Mountain Fault Zone Research Articles

Analysing the Relationship Between the Fault’s Distribution and the Earthquake Actives Near Gongxian China Based on Gravity Data

Huaying Mountain fault zone, the largest fault zone in the Sichuan Basin, is playing an important role to geological studies, such as the characterization of structural features, earthquakes occurrence, oil and gas reservoir distribution. However, the complexity and lack of consensus on deformation mechanisms necessitate further investigation of fold and thrust belts in the SB. Therefore, based on the real areal gravity exploration data and magneto-telluric (MT) data, it characterizes the spatial distribution of HMFZ in this region. Further, it gives the analysis combining the gravity anomaly, fault distribution data, and frequent earthquake activity around the study area, which provides insights that can be exploited for the evaluation of seismogenic tectonics in this region.

Geophysical Characterisation and Oil–Gas Resource Analysis of the Southern Huaying Mountain Fault Zone, Sichuan Basin, China

As the west rim of an ejective fold zone, the Huaying Mountain fault zone (HMFZ) in the eastern Sichuan Basin (SB) plays an important role in the tectonic evolution of the SB. The distribution and characteristics of HMFZ are strongly associated with tectonic activities and have greatly impacted the distribution of oil and gas reservoirs. However, its distribution and characteristics have remained poorly understood due to a lack of geophysical data, especially areal gravity survey and magnetotellurics (MT) survey, which are admittedly advantageous for detecting the edges of geological structures. Therefore, we carried out the ground geophysical surveys that areal gravity survey and MT survey, and acquired 1:250,000-scale real gravity data and MT data for the first time in this area. Optimized edge-detection methods were adopted to process the areal gravity data, allowing us to characterize the planar distribution of faults more reliably and convincingly. We found that the southern HMFZ is well developed and primarily trends in NNE and NE, whereas the subordinate faults trend in N-S and W-E. Vertical information for the faults extracted using the improved depth from the extreme points method revealed that the fault dominantly dipped to the SE, which was consistent with the results of MT inversion. Based on the spatial distribution of the faults, we further discussed the gravity anomaly, fault distribution, Luzhou palaeo uplift, and the distribution and characteristics of oil-gas resources, and found the convincing evidence to analysis the distribution of oil and gas resources in this region.

Comparative analysis on seismicity and stress triggering of strong earthquakes sequence in Central Tibet

Central Tibet constitutes part of the central part of the Qinghai–Tibetan Plateau, which is one of the highest seismically active areas in China. This paper discusses the regularity of seismic activity in this area. Based on a stratified viscoelastic earth model, we calculated the Coulomb stress changes imparted from four strong earthquakes (M ≥ 6.3) along the Bengco–southeastern piedmont of the Nyainqentanglha mountain fault zone in this region. The result shows that the study area may be entering a new active period from 2020. There was a trigger between the strong earthquakes (M ≥ 6.3) on the Bengco fault zone. The post-seismic viscous relaxation effect of a strong earthquake had a significant impact on subsequent earthquakes (M ≥ 6.3). In the next 100 years, the Coulomb stress loading is more than 1.0 MPa in the northwest section of the Bengco fault and the central part of Nimu segment of the southeastern piedmont of the Nyainqentanglha mountain fault. Thence, strong earthquakes may occur along these fault segments. The maximum magnitude of the earthquake could be M6.7 in the next 100 years.

Structural analysis of the Casper Mountain fault zone and area, Wyoming surrounding area, Wyoming: Implications for Laramide kinematics and structural inheritance across the Wyoming Province

