Earth Rotation Parameters 1899.7–:1992.0 After Reanalysis Within The Hipparcos Frame

Earth rotation parameters (ERP) in the interval 1899.7–1992.0 are obtained from re-analysis of the observed latitude/universal time variations by optical astrometry. Hipparcos Catalogue is used to define the celestial reference frame, within which the ERP are described, with special care devoted to 'problematic' double and/or multiple stars. The terrestrial reference frame is defined by the adopted latitudes/longitudes of participating instruments and their secular motions as given by the NUVEL-1 NNR model of plate motions, and it is chosen to be very close to the International Terrestrial Reference Frame (ITRF). More than four million observations made with 48 different instruments at 31 observatories, located all over the world, are utilized to determine polar motion, celestial pole offsets and (after 1956) universal time UT1, all at 5-day intervals. Along with these parameters, the combinations of Love and Shida numbers, governing the tidal variations of the local verticals at individual observatories, are also determined. The analysis of the results covering almost a century, namely the long-periodic polar motion and length-of-day changes, is presented.     

The International Latitude Service – A Historical Review, From The Beginning To Its Foundation In 1899 And The Period Until 1922

The study of Polar Motion (PM) is associated withthe work of the InternationalLatitude Service (ILS), the International PolarMotion Service (IPMS) and theBureau International de l'Heure (BIH), andthen the International Earth RotationService (IERS). After reviewing briefly thehistory of early PM research, thescientific objective of the ILS including itsrealizing, and the activities of all thecited services and giving information on thePM time series available from mid-19thcentury to the present, important details onthe events and results are presentedover the period until 1922. It is shown, inwhich way the Geodetic Institute Potsdamcontributed to the foundation of the ILS andtook part in its activities through thework of Helmert, Albrecht, Wanach and Mahnkopf.Based on international scientificcooperation, the results were the rectangularcoordinates of the Polar Motion from1890.0 to 1922.7 at 0.1 year intervals, inparticular those derived from the latitudeobservations at independent stations from 1890.0to 1899.8, and those derived fromthe latitude observations at the ILS stationsfrom 1899.9 to 1922.7.     

基于实测记录分析场地因素对海底地震动特性影响

目前基于海底实测记录的分析发现海底与陆地地震动特性存在明显差异,但难以进一步确定海底地震动特性的影响因素。在以往研究的基础上对比同次地震中相邻海底台站间地震动特性的差异,并分析造成差异的原因。以日本K-NET地震台网中6个海底强震台站及其相邻不同场地条件陆地台站监测的8次强震记录为研究对象,通过分析强震记录的峰值加速度、水平放大系数谱、竖向与水平反应谱的比谱等,对比分析不同海底台站地震动的特性,以及海底台站与相邻不同场地条件陆地台站地震动特性的差别。研究发现：（1）海底与陆地竖向地震动存在明显差异;（2）不同海底台站间地震动特性亦存在较大差异和明显的规律性,海底场地条件、地形等场地因素对海底地震动特性的影响较大;（3）海底水平向地震动反应谱的特征周期较大,谱特性介于陆地中硬土与软土场地间。     

Variation of earthquake ground motions within a very small distance

At present, dense strong motion observation networks have been established in Japan. One of the important findings based on these networks is that strong ground motions are quite site-dependent. Characteristics of observed ground motions at nearby stations can exhibit a significant variation, even when the stations are within several hundreds of meters. These observations raise one important question; if characteristics of strong ground motions exhibit large variations even for smaller regions, we should be concerned about the application of observed or predicted ground motions for the assessment of structures. In particular, if ground-motion parameters such as PGA, PGV, Spectral Intensity, etc., exhibit large variations for smaller regions, their use for the seismic design and practice will be subject to restriction. In other words, the evaluation of variation of these parameters is an important issue. From such a point of view, the authors investigated the variation of observed ground motions within a very small distance in this study. First of all, fifteen couples of adjacent strong motion stations in Japan, where the distance is within 100 m, were listed up based on our field reconnaissance. Then, microtremor measurements were carried out at each of the station pairs. Next, variation of recorded earthquake ground motions for the station pairs was examined based on various ground motion parameters and response spectra. Moreover, we investigated the key factor which is affecting the variation of observed ground motions.     

