Stephen J. Angster

Field Reconnaissance after the 25 April 2015 M 7.8 Gorkha Earthquake

Fault scarps and uplifted terraces in young alluvium are frequent occurrences along the trace of the northerly dipping Himalayan frontal thrust (HFT). Generally, it was expected that the 25 April 2015 M xa07.8 Gorkha earthquake of Nepal would produce fresh scarps along the fault trace. Contrary to expectation, Interferometric Synthetic Aperture Radar and aftershock studies soon indicated the rupture of the HFT was confined to the subsurface, terminating on the order of 50xa0km north of the trace of the HFT. We undertook a field survey along the trace of the HFT and along faults and lineaments within the Kathmandu Valley eight days after the earthquake. Our field survey confirmed the lack of surface rupture along the HFT and the mapped faults and lineaments in Kathmandu Valley. The only significant ground deformation we observed was limited to an ∼1-km-long northeast-trending fracture set in the district of Kausaltar within Kathmandu. This feature is interpreted not to be the result of tectonic displacement, but rather a localized extension along a ridge. Our survey also shows the ubiquitous presence of fallen chimneys of brick kilns along the HFT and within the Kathmandu Valley. Measurements of a small subset of fallen chimneys across the region suggest a degree of systematic fall direction of the chimneys when subdivided geographically.

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 1050–790u2009calu2009yr B.P. and subsidence in A.D. 1964 and circa 640–510u2009calu2009yr B.P. Radiocarbon, 137Cs, and 210Pb ages bracketing a sand bed traced 1.5u2009km 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.

Latest Quaternary paleoseismology and evidence of distributed dextral shear along the Mohawk Valley fault zone, northern Walker Lane, California

The dextral-slip Mohawk Valley fault zone (MVFZ) strikes northwestward along the eastern margin of the Sierra Nevada in the northern Walker Lane. Geodetic block modeling indicates that the MVFZ may accommodate ~3 mm/yr of regional dextral strain, implying that it is the highest slip-rate strike-slip fault in the region; however, only limited geologic data are available to constrain the systems slip rate and earthquake history. We mapped the MVFZ using airborne lidar data and field observations and identified a site near Sulphur Creek for paleoseismic investigation. At this site, oblique dextral-normal faulting on the steep valley margin has created a closed depression that floods annually during spring snowmelt to form an ephemeral pond. We excavated three fault-perpendicular trenches at the site and exposed pond sediment that interfingers with multiple colluvial packages eroded from the scarp that bounds the eastern side of the pond. We documented evidence for four surface-rupturing earthquakes on this strand of the MVFZ. OxCal modeling of radiocarbon and luminescence ages indicates that these earthquakes occurred at 14.0 ka, 12.8 ka, 5.7 ka, and 1.9 ka. The mean ~4 kyr recurrence interval is inconsistent with slip rates of ~3 mm/yr; these rates imply surface ruptures of more than 10 m per event, which is geologically implausible for the subdued geomorphic expression and 60 km length of the MVFZ. We propose that unidentified structures not yet incorporated into geodetic models may accommodate significant dextral shear across the northern Walker Lane, highlighting the role of distributed deformation in this region.

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 ~22° has produced vertical separation across a scarp equal to 5.5 m. Stratigraphic, structural, geometrical, and radiocarbon observations are interpreted to indicate the displacement is the result of a single earthquake of 11.3 ± 3.5 m of dip-slip displacement that occurred 1146 – 1256 AD. Empirical scaling laws indicate that thrust earthquakes characterized by average displacements of this size may produce rupture lengths of 450 - > 800 km and moment-magnitudes Mw of 8.6 to > 9. Sufficient strain has accumulated along this portion of the Himalayan arc during the roughly 800 years since the 1146 – 1256 AD earthquake to produce another earthquake displacement of similar size.

3D Ground‐Motion Simulations of Mw 7 Earthquakes on the Salt Lake City Segment of the Wasatch Fault Zone: Variability of Long‐Period (T≥1 s) Ground Motions and Sensitivity to Kinematic Rupture Parameters

Abstract We examine the variability of long‐period ( T ≥1u2009u2009s) earthquake ground motions from 3D simulations of M w xa07 earthquakes on the Salt Lake City segment of the Wasatch fault zone, Utah, from a set of 96 rupture models with varying slip distributions, rupture speeds, slip velocities, and hypocenter locations. Earthquake ruptures were prescribed on a 3D fault representation that satisfies geologic constraints and maintained distinct strands for the Warm Springs and for the East Bench and Cottonwood faults. Response spectral accelerations (SA; 1.5–10xa0s; 5% damping) were measured, and average distance scaling was well fit by a simple functional form that depends on the near‐source intensity level SA 0 ( T ) and a corner distance R c :SA( R , T )=SA 0 ( T )(1+( R / R c )) −1 . Period‐dependent hanging‐wall effects manifested and increased the ground motions by factors of about 2–3, though the effects appeared partially attributable to differences in shallow site response for sites on the hanging wall and footwall of the fault. Comparisons with modern ground‐motion prediction equations (GMPEs) found that the simulated ground motions were generally consistent, except within deep sedimentary basins, where simulated ground motions were greatly underpredicted. Ground‐motion variability exhibited strong lateral variations and, at some sites, exceeded the ground‐motion variability indicated by GMPEs. The effects on the ground motions of changing the values of the five kinematic rupture parameters can largely be explained by three predominant factors: distance to high‐slip subevents, dynamic stress drop, and changes in the contributions from directivity. These results emphasize the need for further characterization of the underlying distributions and covariances of the kinematic rupture parameters used in 3D ground‐motion simulations employed in probabilistic seismic‐hazard analyses. Electronic Supplement: Description of seismic‐velocity model and wave propagation code and interpretation of directivity effects, and figures of fault geometry, Z1 and Z2.5 depths, effects of rupture speed variations on directivity effects and amplifications.

