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1.
The bane of conventional Thellier-type palaeointensity experiments is the thermal alteration of the samples during experimentation. However, high-frequency microwaves can be used to stimulate the magnetic minerals directly, thus eliminating the need for the bulk sample to be heated ( Walton et al . 1993 ). The microwave technique has been successfully applied to ceramics ( Shaw et al . 1996 , 1999 ), and here we present results for historic lavas from Mt Etna, Sicily. 20 samples were randomly selected from 10 different historic flows dating from 1843 to 1983. Hysteresis parameters were monitored as a check for alteration during experimentation and minimal alteration was found. Rock-magnetic analysis and AF plus microwave demagnetizations were carried out on all samples, and microwave intensity analysis was carried out on those samples that were stable to microwave demagnetization (16 in total). With one exception, all samples gave high-quality intensity results. Two or three subsamples from each sample were investigated and a weighted average calculated to give a single estimate of the field. When the intensity values were compared with the SV model based on direct observatory measurements ( Bloxham & Gubbins 1985 ), it was found that those samples that contained a high multidomain (MD) component gave intensity values around 20 per cent lower than expected. This can be explained by the cooling rate effect ( Dodson & McClelland-Brown 1980 ) and/or the presence of an MD component ( McClelland et al. 1996 ). Those samples with the lowest numbers of MD grains gave field values which, within error, were the same as those for the model, or slightly too high: this is compatible with the cooling rate effect. 相似文献
2.
The microwave palaeointensity technique has been used to determine palaeointensity during the R3–N3 geomagnetic field reversal, using lavas from the Esja region of southwestern Iceland. The resulting intensity determinations have been compared to two previous studies which used the Shaw and Thellier techniques. Both reported low field intensities during the reversal except for four flows that produced high values (20.4–36.8 μT) using the Shaw technique compared to a single maximum intermediate value of 15.9 μT using the Thellier method. In this paper, an average microwave transitional palaeointensity of 6.95 ± 2.07 μT is found for samples used in the Shaw technique study, and of 7.80 ± 1.61 μT for samples used in the Thellier study, demonstrating that there is no evidence for strong fields during this reversal. 相似文献
3.
A collection of ceramics and samples, collected from north-east China with ages ranging from 1000 to 7000 years, have been investigated using a modified version of the Shaw palaeointensity techniques (Shaw 1974; Rolph & Shaw 1985) in which only partial NRMs and TRMs (PNRMs and PTRMs) with blocking temperatures (Tb ) above 300 C are used after pre-selection of samples by mineral magnetic analysis. A secular variation curve obtained from this study is quite consistent with previous results from other areas of China (Wei et al. 1987; Tang et al. 1991), as well as with the global model of McElhinny & Senanayake (1982). Comparison of the Chinese results with contemporaneous results from Greece (Aitken et al. 1989) has allowed us to track the movement of a large non-dipole anomaly as it drifts westwards. 相似文献
4.
An analysis has been made of archaeointensity data for the past 12 000 years. There are 3243 results from different areas of the world covering the past 12 000 years. Of these, 2203 are from the European region and 1040 are from the rest of the world. The archaeointensity data set analysed in the present study is almost three times larger than that used by McElhinny & Senanayake (1982 ). Although there is no major difference between our global data and the earlier data, the data for the non-European region have been improved and we now have a data set for Asia. 相似文献
5.
Mammoth Mountain is a seismically active volcano 200 000 to 50 000 years old, situated on the southwestern rim of Long Valley caldera, California. Since 1989 it has shown evidence of unrest in the form of earthquake swarms (Hill et al. 1990), volcanic 'long-period' earthquakes (Pitt & Hill 1994), increased output of magmatic 3 He (Sorey et al. 1993) and the emission of about 500 tonnes day −1 of CO2 (Farrar et al. 1995; Hill 1996; M. Sorey, personal communication, 1997), which has killed trees and poses a threat to human safety. Local-earthquake tomography shows that in mid-1989 areas of subsequent tree-kill were underlain by extensive regions where the ratio of the compressional and shear elastic-wave speeds VP /VS was about 9 per cent lower than in the surrounding rocks. Theory (Mavko & Mukerji 1995), experiment (Ito, DeVilbiss & Nur 1979), and experience at other geothermal/volcanic areas (Julian et al. 1996) and at petroleum reservoirs (Harris et al. 1996) indicate that VP /VS is sensitive to pore-fluid compressibility, through its effect on VP . The observed VP /VS anomaly is probably caused directly by CO2 , and seismic VP /VS tomography is thus a promising tool for monitoring gas concentration and movement in volcanoes, which may in turn be related to volcanic activity. 相似文献
6.
