地震共轭破裂及极限主应力随地壳深度的变化——以1975年海城7.3级地震为例

许多震例表明 ,地震可以形成共轭破裂。地震震源处 ,其温度和压力与地壳浅处及地表不同 ,温、压越高 ,共轭破裂角越大。随着深度的变浅 (温、压减小 ) ,地震共轭破裂角减小 ,地震烈度共轭角也减小。本文用数学力学方法和岩石力学实验结果 ,探讨了海城地震共轭破裂及极限主应力随地壳深度的变化。

利用接收函数探讨新疆几次中强地震的地壳孕育深度

在分析2009~2012年新疆地区的远震P波接收函数过程中,发现部分台站的接收函数中存在清晰且能量较强的壳内间断面转换波。本文分别利用接收函数中的壳内间断面和莫霍面转换波,联合接收函数H-κ叠加方法和时间窗滑动方法,分析了2009~2012年新疆6次中强地震震中周围的7个地震台站下方的地壳和上地壳(壳内间断面上方地壳)介质泊松比在地震前后随时间的动态变化情况。结果显示:乌苏、于田和独山子3个地震台站的地壳介质泊松比在对应的3次中强地震前后出现了较为明显且持续的下降过程,下降幅度大于均值误差,变化形态特征呈“V”字型(乌苏台呈双“V”字型),地震发生在“V”字形成的尾端,但其上地壳介质泊松比在地震前后没有出现明显低值异常变化;其他4个地震台站下方的地壳和上地壳介质泊松比在地震前后没有观测到明显低值异常。根据观测结果,推测3次中强地震前后地壳介质泊松比出现低值异常主要源于中下地壳,地壳孕育深度位于中下地壳。

Exploration and Research of Deep Crustal Structures in the Zhangzhou Basin and Its Vicinity

The Zhangzhou basin is located at the middle section of the southeast coast seismic zone of the mainland of China. Using high-resolution refraction and wide-angle reflection/refraction seismic profiling of Zhangzhou basin and its vicinity, we have obtained the crustal geometric structure and velocity structure as well as the geometric configuration and structural relationship between the deep and shallow fractures. The results show that the crust in the region is divided into the upper crust and lower crust. The thickness of the upper crust is 16.5km- 18.8km, and that of the lower crust is 12.0km- 13.0km. The upper crust is further divided into an upper and lower section. In the lower section of the upper crust, there is a low-velocity layer with a velocity of about 6.00km/s; the depth of the top surface of the low-velocity layer is about 12.0km, and the thickness is about 5.0km. The lower crust is also divided into an upper and lower section. The depth of Moho is 29.0km- 31 .8km There are 6 normal faults in the shallow crust in this region, and most of them extend downwards to a depth of less than 4kin, the maximum depth is about 5km. Below the shallow normal faults, there is a conjectural high-dip angle deep fault zone. The fault zone extends downwards till the Moho and upwards into the low-velocity layer in lower section of the upper crust. The deep and shallow faults are not tectonically connected. The combination character of deep and shallow structures in the Zhangzhou basin indicates that the Jiulongjiang fault zone is a deep fault zone with distinct characteristics and a complex deep and shallow structure background. The acquisition of deep seismic exploration results obviously enhanced the reliability of explanation of deep-structural data and the exploration precision of the region. The combination of deep and shallow structures resulted in uniform explanation results. The delamination of the crust and the characteristic of the structures are more precise and explicit. We discovered for the first time the combination characteristics of extensional structures and listric faults in the upper crust. This is not only helpful to the integrative judgment of earthquake risk in Zhangzhou and its vicinity, but also of importance for deepening the knowledge of deep dynamic processes in the southeast coast seismic zone.

VARIATIONS IN CRUSTAL THICKNESS OF THE KANE TRANSFORM IN THE NORTH ATLANTIC OCEAN

A new approach was taken to investigate the crustal stucture of the Kane transform and its aseismic extension, using high quality bathymetry and gravity data. The gravity signatures associated with variations in crustal thickness of the transform were isolated from the observed free air anomaly, was continued downward to the mean depth of the crust/mantle interface and converted onto the relief on that surface. The crustal thickness of the transform was then calculated by subtracting seawater depth from the depth of the gravity inferred crust/mantle interface.3 D gravity investigation results indicate that the Kane transform and adjacent areas are associated with a crust thinner than normal oceanic crust. The transform trough is largely underlain by a crust less than 4.5km thick and in the nodal basins the crust may be as thin as 3 km. The crust beneath the fracture zone valley is 4-5.5 km thick. The rift valleys on the spreading segments are also characterized by thin crust (4-5 km thick). Thin oceanic crust extends to 20-30 km from the transform axis,except for some localized places such as the inside corner highs adjoining the ridge transform intersections. These gravity inferred results match fairly well with limited published seismic results. Thinning of the crust is mainly attributable to a thin layer 3, which in turn may be explained by the combined effects of reduced magma supply at the ends of the spreading segments and tectonic activities in the region.

