共查询到20条相似文献，搜索用时 31 毫秒
1.
2.
O. K. Kedrov E. O. Kedrov N. A. Sergeyeva L. P. Zabarinskaya V. R. Gordon 《Izvestiya Physics of the Solid Earth》2008,44(5):364-380
The dynamic calibration method (DCM), using natural seismicity data and initially elaborated in [Kedrov, 2001; Kedrov et al., 2001; Kedrov and Kedrov, 2003], is applied to International Monitoring System (IMS) stations in Central Asia. The algorithm of the method is refined and a program is designed for calibrating diagnostic parameters (discriminants) that characterize a seismic source on the source-station traces. The DCM calibration of stations in relation to the region under study is performed by the choice of attenuation coefficients that adapt the diagnostic parameters to the conditions in a reference region. In this method, the stable Eurasia region is used as the latter. The calibration used numerical data samples taken from the archive of the International Data Centre (IDC) for the IMS stations MKAR, BVAR, EIL, ASF, and CMAR. In this paper, we used discriminants in the spectral and time domains that have the form and are independent of the magnitude m b and the epicentral distance Δ; these discriminants were elaborated in [Kedrov et al., 1990; Kedrov and Lyuke, 1999] on the basis of a method used for identification of events at regional distances in Eurasia. Prerequisites of the DCM are the assumptions that the coefficient a m is regionindependent and the coefficient b Δ depends only on the geotectonic characteristics of the medium and does not depend on the source type. Thus, b Δ can be evaluated only from a sample of earthquakes in the region studied; it is used for adapting the discriminants D(X i ) in the region studied to the reference region. The algorithm is constructed in such a way that corrected values of D(X i) are calculated from the found values of the calibration coefficients b Δ, after which natural events in the region under study are selected by filtering. Empirical estimates of the filtering efficiency as a function of a station vary in a range of 95–100%. The DCM was independently tested using records obtained at the IRIS (Incorporated Research Institutions for Seismology) stations BRVK and MAKZ from explosions detonated in India on May 11, 1998, and Pakistan on May 28, 1998; these stations are similar in location and recording instrumentation characteristics to the IMS stations BVAR and MKAR. This test resulted in correct recognition of the source type and thereby directly confirmed the validity of the proposed calibration method of stations with the use of natural seismicity data. It is shown that the calibration coefficients b Δ for traces similar in the conditions of signal propagation (e.g., the traces from Iran to the stations EIL and ASF) are comparable for nearly all diagnostic parameters. We arrive at the conclusion that the method of dynamic calibration of stations using natural seismicity data in a region where no explosions were detonated can be significant for a rapid and inexpensive calibration of IMS stations. The DCM can also be used for recognition of industrial chemical explosions that are sometimes erroneously classified in regional catalogs as earthquakes.
相似文献
$D_i = X_i - a_m m_b - b_\Delta \log \Delta $
3.
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. 相似文献
4.
v--vS/P amplitude ratios have proven to be a valuable discriminant in support of monitoring a Comprehensive Nuclear Test Ban Treaty. Regional S and P phases attenuate at different rates and the attenuation can vary geographically. Therefore, calibration is needed to apply the S/P discriminant in new regions. Calibration includes application of frequency-dependent source and distance corrections for regional Pn, Pg, Sn, and Lg phases.¶Jenkins et al. (1998) developed Pn, Pg, Sn, and Lg amplitude models for nine geographic regions and two global composite models, stable and tectonic. They determined frequency-dependent source and attenuation corrections from a large data set obtained from the Prototype International Data Center (PIDC). We use their corrections to evaluate calibrated S/P discriminants.¶Our discrimination data set includes >1000 amplitude ratios from earthquakes, industrial explosions, chemical explosions, and nuclear explosions from Lop Nor, India and Pakistan. We find that the calibrated S/P ratio is largest for earthquakes and smallest for the nuclear explosions, as expected. However, the discriminant is not universally valid. In particular, the S/P ratio for the Pakistan nuclear explosion fell within the normal range for the earthquakes. This event was recorded by only a few stations at far-regional distances and appears to have an anomalously high Sn amplitude. The industrial explosions overlap with the earthquake population, however the buried chemical explosions generally register lower S/P ratio than earthquakes. 相似文献
5.
