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Located at the transition between monsoon- and westerly-dominated climate systems, major rivers draining the western North Qilian Shan incise deep, narrow canyons into latest Quaternary foreland basin sediments of the Hexi Corridor. Field surveys and previously published geochronology show that the Beida River incised 130 m at the mountain front over the Late Pleistocene and Holocene at an average rate of 6 m kyr -1. We hypothesize that a steep knickzone, with 3 % slope, initiated at the mountain front and has since retreated to its present position, 10 km upstream. Additional terrace dating suggests that this knickzone formed around the mid-Holocene, over a duration of less than 1.5 kyr, during which incision accelerated from 6 m kyr -1 to at least 25 m kyr -1. These incision rates are much faster than the uplift rate across the North Qilian fault, which suggests a climate-related increase in discharge drove rapid incision over the Holocene and formation of the knickzone. Using the relationship between incision rates and the amount of base level drop, we show the maximum duration of knickzone formation to be ∼700 years and the minimum incision rate to be 50 m kyr -1. We interpret that this period of increased river incision corresponds to a pluvial lake-filling event at the terminus of the Beida River and correlates with a wet period driven by strengthening of the Southeast Asian Monsoon. [ABSTRACT FROM AUTHOR]

Large, multi-fault earthquakes increase the threat of strong ground shaking and reshape the probability of future events across a system of faults. Fault junctions act as conditional barriers, or earthquake gates, that stop most earthquakes but permit junction-spanning events when stress conditions are favorable. Constraining the physical conditions that favor multifault earthquakes requires information on the frequency of isolated events versus events that activate faults through the junction. Measuring this frequency is challenging because dating uncertainties limit correlation of paleoseismic events at different faults, requiring a direct approach to measuring rupture through an earthquake gate. We show through documentation and finite-element modeling of secondary fault slip that co-rupture of the San Andreas and San Jacinto faults (California, USA) through the Cajon Pass earthquake gate occurred at least three times in the past 2000 yr, most recently in the historic 1812 CE earthquake. Our models show that gate-breaching events taper steeply and halt abruptly as they transfer slip between faults. Comparison to independent chronologies shows that 20%-23% of earthquakes on the San Andreas and the San Jacinto faults are co-ruptures through Cajon Pass. [ABSTRACT FROM AUTHOR]

Details about the timing and kinematics of rifting are crucial to understand the conditions that led to strain localization, continental rupture, and formation of the Gulf of California ocean basin. We integrate detailed geologic and structural mapping, basin analysis, and geochronology to characterize transtensional rifting on northeastern Isla Tiburón, a proximal onshore exposure of the rifted North America margin, adjacent to the axis of the Gulf of California. Slip on the Kunkaak normal fault tilted its hanging wall down-to-the-east ~ 70° and formed the non-marine Tecomate basin, deposited across a ~ 20° angular unconformity. From 7.1–6.4 Ma, the hanging wall tilted at 35 ± 5°/Myr, while non-marine sandstone and conglomerate accumulated at 1.4 ± 0.2 mm/yr. At least 1.8 ± 0.1 km of sediments and pyroclastic deposits accumulated in the Tecomate basin concurrent with clockwise vertical-axis block rotation and ~ 2.8 km of total dip-slip motion on the Kunkaak fault. Linear extrapolation of tilting and sedimentation rates suggests that faulting and basin deposition initiated ~ 7.6–7.4 Ma, but an older history involving initially slower rates is permissible. The Kunkaak fault and Tecomate basin are truncated by NW-striking, dextral-oblique structures, including the Yawassag fault, which accrued > 8 km of post-6.4 Ma dextral displacement. The Coastal Sonora fault zone on mainland Sonora, which accrued several tens of kilometers of late Miocene dextral offset, continues to the northwest, across northeastern Isla Tiburón and offshore into the Gulf of California. The establishment of rapid, latest Miocene transtension in the Coastal Sonora fault zone was synchronous with the 8–7 Ma onset of transform faulting and basin formation along the nascent Pacific-North America plate boundary throughout northwestern Mexico and southern California. Plate boundary strain localized into this Gulf of California shear zone, a narrow transtensional belt that subsequently hosted the marine incursion and continental rupture in the Gulf of California. [ABSTRACT FROM AUTHOR]

The Gulf of California rift has accommodated oblique divergence of the Pacific and North America plates in northwestern México since Miocene time. Due to its infancy, its rifted margins preserve a rare onshore record of early continental break-up processes and an opportunity to investigate the role of rift obliquity in strain localization. We map rift-related structures and syn -tectonic basins on southern Isla Tiburón, a proximal onshore exposure of the rifted North America margin. We integrate analysis and geochronology of syn -tectonic sedimentary basins and mapping of crosscutting relationships to characterize the style and timing of fault activity. On southern Isla Tiburón, an early phase of extension initiated between ~ 19–17 Ma and ~ 12.2 Ma. Subsequently, these normal faults and related basins were cut by the La Cruz strike-slip fault and buried by deposits of the La Cruz basin, an elongate, fault-controlled trough coextensive with the La Cruz fault. Crosscutting relationships show that the NW-striking La Cruz fault accrued 5 ± 2 km of dextral slip ~ 8–4 Ma. The La Cruz fault and parallel Tiburón transform were kinematically linked to detachment faulting that accommodated latest Miocene to Pliocene oblique opening of the offshore Upper Tiburón pull-apart basin. The onset of strike-slip faulting on Isla Tiburón was synchronous with the ~ 8–6 Ma onset of transform faulting and basin formation along > 1000 km of the reconstructed Pacific-North America plate boundary. This transition coincides with the commencement of a clockwise azimuthal shift in Pacific-North America relative plate motion that increased the obliquity of the Gulf of California rift and formed the Gulf of California shear zone. The record from the proto-Gulf of California illustrates how highly oblique rift geometries, where transform faults are kinematically linked to pull-apart basins, enhance the ability of continental lithosphere to rupture and, ultimately, hasten the formation of new oceanic rift basins. [ABSTRACT FROM AUTHOR]

