Prototype International Data Center Research Articles

A Method for First-Order Earthquake Depth Estimation Using Superarrays

Source depth estimation is a key process in earthquake location, hazard assessment, and nuclear monitoring. For instance, hypocentral depth is an important parameter affecting the spatial attenuation of ground motions resulting from an earthquake and an important indicator of natural versus man‐made events. Resolving transition zone phase complexity is important in sparsely monitored areas, for improved location and magnitude estimation of a seismic event at regional‐to‐teleseismic distances. In response to this need, we demonstrate a new approach to first‐order depth estimation. Depth phases are notoriously difficult to identify, specifically for shallower events. The usual way of determining the depth of focus of an earthquake is to measure the difference Δ t P − pP between the arrival time of the direct P wave and the arrival time of the surface reflection pP . Once Δ t P − pP is calculated, the depth of the focus can then be estimated closely, assuming that the velocity structure between the surface and the event focus is satisfactorily known. Although this method sounds simple, one major challenge remains: depth‐phase identification. For this reason, many monitoring agencies pick few depth phases from earthquakes shallower than 70 km. For example, depth‐phase observations were reported for only about 5% of the analyst‐reviewed events listed in the revised event bulletins of the Prototype International Data Centre during the Group of Scientific Experts Technical Test No 3 (GSETT‐3) experiment (Murphy and Barker, 2006). Routine depth‐phase identification is complicated by issues including (1) focal mechanism (affecting the relative P and pP or sP amplitude), (2) differential attenuation effects, (3) variations of surface topography and of subsurface structure at the depth‐phase bounce points, (4) earthquake epicentral distance (e.g., complex coda of P phases bottoming at the transition zone of the upper and lower mantle makes pP identification difficult), (5) rupture duration for large earthquakes ( M …

Comparison of Methods for Estimating Station Magnitude Corrections for Improved Seismologic Monitoring of the Comprehensive Nuclear-Test-Ban Treaty

Abstract Accurate estimation of the magnitude of seismic events is essential to effective monitoring of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The standard estimator of body-wave magnitude in the International Data Center (IDC) bulletins is the average m b reading at the detecting stations in a prescribed distance window. Two sources of bias may affect these magnitudes. First, some stations may tend to give high readings and others low readings, so the average may be biased by the particular set of detecting stations. Second, the averaging method ignores eligible stations that could have, but failed to, detect an event. The first source of bias can be reduced by estimating station correction terms. The second source of bias can be handled by applying maximum likelihood methods that take into account both detecting and nondetecting stations. We compare several different methods that have been proposed to estimate station correction terms and show that a joint maximum likelihood method is clearly superior to other alternatives. A least-squares method that was used by Murphy and Barker (2003) to estimate correction terms leads to biased estimates of correction terms because it fails to take into account nondetecting stations. The joint method provides accurate estimates of station correction terms and is also superior to applying maximum likelihood to single events without estimating station corrections or with the least-squares corrections. We illustrate our ideas by analyzing data from about 15,000 seismic events recorded by the prototype IDC. We caution that our correction terms are for primary stations only and are not meant to be final estimates; to obtain final correction terms would require applying our methods to a complete data set including both primary and auxiliary stations.

A Semiautomatic Calibration Method Applied to a Small-Aperture Alaskan Seismic Array

Small-to-medium aperture (25 km or less) seismic arrays are of great importance for event location and characterization. Effective use of these arrays requires calibration, preferably with large numbers of events, to account for local and distant structural and propagation effects. We implement a cross-correlation method as a semiautomatic procedure applicable to any small array, able to process thousands of events with several days of computer time on a Sun Blade 1000 workstation. We analyze a database of 1228 P and PcP arrivals recorded between 1997 and 1998 at a 19-element Alaskan seismic array. The arrivals are picked by the Prototype International Data Centre for events well located by the United States Geological Survey. Backazimuth and horizontal-velocity residuals are calculated for all events. Complicated geology beneath the elements and elevation differences among the array stations make static corrections necessary. We use 328 core phases (including PcP , PKiKP , PKP , PKKP ) to determine the static corrections. We present first-order structural interpretations of our calibration results, including a Moho discontinuity dipping to the north with a 10.5° dip angle and a strike of 109°. Our method allows for resolution of phase arrivals within 2–3 sec and has potential to be used as an automatic detector of primary and secondary seismic phases.