Günter Bock 《Physics of the Earth and Planetary Interiors》1981,25(4):360-371
P-wave travel-time residuals at the Warramunga Seismic Array (WRA) in the Northern Territory, Australia, have been studied from 49 earthquakes with epicenters south of 19°S in the Fiji-Tonga region. Focal depths are between 42 and 679 km as determined from pP-P. Using the Jeffreys-Bullen and the Herrin travel-time tables the epicentral parameters have been redetermined by considering only “normal” seismic stations in the location procedure. These are those stations where P-wave travel times are probably not affected by lateral heterogeneities caused by the lithosphere descending beneath the Tonga trench. Epicenters of deep earthquakes below 300 km have been relocated by using stations at Δ > 25° only. Epicenters from shallower-depth earthquakes have been recalculated without using stations between 35 < Δ < 75° epicentral distance. In both cases focal depths were determined from pP-P times. The resulting pattern of P-residuals at WRA does not show any significant change with depth below 350 km. The residuals become more negative for shallower earthquakes above about 250 km. P-waves to WRA are advanced by approximately 2 s compared with those from deep earthquakes. The results do not essentially differ for the two different travel-time tables used. The observations can be interpreted by P-wave velocities that are higher in the sinking slab down to 350–400 km by 5±2% than in both the Jeffreys-Bullen and Herrin models. Without considering possible elevations of phase boundaries this estimate yields a temperature contrast of 1000±450°C between slab and normal mantle material in this depth range. 相似文献
6.
Improving Regional Seismic Event Location in China 总被引：1，自引：0，他引：1
L.K. Steck A.A. Velasco A.H. CogbillL H.J. Patton 《Pure and Applied Geophysics》2001,158(1-2):211-240
—?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. 相似文献
7.
C. Wright 《Physics of the Earth and Planetary Interiors》1983,32(2):168-181
A method for analysing travel times measured at a large array or a network of seismographs from many earthquakes within a specific region has been developed. Approximate relative station corrections are calculated from the residuals on a least-squares line or least-squares quadratic form fitted through the times for each earthquake, and may be improved by iteration after a preliminary travel-time curve has been derived. Accurate relative baseline corrections for each earthquake are also calculated iteratively, and an optimum slowness-distance curve is determined from the combined corrected travel times from all earthquakes using a trade-off procedure. Calculations using synthetic travel-time data suggest that abrupt changes in slowness of ～ 0.4 s deg?1 due to the presence of triplications are generally resolvable, provided that the effects of lateral variations are small, even with random epicentre mislocations in the range ± 0.5°. Slowness measurements at a network of temporary stations deployed across Australia do not show any discontinuities in slowness greater than 0.2 s deg?1 in the distance range 45–54°. Similar measurements at the Warramunga array from the same source regions, however, suggest the presence of complexity in the slowness curve at distances close to 50°. Relative arrival times at the temporary network generally have standard deviations less than 0.25 s, thus suggesting that details of structure finer than those derived from conventional travel-time studies can be resolved. 相似文献
8.
—?Identification of seismic events is a major scientific issue in the framework of verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Of special interest in this context is the identification of the numerous low-yield mining or blasting events, especially those occurring in the same area as earthquakes, such as the Vogtland area in the border region of Germany and the Czech Republic. Seismic events in this area were investigated by WÜSTER (1993, 1995), who achieved complete discrimination using measures of spectral decay and spectral variance at the GERESS array and a quadratic discrimination function.¶A subset of these events, for which ground-truth information is available, has been analyzed in this study using multivariate statistical analysis. Various parameters based on measurements from seismic waveforms of the broadband stations of the German Regional Seismic Network (GRSN) and short-period elements of the GERESS array are tested for statistical significance in a linear regression analysis, in particular spectral amplitude ratios for the L g phase and P g /S g amplitude ratios. The subset includes a total of 35 explosions and 24 earthquakes. The results of our study argue that identification based on spectral L g and high-frequency P g /S g ratios is promising. However, discrimination success is strongly varying from station to station; thus, weighting according to station success rates could improve the overall identification capability. 相似文献
9.
