Trench Exposures Research Articles

Paleoseismic Investigation of the Thousand Springs Fault, Northwestern Basin and Range, Oregon

ABSTRACT Earthquake recurrence intervals, surface-rupture extents, and interactions between faults provide insight into how faults behave and are critical for seismic hazard mitigation and earthquake forecasting. Investigating the paleoseismology of spatially related faults can reveal strain distribution and whether faults rupture as a system or independently. Summer Lake basin, a graben in the northwestern Basin and Range with four active faults (three of which have prior paleoseismic investigations), provides an opportunity to investigate fault interactions. To expand the paleoseismic record, two trenches were excavated across the previously undocumented Thousand Springs fault, exposing a normal fault zone that offsets a sequence of deep- to shallow-water lake sediments, sand dunes containing reworked Mazama ash, and other Cascades-sourced tephra. Tephra units were correlated to known units by their physical characteristics, stratigraphic sequence, glass chemistry, and two new radiocarbon dates from the uppermost lake sediments. Using trench exposures, measured vertical separations through auguring, colluvial wedges, and extrapolated offsets based on a constant sedimentation rate, we identified at least five surface-rupturing earthquakes with a total offset of 3.4 + 2/−1 m in the past ∼65 ka. The oldest event (EH5) occurred at 63.8 ± 1.5 ka, event horizon 4 at 36.2 ± 12.7 ka (which could be more than one event), and event horizon 3 at 24.6 ± 0.3 ka. Event horizon 2, a warping event at our site, is likely more than one event and occurred between 7.5 and 10 ka; and the most recent event (EH1+), most likely more than one event, occurred between 3.3 and 7.7 ka. Several events correlate, within error, with events on other faults in the Summer Lake basin, suggesting that (1) the faults generally rupture together as a system, (2) the most recent earthquake may have ruptured all faults in the region, and (3) fault rupture is influenced by the rapid regression of Lake Chewaucan (∼13 ka).

First paleoseismic data from the Balkan Range

The normal faults along the Balkan range front integrate great tectonic relief, prominent basin-and-range morphology, historic seismic quiescence, and ongoing dilatation rate varying near zero. The official standards prioritize the instrumental seismicity in a random model of earthquake occurrence, and the region is assessed at low seismic hazard levels. The need of geologic data on earthquake occurrence motivated us to excavate the first paleoseismological trench in the Balkan Range. We trenched the Zlatitsa normal fault, which occupies the westernmost position within a zone of localized strain along the Central Balkan Range. The trench exposure revealed two surface-rupturing earthquakes generated at a long-term slip rate lower than 0.04 mm/yr, probably about 0.02 mm/yr for the past 42 thousand years. A stratigraphic hiatus between the earthquake records, coincident with the Last Glacial Maximum, results in large uncertainties in the estimated inter-event time, yielding 13.2–33.4 kyr (95 % confidence interval). The new data suggest that the strain is released by regionally distributed faulting. The Zlatitsa fault may form spatial clusters with the neighboring faults, analogous to the two M6.8–7.1 earthquake pairs in 1904 and 1928, south the studied location. The time elapsed since the most recent earthquake on the Zlatitsa fault is 1–15 % from the duration of the previous seismic cycle. The geodetic dilatation rate in the fault vicinity most likely exceeds several times the paleoseismic dilatation rate. However, the geodetic rate eastward from the fault is nearly identical to the paleoseismic rate obtained in the trench. Assuming exponentially decreasing fault reloading, the Zlatitsa fault may still undergo rapid stress restoration, whereas the time elapsed since the most recent earthquakes on faults to the east may be longer than the corresponding relaxation time.

Paleoseismic evolution of the central section of the Serteng Shan fault at the northern Ordos Block (North China) since the Late Pleistocene

ABSTRACT Reliable palaeoseismological data are crucial for understanding the rupture history and seismic hazard assessment of a specific seismogenic structure. Many large earthquakes occurred around the Ordos Block of northern China. However, due to insufficient age data and event interpretations, the rupture patterns of the block are still poorly known. This study focuses on the central section of the Serteng Shan fault that is situated at the north margin of the Ordos Block. We present palaeoseismic sequence of faults using trenches combined with dating methods, including optically stimulated luminescence and 14C dating. We collected eleven optically stimulated luminescence (OSL) and seven radiocarbon samples to constrain the timing of palaeoseismic events in the excavated trench exposures along the fault strand. The results show that the studied samples were deposited since the latest Pleistocene, and the ages are, within uncertainty, in the stratigraphic order. The cause of the most recent surface rupture is still not clear. Four other older events, which occurred in 3.1–3.9 ka, 5.8−6.5 ka, 10.8–13.1 ka, and 14.2–16.5 ka, likely extended at different lengths along the Serteng Shan fault.