Overlooked earthquakes on and around the Antarctica plate: identification and location of 1999 shallow depth events

As a contribution to the study of high-latitude seismicity on the Antarctica plate, we have developed a procedure to identify and locate previously unnoticed earthquakes by using broadband records of 29 stations operating in the Southern Hemisphere. A first approach of the method consists of visually examining continuous records from GEOSCOPE and GSN stations for Rayleigh-waves associated with earthquakes missing in the NEIC monthly catalogue. Two hundred ten earthquakes were identified that had no corresponding locations in the 1999 NEIC catalogue. Among them, 88 events, with magnitude ranging from 3.7 to 5.2, and identified on at least four stations, have been located by using an epicenter location algorithm based on regional Rayleigh-wave propagation models. The precision of the location, depending both on the selected propagation model and the phase picking accuracy, is around 100 km. Most of the reported earthquakes occurred on the plate boundaries, principally along the South-Indian and Pacific ridges. A few epicenters have been located inside the Antarctica plate, far from the plate limits. Among them, only two earthquakes occurring on the continent itself are reported. About half of the events located in our study are reported by the PIDC (Prototype International Data Center). Three earthquakes, with magnitudes ranging from 4.7 to 5.2, reported neither by NEIC nor by PIDC, are sufficiently energetic to be recorded in most broad-band stations operating around the world. Considerations concerning the local crustal structure and the physical processes at the source are involved in interpreting these observations.

Revised Distance and Depth Corrections for Use in the Estimation of Short-Period P-Wave Magnitudes

A new short-period seismic magnitude measure based on uniform, automatic processing of digital P -wave data recorded on the standardized International Monitoring System (IMS) global network of stations has been formulated and tested. The test data consisted of over 220,000 single-station P -wave amplitude and period observations from approximately 25,000 events that were reported in the Reviewed Event Bulletin of the prototype International Data Center, which was established to evaluate various concepts proposed for use in the monitoring of nuclear explosions. Large-scale statistical analyses of these data have been used to define improved corrections for the effects of epicentral distance and focal depth, distance weighting factors for use in the estimation of a new, generalized m b measure ( m b ) that incorporates data observed over the expanded epicentral distance range extending from 2° to 180°, as well as preliminary global station corrections for a number of existing and proposed IMS station sites. The station corrections are considered to be preliminary at this time since possible maximum-likelihood effects are not explicitly considered in the present analysis. However, evidence is presented that indicates that such effects are not significantly biasing the derived distance/depth corrections and that the magnitude of their influence on the estimated station corrections is likely to be rather small with respect to the range encompassed by these corrections. Tests of this new, generalized m b magnitude measure have indicated that it can provide consistent and easily reproducible measures of seismic event magnitude, even for events that are not well recorded at traditional teleseismic distances.

An Assessment of Seismic Body-wave Magnitudes Published by the Prototype International Data Centre

A significant initiative in the reporting of global seismicity began in January 1995 with the publication of daily “Reviewed Event Bulletins” (REB's), a few days in arrears. This work was and still is done in the context of international efforts aimed at verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). When it was finalized in 1996 the CTBT specified that the seismic monitoring network of the International Monitoring System (IMS) would consist of 50 primary stations contributing data continuously to an International Data Centre (IDC) in Vienna, plus 120 auxiliary stations that record continuously but contribute data to IDC only on request (Richards, 2002). Until IDC began daily operations on 21 February 2000, the daily REB was published by the Prototype International Data Centre (PIDC) and a predecessor organization. We shall refer to all the REB's from 1 January 1995 to 20 February 2000 as having been published by PIDC. This paper focuses on the PIDC teleseismic body-wave magnitudes ( mb ). They are important as a modern effort to measure short-period magnitudes based on what is expected to become a fixed network of global stations, all operating in a standardized way. Although seismic magnitudes are intended to be a characteristic of the source, it is well known that in practice they are influenced by numerous additional factors such as the particular set of stations reporting individual magnitudes, the instrument responses, and the details of how each station magnitude is assigned. Global bulletins of seismicity, including mb values, have long been reported, months in arrears, by the Preliminary Determination of Epicenters (PDE) and Earthquake Data Report (EDR) of the U.S. Geological Survey's National Earthquake Information Center (USGS NEIC). They have also been reported, about two years in arrears, by the International Seismological Centre (ISC), based in England. To obtain its …

