大西洋中北部双频微地动特征

本文对大西洋中北部两侧五个地震台站2015年记录到的地震数据进行处理,计算噪声功率谱密度和概率密度函数,并通过极化分析对双频微地动不同周期的主导源区方位角分布进行了分析.研究结果显示:大西洋中北部台站双频微地动发生显著分裂,各台站的峰值周期各不同,且来自相同方向和不同方向的双频微地动都有可能产生双频微地动分裂;大西洋中...     

四川地区地震背景噪声特征分析

选取四川省数字测震台网2015年1月1日至2018年12月31日期间60个固定台站的三分量连续波形记录,计算了台站噪声加速度功率谱密度及相应的概率密度函数分布,统计了不同频率下的噪声功率谱密度值分布,对不同区域、不同频率下背景噪声水平的变化特征予以分析。结果表明:大部分地震台站的高频段噪声由于受到台站附近人为的、规律的作息生活和生产方式的影响,呈现明显的季节性变化和日变化,即夏季噪声水平升高,冬季降低,在农历春节期间达到全年最低值,地理空间分布特征不明显;第二类地脉动冬季噪声水平升高,夏季降低,季节性变化明显,平均变化为1—5 dB,且冬季峰值出现的频率向长周期方向移动1—2 s,呈现明显的地理空间分布特征,川东地区平均噪声水平最高,攀西地区次之,川西高原最低;与第二类地脉动相比,第一类地脉动观测到的噪声能量较弱,季节性变化不明显,地理空间分布的噪声水平差异明显减小;在20 s以上的长周期部分,台站噪声未呈明显的季节性和地理空间分布差异。此外,将地震计安置在山洞和井下,可以有效地降低台站周围干扰源、温度和压强对高频段和长周期观测的影响,噪声水平低于地表安装方式。      

Analysis of the primary and secondary microseisms in the wavefield of the ambient noise recorded in northern Poland

Seasonal changes of the primary and secondary microseisms were analysed in the wavefield of the ambient noise recorded during the entire 2014 at the “13 BB star” array located in northern Poland, composed of thirteen, symmetrically arranged, broadband seismic stations. To that, spectral analysis, seismic interferometry, surface scalar wind speed distribution, and beamforming were used. Spectral analysis allowed to observe that a splitting of the secondary microseism peak was present in winter and autumn, and that the primary microseism peak was visible in spring, summer and autumn. Using seismic interferometry, the long-term characteristics of the noise wavefield were recognized. The seasonal variations of the secondary microseism source were described by means of the analysis of the surface scalar wind speed for each month. The splitting of the secondary peak was attributed to the interaction of a strong wind blowing from the North Sea with a weak wind blowing from the Baltic Sea. The seasonal variations of the primary microseism peak were characterized through the frequency-domain beamforming. The peak was identified during spring, summer and autumn, when the generated wavefield was coming from the Baltic Sea. The velocity of the wavefield was evaluated within the 2.0–5.0 km/s range. The described mechanism of generation of the microseisms, based on the interaction of the nearby winds, was found to be consistent with the models reported in the literature.     

Ambient noise levels and characterization in Aegean region,Turkey

We assessed the ambient noise level in the Aegean region and analyzed its diurnal variation and its relation to the earthquake detection capability of the Aegean Region Seismic Network (ARSN). We prepared probability density functions (PDFs) for 19 broadband stations in the Aegean region operated by the Earth and Marine Sciences Institute (EMSI) of the Marmara Research Center (MRC) of the Turkish Scientific Research Council (TÜB?TAK). The power spectral densities (PSDs) used to construct PDFs for each station were computed for the periods between ~?0.02 and 180 s. In addition, we generated noise map of the Aegean region for different periods using the PDFs to assess the origin of the noise. We analyzed earthquake activity in the region and found that there are more local events recorded at night than during the day for each station. This difference is strongly related to diurnal variation of background noise level for the period range mostly covering the frequency range for the local events. We observed daytime noise level ~?15 to 20 dB higher than that at the nighttime in high frequencies for almost all stations caused by its proximity to settled areas and roads. Additionally, we observed a splitting peak within the Double Frequency (DF) microseism band; it showed a clear noise increase around the short period DF band at all the stations, decreasing inland. This peak may be related to sea waves locally generated in the Aegean Sea. We also identified a prominent increase related to marble saw companies in some stations’ noise PDFs.     

