Analysis of GPS Measurements in Eastern Canada Using Principal Component Analysis

Continuous Global Positioning System (CGPS) position time series from eastern North America constrain the pattern and magnitude of regional crustal deformation. Initial analysis delineates consistent uplift patterns, as expected from glacial isostatic adjustment (GIA) predictions, but the associated horizontal deformation is not definitive, in part due to the short time periods for a significant number of the available stations. We employ an eigenpattern decomposition in order to define a unique, finite set of deformation patterns for this continuous GPS data. Similar in nature to the empirical orthogonal functions historically employed in the analysis of atmospheric and oceanographic phenomena, the method derives the eigenvalues and eigenstates from the diagonalization of the correlation matrix using a Karhunen–Loeve expansion (KLE). The KLE technique is used to identify the important modes in both time and space for the CGPS data, modes that potentially include signals such as horizontal and vertical GIA, tectonic strain, and seasonal effects. Here we filter both the vertical and horizontal velocity patterns on different spatiotemporal scales in order to study the potential geophysical sources, after the removal of correlated and random noise. The method is successful in disaggregating the linear vertical signal from more variable and less spatially correlated signals. The vertical and horizontal results are compared to the predictions of the ICE-3G GIA loading model with a number of plausible mantle viscosity profiles. The horizontal velocity analysis allows for qualitative differentiation between several potential GIA models and suggests that, with longer time series, this technique can be employed to remove correlated noise and improve estimates of crustal strain patterns and their sources.     

Specific features of variations in the characteristics of VLF signals when the lunar shadow propagated along the path during the solar eclipse of March 29, 2006

The effects of the solar eclipse of March 29, 2006, in the signals of ULF radio stations, in the intensity of regular radio noise at frequencies of 0.3–10 kHz, and in the number of atmospherics received in Yakutsk mostly from the west have been considered. The observations were performed using a multichannel parallel analyzer-recorder (11 channels in the frequency band 0.47–8.7 kHz), one-point lightning direction and range finder (0.3–100 kHz), narrow-sector radio noise direction-finder (0.3–10 kHz), recorder of signals from VLF radio stations, and broadband radio noise recorder (0.3–100 kHz). A GPS clock was used to synchronize a recorder of signals from VLF radio stations. The effect was observed in radio signals, radio noise, and number of atmospherics from the direction 270° ± 20° counted off clockwise from the north during the last stage of the eclipse (～ 1100–1200 UT), when the lunar shadow approached the line of the nighttime terminator and obscured part of the signal propagation path. The effect was observed as an enhancement of the received signals by a factor of ～1.2, a factor of ～1.4 increase in the number of atmospherics, and a change in the radio station phase values.     

云南地区的背景弹性波场分析

利用云南省地震台网44个台站记录的2008年1-9月连续波形数据进行互相关计算,得到了台站间的格林函数,并获取对应的频散曲线,据此分析了该地区的区域背景弹性波场的来源及分布.研究发现,该地区的区域背景弹性波场有着明显的方向性,15s信号的总体传播方向是从东南向西北,也就是说主要来自云南省的东南方向,据此推测区域背景弹性...     

人工源地电场空间变化区域性特征

认识地震电信号特征对地震短临预报具有重要意义.通过陕甘宁晋地区、川滇地区、山东及其周边地区和上海地区的换流站接地极向地下注入2100—4600 A大电流,研究各地区地电场的变化特征,以深入地认识地电场信号的区域性变化实质.研究显示:① 在陕甘宁晋地区,高沙窝大电流源的附加地电场可被分布于鄂尔多斯地块边缘弧形地带的490 km范围内的10个台站观测到,而距该信号源以西150 km的台站均未观测到该信号;② 在川滇地区,丽江信号源的南部地区观测到的信号强度和距离远大于北部地区.对于西昌大电流源,只有距其10 km处的小庙台和540 km处的仙女山台可观测到信号,而其它十几个台站均未观测到;③ 在山东和上海地区,可被观测到的人工源信号的最远距离及其特征各不相同,亦与陕甘宁晋和川滇地区大为不同;④ 对于本文所研究的地区,信号的波形变化存在着随台站与信号源距离的增加畸变程度增大的现象,一些台站只能记录到大电流开始和结束时的尖峰脉冲信息.针对上述变化特征,本文从机理和地质条件等方面进行了一些有益的探讨,以期加深对地震电信号变化特征的认识.      

