Slowness–azimuth corrections of teleseismic events for IMS primary arrays in China

The seismic arrays at Hailar (HILR) and at Lanzhou (LZDM) in China are both primary stations of the International Monitoring System for verifying compliance with the Comprehensive Nuclear Test Ban Treaty. These two stations became operational in 2002 and have since then provided continuous data. In this study, the so-called slowness–azimuth station corrections (SASC) were derived and used to improve the location accuracy of the two arrays. The SASC are found by comparing the back-azimuth and slownesses obtained from array processing to the theoretical values calculated from the reported event locations and the corresponding seismic velocity model. Events reported by the National Earthquake International Center in the time period 2002 to 2006 were used as reference events, and the IASP91 was used as the theoretical velocity model. Small correction vectors with random orientation were found for HILR. Larger correction vectors with systematic vector biases were found for LZDM. The LZDM correction vectors seem to point to the same direction in a large part of the slowness space and may be attributed to local structure. After introducing the SASC for HILR, the standard deviations of back-azimuth and slowness residuals drop from 7.1° to 4.6° and from 1.0 to 0.6 s/°, respectively. For LZDM, these values drop from 22.3° to 10.2° and from 2.9 to 1.1 s/°, respectively. The variations of back-azimuth and slowness residuals were reduced by 32% and 30.2%, respectively, for HILR after SASC and the reductions were 21% and 40.2% for LZDM. The improvements were 77% in back-azimuth and 67% in slowness location for HILR and were 79% and 81% for LZDM after SASC. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Earthquake Location Accuracies in Norway Based on a Comparison between Local and Regional Networks

—?Detailed studies of the low to intermediate seismicity in two coastal regions of Norway have been used in a comparison between earthquake locations from local high-precision networks on the one side and locations using a sparse regional array network on the other side. To this end, a reference set of 32 low-magnitude earthquakes have been located using two local temporary networks in northern and western Norway, with estimated epicenter accuracies better than 5 and 10?km, respectively. Comparisons are made between the local network solutions and the NORSAR Generalized Beamforming (GBF) system, which provides automatic phase association and location estimates using the Fennoscandian regional array network. The median automatic GBF location error is of the order of 20–30?km when four or more arrays detect the event, increasing to about 80–100?km when only two arrays are available, and the automatic GBF bulletin is essentially complete down to magnitude M_L = 2.0. Most of the mislocation vectors of the NORSAR GBF solutions are oriented perpendicular to the Norwegian coast, and with a tendency to pull the location in a southeasternly direction. The GBF performance is clearly better, both in terms of accuracy and completeness, than the performance of the automatic bulletin of the Prototype International Data Center (PIDC) which uses data from essentially the same network. The analyst reviewed NORSAR and PIDC bulletins show, not unexpectedly, an improvement in location accuracy compared to the automatic solutions and appear to be of similar quality for the few common events, with an average mislocation of about 20?km. The NORSAR reviewed bulletin is more complete at low magnitudes compared to PIDC, and there appears to be a potential for significant improvements in the PIDC processing of small seismic events in this region.     

The Applicability of Incoherent Array Processing to IMS Seismic Arrays

The seismic arrays of the International Monitoring System (IMS) for the Comprehensive Nuclear-Test-Ban Treaty (CTBT) are highly diverse in size and configuration, with apertures ranging from under 1 km to over 60 km. Large and medium aperture arrays with large inter-site spacings complicate the detection and estimation of high-frequency phases lacking coherence between sensors. Pipeline detection algorithms often miss such phases, since they only consider frequencies low enough to allow coherent array processing, and phases that are detected are often attributed qualitatively incorrect backazimuth and slowness estimates. This can result in missed events, due to either a lack of contributing phases or by corruption of event hypotheses by spurious detections. It has been demonstrated previously that continuous spectral estimation can both detect and estimate phases on the largest aperture arrays, with arrivals identified as local maxima on beams of transformed spectrograms. The estimation procedure in effect measures group velocity rather than phase velocity, as is the case for classical f–k analysis, and the ability to estimate slowness vectors requires sufficiently large inter-sensor distances to resolve time-delays between pulses with a period of the order 4–5 s. Spectrogram beampacking works well on five IMS arrays with apertures over 20 km (NOA, AKASG, YKA, WRA, and KURK) without additional post-processing. Seven arrays with 10–20 km aperture (MJAR, ESDC, ILAR, KSRS, CMAR, ASAR, and EKA) can provide robust parameter estimates subject to a smoothing of the resulting slowness grids, most effectively achieved by convolving the measured slowness grids with the array response function for a 4 or 5 s period signal. Even for medium aperture arrays which can provide high-quality coherent slowness estimates, a complementary spectrogram beampacking procedure could act as a quality control by providing non-aliased estimates when the coherent slowness grids display significant sidelobes. The detection part of the algorithm is applicable to all IMS arrays, with spectrogram-based processing offering a potential reduction in the false alarm rate for high-frequency signals. Significantly, the local maxima of the scalar functions derived from the transformed spectrogram beams are robust estimates of the signal onset time. High-frequency energy is of greater importance for lower event magnitudes and in the cavity decoupling detection evasion scenario. There is a need to characterize both propagation paths with low attenuation of high-frequency energy and situations in which parameter estimation on array stations fails.     

