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1.
For most previous geomagnetic surveys, the time variations of the observatory closest to the site of observation have been used for reduction. The accuracy of repeat station measurements depends upon the distance between the station and the observatory. Nowadays, the requirements for the accuracy of repeat station measurements are much higher, especially with respect to local secular variation anomalies. On the other hand, transportable variograph stations are available, by means of which time variations can be recorded closer to the station site. The reduction steps for this method are: - the stability of the base-line values of the variographs; - the question as to what degree a quiet night value represents the normal value at that time (e.g. influence of aD st field and/or other fields). A repeat station survey of the Federal Republic of Germany was carried out in 1982. During the same time, eleven stations recorded the time variations in that area. We discuss the aforementioned aspects by way of an example taken from this survey. 相似文献
- Reduction of measured values to the base-line of the nearby variograph station;
- Computation of the nearest quiet night value using the baseline of the variograph;
- Computation of the difference between this value and the corresponding one at the observatory.
2.
S. M. Spottiswoode 《Pure and Applied Geophysics》1989,129(3-4):673-680
This paper summarizes seismic and rockburst research activities related to South African deep-level gold mines over the period 1983 to 1987. It covers continued research in directions that were considered in the Seismicity in Mines Symposium in 1982 as well as in several new areas of research. Five broad areas are identified:
- Seismic data acquisition and processing. Improved seismic systems are being developed. Velocity models related to known stratigraphy are being used to provide more accurate estimates of seismic locations.
- Source mechanisms and near-source effects on seismic wave transmission. This work provides fundamental insights into seismicity and rock behaviour and is being applied in rockburst prediction research.
- Mine layouts. Excess shear stress is being investigated as a design parameter by analyzing mining configurations and resultant seismicity. In addition, better understanding of the behaviour of highly stressed remnants and pillars is also being obtained from seismic studies.
- Strong ground motion studies. Evaluation of the performance of support elements, including recently developed backfill materials, requires better knowledge of ground motion around underground excavations during seismic events and rockbursts.
- Rockburst prediction and control. Rockburst prediction research continues with some reported success. In addition, the feasibility of actively triggering fault slip or conditioning the rock ahead of the stope face to ameliorate the rockburst hazard is currently being investigated.
3.
Microearthquake digital data collected at Campi Flegrei during the recent (1982–1985) ground uplift episode have been analyzed in order to infer source and medium seismic properties. The main results obtained from these analyses are:
- Hypocenter distribution and the size of the seismic zone do not change with time and do not depend on the ground uplift rate. Events occurred clustered in time with no simple causal relations between the cluster occurrences and their energy.
- Anelastic attenuation does not depend strongly on frequency, showing a constant pattern at high frequencies. The observed values of low and high frequency attenuation, due to the short source receiver distances, do not seriously affect the spectral content of signals radiated by the sources.
- A constant Brune stress drop pattern (～4–5 bars) as a function of seismic moment is observed. This indicates that the manner of fracturing is almost independent on magnitude of earthquakes (hypothesis of self-similarity (Aki, 1967)). Seismic processes in a prefractured medium can explain the observed small stress drop values.
- Focal mechanisms from moment tensor estimates show that radiation patterns are mostly well interpreted in terms of double couple source models.
- The scaling of peak ground motion parameters (A max andV max vs seismic moment) can be explained by an ω2 source model (constant stress drop) multiplied by an exponential function with a small decay parameter, which takes into account the measured attenuation.
4.
A modified formula of the cumulative frequency-magnitude relation has been formulated and tested in a previous paper by the authors of this study. Based on the modified relationship, the following reoccurrence formulas have been obtained.
- For the ‘T-years period’ larger earthquake magnitude,M T $$M_T = \frac{1}{{A_3 }}ln\frac{{A_2 }}{{(1/T) + A_1 }}.$$
- For the value of the maximum earthquake magnitude, which is exceeded with probabilityP inT-years period,M PT $$M_{PT} = \frac{{ln(A_2 .T)}}{{A_3 }} - \frac{{ln[A_1 .T - ln(1 - P)]}}{{A_3 }}.$$
- For the probability of occurrence of an earthquake of magnitudeM in aT-years period,P MT $$P_{MT} = 1 - \exp [ - T[ - A_1 + A_2 \exp ( - A_3 M)]].$$
5.