Holocene marine terraces as recorders of earthquake uplift: Insights from a rocky coast in southern Hawke's Bay, New Zealand

AbstractOn rocky tectonic coasts, data from Holocene marine terraces may constrain the timing of coseismic uplift and help identify the causative faults. Challenges in marine terrace investigations include: (1) identifying the uplift datums; (2) obtaining ages that tightly constrain the timing of uplift; (3) distinguishing tsunami deposits from beach deposits on terraces; and (4) identifying missing terraces and hence earthquakes. We address some of these challenges through comparing modern beach sediments and radiocarbon ages with those from a trench excavated across three terraces at Aramoana, central Hikurangi Subduction Margin, New Zealand. Sedimentary analyses identified beach and dune deposits on terraces but could not differentiate specific environments within them. Modern beach shells yielded modern radiocarbon ages, regardless of position or species, showing age inheritance and habitat is likely not an issue when dating shells on these terraces. By integrating terrace mapping, stratigraphy, morphology, and radiocarbon ages we develop a conceptual model of coastal uplift and terrace formation following at least two, possibly three, earthquakes at 5490–5070, 2620–2180, and 950–650 cal. yr bp. A high step and time gap between the upper two terraces raises the possibility that at least one intervening terrace is completely eroded. The trench exposure also showed that terrace stratigraphy may differ from that inferred from surface geomorphology, with apparent beach ridges being of composite origin and draping of younger beach deposits on the outer edge of a previous terrace. Dislocation modelling and comparison of marine terrace and earthquake ages from ~4 km south and ≤ 73 km north confirms that the most likely earthquake source is the nearshore, landward‐dipping, Kairakau Fault. Alternative sources, such as multi‐fault ruptures of the Kairakau‐Waimārama faults or Hikurangi subduction earthquakes, and/or a combination of the two are also possible and should be examined in future studies.

Testing the Synchronicity of Splay-Fault Ruptures in Carson Valley, Nevada, United States

ABSTRACTThe Carson City and Indian Hills faults in Carson City, Nevada, splay northeastward from the major range-bounding Genoa fault. Each splay is part of the Carson range fault system that extends nearly 100 km northward from near Markleeville, California, to Reno, Nevada. Stratigraphic and structural relationships exposed in paleoseismic excavations across the two faults yield a record of ground-rupturing earthquakes. The most recent on the Carson City fault occurred around 473–311 B.P., with the two penultimate events between 17.9 and 8.1 ka. Two trench exposures across the Indian Hills fault record the most recent earthquake displacement after ∼900 yr, preceded by a penultimate surface rupture ≥∼10,000, based on radiocarbon and infrared-stimulated luminescence dating of exposed sediments. The age estimates allow that the Carson City and Indian Hills faults ruptured simultaneously with a previously reported large earthquake on the Genoa fault ∼514–448 B.P. Similar synchronicity of rupture is not observed in the record of penultimate events. Penultimate ages of ruptures on the Carson City and Indian Hills faults are several thousand years older than that of the Genoa fault from which they splay. Together, these observations imply a variability in rupture moment through time, demonstrating the importance of considering multi-fault rupture models for seismic hazard analyses.

