Hans E. Hartse

Amplitude corrections for regional seismic discriminants

Abstract — A fundamental problem associated with event identification lies in deriving corrections that remove path and earthquake source effects on regional phase amplitudes used to construct discriminants. Our goal is to derive a set of physically based corrections that are independent of magnitude and distance, and amenable to multivariate discrimination by extending the technique described in Taylor and Hartse (1998). For a given station and source region, a number of well-recorded earthquakes is used to estimate source and path corrections. The source model assumes a simple Brune (1970) earthquake source that has been extended to handle non-constant stress drop. The discrimination power in using corrected amplitudes lies in the assumption that the earthquake model will provide a poor fit to the signals from an explosion. The propagation model consists of a frequency-independent geometrical spreading and frequency-dependent power law Q. A grid search is performed simultaneously at each station for all recorded regional phases over stress-drop, geometrical spreading, and frequency-dependent Q to find a suite of good-fitting models that remove the dependence on mb and distance. Seismic moments can either be set to pre-determined values or estimated through inversion and are tied to mb through two additional coefficients. We also solve for frequency-dependent site/phase excitation terms. Once a set of corrections is derived, effects of source scaling and distance as a function of frequency are applied to amplitudes from new events prior to forming discrimination ratios. Thus, all the corrections are tied to just mb (or M0) and distance and can be applied very rapidly in an operational setting. Moreover, phase amplitude residuals as a function of frequency can be spatially interpolated (e.g., using kriging) and used to construct a correction surface for each phase and frequency. The spatial corrections from the correction surfaces can then be applied to the corrected amplitudes based only on the event location. The correction parameters and correction surfaces can be developed offline and entered into an online database for pipeline processing providing multivariate-normal corrected amplitudes for event identification. Examples are shown using events from western China recorded at the station MAKZ.

1 Hz Lg Q tomography in central Asia

We have applied tomographic techniques to a data set of over 1700, regional distance, Lg amplitudes from 12 stations for paths inside a 30° by 40° region of Central Asia. Our purpose is to create high resolution path correction maps for use in regional distance source discrimination and magnitude determination, as well as to study Lg propagation characteristics in one of the most heterogeneous portions of the earth. For 1.0 Hz data, tomography reduced variance 33%, relative to a best fit, uniform Q model. LgQ varied with geologic region, with low values in Tibet, the Tien Shan and the Pamir range (Q=200 to 400), intermediate values in basins such as the Tarim (Q=500) and high values for platforms and older crust (Q>800). The relatively efficient propagation across the Tarim differs from the poor propagation that is generally observed across sedimentary basins.

A procedure for estimation of source and propagation amplitude corrections for regional seismic discriminants

We outline a procedure for the estimation of frequency-dependent source and propagation amplitude corrections for regional seismic discriminants (source path amplitude correction (SPAC)). For a given station and phase a number of well-recorded earthquakes are inverted for source and path corrections. The method assumes a simple Brune [1970] earthquake-source model and a simple propagation model consisting of a frequency-independent geometrical spreading and frequency-dependent power law Q. The inverted low-frequency levels are then regressed against mb to derive a set of corrections that are a function of mb and distance. Once a set of corrections is derived, effects of source scaling and distance as a function of frequency are applied to amplitudes from new events prior to forming discrimination ratios. The resulting discriminants are normally distributed and amenable to multivariate feature selection, classification, and outlier techniques. To date, most discrimination studies have removed distance corrections once a particular amplitude ratio is formed (distance corrected ratio (DCR)). DCR generally works well for phase ratios taken in a particular frequency band. However, when different frequency bands are combined (for phase spectral ratios or cross spectral ratios), significant source-scaling effects (e.g., corner-frequency scaling) can remain, causing the discriminants to vary as a function of event size and to be nonnormally distributed. It is then often necessary to construct nonphysical transformations in an attempt to make the discriminants multivariate normal. The SPAC technique can be used to construct discriminants that are multivariate normal by using simple physical seismic source and propagation models. Moreover, phase amplitude residuals as a function of frequency can be spatially averaged and used as additional path-specific corrections to correct for additional propagation effects such as phase blockages.

