The following report summarizes the dating results from sedimentary deposits exposed by soil pits in Panamint Valley, CA. Within this report, we detail the methodology used by the USGS Luminescence Geochronology Laboratory to obtain ages including sample preparation methods, luminescence measurement, equivalent dose determination, and dating-related calculations.

Observations of widespread liquefaction and stratigraphic and structural relationships in a trench across an ambiguous scarp are used to constrain the timing of Holocene earthquakes on the northwest-striking Wallula fault zone in southeast Washington and Oregon. Additional observations and age constraints from OSL analysis of samples collected from large-scale liquefaction features that crosscut the Mount St Helens J tephra (13.8-13.7 ka) exposed at a nearby outcrop suggest up to 3 Holocene regional liquefaction events, any of which were likely triggered by seismic shaking sourced from either the Wallula fault and/or faults of the Yakima fold and thrust belt. Our observations provide plausible evidence supporting that the scarp formed during the M6 1936 Milton-Freewater earthquake. In addition, stratigraphic relationships observed in this study indicate that the end of the Missoula Floods in the southeast Washington region occurred sometime between 13.8–13.5 cal. k.y. B.P., approximately 1,000 years earlier than prior estimates.

Several historic, multi-fault ruptures in the Eastern California Shear Zone (ECSZ) reinforce the need to understand how this rupture style contributes to seismic hazard in complex and diffuse fault zones. Several historic earthquakes in the ECSZ, the 1992 Landers, the 1999 Hector Mine, and the 2019 Ridgecrest rupture sequence, involved complex and multi-fault rupture. However, paleoseismic evidence of multi-fault ruptures in the ECSZ is poorly resolved in the rock record. Here I investigate paleoseismic evidence for complex rupture in Panamint Valley, located ~50 km northeast of the 2019 Ridgecrest ruptures. Late Holocene scarps in the 10 km-wide transtensional relay between the Ash Hill and Panamint Valley faults display surface rupture geometries analogous to those produced during the 1992 Landers and 1999 Hector Mine earthquakes. I produce a 1:4000 scale tectonogeomorphic map of the 40 km² area between the Ash Hill and Panamint Valley faults using my locally-calibrated relative-age alluvial fan chronology and using NCALM lidar DEMs and aerial imagery to identify ruptures. I bracket earthquakes with post-IR feldspar infrared-stimulated luminescence dating of offset deposits. I record vertical and lateral offsets at over 250+ locations using field mapping and backslipped reconstructions of newly generated high resolution (5 cm) drone-based structure from motion digital surface models. My mapping shows that the transtensional relay consists of 100+ fault strands that occur in parallel and en échelon arrays 5-7 km in length, with spacings of 1s to 100s of meters. Using my relative-age fan stratigraphy, geochronologic dating of offset deposits, and relative cumulative offset, I identify four late Holocene ruptures at ~0.3 – ~0.7 ka, ~0.7 – 2.4 ka, ~2.6 – 3.6 ka, and ~3.6 – 4.2 ka. Displacement magnitude per event ranges from 0.6 – 1.0 m of lateral slip and 0 – 0.2 m of dip slip. Displacement-length scaling relationships suggest that these mapped faults cannot rupture independently of a larger fault system. My results show overlap in the timing of ruptures in the transtensional relay, on the Ash Hill and Panamint faults, and that the Ash Hill and transtensional relay are kinematically similar. These similarities suggest this region acts as a zone for complex strain transfer between the Ash Hill and Panamint faults over multiple earthquake cycles. These relationships may support a geometric link at depth or the reoccupation of preexisting weaknesses at depth capable of transferring strain over larger distances.

