Observations of mine seismicity in the eastern Wasatch Plateau,Utah, U.S.A.: A possible case of implosional failure

In the summer of 1984, a three-dimensional, high-resolution microearthquake network was operated in the vicinity of two coal mines beneath Gentry Mountain in the eastern Wasatch Plateau, Utah. During a six-week period, approximately 3,000 seismic events were observed of which the majority were impulsive, higher frequency (>10 Hz), short duration (<2–3 sec) events probably associated with the caving of the roof from a longwall operation. In contrast, 234 of the largest located events appeared to occur predominantlybeneath the mines to a depth of 2 to 3 km consistent with previous studies. The magnitudes of these events ranged from less thanM _c 0 to 1.6. In addition to the unusual depths of these latter events, an anomalous aspect displayed by the events was an apparent dilatational focal mechanism suggesting a non-double-couple, possibly implosional source. Implosional events have been observed in other studies of mine seismicity; however, the generally inadequate instrumental coverage of the focal sphere has cast some doubt on the validity of such mechanisms. Previously suggested source mechanisms for such implosional events have included tensional failure through strata collapse, and a shear-implosional displacement mechanism. Shear failure must be involved in the failure process of the Gentry Mountain implosional events as evidenced by well-defined shear waves in the observed seismograms. Simultaneous monitoring in the East Mountain coal mining area to the south by the University of Utah revealed typical shear failure events mixed with implosional events. The observed double-couple, reverse focal mechanisms at East Mountain were similar to mechanisms determined in previous studies and a composite focal mechanism determined in this study for a sequence outside the mining areas. This suggested that the shear events within the mining areas are being influenced by the regional tectonic stress field. Thus in addition to the seismic events associated with caving of the roof from the longwall operation, there appear to be at least two other types of mining-induced seismic events occurring in the eastern Wasatch Plateau, both submine in origin: (1) events characterized by apparent non-double-couple possibly implosional focal mechanisms and well-defined shear waves; and (2) shear events, which are indistinguishable from tectonic earthquakes and may be considered mining triggered earthquakes. The small mining-induced stress changes that occur beyond a few hundred meters from the mine workings suggest both types of seismic events are occurring on critically stressed, pre-existing zones of weakness. Topography, overburden, method of mining, and mine configuration also appear to be significant factors influencing the occurrence of the implosional submine events.     

临汾盆地现代地震活动特征及其与深部构造关系初探

本文分析了临汾盆地现代地震在时序分布上的阶段性，平面分布上的丛集性，及其震源三维分布与分布地深部构造的关系，指出盆地北部的临汾－洪洞凹陷为该区未来若干年内的主要发震区，而２０－３０ｋｍ深度则是主要发震层次。     

Focal mechanisms of intraplate earthquakes in Bolivia,South America

Intraplate seismic activity in Bolivia is mainly located in the central region (16°–19°S, 63°–67°W) which includes the East Andean Cordillera and the Sub-Andean Sierras. At this region there is a bend in the trend of the main geological structures from NW-SE in the north to N-S in the south. Focal mechanisms have been calculated for 10 earthquakes of magnitudes 4.9–5.6, using first motionP-waves from long period instruments. Their solutions correspond to reverse faulting, some with a large component of strike-slip motion. Their solutions can be grouped into two types; one with pure reverse faulting on planes with azimuth NW-SE and the other with a large strike-slip component on planes with azimuths nearly N-S or WNW-ESE. The maximum stress axis (P-axis) is practically horizontal (dipping less than 5°) oriented in a mean N56°E direction. This orientation may be related with the direction of compression resulting from the collision of the Nazca plate against the western margin of the South American continent. Wave-form analysis of long-periodP-waves for one event restricts the focal depth to 8 km in the Sub-Andean region. Seismic moments and source dimensions determined from spectra of Rayleigh waves are in the range of 10¹⁶–10¹⁷Nm and 17–24 km, respectively. The Central Bolivia region can be considered as a zone of intraplate deformation situated between the Bolivian Altiplano and the Brazil shield.     

Complex time－space spreading and focal mechanism of the 1989 Batang earthquake swarm （M_s＝6．7）

Ｃｏｍｐｌｅｘ　ｔｉｍｅ－ｓｐａｃｅ　ｓｐｒｅａｄｉｎｇ　ａｎｄ　ｆｏｃａｌ　ｍｅｃｈａｎｉｓｍ　ｏｆ　ｔｈｅ　１９８９　Ｂａｔａｎｇ　ｅａｒｔｈｑｕａｋｅ　ｓｗａｒｍ　（Ｍ＿ｓ＝６．７）（程万正）（陈天长）Ｃｏｍｐｌｅｘｔｉｍｅ－ｓｐａｃｅｓｐｒｅ...     

Applying artificial neural networks to achieve quantitative division of potential focal regions

（姜效典，张维岗）Ａｐｐｌｙｉｎｇａｒｔｉｆｉｃｉａｌｎｅｕｒａｌｎｅｔｗｏｒｋｓｔｏａｃｈｉｅｖｅｑｕａｎｔｉｔａｔｉｖｅｄｉｖｉｓｉｏｎｏｆｐｏｔｅｎｔｉａｌｆｏｃａｌｒｅｇｉｏｎｓ￥Ｘｉａｏ－ＤｉａｎＪＩＡＮＧａｎｄＷｅｉ－ＧａｎｇＺＨＡＮＧ（...     

诱发地震中的非双力偶震源

在诱发地震的研究中，用小孔径地震台网在近场观测，其震源机制的Ｐ波初动除Ⅲ象限分布的双力偶外，还有不是Ⅲ象限分布的非双力偶震源机制。双力偶震源机制是剪切破裂机制。非双力偶震源机制为向内源炸型和张破裂机制，以及在适当方向上两个以上双力偶迭加的结果。     

变比例尺地图投影系统

本文对编制城市游览图提出了变比例尺地图投影系统。通过由普通城市平面图向辅助球面作逆投影A,再由辅助球面向平面作非A投影,构成了变比例尺地图的数学基础。由不同性质的投影的组合,能起到适应不同城市街区结构的特点。使本系统具有相当的灵活性。文中还讨论了辅助球适宜的大小和不同方位投影之间的变换公式。文末试作了北京市的变比例尺地图。     

华北地区的共轭地震构造带

本文采用小地震活动图象和４．０级（Ｍｓ）以上地震震源机制资料的构造分析方法，得到一幅华北地区震源构造在地面的投影分布图，它显示４条ＮＮＥ－ＮＥ向和１条ＮＷＷ－ＮＷ向地震构造带交切成的共轭剪切构造格架。每条地震构造带又由一系列共轭剪切构造组成。由发生在带内的５个大震序列共轭破裂特征发现，共轭地震构造的孕震与控震作用是地震构造带形成的机制。     

大同地震序列的震源机制解

使用ＣＤＳＮ长周期地震波形记录，利用Ｈｅｌｍｂｅｒｇｅｒ（１９６８）的广义地震射线理论得到的线性反演地震矩张量的方法，对１９８９年１０月１８日－１９９１年３月２５日发生在大同的４个Ｍｓ＞５．０的地震进行了反演，计算了这些地震矩张量和断层面解。