Explore the vast range of titles available.

Volcanism that occurs far from plate margins is difficult to explain with the current paradigm of plate tectonics. The Changbaishan volcanic complex, located on the border between China and North Korea, lies approximately 1,300 km away from the Japan Trench subduction zone and is unlikely to result from a mantle plume rising from a thermal boundary layer at the base of the mantle. Here we use seismic images and three-dimensional waveform modelling results obtained from the NECESSArray experiment to identify a slow, continuous seismic anomaly in the mantle beneath Changbaishan. The anomaly extends from just below 660 km depth to the surface beneath Changbaishan and occurs within a gap in the stagnant subducted Pacific Plate. We propose that the anomaly represents hot and buoyant sub-lithospheric mantle that has been entrained beneath the sinking lithosphere of the Pacific Plate and is now escaping through a gap in the subducting slab. We suggest that this subduction-induced upwelling process produces decompression melting that feeds the Changbaishan volcanoes. Subduction-induced upwelling may also explain back-arc volcanism observed at other subduction zones. The Changbaishan volcanic complex in China cannot be easily explained as the consequence of a mantle plume. Seismic images from the region identify buoyant mantle material that may have been entrained and dragged downwards by the subducting Pacific Plate, but is now escaping upwards through a gap in the plate and producing the intraplate volcanism.

Generally, when there are only a few boreholes present along a tunnel design alignment, geological understanding of the worksite may not be adequate and the ability to optimise the tunnelling parameters is limited. This lack of boreholes will cause an increased potential of geo-hazards during tunnelling works. This study proposes an alternative method to determine the major and other components of ground under such circumstances. Five factors, cutter wheel torque, sieve residue, flow rate of feedline, pressure in the feed and discharge lines and density of bentonite slurry, are adopted for determining the major and other ground components. Comparisons of the soil types based upon the results of grading and Atterberg limits tests on the spoil and soil samples, respectively, and those resulting from the proposed method indicate good consistency. The proposed method provides an opportunity for establishing a more comprehensive geological structure for refining the tunnelling parameters, reducing the potential of geo-hazards associated with the inappropriate tunnelling parameters.

Shear wave splitting measurements using teleseismic SKS and SKKS phases recorded by the INDEPTH‐IV arrays has revealed a strong upper mantle anisotropic fabric in northeastern Tibet with large delay times of up to 2.2 s, suggesting that anisotropy exists in both the lithospheric and asthenospheric mantle. The coherence among fast polarization orientations of split core phases and the left‐lateral slip on eastern‐striking, southern‐striking faults in eastern Tibet and the surface deformation fields calculated from both GPS observations and Quaternary fault slip rates support the idea that left‐lateral shear strain is the predominant cause of the orientation of the upper mantle petrofabrics. We suggest the bending of the Eastern Himalayan Syntaxis around the foundering Burma‐Andaman‐Sumatra slab also contributes to the observed seismic anisotropy in the Eastern Himalayan Syntaxis region. Two plausible competing processes are proposed for the flow of asthenosphere in eastern Tibet. In the first, the deforming lithosphere glides over the passive asthenosphere inducing flow in the asthenospheric mantle. In the second, the asthenosphere beneath northeastern Tibet is squeezed between the advancing Indian continental lithosphere and the thick Tarim and Qaidam lithospheric blocks to the north. A westward retreat of the Burma slab from Eurasia may induce flow that is toroidal and located exclusively around the northern edge of the slab. The rotation of fast orientations for stations in the Eastern Himalayan Syntaxis region are consistent with the toroidal flow pattern as well as the rotational deformation of the overlying lithosphere. Key Points Anisotropy exists in both the lithospheric and asthenospheric mantle Coherence of SKS shear wave splitting with surficial deformation fields

