Toward a Rayleigh Wave Attenuation Model for Asia and Surrounding Regions

We report on the progress toward the development of attenuation models for short-period (12-22 sec) Rayleigh waves in Asia and surrounding regions. These models are defined by maps of attenuation coefficients across the region of study in the specified period band. These models are designed to calibrate the regional surface-wave magnitude scale and to extend the teleseismic surface-wave magnitude body-wave magnitude (Ms-mb) discriminant to regional distances. In order to obtain accurate attenuation estimates, we must first measure surface-wave amplitudes reliably. Taking advantage of certain characteristics of Rayleigh waves, such as the dispersion and the elliptical particle motion, we employed a suite of techniques in making accurate fundamental-mode Rayleigh-wave amplitude measurements. We first analyze the dispersion of the surface wavetrain using a spectrogram. Based on the characteristics of the data dispersion, we design a phase-matched filter by using either a manually picked dispersion curve, or a model-predicted dispersion curve, or the dispersion of the data, and apply the filter to the seismogram. Intelligent filtering of the seismogram and windowing of the resulting cross-correlation based on the spectrogram analysis and the comparison between the spectrum of phase-match filtered data and raw-data and source spectra effectively reduces amplitude contaminations from surface-wave higher modes, multipathing, body-wave energy and other noise sources, and results in reliable amplitude measurements in many cases. We implemented these measuring techniques in a graphic-user-interface tool called Surface Wave Amplitude Measurement Tool (SWAMTOOL). Using the tool, we collected and processed waveform data for 200 earthquakes occurring throughout 2003-2006 inside and around Eurasia. The records from 135 broadband permanent and temporary stations were used. Presented at the Conference on Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies (30th), held in Portsmouth, VA, on 23-25 Sep 2008. Published in the proceedings of the conference, p108-117, 2008. Prepared in collaboration with Los Alamos National Laboratory. Sponsored in part by the National Nuclear Security Administration (NNSA). The original document contains color images.