River incision into a widespread, upland low-relief landscape, and related patterns of exhumation recorded by low-temperature thermochronology, together underpin geodynamic interpretations for crustal thickening and uplift of the eastern Tibetan Plateau. We report results from a suite of 11 (U–Th–Sm)/He cooling-age samples. Eight samples comprise a 1.2 km relief section collected from elevations up to 4800 m in the Jiulong Shan, an elevated, rugged region located in the hinterland of the Yalong-Longmen Shan Thrust Belt, and surrounded on three sides by upland low-relief landscape surfaces. Zircon and apatite cooling ages record two episodes of rapid exhumation in the early Oligocene and late Miocene, that were separated by a period of stability from ∼30 to 15 Ma. The first episode is consistent with a similar pulse evident from the Longmen Shan. The second episode is ongoing, and when integrated with adjacent cooling-age data sets, shows that doming of the Jiulong Shan has resulted in 2 to 4 km of differential exhumation of the plateau interior. We show from a compilation of glacial landform-mapping that the elevation of the plateau surface closely tracks global last glacial maximum equilibrium line altitude. We hypothesize that smoothing of highlands by efficient glacial and periglacial erosion, coupled with potential river captures and conveyance of sediments via external drainage, can yield an apparently continuous low-relief plateau landscape formed diachronously at high elevation. [ABSTRACT FROM AUTHOR]

Accurate information on the timing of earliest marine incursion into the Gulf of California (northwestern México) is critical for paleogeographic models and for understanding the spatial and temporal evolution of strain accommodation across the obliquely divergent Pacific-North America plate boundary. Marine strata exposed on southwest Isla Tiburón (SWIT) have been cited as evidence for a middle Miocene marine incursion into the Gulf of California at least 7 m.y. prior to plate boundary localization ca. 6 Ma. A middle Miocene interpretation for SWIT marine deposits has played a large role in subsequent interpretations of regional tectonics and rift evolution, the ages of marine basins containing similar fossil assemblages along ~1300 km of the plate boundary, and the timing of marine incursion into the Gulf of California. We report new detailed geologic mapping and geochronologic data from the SWIT basin, an elongate sedimentary basin associated with deformation along the dextral-oblique La Cruz fault. We integrate these results with previously published biostratigraphic and geochronologic data to bracket the age of marine deposits in the SWIT basin and show that they have a total maximum thickness of ~300 m. The 6.44 ± 0.05 Ma (Ar/Ar) tuff of Hast Pitzcal is an ash-flow tuff stratigraphically below the oldest marine strata, and the 6.01 ± 0.20 Ma (U/Pb) tuff of Oyster Amphitheater, also an ash-flow tuff, is interbedded with marine conglomerate near the base of the marine section. A dike-fed rhyodacite lava flow that caps all marine strata yields ages of 3.51 ± 0.05 Ma (Ar/Ar) and 4.13 ± 0.09 Ma (U/Pb) from the base of the flow, consistent with previously reported ages of 4.16 ± 1.81 Ma (K-Ar) from the flow top and (K-Ar) 3.7 ± 0.9 Ma from the feeder dike. Our new results confirm a latest Miocene to early Pliocene age for the SWIT marine basin, consistent with previously documented latest Miocene to early Pliocene (ca. 6.2-4.3 Ma) planktonic and benthic foraminifera from this section. Results from biostratigraphy and geochronology thus constrain earliest marine deposition on SWIT to ca. 6.2 ± 0.2 Ma, coincident with a regional-scale latest Miocene marine incursion into the northern proto-Gulf of California. This regional marine incursion flooded the northernmost, >500-km-long portion of the Gulf of California shear zone, a narrow belt of localized strike-slip faulting, clockwise block rotation, and subsiding pull-apart basins. Oblique Pacific-North America relative plate motion gradually localized in the >1000-km-long Gulf of California shear zone ca. 9-6 Ma, subsequently permitting the punctuated south to north flooding of the incipient Gulf of California seaway. [ABSTRACT FROM AUTHOR]