Several historic, multi-fault ruptures in the Eastern California Shear Zone (ECSZ) reinforce the need to understand how this rupture style contributes to seismic hazard in complex and diffuse fault zones. Several historic earthquakes in the ECSZ, the 1992 Landers, the 1999 Hector Mine, and the 2019 Ridgecrest rupture sequence, involved complex and multi-fault rupture. However, paleoseismic evidence of multi-fault ruptures in the ECSZ is poorly resolved in the rock record. Here I investigate paleoseismic evidence for complex rupture in Panamint Valley, located ~50 km northeast of the 2019 Ridgecrest ruptures. Late Holocene scarps in the 10 km-wide transtensional relay between the Ash Hill and Panamint Valley faults display surface rupture geometries analogous to those produced during the 1992 Landers and 1999 Hector Mine earthquakes. I produce a 1:4000 scale tectonogeomorphic map of the 40 km² area between the Ash Hill and Panamint Valley faults using my locally-calibrated relative-age alluvial fan chronology and using NCALM lidar DEMs and aerial imagery to identify ruptures. I bracket earthquakes with post-IR feldspar infrared-stimulated luminescence dating of offset deposits. I record vertical and lateral offsets at over 250+ locations using field mapping and backslipped reconstructions of newly generated high resolution (5 cm) drone-based structure from motion digital surface models. My mapping shows that the transtensional relay consists of 100+ fault strands that occur in parallel and en échelon arrays 5-7 km in length, with spacings of 1s to 100s of meters. Using my relative-age fan stratigraphy, geochronologic dating of offset deposits, and relative cumulative offset, I identify four late Holocene ruptures at ~0.3 – ~0.7 ka, ~0.7 – 2.4 ka, ~2.6 – 3.6 ka, and ~3.6 – 4.2 ka. Displacement magnitude per event ranges from 0.6 – 1.0 m of lateral slip and 0 – 0.2 m of dip slip. Displacement-length scaling relationships suggest that these mapped faults cannot rupture independently of a larger fault system. My results show overlap in the timing of ruptures in the transtensional relay, on the Ash Hill and Panamint faults, and that the Ash Hill and transtensional relay are kinematically similar. These similarities suggest this region acts as a zone for complex strain transfer between the Ash Hill and Panamint faults over multiple earthquake cycles. These relationships may support a geometric link at depth or the reoccupation of preexisting weaknesses at depth capable of transferring strain over larger distances.

The data release includes 35 new luminescence ages and 3 new radiocarbon ages. The new age data further brackets the ages of the Las Vegas basin Quaternary stratigraphy and provide new constraints on the timing of Quaternary fault activity.

The data release includes 35 new luminescence ages and 3 new radiocarbon ages. The new age data further brackets the ages of the Las Vegas basin Quaternary stratigraphy and provide new constraints on the timing of Quaternary fault activity.

Deep Springs Valley (DSV) is a hydrologically isolated valley between the White (north and west) and Inyo (south and east) Mountains that is commonly excluded from regional paleohydrologic and paleoclimate studies. Previous studies showed that uplift of Deep Springs ridge (informal name) by the Deep Springs fault defeated streams crossing DSV and hydrologically isolating the valley sometime after eruption of the Bishop Tuff. Here we present tephrochronology, clast counts, paleontology, and infrared stimulated luminescence (IRSL) data that reaffirms interruption of the Plio-Pleistocene hydrology and formation of DSV during the Pleistocene. Fossil gastropod, ostracodes, and charophytes along with IRSL dating document the 83.3-61.5 ka freshwater Deep Springs Lake, which roughly coincides with 71-57 ka Marine Isotope State 4 (MIS 4) glacial climate period. Documentation of the MIS-4 glacial climate in southwestern North America is sparse and pluvial Deep Springs Lake is indirect evidence of the MIS 4 glaciation that is corroborated by pluvial lakes in nearby Owens and Searles Valleys. We hypothesize that the MIS-4 Deep Springs Lake overflowed into Eureka Valley via the Soldier Pass wind gap. Hydrologic evolution of DSV has potential implications for understanding Pliocene and Pleistocene biotic dispersal pathways and endemism.

