朝鲜2009年和2013年两次核爆的地震学特征对比研究

利用布设在长白山地区临时地震台站接收到的朝鲜核爆的波形资料,对2009年5月25日和2013年2月12日两次朝鲜核试验的地震学特征进行比较.震中距范围从145km到420km.采用P/S型谱比值方法识别朝鲜核爆,通过与2009年3月20日长春地震和2013年1月23日沈阳地震事件的比较,表明在频率大于3 Hz时P/S型谱比值能够有效识别发生在中朝边境地区的地下核试验.选定参考台站,利用区域震相Pg波的振幅谱比值计算朝鲜核爆至各台站路径上的相对衰减.结合介质速度模型,在一定程度上反映了长白山地区衰减情况,为进一步研究长白山地区衰减层析成像提供初始模型.     

朝鲜核试验相关地震学研究进展及其文献计量学分析

朝鲜自2006年10月9日第一次开展地下核试验以来,分别于2009年5月25日、2013年2月12日、2016年1月6日、2016年9月9日和2017年9月3日相继进行了5次规模较大的核试验.由于核爆炸和天然地震的震源机制不同,可以通过核爆炸产生的地震波来进行核试验的监测,核试验相关地震学研究一直是国内外专家关注的焦点.本文分别从事件定位、性质识别、当量和埋藏深度等几个方面总结了近些年来朝鲜核试验相关地震学的研究进展,并基于文献计量学方法对朝鲜核试验相关地震学研究现状进行分析,综合结果表明,近些年基于朝鲜核试验的相关地震学研究的主要研究方向为核试验定位、当量估算以及震源深度等.     

A Study of Small Magnitude Seismic Events During 1961–1989 on and near the Semipalatinsk Test Site, Kazakhstan

—?Official Russian sources in 1996 and 1997 have stated that 340 underground nuclear tests (UNTs) were conducted during 1961–1989 at the Semipalatinsk Test Site (STS) in Eastern Kazakhstan. Only 271 of these nuclear tests appear to have been described with well-determined origin time, coordinates and magnitudes in the openly available technical literature. Thus, good open documentation has been lacking for 69 UNTs at STS.¶The main goal of our study was to provide detections, estimates of origin time and location, and magnitudes, for as many of these previously undocumented events as possible. We used data from temporary and permanent seismographic stations in the former USSR at distances from 500?km to about 1500?km from STS. As a result, we have been able to assign magnitude for eight previously located UNTs whose magnitude was not previously known. For 31 UNTs, we have estimated origin time an d assigned magnitude — and for 19 of these 31 we have obtained locations based on seismic signals. Of the remaining 30 poorly documented UNTs, 15 had announced yields that were less than one ton, and 13 occurred simultaneously with another test which was detected. There are only two UNTs, for which the announced yield exceeds one ton and we have been unable to find seismic signals.¶Most of the newly detected and located events were sub-kiloton. Their magnitudes range from 2.7 up to 5.1 (a multi-kiloton event on 1965 Feb. 4 that was often obscured at teleseismic stations by signals from an earthquake swarm in the Aleutians).¶For 17 small UNTs at STS, we compare the locations (with their uncertainties) that we had earlier determined in 1994 from analysis of regional seismic waves, with ground-truth information obtained in 1998. The average error of the seismically-determined locations is only about 5?km. The ground-truth location is almost alw ays within the predicted small uncertainty of the seismically-determined location.¶Seismically-determined yield estimates are in good agreement with the announced total annual yield of nuclear tests, for each year from 1964 to 1989 at Semipalatinsk.¶We also report the origin time, location, and seismic magnitude of 29 chemical explosions and a few earthquakes on or near STS during the years 1961–1989.¶Our new documentation of STS explosions is important for evaluating the detection, location, and identification capabilities of teleseismic and regional arrays and stations; and how these capabilities have changed with time.     