Casper Mountain is an E–W trending anticlinal structure that is bound on the north by the oblique-slip Casper Mountain fault. The fault is postulated to reflect preexisting Precambrian structure/fabrics that were reactivated and/or guided deformation during the Laramide orogeny. A structural analysis of the fault zone and surrounding area was conducted to confirm this hypothesis, and to garner insight into both Precambrian origins and Laramide kinematics. Surface and subsurface data for structural analysis was collected and synthesized from numerous published sources along the proposed deformation corridor that roughly coincides with the Oregon Trail structural belt of central Wyoming. The Casper Mountain fault zone is characterized by an E–W rectilinear zone of en échelon, steeply inclined faults. The Casper Mountain fault strikes E–W with smaller faults in the zone striking N65°E. Folds trend to the WNW and are left-stepping. Foliations in Precambrian rocks of Casper Mountain are oriented subparallel to the Casper Mountain fault. The North Granite Mountains fault zone is located due west of Casper Mountain and is similarly oriented E–W with associated faults striking NE, NW/SE, and ENE/WSW, off the dominant master fault. Curvilinear, left-stepping, en échelon folds trend to the northwest and are truncated on the south by the North Granite Mountains fault. Faults in basement rocks of the Popo Agie Primitive Area of the central Wind River Mountains are characterized by moderate to high-angle faults striking E–W, NNW, and NE that coincide with mapped surface lineaments and fabric data. Fabric data suggest that Laramide deformation along the Casper Mountain fault was guided by Precambrian anisotropies. Surface and subsurface mapping of the fault zone and the deformation corridor to the west indicate that the Casper Mountain and North Granite Mountains faults are part of a basement-rooted system (wrench fault) that likely extends westward into the Popo Agie Primitive Area. Here, the steeply inclined (75–90°) proposed master fault is exposed within a WNW-striking corridor of faults that sinistrally offset steeply dipping, NE-striking Proterozoic diabase dikes. The dikes likely intruded older faults that are antithetic to the WNW-striking faults. Other faults strike to the NNW and have shallower dips of 45–65°. These three directions of anisotropy (WNW, NE, and NNW) are proposed to have formed from SW–NE-directed subduction along a long-lived, Neoarchean, active continental margin.

Diffuse Tectonic Deformation in the Drum Mountains Fault Zone, Utah, USA: Testing the Utility of Legacy Aerial Photograph-Derived Topography

The Basin and Range province in the western United States hosts numerous low-slip-rate normal faults with diffuse and subtle surface expressions. Legacy aerial photographs, widely available across the region, can be used to generate high-resolution digital elevation models of these previously uncharacterized fault systems. Here, we test the limits and utility of aerial photograph-derived elevation products on the Drum Mountains fault zone—a virtually unstudied and enigmatic fault system in the eastern Basin and Range province of central Utah. We evaluate a new 2-m digital surface model produced from aerial photographs against other remotely sensed and field survey data and assess the various factors that contribute to noise, artifacts, and distortions. Despite some challenges, the new elevation model captures the complex array of cross-cutting fault scarps well. We demonstrate that the fault zone has variable net east- or west-down sense of displacement across a c. 8-km-wide zone of antithetic and synthetic traces. Optically stimulated luminescence ages and scarp profiles are used to constrain net extension rates across two transects and reveal that the Drum Mountains fault zone has average extension rates of c. 0.1–0.4 mm yr−1 over the last c. 35 ka. These rates are both faster than previously estimated and faster than most other faults in the region, and could be an order of magnitude higher if steep faults at the surface sole into a detachment at depth. Several models have been proposed for local and regional faulting at depth, but our data show that the offsets, rates, and geometries of faulting can be generated by the reactivation of pre-existing, cross-cutting faults in a structurally complex zone between other fault systems. This study highlights how legacy aerial-photograph-derived elevation products, in lieu of other high-resolution topographic datasets, can be used to study active faults, especially in remote regions where diffuse deformation would otherwise remain undetected.

