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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 bymapping 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 aswell 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.

Holocene crustal faulting in the northern Olympic Peninsula of Washington State manifests in a zone of west-northwest-striking crustal faults herein named the North Olympic fault zone, which extends for ∼80  km along strike and includes the Lake Creek-Boundary Creek fault to the east and the Sadie Creek fault and newly discovered scarps to the west. This study focuses on the Sadie Creek fault, which extends for >14  km west-northwest from Lake Crescent. Airborne light detection and ranging (lidar) imagery reveals the trace of the Sadie Creek fault and offset postglacial landforms showing a history of Holocene surface-rupturing earthquakes dominated by dextral displacement along a steeply dipping fault zone. Paleoseismic trenches at two sites on the Sadie Creek fault reveal till and outwash overlain by progressively buried forest and wetland soils developed on scarp-derived colluvial wedges. Trench exposures of complex faulting with subhorizontal slickenlines indicate dextral displacement with lesser dip slip. Correlation of broadly constrained time intervals for earthquakes at the Sadie Creek sites and those to the east along the Lake Creek-Boundary Creek fault is consistent with rupture of much of the length of the North Olympic fault zone three to four times: at about 11, 7, 3, and 1 ka, with a shorter rupture at about 8.5 ka. Dated ruptures from trenches only partially coincide with coseismic landslides and megaturbidites in Lake Crescent, indicating that some earthquakes did not trigger megaturbidites, and some turbidites were unrelated to local fault rupture. Landform mapping suggests single-event dextral displacement of 4±1  m on the Sadie Creek fault. Inferred maximum rupture length and single-event slip imply earthquake magnitudes Mw 7.0-7.5. Dextral slip rates of 1.3-2.3  mm/yr and the ∼11,000  yr slip history suggest that the North Olympic fault zone is a prominent contributor to permanent strain in the northern Cascadia fore-arc. [ABSTRACT FROM AUTHOR]

Prehistoric records of subduction earthquakes are often distinguished by evidence of synchronous widespread coastal deformation, the extent of which negates the plausibility of alternative source faults. At the Hi-kurangi subduction margin in New Zealand, untangling the record of subduction interface ruptures is complicated. Large earthquake age uncertainties inhibit unique solutions of along-strike correlations, and complex patterns of coastal deformation caused by upper-plate faulting prevent reliable indication of source faults. In this work, we improved paleoearthquake reconstructions on the central Hikurangi margin with a new, well-constrained 5000 yr earthquake record from Pakuratahi Valley near Napier, North Island, New Zealand. Evidence of laterally extensive paleoenvironmental changes is consistent with coseismic subsidence and coseismic uplift in large earthquakes. Radiocarbon dates on fragile terrestrial macrofossils and tephra isochrons were used to construct robust age models that yielded earthquake ages of 4839-4601 calibrated (cal.) yr B.P., 3630-3564 cal. yr B.P., 2687-2439 cal. yr B.P., and 1228-823 cal. yr B.P. Integration of these ages with refined earthquake chronology from nearby Ahuriri Lagoon indicated that the next large earthquake impacting the Napier area is more likely to cause coastal subsidence than uplift. Drawing on correlations with cotemporal evidence elsewhere on the central margin, we infer that the overall patterns of coseismic deformation could be generated by either rupture of the subduction interface or upper-plate faults, or both. This inability to separate source faults for past earthquakes limits the efficiency of forecasting future earthquakes. Similar problems of intertwined paleoearthquake signatures likely apply to other plate boundaries, where we recommend cautious interpretation of coastal deformation to accurately address the hazard from both types of source faults. [ABSTRACT FROM AUTHOR]

