods (e.g., InSAR, lidar differencing, and GPS). Current methods, however, either fail to capture patterns of near-field coseismic surface deformation or lack preevent data. Consequently, the characteristics of off-fault deformation and the parameters that control it remain poorly understood. We develop a standardized method to fully measure the surface, near-field, coseismic deformation patterns at high resolution using the COSI-Corr program by correlating pairs of aerial photographs taken before and after the 1992 M w 7.3 Landers earthquake. COSI-Corr offers the advantage of measuring displacement across the entire zone of surface deformation and over a wider aperture than that available to field geologists. For the Landers earthquake, our measured displacements are systematically larger than the field measurements, indicating the presence of off-fault deformation. We show that 46% of the total surface displacement occurred as off-fault deformation, over a mean deformation width of 154 m. The magnitude and width of off-fault deformation along the rupture is primarily controlled by the macroscopic structural complexity of the fault system, with a weak correlation with the type of near-surface materials through which the rupture propagated. Both the magnitude and width of distributed deformation are largest in stepovers, bends, and at the southern termination of the surface rupture. We find that slip along the surface rupture exhibits a consistent degree of variability at all observable length scales and that the slip distribution is self-affine fractal with dimension of 1.56.

Supershear earthquakes with rupture velocity exceeding shear-wave speeds, previously observed in laboratory experiments and large strike-slip events, often have an initial sub-shear stage before they transition to supershear. In this study, integrated geophysical observations of the 2018 Mw 7.5 Palu, Indonesia earthquake, provide robust evidence of an early and persistent supershear rupture speed. Slowness-enhanced back-projection (SEBP) of teleseismic data provides a sharp image of the rupture process, consistently across multiple arrays. The inferred rupture path agrees with the surface rupture trace inferred from the net surface displacement field derived by sub-pixel InSAR image correlation. The SEBP results indicate a sustained rupture velocity of 4.1 km/s from the rupture initiation to the end, despite large fault bends. The persistent supershear speed is further validated by evidence of far-field Rayleigh Mach waves in regional seismograms. The short or absent supershear transition distance can be caused by high initial shear stress or short critical slip-weakening distance, and promoted by fault roughness near the hypocenter. Steady rupture propagation at a supershear speed considered to be unstable, lower than the Eshelby speed, could result from the presence of a damaged fault zone.

S U M M A R YGeodetic slip inversions for three major (M w > 7) strike-slip earthquakes (1992( Landers, 1999 Hector Mine and 2010 El Mayor-Cucapah) show a 15-60 per cent reduction in slip near the surface (depth < 2 km) relative to the slip at deeper depths (4-6 km). This significant difference between surface coseismic slip and slip at depth has been termed the shallow slip deficit (SSD). The large magnitude of this deficit has been an enigma since it cannot be explained by shallow creep during the interseismic period or by triggered slip from nearby earthquakes. One potential explanation for the SSD is that the previous geodetic inversions lack data coverage close to surface rupture such that the shallow portions of the slip models are poorly resolved and generally underestimated. In this study, we improve the static coseismic slip inversion for these three earthquakes, especially at shallow depths, by: (1) including data capturing the near-fault deformation from optical imagery and SAR azimuth offsets; (2) refining the interferometric synthetic aperture radar processing with non-boxcar phase filtering, modeldependent range corrections, more complete phase unwrapping by SNAPHU (Statistical Nonlinear Approach for Phase Unwrapping) assuming a maximum discontinuity and an onfault correlation mask; (3) using more detailed, geologically constrained fault geometries and (4) incorporating additional campaign global positioning system (GPS) data. The refined slip models result in much smaller SSDs of 3-19 per cent. We suspect that the remaining minor SSD for these earthquakes likely reflects a combination of our elastic model's inability to fully account for near-surface deformation, which will render our estimates of shallow slip minima, and potentially small amounts of interseismic fault creep or triggered slip, which could 'make up' a small percentages of the coseismic SSD during the interseismic period. Our results indicate that it is imperative that slip inversions include accurate measurements of near-fault surface deformation to reliably constrain spatial patterns of slip during major strike-slip earthquakes.

, a M w 6.5 earthquake, followed 34 h later by a M w 7.1 event, struck Searles Valley, California. These events are part of a long-lived cluster of historical earthquakes along the Eastern California Shear Zone (ECSZ) which started in 1872 and are associated with temporarily elevated strain rates. We find that the M w 6.5 event initiated on a right-lateral NW striking fault and then ruptured a left-lateral fault to the surface. This event triggered right-lateral slip during the M w 7.1 earthquake. It started as a bilateral, crack-like rupture on a segment brought closer to failure by the M w 6.5 event. The rupture evolved to pulse-like as it propagated at a relatively slow velocity (2 km/s) along a segment that was unloaded by the M w 6.5 event. It stopped abruptly at the Coso volcanic area and at the Garlock Fault and brought some neighbouring faults closer to failure.