Testing the Validity of Simulated Strong Ground Motion from the Dynamic Rupture of a Finite Fault, by Using Empirical Equations

This paper is concerned with testing the validity of the ground motions estimated by combining a boundary integral equation method to simulate dynamic rupture along finite faults with a finite difference method to compute the subsequent wave propagation. The validation exercise is conducted by comparing the calculated ground motions at about 100 hypothetical stations surrounding the pure strike-slip and pure reverse faults with those estimated by recent ground motion estimation equations derived by regression analysis of observed strong-motion data. The validity of the ground motions with respect to their amplitude, frequency content and duration is examined. It is found that the numerical simulation method adopted leads to ground motions that are mainly compatible with the magnitude and distance dependence modelled by empirical equations but that the choice of a low stress drop leads to ground motions that are smaller than generally observed. In addition, the scatter in the simulated ground motions, for which a laterally homogeneous crust and standard rock site were used, is of the same order as the scatter in observed motions therefore, close to the fault, variations in source propagation likely contribute a significant proportion of the scatter in observed motions in comparison with travel-path and site effects.     

Broadband ground motion simulation using a hybrid approach of the May 21, 2021 M7.4 earthquake in Maduo,Qinghai, China

In this study, the broadband ground motions of the 2021 M7.4 Maduo earthquake were simulated to overcome the scarcity of ground motion recordings and the low resolution of macroseismic intensity map in sparsely populated high-altitude regions. The simulation was conducted with a hybrid methodology, combining a stochastic high-frequency simulation with a low-frequency ground motion simulation, from the regional 1-D velocity structure model and the Wang WM et al. (2022) source rupture model, respectively. We found that the three-component waveforms simulated for specific stations matched the waveforms recorded at those stations, in terms of amplitude, duration, and frequency content. The validation results demonstrate the ability of the hybrid simulation method to reproduce the main characteristics of the observed ground motions for the 2021 Maduo earthquake over a broad frequency range. Our simulations suggest that the official map of macroseismic intensity tends to overestimate shaking by one intensity unit. Comparisons of simulations with empirical ground motion models indicate generally good consistency between the simulated and empirically predicted intensity measures. The high-frequency components of ground motions were found to be more prominent, while the low-frequency components were not, which is unexpected for large earthquakes. Our simulations provide valuable insight into the effects of source complexity on the level and variability of the resulting ground motions. The acceleration and velocity time histories and corresponding response spectra were provided for selected representative sites where no records were available. The simulated results have important implications for evaluating the performance of engineering structures in the epicentral regions of this earthquake and for estimating seismic hazards in the Tibetan regions where no strong ground motion records are available for large earthquakes.     

2018年2月6日花莲M_W6.4地震近场地震动方向性效应

利用中国台湾省内222个强震动台站以及Palert地震预警系统520个台站所观测的三分量加速度记录,研究此次花莲M_W6.4地震近场强地震动空间分布和衰减特征,将观测结果与美国NGA-West2地震动经验预测模型进行对比,揭示此次台湾花莲地震近场地震动的长周期特点,基于回归残差分析研究地震动峰值加速度(PGA)、峰值速度(PGV)和不同周期地震动的空间分布差异,定量考察近场地震动的方向性效应.研究结果表明:(1)整体上此次地震的近场PGV观测值和周期1.0s以上的长周期加速度谱值与美国NGA-West2地震动预测模型结果接近,PGA观测值和周期小于1.0s的加速度反应谱略低于预测模型结果.从空间分布来看,周期1.0s以上的长周期地震动在断层的不同方位有系统性差异,在破裂传播前方(震中西南方位),周期大于1.0s时的反应谱明显高于美国NGA-West2地震动经验预测模型,在破裂传播后方(震中东北方位),周期大于1.0s时的反应谱低于经验预测模型,表明此次地震近场地震动具有显著的方向性效应.(2)破裂传播的方向性效应主要影响周期超过1.0s的长周期,而对PGA以及周期小于1.0s的短周期地震动影响较弱.在破裂传播前方,周期1.0～10.0s的加速度反应谱值被增强到整体观测平均水平的1.16～1.52倍;在破裂传播后方,周期1.0～10.0s的加速度反应谱值被减弱到整体观测平均水平的0.36～0.70倍.(3)此次地震破裂方向性效应的影响表现出明显的窄带效应,破裂方向性的影响(包括破裂传播前方的增强作用和破裂传播后方的减弱作用)在周期T=3.0s时达到最大,在该周期破裂传播前方的增强系数为1.52,破裂传播后方的减弱系数为0.36.从周期T=3.0s到10.0s,破裂方向性效应的影响随周期增大总体上呈减弱趋势,这与2016年日本熊本M_W7.0地震破裂方向性效应的影响特点显著不同.     

