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1.
In this paper, authors obtain the spectral peaks of the earth free oscillation and check all normal modes from 0S0 to 0S48 accurately, with the Fourier analysis and the maximum entropy spectrum method dealing jointly with six groups of the observational residual data from five international superconducting gravimeter stations. By comparing the observational results in this paper with three former groups of observations or models, authors notice an extra discrepancy between two observational 0S2 modes excited separately by Peru earthquake and Alaska earthquake, which probably mirrors the anisotropy of the Earth's inner core. The analysis on the splitting 1S2 mode shows that the asymmetric factor of rotationwise spectral splitting is possible to be different from that of anti-rotationwise spectral splitting.  相似文献   

2.
Previous literature has suggested that multiple peaks in sea level anomalies (SLA) detected by two-dimensional Fourier Transform (2D-FT) analysis are spectral components of multiple propagating signals, which may correspond to different baroclinic Rossby wave modes. We test this hypothesis in the South Pacific Ocean by applying a 2D-FT analysis to the long Rossby wave signal determined from filtered TOPEX/Poseidon and European Remote Sensing-1/2 satellite altimeter derived SLA. The first four baroclinic mode dispersion curves for the classical linear wave theory and the Killworth and Blundell extended theory are used to determine the spectral signature and energy contributions of each mode. South of 17°S, the first two extended theory modes explain up to 60% more of the variance in the observed power spectral energy than their classical linear theory counterparts. We find that Rossby wave modes 2–3 contribute to the total Rossby wave energy in the SLA data. The second mode contributes significantly over most of the basin. The third mode is also evident in some localized regions of the South Pacific but may be ignored at the large scale. Examination of a selection of case study sites suggests that bathymetric effects may dominate at longer wavelengths or permit higher order mode solutions, but mean flow tends to be the more influential factor in the extended theory. We discuss the regional variations in frequency and wave number characteristics of the extended theory modes across the South Pacific basin.  相似文献   

3.
In this paper, authors obtain the spectral peaks of the earth free oscillation and check all normal modes from 0S0 to 0S48 accurately, with the Fourier analysis and the maximum entropy spectrum method dealing jointly with six groups of the observational residual data from five international superconducting gravimeter stations. By comparing the observational results in this paper with three former groups of observations or models, authors notice an extra discrepancy between two observational 0S2 modes excited separately by Peru earthquake and Alaska earthquake, which probably mirrors the anisotropy of the Earth’s inner core. The analysis on the splitting 1S2 mode shows that the asymmetric factor of rotationwise spectral splitting is possible to be different from that of anti-rotationwise spectral splitting.  相似文献   

4.
The spectral ratio method is used here for evaluating the frequency dependence of a site amplification factor in the Messina Strait area. All stations exhibit the same features of spectral ratios evidencing two peaks at about 10 Hz and 16 Hz. We relate this observation to the same geological structures. In fact, all the stations (except the referenced one) are situated over pleistocenic sediments with a similar grain size. This causes the disappearing of any site effect when the average spectrum is used as a reference. MES station, situated in the city of Messina, presents a clear site effect at a different frequency (6 Hz) which cannot be related to any geological structure because no substantial difference is observed between the sediments on which this station is situated and the sediments on which the other stations are situated. MT1 station exhibits a spectral peak at about 2 Hz which can be explained with oscillations of a little sedimentary basin.  相似文献   