Slowness Corrections — One Way to Improve IDC Products

—?The first step to identify and locate a seismic event is the association of observed onsets with common seismic sources. This is especially important in the context of monitoring the Comprehensive Nuclear-Test-Ban Treaty (CTBT) at the International Data Center (IDC) being developed in Vienna, Austria. Well-defined slowness measurements are very useful for associating seismic phases to presumed seismic events.¶Shortly after installation of the first seismic arrays, systematic discrepancies between measured and theoretically predicted slowness values were observed, and therefore slowness measurements of seismic stations should be calibrated. The observed slownesses measured with small aperture arrays, some of which will be included in the International Monitoring System (IMS) now being implemented for verifying compliance with the CTBT, show large scatter and deviations from theoretically expected values. However, in this study a method is presented, by which mean slowness corrections can be derived, which show relatively stable patterns specific to each array.¶The correction of measured slowness values of these arrays clearly improved the single array location capabilities. Applying slowness corrections with seismic phases observed by ARCES, FINES, GERES, and NORES, and associated to seismic events in the bulletins of the prototype International Data Center (pIDC) in Arlington, VA, also clearly demonstrates the advantages of these corrections. For arrays with large slowness deviations that are due to the influence of a dipping layer, the corrections were modeled with a sine function depending on the measured azimuth. In addition, the measured values can be weighted with the corresponding uncertainties known from the process of deriving the mean corrections.     

利用人工爆破资料研究福建一维P波速度结构和地震定位

通过人工爆破资料研究地球结构的独特优点是震源时间和位置精确知道.2010—2012年间福建省进行了一系列的爆破实验.本文利用手工拾取来自省地震台网记录的爆破地震初至Pg、Pn以及续至Pg波到时数据,采用联合反演方法构建了新的一维P波速度模型,即福建爆破模型(FJEM).与华南模型相比,FJEM模型对走时的拟合程度提高了45%,有明显改善.利用不同爆破地震数据组合得到稳定类似的福建地区一维速度模型,显示福建地区存在较简单的一维速度结构.对爆破地震的重定位显示传统使用的华南模型在福建地区具有较小的水平定位误差(平均0.52±0.45km),但存在较大深度误差(平均4.7±1.2km).FJEM模型表现出与华南模型相似的水平定位能力,但是震源深度误差更小(1.3±1.1km).对基于FJEM模型的合成天然地震目录的重定位,华南模型显示出相似的定位结果:(1)台站方位覆盖较好的福建中部地区的水平定位误差小;(2)台站方位覆盖差的福建海岸及海峡区域水平定位误差大;(3)震源深度误差则跟台站数目及方位分布没有明显的关系,而是与发震时间误差有互易关系.从中可以看出,地震水平定位误差基本上受台站方位覆盖影响,而受参考速度模型影响不大;而在深度方面,本文改进的FJEM模型不仅更加接近真实的速度结构(拟合走时更好)而且也减小了深度误差.因此建议在福建及其邻近区域的日常定位中用FJEM模型替代华南模型.     

