Xianshuihe-Xiaojiang Fault System Research Articles

Geodetic slip rate partitioning in the central Anninghe and Daliangshan faults: Effects of fault geometry, elastic heterogeneity, and viscoelastic rheology

Geodetic measurements can be employed to estimate the secular slip rates and locking depths of crustal faults, two key parameters which influence assessment of long-term seismic hazard presented by a fault. However, this process requires making assumptions about the rheological properties of the Earth's crust and lithosphere, as well as the fault geometry at depth. Previous geodetic studies have inferred widely varying slip rates and locking depths for the central Xianshuihe-Xiaojiang Fault System, located along the southeastern margin of the Tibetan plateau. In this study, we use updated GNSS measurements, including 13 new velocities, to explore the effects of assumed rheology and fault geometry on the inferred slip rates and locking depths of the subparallel Anninghe (ANH) and Daliangshan (DLS) faults in this region. We demonstrate that only models taking into account the viscoelastic relaxation of the lower crust can resolve the apparent discrepancies. Models assuming a fully elastic rheology suggest very deep locking depths (∼35 km) and a lower slip rate on the ANH fault (∼5 mm/yr) than the DLS fault (∼7 mm/yr). Neither the relative slip rates nor the locking depths agree with seismic or geologic observations. We also consider models assuming a heterogeneous elastic rheology, but find they are indistinguishable from the homogeneous models. Modifying the fault geometry can impact the relative slip rates of the two faults but does not produce a reasonable locking depth either. In contrast, viscoelastic models accounting for seismic cycle effects indicate a faster ANH slip rate (∼5–7 mm/yr), a slower DLS rate (∼4–5 mm/yr), and a shallower locking depth for both faults (∼18 km), all of which are more consistent with independent seismic and geological observations. Our study suggests that steady long-term motion on these faults is modulated by long-lasting viscoelastic effects over a relatively broad region.

Extent of the lower crustal flow constrained by the rheological structure at the eastern boundary of the Chuan-Dian Rhombus Block

The Chuan-Dian Rhombus Block is the main channel for materials escaping southeastward from the Tibetan Plateau. Verifying the extent of the lower crustal flow and its impact on the lithospheric deformation, isostatic state, and seismicity in the Chuan-Dian Rhombus Block is of vital importance. Based on data from the China Seismic Experimental Site, the current study analyzes the rheological structure of the eastern boundary of the Chuan-Dian Rhombus Block (i.e., the Xianshuihe–Xiaojiang Fault System). The results reveal a low-viscosity zone of 1020 Pa∙s, with a thickness of approximately 20 km in the middle–lower crust beneath the Xianshuihe fault. The low-viscosity zone extends southward and gradually thins until it basically disappears at the Xiaojiang fault. Our results indicate that the extent of the lower crustal flow is restricted in the Xianshuihe fault and the northernmost part of the Anninghe–Zemuhe fault and does not extend to the Xiaojiang fault. The crust of the Xianshuihe fault is notably thickened and in an isostatic disequilibrium state. The Xiaojiang fault maintains a relatively thick seismogenic layer and isostatic equilibrium.

Seismological research in Yunnan Province, China, and its tectonic implication between the Xianshuihe-Xiaojiang fault system and the Red River fault zone

The tectonic crosscutting relationship between the two most tectonic important Xianshuihe-Xiaojiang fault system (XFS) and the Red River fault zone (RRF) is a key basic problem but it is controversial now. These obscure further leads to a hot argument about the geodynamic model of the southeast margin of the Tibetan Plateau. In order to answer whether the XFS has cut across the RRF and extended southwardly, multiple seismological methods, including seismic relocation, b-value analysis, seismic energy, density study, focal mechanism inversion, and regional stress field research, are applied in the Yunnan Province area, China, where the XFS and the RRF intersects with each other. The results comprehensively demonstrate that the southern segment of the XFS has not been affected by the RRF, and it has continued for a length after crossing the RRF, but the Dian Bien Phu fault zone should not be an extension fault of the XFS. Along the SW direction, starting from the middle segment of the XFS, and cutting across the Qujiang fault, Shiping-Jianshui fault zone, RRF, Ailaoshan fault zone, Wuliangshan fault zone, and the southern section of the Daluo fault, the belt should be treated as the eastern boundary of the clockwise rotational in geodynamics model of the Tibetan Plateau in this study area. Based on these conclusion above and previous recognitions, a new geodynamic evolution model is proposed.