Primary on fault paleoseismic evidences from trench investigation along the Bathubasti fault, South Andaman, India

Continued structural descent of the Indian lithosphere below the Sunda-Andaman trench produced several destructive large to great earthquakes in the region of Andaman and Nicobar Islands. The pattern of strain accumulation and its release in the context of great plate boundary mega thrust earthquakes and slip partitioning along active faults in the Andaman and Nicobar region during these events is unclear, owing to lack of on-fault primary paleoseismic evidences, structural complexity associated with subduction processes and lack of primary surface ruptures of large to huge magnitude earthquakes. In this context, preliminary paleoseismic trench investigations were performed across a 6-m high fault scarp at Bathubasti (South Andaman) to understand the slip partitioning and rupture dynamics of subduction zone earthquakes. Ground-penetrating radar (GPR) profiles and the trench exposure across the scarp suggest progressive deformation and several episodes of co-seismic scarp generation where Andaman Flysch Sandstone (Oligocene) is in fault contact with the late Holocene sediments. A total of eight stratigraphic units (a to g) including three fault strands (F0, F1, and F2) were observed and mapped in the trench stratigraphy. OSL ages constrain that event 1 occurred along F0 around 2312 ± 302 years BP and thrust unit (g) Andaman Flysch -bedrock over unit (f) paleosols. Event 2 has deformed unit (C)-medium sand bed and the basal portion of unit (b)-fine sand bed along F1 and F2 around 422 ± 57 years BP. Given the scarp height of 6 m and with, a vertical displacement of approximately 3 m of unit (g)- bed rock along low-angle fault (θ = 18°) implies an average slip rate of 4.12 mm/year, shortening rate of 3.88 mm/year, and an uplift rate of 4.54 mm/year.

Методика определения технологических параметров ремонта трубопровода с ненормативной кривизной

Pipe sections with curvatures exceeding the required specified values are often found during the pigging of major pipelines. Process parameters for repairs have to be defined in order to develop a maintenance project that also includes works to restore the specified status of the section found. A corresponding method is known for cases of relocation of an initially straight pipeline. The authors developed a method for determining process parameters for repairing pipelines with out-of-project axis curvature. The method is based on modeling the deformation of a pipeline with initial axis curvature; it takes into account the actual operating conditions of the pipeline and in-line inspection data. Examples of calculating process parameters for repairs and stress-deformed state of pipeline sections with out-of-specs curvature have been presented. The modeling results confirm the possibility of using the method for evaluating the repair process parameters, the length of the required trench exposure, and the magnitude and boundaries of additional digging-in or the need to lift the pipeline in order to determine the stress–strain state of the pipeline section to be repaired during execution and after the completion of repairs.

Segmented Thrust Faulting: Example From the Northeastern Margin of the Tibetan Plateau

AbstractSeismogenic 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 aMw7.0 earthquake. However, we cannot rule out correlation of some paleo‐earthquakes between two segments, corresponding to larger events. A cluster of two 7‐m slip events occurred on the eastern segment in the past 1,000 years, one of which may correspond to a multisegment event.

Quaternary Activity of the Beihewan Fault in the Southeastern Beishan Wrench Belt, Western China: Implications for Crustal Stability and Intraplate Earthquake Hazards North of Tibet

AbstractWe present evidence for the Late Quaternary activity of the Beihewan Fault (BHWF), along the southeastern margin of the Beishan block, western China. Field observations and analysis of UAV‐derived DEMs and Google Earth images reveal an ~10‐km‐long active fault trace in a region previously considered tectonically inactive. Offset landforms and trench exposures reveal that the fault is dominantly strike‐slip, with local thrust or normal displacements. Average rates of left‐lateral motion and thrusting since the Holocene are estimated to be ~2.69 and ~0.35 mm/a, respectively. The most recent surface‐rupturing event generated ~2.5 m of strike‐slip offset since 6.2–8.6 ka; with estimated M = 6.3–6.9. Modeling and two‐dimensional inversion of new magnetotelluric data indicate that the BHWF is a subvertical low‐resistivity zone that penetrates into the lower crust. Existing geological and geophysical evidence do not support subsurface linkage between the BHWF and Altyn Tagh–Jinta‐Nanshan Fault system further south in the Hexi Corridor. Close examination of high‐resolution satellite imagery west and east of the BHWF reveals other Quaternary surface fault scarps, pressure ridges, offset drainages, and truncated lithological strike belts. Collectively, these features constitute a previously unrecognized 150‐km‐long sinistral deforming belt along the southeastern Beishan and northern Hexi Corridor boundary that we call the Southeast Beishan Wrench Belt. Recognition of Quaternary activity along the BHWF within the larger Southeast Beishan Wrench Belt challenges long‐held assumptions that the Beishan region is a relatively stable and tectonically inactive block within the Late Cenozoic Indo‐Eurasian deformation field north of Tibet.