The various useful source-parameter relations between seismic moment and common use magnitude lg(M 0) andM s,M L,m b; between magnitudesMs andM L,M s andm b,M L andm b; and between magnitudeM s and lg(L) (fault length), lg (W) (fault width), lg(S) (fault area), lg(D) (average dislocation);M L and lg(f c) (corner frequency) have been derived from the scaling law which is based on an “average” two-dimensional faulting model of a rectangular fault. A set of source-parameters can be estimated from only one magnitude by using these relations. The average rupture velocity of the faultV r=2.65 km/s, the total time of ruptureT(s)=0.35L (km) and the average dislocation slip rateD=11.4 m/s are also obtained. There are four strong points to measure earthquake size with the seismic moment magnitudeM w.
- The seismic moment magnitude shows the strain and rupture size. It is the best scale for the measurement of earthquake size.
- It is a quantity of absolute mechanics, and has clear physical meaning. Any size of earthquake can be measured. There is no saturation. It can be used to quantify both shallow and deep earthquakes on the basis of the waves radiated.
- It can link up the previous magnitude scales.
- It is a uniform scale of measurement of earthquake size. It is suitable for statistics covering a broad range of magnitudes. So the seismic moment magnitude is a promising magnitude and worth popularization.
6.
Luca Malagnini Kevin Mayeda Stefan Nielsen Seung-Hoon Yoo Irene Munafo’ Christopher Rawles Enzo Boschi 《Pure and Applied Geophysics》2014,171(10):2685-2707
We estimate the corner frequencies of 20 crustal seismic events from mainshock–aftershock sequences in different tectonic environments (mainshocks 5.7 < M W < 7.6) using the well-established seismic coda ratio technique (Mayeda et al. in Geophys Res Lett 34:L11303, 2007; Mayeda and Malagnini in Geophys Res Lett, 2010), which provides optimal stability and does not require path or site corrections. For each sequence, we assumed the Brune source model and estimated all the events’ corner frequencies and associated apparent stresses following the MDAC spectral formulation of Walter and Taylor (A revised magnitude and distance amplitude correction (MDAC2) procedure for regional seismic discriminants, 2001), which allows for the possibility of non-self-similar source scaling. Within each sequence, we observe a systematic deviation from the self-similar \( M_{0} \propto \mathop f\nolimits_{\text{c}}^{ - 3} \) line, all data being rather compatible with \( M_{0} \propto \mathop f\nolimits_{\text{c}}^{ - (3 + \varepsilon )} \) , where ε > 0 (Kanamori and Rivera in Bull Seismol Soc Am 94:314–319, 2004). The deviation from a strict self-similar behavior within each earthquake sequence of our collection is indicated by a systematic increase in the estimated average static stress drop and apparent stress with increasing seismic moment (moment magnitude). Our favored physical interpretation for the increased apparent stress with earthquake size is a progressive frictional weakening for increasing seismic slip, in agreement with recent results obtained in laboratory experiments performed on state-of-the-art apparatuses at slip rates of the order of 1 m/s or larger. At smaller magnitudes (M W < 5.5), the overall data set is characterized by a variability in apparent stress of almost three orders of magnitude, mostly from the scatter observed in strike-slip sequences. Larger events (M W > 5.5) show much less variability: about one order of magnitude. It appears that the apparent stress (and static stress drop) does not grow indefinitely at larger magnitudes: for example, in the case of the Chi–Chi sequence (the best sampled sequence between M W 5 and 6.5), some roughly constant stress parameters characterize earthquakes larger than M W ~ 5.5. A representative fault slip for M W 5.5 is a few tens of centimeters (e.g., Ide and Takeo in J Geophys Res 102:27379–27391, 1997), which corresponds to the slip amount at which effective lubrication is observed, according to recent laboratory friction experiments performed at seismic slip velocities (V ~ 1 m/s) and normal stresses representative of crustal depths (Di Toro et al. in Nature in press, 2011, and references therein). If the observed deviation from self-similar scaling is explained in terms of an asymptotic increase in apparent stress (Malagnini et al. in Pure Appl Geophys, 2014, this volume), which is directly related to dynamic stress drop on the fault, one interpretation is that for a seismic slip of a few tens of centimeters (M W ~ 5.5) or larger, a fully lubricated frictional state may be asymptotically approached. 相似文献
7.