We show that a belt of clockwise vertical-axis block rotation associated with dextral-oblique rifting in the Basin and Range province in Mexico hosted the localization of plate-boundary strain that led to formation of the Gulf of California ocean basin. Paleomagnetism of Miocene ignimbrites distributed widely across the rift reveals the magnitude, distribution, and timing of rotation. Using new high-precision paleomagnetic vectors ( α95 ≈1°) from tectonically stable exposures of these ignimbrites in Baja California, we determine clockwise rotations up to 76° for intrarift sites. Low reference-site error permits isolation of intrarift block rotation during proto-Gulf time, prior to rift localization ca. 6 Ma. We estimate that 48% (locally 0%-75%) of the net rotation occurred between 12.5 Ma and 6.4 Ma. Sites of large (>20°) block rotation define an ∼100-km-wide belt, associated with strike-slip faulting, herein named the Gulf of California shear zone, which was embedded within the wide rift Basin and Range province and kinematically linked to the San Andreas fault. After a protracted history of diffuse extension and transtension, rift localization was accomplished by focusing of Pacifi c- North America dextral shear into the Gulf of California, which increased strain rates and connected nascent pull-apart basins along the western margin of the province. Oblique rifting thus helped to localize and increase the rate of continental break up and strongly controlled the three-dimensional architecture of the resultant passive margins. [ABSTRACT FROM AUTHOR]

Continental rifts require focused strain to rupture and form an ocean basin. In oblique rifts, such as the Gulf of California, focused transtensional strain associated with strike-slip faulting may serve as a catalyst for rupture. To test this hypothesis, we analyzed structural mapping, geochronology, paleomagnetism, and fault kinematics of pre- and synrift rocks exposed in an ~200 km² coastal mountain belt flanking the eastern rift margin of the northern Gulf of California. This coastal Sonora region hosts the onshore portion of the transform boundary between the Upper Tiburón and Adair-Tepoca marine basins--two early-formed oblique rift segments. Extension commenced here between 11.5 Ma and 7 Ma, resulting in 25°-40° of E-NE tilting, initiation of clockwise verticalaxis rotation of fault-bounded blocks, and minor basin sedimentation. Rates of deformation prior to 7 Ma are unconstrained due to a lack of exposed syntectonic deposits. Deformation after 7 Ma was associated with rapid tilting and the majority of observed clockwise vertical-axis rotation and strikeslip faulting. Nonmarine sedimentary basins accumulated coarse sediments above an unconformity eroded across older, tilted strata. By 5-6 Ma, deformation in coastal Sonora must have largely ceased and migrated westward into the Upper Tiburón marine basin. We document up to 120% total extension and total clockwise block rotations up to 53°. In portions of the study area, extension and rotation were supplanted by strike-slip faulting as deformation proceeded. We develop a tectonic model for this Coastal Sonora fault zone, which is bounded by major NW-striking transform faults with >10 km of displacement. Internal to the Coastal Sonora fault zone, the majority of an estimated 6.2 ± 1.1 km of dextral deformation, associated with up to 5.7 km of WNW-directed extension, occurred over the final 1-2 m.y. of its life span, at a strain rate approaching 10-14 s-1. This activity occurred as the plate boundary localized along nascent pull-apart basins in the northern Gulf of California, consistent with the hypothesis that late proto- Gulf of California dextral shear zones, such as the Coastal Sonora fault zone, acted to focus lithospheric-scale strain and promoted continental rupture in the wide-rift setting of the Mexican Basin and Range. [ABSTRACT FROM AUTHOR]

Abstract: We analyze high-resolution (>103 points/m2) terrestrial lidar surveys of the 4 April 2010 El Mayor–Cucapah earthquake rupture (Baja California, Mexico), collected at three sites 12–18 days after the event. Using point cloud-based tools in an immersive visualization environment, we quantify coseismic fault slip for hundreds of meters along strike and construct densely constrained along-strike slip distributions from measurements of offset landforms. Uncertainty bounds for each offset, determined empirically by repeatedly measuring offsets at each site sequentially, illuminate measurement uncertainties that are difficult to quantify in the field. These uncertainties are used to define length scales over which variability in slip distributions may be assumed to reflect either recognizable earthquake mechanisms or measurement noise. At two sites characterized by 2–3m of concentrated right-oblique slip, repeat measurements yield 2σ uncertainties of ±11–12%. Each site encompasses ∼200m along strike, and a smoothed linear slip gradient satisfies all measurement distributions, implying along-fault strains of ∼10−3. Conversely, the common practice of defining the slip curve by the local slip maxima distorts the curve, overestimates along-fault strain, and may overestimate actual fault slip by favoring measurements with large, positive, uncertainties. At a third site characterized by 1–2.5m of diffuse normal slip, repeat measurements of fault throw summed along fault-perpendicular profiles yield 2σ uncertainties of ±17%. Here, a low order polynomial fit through the measurement averages best approximates surface slip. However independent measurements of off-fault strain accommodated by hanging wall flexure suggest that over the ∼200m length of this site, a linear interpolation through the average values for the slip maxima at either end of this site most accurately represents subsurface displacement. In aggregate, these datasets show that given uncertainties of greater than ±11% (2σ), slip distributions over shorter scales are likely to be less uneven than those derived from a single set of field- or lidar-based measurements. This suggests that the relatively smooth slip curves we obtain over ∼102 m distances reflect real physical phenomena, whereas short wavelength variability over ∼100–101 m distances can be attributed to measurement uncertainty. [Copyright &y& Elsevier]

