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

Ivan G. Wong  James R. Humphrey  Janet A. Adams  Walter J. Silva 《Pure and Applied Geophysics》1989,129(3-4):369-405

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.  相似文献   

Mechanisms of seismic quiescences   总被引：7，自引：0，他引：7  

Christopher H. Scholz 《Pure and Applied Geophysics》1988,126(2-4):701-718

In the past decade there have been major advances in understanding the seismic cycle in terms of the recognition of characteristic patterns of seismicity over the entire tectonic loading cycle. The most distinctive types of patterns are seismic quiescences, of which three types can be recognized:post-seismic quiescence, which occurs in the region of the rupture zone of an earthquake and persists for a substantial fraction of the recurrence time following the earthquake,intermediate-term quiescences, which appear over a similar region and persist for several years prior to large plate-rupturing earthquakes, andshort-term quiescences, which are pronounced lulls in premonitory swarms that occur in the hypocentral region hours or days before an earthquake. Although the frequency with which intermediate-term and short-term quiescences precede earthquakes is not known, and the statistical significance of some of the former has been challenged, there is a need, if this phenomena is to be considered a possibly real precursor, to consider physical mechanisms that may be responsible for them.The characteristic features of these quiescences are reviewed, and possible mechanisms for their cause are discussed. Post-seismic quiescence can be readily explained by any simple model of the tectonic loading cycle as due to the regional effect of the stress-drop of the previous principal earthquake. The other types of quiescence require significant modification to any such simple model. Of the possibilities considered, only two seem viable in predicting the observed phenomena, dilatancy hardening and slip weakening. Intermediate-term quiescences typically occur over a region equal to or several times the size of the rupture zone of the later earthquake and exhibit a relationship between the quiescence duration and size of the earthquake: they thus involve regional hardening or stress relaxation and agree with the predictions of the dilatancy-diffusion theory. Short-term quiescences, on the other hand, are more likely explained by fault zone dilatancy hardening and/or slip weakening within a small nucleation zone. Because seismicity is a locally relaxing process, seismicity should follow a behaviour known in rock mechanics as the Kaiser effect, in which only a very slight increase in strength, due to dilatancy hardening or decrease in stress due to slip weakening, is required to cause quiescence. This is in contrast to other precursory phenomena predicted by dilatancy, which require large dilatant strains and complete dilatancy hardening.Lamont-Doherty Geological Observatory  相似文献   

Factors affecting trans-Tasman air services     

Christopher Kissling 《GeoJournal》1993,29(3):291-297

International civil aviation is circumscribed by a plethora of bilateral air service agreements between sovereign nations. In recent years there has been a marked trend to liberalise these agreements as well as moves to create multilateral common markets as appear to be emerging in Europe. Airline managements are being given opportunities to implement operational strategies once severely proscribed in confidential memoranda of understanding lying behind the public bilateral documents. In keeping with the philosophy of extending the Closer Economic Relations (CER) agreement, Australia and New Zealand have been exploring the ramifications of bringing trade in air services within the ambit of CER. This paper examines some of the factors affecting the trans Tasman civil aviation environment and discusses the implications of modifications to air services in and between Australia and New Zealand.  相似文献   

Rock-magnetic proxies of climate change in the loess-palaeosol sequences of the western Loess Plateau of China   总被引：5，自引：0，他引：5  

Christopher P. Hunt  Subir K. Banerjee  Jiamao Han  Peter A. Solheid  Eric Oches  Weiwei Sun  Tungsheng Liu 《Geophysical Journal International》1995,123(1):232-244

Results of the first detailed study of the climate proxy record in the loess-palaeosol sequence at Xining-one of the few palaeoclimate sites in the currently arid western Loess Plateau of China-illustrate the importance of making many types of rock-magnetic measurements other than susceptibility. A multiparameter approach yielded confirmation that here, as elsewhere in the Loess Plateau, the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite. Nevertheless, magnetic enhancement was caused not exclusively by changes in relative grain size, but also by variations in concentration and mineralogy of the magnetic fraction.
The effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence, respectively. the resulting signal was ascribed more confidently to variation in magnetic grain size, which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility. Variations in magnetic mineralogy were also determined to constrain interpretations further. the data were then used to discuss climate history at Xining. Finally, results from Xining were compared with other western sites and contrasted with eastern sites.
In summary: (1) data is presented from a new Loess Plateau site which also appears to yield a global climate signal; (2) a demonstration is made of a more rock-magnetically robust way to separate concentration, composition and grain-size controls on susceptibility and other magnetic parameters; and (3) models are provided for inter-regional comparisons of palaeoclimate proxy records.  相似文献   

Effects of lateral velocity heterogeneity under the Nevada Test Site on short-periodP wave amplitudes and travel times     

Christopher S. Lynnes  Thorne Lay 《Pure and Applied Geophysics》1990,132(1-2):245-267