The forced and free response of the South China Sea to the large-scale monsoon system

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.     

Approximate coseismic displacements and near-field fling pulse from the Mw 6.5 30 October 2016 Central Italy earthquake

A study of the coseismic displacement and fling pulse recorded during the Mw 6.5 30 October 2016 Central Italy earthquake is presented. The near-field has been well documented, owing to the deployment of additional strong-motion stations following the earlier events of the 2016 Central Italy seismic sequence. As a result, there are numerous stations with evidence of coseismic displacement and fling pulse. In this study, 25 records with strike distance of less than 25 km and rupture distance under 28 km are considered. Approximate coseismic displacements have been recovered by a bilinear model to remove the low frequency noise in the records. The bilinear noise model uses two linear regression segments on the velocity trace to remove baseline offsets. After obtaining the coseismic displacement time series, the fling pulse period is examined. Existing methods of obtaining the fling pulse period are reviewed and a proposed algorithm is considered for automatic fling pulse detection. Both horizontal and vertical fling periods are obtained, unlike many studies which neglect the vertical fling. It is shown that the fling pulse period is highly variable (~?2–16 s) in the near-field region but exhibits some trends with various site-to-source distances.     

On the Problem of Deep Earthquakes in Crimea–Black Sea Region

It is a common opinion that only crustal earthquakes can occur in the Crimea–Black Sea region. Since the existence of deep earthquakes in the Crimea–Black Sea region is extremely important for the construction of a geodynamic model for this region, an attempt is made to verify the validity of this widespread view. To do this, the coordinates of all earthquakes recorded by the stations of the Crimean seismological network are reinterpreted with an algorithm developed by one of the authors. The data published in the seismological catalogs and bulletins of the Crimea–Black Sea region for 1970–2012 are used for the analysis. To refine the coordinates of hypocenters of earthquakes in the Crimea–Black Sea region, in addition to the data from stations of the Crimean seismological network, information from seismic stations located around the Black Sea coast are used. In total, the data from 61 seismic stations were used to determine the hypocenter coordinates. The used earthquake catalogs for 1970–2012 contain information on ~2140 events with magnitudes from–1.5 to 5.5. The bulletins provide information on the arrival times of P- and S-waves at seismic stations for 1988 events recorded by three or more stations. The principal innovation of this study is the use of the original author’s hypocenter determination algorithm, which minimizes the functional of distances between the points (X, Y, H) and (x, y, h) corresponding to the theoretical and observed seismic wave travel times from the earthquake source to the recording stations. The determination of the coordinates of earthquake hypocenters is much more stable in this case than the usual minimization of the residual functional for the arrival time of an earthquake wave at a station (the difference between the theoretical and observed values). Since determination of the hypocenter coordinates can be influenced by the chosen velocity column beneath each station, special attention is focused on collecting information on velocity profiles. To evaluate the influence of the upper mantle on the results of calculating the velocity model, two different low-velocity and high-velocity models are used; the results are compared with each other. Both velocity models are set to a depth of 640 km, which is fundamentally important in determining hypocenters for deep earthquakes. Studies of the Crimea–Black Sea region have revealed more than 70 earthquakes with a source depth of more than 60 km. The adequacy of the obtained depth values is confirmed by the results of comparing the initial experimental data from the bulletins with the theoretical travel-time curves for earthquake sources with depths of 50 and 200 km. The sources of deep earthquakes found in the Crimea–Black Sea region significantly change our understanding of the structure and geotectonics of this region.     

Velocity structure of the upper mantle of the East European platform according to seismic noise data

Cross-correlation functions of noise are constructed on 119 interstation paths from seismic noise records at stations of Eastern Europe. Dispersion curves of the group velocity of Rayleigh waves obtained from the cross-correlation functions are used for constructing the three-dimensional distribution of the velocity of transverse waves on the East European platform and in adjacent regions by methods of surface-wave tomography. The mean velocity in the crust is minimum in the region of the Caspian depression and Black Sea basin (<3.3 km/s) and maximum in the Baltic shield area (>3.7 km/s). The upper mantle beneath the Baltic and Ukrainian shields is characterized by increased velocity and the absence of the asthenospheric layer. Reduced velocities are noted in the upper mantle of the Black Sea basin. A low-velocity anomaly in the shape of a vertical column is revealed at depths of 200–300 km in the central part of the Dnieper-Donets aulacogen, which confirms the existence of a paleorift in this region.     