Moment budget and seismic potential of the Xianshuihe-Xiaojiang fault system, southeastern Tibetan Plateau

The Xianshuihe-Xiaojiang fault system (XXFS) is an important boundary fault in the model of tectonic extrusion in the southeastern Tibetan Plateau. It is also seismically active, having produced seven M ≥ 7.5 earthquakes since 1480 CE. We compiled an integrated earthquake catalog to estimate seismic moment release rates along the XXFS since 1480 CE, and compared them with moment accumulation estimated from the updated GPS data. Our results show moment deficits of ∼2.49 × 1020 Nm for the northern (the Xianshuihe fault) and ∼ 1.28 × 1020 Nm for the central (the Aninghe-Zemuhe-Daliangshan fault) parts of the XXFS, equivalent to the moment release of MW 7.5 and Mw 7.4 events, respectively. The southern part (the Xiaojiang fault), on the other hand, has a moment surplus, suggesting a relatively low seismic potential. Analysis of moment budget on segments of the fault system indicates six sections of moment deficits along the XXFS, including the Luhuo segment, the Qanning-Kangding segment, the Moxi-Anninghe segment, the section connecting the Anninghe and Zemuhe faults, the northern Zemuhe segment and the connecting section between the Zemuhe and Xiaojiang faults. Each section of moment deficits could allow an MW ∼ 6.5 event, whereas the Qianning-Kangding segment has the potential for an MW ∼ 7.4 event. The Anninghe segment, where no M ≥ 6.5 event has occurred since 1536 CE, also deserves attention.

New Yuomys rodents from southeastern Qinghai-Tibet Plateau indicate low elevation during the Middle Eocene

Yuomysare medium-sized Hystricomorpha rodents. They are known for coming from areas of low elevation in China during the middle and late Eocene. Two newYuomyswere discovered from a locality near Xueshuo village in Litang County, Sichuan Province. The locality lies in the Gemusi pull-apart basin formed in the Litang Fault System (LTFS) in the Hengduan Mountains. The current average elevation is about 4200 m. One of the two newYuomysis larger and shows clear lophodont and unilateral hypsodont morphology, similar toYuomys yunnanensis, which was discovered as being from the early middle Eocene (Irdinmanhan, Asian Land Mammal Ages) in the Chake Basin of Jianshui County, Yunnan Province. The Chake Basin is one of the small pull-apart basins formed in the Xianshuihe-Xiaojiang Fault system (XSH-XJF). The other newYuomysrodent is smaller, brachydont, and less lophodont than the larger new species. The small newYuomysis smaller than all knownYuomysexceptYuomys huheboerhensis, which is from the early middle Eocene Irdinmanhan of Inner Mongolia in Northern China. Given their narrow biochronological distribution and presumably preferred living environment, the occurrence ofYuomysin the pull-apart basins in LTFS and XSH-XJF suggests that the two deep fault systems probably started strike-slip movement by the early middle Eocene, about 49–45 million years ago. Well-studied middle Eocene mammalian faunas from Henan and Inner Mongolia includeYuomys, primates, and other low elevation forest mammals. We suggest that the two newYuomysspecies reported here probably also lived in a similar low elevation forest environment.

Coseismic deformation of the 2022 Luding <italic>M</italic><sub>S</sub>6.8 earthquake and seismic potential along adjacent major faults