Surface rupture in the 2019 Ridgecrest, California, earthquake sequence occurred along two orthogonal cross faults and includes dominantly left-lateral and northeast-striking rupture in the Mw 6.4 foreshock and dominantly right-lateral and northwest-striking rupture in the Mw 7.1 mainshock. We present &gt;650 field-based, surface-displacement observations for these ruptures and synthesize our results into cumulative along-strike displacement distributions. Using these data, we calculate displacement gradients and compare our results with historical strike-slip ruptures in the eastern California shear zone. For the Mw 6.4 rupture, we report 96 displacements measured along 18 km of northeast-striking rupture. Cumulative displacement curves for the rupture yield a mean left-lateral displacement of 0.3–0.5 m and maximum of 0.7–1.6 m. Net mean vertical displacement based on the difference of down-to-the-west (DTW) and down-to-the-east (DTE) displacement curves is close to zero (0.02 m DTW). The Mw 6.4 displacement distribution shows that the majority of displacement occurred southwest of the intersection with the Mw 7.1 rupture. The Mw 7.1 rupture is northwest-striking and 50 km long based on 576 field measurements. Displacement curves indicate a mean right-lateral displacement of 1.2–1.7 m and a maximum of 4.3–7.0 m. Net vertical displacement in the rupture averages 0.3 m DTW. The Mw 7.1 displacement distributions demonstrate that maximum displacement occurred along a 12-km-long portion of the fault near the Mw 7.1 epicenter, releasing 66% of the geologically based seismic moment along 24% of the total rupture length. Using our displacement distributions, we calculate kilometer-scale displacement gradients for the Mw 7.1 rupture. The steepest gradients (∼1–3  m/km) flank the 12-km-long region of maximum displacement. In contrast, gradients for the 1992 Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes are &lt;0.6  m/km. Our displacement distributions are important for understanding the influence of cross-fault rupture on Mw 6.4 and 7.1 rupture length and displacement and will facilitate comparisons with distributions generated remotely and at broader scales.

Subpixel correlation of preevent and postevent air photos reveal the complete near‐field, horizontal surface deformation patterns of the 1992 Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine ruptures. Total surface displacement values for both earthquakes are systematically larger than “on‐fault” displacements from geologic field surveys, indicating significant distributed, inelastic deformation occurred along these ruptures. Comparison of these two data sets shows that 46 ± 10% and 39 ± 22% of the total surface deformation were distributed over fault zones averaging 154 m and 121 m in width for the Landers and Hector Mine events, respectively. Spatial variations of distributed deformation along both ruptures show correlations with the type of near‐surface lithology and degree of fault complexity; larger amounts of distributed shear occur where the rupture propagated through loose unconsolidated sediments and areas of more complex fault structure. These results have basic implications for geologic‐geodetic rate comparisons and probabilistic seismic hazard analysis.

GPS can track terrestrial water storage following extreme precipitation events, with potential to improve flood planning.

The Mw 6.4 and Mw 7.1 Ridgecrest earthquake sequence occurred on 4 and 5 July 2019 within the eastern California shear zone of southern California. Both events produced extensive surface faulting and ground deformation within Indian Wells Valley and Searles Valley. In the weeks following the earthquakes, more than six dozen scientists from government, academia, and the private sector carefully documented the surface faulting and ground-deformation features. As of December 2019, we have compiled a total of more than 6000 ground observations; approximately 1500 of these simply note the presence or absence of fault rupture or ground failure, but the remainder include detailed descriptions and other documentation, including tens of thousands of photographs. More than 1100 of these observations also include quantitative field measurements of displacement sense and magnitude. These field observations were supplemented by mapping of fault rupture and ground-deformation features directly in the field as well as by interpreting the location and extent of surface faulting and ground deformation from optical imagery and geodetic image products. We identified greater than 68 km of fault rupture produced by both earthquakes as well as numerous sites of ground deformation resulting from liquefaction or slope failure. These observations comprise a dataset that is fundamental to understanding the processes that controlled this earthquake sequence and for improving earthquake hazard estimates in the region. This article documents the types of data collected during postearthquake field investigations, the compilation effort, and the digital data products resulting from these efforts.