Contributions to the Neotectonics of the central and northern Walker Lane

The Walker Lane is a well-known intraplate shear zone located east of the Sierra Nevada that accommodates a significant portion of the North American – Pacific Plate relative transform motion. It its defined by a broad zone of discontinuous active strike-slip and normal faulting that sit within a zone geodetically characterized by transtensional dextral shear. Strain in the Walker Lane is driven by the northwest translation of the Sierra Nevada and long-term geologic constraints on the distribution and partitioning of slip amongst the active faults are limited, thus our understanding of how these faults interact and behave as a system remains understudied. These limitations are a consequences of the slow-slip rate environments, making geomorphic preservation and observation of offset more difficult, and due to the state of scientific advancement. Similarly, at the northern end of the Sierra Nevada, regional north-south oriented contraction is geodetically observed and has been attributed to the interaction of the northwest translating Sierra Nevada with the overriding crust of the southern Cascadia Subduction Zone. Accommodation of this strain remains geologically unaccounted for, thus it is unknown how the contraction is accommodated. The purpose of this dissertation is to place new and more robust constraints on the rates of slip for multiple Walker Lane faults and to identify and characterize potentially active faults within the northern Sierra and southern Cascadia transition zone. Much of this work is motivated by seismic hazard, but scientifically serves to provide a better understanding of how strain is released bordering the Sierra Nevada Mountains. This dissertation consists of 3 chapters. The first study explores a new application of a ground surface modeling technique for the purpose of placing better constraints on the rate of slip for the Pyramid Lake fault, a major strike-slip fault in the northern portion of the Walker Lane. This study is herein presented as published in the Bulletin of the Seismological Society of America (Vol. 106, No. 2, 2016). Chapter 2 builds on this method and incorporates multiple Quaternary dating techniques that place constraints on horizontal slip-rates in a network of strike-slip faults that comprise the central Walker Lane. The observations and new rates obtained in this study provide insight into how slip is accommodated on a variety of spatiotemporal scales. Finally, chapter 3 presents the results from a combination of geophysical, geologic, and geomorphic observations made within the northern Sacramento Valley that show the presence of Quaternary contractional deformation associated with a series of previously identified structures optimally oriented to accommodate north-south contraction. These findings show for the first time, how geodetically observed strain is accommodated within the northern Sierra and southern Cascadia transition zone

A paleoseismic transect across the northwestern Basin and Range Province, northwestern Nevada and northeastern California, USA

We use new and existing data to compile a record of similar to 18 latest Quaternary large-magnitude surface-rupturing earthquakes on 7 fault zones in the northwestern Basin and Range Province of northwestern Nevada and northeastern California. The most recent earthquake on all faults postdates the ca. 18-15 ka last glacial highstand of pluvial Lake Lahontan and other pluvial lakes in the region. These lacustrine data provide a window in which we calculate latest Quaternary vertical slip rates and compare them with rates of modern deformation in a global positioning system (GPS) transect spanning the region. Average vertical slip rates on these fault zones range from 0.1 to 0.8 mm/yr and total similar to 2 mm/yr across a 265-km-wide transect from near Paradise Valley, Nevada, to the Warner Mountains in California. We converted vertical slip rates to horizontal extension rates using fault dips of 30 degrees-60 degrees, and then compared the extension rates to GPS-derived rates of modern (last 7-9 yr) deformation. Our preferred fault dip values (45 degrees-55 degrees) yield estimated longterm extension rates (1.3-1.9 mm/yr) that underestimate our modern rate (2.4 mm/yr) by similar to 21%-46%. The most likely sources of this underestimate are geologically unrecognizable deformation from moderate-sized earthquakes and unaccounted-for coseismic off-fault deformation from large surface-rupturing earthquakes. However, fault dip values of <= 40 degrees yield long-term rates comparable to or greater than modern rates, so an alternative explanation is that fault dips are closer to 40 degrees than our preferred values. We speculate that the large component of right-lateral shear apparent in the GPS signal is partitioned on faults with primary strike-slip displacement, such as the Long Valley fault zone, and as not easily detected oblique slip on favorably oriented normal faults in the region

Characterizing the Quaternary expression of active faulting along the Olinghouse, Carson, and Wabuska lineaments of the Walker Lane

