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• The Riganpei Co fault is divided into four segments based on the distribution of displacements. • The COPD within 0–30 m for each segment reveals that at least four large earthquakes ruptured this fault. • Reconstructed slip distribution shows a complex pattern of strong earthquake activities on different segments of the fault. High-resolution topographic datasets are useful for revealing offset features of fault geomorphology and are thus important for reconstructing the evolution history of faults. The Riganpei Co fault, located in the central Tibetan Plateau, is an important active fault on the northern boundary of the Bangong-Nujiang suture zone. In this study, WorldView-2 stereo images were used to generate high-resolution topographic data in a 1 km range on both flanks of the middle section of the Riganpei Co fault. We interpreted the tectonic geomorphology along the fault, classified it into distinct units, and measured the horizontal displacement of each unit. Within approximately 70 km along the fault, we obtained 396 displacement measurements ranging from 0 to 60 m. Given the evident uneven distribution of displacement, the fault was divided into four segments. In each fault segment, the reconstructed slip distributions revealed a tendency of larger displacements in the center and smaller displacements toward the ends. The cumulative offset probability distribution (COPD) of displacements displays 4–5 offset clusters in the range of 0–30 m for each segment, suggesting that at least four large earthquakes ruptured this fault. The binned COPD demonstrated that there are differences in the displacement accumulation modes among these fault segments, where the middle segment follows a characteristic slip accumulation mode. Reconstructed cumulative slip profiles revealed a complex pattern of strong earthquake activities along the Riganpei Co fault. [ABSTRACT FROM AUTHOR]

Paleoseismic studies are critical for the assessment and prediction of large earthquakes (M ≥ 7). These studies involve excavating and analyzing evidence from surface–rupturing earthquakes. However, they often face limitations in spatiotemporal resolution. The Yuguang Graben, located in the northern Shanxi Grabens of North China, contains an abundance of Quaternary loess that can be utilized for extending paleoseismic records. Fine-grained deposits of loess contrast sharply with gravel deposits of the colluvial wedge, making the wedge shape apparent. To enhance our understanding of earthquake behavior in the Yuguang Graben, we excavated a trench on terrace T2 of the Songzhikou segment of the Yuguang Graben Fault (YGF). We used LiDAR to obtain a high–resolution orthophoto of the trench wall. Through OSL dating of colluvial wedges and loess, we identified at least four large paleoseismic events with a magnitude of M ≥ 7 that occurred along the YGF. These events took place at >77.6 ± 5.1, 63.9 ± 5.1, 51.0 ± 4.3, and 37.3 ± 0.3 kyr, respectively. Combined with previous paleoseismic studies, we identified seven paleoseismic events since the late Pleistocene. These findings reveal that earthquakes occurring on YGF follow a quasiperiodic pattern with an average recurrence interval of about 12 ± 3 kyr. The elapsed time of the most recent earthquake near the recurrence interval suggests an increased risk of a large earthquake along the YGF. We also analyzed earthquake clusters generated by neighboring faults in the active northeastern boundary of the Ordos block. Our analysis indicated that fault interactions can affect the recurrence interval of earthquakes along single faults, while regional seismic activity tends to concentrate in time during a seismically active period. These observations underscore the importance of expanding paleoseismic records and exploring regional seismic hazards to gain a more comprehensive understanding of earthquakes. • Colluvial wedges on normal fault reveal new paleoseismic events. • Establishment of the paleoseismic sequence of the Yuguang Graben Fault for 77,000 years. • Improve the understanding of fault behavior located at the boundary of the Ordos block. [ABSTRACT FROM AUTHOR]

Earthquake-triggered landslides (EQTLs) are affected by both seismogenic faults and topography. Using landslide data associated with the 2008 MW7.9 Wenchuan earthquake, the distribution characteristics of EQTLs were identified. And the influence of topographical changes and fault dip angle changes on the distribution of EQTLs was analyzed. Fault dip angle had a controlling impact on EQTL spatial distribution. When the dip angle increased, EQTL distribution was concentrated in a certain range on both sides of the fault and decayed rapidly with distance from the fault. When dip angle decreased, the hanging wall effect became more obvious, and the number of landslides on the hanging wall gradually decreased with distance from the fault. In terms of topography, EQTLs were closely related to relative elevation differences, forming a single peak for a local relief range of 400–1600 m. For > 70% of statistical units, EQTL density increased with local relief; the landslide distribution conformed to an exponential relationship with relief. Peak local relief and dominant local relief were highly negatively correlated with fault dip angle. Fault geometry and motion characteristics control the propagation of seismic waves, directly affecting the distribution of EQTLs; moreover, as tectonic activity controls local relief, it also indirectly affects the distribution of EQTLs. Our findings provide empirical evidence for the rapid assessment of EQTL distributions. [ABSTRACT FROM AUTHOR]