Short-period teleseismicP waves from the Nevada Test Site (NTS) show systematic variations in amplitudes and travel times, with low amplitudes corresponding to fast travel times, suggesting elastic focussing-defocussing effects. Also, the azimuthal amplitude and travel time patterns for events at the Pahute Mesa subsite are systematically different from those at the Yucca Flat subsite, indicating the presence of a near-source component in both the amplitude and travel-time variations. This component is isolated by removing the mean station pattern for all of NTS from the observations. A very-near-source component in the Pahute Mesa observations is also isolated by removing subsite station means from the measurements, whereas the Yucca Flat observations exhibited no coherent very-near-source component. These anomalies are back-projected through laterally homogeneous structure to form thin lens models at various depths. Travel-time delays are predicted from the amplitude variations using the equation for wavefront curvature. The long-wavelength components of the predicted and observed time delays correlate well, at depths of 25 km for the very-near-source component under Pahute Mesa and 160 km for the regional component under NTS. The time delay surfaces predicted by the amplitudes at these depths are mapped into warped velocity discontinuities suitable for the calculation of synthetic seismograms using the Kirchhoff-Helmholtz integral formulation. Both the intersite (near-source) and intrasite (very-near-source) differences in amplitudes are qualitatively predicted very well, although the range of variation is somewhat underpredicted. This deficiency is likely due to the destructive interference of anomalies inherent in back-projection to a single layer.  相似文献   

Responses of fish and macrobenthic assemblages to hydrologic disturbances in Tijuana Estuary and Los Peñasquitos Lagoon,California     

Christopher S. Nordby  Joy B. Zedler 《Estuaries and Coasts》1991,14(1):80-93

Changes in the assemblages of fishes and benthic macro-invertebrates were evaluated in relation to wastewater inflows at Tijuana Estuary and impounded streamflows and mouth closure at Los Peñasquitos Lagoon. Freshwater from sewage spills or winter rains lowered water salinities and had major impacts on the channel organisms of both southern California coastal wetlands. Benthic infaunal assemblages responded more rapidly to reduced salinity than did fishes, with continued salinity reduction leading to the extirpation of most species. Both the fish and benthic invertebrate assemblages became dominated by species with early ages of maturity and protracted spawning seasons. Between-system comparisons showed that good tidal flushing reduced negative impacts on both the fish and benthic assemblages.  相似文献   

Fluid mechanical aspects of the pollutant transport to coniferous trees     

B. Ruck  E. Adams 《Boundary-Layer Meteorology》1991,56(1-2):163-195

Forest decline in some parts of Europe gave rise to various environmental studies concerning the intake and uptake of pollutants in the ecosphere. As far as fluid mechanics is concerned, the current interest is centered on flow-induced phenomena, e.g., the flow-enhanced deposition of pollutants to trees. In order to understand better the mechanisms of pollutant dispersion and deposition to trees, wind tunnel experiments carried out with small real coniferous trees and model trees are summarized in this paper. The flow around single trees and tree stands, both in flat terrain and on hillsides, has been analysed. The measurements were performed with a two-component laser Doppler anemometer system installed in an atmospheric boundary-layer wind tunnel. A chemical tracer method based on an ammonia-manganese chloride reaction was applied to visualize the deposition patterns around trees and modeled forest stands.  相似文献   

Airborne vector gravimetry using precise,position-aided inertial measurement units     

Christopher Jekeli 《Journal of Geodesy》1994,69(1):1-11

Vector gravimetry using a precise inertial navigation system continually updated with external position data, for example using GPS, is studied with respect to two problems. The first concerns the attitude accuracy requirement for horizontal gravity component estimation. With covariance analyses in the space and frequency domains it is argued that with relatively stable uncompensated gyro drift, the short-wavelength gravity vector can be estimated without the aid of external attitude updates. The second problem concerns the state-space estimation of the gravity signal where considerable approximations must be assumed in the gravity model in order to take advantage of the ensemble error estimation afforded by the Kalman filter technique. Gauss-Markov models for the gravity field are specially designed to reflect the attenuation of the signal at a specific altitude and the omission of the long-wavelength components from the estimation. With medium accuracy INS/GPS systems, the horizontal components of gravity with wavelengths shorter than 250 km should be estimable to an accuracy of 4–6 mgal (µg); while high accuracy systems should yield an improvement to 1–2 mgal.  相似文献   

Eccentric geomagnetic dipole drift     

Joseph C. Cain  Dave R. Schmitz  Christopher Kluth 《Physics of the Earth and Planetary Interiors》1985,39(4):237-242

A recalculation of the rate of westward drift of the eccentric geomagnetic dipole since 1900 has confirmed its correlation with irregularities in the excess length-of-day (LOD) as originally reported by Vestine. The major changes in the LOD curve since 1900 are shown to be well correlated with the westward drift rate from a number of magnetic models covering the interval. A new Magsat model of secular variation for 1980.0 shows no increase in the drift rate which dropped by a factor of three in the interval 1965–1970.  相似文献   

Acid mine drainage contaminates groundwater of a Tennessee watershed     

Christopher J. O’Bara  R. Don Estes 《Environmental Geology》1985,7(3):159-161

Water samples were collected from 18 natural springs within the West Fork of the Obey River watershed. Overton County, Tennessee, to determine if groundwater was adversely affected by runoff from abandoned surface coal mines Six springs were found to be affected severely and deemed unfit as a source of potable water Water quality of the remaining springs was essentially unaffected it appeared that proximity to surface mines, elevation at the outflow, and geology of the surrounding strata determined the quality of the groundwater The unit is jointly supported by Tennessee Technological University, the Tennessee Wildlife Resource Agency, and the US. Fish and Wildlife Service  相似文献