Masaru Kono 《Geophysical Journal International》1978,54(2):241-261
Summary. Using natural volcanic rocks which acquired thermoremanence (TRM) in known fields, reliability of various palaeointensity methods using alternating field (AF) demagnetization were evaluated. Natural remanence (NRM), TRM and anhysteretic remanences (ARM's) before and after heating were stepwisely AF demagnetized following Shaw's method.
It was found that the coercivity spectra of TRM and ARM in these samples are very similar, and that even when changes occurred during heating, the changes for two remanences are quite similar in many samples. Therefore, Shaw's method of palaeointensity determination, which incorporates ARM checks to the conventional comparison of NRM and TRM coercivity spectra, gives results as reliable as those obtained by the Thellier method. Many examples were demonstrated in which TRM and ARM intensities changed substantially by heating, but without changes in the shape of their coercivity spectra. Such changes cannot normally be detected and erroneous palaeointensities with apparent internal consistency would be obtained by usual AF demagnetization methods.
Although ARM is quite similar to TRM, the rate of acquisition of ARM and TRM in weak fields varies by a factor of five among the present samples. To determine palaeointensities from the linear relation between ARM and TRM, it is necessary to determine experimentally the relative acquisition rate of these remanences. Therefore, methods based only on NRM-ARM relations would not give palaeointensities with acceptable errors. 相似文献
It was found that the coercivity spectra of TRM and ARM in these samples are very similar, and that even when changes occurred during heating, the changes for two remanences are quite similar in many samples. Therefore, Shaw's method of palaeointensity determination, which incorporates ARM checks to the conventional comparison of NRM and TRM coercivity spectra, gives results as reliable as those obtained by the Thellier method. Many examples were demonstrated in which TRM and ARM intensities changed substantially by heating, but without changes in the shape of their coercivity spectra. Such changes cannot normally be detected and erroneous palaeointensities with apparent internal consistency would be obtained by usual AF demagnetization methods.
Although ARM is quite similar to TRM, the rate of acquisition of ARM and TRM in weak fields varies by a factor of five among the present samples. To determine palaeointensities from the linear relation between ARM and TRM, it is necessary to determine experimentally the relative acquisition rate of these remanences. Therefore, methods based only on NRM-ARM relations would not give palaeointensities with acceptable errors. 相似文献
7.
J. Urrutia Fucugauchi 《Geophysical Journal International》1980,63(3):601-618
b
Sixteen samples representing eight Tertiary volcanic units from north-east Jalisco, Mexico were studied in an attempt to estimate the palaeointensity of the Earth's magnetic field. The experimental technique used was similar to that proposed by Shaw (1974) and an attempt was made to incorporate further criteria using the directional behaviour of the NRM, TRM and two ARMs during the af treatment and measuring the rate of ARM acquisition before and after heating. The directional information was used for checking the stability of all remanent magnetizations involved and for correcting the ARM test values. In addition the TRM directional behaviour could be a valuable yet simple test to detect the problem of insufficient heating. The use of the ARM acquisition test in the palaeointensity determination permits the full investigation of the coercive force spectrum and could lead to a more reliable palaeointensity method. This combined ARM method was applied to one sample. Further work is needed to understand the NRM—ARM1 and TRM—ARM2 relationships if the ARM tests are to be used for correcting TRM alteration effects. Whole rock K—Ar age determinations were carried out on samples from four selected units. Ten samples, representing six of the units, are considered to yield reliable palaeointensity values. Mean values were computed for each unit and reduced to the palaeoequator. The mean palaeoequatorial values and K—Ar ages determined are: I (13 |Mp 2 Myr), 0.504 Oe; II, 0.453 Oe; III, 0.439 Oe; VI (52 |Mp 10 Myr), 0.074 Oe; VII (14 |Mp 2 Myr), 0.187 Oe and VIII (12 |Mp 2 Myr), 0.251 Oe. These results agree reasonably well with those from previous studies. A large number of palaeointensity estimations, many more than currently available, are required to obtain an average estimate of the behaviour of the Earth's magnetic field during the Tertiary. 相似文献