Comment on “Slowness-azimuth corrections of teleseismic events for IMS primary arrays in China” by Hao and Zheng

Knowledge about backazimuth and slowness deviations at seismic arrays can be used as a tool to study subsurface lateral heterogeneity and improve the ability to locate events. Recently, Hao and Zheng (J Seismol 13:437–448, 2009) estimated the backazimuth and slowness deviations for teleseismic P waves recorded by the HILR array and the LZDM array using f–k analysis. They attributed the significant deviations at the LZDM array to dipping structures beneath the array. However, another possible factor, namely the altitude variations of array elements, was not taken into consideration during the slowness estimation process. For the LZDM array, the maximum altitude difference is ~15% of the array aperture and not negligible. In this study, we made some numerical experiments to investigate the difference between the estimated and theoretical slowness vectors when ignoring the altitude difference. The results reveal that remarkable artificial slowness shift is produced. Assuming a P-wave velocity of 5.4 km/s immediately beneath the array, the magnitude of slowness shift increases from 1.4 to 2.2 s/° when the theoretical slowness decreases from 16 to 4 s/°. For a 10° emergence angle, the backazimuth deviation reaches nearly 40°, and the relative slowness deviation can be greater than 60%. It is also shown that ignoring the altitude difference gives rise to a northeastward slowness shift, opposite to the southwestward shift proposed by Hao and Zheng, suggesting that they have heavily underestimated the slowness residuals at the LZDM array. Note that the elevation of one of the array stations is much lower than others. Avoiding the use of this station, the elevation variation range of array stations decreases by nearly one half, and the artificial backazimuth and slowness deviations decrease by more than one half.     

Locating Seismic Events in the CTBT Context

—?The verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) requires the determination of accurate location of seismic events from a fixed network of seismic stations across the globe. The requirements of possible on-site inspections mean that the goal is to place the location estimate in a zone smaller than 1000 km² that includes the true location. Because a defined set of stations will be used, corrections can be refined to represent the influence of departures from the global reference model IASPEI91. The primary stations in the International Monitoring Scheme (IMS) are mostly seismic arrays and therefore the present location scheme is based on minimisation of a misfit function built from arrival time, azimuth and array slowness residuals. The effective network will change markedly with the magnitude of the event and as a result regional information has to be integrated into the location process.     

Precise Relative Location of 25-ton Chemical Explosions at Balapan Using IMS Stations

—?We test how well low-magnitude (m _bLg 1.8 to 2.6), 25-ton chemical explosions at Balapan, Kazakhstan, can be located using IMS stations and standard earth models, relying on precisely determined relative arrival times of nearly similar, regional and teleseismic waveforms. Three 1997 Balapan explosions were recorded by a number of currently reporting and surrogate IMS stations. Three regional stations and two teleseismic arrays yielded consistent waveforms appropriate for relative picking. Master-event locations based on the AK135 model and ground-truth information from the first, shallowest and best-recorded explosion, fell under 1 km from known locations, for depths constrained to that of the master event. The resulting 90% confidence ellipses covered 12–13?km² and contained the true locations; however, results for depth constrained to true depth were slightly less satisf actory. From predictions based on ground truth, we found a P _g -coda phase at Makanchi, Kazakhstan to be misidentified and poorly modeled. After accounting for this, 90% ellipses shrank to 2–3?km² and true-depth mislocation vectors became more consistent with confidence-ellipse orientations. These results suggest that a high level of precision could be provided by a tripartite array of calibration shots in cases where models are poorly known. We hope that the successful relocation of these small Balapan shots will support the role of calibration explosions in verification monitoring and special event studies, including on-site inspection.     

小江断裂带北段地壳浅层P波速度各向 异性观测的原理与数值试验

以二维情形下观测速度场为各向同性场和各向异性场的叠加为前提, 提出了一种利用走时残差估算地震波速度各向异性的方法, 即剩余慢度矢量法. 利用小江断裂带北段巧家流动地震台阵24个台站记录的3181次地震事件的P波走时残差, 采用剩余慢度矢量法计算了各观测台站周围水平方向上尺度为0.5°×0.5°, 震源深度为0—5 km的剩余慢度矢量, 由此得到了P波快波和慢波方向. 计算结果表明, 大部分观测台站周围的P波速度方向性较为一致, 快波方向为ESE向, 慢波方向为NNE向. 快波方向与小江断裂带北段应力场P轴方向较为一致, 而慢波方向与应力场T轴方向一致, 表明应力的长期作用可能是导致P波速度各向异性的重要原因.      