5.
Recordings of micro- and moderate-size local earthquakes have been used to quantify site effects in the central-west Turkey which contains one of the world’s best examples of a rapid intra-continental extension with its high population and industrial potential. We analyzed 436 earthquakes with local magnitudes ranging between 2.0 and 5.6 using three component digital recordings from 32 stations. Site functions were obtained using two different spectral ratio approaches (horizontal to vertical spectral ratio, HVSR, and standard spectral ratio, SSR). HVSR estimates of transverse and radial S-waves were compared with one another. Epicentral distance, magnitude and back-azimuth dependencies of site functions were also evaluated. In general, HVSR values from transverse and radial S-waves are similar within a factor of 2. The back-azimuth dependencies of transverse S-wave HVSR results are more significant than distance and magnitude dependencies. On the other hand, averaging of transverse and radial S-wave HVSR results eliminates systematic back-azimuth dependencies caused by source radiation effects. Distributions of HVSR estimates along ~N–S linear array, which traversed main grabens in the region with a station spacing of 3–4 km, reflect subsurface geological complexities in the region. The sites located near the basin edges are characterized by broader HVSR curves. Broad HVSR peaks could be attributed to the complexity of wave propagation related to significant 2D/3D velocity variations at the sediment–bedrock interface near the basin edges. The results also show that, even if the site is located on a horst, the presence of weathered zones along the surface could cause moderate frequency dependent site effects. Comparison of HVSR and SSR estimates for the stations on the graben sites showed that SSR estimates give larger values at lower frequencies which could be attributed to lateral variations in regional velocity and attenuation values caused by basin geometry and edge effects.  相似文献   

6.
雅砻江数字水库地震台网技术系统按照流域化建设和运行模式,以科学合理的测震台站布设、多样化及时通信组网方式和现代化台网中心,构成一个台站流域化布局、地震数据传输通信方式多元化、监测成果实时共享的水库地震监测系统,为工程安全监测和区域防震减灾及相关研究工作提供准确详实的基础资料,并为同类工程及大规模水库地震台网的设计和建设提供参考依据。  相似文献   

7.
Previous observations with the Goose Bay HF coherent-scatter radar have revealed structured spectral peaks at ultra-low frequencies. The frequencies of these spectral peaks have been demonstrated to be extremely consistent from day to day. The stability of these spectral peaks can be seen as evidence for the existence of global magneto spheric cavity modes whose resonant frequencies are independent of latitude. Fieldline resonances occur when successive harmonics of the eigenfrequency of the magnetospheric cavity or waveguide match either the first harmonic eigenfrequency of the geomagnetic field lines or higher harmonics of this frequency. Power spectra observed at the SABRE VHF coherent-scatter radar at Wick, Scotland, during night and early morning are revealed to show similarly clearly structured spectral peaks. These spectral peaks are the result of local field-line resonances due to Alfvén waves standing on magnetospheric field lines. A comparison of the spectra observed by the Goose Bay and Wick radars demonstrate that the frequencies of the field-line resonances are, on average, almost identical, despite the different latitudinal ranges covered by the two radars. Possible explanations for the similarity of the signatures on the two radar systems are discussed.  相似文献   

8.
Non-tidal sea level anomalies (SLAs) can be produced by many different dynamical phenomena over many time scales, and they can induce serious damages in coastal regions especially during extreme events. In this work, we focus on the SLAs in the South China Sea (SCS) to understand whether and how they can be related to the large-scale, seasonal monsoon system which dominates the SCS circulation and dynamics. We have two major objectives. The first one is to understand whether the NE (winter) and SW (summer) monsoons can be responsible for the persistent SLAs, both positive and negative, observed at the SCS ends along the main monsoon path. The second objective is to understand the SCS response as a free system upon onset/relaxation or sudden changes in the forcing wind. It is well known that sudden changes in the forcing mechanism induce free oscillations, or seiches, in closed, semi-enclosed basins and harbors, and we want to identify the possible seiche modes of the SCS. To our knowledge, these two objectives have not been previously addressed. We address these objectives both through observational analysis and modeling simulations. Multi-year tide-gauge data from stations along the coastal regions of the SCS are analyzed examining their spatial correlations. Strong negative correlations are found between the northeast and southwest stations at the two ends of the SCS under the path of the NE/SW monsoons. They correspond to wind-induced positive/negative sea level set-ups lasting for the entire monsoon season and changing sign from winter to summer. Short periods of negative correlations are also found between the SLAs at eastern and western stations during El Niño years in which the monsoons are weaker and have an enhanced E/W component inducing corresponding sea level set-ups. The tide-gauge station at Tanjong Pagar at the southwest SCS end near Singapore is chosen to study four extreme SLAs events in the observational record during 1999. Modeling simulations are carried out to reproduce them. The observed and modeled extreme SLAs agree quite well, both in the amplitude of the highest peak and in phase. Three main peaks are identified in the observational energy spectrum of the de-tided SLAs at the same station in 1999. Using Merian’s formula to evaluate the periods of seiches in idealized basins Wilson (Adv Hydrosci 8:1–94, 1972) the first two peaks (24.4 h and 11.9 h) are found to correspond to the first two seiche modes in the direction of the main, longer axis of the SCS. The third peak (8.5 h) is found to correspond to the seiche in the transversal, shorter axis. Finally, modeling simulations are carried out by suddenly dropping a circular bump of water in the quiescent basin at different locations to excite the seiches. The periods of the modeled peaks agree quite well with the observational ones, the first two periods being actually identical.  相似文献   