Faulting within the San Juan–southern Gulf Islands Archipelagos, upper plate deformation of the Cascadia subduction complex

Abstract The San Juan–southern Gulf Islands Archipelago of Washington State, USA and western Canada is located on the upper plate of the Cascadia subduction zone, in the forearc between the trench and volcanic arc. Onland and island investigations show many faults within the region that primarily represent old, inactive faults associated with transport, subduction and accretion of tectonostratigraphic terranes. However, until recently little geologic investigation and mapping have been done in the offshore. From these narrow straits, channels and sounds we have collected and interpreted high-resolution multibeam echosounder bathymetric data, 3.5 kHz sub-bottom and Huntec seismic-reflection profiles, and piston-cores to identify and date recently active faults. Previous studies by us focused on the earlier recognized active Devils Mountain fault zone that bounds the southern part of the Archipelago and the recently reported newly mapped active Skipjack Island fault zone that bounds the northern part. These transcurrent fault zones appear to be deforming and rotating the Archipelago. We concentrate on the unique deformation occurring within the seaways to determine the relationship and styles of faulting associated with these active bounding fault zones and relate the fault geometry and kinematics to one other subduction complex, the New Hebrides island arc of Vanuatu.

Quaternary slip rates on the White Mountains fault zone, eastern California: Implications for comparing geologic to geodetic slip rates across the Walker Lane

AbstractThe White Mountains fault zone in eastern California is a major fault system that accommodates right-lateral shear across the southern Walker Lane. We combined field geomorphic mapping and interpretation of high-resolution airborne light detection and ranging (LiDAR) digital elevation models with 10Be cosmogenic nuclide exposure ages to calculate new late Pleistocene and Holocene right-lateral slip rates on the White Mountains fault zone. Alluvial fans were found to have ages of 46.6 + 11.0/–10.0 ka and 7.3 + 4.2/–4.5 ka, with right-lateral displacements of 65 ± 13 m and 14 ± 5 m, respectively, yielding a minimum average slip rate of 1.4 ± 0.3 mm/yr. These new slip rates help to resolve the kinematics of fault slip across this part of the complex Pacific–North American plate boundary. Our results suggest that late Pleistocene slip rates on the White Mountains fault zone were significantly faster than previously reported. These results also help to reconcile a portion of the observed discrepancy between modern geodetic strain rates and known late Pleistocene slip rates in the southern Walker Lane. The total middle to late Pleistocene slip rate from the southern Walker Lane near 37.5°N was 7.9 + 1.3/–0.6 mm/yr, ∼75% of the observed modern geodetic rate.

Mechanical Mechanism of Fault Dislocation Based on in situ Stress State

Fault dislocation occurs under certain stress conditions. Based on the mechanical relationship between the direction of crustal stress and the occurrence of fault plane, three criterions of fault strike dislocation trend, dip dislocation trend and fault dislocation or not are put forward, According to these three criteria, the fault slip and the type of slip can be inferred. The parameters that have great influence on the characteristics of fault slip are: fault dip angle, angle between horizontal principal stress and fault strike, depth, lateral pressure coefficient, internal friction angle and cohesion of fault plane. Fault slip is more likely to occur in the environment of high deviation stress, low friction angle and dip angle of about 40°. Fault rupture is a point-to-surface and deep-to-shallow process. When the criterion f2 of the local position of the fault is greater than 0, it will lead to the slip of the nearby fault. When the slip range of the fault extends to the surface, it will cause large earthquakes with large-scale surface rupture. The theoretical calculation is basically consistent with the numerical simulation results. According to the theory in this paper, the slip instability state of Longmen Mountain Fault Zone under different stress conditions is calculated, the results show that when the lateral pressure coefficient is greater than 2.5, dislocation occurs in the deep part of the fault.

Dextral, normal, and sinistral faulting across the eastern California shear zone–Mina deflection transition, California-Nevada, USA