Processing and analysis of infrasound data at the Comprehensive Nuclear Test Ban Treaty International Data Center

The Center for Monitoring Research (CMR) in Arlington, VA served as a prototype of the International Data Center (IDC) that is specified in the Comprehensive Nuclear Test Ban Treaty (CTBT). The CMR developed the software that is now being used by the IDC that has been established in Vienna to acquire, process, and analyze data from the global networks of hydroacoustic, infrasonic, radionuclide, and seismic sensors that are being installed to assist verification of the CTBT. The prototype IDC tested this software and related procedures very extensively with seismic and radionuclide data, but the infrasonic aspects of the system are less advanced due to the limited number of sensors available and a less complete knowledge of sources and signal propagation. The infrasonic processing system attempts to detect signals of interest by combining spatial coherence and power, and estimates signal arrival time, direction of approach, velocity, and other parameters. After a signal is detected, further processing is carried out to try to identify ‘‘phases’’ that correspond to reflections from different atmospheric layers. Finally, detections at multiple sensors of all types (seismic and hydroacoustic as well as infrasonic) are combined to estimate the time and location of potential sources.

Was the 16 August 1997 Seismic Disturbance near Novaya Zemlya an Earthquake?

The Reviewed Event Bulletin for 16 August 1997, produced by the prototype International Data Center (pIDC), reported a small seismic disturbance in the vicinity of the northern Russian test site at Novaya Zemlya (at about 02:11 UTC). Initial reports suggested that this disturbance was caused by a small nuclear explosion, whereas others identify the source as an earthquake using vertical-component S/P ratios. However, other authors show evidence that questions the validity of the S/P method for this region and conclude that the disturbance cannot be positively identified as an earthquake using seismological evidence. The apparent attenuation from the 16 August 1997 disturbance to SPITS (Spitsbergen, distance Δ = 1280 km, azimuth = 317°) is reported to be weak. Similar low attenuation is suggested from high-frequency observations at KEV (Kevo, Finland, Δ = 1130 km, azimuth = 268°). Thus, high-frequency P and S waves propagate efficiently across the Barents Sea, leading to the possibility that the radiation pattern of the source may be inferred. We show, using a grid-search method, that the high-frequency SV/SH amplitude ratios observed at SPITS and KEV (measured from the complex envelopes of the free-surface corrected three-component seismograms), combined with simple P n signals (with positive first motion) at HFS (Hagfors, Sweden) and NORES (Norway), are consistent with a double-couple source, with a range of orientations. We compute synthetic seismograms for one such double-couple source oriented at σ = 255°, δ = 115°, ψ = 120° (similar to that reported for the 1 August 1986 Kara Sea earthquake). The synthetic P n seismograms at HFS and NORES and the relative SV/SH amplitudes of the synthetic S n seismograms at KEV and SPITS match those observed reasonably well. Our model also predicts (1) weak P n at NRI (Norilsk, Russia), suggesting that the emergent P n onset and low signal-to-noise ratio observed is related to the focal mechanism, and (2) the negative P first motion and large SH/SV ratio observed at AMD (Amderma, Δ = 360 km, azimuth = 152°). Thus, we conclude that the 16 August 1997 seismic disturbance was an earthquake beneath the Kara Sea and that identification is possible using only data that would normally be available to the pIDC (assuming KEV can be used as an alternative for the nearby station ARCES, which was not working at the time of the disturbance).