Comparison of observed and theoretically calculated seiches of Heda Bay,Japan

We carried out observations of sea-level fluctuations simultaneously at three stations on the coast of Heda Bay, Honshu, Japan, using supersonic-type water level gauges controlled by a personal computer. Analyses of the obtained data showed predominant spectral peaks at periods of 7.6, 2.0 and 1.3 minutes for all three stations. Comparison of the observed data with numerically calculated normal oscillation modes of the bay indicates that these three spectral peaks correspond to the theoretical first, third and seventh normal modes of the basin respectively, judging from the results of cross-spectral analyses. The reason for the absence of the remaining normal modes, especially of the second or the lateral first mode of the basin, is briefly considered.     

与大电流信号有关的华北东部地区地电场空间变化特征的研究

在地震电磁学研究中,电流在地下的流动特征与构造关系是一个非常重要的问题.大范围观测站的分布和强电流源存在的格局为我们研究这个问题提供了机会.本文通过对青岛换流站接地极向地下注入2100~3004 A大电流时华北东部地区21个地电场台站观测的研究,深入认识了地电场信号变化实质.研究表明：①沿着郯庐断裂带方向、跨过郯庐断裂带西南方向、跨过郯庐断裂带北西方向能够记录到地电流信号的特征变化非常大,存在着明显方向性特征和“穴位”现象.②注入大地的电流强度越大,则在观测点记录到的地电流信号强度也越强.③本文研究区域里地电场信号的均匀度并不遵循均匀介质条件下的一般规律,说明地下介质电性非均匀性和各向异性特征明显.④大电流注入地下时能够较正确地反映信号源方位的台站较少,只有安丘台、大山台和新沂台所观测到的方位线可用于确定信号源的位置.本文从点电流源与台站测线布局构成的装置系统、大区域介质非均匀性、台站测区以及附近地区的介质细结构的影响三个方面,通过数值模拟,对上述特征进行了较深入的分析和解释.     

地震勘探組合法之研究（一）

假定随机干扰是对位置平稳的随机变量,利用固定瞬间多道记录统计分析方法,讨论了组合地震法的统计效应及其效益,提出了在线性等距组合形式中组內间距选择以适应信杂比最大的条件,并讨论了选用组合形式和组內间距的方法。     

Event detectability of seismograph stations in Fennoscandia

The event detection probability as a function of ISC magnitude shows significant variations between seismic stations in Fennoscandia. In the teleseismic range, the 50% incremental detection thresholds extend from m_b = 4.5 to 5.5 among those stations that reported to the ISC during 1964–1969; the best stations being located in Finland. Most of this variation, though not all, can be explained from noise level fluctuation and regional signal attenuation effects. A common feature for all stations studied is a significantly better detectatbility for signals from eastern Asia than from western North America at a similar distance range. This is interpreted as resulting from higher attenuation in the mantle of high-frequency signals from the latter region.     

利用地震背景噪声互相关检测流动地震台波形时钟漂移

时钟精度是地震观测中最重要的参数之一,需要通过卫星授时信号来保障。当卫星信号被屏蔽或仪器守时部件出现问题时,地震计内部时钟会逐渐漂移,给后续数据处理带来极大困扰。利用地震背景噪声台站对互相关提取的经验格林函数,不仅可以用于结构成像,也可以用于检测波形时钟是否存在漂移,并获得时钟漂移幅度。使用地震背景噪声互相关方法对2017—2019年云南永胜地区4个流动观测台站的连续波形进行了时钟漂移检测,结果发现部分流动台站在不同时间段存在不同模式的时钟漂移,最大幅度可达到1.75 s。同时,利用云南宾川气枪重复震源激发的信号进行互相关计算,对上述结果进行了验证,发现两种结果具有较好的一致性。研究表明,背景噪声互相关对波形的时钟漂移有较高的灵敏度,能够有效检测出时钟问题,防止波形被误用,可为后续波形时钟校正提供参考信息。     