F. G. Piccinelli M. Mucciarelli P. Federici D. Albarello 《Pure and Applied Geophysics》1995,145(1):97-108
The Ridracoli Dam has been operating since 1981. Around the reservoir ISMES installed and operated for 10 years a seismic network, now reduced to a 3-D station. Earthquakes were recorded with completeness from magnitude 0.8 onwards. In the same period, all the parameters relevant to the dam and the environment were measured. This provided a complete data base for RIS studies, unique in its kind in Italy. The main findings of the analyses performed are the following:
- The filling of the reservoir has not influenced the seismicity of the area for most significant events (M L>3.5).
- Lesser seismicity around the reservoir seems to be correlated with water level in the reservoir, but also shows to be dependent on regional seismicity.
- b value shows a slight increase with time. This may indicate an increase in rock fracturing, which is known to precede the disappearing of Type II RIS.
8.
J. Keller 《Bulletin of Volcanology》1969,33(3):942-959
Rhyolitic pumice deposits on some East Aegean Islands are the remnants of a tuff sheet which covered formerly more than 2500 km2. The rhyolites originated by melting of granitic crustal rocks. The petrological properties leading to this conclusion are as follows: -mineralogical composition of the pumice being 60 wt % of glass and 40 % of relictic granitic minerals (quartz, plagioclase, potash feldspar, biotite) in «phenocrysts» up to 1 cm Ø. -phenomena of strong corrosion, resorption and melting of all light colored «phenocrysts» in the rhyolitic pumice. -mineralogical and chemical identity between pumice and granitic xenolites found in great quantity in the pumice tuff. -structurally transitional types between pumice and the granites in different state of melting. -eutectic composition of the pumice glass for a pressure of 2 kb. 相似文献
9.
Ya-Ju Hsu Jean-Philippe Avouac Shui-Beih Yu Chien-Hsin Chang Yih-Min Wu Jochen Woessner 《Pure and Applied Geophysics》2009,166(10-11):1853-1884
We use preseismic, coseismic, and postseismic GPS data of the 1999 Chi-Chi earthquake to infer spatio-temporal variation of fault slip and frictional behavior on the Chelungpu fault. The geodetic data shows that coseismic slip during the Chi-Chi earthquake occurred within a patch that was locked in the period preceding the earthquake, and that afterslip occurred dominantly downdip from the ruptured area. To first-order, the observed pattern and the temporal evolution of afterslip is consistent with models of the seismic cycle based on rate-and-state friction. Comparison with the distribution of temperature on the fault derived from thermo-kinematic modeling shows that aseismic slip becomes dominant where temperature is estimated to exceed 200° at depth. This inference is consistent with the temperature induced transition from velocity-weakening to velocity-strengthening friction that is observed in laboratory experiments on quartzo-feldspathic rocks. The time evolution of afterslip is consistent with afterslip being governed by velocity-strengthening frictional sliding. The dependency of friction, μ, on the sliding velocity, V, is estimated to be ${{\partial \mu }/{\partial \, {\rm ln}\, V}} = 8 \times 10^{ - 3}$ . We report an azimuthal difference of about 10–20° between preseismic and postseismic GPS velocities, which we interpret to reflect the very low shear stress on the creeping portion of the décollement beneath the Central Range, of the order of 1–3 MPa, implying a very low friction of about 0.01. This study highlights the importance of temperature and pore pressure in determining fault frictional sliding. 相似文献
10.
Four volcanic units have been distinguished on the islanf of Fayal. In order of decreasing age, these are: the eastern rift, and products of the activity preceding the collapse of the caldera visible at the summit of the stratovolcano, characterized by an alkaline series: basalt-hawaiite-mugearite-trachyte; the products of the explosive and postcaldera activity where only evolved lavas occur (benmoreites and trachytes); the recent basaltic activity of the Horta region; the western fissural activity — recent and historical. The two last units are characterized by exclusively basaltic, frequently picritic, eruptions. The lava groups cannot be distinguished by chemical criteria and have thus been treated as a single suite. Ninety samples have been analysed by X-ray fluorescence, and the mineralogy of 6 representative specimens has been determined by microprobe. The data were used to work out the evolution of the lava. The series is shown to have been produced by crystal fractionation under moderate water pressure from an alkali basalt. Moderate fractionation of amphibole during the last stages allow the liquids to remain weakly undersaturated from initial basalts until final trachytes. Mineralogical and chemical diversity between the most evolved lavas, benmoreites and trachytes, is an evidence of the strong influence ofpH2O and/orfO2 on the composition of such residua. 相似文献
11.
