2020年5月6日新疆乌恰5.0级和5月9日柯坪5.2级地震总结

2020年5月6日、5月9日,新疆地区南天山西段先后发生乌恰5.0级和柯坪5.2级地震,系统总结2次地震发生前出现的地震活动和地球物理观测异常,其中:①地震活动:震前存在调制地震集中、地震窗、5级以上地震成组等中短期异常;②地球物理观测:2次地震震中附近震前出现形变、电磁和流体观测异常,其中形变异常3项、电磁异常4项、流体异常1项,主要分布在柯坪5.2级地震震中附近。通过对2次地震序列进行跟踪,发现:乌恰5.0级地震余震较少,震后60天内共记录M_L 3.0以上余震4次,最大震级为M_L 4.5;柯坪5.2级地震后余震较丰富,震后60天内共记录M_L 3.0以上余震10次,最大震级为M_L 4.7,计算得到序列h值为1.6,b值为0.73。综合分析认为,2020年5月新疆地区2次5级以上地震前存在的地震活动异常较少,但区域地震活动水平较强,主要存在具有中短期指示意义的地球物理观测异常。     

Redundant and Nonredundant Information for Model Calibration or Hydraulic Tomography

Drawdown data from independent pumping tests have widely been used to validate the estimated hydraulic parameters from inverse modeling or hydraulic tomography (HT). Yet, the independent pumping test has not been clearly defined. Therefore, the goal of this paper is to define this independent pumping test concept, based on the redundant or nonredundant information about aquifer heterogeneity embedded in the observed heads during cross-hole pumping tests. The definition of complete, moderate redundancy and high nonredundancy of information are stipulated using cross-correlation analysis of the relationship between the head and heterogeneity. Afterward, data from numerical experiments and field sequential pumping test campaigns reinforce the concept and the definition.     

Hydraulic Performance of the Horizontal Reactive Media Treatment Well: Pilot and Numerical Study

This study is focused on a passive treatment system known as the horizontal reactive treatment well (HRX Well®) installed parallel to groundwater flow, which operates on the principle of flow focusing that results from the hydraulic conductivity (K) ratio of the well and aquifer media. Passive flow and capture in the HRX Well are described by simplified equations adapted from Darcy's Law. A field pilot-scale study (PSS) and numerical simulations using a finite element method (FEM) were conducted to verify the HRX Well concept and test the validity of the HRX Well-simplified equations. The hydraulic performance results from both studies were observed to be within a close agreement to the simplified equations and their hydraulic capture width approximately five times greater than the well diameter (0.20 m). Key parameters affecting capture included the aquifer thickness, well diameter, and permeability ratio of the HRX Well treatment media and aquifer material. During pilot testing, the HRX Well captured 39% of flow while representing 0.5% of the test pit cross-sectional volume, indicating that the well captures a substantial amount of surrounding groundwater. While uncertainty in the aquifer and well properties (porosity, K, well losses), including the effects of boundary conditions, may have caused minor differences in the results, data from this study indicate that the simplified equations are valid for the conceptual design of a field study. A full-scale HRX Well was installed at Site SS003 at Vanderberg Air Force Base, California, in July/August 2018 based on outcomes from this study.     

城市浅层地震勘探过障碍变观设计

在城市浅层地震勘探数据采集中,地震测线经常会遇到河流、桥、交叉路口等地表障碍。为了尽量减小地表障碍造成的影响,需要对观测系统作变观设计。作者分析总结了城市浅层地震勘探中常见障碍以及过障碍变观模式,并开发了浅层地震过障碍变观设计软件,利用此软件可在采集现场快捷地进行过障碍变观。将这种方法应用于实际工作中,有效减小了资料缺失造成的影响,提高了资料的信噪比。该软件能够根据实际工作环境和设备条件在野外现场对观测系统进行灵活变观,设计出合理的过障碍观测系统,有效地提高了变观设计的效率。     

The Study on Seismicity and Relocation of the July 3,2015 Pishan MS6.5 Earthquake

The July 3, 2015 Pishan M_S6.5 earthquake occurred in the intersection area of the Tarim block and West Kunlun block where the moderate-strong earthquakes have become active in recent years. This paper has studied the seismicity parameters of the earthquake sequences such as the b-value in the Pishan region and its vicinity. In addition, we also relocated the aftershocks of the Pishan M_S6.5 earthquake using the seismic phase report by the double-difference method. The temporal and spatial variation characteristics of the Pishan earthquake sequence in the rupture zone are analyzed. The study is of great significance in the seismic hazard assessment in this region.     