Earthquakes reported in the frontal belts of active orogenic wedges such as the 2013 M w 6.6 Lushan and 1999 M w 7.6 Chi-Chi earthquakes, highlight the importance of loading exerted by mountainous topography and inherited fault zones. However, their effects on formation and activation of these wedge front fault systems remain poorly understood. This study presents three sets of sandbox experiments in which topographic loading and geometry of inherited faults were first separately investigated, then explored jointly. Experiments show that smaller topographic loading favors gentle-dipping wedge front thrusts, while larger topographic loading promotes the development of steep wedge front faults. Introduced as zones of frictional weakness, all inherited faults promote deformation localization, though the steep ones are characterized by buttressing nearby rather than direct slip on themselves. When the two factors were tested integrally, four major deformation modes arose, including (1) direct reactivation; (2) fault truncation in the initial stage of compression; (3) stacking of small thrusts against the inherited fault and possible truncation in a later evolutionary stage; (4) behavior as a fossil structure. These modes show differences not only in first-order deformation styles, but also in the distribution of secondary structures, defining a basic framework and providing new guides for the interpretation of complex wedge front fault systems and their activities, in areas such as the Longmen Shan (eastern Tibet), the Tian Shan and the western margin of the Taiwan orogen. • Mountain front deformation and resultant faults are simulated. • Smaller topographic loading favors gentler mountain front fault. • Behavior of inherited faults varies with imposed mountainous topographic loading. • Four mountain front deformation modes are identified. • Simple shear kinematics in several mountain front earthquakes is reproduced. [ABSTRACT FROM AUTHOR]

We studied the 2019 Mw6.4 and Mw7.1 Ridgecrest, California, earthquake sequence, using Sentinel‐1 and ALOS‐2 coseismic interferograms and subpixel offsets to retrieve the three‐dimensional (3‐D) surface displacements. By inverting the 3‐D displacements, optimal dip angles of the earthquake faults and the slip model were obtained. The interferometric synthetic aperture radar‐based slip model supplemented with the analysis of GPS data shows that the Mw6.4 event ruptured two orthogonal faults and its major geodetic moment was released by sinistral motion on a NE‐SW trending fault that dips 78° NW. Right‐lateral slip on NW‐SE trending subvertical faults was responsible for the Mw7.1 earthquake. Postseismic analysis with U.S. Geological Survey earthquake catalog and GPS time series at site P595 shows that the postseismic surface deformation following the Mw7.1 event has similar temporal patterns with the postseismic moment release, but requires more energy. This implies that the early aftershocks were likely controlled by rapid afterslip following the mainshock. Key Points: The 3‐D surface displacements of the 2019 Ridgecrest earthquakes are derived from Sentinel‐1 and ALOS‐2 InSAROrthogonal fault ruptures of the Mw6.4 event are revealed by using GPS horizontal measurementsThe rapid decay of aftershocks is explained by the afterslip following the mainshocks [ABSTRACT FROM AUTHOR]

Optical imagery has been proven to be an effective tool for measuring earthquake deformation in continental regions since its first application in the 1999 Izmit earthquake. In this article, we compile and analyze all the earthquakes that have been investigated with optical image matching by 2019, based on which we comment on various issues regarding measuring earthquake deformation with optical imagery. New generations of very high-resolution (VHR) data are effective for earthquake studies, but orthorectification of the VHR images is the major source of error, which is often ignored. We found that the displacements derived from the WorldView images strongly correlate with the errors in the Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) that was used in orthorectification. Based on the observed correlation between displacements and topography, we propose a new DEM-based method using the Advanced Land Observing Satellite (ALOS) World 3-D DEM to reduce the orthorectification errors. Combining the published optical data of earthquake deformation, we re-analyze the coseismic slip distribution and shallow slip deficit (SSD). The SSD model states that the coseismic slip in many strike-slip earthquakes decreases in magnitude toward the surface, but this model remains arguable because the interferometric synthetic aperture radar (InSAR)-derived slip is usually not well-constrained at shallow depths due to decorrelation. Because optical matching directly measures the surface slip, we re-examine the slip distribution of 11 strike-slip earthquakes and find that the SSD model may primarily be artifacts in the InSAR measurements. It is therefore of great importance to include the optical data in earthquake studies to constrain coseismic slip inversions. [ABSTRACT FROM AUTHOR]