Weiping Qin 《地震科学（英文版）》1992,5(1):177-185
This paper gives a complete commentary on some popular gradings which based on the 2×2 contingency table, for one—dimensional seismologic prediction. It contains:
- In view of the statistical correlation, the Wallen gradingV=√QS is the most equitable one amomg all gradings.
- Each one of the three gradings: Zhenchao Gu gradingS, Obuhov gradingQ and Shaoxie Xu gradingR, has its own inadequacy although they are so easy for use.
- A set of gradings can be derived fromV. The harmonic mean ofS andQ, H(S,Q) and the Hedike gradingS H are the members of this set. The order of some popular gradings are given in Theorem 5. 3.
- In view of the measure of the overlapping part a gradingW is introduced. Another set of gradings may be derived from it. The index of efficiencyh is the severest one among this set.
12.
László Szarka 《Surveys in Geophysics》1987,9(3-4):287-318
The paper gives a summary of geophysical aspects of man-made electromagnetic noise in the Earth as follows: - EM distortion effects of man-made constructions below and over the Earth's surface defined as ‘passive-noise’, - field observation of EM disturbances due to ‘active’ man-made sources, - EM source mechanism of some important active sources from a geophysical point of view, - efforts in order to improve the signal-to noise ratio by instrumental, methodological and dataprocessing ways, - application of man-made EM noise for geophysical prospecting. The paper is based on world-wide EM noise survey studies published mainly in geophysical journals. 相似文献
13.
Viewing from the energy angle and taking the Beijing depression as an example, this paper studies the effects of underlying geological structures, mainly bedrock topography and bedrock faults, on the propagation of seismic waves and discusses the effects of the overlying soil layer on seismic waves. From the study, some conclusions are drawn as follows:
- Underlying bedrock faults affect the duration, frequency spectra and characteristics of energy distribution of seismic waves.
- Underlying bedrock topography changes the field of ground motion not only because the bedrock at different places receives different amounts of energy from the same source but also because its asperities diverge or converge seismic waves.
- Overlying soil layer is able both to absorb and to amplify seismic waves.
14.
S. Nielsen E. Spagnuolo M. Violay S. Smith G. Di Toro A. Bistacchi 《Journal of Seismology》2016,20(4):1187-1205
Recent estimates of fracture energy G ′ in earthquakes show a power-law dependence with slip u which can be summarized as G ′ ∝ u a where a is a positive real slightly larger than one. For cracks with sliding friction, fracture energy can be equated to G f : the post-failure integral of the dynamic weakening curve. If the dominant dissipative process in earthquakes is friction, G ′ and G f should be comparable and show a similar scaling with slip. We test this hypothesis by analyzing experiments performed on various cohesive and non-cohesive rock types, under wet and dry conditions, with imposed deformation typical of seismic slip (normal stress of tens of MPa, target slip velocity > 1 m/s and fast accelerations ≈ 6.5 m/s2). The resulting fracture energy G f is similar to the seismological estimates, with G f and G ′ being comparable over most of the slip range. However, G f appears to saturate after several meters of slip, while in most of the reported earthquake sequences, G ′ appears to increase further and surpasses G f at large magnitudes. We analyze several possible causes of such discrepancy, in particular, additional off-fault damage in large natural earthquakes. 相似文献
15.
In this paper, the relations of the changes of earth resistivity (ρ) s recorded at 100 geoelectrical stations in 31 earthquakes occurred in the continent of China, to the active faults (active abyssal faults or badly active faults near the focal zone) and the causative stress fields are discussed and the following conclusions are obtained:
- On the condition that a station is near the active fault and in the direction of the causative stress (DCS) of an earthquake (EQ), the immediate variation ofρ s to the earthquake (called “immediate variation” for short) could be recorded generally at the station.