Large [moment magnitude (Mw) ≥ 7] continental earthquakes often generate complex, multifault ruptures linked by enigmatic zones of distributed deformation. Here, we report the collection and results of a high-resolution fenine returns per square meter) airborne light detection and ranging (LIDAR) topographic survey of the 2010 /Mw 12 El Mayor-Cucapah earthquake that produced a 120-kilometer-long multifault rupture through northernmost Baja California, Mexico. This differential LIDAR survey completely captures an earthquake surface rupture in a sparsely vegetated region with pre-earthquake lower-resolution (5-meter-pixel) LIDAR data. The postevent survey reveals numerous surface ruptures, including previously undocumented blind faults within thick sediments of the Colorado River delta. Differential elevation changes show distributed, kilometer-scale bending strains as large as ~10³ microstrains in response to slip along discontinuous faults cutting crystalline bedrock of the Sierra Cucapah. [ABSTRACT FROM AUTHOR]

• We present a probabilistic inheritance model for cosmogenic nuclide exposure ages. • Distributions reveal sediment contributions from episodic and diffusive processes. • We analyze global datasets and show landform ages are reduced by our model. • Impact of age reduction on slip rates must be analyzed on a case-by-case basis. • Our analysis shows landslide recurrence must be spatiotemporally correlated. Terrestrial cosmogenic nuclides (TCNs), produced by the bombardment of Earth's surface by cosmic rays, are widely used for age-dating and pacing surface processes. Sediments carry an inherited TCN concentration, useful for quantifying erosion and transport rates, but that must be subtracted when age-dating sedimentary landforms, such as alluvial fans. Here we present a mechanistic model of inheritance based on the contributions of episodic erosion by landsliding and steady, background erosion due to soil formation. The balance of these processes, revealed by the distribution of inheritance recorded by a population of individual surface clasts, affects rates of soil generation and the cycling of material through the Earth's critical zone – the surficial layer upon which all terrestrial life depends. We test our inheritance model on alluvial fan TCN datasets drawn from a global compilation of active-fault slip-rate studies. Inheritance-corrected landform ages are systematically younger than published ages. Our results reveal a consistent signature of spatiotemporal clustering of landslides, important for quantifying hazard and for understanding the coupling of physical and chemical erosion. Application of our inheritance model provides a rigorous approach to correcting landform ages for inheritance and reveals information on landslide frequency, with broad implications for hazard and land use. [ABSTRACT FROM AUTHOR]

The seismic cycle model is roughly constrained by limited offset data sets from the eastern Altyn Tagh fault with a low slip rate. The recent availability of high-resolution topographic data from the eastern Altyn Tagh fault provides an opportunity to obtain distinctly improved quantitative, dense measurements of fault offsets. In this paper, we used airborne light detection and ranging data and unmanned aircraft vehicle photogrammetry to evaluate fault offsets. To better constrain the large earthquake recurrence model, we acquired dense data sets of fault displacements using the LaDiCaoz_v2.1 software. A total of 321 offset measurements below 30 m highlight two new observations: (1) surface-slip of the most recent earthquake and multiple events exhibit both short-wavelength (m-scale) and long-wavelength (km-scale) variability; and (2) synthesis of offset frequency analysis and coefficient of variation indicate regular slip events with ~6 m slip increment on fault segments to the west of the Shulehe triple junction. The distribution of offsets and paleoseismological data reveal that the eastern Altyn Tagh fault exhibits characteristic slip behavior, with the characteristic slip of ~6 m and a recurrence period ranging from 1170 to 3790 years. Paleoearthquake recurrence intervals and slip increments yield mean horizontal slip-rate estimates of 2.1-2.6 mm/yr for fault segments to the west of the Shulehe triple junction. Assuming a 10 km rupture depth and a 30 GPa shear modulus, we estimated a characteristic slip event moment magnitude (Mw) of ~7.6. Finally, we discuss the interaction mechanism between Altyn Tagh fault (strike fault) and the NW-trending thrust faults (reverse faults) that caused the sudden decrease of sinistral slip rate at the Shulehe and Subei triple junctions; our results support the eastward "lateral slip extrusion" model. [ABSTRACT FROM AUTHOR]

Seismogenic faults are commonly composed of a series of segments, revealed by geometric barriers and along-strike gradients in fault activity. At the northeastern margin of the Tibetan Plateau, the Minle-Damaying Fault (MDF), bounding a portion of the eastern Qilian Shan, illustrates segmented paleoseismic behavior of a major seismogenic reverse fault. Field surveys, trench excavations, and drone-surveyed topography along the MDF reveal scarps with diverse cumulative displacements. Multiple events, with timing constrained by radiocarbon dating, are identified by both trench exposures and analysis of deformed landforms. Both individual coseismic displacements and slip rates taper toward the ends, revealing repeated rupture of the western MDF segment. Not all events present at the center of this segment appear at sites closer to the fault tips, suggesting that some earthquakes did not everywhere rupture to the surface. Isolated rupture of the western segment would produce a Mw7.0 earthquake. However, we cannot rule out correlation of some paleo-earthquakes between two segments, corresponding to larger events. A cluster of two 7-mslip events occurred on the eastern segment in the past 1,000 years, one of which may correspond to a multisegment event. [ABSTRACT FROM AUTHOR]