Spectral classification methods in monitoring small local events by the Israel seismic network 总被引：1，自引：1，他引：0
We use the dense Israel Seismic Network (ISN) to discriminate between low magnitude earthquakes and explosions in the Middle East region. This issue is important for CTBT monitoring, especially when considering small nuclear tests which may be conducted under evasive conditions. We explore the performance of efficient discriminants based on spectral features of seismograms using waveforms of 50 earthquakes and 114 quarry and underwater blasts with magnitudes 1.0–2.8, recorded by ISN short-period stations at distances up to 200 km. The single-station spectral ratio of the low and high-frequency seismic energy shows an overlap between explosions and earthquakes. After averaging over a subnet of stations, the resolving power is enhanced and the two classes of events are separated. Different frequency bands were tested; the (1–3 Hz)/(6–8 Hz) ratio provided the best discriminant performance. We also estimated normalized r.m.s. spectral amplitudes in several sequential equal frequency windows within the 1–12 Hz band and applied multiparametric automatic classification procedures (Linear Discrimination Function and Artificial Neural Network) to the amplitudes averaged over a subnetwork. A leave-one-out test showed a low rate of error for the multiparametric procedures. An innovative multi-station discriminant is proposed, based on spectral modulation associated with ripple-firing in quarry blasts and with the bubbling effect in underwater explosions. It utilizes a distinct azimuth-invariant coherency of spectral shapes for different stations in the frequency range (1–12 Hz). The coherency is measured by semblance statistics commonly used in seismic prospecting for phase correlation in the time domain. After modification, the statistics applied to the network spectra provided event separation. A new feature of all the above mentioned procedures is that they are based on smoothed (0.5 Hz window), instrument-corrected FFT spectra of the whole signal; they are robust to the accuracy of onset time estimation and, thus well suited to automatic event identification. 相似文献
10.
O. K. Kedrov E. O. Kedrov N. A. Sergeyeva L. P. Zabarinskaya V. R. Gordon A. B. Chulkov 《Izvestiya Physics of the Solid Earth》2010,46(11):974-999
The Method of Dynamic Calibration (MDC) of stations of the International Monitoring System (IMS) was developed for calibrating
regions where no underground nuclear explosions were carried out, with the purpose of providing conditions for implementation
of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) in nontrivial cases. Initially, the MDC had been presented in [Kedrov,
2001; Kedrov et al., 2001; Kedrov O.K. and Kedrov E.O., 2003] and then considered in detail in [Kedrov et al., 2008]. The
core of MDC relates to adapting diagnostic parameters for the identification of underground nuclear explosions (UNE) and earthquakes
elaborated for the region of Eurasia, taken as a basic region (BR), for other researched regions that differ from BR in the
character of the attenuation of seismic waves. The unique characteristic of this method lies in the fact that calibration
of diagnostic parameters with the help of attenuation coefficients b
Δ at varied source-station traces is implemented using only natural seismicity data within the limits of an explored region
and does not require special underground chemical explosions. The MDC algorithm is implemented in the research program ”Kalibr”,
which was tested by using the experimental data from Eurasia region. It is shown in this work that MDC can be used for calibration
of regions where a very low level of natural seismicity is observed. According to the results of the calibration of diagnostic
parameters at IMS stations in several regions of North America, Africa, and Asia, the approximate classification of propagation
conditions for seismic signals at source-station traces in platform and tectonically active regions is made. The results for
the development of two research programs, “Spektr” and “Signal”, are presented; this software is intended for automation of
calculation procedures for spectral diagnostic parameters of UNEs’ and earthquakes’ identification by amplitude spectra of
P waves and by the maximal amplitudes of P, S, and LR signals. The application of these programs allowed us to accelerate the whole calibration procedure for a particular source-station
trace using the ”Kalibr” program. 相似文献
11.