Despite its subdued expression and isolated location within the Great Plains of southeastern Colorado, the 80-km-long Cheraw fault may be one of the most active faults in North America east of the Southern Rocky Mountains. We present geomorphic analyses, geochronology, and paleoseismic trenching data to 1) document the rupture history of the ~45-km-long southwestern section of the Cheraw fault over the past ~19 ka, and 2) evaluate slip-rate changes for the entire fault over the past ~200 ka. Results from new trenches excavated at the Old Ranch site show evidence of four surface-rupture events since ~19 ka, each with an average vertical displacement of 0.75 m. An additional event is likely only slightly older than ~19 ka. Evidence for relatively small displacements at and near the Old Ranch site suggests that most of these earthquakes were M 7 or less and likely did not rupture the full length of the Cheraw fault. Since ~19 ka, the average slip rate is ~0.16 mm/yr near the Old Ranch site with an average interevent time of 3 - 5 kyr. New geochronologic data for mid- to late Quaternary geomorphic surfaces cut by the Cheraw fault imply rapid incision by local Arkansas River tributaries from ~145 ka to ~100 ka. Maximum vertical offsets of 7 to 9 m for these surfaces indicate that from ~19 to >200 ka the average slip rate was no greater than ~0.03 mm/yr. The accelerated slip rate since ~19 ka suggests a possible response to rapid erosional unloading and/or a limited late Cenozoic, <40 kyr, paleoseismic history for the Cheraw fault.

Despite its subdued expression and isolated location within the Great Plains of southeastern Colorado, the 80-km-long Cheraw fault may be one of the most active faults in North America east of the Southern Rocky Mountains. We present geomorphic analyses, geochronology, and paleoseismic trenching data to 1) document the rupture history of the ~45-km-long southwestern section of the Cheraw fault over the past ~19 ka, and 2) evaluate slip-rate changes for the entire fault over the past ~200 ka. Results from new trenches excavated at the Old Ranch site show evidence of four surface-rupture events since ~19 ka, each with an average vertical displacement of 0.75 m. An additional event is likely only slightly older than ~19 ka. Evidence for relatively small displacements at and near the Old Ranch site suggests that most of these earthquakes were M 7 or less and likely did not rupture the full length of the Cheraw fault. Since ~19 ka, the average slip rate is ~0.16 mm/yr near the Old Ranch site with an average interevent time of 3 - 5 kyr. New geochronologic data for mid- to late Quaternary geomorphic surfaces cut by the Cheraw fault imply rapid incision by local Arkansas River tributaries from ~145 ka to ~100 ka. Maximum vertical offsets of 7 to 9 m for these surfaces indicate that from ~19 to >200 ka the average slip rate was no greater than ~0.03 mm/yr. The accelerated slip rate since ~19 ka suggests a possible response to rapid erosional unloading and/or a limited late Cenozoic, <40 kyr, paleoseismic history for the Cheraw fault.

Significant uncertainty remains in how and where crustal shortening occurs throughout the eastern Cascade Range in Washington State. Using lidar imagery, we identified a ~5 km long lineament in Swakane canyon near Wenatchee, roughly coincident with a strand of the Entiat fault. Topographic profiles across the lineament reveal a southwest-side-up break in slope with an average of ~3 m of vertical separation of the hillslope surface. We consider a range of possible origins for this feature, including differential erosion across a fault-line scarp, slope failure (sackung or landslide), and surface deformation across an active fault strand. Based on trenching, radiocarbon and luminescence dating, and ground penetrating radar (GPR) across the lineament, we conclude that warped saprolite observed in the shallow subsurface is most consistent with southwest-side-up folding caused by blind reverse faulting at depth. Following this reasoning, dating of overlying colluvial deposits suggests at least one Holocene earthquake occurred on this strand of the southern Entiat fault with an approximate vertical separation of ≥1 m. GPR reveals up to 4 m of cumulative vertical separation of the saprolite, suggesting a history of multiple earthquakes on the structure. Taken in context with other potential fault-related lineaments along the Entiat fault, our interpretation of Holocene earthquakes in Swakane canyon could suggest reactivation of longer sections of the Entiat fault, as well as other bedrock faults in the eastern Cascades. Although active erosion and slow strain rates lead to a subdued geomorphic expression of recent deformation, we conclude that the reactivated Entiat fault represents a seismogenic structure that should be considered in regional seismic hazard analyses. The difficulty of recognizing low slip-rate structures in forested and mountainous terrain underscores the importance of additional lidar surveys and geological and geophysical studies for fully understanding seismic hazard in regions with infrequent but potentially large earthquakes.