Regional P-Coda for Stable Estimates of Body Wave Magnitude: Application to Novaya Zemlya and Nevada Test Site Events PDF Download
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Author: Publisher: ISBN: Category : Languages : en Pages : 24
Book Description
Regional seismic explosion monitoring requires the discrimination of small clandestine nuclear explosions from background earthquakes. most successful teleseismic discriminant, the so-called Ms:mb, discriminant, compares the long-period surface waves magnitude (Ms) with the period P-based body wave magnitude (mb). There are many studies underway to try and extend surface wave magnitude (Ms) estimation to re distances and smaller magnitudes. Another problem that is encountered is how to estimate mb so that the Ms:mb discriminant is meaningful a consistent with teleseismic measures. For small-to-moderate sized events, the teleseismic body wave magnitude, mb(P), cannot be effectively measured due to low signal-to-noise ratio. We develop a stable regional alternative based on the P-coda that scales 1-to-1 with the teleseismic mb(P), but with the advantage of lower variance. Though mb(Lg) and mb(Lg-coda) can be tied to mb(P) for explosions, they overpredict earth magnitudes by ~0.5-1 magnitude units and degrade the performance of the Ms:mb discriminant. In contrast, mb(P-coda) does not exhibit this and can be used to extend Ms:mb to smaller regional events.
Author: Publisher: ISBN: Category : Languages : en Pages : 24
Book Description
Regional seismic explosion monitoring requires the discrimination of small clandestine nuclear explosions from background earthquakes. most successful teleseismic discriminant, the so-called Ms:mb, discriminant, compares the long-period surface waves magnitude (Ms) with the period P-based body wave magnitude (mb). There are many studies underway to try and extend surface wave magnitude (Ms) estimation to re distances and smaller magnitudes. Another problem that is encountered is how to estimate mb so that the Ms:mb discriminant is meaningful a consistent with teleseismic measures. For small-to-moderate sized events, the teleseismic body wave magnitude, mb(P), cannot be effectively measured due to low signal-to-noise ratio. We develop a stable regional alternative based on the P-coda that scales 1-to-1 with the teleseismic mb(P), but with the advantage of lower variance. Though mb(Lg) and mb(Lg-coda) can be tied to mb(P) for explosions, they overpredict earth magnitudes by ~0.5-1 magnitude units and degrade the performance of the Ms:mb discriminant. In contrast, mb(P-coda) does not exhibit this and can be used to extend Ms:mb to smaller regional events.
Author: Publisher: ISBN: Category : Languages : en Pages : 5
Book Description
We use regional waveform data from the Nevada Test Site (NTS) to investigate phenomenological relationships between recorded amplitude and explosion yield as well as test regional depth estimation procedures. Our goal is to better understand the performance of seismic observables in other regions of monitoring interest, especially at small magnitudes (mb much less than 4.5). Some of the topics we are studying include: stable yield estimation, depth estimation, and Ms:mb performance. We use Lawrence Livermore National Laboratory's NTS explosion database, which consists of several hundred events ranging from approx. 200- to approx. 1500-m depth and yields ranging from a few tenths of a kiloton to the megaton range. In addition to the broadband explosion data, we have a large dataset of well-located earthquakes on the test site with depths ranging from 2 to 17 km and magnitudes ranging between MW 1.5 and 5.7. For yield estimation the relation between teleseismic body wave magnitude (mb) and nuclear explosion yield has been studied extensively over the past several decades for a number of test sites for large (greater than 1 kt) explosions. In this paper we will look at broadband coda, Pg, and Lg from over 260 nuclear explosions to study yield estimation capability by comparing F-factors. For monitoring compliance with a CTBT, small events that are recorded only at regional distances will be used to estimate magnitude and equivalent yield. Past coda studies show that coda-derived magnitudes of earthquakes and explosions are more stable than any direct phase method, including mb(Lg). In fact, single-station coda measurements can be equivalent to a network average of at least ten direct phase measurements over a broad range of frequencies.