台湾费丛峡谷对强地面运动的影响研究──利用单台资料及2．5维SH混合方法

本文通过格林函数反褶积方法，由台湾峡谷附近的记录资料预测峡谷区的强地面运动的时程曲线．峡谷区的理论格林函数应用２．５维ＳＨ混合方法求解．通过格林函数反褶积方法得到的峡谷区费丛１及费丛２台的位移、速度、加速度和实际资料对比，取得了满意结果．计算结果表明，峡谷底部的峰值加速度相对峡谷边缘为最小；在靠近震中的一侧，峡谷的加速度的最大振幅相对比另一侧大．还给出了其余３个台的预测结果，研究了它们的加速度傅里叶谱和反应谱．     

台湾费丛峡谷对强地面运动的影响研究──利用单台资料及2．5维SH混合方法

本文通过格林函数反褶积方法，由台湾峡谷附近的记录资料预测峡谷区的强地面运动的时程曲线．峡谷区的理论格林函数应用２．５维ＳＨ混合方法求解．通过格林函数反褶积方法得到的峡谷区费丛１及费丛２台的位移、速度、加速度和实际资料对比，取得了满意结果．计算结果表明，峡谷底部的峰值加速度相对峡谷边缘为最小；在靠近震中的一侧，峡谷的加速度的最大振幅相对比另一侧大．还给出了其余３个台的预测结果，研究了它们的加速度傅里叶谱和反应谱．     

Stochastic finite-fault simulation of ground motion from the August 11, 2012, <Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">w</Emphasis></Subscript> 6.4 Ahar earthquake,northwestern Iran

In this study, the 11 August 2012 M _w 6.4 Ahar earthquake is investigated using the ground motion simulation based on the stochastic finite-fault model. The earthquake occurred in northwestern Iran and causing extensive damage in the city of Ahar and surrounding areas. A network consisting of 58 acceleration stations recorded the earthquake within 8–217 km of the epicenter. Strong ground motion records from six significant well-recorded stations close to the epicenter have been simulated. These stations are installed in areas which experienced significant structural damage and humanity loss during the earthquake. The simulation is carried out using the dynamic corner frequency model of rupture propagation by extended fault simulation program (EXSIM). For this purpose, the propagation features of shear-wave including \( {Q}_s \) value, kappa value \( {k}_0 \), and soil amplification coefficients at each site are required. The kappa values are obtained from the slope of smoothed amplitude of Fourier spectra of acceleration at higher frequencies. The determined kappa values for vertical and horizontal components are 0.02 and 0.05 s, respectively. Furthermore, an anelastic attenuation parameter is derived from energy decay of a seismic wave by using continuous wavelet transform (CWT) for each station. The average frequency-dependent relation estimated for the region is \( Q=\left(122\pm 38\right){f}^{\left(1.40\pm 0.16\right)}. \) Moreover, the horizontal to vertical spectral ratio \( H/V \) is applied to estimate the site effects at stations. Spectral analysis of the data indicates that the best match between the observed and simulated spectra occurs for an average stress drop of 70 bars. Finally, the simulated and observed results are compared with pseudo acceleration spectra and peak ground motions. The comparison of time series spectra shows good agreement between the observed and the simulated waveforms at frequencies of engineering interest.     

A note on short-term core-mantle coupling,geomagnetic secular variation impulses,and potential magnetic field invariants as Lagrangian tracers of core motions

This note summarizes recent studies of atmospheric excitation of short-term changes in the length of the day and polar motion which set useful limits on the timescales associated with angular momentum transfer between the Earth's core and mantle. It also speculates about the nature of the recently-discovered phenomenon of “impulses” or “jerks” in the geomagnetic secular variation, proposing that they might be manifestations of “loop” instability of the magnetic field within the core. Finally, it outlines novel properties of high magnetic Reynolds number flows that bear on the inverse problem of deducing core motions from geomagnetic secular variation data.     