Seismological identification of the 1998 May 28 Pakistan nuclear test

On 1998 May 28 Pakistan announced that it had conducted an underground nuclear test. Here we assess whether seismological data, recorded by the International Monitoring System (IMS) being set up to help verify the Comprehensive Test Ban Treaty (CTBT), can be used to identify the Pakistani test as a possible underground explosion. The prototype International Data Centre (pIDC) automatically determined the network-averaged body wave ( $\bar m_\mathrm {b}$ ) and surface wave ( $\bar M_\mathrm {s}$ ) magnitudes to be 4.9 and 3.6, respectively. One of the most reliable methods of identifying possible underground explosions is the m b : M s criterion. However, m b : M s is calibrated using conventional magnitudes from historical earthquakes and explosions. We calculate $\bar m_\mathrm {b}=4.88$ , in the conventional way, using P waves from the Pakistani test recorded by a simulated standard short-period seismograph and read by an experienced analyst. We also analyse the three components of the surface waves from the Pakistani test to confirm that these are correctly associated, and calculate $\bar M_\mathrm {s} = 3.44$ . On m b : M s the Pakistani test falls between the historical Eurasian underground explosion and earthquake populations. Thus, while the source may arouse suspicion on m b : M s , its signature is typical of both explosions and deep-lithospheric Eurasian earthquakes. The vast majority of the seismic P signals from the Pakistani test, recorded at long range, are complex. However, simple P seismograms are recorded by at least three of the IMS stations. Analysis, using the relative amplitude method, of three of the simple P seismograms suggests that the source is shallow (less than 5 km). We conclude that the combination of the m b : M s signature and shallow depth are sufficient to classify the Pakistani test as a possible explosion. Under the CTBT an on-site inspection would be required to determine whether the explosion was nuclear.

Application of a Cepstral F Statistic for Improved Depth Estimation

The depth of a seismic event can be determined using peaks corresponding to depth phase-delay times found in the cepstrum of the seismic signal. Until now, however, there has been no method available to determine the significance of various peaks in the cepstrum. We have formulated a cepstral F statistic by using a classic approach to detect a signal in a number of stationarily correlated time series. The method attaches a statistical significance level to peaks in the cepstrum of seismic data caused by echoes in the signal. The method is particularly well suited for detecting depth phase echoes ( pP and sP ) recorded on regional seismic arrays. Detections determined from the peaks in the cepstral F statistic are then stacked as a function of pP-P and sP-P travel times predicted from the IASPEI global 1-D model (Kennett and Engdahl, 1991), using a modification of the method of depth-phase beamforming (Woodgold, 1998; Murphy et al. , 1999). Tests on synthetic data show the method is most successful when the P -wave arrival has a signal-to-noise ratio (SNR) greater than 4-7 and the depth phase exhibits an SNR greater than ∼2. Tests on complex regional data suggest that the depth-phase SNR must be between 4 and 8 for consistent successful application of the method. We tested the method by using events from the Hindu-Kush region of Afghanistan with well-determined depths as recorded on arrays at teleseismic distances. We determined the correct depths for 14 of the 19 events in this dataset (74% success rate), and we determined incorrect depths for two events, for a false-alarm rate of 10.5%. The difference between the depths calculated by the cepstral method and the prototype International Data Center (pIDC) decrease with increasing magnitude. For events with m b > 3.8, the average difference between the cepstral and pIDC depths was 7.9 km, with no false alarms. To test the operational capabilities of this method as a tool for data center use, we first analyzed 32 events that occurred on 12 February 2000 as located by the pIDC. Additionally, we analyzed 29 events located in Central and South America between 05 May and 15 June 2000 with event characteristics published by the International Data Center (IDC). Most of these events were also located by the National Earthquake Information Center. Our method determined statistically significant depths for 40 of these 61 events, with 11 having a low SNR at three or more recording arrays, while another 10 were either too shallow for analysis or did not exhibit depth phases. Only seven of the 61 events had depth phases listed in the Reviewed Event Bulletin produced by the pIDC and IDC. For these events, the average difference between the reported and cepstral F -statistic depths was 4.0 km. The pIDC/IDC fixed the depth of 26 of 61 events to 0.0. The cepstral F -statistic method determined depths ranging from 16 to 92 km for 15 of the events that had been constrained as surface foci. Overall, we believe the method would be most valuable when used by analysts to detect possible depth phases that could then verify or improve network-calculated focal depths. Manuscript received 25 February 2001.