源自多个大电流源的华东地区地电场空间变化特征

在地震电磁学研究中,电流在地下流动特征与构造关系是一个非常重要的问题.多个强电流源的存在和大范围地电场观测站的分布格局为我们研究这个问题提供了更好的机会.本文通过利用上海及其周边地区4个地点不同的高压换流站接地极向地下注入的大电流(1200~4780A)信号,研究了华东地区8个台站组成的地电场观测台网接收到的附加地电场信号,推进了对地电流在地下传播特征的认识.研究显示:1从幅度特征而言,对于源自不同的大电流信号,各台站观测到的附加地电场信号幅度特征有很大不同,而近处的浦东台只能观测到南桥大电流源的信号而观测不到其他3个大电流源发出的信号,存在着所谓的"敏感点"效应;2对于上海地区4个接地极大电流而言,能够观测到源自它们的附加地电场的台站最远为350km,在更远的台站则观测不到.3从均匀度方面而言,在每个台站对于不同的信号源,记录到的附加地电场信号在各方向上的长极距和短极距信号幅度之比可以有很大变化;4在方位特征上,只有崇明台站接收到的同里接地极和华新接地极大电流信号的计算方位度数与实际方位度数误差最小,分别为0.2°和0.8°.而在距离各个信号源最近的青浦台,这种误差要比崇明台站的大;5极化方向特征显示,在青浦台和崇明台所观测到的附加地电场信号中,存在着同一方向上长、短极距信号极化方向是反向的现象,这与常理相悖,怎样解释该现象有待于今后进一步研究.本文从点电流源与台站测线布局构成的装置系统、大区域介质非均匀性、台站测区以及附近地区的介质细结构的影响三个方面,对上述特征进行了较深入的分析和解释.     

Characteristics of coastal trapped waves along the northern coast of the South China Sea during year?1990

The structures and evolution of the coastal-trapped waves (CTW) along the northern coast of the South China Sea (SCS) in the year?1990 are studied using observed hourly sea level records collected from four sites around the northern SCS and a three-dimensional numerical model with realistic bathymetry and wind forcing. Analysis of the yearlong records of the observed sea level data indicates that the sea level variations are highly correlated between the stations and the sea level variability propagates southwestward along the coast. The sea level signals traveling from northeast to southwest along the coast with a propagation speed of 5.5–17.9?m?s^?1 during both the typhoon season and the winter month show the characteristics of a CTW. The wave speed is faster between stations Shanwei and Zhapo than that between Xiamen and Shanwei. Sea level variations during both typhoon season and winter month are reasonably well represented by the numerical model. The model runs focused on the wave signals related to typhoons and winter storm show that the CTW propagating southwestward along the coast can be reinforced or decreased by the local wind forcing during its propagation and there are apparent differences in the propagation characteristics between the waves along the mainland and those traveling around Hainan Island. The abrupt change of the shelf width and coastline around Leizhou Peninsula and Hainan Island are responsible for strong scattering of CTWs from one mode into higher modes. The alongshore velocities across different transects associated with CTW are investigated to examine the vertical structures of the waves. The alongshore velocity structures at transects during different events are related to the combined effect of stratification and shelf profile, which can be estimated using the Burger number. The empirical orthogonal function analysis of alongshore velocity and nodal lines of the mode structure suggest mode two CTWs in transect S2 during typhoon season and mode 1 CTWs during winter. Sensitivity model experiments are also performed to demonstrate the effects of local wind and topography on the wave propagation.     