Regional characteristics of the effects of the El Niño-Southern Oscillation on the sea level in the China Sea

Based on coastal tide level, satellite altimetry, and sea surface temperature (SST) data of offshore areas of China’s coast and the equatorial Pacific Ocean, the regional characteristics of the effects of the El Niño-Southern Oscillation (ENSO) on the sea level in the China Sea were investigated. Singular value decomposition results show a significant teleconnection between the sea level in the China Sea and the SST of the tropical Pacific Ocean; the correlation coefficient decreases from south to north. Data from tide gauges along China’s coast show that the seasonal sea-level variations are significantly correlated with the ENSO. In addition, China’s coast was divided into three regions based on distinctive regional characteristics. Results obtained show that the annual amplitude of sea level was low during El Niño developing years, and especially so during the El Niño year. The ENSO intensity determined the response intensity of the annual amplitude of the sea level. The response region (amplitude) was relatively large for strong ENSO intensities. Significant oscillation periods at a timescale of 4–7 years existed in the sea level of the three regions. The largest amplitude of oscillation was 1.5 cm, which was the fluctuation with the 7-year period in the South China Sea. The largest amplitude of oscillation in the East China Sea was about 1.3 cm. The amplitude of oscillation with the 6-year period in the Bohai Sea and Yellow Sea was the smallest (less than 1 cm).     

Imaging heterogeneity of the crust adjacent to the Dead Sea fault using ambient seismic noise tomography

For the purpose of studying the Earth’s crust by means of tomography, we investigated cross-correlation functions emerging from long-term observations of propagating ambient seismic noise at pairs of broadband stations in Israel and Jordan. The data was provided by the eight permanent broadband stations of the Israel Seismic Network evenly distributed over Israel and the 30 stations of the DESERT2000 experiment distributed across the Arava Fault (South of the Dead Sea basin). To eliminate the influence of earthquakes and explosions, we have applied the methodology of Bensen et al. (Geophys J Int 169:1239–1260, 2007), including bandpass filtering and amplitude normalization in time and frequency domain. The cross-correlation functions estimated from continuous recordings of a few months were used to extract Rayleigh waves group velocity dispersion curves using automatic version of the frequency–time analysis procedure. Subsequently, these curves have been converted into the Rayleigh wave group velocity maps in the period range 5–20 s and S waves velocity maps in the depth range 5–15 km. In these maps, four velocity anomalies are prominent. Two of them are outlined by the previous reflection-refraction profiles and body wave tomography studies, i.e. a low velocity anomaly corresponds to the area of the extremely deep (down to 14 km) sedimentary infill in the Southern Dead Sea Basin and a high velocity anomaly in the Southern Jordan corresponds to the area of the Precambrian crystalline rocks of the Nubian Shield on the flanks of the Red Sea. The two other anomalies have not been reported before - the high velocity zone close to the Beersheba city and the low velocity anomaly in the region of Samaria-Carmel mountains - Southern Galilee. They have relatively low resolution and hence need further investigations for approving and contouring. The highest contrast between the average Rayleigh wave group velocity (2.7 km/s) and the anomalies is 10–13 %, comparable, however, to the level of noise in the data. The results have been verified by modeling the revealed anomalies which showed that all the four zones mentioned above could be detected by the tomography study.     