Detection and Analysis of Near-Surface Explosions on the Kola Peninsula

Seismic and infrasonic observations of signals from a sequence of near-surface explosions at a site on the Kola Peninsula have been analyzed. NORSAR’s automatic network processing of these events shows a significant scatter in the location estimates and, to improve the automatic classification of the events, we have performed full waveform cross-correlation on the data set. Although the signals from the different events share many characteristics, the waveforms do not exhibit a ripple-for-ripple correspondence and cross-correlation does not result in the classic delta-function indicative of repeating signals. Using recordings from the ARCES seismic array (250 km W of the events), we find that a correlation detector on a single channel or three-component station would not be able to detect subsequent events from this source without an unacceptable false alarm rate. However, performing the correlation on each channel of the full ARCES array, and stacking the resulting traces, generates a correlation detection statistic with a suppressed background level which is exceeded by many times its standard deviation on only very few occasions. Performing f-k analysis on the individual correlation coefficient traces, and rejecting detections indicating a non-zero slowness vector, results in a detection list with essentially no false alarms. Applying the algorithm to 8 years of continuous ARCES data identified over 350 events which we confidently assign to this sequence. The large event population provides additional confidence in relative travel-time estimates and this, together with the occurrence of many events between 2002 and 2004 when a temporary network was deployed in the region, reduces the variability in location estimates. The best seismic location estimate, incorporating phase information for many hundreds of events, is consistent with backazimuth measurements for infrasound arrivals at several stations at regional distances. At Lycksele, 800 km SW of the events, as well as at ARCES, infrasound is detected for most of the events in the summer and for few in the winter. At Apatity, some 230 km S of the estimated source location, infrasound is detected for most events. As a first step to providing a Ground Truth database for this useful source of infrasound, we provide the times of explosions for over 50 events spanning 1 year.     

Ground truth seismic events and location capability at Degelen mountain, Kazakhstan

We utilized nuclear explosions from the Degelen Mountain sub-region of the Semipalatinsk Test Site (STS), Kazakhstan, to assess seismic location capability directly. Excellent ground truth information for these events was either known or was estimated from maps of the Degelen Mountain adit complex. Origin times were refined for events for which absolute origin time information was unknown using catalog arrival times, our ground truth location estimates, and a time baseline provided by fixing known origin times during a joint hypocenter determination (JHD). Precise arrival time picks were determined using a waveform cross-correlation process applied to the available digital data. These data were used in a JHD analysis. We found that very accurate locations were possible when high precision, waveform cross-correlation arrival times were combined with JHD. Relocation with our full digital data set resulted in a mean mislocation of 2 km and a mean 95% confidence ellipse (CE) area of 6.6 km² (90% CE: 5.1 km²), however, only 5 of the 18 computed error ellipses actually covered the associated ground truth location estimate. To test a more realistic nuclear test monitoring scenario, we applied our JHD analysis to a set of seven events (one fixed) using data only from seismic stations within 40° epicentral distance. Relocation with these data resulted in a mean mislocation of 7.4 km, with four of the 95% error ellipses covering less than 570 km² (90% CE: 438 km²), and the other two covering 1730 and 8869 km² (90% CE: 1331 and 6822 km²). Location uncertainties calculated using JHD often underestimated the true error, but a circular region with a radius equal to the mislocation covered less than 1000 km² for all events having more than three observations.     

Velocities of earthquake-generated Pn waves in Japan measured by the time-term method

Velocities of upper-mantle P waves (P_n) generated by large shallow earthquakes in the Japan area have been studied by the time-term method in three areas of the Japanese islands. Data reported from routinely recording seismic stations, grouped to act as elongated arrays, were used. The velocities observed correspond to the P-wave velocity in the top layer of the mantle. For a region in southwestern Japan not spanned by active volcanoes the value obtained was 8.0 km/s, while for central Japan and on northern Honshu Island the corresponding values were as low as 7.6–7.7 km/s. After allowance for the influence of systematic earthquake mislocation, the probable error of the measurements is estimated not to exceed 0.1 km/s. The differences in P-wave velocity are attributable to the regional variation of subcrustal temperature in Japan reported by Uyeda and Horai, with high temperatures and low velocities in the volcanic zone.The time terms tend to be larger on the Japan Sea side than on the Pacific side of the Japanese islands. We suggest that this is due to an increase in P velocity in the mantle towards the Pacific, perpendicular to our profiles of measurement.     