K. Koch 《Pure and Applied Geophysics》2002,159(4):759-778
12.
Application of 3-D Crustal and Upper Mantle Velocity Model of North America for Location of Regional Seismic Events 总被引：1，自引：0，他引：1
—?Seismic event locations based on regional 1-D velocity-depth sections can have bias errors caused by travel-time variations within different tectonic provinces and due to ray-paths crossing boundaries between tectonic provinces with different crustal and upper mantle velocity structures. Seismic event locations based on 3-D velocity models have the potential to overcome these limitations. This paper summarizes preliminary results for calibration of IMS for North America using 3-D velocity model. A 3-D modeling software was used to compute Source-Station Specific Corrections (SSSCs(3-D)) for Pn travel times utilizing 3-D crustal and upper mantle velocity model for the region. This research was performed within the framework of the United States/Russian Federation Joint Program of Seismic Calibration of the International Monitoring System (IMS) in Northern Eurasia and North America.¶An initial 3-D velocity model for North America was derived by combining and interpolating 1-D velocity-depth sections for different tectonic units. In areas where no information on 1-D velocity-depth sections was available, tectonic regionalization was used to extrapolate or interpolate. A Moho depth map was integrated. This approach combines the information obtained from refraction profiles with information derived from local and regional network data. The initial 3-D velocity model was tested against maps of Pn travel-time residuals for eight calibration explosions; corrections to the 3-D model were made to fit the observed residuals. Our goal was to find a 3-D crustal and upper mantle velocity model capable predicting Pn travel times with an accuracy of 1.0–1.5 seconds (r.m.s.).¶The 3-D velocity model for North America that gave the best fit to the observed travel times, was used to produce maps of SSSCs(3-D) for seismic stations. The computed SSSCs(3-D) vary approximately from +5 seconds to ?5 seconds for the western USA and the Pre-Cambrian platform, respectively. These SSSCs(3-D) along with estimated modeling and measurement errors were used to relocate, using regional data, an independent set of large chemical explosions (with known locations and origin times) detonated within various tectonic provinces of North America. Utilization of the 3-D velocity model through application of the computed SSSCs(3-D) resulted in a substantial improvement in seismic event location accuracy and in a significant decrease of error ellipse area for all events analyzed in comparison both with locations based on the IASPEI91 travel times and locations based on 1-D regional velocity models. 相似文献
13.
A. B. Baba E. E. Papadimitriou B. C. Papazachos C. A. Papaioannou B. G. Karakostas 《Pure and Applied Geophysics》2000,157(5):765-783
14.
This study analyzes and compares the P- and S-wave displacement spectra from local earthquakes and explosions of similar magnitudes. We propose a new approach to discrimination between low-magnitude shallow earthquakes and explosions by using ratios of P- to S-wave corner frequencies as a criterion. We have explored 2430 digital records of the Israeli Seismic Network (ISN) from 456 local events (226 earthquakes, 230 quarry blasts, and a few underwater explosions) of magnitudes Md?=?1.4–3.4, which occurred at distances up to 250 km during 2001–2013 years. P-wave and S-wave displacement spectra were computed for all events following Brune’s source model of earthquakes (1970, 1971) and applying the distance correction coefficients (Shapira and Hofstetter, Teconophysics 217:217–226, 1993; Ataeva G, Shapira A, Hofstetter A, J Seismol 19:389-401, 2015), The corner frequencies and moment magnitudes were determined using multiple stations for each event, and then the comparative analysis was performed.The analysis showed that both P-wave and especially S-wave displacement spectra of quarry blasts demonstrate the corner frequencies lower than those obtained from earthquakes of similar magnitudes. A clear separation between earthquake and explosion populations was obtained for ratios of P- to S-wave corner frequency f 0(P)/f 0(S). The ratios were computed for each event with corner frequencies f 0 of P- and S-wave, which were obtained from the measured f 0 I at individual stations, then corrected for distance and finally averaged. We obtained empirically the average estimation of f 0(P)/f 0(S)?=?1.23 for all used earthquakes, and 1.86 for all explosions. We found that the difference in the ratios can be an effective discrimination parameter which does not depend on estimated moment magnitude M w .The new multi-station Corner Frequency Discriminant (CFD) for earthquakes and explosions in Israel was developed based on ratios P- to S-wave corner frequencies f 0(P)/f 0(S), with the empirical threshold value of the ratio for Israel as 1.48. 相似文献
15.