<p indent="0mm">The 2022 Luding (Sichuan) <italic>M</italic>6.8 earthquake occurred on the Moxi segment of the Xianshuihe fault, one of the most seismically active faults in mainland China. Since 1900, six moderate to strong earthquakes (&gt; <italic>M</italic>6.5) have ruptured different sections of the Xianshuihe fault. This Moxi segment has a complex tectonic setting as it forms an intersection zone of the Anninghe, Daliangshan, Xiaojin, and Longmenshan faults. In this study, we investigated the distribution of coseismic deformation of the 2022 Luding earthquake in order to improve understanding of the seismogenic fault and gain insight into the kinematic behaviors of regional faults and seismogeneity of future strong earthquakes. We collected continuous global positioning system (GPS) and strong motion data within 200 and <sc>50 km</sc> from the epicenter, respectively, and performed high-precision processing to extract the coseismic deformation field of this earthquake. We also collected Sentinel-1 and Advanced Land Observing Satellite-2 (ALOS-2) ScanSAR model descending data covering the epicenter area, and processed these data using differential interferometric synthetic aperture radar (D-InSAR) technology. The GPS measurements confirm that deformation from this earthquake occurred as far as <sc>200 km</sc> from the fault. This coseismic deformation was distributed in four quadrants, indicating that the focal mechanism of the Luding earthquake was left-lateral strike-slip. A strong motion station <sc>16 km</sc> from the epicenter recorded a significant displacement waveform; peak ground displacement was <sc>14 cm</sc> and the permanent coseismic displacement was <sc>12 cm.</sc> InSAR deformation maps show significant coseismic deformation in a 30 km×<sc>30 km</sc> area along the fault. The maximum line-of-sight displacement reached <sc>15 cm</sc> to the west of the fault. In addition, significant local deformation (&gt; <sc>15 cm)</sc> was observed near the Wajiao township. Field investigations suggest that surface ruptures developed along the fault segment between Ertaizi and Aiguocun. However, it is difficult to identify rupture information from the InSAR maps owing to a loss of coherence in the near-field of the seismogenic fault. Detailed field investigations are needed to determine if coseismic slip reached the surface. Finally, we inverted interseismic coupling along the northeastern boundary faults of the Sichuan–Yunnan block, and quantitatively calculated the seismic moment budget on these faults. Based on the historical earthquake distribution of the past <sc>300 years</sc> and present-day fault locking conditions along the Xianshuihe fault, the Luding earthquake accommodated clockwise vertical-axis rotation of the expanding Qinghai-Tibetan Plateau relative to the South China Block. The earthquake fully released the strain accumulated on the Moxi segment since the 1786 Moxi <italic>M</italic>7³/₄ earthquake; as such, it is unlikely that large earthquakes will occur on the Moxi segment in the near future. However, we also performed a comprehensive investigation of seismic hazard along the Xianshuihe-Xiaojiang fault system based on the mapped seismic gap, asperity distribution, seismic moment deficit, and Coulomb failure stress caused by surrounding earthquakes. We found that the seismic moment accumulated on the Anninghe, Zemuhe, Daliangshan, and southern Longmenshan faults can be balanced by <italic>M</italic>_w7.0–7.7 earthquake. The Luding earthquake caused significant Coulomb stress loading (&gt; <sc>0.01 MPa)</sc> on the Daliangshan, Anninghe, and southern Longmenshan faults, bringing those faults closer to failure. In particular, Coulomb stress loading at the northern end of the Anninghe fault exceeded the conventional triggering threshold <sc>(0.1 MPa)</sc> for earthquakes; therefore, the risk of future strong earthquakes on the Anninghe fault deserves special attention.

InSAR-derived present-day crustal movement of Daliangshan on the southeastern margin of the Qinghai–Tibetan Plateau

SUMMARY The Daliangshan area of Sichuan, China, is located on the southeastern margin of the Qinghai–Tibetan Plateau. It marks the central section of the left-lateral Xianshuihe–Xiaojiang Fault System (XXFS), which plays a crucial role in accommodating clockwise vertical-axis rotation of the expanding Qinghai–Tibetan Plateau relative to the South China Block. The area is seismically and tectonically active, and three major active faults—the Anninghe fault (ANHF), Zemuhe fault (ZMHF) and Daliangshan fault (DLSF)—together accommodate a significant amount of left-lateral strike-slip motion. Here, we present a geodetic study of the Daliangshan area mainly based on satellite interferometric synthetic aperture radar (InSAR). We processed Advanced Land Observing Satellite-2 (ALOS2) Phased-Array L-band Synthetic Aperture Radar-2 (PALSAR2) imagery to reconstruct the present-day interseismic deformation field. We developed an optimal atmospherics phase correction strategy to overcome the significant artefacts caused by ionospheric and tropospheric. By applying a multitemporal analysis on corrected interferograms, for the first time we generated a spatially continuous deformation rate field for the Daliangshan area. The satellite line-of-sight (LOS) rate field agrees with Global Positioning System (GPS) and levelling data to ∼2 mm yr–1, confirming that our processing approach is suitable for use in heavily vegetated areas. The InSAR rate map reveals displacements along the central and southern segments of the DLSF and ZMHF. By applying an inversion based on the Okada model, we quantitatively estimated the kinematic parameters of fault segments. Assuming a simple rectangular fault plane for each fault segment and constrained with the prior knowledge of the left-lateral slip, we determined that the ZMHF has a slip rate of 5.06$\pm 0.99{\rm{\ mm}}\,{\rm{yr}}^{-1}$ with a rake angle of ∼$38^\circ $ gently dipping to the SW. The southern section of the DLSF has a rake of $- 48^\circ \,\,{\rm to}\,\, - 41^\circ $ with the total slip rate of $4.93 \pm 0.4{\rm{\ mm}}\,{\rm{yr}}^{-1}$, dipping towards to the NE at $63.50^\circ \pm 2.31^\circ $. The modelled 3-D rate field has been validated with GPS and levelling measurements. Additionally, the inversion model and strain field suggest that the middle section of the DLSF undergoes strong deformation. We considered the implications of three factors—oblique convergence, gravity-driven movement, and the ELIP beneath the Sichuan–Yunnan area—for complex 3-D velocities in the transitional Daliangshan area.

