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Kjeller, Norway

Gibbons S.J.,NORSAR
Pure and Applied Geophysics | Year: 2014

The seismic arrays of the International Monitoring System (IMS) for the Comprehensive Nuclear-Test-Ban Treaty (CTBT) are highly diverse in size and configuration, with apertures ranging from under 1 km to over 60 km. Large and medium aperture arrays with large inter-site spacings complicate the detection and estimation of high-frequency phases lacking coherence between sensors. Pipeline detection algorithms often miss such phases, since they only consider frequencies low enough to allow coherent array processing, and phases that are detected are often attributed qualitatively incorrect backazimuth and slowness estimates. This can result in missed events, due to either a lack of contributing phases or by corruption of event hypotheses by spurious detections. It has been demonstrated previously that continuous spectral estimation can both detect and estimate phases on the largest aperture arrays, with arrivals identified as local maxima on beams of transformed spectrograms. The estimation procedure in effect measures group velocity rather than phase velocity, as is the case for classical f-k analysis, and the ability to estimate slowness vectors requires sufficiently large inter-sensor distances to resolve time-delays between pulses with a period of the order 4-5 s. Spectrogram beampacking works well on five IMS arrays with apertures over 20 km (NOA, AKASG, YKA, WRA, and KURK) without additional post-processing. Seven arrays with 10-20 km aperture (MJAR, ESDC, ILAR, KSRS, CMAR, ASAR, and EKA) can provide robust parameter estimates subject to a smoothing of the resulting slowness grids, most effectively achieved by convolving the measured slowness grids with the array response function for a 4 or 5 s period signal. Even for medium aperture arrays which can provide high-quality coherent slowness estimates, a complementary spectrogram beampacking procedure could act as a quality control by providing non-aliased estimates when the coherent slowness grids display significant sidelobes. The detection part of the algorithm is applicable to all IMS arrays, with spectrogram-based processing offering a potential reduction in the false alarm rate for high-frequency signals. Significantly, the local maxima of the scalar functions derived from the transformed spectrogram beams are robust estimates of the signal onset time. High-frequency energy is of greater importance for lower event magnitudes and in the cavity decoupling detection evasion scenario. There is a need to characterize both propagation paths with low attenuation of high-frequency energy and situations in which parameter estimation on array stations fails. © 2012 Springer Basel. Source

Erduran E.,NORSAR | Dao N.D.,University of Nevada, Reno | Ryan K.L.,University of Nevada, Reno
Earthquake Engineering and Structural Dynamics | Year: 2011

In this study, the multi-intensity seismic response of code-designed conventional and base-isolated steel frame buildings is evaluated using nonlinear response history analysis. The results of hazard and structural response analysis for three-story braced-frame buildings are presented in this paper. Three-dimensional models for both buildings are created and seismic response is assessed for three scenario earthquakes. The response history analysis results indicate that the design objectives are met and the performance of the isolated building is superior to the conventional building in the design event. For the Maximum Considered Earthquake, isolation leads to reductions in story drifts and floor accelerations relative to the conventional building. However, the extremely high displacement demands of the isolation system could not be accommodated under normal circumstances, and creative approaches should be developed to control displacements in the MCE. © 2010 John Wiley & Sons, Ltd. Source

Erduran E.,NORSAR | Ryan K.L.,University of Nevada, Reno
Earthquake Engineering and Structural Dynamics | Year: 2011

In this study, the torsional response of buildings with peripheral steel-braced frame lateral systems is evaluated. A three-dimensional model of a three story braced frame with various levels of eccentricity is created and the effects of torsion on the seismic response is assessed for four hazard levels. The response history analysis results indicate that, unlike frame structures, the torsional amplifications in the inelastic systems exceed those of corresponding elastic systems and tend to increase with an increase in the level of inelasticity. The ability of two simplified procedures, elastic response spectrum analysis and pushover analysis, to capture the torsional amplifications in steel-braced frames is evaluated. Copyright © 2010 John Wiley & Sons, Ltd. Source

Erduran E.,NORSAR | Kunnath S.K.,University of California at Davis
Earthquake Spectra | Year: 2010

The displacement coefficient method proposed in FEMA 440 was evaluated using response statistics from a comprehensive set of nonlinear simulations of multi degree of freedom systems under both far-fault and near-fault ground motions. The study finds that it is practically difficult to achieve high relative strength factors (R values equal to or greater than 6.0) for very stiff systems thereby dictating the need to define R-dependent demand coefficients. The approximate expressions proposed in FEMA 440 for the C2 coefficient is shown to underestimate the displacement demand of stiffness-degrading short period systems. Additional nonlinear simulations were performed to investigate the combined effect of strength degradation and P-Delta effects on the displacement demands of MDOF systems. A new expression for the modification factor that reflect combined P-Delta and degrading effects for the estimation of displacement demands is proposed. © 2010, Earthquake Engineering Research Institute. Source

Harris D.B.,Lawrence Livermore National Laboratory | Kvaerna T.,NORSAR
Geophysical Journal International | Year: 2010

Scattering and refraction of seismic waves can be exploited with empirical-matched field processing of array observations to distinguish sources separated by much less than the classical resolution limit. To describe this effect, we use the term 'superresolution', a term widely used in the optics and signal processing literature to denote systems that break the diffraction limit. We illustrate superresolution with Pn signals recorded by the ARCES array in northern Norway, using them to identify the origins with 98.2 per cent accuracy of 549 explosions conducted by closely spaced mines in northwest Russia. The mines are observed at 340-410 km range and are separated by as little as 3 km. When viewed from ARCES many are separated by just tenths of a degree in azimuth. This classification performance results from an adaptation to transient seismic signals of techniques developed in underwater acoustics for localization of continuous sound sources. Matched field processing is a potential competitor to frequency-wavenumber (FK) and waveform correlation methods currently used for event detection, classification and location. It operates by capturing the spatial structure of wavefields incident from a particular source in a series of narrow frequency bands. In the rich seismic scattering environment, closely spaced sources far from the observing array nonetheless produce distinct wavefield amplitude and phase patterns across the small array aperture. With observations of repeating events, these patterns can be calibrated over a wide band of frequencies (e.g. 2.5-12.5 Hz) for use in a power estimation technique similar to frequency-wavenumber analysis. The calibrations enable coherent processing at high frequencies at which wavefields normally are considered incoherent under a plane-wave model. No claim to original US government works Journal compilation © 2010 RAS. Source

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