Abstract: The detailed features of an earthquake surface rupture may decay quickly in the landscape due to erosion or repair, such that primary displacements are seldom available for repeated study within the weeks following an earthquake. Here we describe a rapidly deployable field workflow using structure‐from‐motion (SfM) photogrammetry to produce three‐dimensional models of offset features and apply the technique to the 24 August 2014 Mw 6.0 South Napa earthquake surface rupture. We use repeated analyses of SfM point clouds using 3‐D interactive virtual reality tools to derive precise and reproducible fault‐slip measurements along the surface rupture and document the temporal evolution of afterslip at two sites along its trace. The SfM technique, combined with 3‐D virtual reality, enables archiving of primary field relationships and assessment of uncertainty in those data to a degree that was previously inaccessible. [ABSTRACT FROM AUTHOR]

Abstract: Fault geometry is recognized as an important control on earthquake rupture extent, magnitude, and recurrence behavior. However, multicycle earthquake rupture simulations indicate that geometrical barriers do not always stop rupture propagation. Stress buildup over multiple prior earthquakes may promote an occasional breach of a barrier, producing a significantly larger rupture. Here we present paleoseismic data from the Aksay restraining double bend of the Altyn Tagh fault, which supports such breaching. The Aksay bend consists of two main strands, which bend together in a broad transpressional configuration. We investigate the paleo‐earthquake histories of both strands to assess the correlation of events within and beyond the bend. The south strand exhibits four very likely paleo‐earthquakes with an average interval of 1,433 ± 57 years and a coefficient of variation (COV) of ~ 0.5. The north strand records four highest‐quality events after 3650 Before Common Era with an average interval of ~1,326 ± 179 years and a COV of ~0.3. The varying recurrence intervals and relatively high COVs indicate that faults within the Aksay bend have only quasi‐periodic rupture recurrence. Comparing the event sequences also suggests a complex rupture history. The penultimate event appears to correlate across the bend, as well as to previously published paleoseismic chronologies both to the east and to the west, which supports it being a barrier‐breaching event. The event histories reported here indicate that multiple earthquakes must terminate within the bend, over several earthquake cycles, in order to arrive at a stress condition suitable for a breaching earthquake rupture. [ABSTRACT FROM AUTHOR]

We systematically mapped (scales >1:500) the surface rupture of the 4 April 2010 Mw (moment magnitude) 7.2 El Mayor-Cucapah earthquake through the Sierra Cucapah (Baja California, northwestern Mexico) to understand how faults with similar structural and lithologic characteristics control rupture zone fabric, which is here defined by the thickness, distribution, and internal configuration of shearing in a rupture zone. Fault zone thickness and master fault dip are strongly correlated with many parameters of rupture zone fabric. Wider fault zones produce progressively wider rupture zones and both of these parameters increase systematically with decreasing dip of master faults, which varies from 20° to 90° in our dataset. Principal scarps that accommodate more than 90% of the total coseismic slip in a given transect are only observed in fault sections with narrow rupture zones (<25 m). As rupture zone thickness increases, the number of scarps in a given transect increases, and the scarp with the greatest relative amount of coseismic slip decreases. Rupture zones in previously undeformed alluvium become wider and have more complex arrangements of secondary fractures with oblique slip compared to those with pure normal dip-slip or pure strike-slip. Field relations and lidar (light detection and ranging) difference models show that as magnitude of coseismic slip increases from 0 to 60 cm, the links between kinematically distinct fracture sets increase systematically to the point of forming a throughgoing principal scarp. Our data indicate that secondary faults and penetrative off-fault strain continue to accommodate the oblique kinematics of coseismic slip after the formation of a thoroughgoing principal scarp. Among the widest rupture zones in the Sierra Cucapah are those developed above buried low angle faults due to the transfer of slip to widely distributed steeper faults, which are mechanically more favorably oriented. The results from this study show that the measureable parameters that define rupture zone fabric allow for testing hypotheses concerning the mechanics and propagation of earthquake ruptures, as well as for siting and designing facilities to be constructed in regions near active faults. [ABSTRACT FROM AUTHOR]

