Journal of Earth Science

Currently available earthquake attenuation equations are locally applicable, and methods based on observation data are not applicable in areas without available observation data. To solve the above problems and further improve the prediction accuracy of ground motion parameters, we present a prediction model referred to as a light gradient boosting machine with feature selection (LGB-FS). It is based on a light gradient boosting machine (LightGBM) constructed using historical strong motion data from the NGA-west2 database and can quickly simulate the distribution of strong motion near the epicenter after an earthquake. Cases study shows that compared with GMPE methods and those based on real-time observation data, the model has a better prediction effect in areas without available observation data and can be applied to Yangbi Earthquake and Maduo Earthquake. The feature importance evaluation based on both information gains and partial dependence plots (PDPs) reveals the complex relationships between multiple factors and ground motion parameters, allowing us to better understand their mechanisms and connections.

We firstly present the description of the river terrace at Tangjia (唐家) Village in Lhasa, Tibet, collect soil samples, and select the climate indicators including δ 13C, total organic carbon (TOC), and the Rb/Sr ratios to study its paleoclimate in this area. Ancient climate changes have been reconstructed since the last glacier period. The results show that the δ 13C, TOC, and the Rb/Sr ratio are good indicators of ancient climate fluctuations. Paleoclimatic evolution in the Lhasa Tangjia region could be divided into seven stages. In stages II (11.7–10.2 kaB.P.) and IV (8.1–6.1 kaB.P.), δ 13C was positive and TOC was high, indicating that the climates in these two stages were relatively warm and humid. In stages III (10.2–8.1 kaB.P.) and V (6.1–4.9 kaB.P.), δ 13C showed cyclical fluctuations, but TOC exhibited less change, suggesting that the climates displayed variation on the millennial scale. Moreover, the climatic variations were on a century-long scale during the later Middle Holocene. Compared with δ 13C from Sumxi Co (松木希错) and δ 18O from the Guliya (古里雅) ice core, the study confirmed that four cold events occurred during the Holocene (9.4, 8.2, 5.4, and 4.2 kaB.P.). The climate indicators were limited to the river terrace based on the geological characteristics of the Lhasa region. Unexpectedly, δ 13C was a sensitive indicator of climate change.

Located on the western Yangtze Block, the Sichuan (四川)-Yunnan(云南)-Guizhou(贵州) (SYG) Pb-Zn metallogenic province has been a major source of base metals for China. In the southeastern SYG province, structures are well developed and strictly control about 100 Pb-Zn deposits. Almost all the deposits are hosted in Devonian to Permian carbonate rocks. Lead-zinc ores occur either as veinlets or disseminations in dolomitic rocks with massive and disseminated textures. Ore minerals are composed of pyrite, sphalerite and galena, and gangue minerals are calcite and dolomite. Sulfide minerals from four typical Pb-Zn deposits are analyzed for sulfur isotope compositions to trace the origin and evolution of hydrothermal fluids. The results show that δ34S values of sulfide minerals range from +3.50‰ to +20.26‰, with a broad peak in +10‰ to +16‰, unlike mantle-derived sulfur (0±‰). However, the mean δ34Ssulfide and δ34SΣS-fluids values are similar to that of sulfate-bearing evaporites in the host rocks (gypsum: ⊃+15‰ and barite: +22‰ to +28‰) and Cambrian to Permian seawater sulfate (+15‰ to +35‰). This suggests that reduced sulfur in hydrothermal fluids was likely derived from evaporates in the host rocks by thermo-chemical sulfate reduction (TSR). Calculated δ34SΣS-fluids values of the Shanshulin(杉树林), Qingshan(青山), Shaojiwan(筲箕湾) and Tianqiao(天桥) Pb-Zn deposits are +21.59‰, +18.33‰, +11.43‰ and +10.62‰, respectively, indicating sulfur-bearing hydrothermal fluids may be evolved from the Shanshulin to Qingshan and then the Shaojiwan to Tianqiao deposition sites along the Yadu(垭都)-Ziyun(紫云) lithospheric fracture in the southeastern SYG province.

The stability of the anchorage slope on the Baiyang Yangtze River Highway Bridge in Yichang, China, was investigated under different rainfall conditions using model test, numerical simulation, and factor analysis. The results of the study are as follows: (1) with the increase of rainfall intensity, the change of earth pressure can be divided into three stages. However, when the rainfall intensity was larger than a certain value, the change of earth pressure of cut slope became two stages; with the increase of rainfall intensity, pore water pressure increased with the increase of rainfall time, while at a certain stage after the rainfall, the pore water pressure in the cut slope did not decrease immediately, but increased for a period of time. (2) When the rainfall stopped, the stability coefficient of the anchorage slope continued to decrease, then slowly increased, and finally tended to be gentle. Meanwhile, when the rainstorm reached a certain intensity, the main factor that restricted the rainfall infiltration rate became the geotechnical permeability coefficient of the cut slope, which was no longer the rainfall intensity. (3) Factor analysis shows that the rainfall intensity and rainfall duration were the most important factors for anchorage slope stability, while earth pressure, pore water pressure and slope displacement were much less significant.