Seismic waves from large explosions reveal periodical deformations at lithospheric depths

Seismic signals from large explosions recorded by Soviet seismic stations in the distance range from 100 to 2800 km during 12–14 years are analysed. Periodic variations of travel times and amplitudes of P and S waves corresponding to different boundaries in the lithosphere have been discovered. Characteristic periods are close to 2 and 6 years. Since the variations of amplitudes and velocities of seismic waves may be caused by dilatational deformations of rocks, the results obtained reveal a possible existence of slow tectonic stress waves. Tectonic waves were detected not only at seismically active zones but also at aseismic regions of the Russian and Kazakh platforms. At almost all seismic stations these variations increase with the seismic ray depth in the lithosphere. This increase can be explained by the presence of bending deformations and by their attenuation due to the rheological stratification of the lithosphere. Accordingly, the theory of Elsasser's waves is generalized to include bending deformations and the presence of stationary asthenosphere flows.     

场地效应研究的传递台站线性反演法——以芦山地震为例

采用线性反演法研究了地震动的场地效应,并针对线性反演法的局限性,提出了"传递台站"的方法来扩展其应用.通过传递台站,地震事件不再局限于必须被参考台记录到,台站也不再局限于须与参考台站同时获取到地震记录,改进后可大大增加数据样本,使得结果更趋稳定.同时,对于原来难以参与分析的台站,也可得到其场地反应.研究中,还采用了基于遗传算法的非参考台方法,进一步分析了场地反应和验证所提方法的合理性.将所提方法应用于2013年芦山7.0级地震周边区域,分析了这一地区的强震台站的场地反应和地震波衰减因子Q_s值.研究结果显示,当采用一个和二个传递台站进行分析时,所能利用的地震事件和所能求解场地反应的台站数目逐步增加,随着数据样本的增加,分析结果趋于稳定.研究区域的Q_s值拟合结果为Q_s(f)=334.16f~(0.73).遗传算法结果表明,线性反演法中所选取的参考台站有其自身的场地反应.当利用遗传算法获得的参考场的场地反应去校正线性反演法的结果时,对于多数场地,两者的结果符合得很好,说明了遗传算法的结果可以合理地认为是场地较为真实的反应.     

OBC signal fidelity

A suite of three tests was performed to characterize the signal fidelity of OBC 4C acquisition systems. The test methodology was to evaluate individual sensor stations by acquiring source lines that were parallel to the in‐line and cross‐line horizontal sensors and source lines that were at 45° to the in‐line and cross‐line sensors. This technique provides constant azimuth gathers with a uniform offset range and removes issues associated with source array directivity. Characterization of the test data identified the frequency content of the geophone signals and the correlation between the vertical and cross‐line geophones as the most sensitive indicators of signal infidelity. In the former case, stations with questionable signal fidelity exhibited a very reverberatory signal. This signal was most evident on the cross‐line sensor. In the latter case, when normalized cross‐correlation coefficients are computed in a moving window, the cross‐line sensor and the vertical sensor are highly correlated, beginning several hundred milliseconds after the first arrivals. These characteristics can be exploited to allow stations with questionable signal fidelity to be programmatically identified. One means of identifying questionable stations is to compute the histogram of the instantaneous frequency. The frequency distributions from questionable stations are unambiguously distinguishable from stations that exhibit better signal fidelity. It was noted that signal fidelity appeared as a range, between acceptable and poor. To characterize the signal fidelity of an acquisition system adequately, the number of test samples must be statistically significant.     

Results from 44 months of observations with a superconducting gravimeter at Moxa/Germany