朝鲜核爆的Rayleigh波震级测量

利用1995年至2009年中国东北及邻近地区11个宽频带台站记录到的77个地震事件、3个化学爆炸和2次朝鲜核爆的区域地震资料,标定该区域台网的Rayleigh波震级.通过对8~25 s 周期的垂直分量Rayleigh波形进行分析,获取基于最大振幅的面波震级.计算82个区域事件不同周期的台基响应,经过台基校正后取最大振幅的面波震级为事件震级.2006年和2009年两次朝鲜核爆的面波震级分别为2.93±0.19和3.62±0.21.将地震和核爆事件的面波震级M_s与体波震级m_b(Lg)进行比较,发现根据该区域台网的数据利用M_s-m_b识别方法无法鉴别朝鲜地区的核爆与地震.朝鲜核爆的面波震级相对较大,使M_s-m_b识别方法失效,其原因可能是源区介质的不均匀性、由核爆炸冲击引发的深部的拉伸破坏被抑制,或者是近爆源区存在张性的构造预应力.假定核爆可能的埋藏深度范围是0.01~1.0 km,用Rayleigh波震级估计朝鲜核爆的当量,对2006年和2009年核爆当量的估值范围分别为0.42~3.17 kt和2.06~15.53 kt.     

Underground Nuclear Explosions and Release of Radioactive Noble Gases

Over a period in 1961–1990 496 underground nuclear tests and explosions of different purpose and in different rocks were conducted in the Soviet Union at Semipalatinsk and anovaya Zemlya Test Sites. A total of 340 underground nuclear tests were conducted at the Semipalatinsk Test Site. One hundred seventy-nine explosions (52.6%) among them were classified as these of complete containment, 145 explosions (42.6%) as explosions with weak release of radioactive noble gases (RNG), 12 explosions (3.5%) as explosions with nonstandard radiation situation, and four excavation explosions with ground ejection (1.1%). Thirty-nine nuclear tests had been conducted at the Novaya Zemlya Test Site; six of them – in shafts. In 14 tests (36%) there were no RNG release. Twenty-three tests have been accompanied by RNG release into the atmosphere without sedimental contamination. Nonstandard radiation situation occurred in two tests. In incomplete containment explosions both early-time RNG release (up to ~1 h) and late-time release from 1 to 28 h after the explosion were observed. Sometimes gas release took place for several days, and it occurred either through tunnel portal or epicentral zone, depending on atmospheric air temperature.     

Mikhnevo: from seismic station no. 1 to a modern geophysical observatory

The Mikhnevo seismic station was founded in accordance with directive no. 1134 RS of the Council of Ministers of the Soviet Union of February 6, 1954. The station, installed south of Moscow, began its operations on monitoring nuclear tests in the United States and England in 1954. For dozens of years this station was the leading experimental base for elaborating new technical solutions and methods for monitoring nuclear explosions, equipped with modern seismological instruments. At present, the focus of activities has been moved from military applications to fundamental geophysical research. The station preserves its leading position in seismological observations due to the development of national high-performance digital instruments and creation of the small-aperture seismic array, the only one in the central part of European Russia, which is capable of recording weak seismic events with M_L ≥ 1.5 within a distance of 100 km.     

Regional coda magnitudes of underground nuclear explosions

The concept of determining magnitudes, M_τ, of regional events (Δ < 1000 km) by means of coda-duration measurements is re-examined by using short-period vertical-component seismograms from Nevada Test Site explosions. The duration is specified as the time interval between the expected arrival of the S-wave and the time when coda waves fall and stay below 80 μm peak-to-peak ground displacement. The suggested procedure requires that for M_τ = 4.0 the coda duration at a distance of 100 km is 120 s. The adaptivity of the method is examined in terms of the single-station magnitude scatter, and with respect to the potential accuracy of the yield estimation of the explosions.The derived magnitude formula for underground nuclear explosions in granite is of the form: M_τ =0.18+0.001Δ+1.79 log τ+C_s, where C_s is a station correction coefficient for non-WWNSS stations.     

Spatio-temporal variations in the structure of the attenuation field of the S-wave in the region of Nevada nuclear test site

The characteristics of the attenuation field of short-period shear waves in the region of Nevada nuclear test site (NNTS) are studied. The seismograms of underground nuclear explosions (UNEs) and earthquakes recorded by three seismic stations in 1975–2012 at the epicentral distances of up to 1000 km are processed by the methods based on the analysis of the amplitude ratios of Sn to Pn and Lg to Pg waves, as well as the S-coda envelopes for close events. It is shown that the structure of the attenuation field in the Earth’s crust and upper mantle in the NNTS region experienced significant temporal variations during the interval of nuclear operations. The strongest variations were associated with UNEs conducted in the Pahute Mesa area, which held about two-thirds of the most intense explosions. Our data indicate that temporal variations in the structure of the attenuation field are related to the migration of deep fluids. A comparison of the general characteristics of the attenuation field in the regions of the three large nuclear test sites is presented.     