9.
We report the statistical and wavelet analyses of the 21 May 2003 tsunami produced by an M w 6.8–6.9 thrust earthquake in the western Mediterranean Sea using 19 tide gauge records. The largest trough-to-crest wave height was 196 cm recorded at the Sant Antoni station in the lee of the incoming tsunami wave. Except at one station, the first wave was not the largest wave at all the analyzed stations, and the largest wave arrived several hours after the first arrival. In addition, the tsunami waves persisted for more than 1 day at most stations. As the spectra of coastal tide gauge stations are strongly influenced by topographic features, special care was taken here while interpreting the results of spectral and wavelet analysis. Our wavelet analysis shows that only a peak at around 23 min is persistent for long duration, and other peaks at 14, 30, 45, and 60 min appeared at short durations. The 23-min signal is possibly associated with the width of the source fault whereas the fault length contributed to the 45-min signal. Based on these dominant periods, the tsunami source dimensions are estimated as 95 km × 45 km. The statistical and wavelet analyses performed here provide some new insights into the characteristics of the tsunami that was generated and propagated in the western Mediterranean basin.  相似文献   

10.
A computational analysis of the periods and structure of surface seiches of the southern basin of Lake of Lugano and its experimental verification from three simultaneous water gauge recordings, mounted along the shores in Agno, Morcote and Riva S. Vitale, is given. The first five theoretical modes are calculated with a finite element code of the tidal equations; it yields the eigenperiods and co-range and co-tidal lines, which are graphically displayed and discussed in detail. Experimental verification is from recordings taken during February/March 1982. Inspection by eye allows identification of the five lowest order modes, partly including interstation phase shift. Power spectral analysis of three-time series and interstation phase difference and coherence spectra allow identification of higher order modes, probably up to order 13. Agreement between the theoretically predicted and the experimentally determined periods is excellent for all calculated modes. Computational results for the four lowest modes and their structure of the northern basin of Lake of Lugano are also presented and experimentally verified with records taken from an event in August 1979. Agreement is again excellent.  相似文献   

11.
The Soil and Water Assessment Tool (SWAT) model is generally applied in alpine catchments using a unique set of snow parameters for the entire basin, and calibration is based on discharge records only. This technical note presents three calibration procedures for snow parameters of SWAT considering snow water equivalent (SWE) values computed using a dense network of snow depth measurement stations available in the Upper Adige River basin, Italy. The first two procedures calibrate snow parameters according to the average sub-basin SWE: the first one defines a unique set of parameters for the entire basin, while the second allows for sub-basin variability. The last approach includes the elevation band SWE output in the calibration for each sub-basin and qualitatively compares it to the SWE computed from the available snow depth monitoring stations. This last method provides the best agreement between SWAT model results and SWE data.  相似文献   