Results for more than 42 months of observations with the superconducting gravimeter CD-034 at the Geodynamic Observatory Moxa are discussed. Moxa observatory is one of the newer stations within the ‘Global Geodynamics Project’ (GGP). A special feature of the gravimeter at Moxa is its dual sensor system; differences in the results obtained from the two sensor recordings are generally well within the standard deviations of the tidal analysis. One significant difference concerns the slightly different drift rates of 31 and 49.5 nm/s² per year for upper and lower sensor; both sensor drifts can be fitted by a linear function. We find that the noise levels are close to the ‘New Low Noise Model’ for the seismic-modes and are also low in the tidal bands. Due to this low noise, Moxa is a station well suited to search for small geodynamic signals. The long-period variation in the gravity residuals correlates well with the polar motion.The difference signal between the two sensor recordings has a peak-to-peak amplitude of about 6 nm/s² and shows systematic variations. Its spectrum is characterised by instrumental noise between 0.2 and 0.4 cph. The noise level of the difference and of the sum of the two residual datasets are clearly lower, respectively, higher than the noise contents of the gravity residuals themselves. This is a strong indication for the existence of broadband signals common to the two residual datasets, leading to the conjecture that the reduction of environmental effects is still not sufficient.Our results once more emphasize the necessity to correct the data for barometric pressure effects when analyzing the data for seismic modes. The reduction visibly increases the signal-to-noise ratio in the low frequencies of the mode band and helps to avoid misinterpreations of peaks. Besides the well known barometric pressure influence we can establish hydrological effects in the data which are probably caused by soil moisture and groundwater table variations as well as by batch-wise water movement within the weathering layer. As the major part of the observatory surroundings is above gravimeter level, an anticorrelation between hydrological and gravity changes is observed. In addition, it can be shown that global hydrological effects reach an order of magnitude that makes it necessary to consider these effects when analyzing long-period signals like polar motion. Vice versa these effects are large enough to be detectable in the gravity data. A first joint analysis of five datasets from the GGP network shows no indications for signals related to the Slichter triplet or core modes.     

利用固定台站分析长江激发气枪信号特征

“地学长江计划”安徽实验是以气枪震源为核心的大型主动源探测实验。通过在长江安徽段20个固定点定点激发气枪震源,结合109个固定台站、11条流动测线组成的观测网络,首次利用主动源实现了对长江流域安徽段约6万km2面积的三维地下结构探测。本文利用固定台站对长江激发气枪信号进行了分析,结果表明,长江中气枪信号激发效果良好,固定台记录中气枪信号可识别的最远距离达300km。对气枪信号绝对振幅的研究结果表明：① 50km处的气枪信号约为10nm量级,200km处的气枪信号小于1 nm;② 气枪信号强度的空间分布存在一定的方位各向异性,可能与长江的几何形状有关;③ 台站背景噪声对于提取气枪信号至关重要,高质量的固定台网为识别nm量级气枪信号提供了可能。     

不同深度布设台站的噪声及信号幅值特征分析

为探究地震观测中地震计检测到的噪声和信号强度均受其布设深度的影响,本文首先对CPUP和LPAZ两台站布设的不同深度地震计所得到的数据进行噪声水平和地震信号对比;其次采用对比功率谱密度的方法对两台站不同通道采集到的不同时段的噪声数据进行分析;最后比较两台站不同通道采集到的整月数据的噪声幅值、信号幅值、信噪比特征。结果显示:深度较大的通道,其噪声功率均值较小;当事件信号到来时,较深通道的地震计检测到的信号和噪声幅值比较浅通道均有所减小,在信号和噪声幅值均减小的共同影响下,信噪比有一定程度的变化,其中LPAZ台站的信噪比提高较为明显。      

VLF向西传播TE模激励的实验研究

本文分析了在新乡和临潼两地同时接收Omega导航台的H台和C台信号的相位变化。从四条传播路径的测试结果中,分析了TE模的存在以及特点。通过一组导出的方程组,从C台信号的测试数据中,用实验回归法确定出TE₁模的激励因子。结果表明,VLF信号夜间向西传播时,测出的TE₁模的激励因子看来比理论给出的数值要大。     

地震监测人工源极低频电磁技术(CSELF)新试验

2009年利用大功率人工源极低频电磁技术(CSELF)在位于华北、华南、东北、西北和西南的12个地震台站和几个流动观测点进行了连续30天新的观测试验.结果表明,利用CSELF技术可以在1700 km之外测量到人工源电磁场信号,计算得到的电、磁场功率谱密度和视电阻率与天然源信号相比,抗干扰能力更强,观测信号更稳定,特别有利于识别和捕捉地震等诱发的电磁异常现象,在地震预测监测中具有很大的研究应用潜力.试验还发现,各地震台站和流动测点观测的CSELF信号的强度,与台站或者测量的电、磁场分量相对于发射源的距离、方位有明显的关系;发现在离开发射源的相对近区,场强随距离的衰减,比在相对远区更快;发现接收磁场信号比电场信号较易获得更高信噪比的数据.