AbstractStrike-slip faults commonly include extensional and contractional bends and stepovers, whereas rotational stepovers are less common. The Volcanic Tableland, Black Mountain, and River Spring areas (California and Nevada, USA) (hereafter referred to as the VBR region) straddle the transition from the dominantly NW-striking dextral faults that define the northwestern part of the eastern California shear zone into a rotational stepover characterized by dominantly NE-striking sinistral faults that define the southwestern Mina deflection. New detailed geologic mapping, structural studies, and 40Ar/39Ar geochronology across the VBR region allow us to calculate Pliocene to Pleistocene fault slip rates and test predictions for the kinematics of fault slip transfer into this rotational stepover. In the VBR, Mesozoic basement is nonconformably overlain by a Miocene sequence of rhyolite, dacite, and andesite volcanic rocks that yield 40Ar/39Ar ages between 22.878 ± 0.051 Ma and 11.399 ± 0.041 Ma. Miocene rocks are unconformably overlain by an extensive sequence of Pliocene basalt and andesite lava flows and cinder cones that yield 40Ar/39Ar ages between 3.606 ± 0.060 Ma and 2.996 ± 0.027 Ma. The Pliocene sequence is, in turn, unconformably overlain by Quaternary tuffs and sedimentary rocks. This sequence of rocks is cut by NS- to NW-striking normal faults across the Volcanic Tableland that transition northward into NS-striking normal faults across the Black Mountain area and that, in turn, transition northward into NW-striking dextral and NE-striking sinistral faults in the River Spring area. A range of geologic markers were used to measure offset across the faults in the VBR, and combined with the age of the markers, yield minimum ∼EW-extension rates of ∼0.5 mm/yr across the Volcanic Tableland and Black Mountain regions, and minimum NW-dextral slip and NE-sinistral slip rates of ∼0.7 and ∼0.3 mm/yr, respectively, across the River Spring region. In the River Spring area, our preferred minimum dextral slip and sinistral slip rates are 0.8–0.9 mm/yr and 0.7–0.9 mm/yr, respectively. We propose three kinematic fault slip models, two irrotational and one rotational, whereby the VBR region transfers a portion of dextral Owens Valley fault slip northwestward into the Mina deflection. In irrotational model 1, Owens Valley fault slip is partitioned into two components, one northeastward onto the White Mountain fault zone and one northwestward into the Volcanic Tableland. Slip from the two zones is then transferred northward into the southwestern Mina deflection. In irrotational model 2, Owens Valley fault slip is partitioned into three components, with the third component partitioned west-northwest onto the Sierra Nevada frontal fault zone. In the rotational model, predicted sinistral slip rates across the southwestern Mina deflection are at least 115% greater than our observed minimum slip rates, implying our minimum observed rates underestimate true sinistral slip rates. A comparison of summed geologic fault slip rates, parallel to motion of the Sierra Nevada block relative to the central Great Basin, from the Sierra Nevada northeastward across the VBR region and into western Nevada are the same as geodetic rates, if our assumptions about the geologic slip rate across the dextral White Mountain fault zone is correct.

Effects of static stress triggering of the strongest earthquakes along the Bengco—southeastern Piedmont of Nyainqentanglha mountain fault zone, Tibet

In this paper, we calculate the Coulomb stress changes triggered by the four strongest earthquakes (M ≥ 6.3) occurred along the Bengco—southeastern piedmont of Nyainqentanglha mountain fault zone in 1411 (M8.0), in 1951 (7.7), in 1952 (M7.4), and 2008 (M6.3). We demonstrate that each strong earthquake occurred in the area where Coulomb stress had been increased (0.018~1.353 bar) by the prior major earthquakes. For example, the Coulomb stress of M7.7 earthquake in 1951 increased by 0.874 bar by the M8.0 earthquake in 1411. The Coulomb stress of M7.4 earthquake in 1952 increased by 1.353 bar by the M7.7 earthquake in 1951. However, the Coulomb stress of M6.3 earthquake in 2008 increased only by 0.018 bar by the M7.4 earthquake in 1952. The 1411, 1951, and 1952 prior strong earthquakes brought cumulative Coulomb stress triggering effects on the Nimu segment of the southeastern piedmont fault of Nyainqentanglha mountain, which is related with the occurrence of the M6.3 earthquake in 2008. The result shows that all of the strong earthquakes on the Bengco—southeastern piedmont of Nyainqentanglha mountain fault zone since 1411 were triggered by a series of prior strong earthquakes and tectonic stress loading.