Sixteen samples representing eight Tertiary volcanic units from north-east Jalisco, Mexico were studied in an attempt to estimate the palaeointensity of the Earth's magnetic field. The experimental technique used was similar to that proposed by Shaw (1974) and an attempt was made to incorporate further criteria using the directional behaviour of the NRM, TRM and two ARMs during the af treatment and measuring the rate of ARM acquisition before and after heating. The directional information was used for checking the stability of all remanent magnetizations involved and for correcting the ARM test values. In addition the TRM directional behaviour could be a valuable yet simple test to detect the problem of insufficient heating. The use of the ARM acquisition test in the palaeointensity determination permits the full investigation of the coercive force spectrum and could lead to a more reliable palaeointensity method. This combined ARM method was applied to one sample. Further work is needed to understand the NRM—ARM1 and TRM—ARM2 relationships if the ARM tests are to be used for correcting TRM alteration effects. Whole rock K—Ar age determinations were carried out on samples from four selected units. Ten samples, representing six of the units, are considered to yield reliable palaeointensity values. Mean values were computed for each unit and reduced to the palaeoequator. The mean palaeoequatorial values and K—Ar ages determined are: I (13 |Mp 2 Myr), 0.504 Oe; II, 0.453 Oe; III, 0.439 Oe; VI (52 |Mp 10 Myr), 0.074 Oe; VII (14 |Mp 2 Myr), 0.187 Oe and VIII (12 |Mp 2 Myr), 0.251 Oe. These results agree reasonably well with those from previous studies. A large number of palaeointensity estimations, many more than currently available, are required to obtain an average estimate of the behaviour of the Earth's magnetic field during the Tertiary. 相似文献
8.
Denis Hatzfeld Vassilis Karakostas Maria Ziazia Iannis Kassaras Elephteria Papadimitriou Kostas Makropoulos Nikos Voulgaris and Christos Papaioannou 《Geophysical Journal International》2000,141(2):438-456
During the summer of 1993, a network of seismological stations was installed over a period of 7 weeks around the eastern Gulf of Corinth where a sequence of strong earthquakes occurred during 1981. Seismicity lies between the Alepohori fault dipping north and the Kaparelli fault dipping south and is related to both of these antithetic faults. Focal mechanisms show normal faulting with the active fault plane dipping at about 45° for both faults. The aftershocks of the 1981 earthquake sequence recorded by King et al . (1985 ) were processed again and show similar results. In contrast, the observations collected near the western end of the Gulf of Corinth during an experiment conducted in 1991 ( Rigo et al . 1996 ), and during the aftershock studies of the 1992 Galaxidi and the 1995 Aigion earthquakes ( Hatzfeld et al . 1996 ; Bernard et al . 1997 ) show seismicity dipping at a very low angle (about 15°) northwards and normal faulting mechanisms with the active fault plane dipping northwards at about 30°. We suggest that the 8–12 km deep seismicity in the west is probably related to the seismic–aseismic transition and not to a possible almost horizontal active fault dipping north as previously proposed. The difference in the seismicity and focal mechanisms between east and west of the Gulf could be related to the difference in the recent extension rate between the western Gulf of Corinth and the eastern Gulf of Corinth, which rotated the faults dipping originally at 45° (as in the east of the Gulf) to 30° (as in the west of the Gulf). 相似文献
9.
P. P. Kruiver Y. S. Kok M. J. Dekkers C. G. Langereis & C. Laj 《Geophysical Journal International》1999,136(3):757-770
In the pseudo-Thellier method for relative palaeointensity determinations (Tauxe et al. 1995) the slope of the NRM intensity left after AF demagnetization versus ARM intensity gained at the same peak field is used as a palaeointensity measure. We tested this method on a marine core from the Azores, spanning the last 276 kyr. We compared the pseudo-Thellier palaeointensity record with the conventional record obtained earlier by Lehman et al . (1996 ), who normalized NRM by SIRM. The two records show similar features: intensity lows with deviating palaeomagnetic directions at 40–45 ka and at 180–190 ka. The first interval is associated with the Laschamps excursion, while the 180–190 ka low represents the Iceland Basin excursion (Channell et al. 1997). The pseudo-Thellier method, in combination with a jackknife resampling scheme, provides error estimates on the palaeointensity.