Effective elastic thickness of the lithosphere from joint inversion in western China and its implications*

The western China lies in the convergence zone between Eurasian and Indian plates. It is an ideal place to study the lithosphere dynamics and tectonic evolutions on the continental Earth. The lithospheric strength is a key factor in controlling the lithosphere dynamics and deformations. The effective elastic thickness (T_e) of the lithosphere can be used to address the lithospheric strength. Previous researchers only used one of the admittance or coherence methods to investigate the T_e in the western China. Moreover, most of them ignored the internal loads of the lithosphere during the T_e calculation, which can produce large biases in the T_e estimations. To provide more reliable T_e estimations, we used a new joint inversion method that integrated both admittance and coherence techniques to compute the T_e in this study, with the WGM2012 gravity data, the ETOPO1 topographic data, and the Moho depths from the CRUST1.0 model. The internal loads are considered and investigated using the load ratio (F). Our results show that the joint inversion method can yield reliable T_e and F values. Based on the analysis of T_e and F distributions, we suggest (1) the northern Tibetan Plateau could be the front edge of the plate collision of Eurasian and Indian plates; (2) the southern and part of central Tibetan Plateau have a strong lithospheric mantle related to the rigid underthrusting Indian plate; (3) the southeastern Tibetan Plateau may be experiencing the delamination of lithosphere and upwelling of asthenosphere.     

Multi‐dimensional analyses of the SEAM controlled source electromagnetic data—the story of a blind test of interpretation workflows

Using a subset of the SEG Advanced Modeling Program Phase I controlled‐source electromagnetic data, we apply our standard controlled‐source electromagnetic interpretation workflows to delineate a simulated hydrocarbon reservoir. Experience learned from characterizing such a complicated model offers us an opportunity to refine our workflows to achieve better interpretation quality. The exercise proceeded in a blind test style, where the interpreting geophysicists did not know the true resistivity model until the end of the project. Rather, the interpreters were provided a traditional controlled‐source electromagnetic data package, including electric field measurements, interpreted seismic horizons, and well log data. Based on petrophysical analysis, a background resistivity model was established first. Then, the interpreters started with feasibility studies to establish the recoverability of the prospect and carefully stepped through 1D, 2.5D, and 3D inversions with seismic and well log data integrated at each stage. A high‐resistivity zone is identified with 1D analysis and further characterized with 2.5D inversions. Its lateral distribution is confirmed with a 3D anisotropic inversion. The importance of integrating all available geophysical and petrophysical data to derive more accurate interpretation is demonstrated.     

Paleoenvironments of the evolving Pliocene to early Pleistocene foreland basin in northwestern Taiwan: An example from the Dahan River section

The overriding of the Luzon volcanic arc atop the underlying Chinese rifted‐continental margin has caused the formation of the Taiwan mountain belts and a peripheral foreland basin west of the orogen since the late Miocene. In this study, lithofacies analysis and calcareous nannofossil biostratigraphic investigations of the Dahan River section in northwestern (NW) Taiwan were performed. Our results offer insights into the temporal evolution of the sedimentary environments and the competing effects of the sedimentation and basin tectonics of the NW Taiwan foreland basin from the Pliocene to early Pleistocene. Nannofossil biostratigraphic studies showed that the upper Kueichulin Formation and the overlying Chinshui Shale can be assigned to the NN15 biozone of the Pliocene age, and the Cholan Formation pertains to NN16–NN18 of the early Pleistocene. The NN15–NN16 boundary coincides roughly with the boundary of the Chinshui Shale and Cholan Formation. We recognized three major sedimentary environments in the studied foreland succession comprising the upper Kueichulin Formation, Chinshui Shale, Cholan Formation and Yangmei Formation, in ascending order. During the deposition of the upper Kueichulin Formation in the early Pliocene, the dominant environment was a wave‐ and tide‐influenced open marine setting. During the late Pliocene, the environment deepened to an outer‐offshore setting when the sediments of Chinshui Shale were accumulated. In the Pleistocene, the environment then shallowed to wave‐dominated estuaries during the deposition of the lower Cholan Formation, and the basin was rapidly filled, generating a meandering and sandy braided river environment during the deposition of the upper Cholan to the Yangmei Formation. In summary, the evolution of sedimentary environments in the studied succession shows a deepening then a shallowing and coarsening upward trend during the period from the Pliocene to the Pleistocene, spanning the age from approximately 4 to 1 Ma.     

Characteristics of strong motions and damage implications of M S6.5 Ludian earthquake on August 3, 2014

The Ludian County of Yunnan Province in southwestern China was struck by an MS6.5 earthquake on August 3, 2014, which was another destructive event following the MS8.0 Wenchuan earthquake in 2008, MS7.1 Yushu earthquake in 2010, and MS7.0 Lushan earthquake in 2013. National Strong-Motion Observation Network System of China collected 74 strong motion recordings,which the maximum peak ground acceleration recorded by the 053 LLT station in Longtoushan Town was 949 cm/s2 in E–W component. The observed PGAs and spectral ordinates were compared with ground-motion prediction equation in China and the NGA-West2 developed by Pacific Earthquake Engineering Researcher Center. This earthquake is considered as the first case for testing applicability of NGA-West2 in China. Results indicate that the observed PGAs and the 5 % damped pseudo-response spectral accelerations are significantly lower than the predicted ones. The field survey around some typical strong motion stations verified that the earthquake damage was consistent with the official isoseismal by China Earthquake Administration.