Elisabeth Dologlou 《Acta Geophysica》2008,56(4):1015-1024
A 12-year period experimental data, from 1 January 1995, to 20 August 2007, have been examined for possible correlations between Seismic Electric Signals (SES) of the VAN method and source parameters of the corresponding earthquakes in Western Greece. During that period 13 earthquakes of magnitude M W ≥ 5 with epicenters in the area 19–24°E, 36–41°N and available the CMT solutions (Centroid Moment Tensor focal mechanism solutions) have been found to be preceded by a SES each time at one of the three VAN stations of IOA, PIR or PAT. The results of IOA and PIR stations are compared to those reported by Uyeda et al. 1999, for the previous period 1983–1994. The IOA station seems to be sensitive to earthquakes with thrust type mechanisms being mainly consistent to its past behavior. The PIR station, which is moved from its previous position by few kilometers, exhibits some changes. It detects now both strike slip and thrust type earthquakes and becomes sensitive to new areas indicating probably the strong dependence of the SES station detection ability upon its location. 相似文献
16.
利用2013年10月—2015年6月山东乳山震群的地震波形资料,基于距离乳山震群最近的3个台站所记录的地震波形互相关系数c≥0.9的地震即为重复地震的原则,识别了乳山震群的重复地震．利用波形互相关时延法,计算分析了文登台和招远台这两个固定台站的走时差随时间的变化特征．结果表明:乳山震群的地震相似性很好,且震中位置较为集中,重复地震的时间跨度较大,有利于提高观测结果的时间分辨率;乳山震群中3次MS＞4.0地震前均出现了短期的走时差低值异常现象,反映了乳山震群震源区在中强震前会出现短期地壳介质速度明显升高的过程,可以为该区域的中强震预测提供一定依据． 相似文献
17.
Forensic seismology revisited 总被引：1，自引：0，他引：1
A. Douglas 《Surveys in Geophysics》2007,28(1):1-31
The first technical discussions, held in 1958, on methods of verifying compliance with a treaty banning nuclear explosions,
concluded that a monitoring system could be set up to detect and identify such explosions anywhere except underground: the
difficulty with underground explosions was that there would be some earthquakes that could not be distinguished from an explosion.
The development of adequate ways of discriminating between earthquakes and underground explosions proved to be difficult so
that only in 1996 was a Comprehensive Nuclear Test Ban Treaty (CTBT) finally negotiated. Some of the important improvements
in the detection and identification of underground tests—that is in forensic seismology—have been made by the UK through a
research group at the Atomic Weapons Establishment (AWE). The paper describes some of the advances made in identification
since 1958, particularly by the AWE Group, and the main features of the International Monitoring System (IMS), being set up
to verify the Test Ban.
Once the Treaty enters into force, then should a suspicious disturbance be detected the State under suspicion of testing will
have to demonstrate that the disturbance was not a test. If this cannot be done satisfactorily the Treaty has provisions for
on-site inspections (OSIs): for a suspicious seismic disturbance for example, an international team of inspectors will search
the area around the estimated epicentre of the disturbance for evidence that a nuclear test really took place.