Spectral analysis of the rock magnetic parameters and the palaeointensity estimates shows orbitally forced periods, particularly 23 kyr for climatic precession. This suggests that palaeointensity is still slightly contaminated by climate. Fuzzy c -means cluster analysis of rock magnetic and geochemical parameters yields a seven-cluster model of predominantly calcareous clusters and detrital clusters. The clusters show a strong correlation with climate, for example samples from detrital clusters predominantly appear during rapid warming. Although both the pseudo-Thellier palaeointensity ma and fuzzy clusters show climatic influences, we have not been able to find an unambiguous connection between the clusters and m a . 相似文献
Spectral analysis of the rock magnetic parameters and the palaeointensity estimates shows orbitally forced periods, particularly 23 kyr for climatic precession. This suggests that palaeointensity is still slightly contaminated by climate. Fuzzy c -means cluster analysis of rock magnetic and geochemical parameters yields a seven-cluster model of predominantly calcareous clusters and detrital clusters. The clusters show a strong correlation with climate, for example samples from detrital clusters predominantly appear during rapid warming. Although both the pseudo-Thellier palaeointensity m
10.
Shaul Levi 《Geophysical Journal International》1979,59(1):205-218
Summary. Experiments were done to test the additivity of partial thermal remanent magnetizations (PTRMs) for prepared samples containing magnetite particles whose sizes range from SD (single domain) to MD (multidomain). The experiments compare the sum of two PTRMs with total-TRM, all produced by the same external field of 0.47 oe. The most significant conclusion of this paper is that, to first order, the additivity of PTRMs is obeyed for the magnetites of this study regardless of particle size. However, small, higher order deviations from additivity occur such that ΣPTRM > TRM by an average of about 1 per cent. Though small, these departures from additivity are significant at the 99 per cent confidence level, and they can be understood in terms of Néel's theory for SD particles. The small departures from additivity are intrinsic to the experimental procedure in which some particles acquire remanence twice, in each of the two PTRM steps. In the limit of small inducing fields additivity should be obeyed exactly for the magnetites of this study and for samples of interest in palaeomagnetism. The deviations from additivity should have no effect on palaeointensity determinations by the Thelliers' version of the Thellier palaeointensity method. For palaeointensity determinations by Coe's version of the Thellier method the effects of deviations from additivity would be very small, less than 4 per cent on the average for a worst-case experimental configuration, and these effects can be minimized by producing PTRMs parallel to the original NRM and by using weak laboratory fields. 相似文献
11.
The coupled rotation of the inner core 总被引：1，自引：0，他引：1
The rotation of the inner core (IC) is influenced by the rest of the Earth through a number of coupling mechanisms. Among four possible coupling mechanisms, gravitational, pressure, viscous and electromagnetic, the first two torques are dominant. Numerous existing IC gravitational torque estimates have been shown to agree very well with one another Xu & Szeto 1996 ). It is shown in this paper that different estimates of the IC pressure torque are also in good agreement.
The coupled rotation of the IC has been investigated in the frequency domain by several research groups (Mathews et al. 1991a,b; De Vries & Wahr 1991 ; Dehant et al. 1993; Jiang 1993 ). Not all of these efforts obtained two IC-related rotational modes, the inner-core wobble (ICW) and the free-inner-core nutation (FICN). We investigate this problem in the time domain and confirm the existence of the two modes. The periods of ICW and FICN are in good agreement with those obtained by other researchers. In studying the effects of coupling torques on the IC rotational modes we have found that depending on whether the IC net torque is 'restoring' or otherwise, an increased torque magnitude will respectively shorten or lengthen the ICW period. We have also found that the sense of FICN is determined by the orientation of the net coupling torque on the IC. 相似文献
The coupled rotation of the IC has been investigated in the frequency domain by several research groups (Mathews et al. 1991a,b; De Vries & Wahr 1991 ; Dehant et al. 1993; Jiang 1993 ). Not all of these efforts obtained two IC-related rotational modes, the inner-core wobble (ICW) and the free-inner-core nutation (FICN). We investigate this problem in the time domain and confirm the existence of the two modes. The periods of ICW and FICN are in good agreement with those obtained by other researchers. In studying the effects of coupling torques on the IC rotational modes we have found that depending on whether the IC net torque is 'restoring' or otherwise, an increased torque magnitude will respectively shorten or lengthen the ICW period. We have also found that the sense of FICN is determined by the orientation of the net coupling torque on the IC. 相似文献
12.