The tectonic domains of Basin and Range extension, Cascadia subduction zone contraction, and Walker Lane dextral transtension converge in the Mushroom Rock region of northeastern California, USA. We combined analysis of high- resolution topographic data, bedrock mapping, 40Ar/39Ar geochronology, low- temperature thermochronology, and existing geologic and fault mapping to characterize an extensive dextral- normal-oblique fault system called the Pondosa fault zone. This fault zone extends north- northwest from the Pit River east of Soldier Mountain, California, into moderately high- relief volcanic topography as far north as the Bartle (California) townsite with normal and dextral offset apparent in geomorphology and fault exposures. New and existing 40Ar/39Ar and radiocarbon dating of offset lava flows provides ages of 12.4 ka to 9.6 Ma for late Cenozoic stratigraphic units. Scarp morphology and geomorphic expression indicate that the fault system was active in the late Pleistocene. The Pondosa fault zone may represent a dextral- oblique accommodation zone between north-south–oriented Basin and Range extensional fault systems and/or part of the Sierra Nevada–Oregon Coast block microplate boundary. [ABSTRACT FROM AUTHOR]

We excavated trenches at two paleoseismic sites bounding a trans-basin bedrock ridge (the Willow Creek Hills) along the northern Lost River fault zone to explore the uniqueness of the 1983 Mw 6.9 Borah Peak earthquake compared to its prehistoric predecessors. At the Sheep Creek site on the southernmost Warm Springs section, two earthquakes occurred at 9.8--14.0 ka (95% confidence) and 6.5--7.1 ka; each had -1.9 m of vertical displacement. About 4 km to the southeast, across the Willow Creek Hills, two ruptures at the Arentson Gulch site on the northernmost Thousand Springs section occurred at 9.0--14.7 ka and 6.1--7.5 ka with -1.9 m of vertical displacement each. We synthesize these and previous paleoseismic results into a model of five postglacial (<15 ka) ruptures along a -65 km reach of the northern Lost River fault zone. Our results show that the Borah Peak earthquake (34 km; 0.9 m mean displacement) was unique compared to previous ruptures that had both longer and shorter rupture lengths (-25--38 km), more displacement (mean of -1.3--1.4 m), and equal or greater magnitude (Mw 6.9--7.1) than that in the 1983 earthquake. These ruptures support a hypothesis of variable rupture length and displacement on the northern Lost River fault zone and show that predecessors to the 1983 rupture have passed unimpeded through the Willow Creek Hills. Our work demonstrates that normal faults are capable of producing variable spatial-temporal patterns of rupture that, together with comparisons of fault geometry and historical rupture length, improve our understanding of fault segmentation and help inform models of earthquake rupture probability. [ABSTRACT FROM AUTHOR]

The 72-km-long Teton normal fault bounds the eastern base of the Teton Range in northwestern Wyoming, USA. Although geomorphic surfaces along the fault record latest Pleistocene to Holocene fault movement, the postglacial earthquake history of the fault has remained enigmatic. We excavated a paleoseismic trench at the Buffalo Bowl site along the southernmost part of the fault to determine its Holocene rupture history and slip rate. At the site, 6.3 m of displacement postdates an early Holocene (ca. 10.5 ka) alluvial-fan surface. We document evidence of three surface-faulting earthquakes based on packages of scarp-derived colluvium that postdate the alluvial-fan units. Bayesian modeling of radiocarbon and luminescence ages yields earthquake times of ca. 9.9 ka, ca. 7.1 ka, and ca. 4.6 ka, forming the longest, most complete paleoseismic record of the Teton fault. We integrate these data with a displaced deglacial surface 4 km NE at Granite Canyon to calculate a postglacial to mid-Holocene (14.4-4.6 ka) slip rate of 1.1 mm/yr. Our analysis also suggests that the postglacial to early Holocene (14.4-9.9 ka) slip rate exceeds the Holocene (9.9-4.6 ka) rate by a factor of 2 (maximum of 3); however, a uniform rate for the fault is possible considering the 95% slip-rate errors. The 5 k.y. elapsed time since the last rupture of the southernmost Teton fault implies a current slip deficit of 4-5 m, which is possibly explained by spatially/temporally incomplete paleoseismic data, irregular earthquake recurrence, and/or variable per-event displacement. Our study emphasizes the importance of minimizing slip-rate uncertainties by integrating paleoseismic and geomorphic data sets and capturing multiple earthquake cycles. [ABSTRACT FROM AUTHOR]