12.
This paper presents the effects of impedance contrast (IC) across the basin edge, velocity contrast between the basin and underlying bedrock, Poisson’s ratio and soil thickness on the characteristics of basin-transduced Rayleigh (BTR) waves and associated differential ground motion (DGM). Analysis of simulated results for a two-dimensional (2D) basin revealed complex mode transformation of Rayleigh waves after entering the basin. Excellent correlation of frequencies corresponding to different spectral ratio peaks in ellipticity curves of BTR waves and spectral amplification peaks was obtained. However, such correlation was not observed between values of peaks in ellipticity curves and spectral amplification at the corresponding frequencies. An increase of spectral amplification with IC was obtained. The largest spectral amplification was more than twice the IC in the horizontal component and more than the IC in the vertical component in the case of large and same impedance contrast for P- and S-waves. It was concluded that the frequency corresponding to the largest spectral amplification was greater than the fundamental frequency of soil by around 14% and 44% in the vertical and horizontal components, respectively. Spectral amplification of the vertical component was negligible when soil thickness was less than around 15–20 times the S-wave wavelength in the basin. The largest values of peak ground displacement (PGD) and peak differential ground motion (PDGM) were obtained very near the basin edge, and their values with offset from the edge were strongly dependent on the IC across the basin edge, Poisson’s ratio, velocity contrast between the basin and underlying bedrock (dispersion), damping and soil thickness. The obtained value of PDGM for a span of 50 m in the horizontal and vertical components due to the BTR wave was of the order of 0.75 × 10?3 and 1.32 × 10?3 for unit amplitude (1.0 cm) in the horizontal component of the Rayleigh wave at rock very near the basin edge.  相似文献   

13.
The new accelerographic network of Santa Fe de Bogotá is composed of 29 three-component stations with sensors at the surface and three additional six-component borehole stations with three sensors at the surface and three at depth (115, 126 and 184 m). In total, 32 stations have been operative in the metropolitan area of Bogotá since 1999. During this period of time, a significant number of weak motion are recorded and used for a preliminary analysis of local site effects. Using the SH-wave response spectra we verify the behavior of the different seismic zones proposed by the previous microzonation study of the city. A comparison between normalized SH-wave response spectra and the normalized design spectra for each zone clearly depicts that parts of the design spectra should be revised, as well as the boundaries between different zones may require some changes. The spectral amplification levels reach up to a factor of 5. The predominant periods obtained by the amplification spectra in different stations in the city, show variability from 0.3 to 3.0 s. A comparison is also made between the predominant periods obtained using H/V spectral ratios of microtremors and those using weak motion. In general, microtremors tend to predict slightly lower values of dominant periods than those calculated by the weak motion spectra. However, there is a general correlation between the two data sets. Using the data recorded by one of the borehole station, an equivalent linear seismic response analysis was conducted. The modeled and recorded response spectra show similarities in period peaks, however, the modeled soil amplification is underestimated for periods less than 0.8 s. Since the available record is weak motion which represents mostly the linear response of the soils, further analysis is required.  相似文献   

14.
The objective of the study was to compare the relative accuracy of three methodologies of regional flood frequency analysis in areas of limited flood records. Thirty two drainage basins of different characteristics, located mainly in the southwest region of Saudi Arabia, were selected for the study. In the first methodology, region curves were developed and used together with the mean annual flood, estimated from the characteristics of drainage basin, to estimate flood flows at a location in the basin. The second methodology was to fit probability distribution functions to annual maximum rainfall intensity in a drainage basin. The best fitted probability function was used together with common peak flow models to estimate the annual maximum flood flows in the basin. In the third methodology, duration reduction curves were developed and used together with the average flood flow in a basin to estimate the peak flood flows in the basin. The results obtained from each methodology were compared to the flood records of the selected stations using three statistical measures of goodness-of-fit. The first methodology was found best in a case of having short length of record at a drainage basin. The second methodology produced satisfactory results. Thus, it is recommended in areas where data are not sufficient and/or reliable to utilise the first methodology.  相似文献   