- The active fault which lies between a station and the epicenter of an earthquake seems to play a role in “obstructing” the recording of the imminent variation when the strike of the fault is close to the DCS of the earthquake. When that is parrallel with the DCS the “obstructing” function of the fault is strongest; when normal with the DCS, weakest.
16.
René Gächter 《Aquatic Sciences - Research Across Boundaries》1987,49(2):170-185
Data obtained from Lakes Baldegg and Sempach, two artificially mixed and oxygenated lakes, show that it is technically possible to significantly improve redox conditions, even in large eutrophic lakes. It is discussed why - the well-known release of phosphorus from anoxic sediments does not necessarily result from an abiotic reduction of phosphorus containing iron compounds, - net phosphorus retention by sediments is not simply linearly related to the phosphorus content of a lake, - artificial oxygenation of a previously anoxic hypolimnion does not permanently increase phosphorus retention capacity of lake sediments. From this it is concluded that improvement of hypolimnic redox conditions by lake internal measures such as aeration or oxygenation may accelerate the rate of recovery induced by a reduction of the external phosphorus loading, but that oxygenation per se will hardly be able to cause a reduction of trophic state. 相似文献
17.
18.
Liparitic volcanism is a typical feature of the orogenic phase giving rise to the Kazakhstan, Middle Asia and Caucasus folded systems. The main characteristics of the liparitic volcanism common to these three regions are the following:
- Geo-structural zonation of the volcanic structures.
- Dismembered Moho surface within the volcanic structures.
- Synchronous, yet independent evolution of liparitic and andesitic volcanisms.
- Ignimbritic character of the liparitic volcanism.
- Lateral petrochemical zonation with some features common to the liparitic and andesitic rock series.
19.
Steam samples from six wells (Colombaia, Pineta, Larderello 57, Larderello 155, Gabbro 6, and Gabbro 1) in a south to north section across the Larderello geothermal field have been analyzed for inorganic and hydrocarbon gases and for oxygen-18 and deuterium of steam. The wells generally decrease in depth and increase in age toward the south. The steam samples are generally characterized by
- Total gas contents increasing south to north from 0.003 to 0.05 mole fraction;
- Constant CO2 (95±2 percent); near constant H2S (1.6±0.8), N2 (1.2±0.8), H2 (2±1), CH4 (1.2±1), and no O2 in the dry gas;
- Presence of numerous, straight chain and branched C2 to C6 hydrocarbons plus benzene in amounts independent of CH4 contents with highest concentrations in the deeper wells;
- Oxygen-18 contents of steam increasing south to north from ?5.0‰ to ?0.4‰ with little change in deuterium (?42±2‰).
- Decreasing gas contents with amount of production because the proportion of steam boiled from liquid water increases with production;
- Synthesis of CH4 from H2 and CO2 with CO2 and H2 produced by thermal metamorphism and rock-water reactions;
- Extraction of C2 to C6 hydrocarbons from rock organic matter;
- Either oxygen isotope exchange followed by distillation of steam from the north toward the south (2 plates at ～220°C) or mixture of deeper more-exchange waters from the north with shallow, less-exchanged recharging waters from the south.
20.
Federico Von Giovanoli André Lambert 《Aquatic Sciences - Research Across Boundaries》1985,47(2):159-178
Results obtained by means of an ultrasonic current meter in the plume of the Rhone river are summarized as follows:
- Currents of Rhone river water entering the lake were clearly discernible up to a distance of about 1 km from the river mouth. The interflow was observed at depth of 10 to 30 m.
- Interflow velocities decreased with increasing distance from the river mouth: from 40cm/s at a distance of 350 m to about 15 cm/s at 1 km.
- Short-term variations of current velocities and directions documented the highly turbulent nature of the interflow.
- The entering river water (inflow direction to the NNW) interfered with a persisting northeastward current of the lake water. At a distance of 1–2 km from the mouth the interflow gradually assumed the same direction, possibly due to deflection by Coriolis forces.
- Current velocities showed considerable variations within a time scale in the order of hours at the same measuring position. The reasons for these fluctuations remain unclear. Possible causes may be lateral oscillations of the entering river water or its deviation by river mouth bars during periods of reduced river discharge. Variations of the discharge alone cannot explain these current fluctuations.