Heterogeneity of Paleozoic Wufeng-Longmaxi formation shale and its effects on the shale gas accumulation in the Upper Yangtze Region, China

Shale gas as an unconventional gas has made a significant impact on worldwide energy supply. The heterogeneity of the shale reservoir has a dramatic effect on the shale gas accumulation. The heterogeneity characterization of the Paleozoic Wufeng-Longmaxi Formation Shale in the Upper Yangtze Region is studied within a sequence stratigraphic framework by testing in situ gas content, organic matter, minerals, porosity, permeability, and the observation and description of core and thin sections, and data statistics. Results show that the shelf depocenter has higher porosity than the shelf margin and the permeability is obviously lower at the transitional zone compared to neighboring areas. The most obvious characteristic of organic matter and mineral composition is that the TOC and quartz content at the lower section of the formation deposited during transgressive systems tract is higher than that at the upper section of the formation deposited during highstand systems tract throughout the whole deposition zone. The laminae are primarily developed at the upper section of the formation. The natural fracture is mainly developed at the Qiyueshan fault zone and the Xuefeng Mountain fault zone. The primary controlling factors of shale gas accumulation are organic matter and porosity. The transgressive systems tracts with high organic matter content and porosity at the lower section of the formation are more favorable for gas accumulation. The shale gas resources and exploration potential are mainly concentrated in the shelf depocenter.

The Skipjack Island fault zone: An active transcurrent structure within the upper plate of the Cascadia subduction complex

The Skipjack Island fault zone has been mapped in the San Juan Islands between Vancouver Island, Canada, and the Washington State mainland, USA. A decade ago, interpretation of multibeam sonar seafloor imagery revealed that Skipjack Island, an east–west striking sedimentary bedrock outcrop, was a fault-controlled structural feature. A major fault separates Skipjack Island from a deformed sedimentary bedrock outcrop on the seafloor to the north. Recently the Skipjack Island fault zone's morphology and extent have been explored both to the west and east of the island using multibeam echosounder bathymetry, seismic reflection profiles, and sediment cores. The character of the Skipjack Island fault zone is well defined locally by the interpreted seismic-reflection profiles, which show active faults that displace sediments deposited since the Last Glacial Maximum. The central part of the fault zone, near Skipjack Island, appears as a near-vertical structure in the shallow subsurface that has been subjected to left-lateral motion as evidenced by a bedrock exposure on the seabed north of the island where folded strata bend eastward against the fault, the result of drag from fault motion. Interpretations of recent geophysical data suggest an extension of the Skipjack Island fault zone farther to the west, where it trends towards, and possibly connects with, the Fulford Fault on Vancouver Island, and to the east, where it either terminates north of Lummi Island, or continues eastward onto the mainland. The Skipjack Island fault zone is interpreted to be the northern boundary of the San Juan Archipelago with the Devils Mountain fault zone being the southern boundary. Both fault zones represent the longest continuous faults of the San Juan Archipelago. This newly mapped fault zone and its zone of deformation to the north may represent the northern boundary of the Cascadia forearc. Potential sesimicity of the Skipjack Island fault zone and its proximity to the steep unstable northeastern side of Orcas Island and the southern front of the Fraser River Delta poses a geohazads problem in the form of earthquakes, landslides, submarine slides, and tsunamis.

Dynamic monitoring of soil erosion in the upper Minjiang catchment using an improved soil loss equation based on remote sensing and geographic information system