The moment magnitude (Mw) 7.0 12 January 2010 Haiti earthquake is the first major earthquake for which a large-footprint LiDAR (light detection and ranging) survey was acquired within several weeks of the event. Here, we describe the use of virtual reality data visualization to analyze massive amounts (67 GB on disk) of multiresolution terrain data during the rapid scientifi c response to a major natural disaster. In particular, we describe a method for conducting virtual fi eld work using both desktop computers and a 4-sided, 22 m3 CAVE immersive virtual reality environment, along with KeckCAVES (Keck Center for Active Visualization in the Earth Sciences) software tools LiDAR Viewer, to analyze LiDAR pointcloud data, and Crusta, for 2.5 dimensional surfi cial geologic mapping on a bare-earth digital elevation model. This system enabled virtual fi eld work that yielded remote observations of the topographic expression of active faulting within an ~75-km-long section of the eastern Enriquillo-Plantain Garden fault spanning the 2010 epicenter. Virtual fi eld observations indicated that the geomorphic evidence of active faulting and ancient surface rupture varies along strike. Landform offsets of 6-50 m along the Enriquillo- Plantain Garden fault east of the 2010 epicenter and closest to Port-au-Prince attest to repeated recent surface-rupturing earthquakes there. In the west, the fault trace is well defi ned by displaced landforms, but it is not as clear as in the east. The 2010 epi center is within a transition zone between these sections that extends from Grand Goâve in the west to Fayette in the east. Within this transition, between L'Acul (lat 72° 40 W) and the Rouillone River (lat 72° 35 W), the Enriquillo-Plantain Garden fault is undefi ned along an embayed low-relief range front, with little evidence of recent surface rupture. Based on the geometry of the eastern and western faults that show evidence of recent surface rupture, we propose that the 2010 event occurred within a stepover that appears to have served as a long-lived boundary between rupture segments, explaining the lack of 2010 surface rupture. This study demonstrates how virtual reality-based data visualization has the potential to transform rapid scientific response by enabling virtual fi eld studies and real-time interactive analysis of massive terrain data sets. [ABSTRACT FROM AUTHOR]

To better constrain the long-term millennial slip rate of the Haiyuan fault in its central part, we revisited the site of Daqing, where there are multiple paired offset terraces. We used 0.1-m-resolution terrestrial light detection and ranging (LiDAR) and uncrewed aerial vehicle imagery to survey the offset terraces, quantify their geomorphology, and map the fault trace. From these observations, we refined the geomorphological interpretation of the site, measured terrace riser offsets, and determined their relation to terrace formation. The well-constrained age of the highest terrace, T3, at 13.7 ± 1.5 ka, determined from a combination of surface and subsurface optically stimulated luminescence, 14C, and terrestrial in situ 10Be cosmogenic radionuclide dating, associated with an offset of 88 m, yields a late Pleistocene minimum slip rate of 6.4 ± 1.0 mm/yr. The less-well-constrained offset (72 ± 3 m) of the T3/T2 riser base and the age (>9.3 ± 0.6 ka) of terrace T2 yield a maximum slip rate of 7.7 ± 0.6 mm/yr. The smallest offset of a gully incised into T1 of 6.0 ± 0.5 m is potentially associated with the most recent slip event that occurred in the last millennia. Overall, these offsets and ages constrain a geological rate of 5-8 mm/yr (preferred rate >6.4 mm/yr), similar to geodetic estimates. Our collocated high-resolution topography and precise chronology make it possible to reveal the geomorphic complexities of terrace riser offsets and their postformational evolution, and to show how previously determined geological rates along the fault were both under- and overestimated. [ABSTRACT FROM AUTHOR]

We investigate the 4 April 2010 Mw 7.2 El Mayor‐Cucapah (Mexico) earthquake using three‐dimensional surface deformation computed from preevent and postevent airborne lidar topography. By profiling the E‐W, N‐S, and vertical displacement fields at densely sampled (∼300 m) intervals along the multisegment rupture and computing fault offsets in each component, we map the slip vector along strike. Because the computed slip vectors must lie on the plane of the fault, whose local strike is known, we calculate how fault dip changes along the rupture. A principal goal is to resolve the discrepancy between field‐based inferences of widespread low‐angle (<30°) oblique‐normal slip beneath the Sierra Cucapah, and geodetic and/or seismological models which support steeper (50°–75°) faulting in this area. Our results confirm that low‐angle slip occurred along a short (∼2 km) stretch of the Paso Superior fault—where the three‐dimensional rupture trace is also best fit by gently inclined planes—as well as along shorter (∼1 km) section of the Paso Inferior fault. We also characterize an ∼8‐km fault crossing the Puerta accommodation zone as dipping ∼60°NE with slip of ∼2 m. These results indicate that within the northern Sierra Cucapah, deep‐seated rupture of steep faults (resolved by coarse geodetic models) transfers at shallower depths onto low‐angle structures. We also observe a statistically significant positive correlation between fault dip and slip, with slip pronounced along steep sections of fault and inhibited along low‐angle sections. This highlights the important role of local structural fabric in controlling the surface expression of large earthquakes. Key Points: We use differential lidar to map the slip vector distribution and shallow fault geometry for the 2010 El Mayor‐Cucapah earthquake ruptureWe observe localized subsurface low‐angle (<30°) oblique‐normal slipWe observe a statistical correlation between dip and slip magnitude along the low‐angle structures [ABSTRACT FROM AUTHOR]