Infrasonic Signal Detection and Source Location at the Prototype International Data Centre

This paper describes an automatic and interactive data processing system designed to locate impulsive atmospheric sources with a yield of at least one kiloton by detecting and characterizing the airborne infrasound radiated by the source. The infrasonic processing subsystem forms part of a larger system currently under development at the Prototype International Data Center (PIDC) in Arlington, Virginia where seismic, hydroacoustic, radionuclide and infrasonic methods are used to detect and locate impulsive sources in any terrestrial environment. Infrasonic signal detection is achieved via a coincidence detector which requires both the normalized cross correlation and the short-term-average/ long-term-average ratio of a beam in the direction of maximum correlation to exceed predetermined threshold values simultaneously before a detection is declared.The infrasound propagation model currently used to infer travel-time information assumes the horizontal sound speed across the ground to be 320.0 m/s. This crude model is currently being replaced by a model which predicts travel-time information through a ray-tracing algorithm for acoustic waves in an atmosphere with seasonal representations for temperature and wind. A novel feature of the source location process is the fusion of all available arrival information, whether it be seismic, hydroacoustic or infrasonic to locate a single source where it is reasonable to hypothesize a common source. In its final configuration the infrasonic subsystem will routinely process data from the global 60-station International Monitoring System (IMS) infrasonic network currently under development.

Estimation of Background Noise for International Monitoring System Seismic Stations

The prototype International Data Centre (IDC) in Arlington, Virginia has been acquiring data from seismic stations at locations designated in the Comprehensive Test-Ban Treaty for the International Monitoring System (IMS) since the start of 1995. A key characteristic of these stations is their background noise levels and their seasonal and diurnal variability. Since June 1997 an automated sample selection effort has collected over 700,000 individual noise sample spectra from 39 primary and 57 auxiliary stations. Monthly median and 5 and 95 percentile estimates have been calculated for each channel of every station. Compatibility of median spectra obtained for the same station and channel in the same month for two different years confirms the consistency of the noise-sampling algorithm used. A preliminary analysis of the results shows strong (more than a factor of two) seasonal variation at a quarter of all stations. Strong diurnal variations at half of the sites indicate that many of the selected sites are poorly located with respect to cultural noise sources. The results of this study are already being used to evaluate station quality, improve those processes that require background noise values, such as automatic association and requesting auxiliary station data, and to improve the estimation of station and network detection and location thresholds.

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.

Global Test of Seismic Event Locations Using Three-Dimensional Earth Models

To prepare to update location procedures at the International Seismological Centre (ISC), we conduct a global test of location accuracy using two 3D Earth models and three 1D models (Jeffreys-Bullens [J-B], Preliminary Reference Earth Model [PREM], ak135) with initial phase arrival times from different distance ranges. The 3D models include one spherical harmonic model and one irregular grid model, representing two important alternative model parameterizations. Our tests include a worldwide distribution of reference events from a previous study by Smith and Ekstrom (1996) and the ground-truth database of the prototype International Data Centre. For each test we use phase arrival times associated with the events in the ISC Bulletin and recompute hypocenters with the same algorithm used in the ISC operations. With teleseismic arrival times alone (Δ ≥ 25°), the overall accuracy of our recomputed hypocenters is similar using either of the 3D models. The teleseismic hypocenters from 3D models are, on an average, closer to the reference locations than the hypocenters computed using any of the 1D models; but the improvement in results using ak135 is small. When we use the irregular grid model with teleseismic, regional, and local initial arrival times together and fix to reference depths, the accuracy of the epicenters is improved over the results with teleseismic times alone. With a 3D model we are able to invert for depth for a larger fraction of earthquakes, and our newly computed depths are more tightly clustered in a subduction zone than depths from the ISC Bulletin, but not as tightly as depths computed by Engdahl et al. (1998) who used depth phases as well as direct phases. In a test of all the locatable events from 1 month of the ISC Bulletin the irregular grid model improved residuals and formal location uncertainties without introducing large systematic changes in the hypocenters.