AbstractThe upper Minjiang catchment has suffered intensive soil erosion due to frequent geologic hazards and its fragile ecosystem zone with steep slopes. This study fully considers the topographic features and surface cover and introduces improved methods of K, LS, and C to establish a soil loss equation for the upper Minjiang catchment based on remote sensing and geographic information system. Spatial–temporal change patterns of soil erosion intensity and its driving mechanisms are then analyzed and discussed. Results show that (a) the soil erosion modulus of the upper Minjiang catchment for 2005 and 2015 were 1,577.29 t km−2 a−1 and 1,619.77 t km−2 a−1, both of which belong to a level of mild erosion. The slight and mild erosion zones covered the largest area and were widely distributed in the northern part of the study area, whereas zones of intensive and severe erosion were mostly concentrated in Wenchuan County and the lower reaches of the Heishui and Zagunao Rivers. (b) During 2005–2015, changes in erosion intensity showed a trend of “overall stability, local deterioration.” Zones of mild, moderate, and intensive increase were mainly concentrated in the Longmen Mountain Fault Zone, such as southern Maoxian and western Wenchuan Counties. (c) Returning cultivated land to forest and grasslands greatly reduced the erosion intensity and erosion amount, whereas geologic hazards aggravated the soil erosion condition. Zones with a slope of <35° had a positive relationship with soil erosion intensity; these areas are crucial control areas for soil preservation and containing soil loss. In addition, grassland is more effective in conserving soil and water than forestland in the upper Minjiang catchment in areas of steep terrain. These results provide an important reference for estimating soil loss intensity in southwest mountainous regions of China, particularly in the Hengduan Mountains, and greatly contribute to the planning of soil and water conservation.

The Devils Mountain Fault zone: An active Cascadia upper plate zone of deformation, Pacific Northwest of North America

The Devils Mountain Fault Zone (DMFZ) extends east to west from Washington State to just south of Victoria, British Columbia, in the northern Strait of Juan de Fuca of Canada and the USA. Recently collected geophysical data were used to map this fault zone in detail, which show the main fault trace, and associated primary and secondary (conjugate) strands, and extensive northeast-southwest oriented folding that occurs within a 6km wide deformation zone. The fault zone has been active in the Holocene as seen in the offset and disrupted upper Quaternary strata, seafloor displacement, and deformation within sediment cores taken close to the seafloor expression of the faults. Data suggest that the present DMFZ and the re-activated Leech River Fault may be part of the same fault system. Based on the length and previously estimated slip rates of the fault zone in Washington State, the DMFZ appears to have the potential of producing a strong earthquake, perhaps as large as magnitude 7.5 or greater, within 2km of the city of Victoria.

Electromagnetic Spider Web Placement in the Longmen Mountain Fault Zone

The design of the electromagnetic spider web deployed in the Longmen Mountain fault zone, Sichuan province, is described. Instrument every year to replace the battery life can reach 20 years. In seismically active areas, the principle for deployment is placing multiple groups of wireless transmitting modules. These spider webs of multiple modules are capable of detecting about 30 square kilometers within the scope of magnetotelluric field changes. Each module is usually equipped with an electronic compass for detection of the magnetic anomaly; using a special timer it opens four times per minute for about 5 to 15 s. If the signal is found to be abnormal, it can repeatedly scan many signals per hour. The signal is assessed by the GPRS network, which detects the seismic anomaly in the signal a few days before the earthquake as an early warning. Here, we focus on using ten tactics of ancient China as models for spatially deploying the electromagnetic spider web for further analysis of the quake epicenter. The equipment has low cost and low power consumption, so it is a good method for earthquake prediction.

Characteristics of isostatic gravity anomaly in Sichuan-Yunnan region, China

Sichuan-Yunnan region in China, a tectonic transition belt where earthquakes occurred frequently and intensely, has a distinct variation characteristic of gradient zone of Bouguer gravity anomaly (BGA). Many deep faults and epicenters of severe earthquake scatter along the BGA gradient zones. Here we apply two forward models (Airy model and Vening Meinesz model) of isostatic gravity mechanisms (local versus regional) in this region to calculated the isostatic gravity anomaly (IGA). Afterwards, the relationship between IGA and distribution of faults as well as seismicity is also illustrated. The IGA results show that the two models are similar and most parts of the study area are in an isostatic state. Most featured faults are distributed along the steep anomaly gradient zones; earthquakes tend to occur in the non-isostatic area and steep gradient belt of IGA. The distribution of root thickness based on regional mechanism can be associated with the main trend of BGA variation. The regional mechanism is more plausible and closer to the reality because of its relatively further consideration of the horizontal forces derived from adjacent particles in the crust. Then we analyze the effect of isostasy on the tectonic movements and find that the isostatic adjustment is not the main cause of the continuous uplift process of Longmenshan Mountain fault zone, which is due to the Indian-Eurasian continental collision.