Ambient noise tomography is applied to the significant data resources now available across Tibet and surrounding regions to produce Rayleigh wave phase speed maps at periods between 6 and 50 s. Data resources include the permanent Federation of Digital Seismographic Networks, five temporary U.S. Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiments in and around Tibet, and Chinese provincial networks surrounding Tibet from 2003 to 2009, totaling ∼600 stations and ∼150,000 interstation paths. With such a heterogeneous data set, data quality control is of utmost importance. We apply conservative data quality control criteria to accept between ∼5000 and ∼45,000 measurements as a function of period, which produce a lateral resolution between 100 and 200 km across most of the Tibetan Plateau and adjacent regions to the east. Misfits to the accepted measurements among PASSCAL stations and among Chinese stations are similar, with a standard deviation of ∼1.7 s, which indicates that the final dispersion measurements from Chinese and PASSCAL stations are of similar quality. Phase velocities across the Tibetan Plateau are lower, on average, than those in the surrounding nonbasin regions. Phase velocities in northern Tibet are lower than those in southern Tibet, perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained. At short periods (<20 s), very low phase velocities are imaged in the major basins, including the Tarim, Qaidam, Junggar, and Sichuan basins, and in the Ordos Block. At intermediate and long periods (>20 s), very high velocities are imaged in the Tarim Basin, the Ordos Block, and the Sichuan Basin. These phase velocity dispersion maps provide information needed to construct a 3‐D shear velocity model of the crust across the Tibetan Plateau and surrounding regions.

The Q of regional seismic phases Lg and Pg within the crust is assumed as a proxy for crustal Qβ and Qα, which is used as a constraint of crustal rheology. We measure regional‐phase Q of the eastern Tibetan Plateau and adjacent areas. This method eliminates contributions from source and site responses and is an improvement on the Two‐Station Method (TSM). We have generated tomographic images of crustal attenuation anomalies with resolution as high as 1°. In general we observe low Q in the northernmost portions of the Tibetan Plateau and high Q in the more tectonically stable regions such as the interior of the Qaidam basin. The calculated site responses appear to correlate with topography or sediment thickness. Furthermore the relationship between earthquake magnitudes and calculated source terms suggest that the RTM method effectively removes the source response and may be used as an alternative to source magnitude. Key Points We use a very accurate method to measure Lg and Pg Q in Tibet based on new data Variations of Q are correlated with tectonics and probably thermal structures Site and source term solution have scientific meanings and wide applications

We use receiver functions calculated for data collected by the INDEPTH‐IV seismic array to image the three‐dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern Tibet. Our results indicate an average crustal thickness of 65 to 70 km in northern Tibet. In addition, we observe a 20 km Moho offset beneath the northern margin of the Kunlun Mountains, a 10 km Moho offset across the Jinsha River Suture and gently northward dipping Moho beneath the Qaidam Basin. A region in the central Qiangtang Terrane with higher than normal crustal Vp/Vs ratio of ∼1.83 can be the result of the Eocene magmatic event. In the Qiangtang Terrane, we observe a significant lithospheric mantle discontinuity beneath the Bangong‐Nujiang Suture at 80 km depth which dips ∼10° to the north, reaching ∼120 km depth. We interpret this feature as either a piece of Lhasa Terrane or remnant oceanic slab underthrust below northern Tibet. We detect a ∼20 km depression of the 660‐km discontinuity in the mantle transition zone beneath the northern Lhasa Terrane in central Tibet, which suggests this phase transition has been influenced by a dense and/or cold oceanic slab. A modest ∼10 km depression of the 410‐km discontinuity located beneath the northern Qiangtang Terrane may be the result of localized warm upwelling associated with small‐scale convection induced by the penetration of the sinking Indian continental lithosphere into the transition zone beneath the central Tibetan Plateau. Key Points We obtained a 2D Moho depth map view of the NE Tibet plateau We observed a preserved Lhasa block lithosphere slab in the upper mantle The deflection of the 660 km and 410 km interface supports a subducting model

