The presence, characteristics and earthquake implications of the St. Lawrence fault zone within and near Lake Ontario (Canada–USA)

Questions persist concerning the earthquake potential of the populous and industrial Lake Ontario (Canada–USA) area. Pertinent to those questions is whether the major fault zone that extends along the St. Lawrence River valley, herein named the St. Lawrence fault zone, continues upstream along the St. Lawrence River valley at least as far as Lake Ontario or terminates near Cornwall (Ontario, Canada)–Massena (NY, USA). New geological studies uncovered paleotectonic bedrock faults that are parallel to, and lie within, the projection of that northeast-oriented fault zone between Cornwall and northeastern Lake Ontario, suggesting that the fault zone continues into Lake Ontario. The aforementioned bedrock faults range from meters to tens of kilometers in length and display kinematically incompatible displacements, implying that the fault zone was periodically reactivated in the study area. Beneath Lake Ontario the Hamilton–Presqu'ile fault lines up with the St. Lawrence fault zone and projects to the southwest where it coincides with the Dundas Valley (Ontario, Canada). The Dundas Valley extends landward from beneath the western end of the lake and is marked by a vertical stratigraphic displacement across its width. The alignment of the Hamilton–Presqu'ile fault with the St. Lawrence fault zone strongly suggests that the latter crosses the entire length of Lake Ontario and continues along the Dundas Valley.The Rochester Basin, an east–northeast-trending linear trough in the southeastern corner of Lake Ontario, lies along the southern part of the St. Lawrence fault zone. Submarine dives in May 1997 revealed inclined layers of glaciolacustrine clay along two different scarps within the basin. The inclined layers strike parallel to the long dimension of the basin, and dip about 20° to the north–northwest suggesting that they are the result of rigid-body rotation consequent upon post-glacial faulting. Those post-glacial faults are growth faults as demonstrated by the consistently greater thickness, unit-by-unit, of unconsolidated sediments on the downthrown (northwest) side of the faults relative to their counterparts on the upthrown (southeast) side. Underneath the western part of Lake Ontario is a monoclinal warp that displaces the glacial and post-glacial sediments, and the underlying bedrock–sediment interface. Because of the post-glacial growth faults and the monoclinal warp the St. Lawrence fault zone is inferred to be tectonically active beneath Lake Ontario. Furthermore, within the lake it crosses at least five major faults and fault zones and coexists with other neotectonic structures. Those attributes, combined with the large earthquakes associated with the St. Lawrence fault zone well to the northeast of Lake Ontario, suggest that the seismic risk in the area surrounding and including Lake Ontario is likely much greater than previously believed.     

2-D viscoelastic FEM simulation on stress state in the deep part of a subducted slab

Introduction It is found that there are some relationships between the thermal structures of subduction zones and the deep seismicity, while the mechanism relates the thermal structure and the deep seismicity is still unsure (Helffrich, Brodholt, 1991; Furukawa, 1994; Kirby, et al, 1996). From 1980s, geoscientists have constituted a series of numerical simulations on the stress states of subduction slabs. Based on the kinetic computation of Sung and Burns (1976a, b), Goto, et al (1983, 1987…     

Study and review on crust-mantle velocity structure in Bohai Bay and its vicinity

Introduction Bohai Bay, along with its adjacent areas, is one of the seismically active areas in North China. Understanding its crust/upper-mantle structural characteristics and lateral heterogeneity of the medium in this area is of great significance to the study of seismogenic environment, thus improvimg the level of earthquake prediction. For years, scientists have studied the area by gravity and magnetic methods (FENG, et al, 1989), geothermal field (WU, et al, 1988; TIAN, ZHANG, 19…     

辽宁城市活断层探测思路与方法探讨

介绍了开展地震活断层探测的意义及在辽宁省开展地震活断层探测的必要性，对在辽宁城市开展活断层探测的基本思路和方法进行了探讨，选取沈阳，抚顺作为试点城市，设定目标区和目标断裂，分别对活断层危险性，危害性和深部发震构造环境进行评价。在断裂探测过程中，确定初步探测和详细探测两个阶段，主要采用人工地震方法并结合钻探，地震划分结果，对目标区内的目标断裂进行评价。     

