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Slip distribution, slip rate, and slip per event for strike‐slip faults are commonly determined by correlating offset stream channels—under the assumption that they record seismic slip—but offset channels are formed by the interplay of tectonic and geomorphic processes. To constrain offset channel development under known tectonic and geomorphic conditions, we use numerical landscape evolution simulations along a theoretical strike‐slip fault with uniform and steady uplift, erosion, and diffusion. We investigate the influence of four tectonic parameters (fault zone width, earthquake recurrence interval, variance of the recurrence interval, and total slip relative to channel spacing) on offset channel development through multiple earthquake cycles. Analysis of >3,000 automatically measured offsets from >135 simulations suggests ~30% variability in individual measurements, but modeled displacement is recovered by averaging multiple measurements. However, the average of multiple offset measurements systematically underestimates modeled slip except when the fault zone is less than ~5 m wide, total slip is less than channel spacing, and offsets are measured shortly after an earthquake. In these simulations, postearthquake landscape evolution widens the geomorphic expression of the fault zone and modifies apparent channel offsets. We distinguish this "geomorphic fault zone" from the tectonic fault zone (zone of coseismic distributed deformation). This study highlights the capability of landscape evolution models to explore a range of conditions not easily defined in natural examples and the importance of averaging multiple measurements. Our results verify that paleoseismic studies must consider how geomorphic change has modified offset markers and use caution interpreting slip histories with multiple earthquakes. Plain Language Summary: We use landscape evolution simulations to investigate how geomorphic markers (e.g., stream channels) record displacement from earthquakes on strike‐slip faults. Stream channels offset across a fault are commonly used to determine the amount of displacement and slip rate of a fault, critical inputs to seismic hazard calculations. For example, Wallace Creek on the San Andreas Fault was famously calculated by Sieh and Jahns (1984, https://doi.org/10.1130/0016‐7606(1984)95<883:HAOTSA>2.0.CO;2) to be offset ~128 m in ~3,750 years (~34 mm/year slip rate). However, actual displacement is rarely known, and displaced stream channels are created and modified by both tectonic processes, such as earthquakes, and geomorphic processes, such as erosion. We run landscape evolution simulations to characterize how offset channels develop under certain tectonic and geomorphic conditions. After an earthquake, the initially discrete channel offset is gradually smeared out and diminished. The results imply that offset stream channels accurately record modeled slip only when the zone of faulting is narrow, cumulative slip is less than the distance between stream channels, and offset distances are measured soon after an earthquake. Otherwise, average channel offset distances underestimate modeled slip, which indicates that slip rates and associated seismic hazard may be underestimated on real faults. Key Points: Mean‐measured offset records modeled slip only if fault zone width is <5 m, total slip is less than channel spacing, and short time since earthquakePostearthquake landscape evolution widens the geomorphic fault zone and smears out initially discrete channel offsetsOffset measurements have ~30% natural variability, but modeled slip is recovered by taking the mean of multiple offset measurements [ABSTRACT FROM AUTHOR]

Analyzes the emergence of marine terraces in the San Joaquin Hills, in the southern Los Angeles Basin in California. Potential earthquake intensity; Earthquake magnitude potential; Measurement of uplift rate during the late Quaternary.

Focuses on indications of a polyphase history dated from Tertiary Period extension in the volcanic strata of the Windermere Hills and Pequop Mountains in Nevada. Records of early extension by a west-tilted half graben filled with sedimentary rocks; Chemical composition of the volcanic rocks; Interpretations of the east-west-striking faults of the hills.

Determines the slip history of the Laguna Salada fault zone in northern Baja California, Mexico. Earthquake of February 23, 1892 as the most recent event along the fault zone; Length of ground rupture and amount of displacement; Timing of the most recent event; Estimation of the recurrence interval and slip rate.