We report on a detailed palaeomagnetic study of the Miocene Farellones volcanic formation in the Chilean Andes near Santiago (two sections, 37 sites, about 400 orientated cores). Petrological observations show evidence of low-grade metamorphism increasing downwards through the volcanic sequence. Optical observations of opaque minerals and magnetic experiments suggest that in many cases maghemitization is associated with hydrothermal alteration. However, thermal demagnetization data indicate that the low-grade metamorphism did not significantly modify the direction of the primary remanent magnetization recorded at the time of emplacement of the volcanic lava flows. Four intervals of polarity with two intermediate palaeodirections were observed in the ~650-m-thick composite section. According to the dispersion of flow average directions, palaeosecular variation was slightly larger than that observed in general during the Upper Cenozoic. The site mean directions obtained in this study differ significantly from the expected Miocene direction. Clockwise rotations of up to 20° of small blocks are probably associated with the deformation of the Andean Cordillera since middle Miocene times. Geomagnetic palaeointensity data were obtained, using the Thellier method, on 24 samples from eight distinct lava flows. The flow mean VDM varies from 1.4 to 4.0 × 1022 A m−2 . Altogether, our data seem to suggest the existence of a relatively low geomagnetic field undergoing large fluctuations. Although a linear relationship was observed between the natural remanent magnetization and the thermal remanent magnetization acquired during the Thellier–Thellier experiments, undetected chemical alteration of the magnetic minerals during hydrothermalism may also explain the unusually low palaeointensity obtained. 相似文献
13.
The crustal structure beneath the northwest fjords, Iceland, from receiver functions and surface waves 总被引：1，自引：0，他引：1
Five broad-band seismic stations were operated in the northwest fjords area of Iceland from 1996 to 1998 as part of the Iceland Hotspot project. The structures of the upper 35 km or so beneath these stations were determined by the modelling and joint inversion of receiver functions and regional surface wave phase velocities. More than 40 teleseismic events and a few regional events containing high-quality surface wave trains were used. Although the middle period passband of the seismograms is corrupted by oceanic microseismic noise, which hinders the interpretation of structural details, the inversions reveal the overall features. Many profiles obtained exhibit large velocity gradients in the upper 5 km or so, smaller zero gradients below this, and, at ~23 km depth, a zone 2–4 km thick with higher velocity gradients. The two shallower intervals are fairly consistent with the 'upper' and 'lower' crust, defined by Flovenz (1980 ). The deep zone of enhanced velocity gradient seems to correspond to the sharp reflector first reported by Bjarnason et al . (1993 ) and identified by them as the 'Moho'. However, this type of structure is not ubiquitous beneath the northwest fjords area. The distinctiveness of the three intervals is variable, and in some cases a structure with velocity gradient increasing smoothly with depth is observed. We term these two end-members structures of the first and second types respectively. Structures of the second type correlate with older areas. Substantial variation in fundamental structure is to be expected in Iceland because of the great geological heterogeneity there. 相似文献
14.
In December of 1994 a fluid injection experiment which triggered several hundreds of microearthquakes was conducted at the KTB main borehole (Oberpfalz, Germany). These events were recorded with a temporal seismic network at the surface. Out of the complete data set, a cluster of five events recorded at four mini-arrays consisting of eight or nine stations was used to investigate the crustal scattering properties in the vicinity of the KTB. For this purpose, the 'Double Beam Method' (DBM; Krüger et al . 1993 , 1995 , 1996) and the 'Double Beam Imaging Method' (Scherbaum, Krüger & Weber 1997) were extended to curved wave fronts to drop the restriction of plane-wave propagation. This technique is used for imaging the crustal scattering strength using earthquake clusters recorded at close-by mini-arrays. The results of the array analysis show that the composition of the P coda is mainly affected by the site location of the arrays. Near-surface and deeper crustal scattering contribute in a very complicated pattern. Furthermore, with the present data set it was possible to identify reflections from the top of the Erbendorf Body. This is a very pronounced arrival in most of the recorded traces. In one of the arrays its amplitudes are even greater than the direct P phases. Five to eight coherent phases could be identified by the mini-arrays. Using only these phases, synthetic P -coda traces were constructed, which only contain the coherent part of the observed wavefield. By subtracting the synthetic coherent wavefield from the original traces we achieve a variance reduction in the P coda of up to 37 per cent. This leads to the conclusion that a large amount of the P coda at the KTB can be modelled by a simple deterministic single-scattering model using a small number of individual scatterers. 相似文献
15.