The detection and recognition of underground nuclear explosions

This paper reports on a joint meeting of the Royal Astronomical Society's Joint Association for Geophysics and VERTIC (the Verification Technology Information Centre) held in London in 1992. The topics presented focused on the detection and recognition of underground nuclear explosions. The objective of the meeting was to emphasize the multi-methodological approach that is important in verifying compliance with test-ban treaties. An overview of seismological monitoring was followed by a discussion of the technical and scientific aspects of a global seismic monitoring network, and in particular of the 1991 experiment to test the large-scale international exchange of seismic data between recording stations and data centres world-wide. The current capabilities of satellite remote-sensing were presented, and their use explained in terms of both the provision of information for monitoring the development of foreign nuclear testing programmes and also for providing sufficient information for the evaluation of treaty compliance. A review of radio-isotope sampling showed how the isotopic signature of both air and ground based sampling programmes can be diagnostic of the nuclear source. Finally, previously classified research on the ionospheric effects of underground nuclear explosions was presented, the generated acoustic waves disturbing the ionosphere and producing detectable changes in the reflection of radio and radar signals which have potential as a monitoring technique.     

禁核试核查国际监测系统中的USRK及KSRS台阵对朝鲜地下核试验场的地震信号检测能力评估

根据禁核试核查国际监测系统中的USRK及KSRS台阵的实际记录数据,统计建立了这两个台阵的本底噪声水平分布概率模型。基于地下爆炸的源频谱模型及比例关系,对2009年5月25日朝鲜核试验在这两个地震台阵上的实际记录信号幅值进行了比例折算,得到了更小震级的“朝鲜地下核试验”在这两个地震台阵上的多频带信号幅值。在3倍信噪比检测条件下,利用概率模型法分别得到了这两个地震台阵对朝鲜地下核试验场小震级地下核试验的多频带信号检测能力。结果表明,震中距在400 km左右的两台阵对朝鲜地下核试验地震信号的检测阈值均随着频率的升高而逐渐降低,即高频段的检测能力强于低频段。这两个地震台阵的最高信号检测能力均出现在6.0—9.0 Hz频带,90%检测概率条件下,USRK和KSRS在该频带的检测阈值分别为m_b1.5和m_b2.2。      

Yield Estimation for Semipalatinsk Underground Nuclear Explosions Using Seismic Surface-wave Observations at Near-regional Distances

—?A statistical procedure is described for estimating the yields of underground nuclear tests at the former Soviet Semipalatinsk test site using the peak amplitudes of short-period surface waves observed at near-regional distances (Δ < 150 km) from these explosions. This methodology is then applied to data recorded from a large sample of the Semipalatinsk explosions, including the Soviet JVE explosion of September 14, 1988, and it is demonstrated that it provides seismic estimates of explosion yield which are typically within 20% of the yields determined for these same explosions using more accurate, non-seismic techniques based on near-source observations.     

Spectral time method of identification of seismic events at distances of 15°–40°

The spectral time method is applied to the identification of underground nuclear explosions (UNEs) and earthquakes from P wave records obtained at stations at epicentral distances of 15°–40°. It is necessary to utilize data at such great distances because the density of stations of the International Monitoring System is small in several areas of the Earth. At such distances, it is difficult to detect S and surface LR waves and, therefore, the efficiency of the identification using the diagnostic parameters (discriminants) S/P and LR/P is drastically reduced. As a result, the role of spectral and spectral time methods in the identification of events from P waves becomes much more significant. An advantage of the spectral time method (STM) is that it is applicable in the region of interest, provided that data from one or two reference underground chemical explosions and a representative sample of earthquake records obtained at network stations of the region are available. Moreover, in the majority of cases, the STM identifies earthquakes that could not be recognized with the use of spectral diagnostic parameters. We used records of UNEs and earthquakes in Central Asia received from the archives of the Center of the Incorporated Research Institution for Seismology (IRIS) in Washington, the National Data Center in Dubna (Russian Federation), and the International Data Center (IDC) in Vienna. The effectiveness of earthquake record selection using the given method amounted to about 80%. Supposedly, the application of this method of identification can increase the percentage of filtration (screening) of regional events having a natural origin.     