Contemporary kinematics along the Xianshuihe-Xiaojiang fault system: Insights from numerical simulation

The slip rates on the Xianshuihe-Xiaojiang fault system (XXFS) are crucial for understanding the strain partitioning and evaluating its seismic risk. Here, we establish a 3D viscoelastic FEM model incorporating the faults as contact discontinuities to gain a numerical image of the XXFS's continuous slip rate. The viscosity of Southeastern Tibet and the locking depth of the XXFS were investigated and constrained by interseismic GPS observations. We estimated that the viscosities of the lower crust and the mantle lithosphere of southeastern Tibet were 1 × 1018 and 5 × 1019 Pa s, respectively. The interaction between the Daliangshan fault and the Anninghe-Zemuhe fault governs slip partitioning around the middle part of the XXFS. The XXFS’ overall mechanical efficiency is improved by the initiation and development of the Daliangshan fault. The interseismic strain buildup rate appears to be spatially heterogeneous, depending on the changes in slip rate along the strike of the XXFS. The accumulated strain energy in the Shimian-Miannning and Mianning-Xichang seismic gaps has enhanced the seismic hazard, which should be closely monitored.

Three-dimensional finite element modelling on strain localization around the Mabian earthquake swarm, Sichuan Province

SUMMARYThe Mabian fault zone, distanced ∼200 km to the east of the Xianshuihe–Xiaojiang fault system, is located in the western vicinity of the relatively stable South China Block. Since 1917, about 54 M &amp;gt; 4.7 earthquakes, including the 1974 Ms = 7.1 Mabian event have occurred around this fault zone, suggesting that significant strain is localized within the Mabian fault zone. Here, we built a 3-D finite element model to investigate the main parameters that possibly control strain localization around the Mabian fault zone averaged over the active deformation timescale. In the model, the Xianshuihe–Xiaojiang fault system is specified as a discontinuous contact interface for its motion governed by a Coulomb-friction law, and the crustal rheology is simplified as a frictional upper crust underlain by a viscoelastic lower crust. In addition, global positioning system (GPS) data are used to mimic the horizontal tectonic loading, and the model base is supported by a hydrostatic pressure. Numerical results show that with the weak fault strength and the low viscosity contrast between the Tibetan plateau and the South China Block, strain rates from motion of the southeastern Tibetan plateau could be propagated across the Xianshuihe–Xiaojiang fault system more widely within the Mabian fault zone. Constrained by the estimates on slip rates of the faults and on rheological structures of the crust, our optimal model predicts the effective friction coefficient of the Xianshuihe–Xiaojiang fault of 0.05–0.1. Under this condition, relative motion across the Xianshuihe–Xiaojiang fault system is largely partitioned by the geometric bend near the central portion of the fault system, resulting in a relatively high strain rate of 2.1–3×10–8 yr–1 accumulating around the Mabian fault zone. Keeping the weak strength of the fault, numerical results also show that if the central portion of the Xianshuihe–Xiaojiang fault system follows the Daliangshan fault, strain accumulation around the Mabian fault zone could be significantly reduced. It thus can be concluded that the strain partitioning from the weak strength and the special geometry of the Xianshuihe–Xiaojiang fault system must play a crucial role in active deformation around the Mabian area out of the Tibetan plateau deformation domain. This in turn means that in the Xianshuihe–Xiaojiang fault system, the Anninghe–Zemuhe fault is still the main boundary between the southeastern Tibetan plateau and the South China Block.