2013年芦山地震震源区地壳介质地震波速变化的特征分析

为探究芦山M7.0级地震后5年多来,震源区龙门山断裂带西南段介质波速的变化规律,本文基于2012年4月至2018年4月共6年的连续波形数据,运用移动窗互谱与频域偏振等分析方法,结合背景噪声源的特性,对不同深度范围内的相对波速变化以及震后的恢复过程与机制进行了研究.获得的主要认识包括：（1）年尺度而言,震源区周期为1~20 s的背景噪声场相对稳定,但成分复杂、2~10 s频带内至少存在2个能量相对稳定的噪声源;不同周期噪声的能量,在月变与季节性上的变化特征差异明显.（2）获得了长时间尺度、不同频带内介质相对波速的背景变化水平,1~2 s、2~4 s的波动幅度（约为±0.04%）与季节性变化规律强于4~10 s、10~20 s的,结合与降雨量相关的地下水位模型能很好地解释其变化规律.（3）震源区的同震波速降低现象清晰,降幅约为0.08%~0.1%;空间上,波速下降最为显著的区域主要集中在龙门山断裂带两侧约70 km范围内,其中四川盆地一侧平均约为0.1%,略高于青藏高原（0.08%）一侧;在断裂带内的降速不显著.对不同子频带进行测量的结果显示,震后除10~20 s外,其余3个子频带的相对波速在震后较短时间内（约20天左右）均出现较大幅度的波速降低现象,其中4~10 s的平均降速最大（约为0.08%）,分析认为主震及大量余震的松弛效应是引起介质波速下降的主要原因.（4）震后大约1年左右,波速变化基本恢复到震前水平,且至2018年4月前未观察到大幅的波速变化现象,总体上各频带内的结果均沿零线小幅波动.

     

Distribution of the low velocity bulk in the middle-to-lower crust of southern Tibet: implications for formation of the north–south trending rift zones

We conducted the ambient noise tomography to image the shallow crustal structure of southern Tibet. The 2D maps of phase velocity anomalies at the periods of 10–16 s show that the low velocities are mainly confined along or near some of the rift zones. While the maps at the periods of 18–25 s show that the coherent patterns that the low velocities expand outside of the rift zones. It means that the low velocities are prevailing in the middle crust of southern Tibet. According to the previous study of surface wave tomography with teleseismic data, we find that the low velocities from the lower crust to the lithospheric mantle are also restricted to the same rift zones. Thus, the integrated knowledge of the distribution of the low velocities in southern Tibet provides some new insight on the formation of the north–south trending rift zones. Compiling the multidiscipline evidences, we conclude that the rifting was an integrated process of the entire lithosphere in the early stage (~26–10 Ma), but mainly occurred within the upper crust due to the weakening a decoupling in the low velocity middle crust in the late stage (later than ~8 Ma).     

Temperature Trends and Changes in Rize,Turkey, for the Period 1975 to 2007

There is increasing evidence that the global climate is changing as a result of anthropogenic activity. Short‐term mean, maximum, and minimum temperatures of the city Rize located at the Eastern Black Sea Coast of Turkey were analyzed to reveal trends, change points, significant warming (cooling) periods, and trend rates per year. An increasing trend of approximately 1.27°C/33 years (α = 0.001) in the annual mean temperatures is found during the period from 1975 to 2007. Two periods, averaging 13.78 and 14.66°C, respectively, were detected from fluctuation in the annual mean temperatures. The trend of the first period (1975–1993) is towards a cooler climate, whereas the trend of the second period (1994–2007) is towards a warmer climate. Summer, autumn and, particularly, the spring mean temperatures have tended to increase strongly, whereas the winter mean temperatures have increased slightly over the whole period. For the winter mean temperature, the trend rate indicates a slight increase, which is insignificant. Maximum temperatures have dramatically increased with 1.61°C (α = 0.001) over the last 33 years. However, annual minimum temperatures have increased by 0.99°C (α = 0.01) over the same period.     

Periodic modulation of Pc3 and Pc4 pulsations in the polar cap by interplanetary and atmospheric processes

The variations in the daily average energy of geomagnetic pulsations and noise in the Pc3 (20–60 mHz) and Pc4 (10–19 mHz) frequency bands in the polar cap have been studied based on the data from P5 Antarctic station (corrected geomagnetic latitude ?87°) from November 1998 to November 1999. The daily average pulsation energy has been calculated using the method for detecting the wave packets, the spectral amplitude of which is higher than the threshold level, from the dynamic spectrum. A spectral analysis of the energy of pulsations and noise in the Pc3 and Pc4 bands, performed using the maximal entropy method, has revealed periodicities of 18 days in the local winter and 26, 13, and 7–9 days during the local summer. The simultaneous and coherent variations with periods of 26, 13, and 7–9 days in the solar wind velocity and IMF orientation indicate that the variations in the Pc3–4 wave energy in the polar cap at a sunlit ionosphere are mainly controlled by the parameters of the interplanetary medium. The variations in the Pc3–4 wave energy with a period of 18 days are observed only during the local winter and are supposedly related to the variations in the ionospheric conductivity modulated by planetary waves.     