Long records of paleoearthquakes are essential for understanding earthquake recurrence behavior of active faults and for evaluating regional seismic hazard. However, paleoseismic data on the Altyn Tagh fault (ATF), one of the longest strike-slip fault in Asia, are scarce. We document a long paleoseismic record along the Xorkoli section of the central ATF. Eight and probably nine earthquakes are identified based on event evidence in the form of open fissures, folds, unconformities, and upward fault terminations, with modeled mean (95% confidence) ages of A.D. 1598 (1491–1741) yr (event A), A.D. 797 (676–926) yr (B), B.C. 668 (732–589) yr (C), B.C. 956 (1206–715) yr (D), B.C. 1301 (1369–1235) yr (E), B.C. 2105 (2233–1987) yr (F, probable), B.C. 2663 (2731–2601) yr (G), B.C. 2818 (2878–2742) yr (H), B.C. 3396 (3522–3205) yr (I). The mean recurrence interval is 620 ± 410 yr with a coefficient of variation (COV) of 0.67, indicating that earthquake recurrence is weakly periodic, with individual intervals ranging from as short as 150 yr to as long as 1460 yr. A global compilation of 35 strike-slip paleoseismic sites yields a similar average COV of 0.69. Synthesis of paleoseismic sites from the central ATF indicates that not all earthquakes ruptured to the eastern end of the Xorkoli section, within the Aksay restraining bend. Given that the 420-yr elapsed time since the most recent event, well within a COV of mean interval, a large surface-rupturing earthquake could occur at any time along the central ATF. [ABSTRACT FROM AUTHOR]

The geologic record of the formation of marine basins during continental rifting is uncommonly preserved. Using GIS-based paleotectonic maps, we show that marine basin formation in the Gulf of California-Salton trough oblique rift (Mexico and the United States) occurred in a stepwise manner as crustal thinning lowered elevations within the Gulf of California Shear Zone, and subsidence along strike-slip and transtensional faults linked isolated pull-apart basins. At 8 Ma, the earliest marine conditions in the Gulf of California were restricted to an embayment at its southern mouth. Farther north, the plate boundary was a set of continental strike-slip faults and linked pull-apart basins, similar to the modern Walker Lane in Nevada and California. By ca. 7 Ma, a series of marine incursions breached across strike-slip faults to the Pescadero and Farallon basins. Marine waters then breached a 75-100 km-long transtensional fault zone between the Farallon and Guaymas basins, with intermittent flooding that led to accumulation of extensive evaporite deposits in the Guaymas basin. Marine incursion north of the Guaymas basin via breaches across the Guaymas and Tiburón strike-slip faults and transtensional zones resulted in flooding of the northern >500 km of the oblique rift by 6.5-6.3 Ma. Thus, strike-slip and transtensional faulting promoted localization of plate boundary strain and guided punctuated marine flooding of the Gulf of California seaway. Inception of the narrow, 1500-km-long Gulf of California at ca. 6.3 Ma was followed by complete continental rupture in the Guaymas basin at ca. 6.0 Ma. [ABSTRACT FROM AUTHOR]

High-resolution topographic or imagery data effectively reveal geomorphic offsets along faults that can be used to deduce slip-per-event of recurrent rupture events. Documentation of patterns of geomorphic offsets is scarce on faults that undergo both creep and coseismic rupture. In this paper, we used newly acquired high-resolution light detection and ranging (LiDAR) data to compile geomorphic offsets along the Laohu Shan section of the Haiyuan fault, in the northern Tibetan Plateau, where interferometric synthetic aperture radar (InSAR) data suggest creep presently occurs over a 35-km-long stretch at a rate comparable to the long-term geological slip rate, despite evidence for past coseismic fault rupture. Numerous offset gullies identified using the LiDAR data yield a range of offsets from less than 2 m up to 50 m. These offsets have well-separated probability density peaks at 2-3 m, ~7 m, and ~14 m, with increments of 2-3 m, 4-6 m, and 5-7 m. The sequence of paleoseismic events along the Laohu Shan section indicates that the gullies with offsets of 2-3 m are likely related to surface rupture of the historical 1888 Jingtai earthquake, plus subsequent creep. Offset increments of 4-6 m and 5-7 m may represent coseismic slip in past paleoseismic events plus creep during the interseismic period. The creeping Laohu Shan section preserves numerous discrete cumulative offsets, with an offset clustering pattern indistinguishable from that on a locked fault with recurrent earthquake ruptures. Association of offset increments with known paleoseismic events yields a slip rate of 3-5 mm/yr during the past 200 years, roughly similar to the ~5 mm/yr creep rate. If the ratio of surface creep rate to the total fault slip rate has been continuous, then seismic moment release by brittle ruptures, and thus seismic hazard, would be much reduced on the Laohu Shan section of the Haiyuan fault. Alternatively, the current high creep rate may be a transient phenomenon, perhaps after slip following the 2000 Jingtai Mw 5.6 earthquake or in response to the adjacent 1920 M ***8 Haiyuan earthquake rupture that terminated immediately to the east. [ABSTRACT FROM AUTHOR]

The article offers information of redefining the age of the lower Colorado River, southwestern U.S. Topics discussed include Crow et al. appeal to—but give scant evidence for structural complications to reconcile the new ages with existing magneto stratigraphy; and they assert that "initially missed right-oblique fault strands subparallel to bedding" probably explain the discrepancy, and predict "further work will likely show that the Earthquake Valley.