Electromagnetic Spider Web Placement in the Longmen Mountain Fault Zone

The design of the electromagnetic spider web deployed in the Longmen Mountain fault zone, Sichuan province, is described. Instrument every year to replace the battery life can reach 20 years. In seismically active areas, the principle for deployment is placing multiple groups of wireless transmitting modules. These spider webs of multiple modules are capable of detecting about 30 square kilometers within the scope of magnetotelluric field changes. Each module is usually equipped with an electronic compass for detection of the magnetic anomaly; using a special timer it opens four times per minute for about 5 to 15 s. If the signal is found to be abnormal, it can repeatedly scan many signals per hour. The signal is assessed by the GPRS network, which detects the seismic anomaly in the signal a few days before the earthquake as an early warning. Here, we focus on using ten tactics of ancient China as models for spatially deploying the electromagnetic spider web for further analysis of the quake epicenter. The equipment has low cost and low power consumption, so it is a good method for earthquake prediction.

Regional fault deformation characteristics before and after the Menyuan Ms6.4 earthquake

This study analyzes data regarding cross-fault deformations within the seismogenic zone of the 2016 Qinghai Menyuan Ms6.4 earthquake and its surrounding area. The results showed that the tendency anomaly sites near the epicenter had relatively long anomaly durations prior to the earthquake, while sudden-jumping anomaly sites started to increase in the middle eastern Qilian Mountains approximately a year before the earthquake and continued to increase and migrate towards the vicinity of the epicenter two to six months before the earthquake. Intensive observations a few days after the earthquake indicated that abnormal returns and turns before the earthquake were significant, but all had small amplitudes, and the coseismic effect was generally minor. In addition, the post-seismic tendency analysis of individual cross faults in the Qilian Mountain fault zone revealed an accelerating thrust tendency at all cross-fault sites in the middle Qilian Mountains after the 2008 Wenchuan Ms8.0 earthquake. This indicates that the Wenchuan mega-earthquake exerted a great impact on the dynamic environment of the northeastern margin of the Qinghai-Tibet plate and significantly enhanced the extrusion effect of the Indian plate on the middle Qilian Mountains, generating favorable conditions for the occurrence of Menyuan thrust earthquakes.

Timing and rates of Holocene normal faulting along the Black Mountains fault zone, Death Valley, USA

Alluvial fans displaced by normal faults of the Black Mountains fault zone at Badwater and Mormon Point in Death Valley were mapped, surveyed, and dated using optically stimulated luminescence (OSL) and 10 Be terrestrial cosmogenic nuclide (TCN) methods. Applying TCN methods to Holocene geomorphic surfaces in Death Valley is challenging because sediment flux is slow and complex. However, OSL dating produces consistent surface ages, yielding ages for a regionally recognized surface (Qg3a) of 4.5 ± 1.2 ka at Badwater and 7.0 ± 1.0 ka at Mormon Point. Holocene faults offsetting Qg3a yield horizontal slip rates directed toward 323° of 0.8 +0.3/–0.2 mm/yr and 1.0 ± 0.2 mm/yr for Badwater and Mormon Point, respectively. These slip rates are slower than the ∼2 mm/yr dextral slip rate of the southern end of the northern Death Valley fault zone and are half as fast as NNW-oriented horizontal rates documented for the Panamint Valley fault zone. This indicates that additional strain is transferred southwestward from northern Death Valley and Black Mountains fault zones onto the oblique-normal dextral faults of the Panamint Valley fault zone, which is consistent with published geodetic modeling showing that current opening rates of central Death Valley along the Black Mountains fault zone are about three times slower than for Panamint Valley. This suggests that less than half of the geodetically determined ∼9–12 mm/yr of right-lateral shear across the region at the latitude of central Death Valley is accommodated by slip on well-defined faults and that distributed deformational processes take up the remainder of this slip transferred between the major faults north of the Garlock fault.

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.