Use of GSETT-3 gamma data in the Slowness-Azimuth Calibration of IMS primary arrays at regional distances

For the purpose of verifying compliance with the CTBT seismic monitoring is one of the four techniques used by the IDC. In order to improve the accuracy of the automatic and the reviewed bulletin epicenter locations the IDC uses SASC for the IMS seismic stations. SASC determination is a straightforward calculation done by comparing for selected events the azimuth and slowness from the waveform processing using array techniques to the theoretical values based on the event locations and the velocity model.The main problem, however, is to build a set of reference events, whose locations are accurate enough and not based on information from the stations to be calibrated. A reference event list assumed to meet this requirement is the Gamma bulletin, which was collected since 1993 and was compiled in the framework of the GSETT-3. In this work calculation of SASC for regional to teleseismic distances (up to 30 degrees) was performed for 11 IMS primary arrays. The calculation was done using Pg, Pn, P, Sg, Sn, and S phases based on the detection list obtained from the pIDC and the Gamma bulletin for 6 years (1993–1999). The number of Gamma events varies from several hundreds for some arrays (BRAR) to several tens of thousands for others (i.e. ARCES, ILAR). Due to the fact that the Gamma bulletin is purely voluntary, the coverage is non-uniform both in time and in space and the location accuracy is non-uniform. This drawback can be overcome by encouraging signatory states to submit quality Gamma bulletin data to the IDC. The work presented here can be used as a routine procedure for improving IMS array performance, especially at regional distances.     

Gradients and phase velocities of ULF geomagnetic disturbances used to determine the source of an impending strong earthquake

Results of studying the behavior of the vectors of gradients and phase velocities of ULF geomagnetic disturbances (F < 1 Hz) in the Japan seismic zone are presented. The gradient and phase velocity vectors along the Earth’s surface were determined using data of the group of three high-sensitivity three-component magnetovariation stations (MVC-3DS) located at triangle vertices at a small (～5 km) distance from one another (magnetic gradiometer). Two such groups of stations were installed in 1999 southwest and southeast of Tokyo at a distance of ～150 km from each other. It has been indicated that, several months before strong earthquakes (M > 5), the values of gradients and phase velocities start anomalously changing, and directions toward sources of impending strong earthquakes appear in the distribution of gradient vector directions. Directions from sources of impending earthquakes appear in the distribution of phase velocity vector directions. It is proposed to use gradients and phase velocities of ULF and ELF geomagnetic disturbances as one of the factors in a short-term prediction of strong earthquakes.     

基于三分向台站波形的重复地下爆炸相关检测

在将相关检测方法应用于三分向台站记录数据时不能采用台阵数据检测时所使用的基于相关系数束慢度估计的虚假触发筛查方法来控制误检测.为此,本文根据重复事件的震中位置本身固定,各台站记录到的重复事件信号之间的到时差与相应参考事件的信号到时差基本相同的特性,根据两个以上三分向台站的相关检测触发到时差筛查相关检测虚假触发,从而解决了相关检测方法在应用于三分向台站数据时虚假触发过多的问题.利用新疆的三个三分向台站一个月的连续数据对该方法进行测试的结果表明,该方法能在接近零误检率的情况下对重复地下爆炸进行检测.     

Seismic detections of the 15 February 2013 Chelyabinsk meteor from the dense ChinArray

Chin Array is a dense portable broadband seismic network to cover the entire continental China, and the Phase I is deployed along the north-south seismic belt in southwest China. In this study, we analyze seismic data recorded on the Chin Array following the February 15,2013 Chelyabinsk(Russia) meteor. This was the largest known object entering the Earth's atmosphere since the1908 Tunguska meteor. The seismic energy radiated from this event was recorded by seismic stations worldwide including the dense Chin Array that are more than 4000 km away. The weak signal from the meteor event was contaminated by a magnitude 5.8 Tonga earthquake occurred *20 min earlier. To test the feasibility of detecting the weak seismic signals from the meteor event, we compute vespagram and perform F-K analysis to the surface-wave data. We identify a seismic phase with back azimuth(BAZ) of 329.7° and slowness of 34.73 s/deg, corresponding to the surface wave from the Russian meteor event(BAZ *325.97°). The surface magnitude(MS) of the meteor event is 3.94 ± 0.18. We also perform similar analysis on the data from the broadband array F-net in Japan, and find the BAZ of the surface waves to be316.61°. With the different BAZs of Chin Array and F-net,we locate the Russian meteor event at 58.80°N, 58.72°E.The relatively large mislocation(*438 km as compared with 55.15°N, 61.41°E by others) may be a result of thebending propagation path of surface waves, which deviates from the great circle path. Our results suggest that the dense Chin Array and its subarrays could be used to detect weak signals at teleseismic distances.     