Maria Victoria Ortega Espaldon 《地理学报(英文版)》2012,22(3):575-576
Land system change has never been out of human concerns. It is always one of the hottest themes in global environmental change research (Seto et al., 2002; Gutman et al., 相似文献
16.
D. N. Thomas T. C. Rolph J. Shaw S. Gonzalez de Sherwood & Z. Zhuang 《Geophysical Journal International》1998,134(3):856-866
Palaeomagnetic results are presented from a volcanic sequence in Zhijin County, Guizhou Province, Southern China. The lavas and associated volcanic breccias comprising the sequence represent a southern extension of the Emeishan volcanic province. Biostratigraphic dating of interbedded limestone units and stratigraphic constraints indicate that the section formed during the late Permian (∼263–255 Ma), and is thus somewhat older than the Emei stratotype section in Sichuan Province, and close in age to reported estimates of the termination of the Permo-Carboniferous (Kiaman) reverse superchron. Rock magnetic analyses and reflected light microscopy indicate that the magnetic mineralogy of the lava units is dominated by fresh, primary magnetites containing a significant fraction of single-domain grains. Thermal demagnetization behaviour of the breccia units is poor, but most lava samples have one or two components of remanence above 250 °C. The normal polarity characteristic remanence held by the lavas implies a post-Kiaman age for this succession and suggests that the termination of the Kiaman occurred prior to 263 Ma, supporting recently published estimates. According to standard criteria, Thellier palaeointensity results from the lavas are of good quality and reveal that the dipole field strength was comparatively low shortly after the termination of the superchron. 80 per cent of samples record relative VDM values in the range 42–52 per cent of the present-day value, supporting recent studies of mid-Kiaman field intensity. This suggests that a low-energy dipole existed at least between 300 and 255 Ma and does not appear to have been confined to the stable reverse polarity interval. 相似文献
17.
V. N. Pham P. Bernard D. Boyer G. Chouliaras J. L. Le Mouël G. N. Stavrakakis 《Geophysical Journal International》2000,142(3):948-969
A deep magnetotelluric sounding (MTS) investigation in the western part of the Gulf of Corinth has revealed a complex electrical image of the crustal structure. The geotectonic structure of the Parnassos unit and the Transition zone in the central Hellenides, overthrusting the Pindos zone both towards the west and towards the south, has been clearly identified by its higher resistivity and its intrinsic anisotropy related to the N–S strike of the Hellenides range. Subsequent N–S extension of the Gulf introduced another heterogeneous anisotropy characteristic that corresponds to E–W-trending normal faults on both sides of the Gulf. The 2-D modelling of the MTS results reveals the existence of a relatively conductive layer about 4 km thick at a depth greater than 10 km in the middle crust. It corresponds to a ductile detachment zone suggested by microseismic and seismic studies ( King et al . 1985 ; Rigo et al . 1996 ; Bernard et al . 1997a ). It may be attributed to the phyllite series lying between the allochthonous Hellenic nappes and the autochthonous Plattenkalk basement. Towards the east, under the Pangalos peninsula, approaching the internal Hellenides, the detachment zone could root deeply into the lower crust.
Some strong local electrical anomalies are observed, reaching the conductive layer in the middle crust, such as that under the Mamousia fault and under the front of the overthrust of the Transition zone on the Pindos zone. Other anomalies affect only the shallower zones such as that beneath the Helike fault and in the Psaromita peninsula. These shallower anomalies provide complementary information to the study of spatial and temporal variations of the seismic anisotropy in relation to the short- and long-term tectonic activity of the Gulf ( Bouin et al . 1996 ; Gamar et al . 1999 ). 相似文献
Some strong local electrical anomalies are observed, reaching the conductive layer in the middle crust, such as that under the Mamousia fault and under the front of the overthrust of the Transition zone on the Pindos zone. Other anomalies affect only the shallower zones such as that beneath the Helike fault and in the Psaromita peninsula. These shallower anomalies provide complementary information to the study of spatial and temporal variations of the seismic anisotropy in relation to the short- and long-term tectonic activity of the Gulf ( Bouin et al . 1996 ; Gamar et al . 1999 ). 相似文献
18.