Neotectonics and related crustal deformation along Carbyn thrust fault, South Andaman, India: implications of the frontal surface faulting and propagation of tectonic activity towards Andaman trench

Paleoseismic investigations have been carried out across an ~ 10-m-high scarp along the Carbyn thrust fault, that marks the structural contact between the overriding Ophiolite suite of the Cretaceous age and under riding Andaman Flysch group (Oligocene) in the outer-arc setting of the Northern segment of Sunda-Andaman subduction zone. The Carbyn thrust fault is one of the important concomitant faults in the accretionary prism of the Andaman subduction zone and the surface trace marks a N-S trending thrust fault with Easterly dip which extends for a length of about 30 km in the Eastern half of South Andaman, starting from Carbyn area up to Chidiyatapu (Southernmost tip of the South Andaman) through Kodiyaghat and Burmanallah areas. The thrust has emplaced dismembered Cretaceous Ophiolite slices over Oligocene siliciclastic turbidites (Andaman Flysch) almost in a N-S trending linear zone of deformation. The 10-m-high scarp expressed at the interface between Ophiolite suite and Andaman Flysch group borders the topographic front of Ophiolite suite and breaks the Quaternary and late Holocene succession almost up to a length of 3.7 km from Carbyn to Burmanallah area, registers the youngest surface deformation along the thrust. The scarp is abrupt and sharp trending in N-S direction shows variations in displacement along the strike and has preserved its initial geometry without erosion. Trench exposure and Georadar survey across the scarp at Carbyn area reveals a very low angle thrust fault that cuts the bedrock surface (Ultramafics of Ophiolite Suite) rocks against the young alluvial surface deposited by Prothrapur Nallah and Carbyn creek at Carbyn area. The thrust fault shows flattening of the fault geometry around 16° at the base of the trench and dips 8–10° at near surface. The expression of the scarp at the frontal position of Ophiolite Suite and verging towards the West displacing late Holocene sediments reflects advancing of thrusting and propagation of tectonic activity towards Andaman trench.

A 3700 yr paleoseismic record from the northern San Jacinto fault and implications for joint rupture of the San Jacinto and San Andreas faults

The San Andreas and San Jacinto faults are the primary plate-boundary structures in southern California and present a large earthquake hazard for the region. They approach each other in the Cajon Pass area between the San Gabriel and San Bernardino Mountains, where the northern end of the San Jacinto fault forms a 2-km-wide releasing step with the San Andreas fault. In this study, we used paleoseismic data from sites on the San Jacinto and San Andreas faults near their juncture to evaluate spatial and temporal patterns of surface rupture between these major structures of the North American–Pacific transform plate boundary. We present a new 3700 yr paleoseismic record from the northern San Jacinto fault at Mystic Lake, where trench excavations exposed evidence for at least 16 surface ruptures. A sedimentary gap in our trench exposures separates three ruptures in the oldest part of the record from 13 ruptures during the past 2000 yr. For the past 2000 yr, the mean recurrence times varied from 86 to 312 yr, with a mean recurrence interval of 160 yr. This rate of surface rupture is roughly equal to that of the southern San Andreas fault south of the Cajon Pass juncture, but it is half that of the San Andreas fault north of the juncture, indicating that coseismic strain on the San Andreas fault is split between the southern San Andreas and San Jacinto faults south of Cajon Pass. Comparison of the past 2000 yr of the Mystic Lake record to similar paleoseismic records from nearby sections of the San Andreas fault suggests that: (1) the current open interval on these two faults in the study area is longer than their average recurrence intervals, but that similar intervals of quiescence have occurred in the past 2000 yr; (2) the San Andreas and San Jacinto faults have probably ruptured together multiple times in the past 2000 yr; and (3) a joint rupture of the San Jacinto fault with the Mojave section of the San Andreas fault may be a more likely source of a major earthquake in southern California than a rupture on the San Andreas fault from the Mojave segment to the southern end of the fault.