Heterogeneous Interseismic Coupling Along the Xianshuihe‐Xiaojiang Fault System, Eastern Tibet

AbstractWe use Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data to quantify the interseismic coupling along the Xianshuihe‐Xiaojiang fault system (XXFS). Our results confirm 7–11 mm/a of left‐lateral strike‐slip motion along the XXFS. South of the Shimian County, high interseismic coupling is found down to 20 km‐depth along the Anninghe‐Zemuhe fault. A second 130 km‐long section, highly coupled down to 10–15 km‐depth, is identified along the northern Xiaojiang fault. North of the Kangding County, the Xianshuihe fault appears to be predominantly creeping at depth. Previously identified long‐lasting surface creep following the 1973 M 7.6 Luhuo earthquake has now ended, possibly marking the initiation of a new period of stress accumulation and progressive relocking of the fault. Farther south along the fault, we identify a ∼30 km‐long section (∼30.2°–30.4°N) creeping at ∼7 mm/yr. Furthermore, creep accelerated by ∼2 mm/yr during the 2008–2014 period, possibly as a result of the static Coulomb failure stress increment induced by the 2008 Mw 7.9 Wenchuan earthquake, that occurred ∼200 km northeast of it. The 2014 Mw 5.9 Kangding earthquake occurred along the creeping section at the end of the period of accelerated creep. Finally, taking advantage of the long historical records of past large earthquakes available since 1327, we quantitatively compare the moment budget along the XXFS. Results highlight that along the Anninghe, Zemuhe, and Xiaojiang faults, moment deficit for Mw &gt; 7 earthquakes has now accumulated.

Fault interaction and active crustal extrusion in the southeastern Tibetan Plateau: Insights from geodynamic modeling

• The 3D model is used to investigate the fault interaction and crustal deformation . • Faults in southeastern Tibet are kinematically linked and mechanically coupled. • Fault interaction has affected fault activities and crustal extrusion rates. In the southeastern Tibetan Plateau, the Xianshuihe-Xiaojiang fault system (XXFS) and its neighboring fault systems collectively accommodate the crustal extrusion of the Tibetan Plateau. However, we do not clearly understand how these faults interact with each other and how the fault interaction impacts fault slip rate, strain partitioning and crustal extrusion in this region. Through building a 3D viscoelastoplastic finite element model, we investigate the effects of fault geometry, fault configuration, lithospheric rheology and topographic loading on fault slip rate and crustal deformation. We find that the geometrical irregularity of Anninghe-Zemuhe faults in XXFS (a concave bend) prevents the crustal extrusion of the Chuandian (Sichuan-Yunnan) block, and tends to cause the inception of their subparallel Daliangshan fault on their east. On the other hand, the active Daliangshan fault decreases the slip rates on the Anninghe-Zemuhe faults, causing faster crustal extrusion of the Chuandian block. The active Lijiang-Xiaojinhe fault decreases the slip rates of faults in the south and hence hinders the crustal extrusion of the Chuandian block. Besides, the topographic loading plays an important role in the crustal extrusion of the southeastern Tibetan Plateau, by redistributing regional deformation and fault slip rates. Modeling results demonstrate that all faults in this region are kinematically connected and mechanically coupled: the slip rate change of one fault, both induce strain repartitioning and variations of slip rates on all other faults in such a system. Our research reveals the significant impact of fault interaction on fault activity and crustal extrusion in the southeastern Tibetan Plateau.

Soil Degassing From the Xianshuihe–Xiaojiang Fault System at the Eastern Boundary of the Chuan–Dian Rhombic Block, Southwest China

The Chuan–Dian region, situated in the middle part of the north-south seismic zone of mainland China in a highly deformed area of the eastern margin of the Tibetan Plateau, is one of the principal areas for monitoring earthquake activities in China. In this study, the geochemical characteristics of soil degassing (of CH4, H2, CO2, Rn, and Hg) and, the relationship between degassing and fault activity, were investigated in the Xianshuihe–Xiaojiang fault system (XXFS) at the eastern boundary of the Chuan–Dian rhombic block. The mean soil-gas concentrations of CH4, H2, CO2, Rn, and Hg in the XXFS were 8.1 ppm, 9.9 ppm, 0.5%, 15.1 kBq/m3 and 12.9 ng/m3, respectively. The δ13CCO2 and δ13CCH4 values of the hot-spring gases varied from −11.9‰ to −3.7‰ and −62.5‰ to 17‰, respectively. The He-C isotopic ratios indicate that the carbon in the northern and middle parts of the XXFS may have originated from deep fluids, whereas the carbon in the southern part of the XXFS is of organic origin. The high concentrations of soil gas were distributed near the faults, indicating that the faults could act as channels for gas migration. The distributions of the high soil-gas concentrations in the XXFS coincide with the highest stress and maximum strain rates, indicating that the fault activity enhanced permeability and increased the emission rates of the gases. The results of this study will be helpful for degassing in active fault zones and earthquake monitoring.