《Norsk geografisk tidsskrift. Norwegian journal of geography》2012,66(1):75-79
The well-preserved Late Stone Age mummy, which was discovered at an altitude of more than 3200 m a.s.1. has astonished the whole world (Fig. I). Never before had a body of such age been discovered in Europe, not to mention having been completely conscrved in a glacier. The dating to about 3300-3200 years BC was not definite until after a large number of C-dates had been carried out (Bonani et al. 1992). Before these find dates, the possibility that a corpse would be preserved in a glacier for such a period of time had been considered unrealistic. Even after the radiocarbon dating, disbelievers considered the find to be a hoax, though the dating of the body was not disputed. A book was even published to ‘prove’ that it was a hoax (Heim & Nosko 1993). This claim was based on the argument that the find was so unlikely that it had to be a fake, and by twisting facts and arguments they tried to prove that this was the case (Spin-dler 1993). The body itself was purported to be some 5000 years old South American, Egyptian or Asian mummy that had been deliberately deposited on the spot sometime during the 20th century. Genetical analysis, however, proved the body beyond any doubt to be that of a European (Handt et al. 1994). The preservation of a body in the glacier for such a period of time therefore calls for an explanation. 相似文献
19.
杭州城市形态和土地利用结构的时空演化 总被引：53，自引：3，他引：50
根据分形理论研究杭州1949~1996年间城市形态和土地利用结构的演化特征,发现杭州城市具有明确的自相似规律。研究表明,杭州城市形态和土地利用结构的分形性态逐渐变好,这与国外学者“演化的城市分形”观相互印证。各类土地形态的维数都小于整个城市形态的维数,从而证实了国内学者“城市化地区的分维大于各职能类土地空间分布维数”的理论推断。从时空变化来看,杭州城市形态的分维呈上升趋势,1996年接近Batty等提出的理论预期维数D = 1.71;居住用地、工业用地和对外交通用地的分维近20年来趋于增大,而教育用地和绿化用地的分维则有所减小。杭州市的分形演化和分维变化总体上揭示了城市自组织演化的特征,但工业用地维数的大幅度上升和绿化用地维数的下降显然暗示该城市在进化过程中的局部退化倾向。 相似文献
20.
C. Wang 《Geophysical Journal International》1979,56(1):199-209
Summary. In examining the effect of discontinuities in the Earth's interior on free oscillations, McNabb, Anderssen & Lapwood derived an equation for the asymptotic behaviour of torsional overtone eigenfrequencies of a discontinuous earth model, the constants in their equation being explicitly determined only for the case of one internal discontinuity. Since Brune's phase correlation method for the evaluation of eigenfrequencies from body-wave data implies a ray-mode duality only for continuous earth models, it is desirable to justify the McNabb et al. formulation from the point of view of ray theory.
By a novel method of ray analysis, Wang, Cleary & Anderssen showed that, for earth models with a single discontinuity between the Earth's surface and the core—mantle boundary, the McNabb et al. formulation can be derived from an adaptation of Brune's method to multiply reflected SH body waves recorded at small epicentral distances. In this paper, the technique of Wang et al. is extended to derive the McNabb et al. formulation (with constants explicitly determined) for the general case of earth models with N discontinuities. This establishes a basis for a ray-mode duality for discontinuous earth models. 相似文献
By a novel method of ray analysis, Wang, Cleary & Anderssen showed that, for earth models with a single discontinuity between the Earth's surface and the core—mantle boundary, the McNabb et al. formulation can be derived from an adaptation of Brune's method to multiply reflected SH body waves recorded at small epicentral distances. In this paper, the technique of Wang et al. is extended to derive the McNabb et al. formulation (with constants explicitly determined) for the general case of earth models with N discontinuities. This establishes a basis for a ray-mode duality for discontinuous earth models. 相似文献