Frontal fault location and most recent earthquake timing for the Alpine Fault at Whataroa, Westland, New Zealand

ABSTRACTActive traces of the reverse dextral Alpine Fault were poorly located on the true left (southwest) side of the Whataroa River. Mapping using airborne LiDAR-derived high resolution topographic data has identified a 300 m wide zone of partitioned faulting including the northeast-striking frontal trace of the Alpine Fault, c. 1.3 km northwest of the DFDP-2 drillsite. Excavation into a 1.2 m high, northeast-trending scarp exposed sheared gravels draped by folded fine-grained alluvial deposits relating to only the most recent faulting event. Sheared gravels within the trench exposure have fault fabrics dipping in the range of c. 45–65° to the southeast. Radiocarbon dates used in an OxCal model that incorporates dendrochronological results from trees that colonised the river floodplain following migration of the river constrain the timing of this faulting event to c. AD 1706–1823. Thus, faulting that formed this single-event scarp has been correlated to the well-known AD 1717 earthquake. The faulted terrace (tw4) was formerly the active floodplain of the Whataroa River, which reworked the surface post-AD 1717 before incising to its current bed level. The preferred fault dip from this frontal trace to the Alpine Fault target depth beneath the DFDP-2 drillsite is c. 46° to the southeast, which is consistent with the range of dips observed in the trench.

Primary surface rupture of the 1950 Tibet-Assam great earthquake along the eastern Himalayan front, India

The pattern of strain accumulation and its release during earthquakes along the eastern Himalayan syntaxis is unclear due to its structural complexity and lack of primary surface signatures associated with large-to-great earthquakes. This led to a consensus that these earthquakes occurred on blind faults. Toward understanding this issue, palaeoseismic trenching was conducted across a ~3.1 m high fault scarp preserved along the mountain front at Pasighat (95.33°E, 28.07°N). Multi-proxy radiometric dating employed to the stratigraphic units and detrital charcoals obtained from the trench exposures provide chronological constraint on the discovered palaeoearthquake surface rupture clearly suggesting that the 15th August, 1950 Tibet-Assam earthquake (Mw ~ 8.6) did break the eastern Himalayan front producing a co-seismic slip of 5.5 ± 0.7 meters. This study corroborates the first instance in using post-bomb radiogenic isotopes to help identify an earthquake rupture.

Application of laser scanning for rapid geologic documentation of trench exposures

Trenching of a known or inferred fault is a traditional technique to verify the presence of the fault and potentially determine relative palaeoseismic slip rates and magnitudes. These procedures have been employed and improved over the past 50years and have been particularly applied by engineering geologists concerned with local and regional tectonics that affect large structures such as dams and power plants as well as local habitable structures. Traditionally, however, trench excavation and documentation (logging) is time-consuming and laborious, requiring experienced geologists to identify plausible trench locations and to manually document the exposures in the field. Differences of interpretation are common; hence, many jurisdictions require an independent reviewer to field check trench exposures and to assess the reasonableness of the geologic interpretations. In this paper, we describe a less costly, novel technique to enhance the quality and rapidity of geologic trench documentation. Specifically, we use terrestrial Light Detection and Ranging (LiDAR) scans, coupled with camera imagery, to collect three-dimensional point clouds. These data are then converted into trench-wall images that are readily accessible for analysis even after trench closure. The procedure also provides grain-size (texture) data that are useful for identification and correlation of discrete, trench-exposed stratigraphic markers that are offset or that overlie an imaged fault without being broken. Then, depending on sediment dating, a fault can be deemed “active” or “hazardous” depending on the local classification, and this information can thus be used by planners and others involved in hazard and risk reduction.

Paleoseismic evidence of a giant medieval earthquake in the eastern Himalaya

AbstractWe present here the results of a paleoseismic investigation carried across a ~10 m high fault scarp at Panijhora village, West Bengal in northeastern India. Accelerator Mass Spectrometer analyzed 14C radiocarbon age constraints from six detrital charcoal samples ranging between 1688 B.C. and A.D. 1152 are consistent with the great medieval earthquake of A.D. 1255 that is interpreted to have produced a minimum observed fault slip of ~5 m in the trench exposure. Recalibration of radiocarbon ages from previous studies at Harmutty, Nameri, and Marha in the eastern Himalaya using Bayesian statistical analyses further substantiates the possibility that the A.D. 1255 earthquake might have ruptured the Himalayan front over a length of ~800 km from ~85.87° to 93.76°E longitudes.