Path Corrections for Source Discriminants: A Case Study at Two International Seismic Monitoring Stations

—?Improving the performance of short-period regional seismic discriminants by applying propagation corrections is explored using observations from two seismic monitoring stations in Asia. Frequency-dependent regional phase amplitude ratio measurements at stations NIL and ZAL for earthquakes and underground nuclear explosions were obtained from the prototype-International Data Center (pIDC) that has been established for developing monitoring capabilities of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The pIDC discriminant measurements have large scatter, much of which is attributed to wave propagation effects in the heterogeneous crustal waveguide. Linear regressions indicate that the phase ratios are correlated with topographic characteristics along the individual paths, providing an empirical means for correcting for path effects beyond conventional distance corrections. Kriging, a spatial multiple regression algorithm, also reveals coherent spatial patterns in the data indicative of regional path effects. Using available high-resolution topography data, correction of regional P/S ratios for the best models obtained from multivariate regressions systematically reduces the data variance relative to distance corrections alone, as has been observed for other data sets. The reduced scatter in the measurements increases the separation between earthquake and explosion populations in most cases, enhancing the regional discriminant performance. The path-corrected discriminants isolate explosions better for NIL than for ZAL, even though some of the explosion sources are located in a common source area. Kriging achieves comparable or superior variance reduction for the discriminant measures, without requiring knowledge of the path structure, although this may not result in improved discriminant performance. While always desirable, corrections for heterogeneous path effects may prove inadequate in some cases, notably when phase blockage occurs or when strong attenuation eliminates the diagnostic high-frequency energy.     

对朝鲜2006年、2009年和2013年3次地下核试验的相对定位

采用双差定位方法, 对朝鲜2006年、2009年和2013年的3次核试验进行相对定位.根据19个区域地震台和8个远震台的垂直分量波形资料, 计算了多种震相(P波、S波和LR波等)的互相关时差, 并由测量时差反演两两核爆的相对空间位置, 通过随机生成部分台站组合进行扰动实验以估算定位的不确定度.结果表明, 相对于朝鲜2009年核试验, 2013年核试验位于其南257 m、西385 m, 2006年核试验位于其南503 m、东2589 m.当扰动实验中随机选取的台站数目为20个时, 在95%置信度下, 朝鲜2013年和2006年核试验相对于2009年核试验的地表定位结果的不确定度约为150 m.      

Does more mean less in epicentre estimation?

The epicentres of explosions at two test sites – Balapan (Shagan River), Former Soviet Union and Lop Nor, China – are estimated using the onset times of P from only three or four array stations at teleseismic distances. The epicentres of the explosions are known to within about 1 km from studies that make use of information from satellite imagery; these estimates are taken to be the true epicentres. With the true epicentres, differences between the true travel times and the times from travel-time tables are estimated. The differences include a component – path effects – that results in epicentre bias. Comparing our estimates using three or four stations with the true epicentres shows that with correction for path effects most of the epicentres are within 5 km of true and even without correction most estimated epicentres are within 10 km of true. The results confirm the conclusion of Evernden that if reading error in P times has a standard deviation of a few tenths of a second, reliable epicentres can be obtained given readings from only a few stations. This implies, what has been noted by others, that for epicentre estimation, better results can be obtained with a few well read P times from a constant network of the most reliable and sensitive stations, than by using uncritically all the available times. Even without correction for path effects none of the explosions (with times free from possible clock errors) falls outside a circular 1000 km² region; 1000 km² being the search area allowed for an on-site inspection under the Comprehensive Test Ban Treaty. The results suggest that rather than try and calibrate the whole of the International Monitoring System, being set up to verify the Test Ban, it would be better initially to concentrate on calibrating the few stations with the longest recording history and lowest detection thresholds.     

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.     

On the Use of Calibration Explosions at the Former Semipalatinsk Test Site for Compiling a Travel-time Model of the Crust and Upper Mantle

—?Two chemical calibration explosions, conducted at the former Semipalatinsk nuclear test site in 1998 with charges of 25 tons and 100 tons TNT, have been used for developing travel-time curves and generalized one-dimensional velocity models of the crust and upper mantle of the platform region of Kazakhstan. The explosions were recorded by a number of digital seismic stations, located in Kazakhstan at distances ranging from 0 to 720?km. The travel-time tables developed in this paper cover the phases P, Pn, Pg, S, Sn, Lg in a range of 0–740?km and the velocity models apply to the crust down to 44?km depth and to the mantle down to 120?km. A comparison of the compiled travel-time tables with existing travel-time tables of CSE and IASPEI91 is presented.     