Forensic seismology and the sinking of the Kursk

On August 10, 2000, Russias Northern Fleet began its largest naval exercise in more than a decade. Among the vessels taking part was the heavily-armed Kursk, an Oscar class submarine that was the most modern cruise-missile sub in the fleet. Beginning on August 14, a series of reports in the press indicated that the Kursk had been severely damaged during the exercise and that the crew were likely dead. By August 17, news agencies were reporting that seismic networks in the Baltic area had detected two seismic events which appeared to correspond to the Kursk disaster in time and space (Figure 1). Specifically the seismic events were consistent with reports from the British Broadcasting Corporation on the location of ongoing rescue efforts. The fact that this section of the Barents Sea is essentially aseismic added credence to the assertion that the seismic events were directly related to the sinking of the Kursk.

An evaluation of generalized likelihood Ratio Outlier Detection to identification of seismic events in Western China

The Generalized Likelihood Ratio Outlier Detection Technique for seismic event identification is evaluated using synthetic test data and frequency-dependent P{sub g}/L{sub g} measurements from western China. For most seismic stations that are to be part of the proposed International Monitoring System for the Comprehensive Test Ban Treaty, there will be few or no nuclear explosions in the magnitude range of interest (e.g. M{sub b} < 4) on which to base an event-identification system using traditional classification techniques. Outlier detection is a reasonable alternative approach to the seismic discrimination problem when no calibration explosions are available. Distance-corrected P{sub g}/L{sub g} data in seven different frequency bands ranging from 0.5 to 8 Hz from the Chinese Digital Seismic Station WMQ are used to evaluate the technique. The data are collected from 157 known earthquakes, 215 unknown events (presumed earthquakes and possibly some industrial explosions), and 18 known nuclear explosions (1 from the Chinese Lop Nor test site and 17 from the East Kazakh test site). A feature selection technique is used to find the best combination of discriminants to use for outlier detection. Good discrimination performance is found by combining a low-frequency (0.5 to 1 Hz) P{sub g}/L{sub g} ratio with high-frequency ratios (e.g. 2 to 4 and 4 to 8 Hz). Although the low-frequency ratio does not discriminate between earthquakes and nuclear explosions well by itself, it can be effectively combined with the high-frequency discriminants. Based on the tests with real and synthetic data, the outlier detection technique appears to be an effective approach to seismic monitoring in uncalibrated regions.

Small-scale stress heterogeneity in the Anza seismic gap, southern California

Focal mechanism inversions reveal significant lateral variations in stress orientations along the Anza segment of the San Jacinto fault zone. The most notable stress anomaly is within the 20-km aseismic (seismic gap) portion of the fault zone, where σˆ1, the maximum compressive stress, is nearly horizontal and is oriented at 74°±13° relative to the fault strike. This contrasts with orientations ranging from 62°±11° to 49°±7° along the more seismically active portions of the fault zone immediately to the northwest and southeast of the seismic gap. Regional stress results, found by inverting all focal mechanisms simultaneously, indicate that σˆ1 is horizontal and trends north-south, while σˆ3 is horizontal and trends east-west. Approximately, 15 km west of the seismic gap, in the off-fault Cahuilla swarm area, σˆ1 and σˆ3 solutions are rotated clockwise by about 25° relative to the regional model. Roughly, 10 km southeast of the seismic gap near the Buck Ridge fault, σˆ1 and σˆ3 are rotated counterclockwise by about 10° relative to the regional solution. Northwest of the seismic gap along the fault zone, σˆ3 plunges about 30° from the horizontal, correlating with a local increase in reverse faulting between the Hot Springs and San Jacinto faults. Southeast of the seismic gap, σˆ1 plunges about 45° from the horizontal, correlating with a local increase in normal faulting in the trifurcation region of the Buck Ridge, Clark, and Coyote Creek faults. We propose a simple mechanical model in which a block rotation superimposed on the dominant right-lateral strikeslip motion of the fault zone satisfies the first-order observations of stress orientation, faulting, and horizontal surface strain. Under this model the Anza seismic gap is the region of zero convergence between the northeast and southwest sides of the fault, and the fault zone strength within the seismic gap is either comparable to or exceeds the fault zone strength adjacent to the gap.