Continental thrust faulting earthquakes pose severe threats to megacities across the world. Recent events show the possible control of fault structures on strong ground motions. The seismogenic structure of the 2008 Wenchuan earthquake is associated with high-angle listric reverse fault zones. Its peak ground accelerations (PGAs) show a prominent feature of fault zone amplification: the values within the 30- to 40-km-wide fault zone block are significantly larger than those on both the hanging wall and the footwall. The PGA values attenuate asymmetrically: they decay much more rapidly in the footwall than in the hanging wall. The hanging wall effects can be seen on both the vertical and horizontal components of the PGAs, with the former significantly more prominent than the latter. All these characteristics can be adequately interpreted by upward extrusion of the high-angle listric reverse fault zone block. Through comparison with a low-angle planar thrust fault associated with the 1999 Chi-Chi earthquake, we conclude that different fault structures might have controlled different patterns of strong ground motion, which should be taken into account in seismic design and construction. [ABSTRACT FROM AUTHOR]

We present results from a detailed investigation of the horizontal displacement distribution, timing of paleoearthquakes and left-lateral slip rate on the western Tianjingshan fault. Measurements of 240 offset streams and ridges confirm that the fault is left-lateral and record evidence of repeated ∼3–4 m coseismic offsets along the 60–km–long fault. This suggests that ∼6 earthquakes may have occurred along the entire western Tianjingshan fault with repeated occurrence of earthquakes of Mw 7.2–7.5. Structural and stratigraphic relationships exposed in our five trenches in combination with previously reported studies further indicate that the fault has ruptured in as many as six paleoearthquakes since the late Quaternary. Paleoseismic data show that the average recurrence interval for Holocene earthquakes is approximately 5,000 yr. The most recent earthquake along the western Tianjingshan fault occurred ∼1.2 ± 0.1 kyr BP, indicating that this fault segment did not rupture in the M 7.5 historical earthquake of 1709 that ruptured the central Tianjingshan fault. We estimate that the Holocene slip rate of the western Tianjingshan fault is ∼1.1–1.2 mm/yr based on measurements of the age and offset of stream channels. Compared with the relatively fast slip rate of the Haiyuan fault (∼4–6 mm/yr), we suggest that the Tianjingshan fault acts as an essential active fault accommodating the sinistral displacement and crustal shortening deformation in NE Tibet. [ABSTRACT FROM AUTHOR]

The West Helanshan fault is situated at the junction of the northeastern Tibetan Plateau, Alxa, and Ordos blocks, providing an opportunity to investigate the deformation in the transitional zone between the contractional northeastern Tibetan Plateau and extensional North China. Using field investigations and satellite imagery interpretations, we determined detailed geometry and kinematics of the West Helanshan fault. The West Helanshan fault is divided into three segments, including the northern, middle, and southern segments, based on varying fault strike, slip sense, and geomorphic expression. The northern and middle segments are dominated by right-lateral strike-slip motion as indicated by linear fault traces, horizontally offset streams and terrace risers, relatively insignificant vertical offset, and frequently changing facing direction of fault scarps. The late Quaternary right-slip rate is estimated to be 0.2–0.4 mm/yr, which is relatively low comparing with other strike-slip faults in the northeastern Tibetan Plateau, based on 10Be-dated offset fluvial/alluvial surfaces. The southern segment shows conspicuous fault scarps with significant systematic vertical offset, which has a reverse component as revealed in the trench. The timing of the most recent earthquake revealed in the trench is constrained to be between 6.2 ± 0.7 ka and 5.9 ± 0.4 ka. Notably, the West Helanshan fault cuts through the city center of Azuoqi town, which is the capital of Azuoqi county and has about 100,000 residents, carrying high potential earthquake hazard in the future. Finally, combining the regional fault geometry and kinematics with GPS observations, we propose a limited north-northeastward extrusion model for the southern Alxa Block. • The West Helanshan fault is dominated by right-lateral strike-slip motion. • The most recent earthquake occurred between 6.2 ± 0.7 ka and 5.9 ± 0.4 ka. • A limited north-northeastward extrusion model is proposed to explain the geometry and kinematics of faults in the southern Alxa Block. [ABSTRACT FROM AUTHOR]