High‐Resolution Trench Photomosaics from Image‐Based Modeling: Workflow and Error Analysis

Photomosaics are commonly used to construct maps of paleoseismic trench exposures, but the conventional process of manually using image‐editing software is time consuming and produces undesirable artifacts and distortions. Herein, we document and evaluate the application of image‐based modeling (IBM) for creating photomosaics and 3D models of paleoseismic trench exposures, illustrated with a case‐study trench across the Wasatch fault in Alpine, Utah. Our results include a structure‐from‐motion workflow for the semiautomated creation of seamless, high‐resolution photomosaics designed for rapid implementation in a field setting. Compared with conventional manual methods, the IBM photomosaic method provides a more accurate, continuous, and detailed record of paleoseismic trench exposures in approximately half the processing time and 15%–20% of the user input time. Our error analysis quantifies the effect of the number and spatial distribution of control points on model accuracy. For this case study, an ∼87 m2 exposure of a benched trench photographed at viewing distances of 1.5–7 m yields a model with <2 cm root mean square error (rmse) with as few as six control points. Rmse decreases as more control points are implemented, but the gains in accuracy are minimal beyond 12 control points. Spreading control points throughout the target area helps to minimize error. We propose that 3D digital models and corresponding photomosaics should be standard practice in paleoseismic exposure archiving. The error analysis serves as a guide for future investigations that seek balance between speed and accuracy during photomosaic and 3D model construction. Online Material: Image‐based modeling workflow for paleoseismic trench photomosaics, 3D trench model, example photomosaic log, table of error analysis data, and Python script for exporting photomosaics in a vertical plane.

Paleoseismic Evidence for Late Holocene Tectonic Deformation along the Saddle Mountain Fault Zone, Southeastern Olympic Peninsula, Washington

Abstract Trench and wetland coring studies show that northeast‐striking strands of the Saddle Mountain fault zone ruptured the ground about 1000 years ago, generating prominent scarps. Three conspicuous subparallel fault scarps can be traced for 15 km on Light Detection and Ranging (LiDAR) imagery, traversing the foothills of the southeast Olympic Mountains: the Saddle Mountain east fault, the Saddle Mountain west fault, and the newly identified Sund Creek fault. Uplift of the Saddle Mountain east fault scarp impounded stream flow, forming Price Lake and submerging an existing forest, thereby leaving drowned stumps still rooted in place. Stratigraphy mapped in two trenches, one across the Saddle Mountain east fault and the other across the Sund Creek fault, records one and two earthquakes, respectively, as faulting juxtaposed Miocene‐age bedrock against glacial and postglacial deposits. Although the stratigraphy demonstrates that reverse motion generated the scarps, slip indicators measured on fault surfaces suggest a component of left‐lateral slip. From trench exposures, we estimate the postglacial slip rate to be 0.2 mm/yr and between 0.7 and 3.2 mm/yr during the past 3000 years. Integrating radiocarbon data from this study with earlier Saddle Mountain fault studies into an OxCal Bayesian statistical chronology model constrains the most recent paleoearthquake age of rupture across all three Saddle Mountain faults to 1170–970 calibrated years (cal B.P.), which overlaps with the nearby M w 7.5 1050–1020 cal B.P. Seattle fault earthquake. An earlier earthquake recorded in the Sund Creek trench exposure, dates to around 3500 cal B.P. The geometry of the Saddle Mountain faults and their near‐synchronous rupture to nearby faults 1000 years ago suggest that the Saddle Mountain fault zone forms a western boundary fault along which the fore‐arc blocks migrate northward in response to margin‐parallel shortening across the Puget Lowland.

Holocene Paleoearthquakes and Early–Late Pleistocene Slip Rate on the Sulmona Fault (Central Apeninnes, Italy)