Early observations made at epicentral distances out to 2,000 km from the Nevada Test Site showed that there is little to distinguish
explosion seismograms from those of nearby earthquakes: for both source types the short-period (SP: ∼1 Hz) seismograms are
complex showing multiple arrivals. At long range, say 3,000–10,000 km, loosely called teleseismic distances, the AWE Group
noted that SP P waves—the most widely and well-recorded waves from underground explosions—were in contrast simple, comprising
one or two cycles of large amplitude followed by a low-amplitude coda. Earthquake signals on the other hand were often complex
with numerous arrivals of similar amplitude spread over 35 s or more. It therefore appeared that earthquakes could be recognised
on complexity. Later however, complex explosion signals were observed which reduced the apparent effectiveness of complexity
as a criterion for identifying earthquakes. Nevertheless, the AWE Group concluded that for many paths to teleseismic distances,
Earth is transparent for P signals and this provides a window through which source differences will be most clearly seen.
Much of the research by the Group has focused on understanding the influence of source type on P seismograms recorded at teleseismic
distances. Consequently the paper concentrates on teleseismic methods of distinguishing between explosions and earthquakes.
One of the most robust criteria for discriminating between earthquakes and explosions is the m
b : M
s criterion which compares the amplitudes of the SP P waves as measured by the body-wave magnitude m
b, and the long-period (LP: ∼0.05 Hz) Rayleigh-wave amplitude as measured by the surface-wave magnitude M
s; the P and Rayleigh waves being the main wave types used in forensic seismology. For a given M
s, the m
b for explosions is larger than for most earthquakes. The criterion is difficult to apply however, at low magnitude (say m
b < 4.5) and there are exceptions—earthquakes that look like explosions.
A difficulty with identification criteria developed in the early days of forensic seismology was that they were in the main
empirical—it was not known why they appeared to work and if there were test sites or earthquakes where they would fail. Consequently
the AWE Group in cooperation with the University of Cambridge used seismogram modelling to try and understand what controls
complexity of SP P seismograms, and to put the m
b : M
s criterion on a theoretical basis. The results of this work show that the m
b : M
s criterion is robust because several factors contribute to the separation of earthquakes and explosions. The principal reason
for the separation however, is that for many orientations of the earthquake source there is at least one P nodal plane in
the teleseismic window and this biases m
b low. Only for earthquakes with near 45° dip-slip mechanisms where the antinode of P is in the source window is the m
b:M
s criterion predicted to fail. The results from modelling are consistent with observation—in particular there are earthquakes,
“anomalous events”, which look explosion-like on the m
b:M
s criterion, that turn out to have mechanisms close to 45° dip-slip. Fortunately the P seismograms from such earthquakes usually
show pP and sP, the reflections from the free surface of P and S waves radiated upwards. From the pP–P and sP–P times the
focal depth can be estimated. So far the estimated depth of the anomalous events have turned out to be ∼20 km, too deep to
be explosions.
Studies show that the observation that P seismograms are more complex than predicted by simple models can be explained on
the weak-signal hypothesis: the standard phases, direct P and the surface reflections, are weak because of amongst other things,
the effects of the radiation pattern or obstacles on the source-to-receiver path; other non-standard arrivals then appear
relatively large on the seismograms.
What has come out of the modelling of P seismograms is a criterion for recognising suspicious disturbances based on simplicity
rather than complexity. Simple P seismograms for earthquakes at depths of more than a few kilometres are likely to be radiated
only to stations that lie in a confined range of azimuths and distances. If then, simple seismograms are recorded over a wide
range of distances and particularly azimuths, it is unlikely the source is an earthquake at depth. It is possible to test
this using the relative amplitudes of direct P and later arrivals that might be surface reflections. The procedure is to use
only the simple P seismograms on the assumption that whereas the propagation through Earth may make a signal more complex
it is unlikely to make it simpler. From the amplitude of the coda of these seismograms, bounds can be placed on the size of
possible pP and sP. The relative-amplitude method is then used to search for orientations of the earthquake source that are
compatible with the observations. If no such orientations are found the source must be shallow so that any surface reflections
merge with direct P, and hence could be an explosion.