Application of UAV Photography to Refining the Slip Rate on the Pyramid Lake Fault Zone, Nevada

The Pyramid Lake fault zone (PLFZ) is a 50‐km‐long, active northwest‐trending right‐lateral fault in the northern Walker Lane, located ∼30u2009u2009km east of Reno, Nevada. Previous paleoearthquake and slip‐rate studies report that the Pyramid Lake fault has produced four surface‐rupturing paleoearthquakes since 15,475±720u2009u2009calu2009B.P., three of which occurred after 8980±260u2009u2009calu2009B.P., and has had an average minimum slip rate of 2.6±0.3u2009u2009mm/yr since the late Pleistocene. These observations imply that coseismic offset for each paleoearthquake averaged 7–9xa0m, larger than expected from empirical scaling relationships for a strike‐slip fault of its length. To reconcile this discrepancy, we used a small camera‐mounted unmanned aerial vehicle to develop high‐resolution digital elevation models and interpret previously unreported right‐laterally offset geomorphic features along the northern section of the PLFZ. Offset measurements at seven sites range from 8 to 21xa0m. The ages of displaced features are interpreted from previous lake level and mapping studies of Lake Lahontan. From these observations, slip‐rate estimates at the sites range between 0.5 and 1.6u2009u2009mm/yr. These lower values of slip rate require that coseismic displacements associated with previously reported paleoearthquakes average only 3–5xa0m, within the range that would be predicted from empirical scaling relationships of rupture length and coseismic displacement.

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 ∼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. 18–15xa0ka 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 0.8xa0mm/yr and total ∼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°–60°, and then compared the extension rates to GPS-derived rates of modern (last 7–9xa0yr) deformation. Our preferred fault dip values (45°–55°) yield estimated long-term extension rates (1.3–1.9 mm/yr) that underestimate our modern rate (2.4xa0mm/yr) by ∼21%–46%. The most likely sources of this underestimate are geologically unrecognizable deformation from moderate-sized earthquakes and unaccounted-xadfor coseismic off-fault deformation from large surface-rupturing earthquakes. However, fault dip values of ≤40° yield long-term rates comparable to or greater than modern rates, so an alternative explanation is that fault dips are closer to 40° 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 ~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. INTRODUCTION The Walker Lane (southwestern USA) is a major intraplate shear zone defined by a northwest-trending zone of discontinuous active faults, basins, and ranges that is between the Sierra Nevada to the west and the north-northeast– striking faults and ranges of the Basin and Range to the east (Fig. 1). It is unique in its width and discontinuous character when compared to other major continental strike-slip faults observed around the globe, including well-known examples such as the San Andreas, which is located just to the west (Fig. 1), the Anatolian fault of Turkey (e.g., Şengör et al., 2005), the Altyn Tagh of Tibet (e.g., Yin et al., 2002), the Denali fault system of Alaska (e.g., Hickman et al., 1978), and the Alpine fault system of New Zealand (Zealandia) (e.g., Norris and Toy, 2014; Mortimer et al., 2017). Cumulative right-lateral slip taken up by the Walker Lane ranges from ~30 km in the north to >50 km in the south. Geodesy shows the Walker Lane corresponds to a well-defined zone of northwestdirected transtensional dextral shear ranging from ~12 mm/yr in the south to ~5–7 mm/yr in the north (Bennett et al., 2003; Hammond and Thatcher, 2007). In Wesnousky (2005b) it was conjectured that the discontinuous nature of faulting in the Walker Lane as compared to the San Andreas may be attributed to the greater cumulative offset having accrued along the San Andreas, and that the San Andreas is transpressional along most of its length, in contrast to the transtension that characterizes the Walker Lane. In this regard, efforts to characterize the varying modes of deformation observed in the Walker Lane may provide clues to understanding the early development of continental shear zones exhibiting larger displacement during periods of time in their development where observations show they were characterized by transtension. The Carson and Excelsior domains of the Walker Lane are each characterized by the presence of left-lateral faults that strike approximately transverse to the general northwest trend of the Walker Lane. In this paper we focus on presenting new observations bearing on the recency, sense, and style of slip of faults within the Carson domain. These observations are then compared to those of others collected within the Excelsior domain to show that the developments of the left-lateral systems differ significantly in the two regions. GEOSPHERE GEOSPHERE; v. 13, no. 6 doi:10.1130/GES01483.1