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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]

Constraining the shortening rate, timing, and deformation pattern in the frontal parts of the southern Qilian Shan (China) is helpful to further understand the strain distribution model and the tectonic deformation mechanism of the Qilian Shan. The Santai fold belt is one of the active thrust and fold belts parallel to southern Qilian Shan in the northern margin of the Qaidam Basin. It has experienced the structural evolution and interactions between itself and surrounding areas throughout the Late Cenozoic. Based on the geological and geomorphic mapping, landform profile surveys, and seismic line interpretations, the Santai anticline is considered to be a fault-propagation fold that is controlled by basement-involved faulting and limb rotation. In the late Quaternary, the shortening rate of the Santai anticline was almost 0.4 ± 0.17 mm/a, accounting for about 8% of the crustal shortening across Qilian Shan, and with the assumption of a constant shortening rate, latest rapid deformation occurred at about 3 ± 1.1 Ma. This indicates that the loci of deformation sequentially migrated southward in the foreland. • Santai anticline is basement-involved fault-propagation fold and raised by progressive limb rotation • The horizontal shortening of the Santai anticline was 0.4 ± 0.17 mm/a in the late Quaternary • The latest deformation of the Santai anticline is 3 ± 1.1 Ma in the Northern Qaidam Basin • The Qilian Shan propagated southward in four stages in the late Cenozoic [ABSTRACT FROM AUTHOR]

The Langshan range-front fault (LRF) is an active Holocene normal fault that borders Langshan Mountain and the Hetao Basin, northwest of the Ordos Plateau, China. In this study, paleoseismic trenching was undertaken at three sites (North–South): Dongshen village (TC1), Qingshan (TC2), and Wulanhashao (TC3). The paleoevents ED1, ED2, ED3 from TC1 were constrained to 6.0 ± 1.3, 9.6 ± 2.0, and 19.7 ± 4.2 ka, respectively. The single paleoevent (EQ1) from TC2 was constrained to about 6.7 ± 0.1 ka, and the paleoevents EW1, EW2, and EW3 from TC3 were constrained to 2.3 ± 0.4, 6.0 ± 1.0, and before 7.0 ka, respectively. With reference to previous research, the Holocene earthquake sequence of the LRF can be established as 2.30–2.43 (E1), 3.06–4.41 (E2), 6.71–6.80 (E3), 7.60–9.81 (E4), and 19.70 ± 4.20 (E5) ka BP. Events E1, E3, and E4 might have been caused by events with magnitudes of Mw 7.6–7.8 that ruptured the entire LRF. Event E2 might have been smaller magnitude, about M7.0, and ruptured only a portion of the fault. The vertical slip rate of the LRF at the Qingshan site is inferred as 0.9 or 1.4–1.6 mm/year in the last 6.8 ka. The slip rate at Wulanhashao is considered to have been close to, but not <0.66 mm/year since 15 ka BP. We deem the LRF is an unsegmented active fault. Although the possibility of missing events in Late Pleistocene can not be dismissed, we argue the Holocene paleoearthquake history is complete, indicating an average recurrence interval of ~2500 years. [ABSTRACT FROM AUTHOR]

Abstract The left-lateral slip rate of the Tianjingshan fault (TJSF) has been debated for several decades. Here, we measured displacements of geomorphic landforms and dated landform ages at two sites along the TJSF, to determine a late Pleistocene to Holocene slip rate of ~1.1 ± 0.2 mm/yr. We also derived a slip rate of ~1.1 ± 0.5 mm/yr using modern GPS data. Finally, by correlating available offset measurements with trenching-derived paleoseismic data, we obtained an average Holocene slip rate of ~1.2 ± 0.1 mm/yr for the TJSF. These fault slip rates over different time scales are in good agreement and are well constrained to 1.1 ± 0.2 mm/yr. The kinematics of the TJSF suggest that the TJSF has played an important role in accommodating the tectonic deformation of the northeastern Tibetan Plateau. Highlights • The slip rates of the TJSF over different time scales are in good agreement and are well constrained to 1.1 ± 0.2 mm/yr; • The fault behavior of the TJSF is characterized by a lower left-lateral slip rate. • The TJSF accommodates complex deformation between the Tibetan Plateau, Alashan block, and Ordos block. [ABSTRACT FROM AUTHOR]

We examined the regional scale topography of the Qilian Shan on the northeastern margin of the Tibetan plateau. Longitudinal profiles and geomorphometric indices, such as slope, local relief and channel steepness reveal that the Qilian Shan was developed as a local plateau with high-relief steep marginal ranges, and interior low-relief topography. Landscape mapping across this local plateau revealed spatially varied origins of the low-relief landscape in the Qilian Shan. North of the Haiyuan fault, universal massive intermontane aggradation was identified. However, the low-relief plateau to the south of the Haiyuan fault was dominated by relict erosional surfaces, even though accompany basin-filling still contributes. The geomorphologic contrasts led us to integrate both the basin-filling processes and antecedent low-relief erosional surfaces into a new insight into the development of the Qilian Shan plateau in the northeastern Tibet. Our present study highlights that explanation for the low-relief plateau formation needs to consider the complex geological evolution of the Qilian Shan, and the Tibetan Plateau itself. [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]

High-resolution images of the crust were obtained along two high-density temporary seismic arrays that cross tectonic blocks in the southwest boundary of the Ordos block and its adjacent regions. Dense stations with <1-km station spacings allowed teleseismic receiver functions to unveil the unprecedented details of crustal structures from the northeast margin of the Tibetan Plateau and the Qinling orogenic belt to the southwest Ordos block. The observations from nearby broadband seismic stations and two-dimensional numerical simulation were used to verify the reliability and validity of our results from the temporary dense array data. The receiver function profiles and the migration images revealed the contrasting crustal structures beneath the southwest Ordos block and the adjacent regions. Liupanshan mountain is a boundary that resists the lateral growth of the Tibetan Plateau; however, the slight bending of the Moho indicates absence of large-scale deformation in west Liupanshan. The Moho transition zone beneath the Weihe graben and southwest corner of the Ordos block represented by the complicated P-to-S converted phases can be contributed to the eastward extrusion of hot uppermost mantle from the Tibetan Plateau driven by the mantle flow. [Display omitted] • High density seismic array revealed the crustal structures beneath the southwest Ordos block and its adjacent regions. • The results indicate the contrasting crustal structures in the southwest boundary region of Ordos block. • The mantle flow from Tibetan Plateau caused the complicated Mohobeneath Weihe Graben and Southwest corner of Ordos block. • No large-scale deformation beneath the Liupanshan imply the strong-strong convergence of the Longxi basin and Ordos block. [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]