15.
为基于谱比方法研究海底地震动场地效应,选取日本DONET1台网的20个海底台站2014—2021年记录的1634组地震数据,对其进行筛选和处理后,利用水平与竖向谱比（HVSR）方法考虑不同布设对海底5组节点台站（KMA、KMB、KMC、KMD、KME）谱比特征的影响。研究结果表明:KMA与KME节点台站具有相似的场地特征,KMB与KMD节点台站分散布置在2种场地,KMC节点台站场地与其他节点均不相似,这与长期地质调查结果相似;海底台站谱比曲线呈多峰值现象,其中KMB、KMC、KMD分组台站利用HVSR方法识别的主频变异性较高,KMA、KME分组台站主频较稳定;相同地形条件下,布设方式相同的海底台站谱比曲线随频率分布相似,海底复杂场地条件下,采用装沙沉底方式布置的台站识别场地条件时出现偏差;海底复杂因素对掩埋沉箱方式布设的台站谱比曲线的影响主要集中在频率<5 Hz的低频处;海底复杂因素对未埋入海底台站谱比曲线的影响主要集中在频率为5—10 Hz的高频处。研究结果可为海底地震动场地效应研究提供参考。  相似文献   

16.
Seismic characterization and monitoring of Fucino Basin (Central Italy)   总被引：1，自引：1，他引：0  
The Fucino basin (Central Italy) is one of the largest intramountain alluvial plain in the Apennines range. It has a tectonic origin related to the presence of important systems of faults located in its northern and eastern edges. Some of these faults are still active and capable of generating strong seismic events. Site effects related to the soft soils filling the basin can be very important. In this paper we show the preliminary results of a seismic network installed in the Fucino area in order to collect information about site amplification effects and geometry of the basin. We analyze ambient seismic vibrations and recordings of about 150 local earthquakes mainly related to the seismic sequence of the April 6th 2009 Mw 6.3 L’Aquila event. Moreover the strongest events of L’Aquila sequence were analyzed at the three permanent strong-motion stations operating in the area. Using standard spectral techniques we investigate the variation of resonance frequencies within the basin. The ground motion recorded in the Fucino plain is mainly characterized by strong energy at low-frequencies (f < 1 Hz) affecting both horizontal and vertical components. This is particularly evident for stations deployed in correspondence of very thick deposits of sedimentary filling, where a significant increase of ground-motion amplitude and duration is likely caused by locally generated surface waves. The amplification at low-frequencies (<1 Hz) on the horizontal components can reach up a factor of 10 in comparison to nearby stiff sites. However, we found evidences of seismic amplification phenomena also for stiff sites surrounding the basin, including stations of the Italian strong motion network. The independent geological information and the shallow shear-velocity profiles available for the basin can be combined with resonance frequencies for deriving representative geological sections to be used as base for future numerical 2D–3D modeling of the basin.  相似文献   

17.
A deep spectral investigation of the monthly time series of Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI) in 45 meteorological stations in the Ebro basin (Spain) from 1950 to 2006 for timescales ranging from 1 to 48 months was performed. In order to summarize the results for the whole basin, the spectral analysis was also carried out on the four principal components of SPI and SPEI. Results confirm that SPI and SPEI presents very similar spectral characteristics. At the shorter time scales, the signal of SPI and SPEI is characterized by purely random temporal fluctuations. The longer time scales tend to feature the signal as a smoothly varying time series or persistent, mostly due to the aggregated nature of the indices calculation. The comparative analysis of the spectral properties of the drought indices for all the 45 sites in the Ebro basin lead to the identification of global or regional effects discriminated by local effects. It was found that some periodical signals are common to almost all the sites, while others where only identified in specific meteorological stations.  相似文献   