Author: Publisher: ISBN: Category : Languages : en Pages : 18
Book Description
We have applied the regional S-wave coda calibration technique of Mayeda et al. (2003) to earthquake data in and around the Nevada Test Site (NTS) using 4 regional broadband stations from the LLNL seismic network. We applied the same path and site corrections to tamped nuclear explosion data and averaged the source spectra over the four stations. Narrowband coda amplitudes from the spectra were then regressed against inferred yield based on the regional m{sub b}(Pn) magnitude of Denny et al. (1987), along with the yield formulation of Vergino and Mensing (1990). We find the following: (1) The coda-derived spectra show a peak which is dependent upon emplacement depth, not event size; (2) Source size estimates are stable for the coda and show a dependence upon the near-source strength and gas porosity; (3) For explosions with the same m{sub b}(Pn) or inferred yield, those in weaker material have lower coda amplitudes at 1-3 Hz.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Our identification research for the past several years has focused on the problem of correctly discriminating small-magnitude explosions from a background of earthquakes, mining tremors, and other events. Small magnitudes lead to an emphasis on regional waveforms. The goal is to reduce the variance within the population of each type of event, while increasing the separation between the explosions and the other event types. We address this problem for both broad categories of seismic waves, body waves, and surface waves. First, we map out the effects of propagation and source size in advance so that they can be accounted for and removed from observed events. This can dramatically reduce the population variance. Second, we try to optimize the measurement process to improve the separation between population types. For body waves we focus on the identification power of the short-period regional phases Pn, Pg, Sn and Lg, and coda that can often be detected down to very small magnitudes. It is now well established that particular ratios of these phases, such as 6- to 8-Hz Pn/Lg, can effectively discriminate between closely located explosions and earthquakes. To extend this discrimination power over broad areas, we developed a revised Magnitude and Distance Amplitude Correction (MDAC2) procedure (Walter and Taylor, 2002). This joint source and path model fits the observed spectra and removes magnitude and distance trends from the data. It allows for the possibility of variable apparent stress scaling in earthquakes, an unresolved issue that is the subject of investigation under separate funding. The MDACZ procedure makes use of the extremely stable coda estimates of Mw for source magnitude and can also use independent Q tomography to help reduce trade-offs in fitting spectra. We can then apply the kriging operation to the MDAC2 residuals to provide full 2-D path corrections by phase and frequency band. These corrections allow the exploration of all possible ratios and multivariate combinations of ratios for their discrimination power. We also make use of the MDAC2 spectra and the noise spectra to determine the expected signal-to-noise value of each phase and use that to optimize the multivariate discriminants as a function of location. We quantify the discrimination power using the misidentified event trade-off curves and an equi-probable measure. In addition to the traditional phases, we are also exploring the application of coda amplitudes in discrimination. Coda-derived spectra can be peaked due to Rg-to-coda scattering, which can indicate an unusually shallow source. For surface waves we have a new high-resolution regional Rayleigh-Wave tomography for the Yellow Sea and Korean Peninsula Region, based on measuring thousands of seismograms. We also continue to make new measurements for our regional Rayleigh and Love wave group velocity tomography models of Western Eurasia and North Africa. These tomography models provide high-resolution maps of group velocity from 10- to 100-s period. The maps also provide estimates of the expected phase spectra of new events that can be used in phase-match filters to compress the expected signals and improve the signal-to-noise ratio on surface wave magnitude (Ms) estimates. Phase match filters in combination with regional Ms formulas can significantly lower the threshold at which Ms can be measured, extending the Ms-mb discriminant. We have measured Ms in western Eurasia for thousands of events at tens of stations, with and without phase match filtering, and found a marked improvement in discrimination. Here we start to quantify the improvement to both discrimination performance and the Ms threshold reduction. The group velocity models also provide constraints on velocity structure, particularly in low seismicity regions. For example we are working with Dr. Bob Henmann and Dr. Charles Ammon to combine tomography derived group velocity curves with station based receiver functions in joint inversions to estimate structure.
Author: Bradley B. Woods Publisher: ISBN: Category : Languages : en Pages : 8
Book Description
Seismic moments of Nevada Test Site (NTS) explosions were determined from regional surface wave spectra. Two methods were used. In one the moment is solved for assuming only an explosive source, or average scalar moment; in the other a joint inversion for an isotropic (explosive) source plus a constrained double couple moment component representing tectonic strain release (TSR). Although the general moment tensor solution to this joint inversion problem is non-unique, if some assumptions are made concerning the non-isotropic moment components, then the remaining source parameters can be solved by a linear least-squares inversion scheme. We examined the errors in determining the isotropic moment component (M sub I) by this latter method of constrained linear inversion solutions in a canonical study using a theoretical network of long-period (6-60 sec.) surface wave data. The network azimuthal coverage was chosen to represent that of a long-period North American super-network of 55 stations used for the actual NTS events. We compared these errors in moment estimate to those obtained from surface wave magnitude (M sub s) and spectral scalar moment (M sub 0) measurements for the same surface wave observations. For a ratio of M sub expl/M sub eq less than 1.0 we found that the inverted M sub I solution is a much better estimate of the actual isotropic moment than either M sub s or M sub O, and the standard deviation in this estimate is substantially less than that using the other two methods for the great majority of isotropic source + double couple sources. Even when the inversion constraints are off in dip and rake each by 30 deg, the mis-estimate of the isotropic moment is less than 35 percent of the actual value.
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Author: United States. Arms Control and Disarmament Agency
Author: United States. Arms Control and Disarmament Agency
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Author: Bradley B. Woods
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Author: Bradley B. Woods
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