The northern Walker Lane (southwestern USA) accommodates similar to 5-7 mm/yr of right-lateral Pacific-North America relative plate motion. The northwest trend of major right-lateral faults in the Walker Lane is interrupted by the presence of northeast-striking left-lateral faults within the Carson and Excelsior domains. Previous studies in the Carson domain have suggested that left-lateral slip on the northeast-striking Olinghouse, Carson, and Wabuska lineaments accommodates Walker Lane transtensional dextral shear through the clockwise rotation of intervening crustal blocks. Our observations confirm and document the presence of late Pleistocene-Holocene faulting along each of these lineaments. Fault scarps along the Carson and Wabuska lineaments are discontinuous and sparse, and show evidence for left-lateral faulting, locally including linear fault traces, alternating scarp face directions, and lateral offsets of small gullies and ridges. The trends of scarps that define these lineaments link at their western ends with north-trending active normal faults. In this manner, it appears that the 5-7 mm/yr of right slip taking place across the northern Walker Lane is being accommodated by the combined processes of basin opening in the west and block rotation to the east. This mode of slip transfer differs from the Excelsior domain, where active left-slip faults and clockwise rotation of crustal blocks are confined to, and the result of, a distinct right step between right-lateral faults of the southern Walker Lane and central Walker Lane, respectively. The observation of these apparently diverse modes of development of left-slip faults and vertical axis rotations provides an example of the complexity that may be expected in the structural development of continental shear zones that have been characterized by transtension

Large paleoearthquake timing and displacement near Damak in eastern Nepal on the Himalayan Frontal Thrust

An excavation across the Himalayan Frontal Thrust near Damak in eastern Nepal shows displacement on a fault plane dipping similar to 22 degrees has produced vertical separation across a scarp equal to 5.5m. Stratigraphic, structural, geometrical, and radiocarbon observations are interpreted to indicate that the displacement is the result of a single earthquake of 11.33.5m of dip-slip displacement that occurred 1146-1256A.D. Empirical scaling laws indicate that thrust earthquakes characterized by average displacements of this size may produce rupture lengths of 450 to >800km and moment magnitudes M-w of 8.6 to >9. Sufficient strain has accumulated along this portion of the Himalayan arc during the roughly 800years since the 1146-1256A.D. earthquake to produce another earthquake displacement of similar size. Plain Language Summary The densely populated country of Nepal sits above the Himalayan Frontal Thrust fault. It is repeated displacements on this fault that are responsible for the uplift of the Himalaya mountains and considered capable of producing great earthquakes. Here we excavate a trench across the fault to show a great earthquake occurred 1146 -1256 AD in eastern Nepal. It has been a sufficiently long time since then that stresses have accumulated to a level capable of producing another such great earthquake

Uplift and subsidence reveal a nonpersistent megathrust rupture boundary (Sitkinak Island, Alaska)

We report stratigraphic evidence of land-level change and tsunami inundation along the Alaska-Aleutian megathrust during prehistoric and historical earthquakes west of Kodiak Island. On Sitkinak Island, cores and tidal outcrops fringing a lagoon reveal five sharp lithologic contacts that record coseismic land-level change. Radiocarbon dates, 137Cs profiles, computerized tomography scans, and microfossil assemblages are consistent with rapid uplift circa 290–0, 520–300, and 1050–790 cal yr B.P. and subsidence in A.D. 1964 and circa 640–510 cal yr B.P. Radiocarbon, 137Cs, and 210Pb ages bracketing a sand bed traced 1.5 km inland and evidence for sudden uplift are consistent with Russian accounts of an earthquake and tsunami in A.D. 1788. The mixed uplift and subsidence record suggests that Sitkinak Island sits above a nonpersistent boundary near the southwestern limit of the A.D. 1964 Mw 9.2 megathrust rupture

Simplifying complex fault data for systems-level analysis: Earthquake geology inputs for U.S. NSHM 2023.

As part of the U.S. National Seismic Hazard Model (NSHM) update planned for 2023, two databases were prepared to more completely represent Quaternary-active faulting across the western United States: the NSHM23 fault sections database (FSD) and earthquake geology database (EQGeoDB). In prior iterations of NSHM, fault sections were included only if a field-measurement-derived slip rate was estimated along a given fault. By expanding this inclusion criteria, we were able to assess a larger set of faults for use in NSHM23. The USGS Quaternary Fault and Fold Database served as a guide for assessing possible additions to the NSHM23 FSD. Reevaluating available data from published sources yielded an increase of fault sections from ~650 faults in NSHM18 to ~1,000 faults proposed for use in NSHM23. EQGeoDB, a companion dataset linked to NSHM23 FSD, contains geologic slip rate estimates for fault sections included in FSD. Together, these databases serve as common input data used in deformation modeling, earthquake rupture forecasting, and additional downstream uses in NSHM development

Documentation of surface fault rupture and ground-deformation features produced by the 4 and 5 July 2019 Mw6.4 and Mw7.1 ridgecrest earthquake sequence

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