Using fundamental mode Rayleigh waves from the INDEPTH‐IV and Namche‐Barwa seismic experiments for periods between 20 and 143 s, we have investigated the lithospheric structure beneath eastern Tibet. We have found a ∼200‐km‐wide high velocity body, starting at ∼60 km depth and roughly centered beneath the Bangong‐Nijuang Suture, which is most likely a piece of the underthrusting Indian continental lithosphere. The sub‐horizontal underthrusting of the Indian lithosphere beneath eastern Tibet appears to be accompanied by its lateral tearing into at least two fragments, and subsequent break‐off of the westernmost portion at ∼91°E‐33°N. The uppermost mantle low velocity zone we observe beneath the N. Qiangtang and Songpan‐Ganzi terranes is most probably due to warmer and thinner lithosphere relative to southern Tibet. We attribute the low velocity zones concentrated along the northern and southern branches of the eastern Kunlun fault at lithospheric depths to strain heating caused by shearing. The azimuthal fast directions at all periods up to 143 s (∼200 km peak sensitivity depth) beneath the N. Qiangtang and Songpan‐Ganzi terranes are consistent, suggesting vertically coherent deformation between crust and uppermost mantle. Furthermore, the low velocity zone below the Kunlun Shan reaching down to >200 km argues against a present southward continental subduction along the southern margin of Qaidam Basin. Key Points We see no indication of southward continental subduction Sub‐horizontal underthrusting of India is accompanied by its lateral tearing VCD appears to be dominant beneath N. Qiangtang and Songpan‐Ganzi

The subduction zone of western Mexico is a unique region on Earth where microplate capture and overriding plate disruption are occurring today. The young, small Rivera plate and the adjacent Cocos plate are subducting beneath the Jalisco block of Mexico. Here, we present a P wave tomographic model of the upper mantle to 400 km depth beneath the Jalisco block and surrounding regions using teleseismic P waves recorded by the Mapping the Rivera Subduction Zone (MARS) and Colima Volcano Deep Seismic Experiment (CODEX) seismic arrays. The inversion used 12,188 P wave residuals and finite‐frequency theory to backproject the 3‐D traveltime sensitivity kernels through the model. Below a depth of 150 km, the tomography model shows a clear gap between the Rivera and Cocos slabs that increases in size with depth. The gap between the plates lies beneath the northern part of the Colima graben and may be responsible for the location of Colima volcano. The images indicate that the deep Rivera plate is subducting more steeply than does the adjacent Cocos plate and also has a more northerly trajection. At a depth of about 100 km, both the Rivera and Cocos slabs have increased dips such that the slabs are deeper than 200 km beneath the Trans‐Mexican Volcanic Belt (TMVB). It is also found that the Rivera plate is at roughly 140‐km depth beneath the young central Jalisco Volcanic lineament. Our images suggest that the Rivera plate and westernmost Cocos plate have recently rolled back toward the trench. This scenario may explain the unusual magmatic activity seen in the TMVB.

A tomographic image of the upper mantle beneath central Tibet from INDEPTH data has revealed a subvertical high-velocity zone from ∼100- to ∼400-kilometers depth, located approximately south of the Bangong-Nujiang Suture. We interpret this zone to be downwelling Indian mantle lithosphere. This additional lithosphere would account for the total amount of shortening in the Himalayas and Tibet. A consequence of this downwelling would be a deficit of asthenosphere, which should be balanced by an upwelling counterflow, and thus could explain the presence of warm mantle beneath north-central Tibet.

Many underground excavation failures are caused by severe water inflow due to the piping through the opening in the diaphragm wall. The opening can be created by slurry pocket or air pocket during concrete placement by tremie method. This type of defect in diaphragm wall can be detected by sonic logging to show the condition of trench wall and by measuring actual concrete dosage during placing. The same detecting method can be used in two very rare case histories where just-completed primary panels in very soft and thick clay deposit were overturned from the adjacent tremie concreting secondary panel during cold winter. The abnormal increasing concrete dosage and the smooth trench wall profile of secondary panel from sonic logging identify the occurrence of this incident. The causes to trigger this incident include (1) panel size, (2) weather condition, (3) tremie concreting speed of secondary panel, (4) initial and final setting times of concrete in trench, (5) roughness of trench wall, etc. Since the temperature for the two case histories is very low during tremie concreting, the initial setting time of concrete in secondary panel, given by equivalent age maturity function, is much longer than the time to place the concrete in secondary panel. To clarify the failure mechanism, the force-equilibrium analysis of inclined primary panel is conducted taking into account the driving force induced by the concrete pressure in the unset secondary panel and the resistant force given by the adhesion between inclined primary panel and soft clayey soil. It can be concluded from this investigation that the concrete in the secondary panel due to lower concrete curing temperature still remains unset or in plastic form and this will generate much larger driving force to push the adjacent just-completed primary panel to tilt. The measures to prevent similar incidents from happening are also suggested in this paper.