This paper studies magnetic properties and composition of granulite-facies rocks of both the Neogene and Archean continental lower crust in the Neogene xenolith-bearing Hannuoba(汉诺坝) alkaline basalt and the exposed lower crustal section in the Archean Huai’an(淮安) terrain (Wayaokou (瓦窑口)-Manjinggou(蔓菁沟 profile), the northern North China Craton. It provides a unique opportunity for a comparative study of magnetic properties and composition of both the Archean and Neogene continental lower crust. We measure magnetic parameters (susceptibility κ and magnetic hysteresis parameters, such as saturation magnetization J s, saturation isothermal remanent magnetization J rs, and intrinsic coercivity H c) of eleven Hannuoba lower crustal xenoliths and nine terrain granulites from the Archean Huai’an terrain. Results indicate that the average values of κ, J s and J rs of Archean granulites are 4 122×10−6 SI, 523.1 A/m and 74.9 A/m, respectively, which are generally higher than those of granulite-facies xenoliths (1 657×10−6 SI, 163.9 A/m and 41.9 A/m, respectively). These two types of granulites contain ilmenite, (titano) magnetite, minor hematite and some “magnetic silicates” (clinopyroxene, plagioclase and biotite). The Mg-rich ilmenite in granulite-facies xenolith is relatively higher than that in terrain granulites. We observe a more evolved character as higher magnetic as well as lower Sr/Nd, Cr/Nd, Ni/Nd, Co/Nd and V/Nd ratios in terrain granulites. These differences in magnetic characteristics reflect their different origins and evolutions. The high magnetization of granulites in the Huai’an terrain represents magnetic properties of the Archean continental lower crust, and low magnetization of granulite-facies xenoliths represents magnetic properties of the Cenozoic lower crusts in the northern North China Craton.

Modeling geomechanical properties of shales to make sense of their complex properties is at the forefront of petroleum exploration and exploitation application and has received much research attention in recent years. A shale’s key geomechanical properties help to identify its “fracibility” its fluid flow patterns and rates, and its in-place petroleum resources and potential commercial reserves. The models and the information they provide, in turn, enable engineers to design drilling patterns, fracture-stimulation programs and materials selection that will avoid formation damage and optimize recovery of petroleum. A wide-range of tools, technologies, experiments and mathematical techniques are deployed to achieve this. Characterizing the interconnected fracture, permeability and porosity network is an essential step in understanding a shales highly-anisotropic features on multiple scales (nano to macro). Well-log data, and its petrophysical interpretation to calibrate many geomechanical metrics to those measured in rock samples by laboratory techniques plays a key role in providing affordable tools that can be deployed cost-effectively in multiple well bores. Likewise, microseismic data helps to match fracture density and propagation observed on a reservoir scale with predictions from simulations and laboratory tests conducted on idealised/simplified discrete fracture network models. Shales complex wettability, adsorption and water imbibition characteristics have a significant influence on potential formation damage during stimulation and the short-term and long-term flow of petroleum achievable. Many gas flow mechanisms and models are proposed taking into account the multiple flow mechanisms involved (e.g., desorption, diffusion, slippage and viscous flow operating at multiple porosity levels from nano- to macro-scales). Fitting historical production data and well decline curves to model predictions helps to verify whether model’s geomechanical assumptions are realistic or not. This review discusses the techniques applied and the models developed that are relevant to applied geomechanics, highlighting examples of their application and the numerous outstanding questions associated with them.

A three-dimensional model of near-surface shear-wave velocity in the deep alluvial basin underlying the metropolitan area of Las Vegas, Nevada (USA), is being developed for earthquake site response projections. The velocity dataset, which includes 230 measurements, is interpolated across the model using depth-dependent correlations of velocity with sediment type. The sediment-type database contains more than 1 400 well and borehole logs. Sediment sequences reported in logs are assigned to one of four units. A characteristic shear-wave velocity profile is developed for each unit by analyzing closely spaced pairs of velocity profiles and well or borehole logs. The resulting velocity model exhibits reasonable values and patterns, although it does not explicitly honor the measured shear-wave velocity profiles. Site response investigations that applied a preliminary version of the velocity model support a two-zone ground-shaking hazard model for the valley. Areas in which clay predominates in the upper 30 m are predicted to have stronger ground motions than the rest of the basin.