On April 20, 2013, a Mw 6.6 earthquake struck Lushan County of Sichuan Province, China, about 80 km southwest of the epicenter of the 12 May 2008 Wenchuan Mw 7.9 event. It triggered a large number of landslides in a broad area, causing causality and injuries and damages to roads, drains, dwellings, and infrastructures. Field investigations of coseismic landslides provide a basis for better understanding and illustrating the spatial distributions and hazards related to the Lushan earthquake-triggered landslides. Therefore, this paper presents results of field investigations of the Lushan earthquake-triggered landslides, which can be classified into two main categories of disrupted landslides and coherent landslides, and constitutes seven types. The disrupted landslides include five types: rock falls, rock slides, rock avalanches, soil falls, and soil slides; and the coherent landslides have two types: soil slumps and slow earth flows. All of the seven types of landslides can be observed in many field photos, aerial photographs, and/or satellite images. Among them, seven typical large-scale landslides, distributed in a northeast trending line consistent with the Shuangshi-Dachuan fault, which is presumably related with the earthquake, are described in detail. We prepared an original, emergency-based coseismic landslide inventory map based on visual interpretation of available post-earthquake aerial photographs and pre-earthquake satellite images, combined with results from emergency-based field investigations. The original inventory map registered 3884 landslides. Later, 11,761 more landslides were identified by much more careful and time-consuming visual interpretation based on the same aerial photographs and satellite images. Subsequently, all the 15,645 coseismic landslides were correlated with six common landslide impact factors, including elevation, slope angle, slope aspect, topographic position, stratum/lithology, and peak ground accumulation (PGA). These observational and compiled coseismic landslide data allow us to make subsequent compiling of a detailed and completed landslide inventory by visual interpretation of remote sensing images as well as further studies of landslides induced by the Lushan earthquake. [ABSTRACT FROM AUTHOR]

On July 22, 2013, an earthquake of Ms. 6.6 occurred at the junction area of Minxian and Zhangxian counties, Gansu Province, China. This earthquake triggered many landslides of various types, dominated by small-scale soil falls, slides, and topples on loess scarps. There were also a few deep-seated landslides, large-scale soil avalanches, and fissure-developing slopes. In this paper, an inventory of landslides triggered by this event is prepared based on field investigations and visual interpretation of high-resolution satellite images. The spatial distribution of the landslides is then analyzed. The inventory indicates that at least 2330 landslides were triggered by the earthquake. A correlation statistics of the landslides with topographic, geologic, and earthquake factors is performed based on the GIS platform. The results show that the largest number of landslides and the highest landslide density are at 2400m-2600m of absolute elevation, and 200m-300m of relative elevation, respectively. The landslide density does not always increase with slope gradient as previously suggested. The slopes most prone to landslides are in S, SW, W, and NW directions. Concave slopes register higher landslide density and larger number of landslides than convex slopes. The largest number of landslides occurs on topographic position with middle slopes, whereas the highest landslide density corresponds to valleys and lower slopes. The underlying bedrocks consisting of conglomerate and sandstone of Lower Paleogene (Eb) register both the largest number and area of landslides and the highest landslide number and area density values. Correlations of landslide number and landslide density with perpendicular- and along-strike distance from the epicenter show an obvious spatial intensifying character of the co-seismic landslides. The spatial pattern of the co-seismic landslides is strongly controlled by a branch of the Lintan-Dangchang fault, which indicates the effect of seismogenic fault on co-seismic landslides. In addition, the area affected by landslides related to the earthquake is compared to the relationship of "area affected by landslides vs. earthquake magnitude" constructed based on earthquakes worldwide, and it is shown that the area affected by landslides triggered by the Minxian-Zhangxian earthquake is larger than that of almost all other events with similar magnitudes. [ABSTRACT FROM AUTHOR]