Latest Quaternary Active Faulting and Paleoearthquakes on the Southern Segment of the Xiaojiang Fault Zone, SE Tibetan Plateau

AbstractThe Xiaojiang fault zone (XJFZ) is an important part of the Xianshuihe-Xiaojiang fault system, acting as the eastern boundary of the Chuan-Dian block on the southeastern margin of the Tibetan Plateau and accommodating the lateral extrusion of the block. The faulting activity and paleoseismic history on the southern segment of the XJFZ remain poorly understood. Here, trench excavations and radiocarbon dating revealed that four recent surface-rupturing paleoearthquakes have occurred on the Jianshui fault (JSF) in the southern segment of the XJFZ since ~15370 yr BP. The ages of these events, labeled E4-E1 from oldest to youngest, are limited to the following time ranges: 15360-12755, 10845-6900, 1455-670, and 635-145 yr BP, respectively. The most recent event E1 was most likely the 1606 Jianshui earthquake. These events appear to occur unregularly in time. The time interval between the last two events is 726±235 yr, and the average recurrence interval for all four events is 4589±3132 yr. The deformed strata show that the JSF is characterized kinematically by transtension, which likely respond to the apparent change in the direction of clockwise rotation of the Chuan-Dian block around the eastern Himalayan syntaxis. Combined with the analysis of the neighboring NW-striking faults, our study suggests that the south-southeastward motion of the Chuan-Dian block is likely to be firstly accommodated in part by the right-lateral shear and dip-slip motions of the Qujiang and Shiping faults and continues across the Red River fault zone, then is transmitted southward along the Dien Bien Phu fault. Therefore, the southern segment of the XJFZ plays a dominant role in the tectonic deformation of the southeastern Chuan-Dian block, with a high seismic hazard.

Short-Term Seismic Precursor Anomalies of Hydrogen Concentration in Luojishan Hot Spring Bubbling Gas, Eastern Tibetan Plateau

The gas compositions (He, H2, CO2, CH4, Ar and N2) and isotope ratios (3He/4He and δ13C) were yearly investigated from April 2010 to April 2019 at the Luojishan spring located in the proximity of the Zemuhe Fault, eastern Tibetan Plateau. The continuous automatic monitoring of hydrogen concentrations in Luojishan hot spring bubbling gas for the purpose of earthquake prediction requires the discrimination of seismic precursor anomalies. Helium isotope ratios (3He/4He) in the bubbling gas of hot springs varied from 0.05 to 0.18 Ra (Ra = 3He/4He = 1.39 × 10−6 in the air), with a maximum mantle-derived He up to 2.2% of the total He measured in the Luojishan hot spring (assuming R/Ra = 8.0 for mantle). This suggests that Zemuhe Fault might act as a conduit for crustal-derived fluid. N2 concentrations in the majority of the hot spring was ≥80 vol%, and δ13CCO2 values varied from −13.2 to −9.3‰ (vs.PDB). Hydrogen concentration time series display a complex temporal pattern reflecting a wide range of different physical processes. There were short-term (5–60 h) seismic precursor anomalies of hydrogen concentration before natural earthquake. The anthropogenically-induced earthquakes provoke only post-earthquake responses. The concentration of hydrogen in bubbling gas of the Luojishan hot spring is sensitive to increase of stress in the Xianshuihe-Xiaojiang fault system. Monitoring the hydrogen concentrations with automatic gas stations may be promising tool for unraveling earthquake mechanisms and for predicting earthquakes.

Strain Accommodation in the Daliangshan Mountain Area, Southeastern Margin of the Tibetan Plateau

AbstractThe Xianshuihe‐Xiaojiang fault system (XXFS) plays a crucial role in accommodating relative motions of the south China block and the expanding Tibetan Plateau. Compared to the narrow southern and northern segments of the XXFS, the central section (also referred to as the Daliangshan mountain area) is broader and dissected by a network of faults. Geologic studies suggest that three major NNW trending faults may dominate slip partitioning of more recent faults in this region. However, due to accessibility issues, sparse geodetic studies have constrained geodetic fault slip rates with contradictory results. In this study, we use 17 new Global Navigation Satellite System sites combined with existing solutions to investigate slip partitioning of the region. We derive a tectonic velocity solution, develop a block kinematic model constrained by Euler pole clustering technique, and calculate geodetic fault slip rates to compare with geologic fault slip rates. We confirm that the Anninghe‐Daliangshan sliver (ADS) is a distinct, rigid block, and we resolve a new, rigid Daliangshan‐Yingjing‐Mabian block (DYB). Kinematic modeling predicts long‐term slip rates that suggest both the ADS and the DYB partition (~10.0–12.0 mm/year) relative motion between the Tibetan Plateau and south China block in the central XXFS, reconciling geologic observations. We suggest that both the continuum and microplate deformation patterns are present in our study area, of which the local tectonics is better explained by the microplate model, but scale of the kinematics makes its motion more consistent with the continuum model.