River morphology and dynamics are strongly influenced by active tectonics. We report channel dynamics for the Peikang River, which flows through the Hsuehshan Range in central Taiwan. Using a digital elevation model and field surveys, we constrain channel morphology for an ∼90 km stretch of river to calculate unit stream power and boundary shear stress along the river path. Incision rates are estimated with optically stimulated luminescence dating of sand deposited on strath terraces. We find a strong correlation between unit stream power/shear stress and incision rate, but only if variation in channel width is considered. A calibrated river incision rule implies river incision rates of ∼9-13.5 mm/yr upstream of the Meiyuan and Tili faults and suggests that one or both of these structures are presently active. Our results indicate that the Shuilikeng fault is also actively deforming, as incision rates increase to ∼6-10 mm/yr across it, compared to 1-4 mm/yr in adjacent reaches. Prominent narrowing across the Shuilikeng fault, and the absence of significant gradient variation indicate that channel width is a first-order morphological adjustment to differential incision. Only when the channel width-to-depth ratio reaches a minimum does the channel slope significantly adjust to local changes in base level, as is the case upstream of the Meiyuan and Tili faults. [ABSTRACT FROM AUTHOR]

Earthquakes and bedrock river incision are fundamental processes in the evolution of tectonically active landscapes, yet little work has focused on understanding how a bedrock river responds to a single large earthquake. Data from the 1999 Mw = 7.6 Chi-Chi earthquake in central Taiwan show dramatic differences in river response that depend on the proximity to the rupture zone. Near the fault scarp, vertical ground deformation intensifies river incision on a time scale of years to decades, while distal to the fault, landslides induced by the earthquake mantle bedrock on the river bed with sediment, impeding incision for decades to centuries as the material is evacuated. This surprising spatial and temporal variability in bedrock incision caused by earthquakes has implications for the paleoseismic interpretation of bedrock terraces and for the evolution of tectonically active landscapes. [ABSTRACT FROM AUTHOR]

Pseudotachylyte veins are solidified frictional melt generated by localized highstrain- rate deformation, and they are commonly considered to be reliable geological indicators of seismic slip. While originally thought to occur primarily in the upper crust, field studies suggest pseudotachylyte can also form far below the frictional-viscous transition. This implies a more complex inter play between brittle and crystal-plastic deformation in crustal-scale fault systems. We report a new example of deep pseudo tachylyte from the Paleoproterozoic granulite-facies strikeslip Cora Lake shear zone in northern Saskatchewan, Canada. Multiple generations of pseudotachylyte have been variably overprinted by plastic deformation and growth of high-grade metamorphic mineral assemblages (including garnet, ortho pyroxene, and/or clinopyroxene). Undeformed pseudotachylyte generation surfaces locally display sinistral offsets, and deformed pseudotachylyte shows unambiguous shear sense indicators compatible with left-lateral Cora Lake shear zone kinematics. The vein networks are commonly proximal to and inherit kinematically compatible semibrittle shear fractures. Thermobarometry and modeling of mineral stability fields for the overprinted pseudotachylyte suggest vein generation between 0.7 and 0.8 GPa and 700 °C and 800 °C (corresponding to depths from 24 to 28 km). Combined with the kinematic compatibility and the well-constrained regional exhumation history, this suggests that the Cora Lake pseudotachylyte formed contemporaneously with the waning stages of deformation in the shear zone and constitutes one of the deepest examples of fault-related pseudotachylyte yet reported in North America. Evidence exists for several possible formation mechanisms, including in situ generation via ductile shear heating--induced plastic instabilities, as well as from interaction with earthquakes nucleating in the overlying brittle fault system. All of these possible mechanisms ultimately suggest a more rheologically complex environment than might typically be expected for these lower-crustal pressure and temperature conditions. [ABSTRACT FROM AUTHOR]

Spatial reasoning is a critical skill in the Geosciences. Using a randomized control study with 592 undergraduate students enrolled in introductory and advanced Geology courses, our data indicates that regular, short interventions throughout an academic semester improve students' spatial thinking skills significantly with a moderate to large effect size when compared to an instruction-as-usual control group. We found evidence for additional gains in students who participated also in hands-on training interventions. We found even larger training effects on students who achieved correct scores of >50% on the practice modules, with gains of almost three-quarter of a standard deviation relative to their baseline scores. Male and female students display significant differences in spatial skills, with males outperforming females. Training resulted in similar magnitudes of improvement in both genders, so we see no evidence that the interventions closed the gender gap. Using the initial performance as a baseline, poor performers' spatial skills appear to improve through practice at the same rate as their peers. We argue that 15.4% of students improve their spatial skills through participation in the training towards a threshold that appears to be critical for participation in STEM careers. On a reflection survey, half of the students reported that they felt their spatial thinking skills improved through their participation, and over a third found the training beneficial for their overall learning in Geology or other classes. We hypothesise that formal training opportunities for spatial reasoning could increase the potential pool of students who successfully enter and succeed in Geoscience careers. [ABSTRACT FROM AUTHOR]

