Geophysical Journal International

SummaryIn this paper, we propose a fully coupled two-phase poroelastic deformation theory for a spherically-layered and self-gravitating Earth. The Earth model consists of a solid inner core, a fluid outer core, and poroelastic mantle and crust. The boundary-value problems are posed in the Laplace-transformed domain using the spherical system of vector functions, and analytical solutions are obtained in each layer using the dual-variable and position matrix method. As an application, the surface loading problem is considered. The undrained and drained limits are discussed with the corresponding governing equations, which are different from the conventional ones where body force is ignored. Numerical examples show that the poroelastic effect can be significant since the difference between the results for undrained and drained limits are large with obvious time-dependent deformation. This newly derived theory for the coupled boundary-value problem will have broad applications, such as displacement and gravity change due to groundwater depletion, poroelastic rebound after an earthquake, and risk evaluation of earthquakes induced by water injection.

SummaryWe investigated sedimentary thickness and shear wave velocity structure in the western part of the Indo-Gangetic Plain (Punjab and Haryana Plain) and adjoining Siwalik Himalaya with the help of receiver function inversion at 20 Broadband seismological stations. This region is one of the most seismically vulnerable zones of the world due to the presence of thick surface sediments in the foreland basin that can amplify seismic waves and cause huge damage due to the earthquakes of the Himalaya. The study reveals a progressive thickening of sediments from southwest to northeast. The basement depth varies from ∼1.5 to 1.7 km in the Central Alluvium Plain, ∼1.8 to 2.8 km in the Zone of Terminal Fans, and attains a maximum of ∼3.8 km near the Himalayan Frontal Thrust. The inverted models show the presence of soft alluvial with extremely low Vs (< 0.5 km/s) and high Vp/Vs (∼2.5-3.0) at the top ∼400-700 m of the surface at most of the stations. A comparatively higher velocity of surface sediments observed at northern stations suggests the presence of compact sediments at the surface. The layered sedimentary structure revealed by the S-wave velocity models supports the previous geophysical investigations using borehole data. The velocity-depth structure obtained in this study is important for evaluating the seismic hazard of the densely populated urban areas spread over this region.

SummaryWe present high-resolution 3-D images of isotropic P wave velocity (Vp), azimuthal anisotropy (AAN), and radial anisotropy (RAN) down to 700 km depth beneath the North China Craton (NCC) and adjacent areas, which are obtained by inverting a great number of high-quality arrival time data recorded at 1 374 portable seismic stations and 635 permanent stations in the study region. Our results reveal new and detailed features of the upper mantle structure beneath the NCC. Varying structural heterogeneities are revealed beneath different tectonic blocks, and differences also exist between northern and southern parts of each block. The fast velocity directions (FVDs) of azimuthal anisotropy are mainly NW-SE under the Alaxa block, and NE-SW beneath the Tibetan Plateau. The FVDs present an arc transition along the boundary faults separating the Tibetan Plateau, the Alaxa block, the western NCC and the Sichuan basin. Low-Vp anomalies with positive RANs (i.e. horizontal Vp > vertical Vp) are revealed at 100-200 km depths under the Tibetan Plateau, reflecting frozen-in anisotropy in the thick lithosphere. Significant low-Vp anomalies with a circular AAN pattern exist at 0-700 km depths beneath the Datong volcano. In addition, negative RAN occurs right below the volcano, whereas positive RANs appear around it, suggesting that the Datong volcano is fed by hot upwelling flow from the lower mantle associated with collapsing of subducted slab materials down to the lower mantle. The eastern NCC shows complex Vp AANs and RANs. Seismic anisotropy exhibits east-west variations in the upper mantle across the Tanlu fault zone. The west of the Tanlu fault shows negative RANs (vertical Vp > horizontal Vp), whereas its east shows positive RANs at 300-500 km depths. The low-Vp anomaly under the Datong volcano is connected with a large low-Vp anomaly beneath the eastern NCC above ∼250 km depth, suggesting that the hot upwelling flow under Datong may migrate laterally to the asthenosphere under the eastern NCC and contribute to the lithospheric delamination and destruction there.

SummaryThe Northern China Domain is located between the Central Asian Orogenic Belts to the north and the Kunlun-Qinling belt to the south, and it comprises the North China, Alxa, and Tarim blocks. The relationships among the Northern China domain and the southern tectonic elements such as the Qaidam Basin/Terrane are debated because of the major modification by crustal deformation in the late Mesozoic–Cenozoic. To address this issue, we conducted a paleomagnetic and high-precision radiometric dating study of Triassic volcanic rocks and Middle Jurassic strata in the Qaidam Terrane. Our objective was to determine the relationship between the Qaidam Terrane with the Tarim Block and the North China Block (NCB) during the late Paleozoic and early Mesozoic. Four volcanic samples yielded zircon U-Pb ages of 236–243 Ma. The characteristic remanent magnetizations (Middle Triassic: D = 40.2°, I = 54.6°, α95 = 3.4°; Middel Jurassic: D = 27.4°, I = 48.0°, ks = 30.8, α95 = 7.9°) passed the fold and reversal tests, and yielded Middle Triassic and Middle Jurassic paleopole positions at 57.6°N, 178.2°E, A95 = 4.0° and 65.8°N, 197.6°E, A95 = 7.8°, respectively. Based on these new poles, combined with other reliable data, we compared the apparent polar wander path (APWP) of the Qaidam Terrane with those of the NCB and Tarim Block. The results show that, from the Carboniferous through Early Cretaceous, the APWP of the Qaidam Terrane resembles that of the Tarim Block, but it is quite different from that of the NCB. Combined with other reported evidence, we conclude that the Qaidam Terrane was an independent dynamic unit during the late Paleozoic until its connection with the Tarim Block, which was followed by continuous eastward motion. During this process, the connection between the Qaidam Terrane and the NCB-Alxa blocks occurredin the Middle Triassic, and subsequently the Qaidam Terrane underwent multiple tectonic responses to collisions with the Qiangtang Terrane, Lhasa Terrane, and the India Plate, before the formation of its modern tectonic configuration.