Investigation and magnitude re‐evaluation of the 1955 Zheduotang earthquake, eastern Tibetan Plateau, China

Rupture lengths and displacements of historical earthquakes are significant components of the accurate magnitude estimates required for understanding fault behaviour and earthquake recurrence patterns. Along the Xianshuihe–Xiaojiang fault system on the eastern Tibetan Plateau, 36 large historical earthquakes have been recorded during the past ~700 years, many of which still require detailed investigation. In this study, we reinvestigated the coseismic surface rupture produced by the 1955 M 7.5 Zheduotang earthquake. The Zheduotang Fault, which is the source fault of the 1955 event, is one of the main faults of the central Xianshuihe fault zone, with a left‐lateral strike‐slip rate of 3.4 mm/year according to our measurements and the ages of offset terraces. High‐resolution Google Earth images and field observations reveal that the surface ruptures of this earthquake were characterized by left‐lateral offsets of gullies, animal paths, alluvial terraces, and fans. The coseismic surface rupture length caused by the 1955 earthquake could be up to ~43 km, with mainly horizontal displacements of 0.6–1.7 m. Based on empirical relationships between magnitude and coseismic surface rupture length for large earthquakes, the magnitude of the 1955 Zheduotang earthquake is re‐evaluated to be MW 7.0. Our study demonstrates the necessity of re‐evaluating previous magnitude estimates of historical earthquakes in western China.

Spatially varying surface seasonal oscillations and 3-D crustal deformation of the Tibetan Plateau derived from GPS and GRACE data

Measurements of 189 continuous and 933 campaign-mode Global Positioning System (GPS) stations with 3–16 yr data spans over the Tibetan Plateau reveal contemporary three-dimensional (3-D) crustal deformation during 1999–2016. The Empirical Orthogonal Function method was used to characterize the spatial variations in the surface deformation with distinct seasonal oscillations at the GPS sites in five regions of the Tibetan Plateau. We find that these surface variations are highly correlated with the corresponding mass load signals observed by the Gravity Recovery and Climate Experiment (GRACE) mission. The improved GPS processing strategy used to determine the 3-D velocity field includes maximum likelihood estimation, removal of common mode errors from GPS time series using Principal Component Analysis (PCA), and power law plus white noise stochastic error modeling. We determined the rates of vertical crustal movement by removing GRACE-observed non-tectonic origin load deformation, 2002–2016. The corrected vertical crustal deformation shows that the Himalaya region is uplifting at an average rate of ∼1.7 mm yr−1, and that the northeastern Tibetan Plateau is uplifting at an average rate of ∼1.3 mm yr−1. In addition, the horizontal velocity relative to the stable Eurasian plate and its corresponding dilatation throughout the Tibetan Plateau suggest that tectonic shortening and crustal thickening is occurring at −90 to −80 nanostrain yr−1 in the southern Tibetan Plateau and −30 to −20 nanostrain yr−1 in the northeastern Tibetan Plateau, which could be related to the geologic shortening and elastic strain accumulation. The interior Tibetan Plateau exhibits crustal thinning and block movement along strike-slip faults. Eastward motion of the crust north of the Xianshuihe-Xiaojiang Fault system relative to crust to its south results in shear strain and reflects eastward escape of plastic crustal material in the southeastern Tibetan Plateau.

Coulomb Stress Evolution History as Implication on the Pattern of Strong Earthquakes along the Xianshuihe-Xiaojiang Fault System, China

Coulomb stress accumulation and releasing history and its relationship with the occurrence of strong historical earthquakes could deepen our understanding of the occurrence pattern of strong earthquakes and hence its seismic potential in future. The sinistral strike-slip Xianshuihe- Xiaojiang fault zone (XXFS) is one of the most dangerous fault zones in China, extending 1 500-km-long from the central Tibetan Plateau to the Red River fault zone. There are 35 M≥6.5 historical earthquakes occurred since 1327, hence it is an ideal site for studying the Coulomb stress evolution history and its relationship with the occurrences of strong earthquakes. In this study, we evaluated the Coulomb stress change history along the XXFS by synthesizing fault geometry, GPS data and historical earthquakes. Coulomb stress change history also revealed different patterns of historical earthquakes on different segments of the XXFS, such as characteristic recurrence intervals along the Salaha-Moxi fault and super-cycles along the Xianshuihe fault. Based on the occurrence pattern of past historical earthquakes and current Coulomb stress field obtained in this study, we suggest positive ΔCFS and hence high seismic potential along the Salaha-Moxi fault and the Anninghe fault.