日本熊本M_W 7.0地震的长周期地震动

利用日本K-NET和KiK-net强震动台网获取的距离发震断层100 km以内136个强震动台站的三分量加速度记录,研究熊本M_W7.0地震地震动的长周期特性.基于残差分析研究不同周期地震动的空间分布差异,将观测分析结果与美国NGA经验模型、汶川和芦山地震观测结果进行对比,揭示此次熊本地震近场强震动的长周期特点及其形成机理.研究结果表明:(1)虽然总体上此次地震的近场地震动水平与美国NGA-West2经验模型的预测结果接近,但周期2 s以上地震动的分布在断层不同方位有系统性差异,在断层的北东方位,周期2.0～10.0 s的反应谱高于NGA-West2经验模型的预测结果,在西南方位,谱值低于经验预测模型.(2)我们认为此次地震2.0～10.0 s的长周期地震动的空间分布差异主要受破裂方向性的影响,在破裂传播的正前方,周期T=2.0 s,3.0 s,5.0 s,7.5 s和10.0 s的加速度谱被放大到整体观测平均水平的1 4～2.0倍.从周期T=2.0 s到10.0 s,破裂向前方向的放大作用和破裂反方向的减弱作用均有所增强,此次地震观测到的速度大脉冲记录均位于断层的东北方位,这与方向性脉冲的产生机理相吻合,速度大脉冲对加速度反应谱有显著的长周期放大作用,放大倍数值可以超过4.0,放大作用的影响主要位于脉冲的特征周期T_p附近.(3)近断层记录在建筑结构敏感的周期(0.5～2.0 s)的反应谱达到芦山地震的3～6倍,虽然与芦山地震震级接近,此次地震近断层地震动破坏力大大超过了芦山M_W6.8地震,甚至超过了汶川W_W7.9地震,这种长周期特点应该引起工程抗震设计和相关研究人员的重视.     

A preliminary investigation of strong-motion data from the French Antilles

Strong-motion networks have been operating in the Caribbean region since the 1970s, however, until the mid-1990s only a few analogue stations were operational and the quantity of data recorded was very low. Since the mid-1990s, digital accelerometric networks have been established on islands within the region. At present there are thought to be about 160 stations operating in this region with a handful on Cuba, 65 on the French Antilles (mainly Guadeloupe and Martinique), eight on Jamaica, 78 on Puerto Rico (plus others on adjacent islands) and four on Trinidad.After briefly summarising the available data from the Caribbean islands, this article is mainly concerned with analysing the data that has been recorded by the networks operating on the French Antilles in terms of their distribution with respect to magnitude, source-to-site distance, focal depth and event type; site effects at certain stations; and also with respect to their predictability by ground motion estimation equations developed using data from different regions of the world. More than 300 good quality triaxial acceleration time-histories have been recorded on Guadeloupe and Martinique at a large number of stations from earthquakes with magnitudes larger than 4.8, however, most of the records are from considerable source-to-site distances. From the data available it is found that many of the commonly-used ground motion estimation equations for shallow crustal earthquakes poorly estimate the observed ground motions on the two islands; ground motions on Guadeloupe and Martinique have smaller amplitudes and are more variable than expected. This difference could be due to regional dependence of ground motions because of, for example, differing tectonics or crustal structures or because the ground motions so far recorded are, in general, from smaller earthquakes and greater distances than the range of applicability of the investigated equations.     

Topographic proxies from 2-D numerical analyses

We use 2D numerical analyses at ground motion stations from the NGA-West2 dataset to develop parameters to predict the effect of surface topography on response spectra. The simplistic numerical analyses use sinusoidal input motions, uniform soil profiles, elastic soil properties, and absorbing boundary conditions. We define several topographic parameters for stations using the natural logarithm of 2D amplifications in peak ground acceleration of a sinusoidal input motion in different orientations. The natural log of 2D amplifications when averaged over multiple orientations is found to have the most predictive power among the studied parameters. We also explore the relationship between the topographic parameters developed in this study, and the topographic parameters developed at the same sites in an earlier study (Rai et al. in Earthq Spectra, 2016b).     

渭河盆地中土层场地对地震动的放大作用

土层场地对地震动的影响较大,建(构)筑物的选址及其抗震设防必须考虑土层场地的放大作用,以避免或减轻其震害.汶川地震中,布设在渭河盆地中的数字强震动台网共有27个台站(包括2个基岩台站和25个土层台站)获得良好的主震加速度时程.利用这些加速度时程,选择汤峪台做为参考场地,基于考虑几何衰减的传统谱比法分析研究了25个土层场...     

