The connection of geodynamics with seismicity patterns

When a study is to be made of seismic risks, the present-day geodynamic conditions are of fundamental importance: Earthquakes do not happen by themselves, they do have a cause. The cause of earthquakes is that the tectonic stresses exceed a critical limit. The build-up of these stresses is conditioned by the geodynamic processes occurring in the region in question. A knowledge of the geodynamics characteristic of a region is therefore fundamental for seismic risk studies. The general methodology for making such a geodynamic study is based on the entire set of manifestations of the plate-tectonic conditions of that region: these include the mechanism of earthquakes, the stresses observed in mines, the orientation of surface joints and even the direction of river valleys. Examples of geodynamic studies and their bearing on seismic risks are shown from various areas of the world, notably from China, India, and Colombia.     

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.     

Subduction and back-arc activity at the Hikurangi convergent Margin,New Zealand

The Hikurangi Margin is a region of oblique subduction with northwest-dipping intermediate depth seismicity extending southwest from the Kermadec system to about 42°S. The current episode of subduction is at least 16–20 Ma old. The plate convergence rate varies along the margin from about 60 mm/a at the south end of the Kermadec Trench to about 45 mm/a at 42°S. The age of the Pacific lithosphere adjacent to the Hikurangi Trench is not known.The margin divides at about latitude 39°S into two quite dissimilar parts. The northern part has experienced andesitic volcanism for about 18 Ma, and back-arc extension in the last 4 Ma that has produced a back-arc basin onshore with high heaflow, thin crust and low upper-mantle seismic velocities. The extension appears to have arisen from a seawards migration of the Hikurangi Trench north of 39°S. Here the plate interface is thought to be currently uncoupled, as geodetic data indicate extension of the fore-arc basin, and historic earthquakes have not exceededM _s=7.South of 39°S there is no volcanism and a back-arc basin has been produced by downward flexure of the lithosphere due to strong coupling with the subducting plate. Heatflow in the basin is normal. Evidence for strong coupling comes from historic earthquakes of up to aboutM _s=8 and high rates of uplift on the southeast coast of the North Island.The reason for this division of the margin is not known but may be related to an inferred increase, from northeast to southwest, in the buoyancy of the Pacific lithosphere.     

Microseismicity in a flooded potash mine,the Hope mine,Federal Republic of Germany

Measuring of microseismic events has been carried out at the former potash mine of Hope since early 1984.Analysis of the data demonstrates an immediate temporal relationship between brine introduction and onset of microseismic activity. The spatial distribution of the sources indicates a dependence on brine level height. Sources are found preferentially immediately next to roads and workings. Source radii, estimated from the frequency ranges in the seismograms, varied with the cross-section of neighbouring cavities and had a similar, order of size.Events occurred rarely in the previously flooded mine sections Similarly, the zone below a depth of approx. 600 m below sea level was virtually inactive.     

On self-similarity of premonitory patterns in the regions of natural and induced seismicity

Anticipating the scale invariance of rock fracturing processes, we applied Keilis-Borok’s algorithm M8, originally designed for identifying times of increased probability (TIPS) of occurrence of strong earthquakes (M < 8.0), retrospectively to Koyna earthquakes which occurred in the region after the impoundment of the Shivaji Sagar reservoir in 1962. The algorithm which enables diagnosis of TIPS from the 7th year onwards after the commencement of the earliest available data set showed that the 5.3 magnitude earthquake of 20 September 1980 indeed occurred within a time of increased probability. This result, apart from its potential application to recognizing future TIPS in the region, points to selfsimilarity between the premonitory patterns of natural and induced earthquakes and to scale-invariant nature of their processes. Further, a typical precursory rise in seismicity followed by a relative quiescence was also found to precede all the three larger earthquakes of the sequence.     

辐射日总量的最热坡度解析模式及其全球分布规律

本文给出了南北向坡面最热坡度的解析模式。（１）在北半球（下同），冬半年南坡存在最热坡度，且随纬度升高而增大，接近极夜时趋于９０°。（２）夏半年，存在两个临界纬度当时，南坡有最热坡度；当或时，北坡出现最热坡度。给出了和的解析式，证明分别是全球水平面上日辐射量最大值和最小值所在的纬度，它们仅依赖于赤纬。（３）最热坡度所在坡面获得的日辐射量，冬半年低纬大于高纬；夏半年低纬北坡随纬度递增，南坡时等于处水平面上日辐射量，极昼区内，北坡等于极点水平面的日辐射量。     

内蒙古呼盟地区地震活动的基本特征

通过对内蒙古呼盟地区地震活动的研究,认为呼盟地区震中分布较稳定;存在20年左右地震活动周期;震源机制两节面分布相对比较稳定,P、T轴近水平向;地震破裂是以水平滑动为主。将来呼盟地区的地震活动主体地区为博克图、满洲里一带,震级为5级左右。     

矿山地震研究述评

就采矿引起的诱发地震活动,即矿山地震的国际研究动态进行评述.重点介绍矿山地震的命名、早期观测与研究、几个主要矿震国家的情况、矿震的分类、矿震的震源机制、矿震震源参数与定标率等问题.还就利用矿山作为天然地震的预报实验场和作为地震技术的推广市场问题进行探讨.