18.
首先采用P波、 SV波和SH波的极性和振幅比联合求解2014年安徽省金寨ML3.9震群序列的震源机制解, 并在此基础上计算得到该震群序列的震源一致性参数和P轴方位角随时间的变化; 然后基于震区附近3个台站记录到的该震群序列的地震波形, 计算其体波谱振幅相关系数, 同时读取震区附近3个台站记录到的该震群序列中115次ML≥1.5地震的极性. 研究结果表明: 金寨ML3.9震群序列的地震震源机制解绝大多数为压性走滑型, P轴方位角较为一致; 其震源一致性参数处于较低水平, 体波谱振幅相关系数较高; 台站所记录到的地震极性也较为一致. 该结果表明金寨ML3.9震群序列的震源一致性程度非常高.   相似文献   

19.
When considering the search for discovery or amplitude estimation of a spectral line with a probabilistic approach, great attention must be paid to the meaning of each step. We give the probability law for the amplitude of a spectral peak in the presence of random noise appearing in a periodogram and discuss the effective probability of the existence of the corresponding wave. We find that the estimated amplitude of a spectral peak is biased and should be corrected when the signal-to-noise ratio is small. As a first application to gravity data, it results in a re-estimation of the gravimetric amplitude factors (delta factors) provided by least-squares tidal analysis. We also estimate the probability of observing a spectral line above a given level in the spectrum of a purely random noise. This allows us to compute for given spectrum the number of peaks expected to overcross the classical levels used in statistical analysis (like nσ, where σ is the standard deviation of the temporal noise distribution and n is an integer with typical values equal to 2 or 3). A specific application to real data is investigating the gravity spectrum derived from a 5 year record of the French superconducting gravimeter and we show that the predicted statistics are indeed in agreement with the observations. We also show the statistical consequence of using longer observing periods to obtain the spectral estimations. The problem of detecting translational motion of the Earth's solid inner core (Slichter modes) in a gravity spectrum is analyzed and the probabilities of having a triplet of random peaks thresholding specific levels in a given frequency window are computed. We show that, in the case of a typical gravity spectrum (1 year of hourly data and a frequency window of 0.03 cycle h−1), the probability of having a random set of three peaks exceeding a level of 3 σ, is very high. This emphasizes the need for a very careful analysis of spectral lines before inferring the existence of a true physical signal.  相似文献   

20.
We report site response in Las Vegas Valley (LVV) from historical recordings of Nevada Test Site (NTS) nuclear explosions and earthquake recordings from permanent and temporary seismic stations. Our data set significantly improves the spatial coverage of LVV over previous studies, especially in the northern, deeper parts of the basin. Site response at stations in LVV was measured for frequencies in the range 0.2–5.0 Hz using Standard Spectral Ratios (SSR) and Horizontal-Vertical Spectral Ratios (HVR). For the SSR measurements we used a reference site (approximately NEHRP B ``rock' classification) located on Frenchman Mountain outside the basin. Site response at sedimentary sites is variable in LVV with average amplifications approaching a factor of 10 at some frequencies. We observed peaks in the site response curves at frequencies clustered near 0.6, 1.2 and 2.0 Hz, with some sites showing additional lower amplitude peaks at higher frequencies. The spatial pattern of site response is strongly correlated with the reported depth to basement for frequencies between 0.2 and 3.0 Hz, although the frequency of peak amplification does not show a similar correlation. For a few sites where we have geotechnical shear velocities, the amplification shows a correlation with the average upper 30-meter shear velocities, V30. We performed two-dimensional finite difference simulations and reproduced the observed peak site amplifications at 0.6 and 1.2 Hz with a low velocity near-surface layer with shear velocities 600–750 m/s and a thickness of 100–200 m. These modeling results indicate that the amplitude and frequencies of site response peaks in LVV are strongly controlled by shallow velocity structure.  相似文献   

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