Application of regional phase amplitude tomography to seismic verification

Abstract — We have applied tomographic techniques to amplitude data to quantify regional phase path effects for use in source discrimination studies. Tomography complements interpolation (kriging) methods by extending our ability to apply path corrections into regions devoid of calibration events, as well as raising levels of confidence in the corrections because of their more physical basis. Our tomography technique solves for resolvable combinations of attenuation, source-generation, site and spreading terms. First difference regularization is used to remove singularities and reduce noise effects.¶In initial tests the technique was applied to a data set of 1488, 1.0 Hz, Pg/Lg amplitude ratios from 13 stations for paths inside a 30° by 40° box covering western China and surrounding regions. Tomography reduced variance 60%, relative to the power-law distance correction traditionally applied to amplitude ratios. Relative Pg/Lg attenuation varied with geologic region, with low values in Tibet, intermediate values in basins and high values for platforms and older crust. Spatial patterns were consistent with previous path effect studies in Asia, especially local earthquake coda-Q. Relative spreading was consistent with expected values for Pg and Lg. Relative site terms were similar to one another, yet some tradeoff with attenuation was evident.¶Tomography residuals followed systematic trends with distance, which may result from the evolution from direct to coda phases, focusing, model tradeoff or data windowing effects. Examination of the residuals using a kriging interpolator showed coherent geographical variations, indicating unmodeled path effects. The residual patterns often follow geological boundaries, which could result from attenuating zones or minor blockages that are too thin to be resolved, or that have anisotropic effect on regional phases. These results will guide efforts to reparameterize tomography models to more effectively represent regional wave attenuation and blockage. The interpolated residuals also can be combined with predictions of the tomographic model to account for path effects in discrimination studies on a station by station basis.

Chapter 12 Source Effects From Broad Area Network Calibration of Regional Distance Coda Waves

Abstract We have applied regional coda techniques to a network of 64 stations across central and east Asia to isolate source effects for bands from 0.03 to 8 Hz. The heterogeneity of the study region required us to determine two‐dimensional (2‐D) path and transfer function corrections. The importance of the 2‐D path corrections increased with frequency and distance, and for continental paths, became critical beyond 500 km for 1 Hz data. We propose a new spreading model for coda amplitudes, termed the extended Street‐Herrmann (ESH) model, to which attenuation can be added, facilitating the use of tomographic techniques for path correction. The 2‐D transfer function varied between continents and oceans, as well as within continents in areas of poor Lg propagation, reflecting differing excitation of Lg and Sn coda. We also demonstrate the use of empirically determined coda shapes, or type curves , to measure coda amplitudes, adding precision and flexibility for source regions of special interest. We applied these techniques to 112,000 records from 35,000 events, magnitudes 2–7, depths 0–50 km, between latitudes 0° and 60° and longitudes 60° and 150°. The resulting coda source spectra were used to derive moments and, for the better recorded events, corner frequencies, allowing computation of apparent stress for just under 6700 earthquakes. Preliminary apparent stress results ranged from 10 −2 to 1 MPa and showed some regional variation. For example, stress increased from south to north across the Tian Shan, perhaps reflecting deformation in varying crustal rheology or effects of prior slip history. Low stress observed in Tibet could be an artifact of under correction for high attenuation; however, the correlation also could be physical. Low stress observed in oceanic regions is inconsistent with local studies and indicates that upgrades to the coda methodology to more explicitly account for mixed Lg and Sn coda will be needed. The regional network coda results should be further tested by comparing to ground‐truth spectra obtained by applying coda techniques to data from local scale networks within the study region.

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

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

Monitoring the comprehensive nuclear-test-ban treaty : seismic event discrimination and identification

Preface: Monitoring the Comprehensive Nuclear-Test-Ban Treaty.- Amplitude Corrections for Regional Seismic Discriminants.- Path Corrections for Source Discriminants: A Case Study at Two International Seismic Monitoring Stations.- Seismic Discrimination of the May 11, 1998 Indian Nuclear Test with Shortperiod Regional Data from Station NIL (Nilore, Pakistan).- Observed Characteristics of Regional Seismic Phases and Implications for P/S Discrimination in the European Arctic.- Study of Regional Surface Waves and Frequency-dependent Ms:mb Discrimination in the European Arctic.- Identification of Earthquakes and Explosions Using Amplitude Ratios: The Vogtland Area Revisited.- Seismic Event Identification of Earthquakes and Explosions in Germany Using Spectral.- Signal Processing for Indian and Pakistan Nuclear Tests Recorded at IMS Stations Located in Israel.- Discriminating Between Large Mine Collapses and Explosions Using Teleseismic P Waves.- Identification of Mining Blasts at Mid- to Far-regional Distances Using Low Frequency Seismic Signals.- Experimental Seismic Event-screening Criteria at the Prototype International Data Center.- Testing for Multivariate Outliers in the Presence of Missing Data.