Investigation of spatial and temporal variability of precipitation in Iran over the last half century

Investigation of the precipitation phenomenon as one of the most important meteorological factors directly affecting access to water resources is of paramount importance. In this study, the precipitation concentration index (PCI) was calculated using annual precipitation data from 34 synoptic stations of Iran over a 50-year period (1961–2010). The trend of precipitation and the PCI index were analyzed using the Mann–Kendall test after removing the effect of autocorrelation coefficients in annual and seasonal time scales. The results of zoning the studied index at annual time scale revealed that precipitation concentration follows a similar trend within two 25-year subscales. Furthermore, the PCI index in central and southern regions of the country, including the stations of Kerman, Bandarabbas, Yazd, Zahedan, Shahrekord, Birjand, Bushehr, Ahwaz, and Esfahan indicates a strong irregularity and high concentration in atmospheric precipitations. In annual time scale, none of the studied stations, had shown regular concentration (PCI < 10). Analyzing the trend of PCI index during the period of 1961–2010 witnessed an insignificant increasing (decreasing) trend in 16 (15) stations for winter season, respectively, while it faced a significant negative trend in Dezful, Saghez, and Hamedan stations. Similarly, in spring, Kerman and Ramsar stations exhibited a significant increasing trend in the PCI index, implying significant development of precipitation concentration irregularities in these two stations. In summer, Gorgan station showed a strong and significant irregularity for the PCI index and in autumn, Tabriz and Zahedan (Babolsar) stations experienced a significant increasing (decreasing) trend in the PCI index. At the annual time scale, 50 % of stations experienced an increasing trend in the PCI index. Investigating the changes in the precipitation trend also revealed that in annual time scale, about 58 % of the stations had a decreasing trend. In winter, which is the rainiest season in Iran, about 64 % of stations experienced a decreasing trend in precipitation that caused an increasing trend in PCI index. Comparing the spatial distribution of PCI index within two 25 years sub-periods indicated that the PCI index of the second sub-period increased in the spring time scale that means irregularity of precipitation distribution has been increased. But in the other seasons any significant variations were not observed. Also in the annual time scale the PCI index increased in the second sub-period because of the increasing trend of precipitation.     

Love and Rayleigh Wave Tomography of the Qinghai-Tibet Plateau and Surrounding Areas

Surface wave data were initially collected from events of magnitude Ms ≥ 5.0 and shallow or moderate focal depth occurred between 1980 and 2002: 713 of them generated Rayleigh waves and 660 Love waves, which were recorded by 13 broadband digital stations in Eurasia and India. Up to 1,525 source-station Rayleigh waveforms and 1,464 Love wave trains have been processed by frequency-time analysis to obtain group velocities. After inverting the path-averaged group times by means of a damped least-squares approach, we have retrieved location-dependent group velocities on a 2° × 2°-sized grid and constructed Rayleigh- and Love-wave group velocity maps at periods 10.4–105.0 s. Resolution and covariance matrices and the rms group velocity misfit have been computed in order to check the quality of the results. Afterwards, depth-dependent SV- and SH-wave velocity models of the crust and upper mantle are obtained by inversion of local Rayleigh- and Love-wave group velocities using a differential damped least-squares method. The results provide: (a) Rayleigh- and Love-wave group velocities at various periods; (b) SV- and SH-wave differential velocity maps at different depths; (c) sharp images of the subducted lithosphere by velocity cross sections along prefixed profiles; (d) regionalized dispersion curves and velocity-depth models related to the main geological formations. The lithospheric root presents a depth that can be substantiated at ~140 km (Qiangtang Block) and exceptionally at ~180 km in some places (Lhasa Block), and which exhibits laterally varying fast velocity very close to that of some shields that even reaches ~4.8 km/s under the northern Lhasa Block and the Qiangtang Block. Slow-velocity anomalies of 7–10% or more beneath southern Tibet and the eastern edge of the Plateau support the idea of a mechanically weak middle-to-lower crust and the existence of crustal flow in Tibet.     