南海地球物理场特征及基底断裂体系研究

南海海域主体可划分为南海北缘、中西沙、南沙南海海盆四块，各块具有明显不同的重磁场特征。反演得到的莫霍面总体趋势由陆向洋抬升，反映陆壳、拉伸陆壳、过渡壳、洋壳的分布。东沙高磁异常含一定的高频成份，与新生代玄武岩及中生代岩浆岩有关，而其低频成份可能反映了发育的下地壳高速层，南海海域断裂极为发育，可分为北东向断裂组、东西向断裂组、北西向断裂组和南北向断裂组，南海北缘、南缘均以北东向张性断裂与北西向张剪性、剪性断裂为主要格架，形成了、南北分带、东西分块”构造格局。     

固体介质中弹性波传播机制的细胞自动机有限深势阱模型

分析了采用细胞自动机研究波动问题的建模方法，针对一维、均匀、各向同性固体介质中弹性纵波的微观机制，借用一经典弹簧振子模型、细胞自动机格子气模型，以及量子力学中的无限深势阱模型，建立了一个细胞自动机有限深势阱模型，从量子力学角度出发，基于介观物理和纳米概念，以微观精子的德布罗意假设为基础，利用薛定谔方程，讨论了该模型中粒子（分子组）的振动速度与粒子物质波波速之间的联系，给出了模型中的波动方程，得出ζ＝Vp（ζ为粒子振动速度，Vp为物质波纵波波速）。同时还讨论了模型中粒子的大小和能量传递问题，引入引力场，得出了能量及引力势的量子化条件，另外，对声波速度、格子气粒子振动速度和本文模型中分子组振动速度进行了比较；还对本文模型中的粒子能量分布作了分析。     

Bedrock structure in Adapazari, Turkey—a possible cause of severe damage by the 1999 Kociaeli earthquake

The 1999 Kocaeli earthquake brought serious damage to downtown of Adapazari. To study why strong motions were generated at the town, a bedrock structure was investigated on the basis of Bouguer gravity anomaly, and SPAC and H/V analyses of microseisms. It was revealed that, the basin consists of three narrow depressions of bedrock with very steep edges, extending in E–W or NE–SW directions along the North Anatolia faults, and the depth to bedrock reaches 1000 m or more. Downtown of Adapazari is located 1–2 km apart from the basin-edge. It is considered that, the specific configuration of bedrock amplifies ground motions at the downtown area by focusing of seismic waves and/or interference between incident S-waves and surface-waves secondarily generated at the basin-edge. Studying 3D bedrock structure is an urgent issue for microzoning an urban area in a sedimentary basin.     

A rifted inside corner massif on the Mid-Atlantic Ridge at 5°S

The structure of the Mid-Atlantic Ridge at 5°S was investigated during a recent cruise with the FS Meteor. A major dextral transform fault (hereafter the 5°S FZ) offsets the ridge left-laterally by 80 km. Just south of the transform and to the west of the median valley, the inside corner (IC – the region bounded by the ridge and the active transform) is marked by a major massif, characterized by a corrugated upper surface. Fossil IC massifs can also be identified further to the west. Unusually, a massif almost as high as the IC massif also characterizes the outside corner (OC) south of the inactive fracture zone and to the east of the median valley. This OC massif has axis-parallel dimensions identical to the IC massif and both are bounded on their sides closest to the spreading axis by abrupt, steep slopes. An axial volcanic ridge is well developed in the median valley both south of the IC/OC massifs and in an abandoned rift valley to the east of the OC massif, but is absent along the new ridge-axis segment between the IC and OC massifs. Wide-angle seismic data show that between the massifs, the crust of the median valley thins markedly towards the FZ. These observations are consistent with the formation of the OC massif by the rifting of an IC core complex and the development of a new spreading centre between the IC and OC massifs. The split IC massif presents an opportunity to study the internal structure of the footwall of a detachment fault, from the corrugated fault surface to deeper beneath the fault, without recourse to drilling. Preliminary dredging recovered gabbros from the scarp slope of the rifted IC massif, and serpentinites and gabbros from the intersection of this scarp with the corrugated surface. This is compatible with a concentration of serpentinites along the detachment surface, even where the massif internally is largely plutonic in nature.