Spatial reasoning is critical for success in science, technology, engineering, and mathematics (STEM) disciplines and particularly for geoscience. To evaluate capabilities among U.S. students, we assessed spatial reasoning skills in 345 introductory geology and upper-level structural geology students. The test measured students' abilities to disembed, visualize, and mentally rotate objects. The results highlight an uneven distribution of spatial skills with a minimum score of 6% and a maximum score of 75% correct responses. Spatial skills are positively correlated with standardized test scores, motivation for learning, STEM major declaration, and number of science courses taken. Our analysis also indicates that the cumulative, informal training of childhood play has the ability to increase spatial reasoning. Spatial skill scores were significantly higher among students who played action, construction, or sports video games in childhood. Male and female students display significant differences in spatial skills, especially for mental rotation, with males outperforming females. However, gender disparities are fully mediated after adjusting for a variety of academic factors and whether students frequently played with construction-based toys. This indicates that gender differences are experiential rather than biological in origin. This study suggests that both formal academic training and extracurricular activities appear to develop spatial skills throughout students' lives and indicates that systematic testing of spatial skills and formal training opportunities for students would facilitate improved spatial reasoning among students. We hypothesize that formal training opportunities for spatial reasoning could increase the potential pool of students who successfully enter STEM careers, including the geosciences, especially among women. [ABSTRACT FROM AUTHOR]

The 4 April 2010 moment magnitude (Mw) 7.2 El Mayor-Cucapah earthquake revealed the existence of a previously unidentified fault system in Mexico that extends ∼120 km from the northern tip of the Gulf of California to the U.S.-Mexico border. The system strikes northwest and is composed of at least seven major faults linked by numerous smaller faults, making this one of the most complex surface ruptures ever documented along the Pacific-North America plate boundary. Rupture propagated bilaterally through three distinct kinematic and geomorphic domains. Southeast of the epicenter, a broad region of distributed fracturing, liquefaction, and discontinuous fault rupture was controlled by a buried, southwest-dipping, dextral-normal fault system that extends ∼53 km across the southern Colorado River delta. Northwest of the epicenter, the sense of vertical slip reverses as rupture propagated through multiple strands of an imbricate stack of eastdipping dextral-normal faults that extend ∼55 km through the Sierra Cucapah. However, some coseismic slip (10-30 cm) was partitioned onto the west-dipping Laguna Salada fault, which extends parallel to the main rupture and defines the western margin of the Sierra Cucapah. In the northernmost domain, rupture terminates on a series of several north-northeast-striking cross-faults with minor offset (<8 cm) that cut uplifted and folded sediments of the northern Colorado River delta in the Yuha Desert. In the Sierra Cucapah, primary rupture occurred on four major faults separated by one fault branch and two accommodation zones. The accommodation zones are distributed in a left-stepping en echelon geometry, such that rupture passed systematically to structurally lower faults. The structurally lowest fault that ruptured in this event is inclined as shallowly as ∼20°. Net surface offsets in the Sierra Cucapah average ∼200 cm, with some reaching 300-400 cm, and rupture kinematics vary greatly along strike. Nonetheless, instantaneous extension directions are consistently oriented ∼085° and the dominant slip direction is ∼310°, which is slightly (∼10°) more westerly than the expected azimuth of relative plate motion, but considerably more oblique to other nearby historical ruptures such as the 1992 Landers earthquake. Complex multifault ruptures are common in the central portion of the Pacific North American plate margin, which is affected by restraining bend tectonics, gravitational potential energy gradients, and the inherently three-dimensional strain of the transtensional and transpressional shear regimes that operate in this region. [ABSTRACT FROM AUTHOR]