2014年新疆于田MS7.3地震强地面运动模拟及震动图预测

基于地震动的时空衰减规律和传播特征,采用邻近地震监测台站地震动时程对1 km×1 km尺度的网格点进行近实时插值计算,同时结合场地效应对震区地震动参数进行修正,并以2014年2月12日新疆于田M_S7.3地震为例,计算震区格网内各点的地震时程,同时以8 s为时间间隔绘制出地震动峰值等值线图并将其连续播放,得到了于田M_S7.3地震峰值地震动(PGV,PGA)的空间分布.结果表明,于田县东部至民丰县北部地区受场地条件影响,震区震害在软弱地基土层及浅地下水位等因素作用下对震区地震动具有明显的放大效应,预测的地震动特征与现场宏观调查结果是一致的.在当前强震台网分布不均匀的情形下,本文方法能较好地描述震区地震动特征,较客观地反映灾区的强地面运动特征.      

Canyon topography effects on ground motion at Feitsui damsite

The free-field accelerograms along Feitsui Canyon are analyzed and modeled by a numerical scheme to study the effect of canyon topography. Since six strong-motion accelerometers (SC1–SC6) were deployed along the Feitsui Canyon in 1991; there are 14 earthquakes (4.9≤M_L≤6.6) recorded by these stations until June 1996, but only five triggered all six stations. The maximum PGA value is 68.6 cm s⁻² recorded at station SC1. According to the present data, the effect of the dam on the ground motions at canyon stations can be negligible. The amplitude of ground motion on the slopes of the canyon is bigger than that at its trough. The integral equation method is applied to a two dimensional model of Feitsui Canyon to study the effects of the canyon topography. We choose the ground motion of SC3 or SC4 station at the trough of the canyon as the input motion for the model, which is then used to predict the ground motion at the other five stations. Apart from the earthquake close to the damsite, the simple model can reproduce the observed accelerations at all frequencies below 4 Hz. Overall, the numerical method can well predict the ground motion along the canyon, although the high-frequency simulation is underestimated.     

A stratified core motion inferred from geomagnetic secular variations

An examination of the westward drift of the geomagnetic field indicates that the drift velocity is almost independent of latitude, suggesting a uniform rigid rotation of spherical shape. When the geomagnetic field is separated into standing and drifting components and expressed in a spherical harmonic series, a lack of sectorial terms is noted in the standing field. It is shown that these features are well explained by a stratified core model.The core is supposed to be stratified near the surface where toroidal fluid motions are predominant. In the deeper part, the fluid motion is two-dimensional, forming Taylor columns. A simplified core model is assumed to represent these features, in which the core is divided into two parts, an outer spherical shell that rotates westwards at a uniform rate of 0.3° y^?1 and a central sphere in which the two-dimensional columnar motions reside. The toroidal motions in the outer spherical interact with the dipole field to induce the drifting field, whereas the columnar motions generate the standing field through interaction with a toroidal field. It follows that a small velocity as 5 × 10^?3 cm s^?1 for the stratified motion is sufficient to create the observed drifting field.     

Estimation of strong ground motions from hypothetical earthquakes on the Cascadia subduction zone,Pacific Northwest

Strong ground motions are estimated for the Pacific Northwest assuming that large shallow earthquakes, similar to those experienced in southern Chile, southwestern Japan, and Colombia, may also occur on the Cascadia subduction zone. Fifty-six strong motion recordings for twenty-five subduction earthquakes ofM _s7.0 are used to estimate the response spectra that may result from earthquakesM _w<81/4. Large variations in observed ground motion levels are noted for a given site distance and earthquake magnitude. When compared with motions that have been observed in the western United States, large subduction zone earthquakes produce relatively large ground motions at surprisingly large distances. An earthquake similar to the 22 May 1960 Chilean earthquake (M _w 9.5) is the largest event that is considered to be plausible for the Cascadia subduction zone. This event has a moment which is two orders of magnitude larger than the largest earthquake for which we have strong motion records. The empirical Green's function technique is used to synthesize strong ground motions for such giant earthquakes. Observed teleseismicP-waveforms from giant earthquakes are also modeled using the empirical Green's function technique in order to constrain model parameters. The teleseismic modeling in the period range of 1.0 to 50 sec strongly suggests that fewer Green's functions should be randomly summed than is required to match the long-period moments of giant earthquakes. It appears that a large portion of the moment associated with giant earthquakes occurs at very long periods that are outside the frequency band of interest for strong ground motions. Nevertheless, the occurrence of a giant earthquake in the Pacific Northwest may produce quite strong shaking over a very large region.