Primary Surface Ruptures of the LudianMw 6.2 Earthquake, Southeastern Tibetan Plateau, China

Online Material: Google Earth images. A M w 6.2 earthquake struck Ludian County at 08:30:11 UTC (16:30:10.2 local time) on 3 August 2014 and is one of the most destructive earthquakes that has occurred on an unmapped active fault east of the Xianshuihe–Xiaojiang fault system between the Chuandian rhomb block (the Sichuan–Yunnan block), southeastern Tibetan plateau, and the South China block, a stable tectonic terrain (Fig. 1; Wen et al. , 2013; X. Xu, Jiang, et al. , 2014; Cheng et al. , 2015). Its hypocenter is located at 27.11° N, 103.35° E, at a depth of ∼12 km, adjacent to Longtoushan Township, ∼29 km southwest of Zhaotong City (Liu et al. , 2014; L. Xu et al. , 2014; Y. Zhang et al. , 2014). The relocation of the aftershocks and a little of the reported fieldwork results may demonstrate the seismogenic fault of the Ludian earthquake is the Baogunao–Xiaohe fault, a northwest-trending left-lateral strike-slip fault, which cut through the Zhaotong–Lianfeng fault zone (http://www.eq-igl.ac.cn/contents/41/867.html, last accessed August 2014; Fang et al. , 2014; Liu et al. , 2014; W. Wang, et al. , 2014; X. Xu, Cheng, et al. , 2014; X. Xu, Jiang, et al. , 2014). We know nothing at all about the former, whereas the latter is a highly developed northeast-trending reverse fault with a right-slip component (Wen et al. , 2013). The finite-source inversion using regional seismic broadband data suggests the hypocenter of the Ludian earthquake was initiated from ∼12 km in the upper crust, then propagated unilaterally to the southeast and upward to the shallow with a peak left slip of ∼0.7 m; it likely offset the ground to form a surface rupture zone 10 km in length (Liu et al. , 2014), but there is no any reliable evidence to support or oppose this view. The significant directivity of the rupture propagation during …

Structure and tectonic geomorphology of the Qujiang fault at the intersection of the Ailao Shan–Red River fault and the Xianshuihe–Xiaojiang fault system, China

The northwest-striking Qujiang fault is located at the southeastern tip of the Sichuan–Yunnan block, which is bounded by the Ailao Shan–Red River fault and the Xianshuihe–Xiaojiang fault system. The Qujiang fault is one of the most seismically active faults in western Yunnan, China and is considered to be the seismogenic fault of the 1970 Ms 7.7 Tonghai earthquake. In this study, we examine the geometry, kinematics, and geomorphology of this fault through field observations, satellite images, and DEMs. It is a Holocene active fault characterized by dextral strike–slip movements with dip–slip components. It can be divided into northwest and southeast segments depending on different kinematics. The northwest segment from Baiyizhai to Wujie shows right-lateral strike slip with normal components, whereas it is characterized by dextral movements with the northeast wall thrusting over the opposite in the southeast segment from Wujie to Miaobeishan. The geophysical data combined with our investigations indicate that the fault dips steeply to the NE at depth, while secondary faults with different dip directions developed at shallow surface owing to fault bifurcation. During the Tonghai earthquake, a blind fault may have developed on the southwest wall, resulting in aftershocks concentrated to the southwest of the surface raptures. Multiple tectonic landforms are developed along the fault. The right-lateral Holocene slip rate was determined to be 2.84–3.27mm/year based on radiometrically dated offset gullies. Our analyses show that the present deformation at the intersection of the Ailao Shan–Red River fault and the Xianshuihe–Xiaojiang fault system is partitioned into a shortening distributed over a region ~150km wide and a right-lateral strike slip localized on the Qujiang and Jianshui faults. Since the late Pleistocene, the Qujiang fault has been playing an important role in accommodating the ongoing SE-directed movements of the Sichuan–Yunnan block, creating strong earthquakes to release strain energy.