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131.
The attenuation of P- and S-waves in Southeastern Sicily was estimated by applying two different methods in time and frequency domains. We analyzed waveforms from about 290 local events (0.6≤ML≤4.6) recorded at a three-component digital network.By applying the pulse broadening method to the first P-wave pulse, we found an average Qp value of ca. 140. The application of the frequency decay method provided a Qp value of ca. 120, in the low-frequency band (3-9 Hz). Conversely, in the high frequency range (16-27 Hz) the average Qp is significantly larger (ca. 640). The frequency decay method was also applied to S-waves spectra. In the low frequency range (2-5 Hz) the estimated average Qs is ca. 190. As for Qp, also Qs, in the high frequency range (16-27 Hz), is larger (ca. 700). These results evidenced a frequency dependence of both the quality factors Qp and Qs, as commonly observed in tectonically active zones characterized by high degree of heterogeneity. 相似文献
132.
Estimating palaeowind strength from beach deposits 总被引:1,自引:0,他引:1
Kenneth D. Adams 《Sedimentology》2003,50(3):565-577
Abstract The geological record of past wind conditions is well expressed in the coarse gravel, cobble and boulder beach deposits of Quaternary palaeolakes in the Great Basin of the western USA and elsewhere. This paper describes a technique, using the particle‐size distribution of beach deposits, to reconstruct palaeowind conditions when the lakes were present. The beach particle technique (BPT) is first developed using coarse beach deposits from the 1986–87 highstand of the Great Salt Lake in Utah, combined with instrumental wind records from the same time period. Next, the BPT is used to test the hypothesis that wind conditions were more severe than at present during the last highstand of Lake Lahontan (≈ 13 ka), which only lasted a decade or two at most. The largest 50 beach clasts were measured at nine beach sites located along the north, west and south sides of Antelope Island in the Great Salt Lake, all of which formed in 1986–87. At these sites, the largest clast sizes range from 10 to 28 cm (b‐axis), and fetch lengths range from 25 to 55 km. Nearshore wave height was calculated by assuming that the critical threshold velocity required to move the largest clasts represents a minimum estimate of the breaking wave velocity, which is controlled by wave height. Shoaling transformations are undertaken to estimate deep‐water wave heights and, ultimately, wind velocity. Wind estimates for the nine sites, using the BPT, range from 6·5 to 17·4 m s?1, which is in reasonable agreement with the instrumental record from Salt Lake City Airport. The same technique was applied to eight late Pleistocene beaches surrounding the Carson Sink sub‐basin of Lake Lahontan, Nevada. Using the BPT, estimated winds for the eight sites range from 9·7 to 27·1 m s?1. The strongest winds were calculated for a cobble/boulder beach with a fetch of 25 km. Instrumental wind records for the 1992–99 period indicate that wind events of 9–12 m s?1 are common and that the strongest significant wind event (≥ 9 m s?1 for ≥ 3 h) reached an average velocity of 15·5 m s?1. Based on this preliminary comparison, it appears that the late Pleistocene western Great Basin was a windier place than at present, at least for a brief time. 相似文献
133.
134.
Reflection and transmission of seismic waves at an interface betwen two saturated soils 总被引:2,自引:0,他引:2
ReflectionandtransmisionofseismicwavesataninterfacebetwentwosaturatedsoilsJUNYANG(杨峻)SHIMINGWU(吴世明)ZhejiangUniversity,Hangz... 相似文献
135.
本文介绍第22太阳活动周峰年期间利用ULF波的观测和分析,对磁层近地空间电磁环境的研究,文中列举了这峰年期间地磁脉动的主要成果。有些成果是国际上首次提出和发表的,目前,地磁脉动的观测和研究已经成为监测近地空间电磁环境的重要手段。 相似文献
136.
137.
X. Lee H.H. Neumann G. Hartog R.E. Mickle J.D. Fuentes T.A. Black P.C. Yang P.D. Blanken 《Boundary-Layer Meteorology》1997,84(3):383-398
In this paper we report the results of the analysis of two 60-min wave events that occurred in a boreal aspen forest during the 1994 BOREAS (Boreal Ecosystems-Atmosphere Study) field experiment. High frequency wind and temperature data were provided by three 3-D sonic anemometer/thermometers and fourteen fine-wire thermocouples positioned within and above the forest. Wave phase speeds, estimated from information revealed by spectral analysis and linear plane wave equations, are 2.2 and 1.3 m s-1 for the two events. The wavelengths are 130 m and 65 m respectively and are much larger than the vertical wave displacements. There is strong evidence from the present analysis and from the literature supporting our postulate that these waves are generated by shear instability. We propose that wind shear near the top of the stand is often large enough to reduce the gradient Richardson number below the critical value of 0.25 and thus is able to trigger the instability. When external conditions are favorable, the instability will grow into waves. 相似文献
138.
The ability of seismological criteria to identify earthquakes from underground explosions depends partly on the orientation of the earthquake source. Well-determined double-couple moment tensor solutions for a large number of earthquakes have been published in the Harvard centroid moment tensor (CMT) and United Slates Geological Survey (USGS) catalogues. Statistical analyses of these catalogues indicate that the distribution of the orientation of earthquake mechanisms is not random. The distribution of the T axes shows significant clustering around the downward vertical, indicating that a larger number of earthquake mechanisms radiate compressional P -wave energy to teleseismic distances from near the maximum of the radiation pattern than is predicted if earthquake sources are randomly oriented double couples. The clustered T axes correspond to compressional dip-slip mechanisms, and it is this type of mechanism which is believed to cause both the m b : M s (the ratio of body-wave to surface-wave magnitude) and first-motion criteria to misidentify an earthquake as an explosion. 相似文献
139.
Delineation of detailed mantle structure frequently requires the separation of source signature and structural response from seismograms recorded at teleseismic distances. This deconvolution problem can be posed in a log-spectral domain where the operation of time-domain convolution is reduced to an additive form. The introduction of multiple events recorded at many stations leads to a system of consistency equations that must be honoured by both the source time functions and the impulse responses associated with propagation paths between sources and receivers. The system is inherently singular, and stabilization is accomplished through the supply of an initial estimate of the source time function. Although alternative choices exist, an effective estimate is derived from the eigenimage associated with the largest eigenvalue in a singular-value decomposition of the suite of aligned seismograms corresponding to a given event. The relation of the deconvolution scheme to simultaneous least-squares deconvolution is examined. Application of the methodology to broadband teleseismic P waveforms recorded on the Canadian National Seismograph Network demonstrates the retrieval of effective Green's functions including secondary phases associated with upper-mantle structure. 相似文献
140.
J. L. Mateos O. Novaro T. H. Seligman J. Flores 《Geophysical Journal International》1997,129(1):212-214
We show that most of the arguments in the above paper are either incorrect or irrelevant to the point the authors are trying to make. We show that their results have no bearing on the model proposed by our group, as they claim. They discuss the seismic response of a valley with a 2-D trapezoidal cross-section in a vertical plane, whereas we dealt with a closed basin with a 2-D cross-section but of arbitrary geometry and in the horizontal plane. Even more significantly, the width of the valley they use is much smaller than the wavelength of the horizontal P waves that can resonate, thereby precluding any possibility of them being trapped. Therefore, their arguments do not clarify the issue posed in the title of their article. 相似文献