In foreland thrust systems, the complex structural geometry creates greater difficulties in constraining the seismotectonics of moderate magnitude earthquakes without pronounced surface ruptures and in assessing future seismic risk. The 2013 M 5.1 Urumqi earthquake was a moderate event that occurred in the Urumqi foreland thrust system. Based on surface investigations, geological mapping, and interpretations of deep seismic reflection profiles, we suggest that the Urumqi foreland thrust belt is a typical active wedge thrust system. Two connected fault segments of the gently south-dipping blind fault ramp and the north-dipping Xishan back-thrust fault merge at a depth of ~11 km and bound the wedge-shaped fault block. Analyses of the focal mechanism and aftershock distribution indicate that the seismogenic fault responsible for the M 5.1 Urumqi event is the south-dipping Xishan fault ramp with a dip angle of ~10–30°, which highlights the potential seismic hazards of the unruptured north-dipping Xishan back-thrust ramp. The four fault branches on the hanging wall of the Xishan fault are interpreted as flexural-slip thrusts. Our results indicate that multiple surface ruptures over a 6 km-wide area may form if a large earthquake occurs along the Xishan fault system. • The seismogenic fault of the 2013 M 5.1 Urumqi event is the south-dipping blind Xishan fault ramp. • Four north-dipping fault branches on the hanging wall of the Xishan fault are flexural-slip thrusts. • Potential seismic hazard of the Urumqi foreland thrust system is estimated. [ABSTRACT FROM AUTHOR]

In the epicenter of the Lushan M7.0 earthquake there are several imbricate active reverse faults lying from northwest to southeast, namely the Gengda-Longdong, Yanjing-Wulong, Shuangshi-Dachuan and Dayi faults. Emergency field investigations have indicated that no apparent earthquake surface rupture zones were located along these active faults or their adjacent areas. Only brittle compressive ruptures in the cement-covered pavements can be seen in Shuangshi, Taiping, Longxing and Longmen Townships, and these ruptures show that a local crustal shortening occurred in the region during the earthquake. Combining spatial distribution of the relocated aftershocks and focal mechanism solutions, it is inferred that the Lushan earthquake is classified as a typical blind reverse-fault earthquake, and it is advised that the relevant departments should pay great attention to other historically un-ruptured segments along the Longmenshan thrust belt and throughout its adjacent areas. [ABSTRACT FROM AUTHOR]

Repeating earthquakes are key evidence for understanding earthquake recurrence and help improve long-term seismic risk assessment. In 1986 and 2016, two Ms6.4 earthquakes occurred in Menyuan region, near the middle segment of Qilian-Haiyuan fault zone, NE Tibetan Plateau. Long period waveforms of the two events at teleseismic stations show high degree of similarity, however, previous solutions on earthquake location and focal mechanism show remarkable discrepancies, especially for the 1986 event, as well the ruptured faults also remain ambiguous. In this study, we determined the source parameters of these two earthquakes, including the relative location, focal depth, focal mechanism, source duration and rupture directivity. The results show that two earthquakes are thrust events in shallow crust and are located within 15 km, and the 2016 earthquake ruptured down-dip for ~6 km along the southwest dipping plane. Combing the source parameters, geological data and deep seismic-reflection profiles, we infer that these two earthquakes occurred on the same secondary fault of Lenglong Ling fault (the middle segment of Qilian-Haiyuan fault), but they ruptured on different sections of the fault. Although the two events are probably not repeating earthquakes of Qilian-Haiyuan fault zone, the seismic risk of ~M6 thrust earthquake on these secondary faults and even larger strike-slip earthquake on Lenglong Ling fault are not negligible based on the source parameters and ten-year GPS observations. • Both earthquakes are thrust events in shallow crust and are located within 15 km. • Occurred on the secondary fault (SW-dipping) and ruptured on different sections • Seismic risk of ~M6 thrust earthquake on these secondary faults is not negligible. [ABSTRACT FROM AUTHOR]

Available data from the Qilian Shan in north-east Tibet suggested that decadal-scale erosion rates were closely correlated with local topographic gradient, but not with climatic factors. However, a climatic change to more arid condition was proposed to explain the discrepancy between short-term and long-term erosion rates. In order to re-evaluate the topographic, tectonic and climatic influences on erosion, we adopted five parameters (slope, mean local relief, historical cumulative seismic moment, runoff coefficient of variation and fault density) to study 11 drainage basins in north-east Tibet. Our results showed that terrain gradient, rock fracture density and rainstorm intensity had strong influence on erosion rates while 60-year cumulative seismic moments of historical earthquakes showed weaker correlations. There was a spatial variation in the erosional mechanisms across the basin, with detachment-limited dominance around the ridges (slope > 20°) and deposition dominant in the flat areas. The variation may lead to the discrepancy between short-term and long-term erosion rates. In general, our study supports the 'bath-tub' model for low relief intermountain basins, hence providing new insights into the landscape evolution of the Qilian Shan in northeastern Tibetan Plateau. [ABSTRACT FROM AUTHOR]