Locating Small Aftershocks Using a Small-Aperture Temporary Seismic Array

In this paper, we developed a specialized method to locate small aftershocks using a small-aperture temporary seismic array. The array location technique uses the first P arrival times to determine the horizontal slowness vector of the incoming P wave, then combines it with S–P times to determine the event location. In order to reduce the influence of lateral velocity variation on the location determinations, we generated slowness corrections using events well-located by the permanent broadband network as calibration events, then we applied the corrections to the estimated slownesses. Applications of slowness corrections significantly improved event locations. This method can be a useful tool to locate events recorded by temporary fault-zone arrays in the near field but unlocated by the regional permanent seismic network. As a test, we first applied this method to 64 well-located aftershocks of the 1992 Landers, California, earthquake, recorded by both the Caltech/USGS Southern California Seismic Network and a small-aperture, temporary seismic array. The average horizontal and vertical separations between our locations and the well-determined catalogue locations are 1.35 and 1.75 km, respectively. We then applied this method to 132 unlocated aftershocks recorded only by the temporary seismic array. The locations show a clear tendency to follow the surface traces of the mainshock rupture.     

Best Practice in Earthquake Location Using Broadband Three-component Seismic Waveform Data

—?We present an earthquake location algorithm, the Broadband Waveform Regional Earthquake Location Program (BW_RELP), which utilizes phase onset times and wave azimuths recorded by three-component broadband seismic stations and an adaptive migrating grid search algorithm to find the global minimum in an arbitrary normed misfit parameter. The performance of BW_RELP is demonstrated using regional (300–800?km distant) broadband recordings to locate events in the 1995 Ridgecrest, California earthquake sequence. The purpose of this study is to introduce the BW_RELP algorithm in detail and to expand on the previous paper by Deger et?al. (BSSA, 88, 1353–1362, 1998), using one Berkeley Digital Seismic Network (BDSN) station (YBH) and two USNSN stations (ELK and MNV) which span 300–800?km in distance and 55 degrees in azimuth, to further investigate the capability of a sparse broadband network of three-component stations at monitoring a region located outside of the network, as will be the case in the monitoring of the Comprehensive Test-Ban-Treaty (CTBT) for low magnitude seismic events. We assess the capability of this sparse three-station broadband network and we compare locations estimated from phase onset time and wave azimuth measurements to a ground-truth catalog of high-quality earthquake locations derived from data recorded by the Southern California Seismic Network (SCSN). The results indicate that in the regional distance range it is possible, when an appropriate calibration event is available, to obtain absolute event locations to within 18?km as is prescribed by the CTBT.     

A Scheme for Initial Beam Deployment for the International Monitoring System Arrays

v--vThe International Monitoring System (IMS) includes a diverse set of seismic arrays with different configurations. These configurations have apertures ranging from less than 1 to more than 25 km and minimum interelement spacings varying from 0.1 to 3.6 km. This paper presents a scheme for initial beam deployment for this variety of seismic arrays. Beamforming is equivalent to a spatiotemporal bandpass filter of which passband is defined by the minimum and maximum wavenumbers, which are functions of the geometry configuration of the array. Deployment of steered-beams for signal detection is based on the wavenumber resolution of the array, slowness and frequency distributions of seismic phases, and coherence properties of seismic signals and noises among sensors. Within the wavenumber passband, all possible slowness values are determined by the resolution for each frequency band, and those that are outside the range of seismological interest are excluded. The appropriate azimuthal distribution for each selected slowness is determined from the azimuthal resolution. Using this approach, detection beams for each array are rationally deployed in the slowness-azimuth and frequency domain.