Path-Dependent Regional Phase Travel-Time Corrections for the International Monitoring System in North America

The software developed for the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Data Centre (IDC) in Vienna, Austria, uses the IASPEI91 seismic travel-time tables as default. However, in reality, travel times as a function of source location are different for each station, especially at regional distances. This is not modeled by a global one-dimensional model such as IASPEI91. The software therefore allows the application of a suite of travel-time corrections for improved travel-time prediction and consequently, more accurate hypocentral locations. To account for path dependency, we have developed gridded source-specific station corrections (SSSCs) for Pn, Sn, Pg , and Lg phases for 22 International Monitoring System (IMS) stations in North America. In deriving the SSSCs and associated modeling errors, we used a methodology that had been successfully applied previously for Fennoscandia. We first compiled regionalized one-dimensional travel times for North America. Then we assigned travel times as weighted sums of the travel times in each region to calculate SSSCs. Validation testing of the SSSCs was conducted using two separate sets of North American ground truth (GT) events with location accuracies within 10 km (GT0–GT10). These events were recorded at IMS stations between 1995 and 1999 and distributed throughout North America. Test event location results with SSSCs applied show reductions in both mislocations and error ellipses. For GT events with location accuracies better than 2 km, the median change in location was a 5-km improvement, and the median area of the 90th percentile error ellipses was reduced from 2550 km 2 to 910 km 2 . Using SSSCs for both data sets, the median improvement is less than 5 km. The testing emphasized that GT events with accuracies 5 km or better are crucial for validating SSSCs. Using SSSCs for all 51 GT0–GT10 events used in the validation testing, the median area of the error ellipses was reduced to less than 1000 km 2 , and the error ellipse reductions were achieved without significant degradation of coverage. The modeling errors used in this study may be somewhat underestimated, however, leaving the target goal of error ellipses less than 1000 km 2 still somewhat in question. We expect the SSSCs, event locations, and uncertainties can be improved by future refinement of travel times and modeling errors. These corrections were implemented in the operational Prototype IDC system located in Arlington, Virginia, in March 2000 and have been delivered to the Vienna IDC in January 2001.

The Kirovskiy Explosion of September 29, 1996: Example of a CTB Event Notification for a Routine Mining Blast

On September 29, 1996, a routine mining blast of about 390 metric tons was detonated underground at the Kirovskiy mine in the central Kola Peninsula. The United States was notified two weeks in advance that the blast was to take place and was given the date, approximate time, location and total charge. The explosion was detected and located by the prototype International Data Center (pIDC) and published in the Reviewed Event Bulletin (REB). Detailed information about the blast, including the type and depth of mining operation, the underground charge configuration, and the blasting delay pattern, is reviewed and combined with a seismological analysis of the event. The seismic analysis points to a possible associated tectonic component to the blast, consisting of a small rock burst or induced tremor, spall, or some combination of these mechanisms, that may have enhanced the shear waves, produced largeRgwaves at low frequency, and smallPn/SnandPn/Lgamplitude ratios at high frequency. While these discriminants might identify the event as an earthquake, the spectral/cepstral analysis of the event clearly shows the ripple-fire delays. This event provides important confidence-building measures for both location calibration, in the form of travel-time corrections for location of mine events in this region, and for improved understanding of seismic discriminants expected for large mine blasts that may have an associated induced tectonic component (e.g., spall, mine tremor or rock burst).

Calibration of Regional S/P Amplitude-ratio Discriminants

— S/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.

Association of Rayleigh Waves Using Backazimuth Measurements: Application to Test Ban Verification

The defining characteristic of a fundamental mode Rayleigh wave in a spherical, isotropic Earth is its elliptical polarization in the plane of propagation. Measurement of this polarization is thus critical both in identification and in determining the backazimuth of a Rayleigh wave from receiver to source and hence the association with a particular seismic disturbance. Currently, the prototype International Data Center (pIDC) and the International Data Center (IDC) measure backazimuth but do not utilize this in association because it is believed that measurement of backazimuth is unreliable. In this article, we demonstrate that an accurate measurement of backazimuth can be obtained for the large majority of Rayleigh waves reported in the bulletins of the pIDC/IDC. A reliable backazimuth measurement confirms the association of a waveform with a seismic disturbance, whereas the inclusion of large backazimuth errors in the bulletin can cast doubt on the association. Because the object of event screening is to positively identify earthquakes, it is vital that surface-wave magnitudes are only measured from positively identified and associated Rayleigh waves.

Operational Processing of Hydroacoustics at the Prototype International Data Center

Operational Processing of Hydroacoustics at the Prototype International Data Center

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

aDetailedstudies 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 ML= 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.