Investigation of the Macroscopic Anomalies before and after the M8. 0 Wenchuan Earthquake

The macroscopic anomalies before and after the May 12,2008 Wenchuan M8. 0 earthquake were collected and analyzed in this paper. The credible anomalies occurring from February 1 to June 10,2008 were treated as macroscopic anomalies,and their space-time distribution was studied. The research indicates that in time distribution, the number of macroscopic anomalies peaked before the earthquake, and these peak values appeared twice after the earthquake. In space distribution, the distribution of macroscopic anomalies before the earthquake was the same as that after the earthquake. In this paper, the mechanism of occurrence of macroscopic anomalies and the role of macroscopic anomalies in forecasting earthquake were also discussed.     

GT-2M型海洋重力仪数据处理若干问题的讨论

GT-2M型海洋重力仪是一种新型的重力测量仪器,其工作原理、输出的原始数据格式及质量均与KSS、LR等常用海洋重力仪有所不同,因此其数据预处理方法也需要调整。选择由GT-2M型海洋重力仪采集的得到的两个工区的原始数据,发现在正常场改正及厄特弗斯改正两个步骤区别于其他重力仪,另外不同的数据提取项也会对处理后的数据精度有一定程度的影响,针对这些问题进行分析研究,总结出适用于GT-2M型重力仪的数据处理流程,为相关人员的工作提供参考依据。     

浅水多波束测深资料处理关键技术探讨

为提高浅水多波束测深资料处理成果质量,基于澳门海域浅水多波束测深资料,提出应用POS/MV大地高数据计算实时潮汐数据,探讨了浅水多波束测深资料声速校正方法。结果表明,经过潮汐改正和声速改正后资料处理成果质量明显提高,解决了浅水多波束测深资料处理中的难题,本方法可为浅水多波束测深资料处理提供重要指导。     

An insight into shallow gas hydrates in the Dongsha area,South China Sea

Previous studies of gas hydrate in the Dongsha area mainly focused on the deep-seated gas hydrates that have a high energy potential, but cared little about the shallow gas hydrates occurrences. Shallow gas hydrates have been confirmed by drill cores at three sites(GMGS2 08, GMGS2 09 and GMGS2 16) during the GMGS2 cruise, which occur as veins, blocky nodules or massive layers, at 8–30 m below the seafloor. Gas chimneys and faults observed on the seismic sections are the two main fluid migration pathways. The deep-seated gas hydrate and the shallow hydrate-bearing sediments are two main seals for the migrating gas. The occurrences of shallow gas hydrates are mainly controlled by the migration of fluid along shallow faults and the presence of deep-seated gas hydrates.Active gas leakage is taking place at a relatively high-flux state through the vent structures identified on the geophysical data at the seafloor, although without resulting in gas plumes easily detectable by acoustic methods.The presence of strong reflections on the high-resolution seismic profiles and dim or chaotic layers in the subbottom profiles are most likely good indicators of shallow gas hydrates in the Dongsha area. Active cold seeps,indicated by either gas plume or seepage vent, can also be used as indicators for neighboring shallow gas hydrates and the gas hydrate system that is highly dynamic in the Dongsha area.     

Temporal and Spatial Evolution of Precursory Anomalies of the Wenchuan MS8.0 Earthquake and Their Mechanical Analysis

In order to study the spatiotemporal evolution of the precursory anomalies 10 years before the Wenchuan M_S8. 0 earthquake in 2008, the epicentral distance of the precursory anomalies is calculated by using the geometric center of the rupture region and the elliptical centerline of the aftershock region. The result shows, precursor anomalies gradually increased about 2 years before the Wenchuan earthquake. The ratio of abnormal items is greater than 25% in the near source area (about twice the source scale) and 17%-24% in the remote area (about 3-5 times the source scale). There are three different stages of spatiotemporal evolution of precursory anomalies. During the α stage (including α_1 and α_2,between 700 to 3000 days before the main earthquake),the anomalies are mainly distributed in the southwest and northwest area of the Wenchuan aftershocks area. It is shown that the precursors of the far source region and the near source area have the characteristics of outward expansion. During the β stage (between 300 to 700 days before the main earthquake), the anomalies are distributed in the southwest and northern region of the aftershock region, showing a large range of anomalies. During the γ stage (including γ_1 and γ_2, 300 days before the main earthquake),the range of anomaly distribution is wide,and the anomalies are distributed in the southwest and northeast of the aftershock area. The anomalies converged to epicenter (γ_1) in the far source region and expand outwards (γ_2) in the near source region. Results of the experimental study and mechanical analysis of earthquake preparation process indicate that the three-stage characteristics of precursory anomalies in the process of earthquake preparation may be controlled by the seismogenic body,which is a form of expression in the process of earthquake preparation and a universal featureduring the earthquake preparation process,which has a certain guiding role in earthquake prediction.