Predictive analysis of stress regime and possible squeezing deformation for super-long water conveyance tunnels in Pakistan

The prediction of the stress field of deep-buried tunnels is a fundamental problem for scientists and engineers. In this study, the authors put forward a systematic solution for this problem. Databases from the World Stress Map and the Crustal Stress of China, and previous research findings can offer prediction of stress orientations in an engineering area. At the same time, the Andersonian theory can be used to analyze the possible stress orientation of a region. With limited in-situ stress measurements, the Hock-Brown Criterion can be used to estimate the strength of rock mass in an area of interest by utilizing the geotechnical investigation data, and the modified Sheorey＇s model can subsequently be employed to predict the areas＇ stress profile, without stress data, by taking the existing in-situ stress measurements as input parameters. In this paper, a case study was used to demonstrate the application of this systematic solution. The planned Kohala hydropower plant is located on the western edge of Qinghai-Tibet Plateau. Three hydro-fracturing stress measurement campaigns indicated that the stress state of the area is SH - Sh 〉 Sv or SH 〉Sv 〉 Sh. The measured orientation of Sn is NEE （N70.3°-89°E）, and the regional orientation of SH from WSM is NE, which implies that the stress orientation of shallow crust may be affected by landforms. The modified Sheorey model was utilized to predict the stress profile along the water sewage tunnel for the plant. Prediction results show that the maximum and minimum horizontal principal stres- ses of the points with the greatest burial depth were up to 56.70 and 40.14 MPa, respectively, and the stresses of areas with a burial depth of greater than 500 m were higher. Based on the predicted stress data, large deformations of the rock mass surrounding water conveyance tunnels were analyzed. Results showed that the large deformations will occur when the burial depth exceeds 300 m. When the burial depth is beyond 800 m, serious squeezing deformations will occur in the surrounding rock masses, thus requiring more attention in the design and construction. Based on the application efficiency in this case study, this prediction method proposed in this paper functions accurately.

Crustal velocity structure of the Shaowu-Nanping-Pingtan transect through Fujian from deep seismic sounding-tectonic implications

The Shaowu-Nanping-Pingtan deep seismic sounding profile is located in northern Fujian Province. High-quality seismic sounding data were acquired by five large explosive blasts received by 133 digital seismic instruments along the profile. Based on seismic facies analysis and travel-time picking on shot record sections, a model of the velocity structure of upper crust was developed by finite-difference tomography of the first breaks; the 2-D P-wave velocity structure and tectonic characteristics of the crust were interpreted further by fitting of waveforms and seismic travel times. The results show that the top of the crystal- line basement is buried at depths of 2.0-4.0 kin, with the deepest buried up to 4.0 km within the Fuzhou Basin. The Moho in- terface was found to be deeper in the west and shallower in the east （i.e., 30.0 km near the coast, increasing to 33.0 km north- westward）. The lower crust on the east side of the Zhenghe-Haifeng Fault Zone has a smoothly varying gradient structure, whereas on the west side it has two distinct layers with a boundary between those layers at a depth of 23 km. Seismic velocities on the west side are generally lower than on the east side; a low velocity layer is observed with a lowest speed of 6.25 km/s at a depth of 22 km on the west side, which may consist of partially molten material. The Zhenghe-Haifeng Fault is a deep crustal fault, and should be a channel for deep material upwelling; it has a direct relationship with multiple stages of continental tectonic movements in Southern China and with multiple magmatic events that started in the Proterozoic and ended in the of late Tertiary in Fujian.

Study on the velocity structure of the crust in Southwest Yunnan of the north-south seismic belt—Results from the Menghai-Gengma-Lushui deep seismic sounding profile

Southwest Yunnan, located in the southern segment of the north-south seismic belt, is one of the regions with strong tectonic movement and seismic activity in China. Study on the characteristics of tectonic setting and deep geophysical field in the region is an important issue in basic science. In 2013, we conducted a 600-km-long Menghai-Gengma-Lushui profile of deep seismic wide-angle reflection/refraction and high-resolution seismic refraction in Southwest Yunnan. In this paper, we use 6 groups of clear intracrustal P-wave phases picked from the seismic record sections of 11 shots to build a velocity structure model of basement and 2D crustal P-wave of the region by using finite difference inversion and ray travel time forward fitting technology. The results show that, from south to north, the crust gradually thickens along the profile and its basement shows a significant lateral heterogeneity. In the vicinity of the Nanting River fault, the basement structure shows the character of alternate depressions and uplifts, and the shallowest basement is about 1.0 kin. In the vicinity of Tengchong and Lancang, the basement is about 5.0 km deep. The velocity of the middle and lower crust in the region generally increases with the increasing of depth. At the block boundary and beneath the fault tectonic belt, the velocity contours show apparent irregularity and the P-wave velocity changes sharply. In this region, the Moho gradually deepens from south to north with relatively large lateral undulations. The shallowest point of the Moho is located near Menghai at a depth of about 32.0 km. The deepest point of the Moho is located near Tengchong at a depth of about 40.5 km. Between Gengma and Yongde, the Moho shows significantly fast uplifting and depressing with an amplitude of about 4.0 km. Beneath the Nanting River fault, Longling-Ruili fault, Dayingjiang fault and Tengchong volcano, the basement velocity structure, 2D crustal P-wave velocity structure, distribution of average profile velocity and intracrustal interface spreading also show significant changes from the basement to the top of the Moho, indicating that the crustal velocity and medium physical properties beneath the fault tectonic belt are apparently different from the crustal materials on its both sides, which suggests that these faults should be in a certain scale and may extend to the lower crust or the top of the upper mantle. The earthquakes in the region mainly occurred at a depth of 10-20 km, and the seismic activity is related to the intracrustal medium velocity difference and fault belt distribution. The results can serve as the important data of the crust-mantle structure for the analysis of the deep tectonic setting, earthquake precise positioning, seismogenic structure modeling of the seismic activities in Southwest Yunnan, as well as the important reference for the evaluation of seismic hazard and the planning of earthquake disaster mitigation of this region.