The IMS when completed will be a global network of 321 monitoring stations, including 170 seismological stations principally
to detect the seismic waves from earthquakes and underground explosions. The IMS will also have stations with hydrophones,
microbarographs and radionuclide detectors to detect explosions in the oceans and the atmosphere and any isotopes in the air
characteristic of a nuclear test. The Global Communications Infrastructure provides communications between the IMS stations
and the International Data Centre (IDC), Vienna, where the recordings from the monitoring stations is collected, collated,
and analysed. The IDC issues bulletins listing geophysical disturbances, to States Signatories to the CTBT.
The assessment of the disturbances to decide whether any are possible explosions, is a task for State Signatories. For each
Signatory to do a detailed analysis of all disturbances would be expensive and time consuming. Fortunately many disturbances
can be readily identified as earthquakes and removed from consideration—a process referred to as “event screening”. For example,
many earthquakes with epicentres over the oceans can be distinguished from underwater explosions, because an explosion signal
is of much higher frequency than that of earthquakes that occur below the ocean bed. Further, many earthquakes could clearly
be identified at the IDC on the m
b : M
s criterion, but there is a difficulty—how to set the decision line. The possibility has to be very small that an explosion
will be classed by mistake, as an earthquake. The decision line has therefore to be set conservatively, consequently with
routine application of current screening criteria, only about 50% of earthquakes can be positively identified as such.
Various methods have been proposed whereby a “determined violator” could avoid the provisions of a CTBT and carry out a test
that would be either undetected or detected but not identified as an explosion. The increase in complexity and cost of such
a test should discourage any State from attempting it. In addition, there is always the possibility of some stations detecting
the test, the test being identified as suspicious, and so subject to an OSI. With time as the IMS becomes more efficient and
effective it will act increasingly to deter anyone contemplating a clandestine test, from going ahead.
What has emerged is several robust criteria. The criteria include: location, which when combined with hydro-acoustic data
can identify earthquakes under the sea; m
b : M
s; and depth of focus. More detailed study is required of any remaining seismic disturbance that is regarded as suspicious:
for example, is close to a site where nuclear tests have been carried out in the past. Any disturbance that is shown to be
explosion-like, may be the subject of an OSI.
One surprise is how little plate tectonics has contributed to resolving problems in forensic seismology. Much of the evidence
for plate tectonics comes from seismological studies so it would be expected that the implications for Earth structure arising
from forensic seismology would be consistent with plate-tectonic models. So far the AWE Group have found little synergy between
plate tectonics and forensic seismology.
It is to be hoped that the large volume of seismological data of high quality now being collected by the IMS and the increasing
number of digital stations, will result in a revised Earth model that is consistent with the findings of forensic seismology,
so that a future review of progress will show that the forensic seismologist can draw on this model in attempting to interpret
apparently anomalous seismograms.
相似文献
| A. DouglasEmail: |
18.
19.
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. 相似文献
20.
v--vOur goal is to develop and test an effective method to detect, identify, extract, and quantify surface wave signals for weak events observed at regional stations. We describe an automated surface wave detector and extractor designed to work on weak surface wave signals across Eurasia at intermediate periods (8 s-40 s). The method is based on phase-matched filters defined by the Rayleigh wave group travel-time predictions from the broadband group velocity maps presented by Ritzwoller and Levshin (1998) and Ritzwoller et al. (1998) and proceeds in three steps: Signal compression, signal extraction or cleaning, and measurement. First, the dispersed surface wave signals are compressed in time by applying an anti-dispersion or phase-matched filter defined from the group velocity maps. We refer to this as the `compressed signal.' Second, the surface wave is then extracted by filtering `noise' temporally isolated from the time-compressed signal. This filtered signal is then redispersed by applying the inverse of the phase-matched filter. Finally, we adaptively estimate spectral amplitude as well as group and phase velocity on the filtered signal. The method is naturally used as a detector by allowing origin time to slide along the time axis. We describe preliminary results of the application of this method to a set of nuclear explosions and earthquakes that occurred on or near the Chinese Lop Nor test site from 1992 through 1996 and one explosion on the Indian Rajasthan test site that occurred in May of 1998. 相似文献