Improving Regional Seismic Event Location in China

—?In an effort to improve our ability to locate seismic events in China using only regional data, we have developed empirical propagation path corrections and applied such corrections using traditional location routines. Thus far, we have concentrated on corrections to observed P arrival times for crustal events using travel-time observations available from the USGS Earthquake Data Reports, the International Seismic Centre Bulletin, the preliminary International Data Center Reviewed Event Bulletin, and our own travel-time picks from regional data. Location ground truth for events used in this study ranges from 25?km for well-located teleseimic events, down to 2?km for nuclear explosions located using satellite imagery. We also use eight events for which depth is constrained using several waveform methods. We relocate events using the EvLoc algorithm from a region encompassing much of China (latitude 20°–55°N; longitude 65°–115°E). We observe that travel-time residuals exhibit a distance-dependent bias using IASPEI91 as our base model. To remedy this bias, we have developed a new 1-D model for China, which removes a significant portion of the distance bias. For individual stations having sufficient P-wave residual data, we produce a map of the regional travel-time residuals from all well-located teleseismic events. Residuals are used only if they are smaller than 10?s in absolute value and if the seismic event is located with accuracy better than 25?km. From the residual data, correction surfaces are constructed using modified Bayesian kriging. Modified Bayesian kriging offers us the advantage of providing well-behaved interpolants and their errors, but requires that we have adequate error estimates associated with the travel-time residuals from which they are constructed. For our P-wave residual error estimate, we use the sum of measurement and modeling errors, where measurement error is based on signal-to-noise ratios when available, and on the published catalog estimate otherwise. Our modeling error originates from the variance of travel-time residuals for our 1-D China model. We calculate propagation path correction surfaces for 74 stations in and around China, including six stations from the International Monitoring System. The statistical significance of each correction surface is evaluated using a cross-validation technique. We show relocation results for nuclear tests from the Balapan and Lop Nor test sites, and for earthquakes located using interferometric synthetic aperture radar. These examples show that the use of propagation path correction surfaces in regional relocations eliminates distance bias in the residual curves and significantly improves the accuracy and precision of seismic event locations.     

Vibration Effect of Near Earthquakes at Different Depths in a Shallow Medieval Mine

The shallow medieval Jeroným Mine is located at a distance of about 25 km southeast of the Nový Kostel focal zone where the most intensive seismic activity in West Bohemia (Czech Republic) has been documented. Permanent seismological monitoring has been carried out since 2004 in this mine. During the 2011 and 2014 seismic swarms, more than 1000 triggered records comprising almost 1500 earthquakes were recorded at the permanent station in the mine. Three short-term seismological experiments were accomplished during these swarms. Several temporary seismic stations were simultaneously placed in different parts of underground spaces which enabled comparison of vibration effect caused by near earthquakes in different parts of the mine. Although the depth of the lowest parts of mine is only about 60 m, a vibration effect generated by earthquakes from the Nový Kostel focal zone is not the same for the whole underground complex.     

Seismological aspects of the 27 June 2015 Gulf of Aqaba earthquake and its sequence of aftershocks

On 27 June 2015, a moderate earthquake with magnitude Mb 5.2 struck the Gulf of Aqaba near Nuweiba City. This event was instrumentally recorded by the Egyptian National Seismic Network (ENSN) and many other international seismological centres. The event was felt in all the cities on the Gulf of Aqaba, as well as Suez City, Hurghada City, the greater Cairo Metropolitan Area, Israel, Jordan and the north-western part of Saudi Arabia. No casualties were reported, however. Approximately 95 aftershocks with magnitudes ranging from 0.7 to 4.2 were recorded by the ENSN following the mainshock. In the present study, the source characteristics of both the mainshock and the aftershocks were estimated using the near-source waveform data recorded by the very broadband stations of the ENSN, and these were validated by the P-wave polarity data from short period stations. Our analysis reveals that an estimated seismic moment of 0.762?×?10¹⁷ Nm was released, corresponding to a magnitude of Mw 5.2, a focal depth of 14 km, a fault radius of 0.72 km and a rupture area of approximately 1.65 km². Monitoring the sequence of aftershocks reveals that they form a cluster around the mainshock and migrated downwards in focal depth towards the west. We compared the results we obtained with the published results from the international seismological centres. Our results are more realistic and accurate, in particular with respect to the epicenteral location, magnitude and fault plane solution which are in accordance with the hypocentre distribution of the aftershocks.