We use two recent Japanese earthquakes to demonstrate the rich potential, as well as some of the challenges, of differencing repeat airborne Light Detection and Ranging (lidar) topographic data to measure coseismic fault zone deformation. We focus on densely-vegetated sections of the 14 June 2008 Iwate–Miyagi ( M w 6.9) and 11 April 2011 Fukushima–Hamadori ( M w 7.1) earthquake ruptures, each covered by 2 m-resolution pre-event and 1 m-resolution post-event bare Earth digital terrain models (DTMs) obtained from commercial lidar providers. Three-dimensional displacements and rotations were extracted from these datasets using an adaptation of the Iterative Closest Point (ICP) algorithm. These displacements remain coherent close to surface fault breaks, as well as within dense forest, despite intervals of ∼2 years (Iwate–Miyagi) and ∼4 years (Fukushima–Hamadori) encompassed by the lidar scenes. Differential lidar analysis is thus complementary to Interferometric Synthetic Aperture Radar (InSAR) and sub-pixel correlation techniques which often break down under conditions of long time intervals, dense vegetation or steep displacement gradients. Although the ICP displacements are much noisier than overlapping InSAR line-of-sight displacements, they still provide powerful constraints on near-surface fault slip. In the Fukushima–Hamadori case, near-fault displacements and rotations are consistent with decreased primary fault slip at very shallow depths of a few tens of meters, helping to account for the large, along-strike heterogeneity in surface offsets observed in the field. This displacement field also captures long-wavelength deformation resulting from the 11 March 2011 Tohoku great earthquake. [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]

Earthquake gates are fault geometric complexities, common in natural fault systems, that conditionally impede earthquake ruptures. This study centers on modeling of multicycle dynamics of the Pingding Shan earthquake gate along the central Altyn Tagh Fault in northwest China. The earthquake gate includes three geometric complexities: a prominent restraining bend, a 4-km wide stepover to the east, and a releasing bend to the west. We use a 2D finite element method to simulate coseismic spontaneous ruptures with interseismic fault stress evolutions computed by an analytic viscoelastic solution. Paleoseismic records and long-term slip-rates are used to constrain the models. We find that fault-geometry-related heterogenous stresses accumulated over earthquake cycles yield complex rupture patterns and help explain earthquake recurrence intervals revealed by paleoseismic records. The three most important contributions to the heterogeneous stresses come from dynamic ruptures passing fault geometric complexities, fault-strike-dependent tectonic loading and relaxation, and stress history from past earthquakes. In the Pingding Shan earthquake gate, the releasing stepover appears to impede ruptures more effectively than the restraining bend. The combined impact of the restraining bend and the releasing stepover makes the Pingding Shan earthquake gate a very effective barrier to 350-km model-spanning ruptures. The best-fit model yields a low recurrence interval of 4.6 kyrs for 350-km-long ruptures, interspersed with more frequent ruptures limited to individual fault segments. A lower static friction tends to reduce the effectiveness of the earthquake gate to impede ruptures. Local fault geometric complexities together with rapid energy release and restrengthening of friction during dynamic ruptures help explain the 0.66 recurrence interval coefficient of variation (COV) recorded at the Copper Mine paleoseismic trench site on the Xorxoli segment. This study provides a method for applying heterogenous initial stresses for single-event dynamic rupture simulations that consider the effect of past earthquakes and fault geometry. • Multicycle dynamics of Pingdingshan earthquake gate on Altyn Tagh fault is modeled. • The model is observation-constrained and predicts an effective Pingdingshan gate. • Dynamic ruptures on realistic fault geometry over cycles yield stress heterogeneity. • The heterogeneity is geometry-related and explains 0.66 COV in paleoseismic record. • The study demonstrates the critical role of fault geometry in earthquake dynamics. [ABSTRACT FROM AUTHOR]

It has been hypothesized that semi-periodic recurrence of earthquakes is more likely to occur on geometrically simple faults or fault sections. Paleoseismology provides geological evidence of the timing and slip at a point due to recurring major earthquakes on a fault, enabling a test of general models of earthquake recurrence behavior. In this study, we document a long paleoseismic record on the geometrically simple Xorkoli segment of the Altyn Tagh fault (ATF). Eight and probably nine earthquakes are identified through event evidence in the form of open fissures, folds, unconformities, and upward fault terminations, with modeled mean (95% confidence) ages of A.D. 1598 (1491-1741) yr (event A), A.D. 797 (676-926) yr (B), B.C. 668 (732-589) yr (C), B.C. 956 (1206-715) yr (D), B.C. 1301 (1369-1235) yr (E), B.C. 2105 (2233-1987) yr (F, probable), B.C. 2663 (2731-2601) yr (G), B.C. 2818 (2878-2742) yr (H), B.C. 3396 (3522-3205) yr (I). The mean recurrence interval is 620±410 years, with individual intervals ranging from as short as 150 years to as long as 1460 years. The coefficient of variation (COV) of intervals is 0.67±0.04 (1σ). This level of irregularity in recurrence pattern appears to be inconsistent with the hypothesis that the remarkable straightness of the Xorkoli segment, bounded by restraining bends, should produce regular events. The significant variation of recurrence interval suggests that the Xorkoli segment does not produce highly periodic earthquakes. A global compilation of 35 strike-slip paleoseismic sites, with long (n ≥ 5) earthquake records, located on thirteen different strike-slip fault; the mean COV of all the sites is 0.69 and COVs at 21 sites are between 0.4 and 0.8, indicating that quasi-periodicity is common for strike-slip faults. [ABSTRACT FROM AUTHOR]