Variations in the total electron content during the powerful typhoon of August 5–11, 2006, near the southeastern coast of China

The variations in the total electron content (TEC), obtained from the data of 11 ground-based GPS stations in the region (5°S–80°N; 110–160°E) in the period August 2–15, 2006, have been analyzed in order to search for possible ionospheric manifestations of the SAOMAI powerful typhoon (August 5–11, 2006) near the south-eastern coast of China. The global TEC maps (GIM) have also been used. In the region of the typhoon action during the magnetic storm of August 7, 2006, an intensification of the TEC variations in the evening local time within the 32–128 min periods range was detected. However, this effect was most probably caused by the dynamics of the irregular structure of the equatorial anomaly and by the disturbed geomagnetic situation (Kp ～ 3–6, Dst varied from ?74 to ?153 nT). The analysis of the diurnal variations in the absolute values of TEC and TEC variations with periods of 2–25 min did not reveal a substantial increase in the intensity and changes in the spectrum of the TEC variations in the period of typhoon action as compared to the adjacent days. Thus, we failed to detect ionospheric disturbances unambiguously related to the SAOMAI typhoon.     

Spectral Structure of Temperature Variations in the Midlatitude Mesopause Region

Long-term series of midnight temperature in the mesopause region have been obtained from spectral observations of hydroxyl airglow emission (OH(6-2) λ840 nm band) at the Tory station (52° N, 103° E) in 2008–2016 and Zvenigorod (56° N, 37° E) station in 2000–2016. On their basis, the Lomb-Scargle spectra of the variations in the period range from ~12 days to ~11 years have been determined. Estimates of the amplitudes of statistically significant temperature fluctuations are made. The dominant oscillations are the first and second harmonics of the annual variation, the amplitudes of which are 23–24 K and 4–7 K, respectively. The remaining variations, the number of which was 16 for the Tory and 22 for Zvenigorod stations, have small amplitudes (0.5–3 K). Oscillations with combinational frequencies, which arise from modulation of the annual variation harmonics, are observed in a structure of the variation spectra in addition to interannual oscillations (periods from ~2 to ~11 years) and harmonics of the annual variation (up to its tenth harmonic).     

On the use of the autocorrelation function: the constraint of using frequency band-limited signals for monitoring relative velocity changes

Correlations of seismic noise are commonly used to monitor temporal variations of relative seismic velocity in period ranges from 1 s up to 100 s. Of particular interest is the detection of small changes in the order of 0.01–0.1 % in propagation speeds. Measuring such small differences can, however, be significantly biased by temporal variations in the properties of the noise sources within the corresponding frequency band. Using synthetic data, we show that apparent relative velocity variations might appear only due to changes in the amplitude and frequency content caused by source variations. Removing such unwanted effects by applying narrow bandpass filters in the preprocessing restricts the high-resolution analysis of any signal due to Gabor’s uncertainty limit, i.e., the correlation function suffers a limited resolution to time delay estimates for small correlation times, low-frequency ranges, and in narrow frequency bands. Better understanding of spatiotemporal noise source properties and the theoretical limitations of time–frequency analysis is critical for accurate and reliable passive monitoring.     

Co-seismic deformation derived from GPS observations during April 20th, 2013 Lushan Earthquake,Sichuan, China

We process the standard 30 s, static GPS data and the 1 s, high-rate GPS (HRGPS) data provided by the Crustal Movement Observation Network of China with GAMIT/GLOBK software package, and obtain the co-seismic displacements of near field and far field, and the epoch-by-epoch time series of HRGPS during Lushan earthquake. GPS data from about 20 sites in Sichuan province, which located between 40 and 450 km from the epicenter, are analyzed so as to study the characteristics of the static displacements and the dynamic crustal deformations, with periods ranging from several minutes to over a month. The result shows that: the static displacements caused by Lushan earthquake are limited to several centimeters; the nearest station SCTQ at 43 km from the epicenter has the largest static displacement of about 2 cm, while the other stations generally have insignificant displacements of less than 5 mm. the stations in the east of Sichuan–Yunnan region shifts 5–10 mm toward the southwest, and the stations in the middle-west of Sichuan Basin moves indistinctively 1–2 mm toward the northwest; station SCTQ has the largest kinematic displacement of about 4 and 3 cm peak-to-peak on the north and east component, respectively, and is much greater than the static permanent displacement; for the stations located at a distance greater than 150 km from the epicenter, the kinematic motions are generally insignificant; exceptionally, station SCNC and station SCSN in central Sichuan Basin have significant kinematic motions although they are more than 200 km away from the epicenter.