Abstract The 23 km long Sulmona normal fault flanks the southwestern slope of the Mount Morrone range in the central Apennines. To date, the recent activity of this structure is uncertain, as the faulting evidence concerns only undated deposits and the fault cannot be associated with any of the strong historical earthquakes ( M w >6.5) of the region. Our observation from ∼1 Ma offset slope breccias, coupled with new tephrochronological data on faulted early Last Glacial lacustrine infilling of the Sulmona basin, implies a vertical slip rate of ∼0.5 mm/yr over both the long term and middle term. Moreover, at the apex of a Late Pleistocene alluvial fan, radiocarbon dating of offset stratigraphy uncovered in four paleoseismological trenches shows that repeated earthquakes resulted in more than 4 m of vertical offset since ∼9 ka, providing again a minimum Holocene vertical slip rate of ∼0.5 mm/yr. In combination with the results of the trench exposure, we define four faulting events in the past ∼9 ky B.P., and at least one before then. The most recent is constrained by robust radiocarbon dates as the middle of the second century A.D. The penultimate event was around the middle of the fifth millennium B.P., whereas the other two events are dated in a time between 8.5 ky to 6.5 ky B.P. and around 9 ky cal B.P. The oldest occurred before 9.5 ky B.P. As well as revealing the unexpected paleoseismic history of the Sulmona silent fault, our data provide a recurrence time for M w ≥6.5 earthquakes of ∼2.4 ky versus an elapsed time of ∼1.85 ky since the last event. The latter matches with an earthquake previously hypothesized through archaeoseismic clues collected from several Roman settlements of the Sulmona Plain, all of which were dated to halfway through the second century A.D. Online Material: Table of major‐element compositions, field photos, and Harker diagrams.

Holocene Earthquakes and Late Pleistocene Slip-Rate Estimates on the Wassuk Range Fault Zone, Nevada

The Wassuk Range fault zone is an 80‐km‐long, east‐dipping, high‐angle normal fault that flanks the eastern margin of the Wassuk Range in central Nevada. Observations from two alluvial fan systems truncated by the fault yield information on the vertical slip rate and Holocene earthquake history along the range front. At the apex of the Rose Creek alluvial fan, radiocarbon dating of offset stratigraphy exposed in two fault trenches shows that multiple earthquakes resulted in 7.0 m of vertical offset along the fault since ∼9400 cal B.P. These data yield a Holocene vertical slip rate of 0.7±0.1 mm/yr. The south trench exposure records at least two faulting events since ∼9400 cal B.P., with the most recent displacement postdating ∼2810 cal B.P. The north trench exposure records an ∼1 m offset between ∼610 cal B.P. and A.D. ∼1850, a 1.3‐m minimum offset prior to ∼1460 cal B.P., and one earlier undated earthquake of a similar size. Variations in stratigraphy and limited datable material preclude a unique correlation of paleoevents between the two trenches. Approximately 25 km north, the range‐front fault has truncated and uplifted a remnant of the Penrod Canyon fan by >40 m since the surface was deposited ∼113 ka, based on cosmogenic dating of two large boulders. These data allow an estimate of the minimum late Pleistocene vertical slip rate at >0.3–0.4 mm/yr for the Wassuk Range fault zone. Online Material: Tables summarizing radiocarbon and cosmogenic analyses, photos of rocks sampled for cosmogenic analyses, and figures summarizing cosmogenic exposure age estimates.

Natural levees and human settlement in the Song Hong (Red River) delta, northern Vietnam

The Song Hong (Red River) delta, northern Vietnam, is characterized by huge natural levees in an area of the delta plain known as the West Floodplain where fluvial sedimentation predominates. The natural levees along the Day River, a major distributary of the Song Hong, are comparable in size with those of the main course of the Song Hong. The Day River levees are 3–8 km wide and rise 2–5 m above the adjacent backswamps and have played an important role in human settlements since the late Metal age. We discussed the relationships among the natural levees of the Day River, delta progradation and the distribution of archaeological sites on the delta plain. During the early Holocene, the accumulation of sediment discharged by the Song Hong enhanced both aggradation of the floodplain and river mouth progradation within the drowned valley of the Song Hong. Radiocarbon dates from cores, trench exposures, and archaeological sites record a dramatic slowing of aggradation when sea level stabilized during 6–4 cal. kyr BP (the Holocene sea-level highstand). As sea level fell to the present level during 4–0 cal. kyr BP, the river mouth prograded rapidly toward the Gulf of Bac Bo (Gulf of Tonkin) and the river channels extended seaward. In the West Floodplain, lateral accretion overtook vertical accretion to generate the present longitudinal profiles of the Song Hong and Day rivers. During this period, human settlements spread across the backswamp and Holocene terrace area, lagging around 2 kyr behind the shoreline migration.