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SIGNALS AND SYSTEMS 2006 IRIS 5-YEAR PROPOSAL The Mantle Magnitude M m and the Slowness Parameter Θ: Five Years of Real-time Use in the Context of Tsunami Warning Stuart A. Weinstein • Pacific Tsunami Warning Center, NOAA Emile A. Okal • Northwestern University We study a database of more than 115,000 measurements of the mantle magnitude M m introduced by Okal and Talandier (1989), obtained since 1999 as part of the operational procedures at the Pacific Tsunami Warning Center. The performance of this method is significantly affected by the seismic instrumentation at the recording station, with the very broad-band STS-1 and KS54000 systems offering the lowest residuals between measured values of M m and those predicted from the Harvard CMT catalog, and also by the period at which spectral amplitudes are measured, with the best results between 70 and 250 s. With such mild restrictions, estimates of seismic moments can be obtained in real time by retaining either the maximum value of M m measured on each record, or its average over the various mantle frequencies, with the resulting residuals on the order of 0.1 ± 0.2 moment magnitude units. M m deficiencies in the case of the two large earthquakes of Peru (2001) and Hokkaido (2003) are attributed to azimuthal bias from an excess of stations (principally in North America) in directions nodal for the focal mechanism and directivity patterns. We further study a group of more than 3000 measurements of the energy-tomoment ratio Θ introduced by Newman and Okal (1998), which allows the real-time identification of teleseismic sources violating scaling laws, and in particular of so-called “tsunami earthquakes”. The use of a sliding window of analysis in the computation of Θ allows the separation of “late earthquakes”, characterized by a delayed, but fast, moment release from truly slow earthquakes. A number of such events are recognized, notably on major oceanic and continental strike-slip faults. STS-1 Only KS-54000 Only M ean Residual r w - 0.4 - 0. 2 0 0 . 2 0 . 4 0 . 0. 6 M ean Residual r w - 0.4 - 0. 2 0 0 . 2 0 . 4 0 . 0. 6 Residual 6 0 0 6 0 0 M ean r w 0 . 0. 6 0 . 4 0 . 2 - 0.4 - 0. 2 0 9 0 1 2 1 5 0 1 8 0 2 1 0 2 4 0 27 0 Period (seconds) 6 0 0 HarvardMw 6.0 6. 4 6. 8 7. 2 7. 6 8. 0 8. 4 STS-2 Only r w 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 M ean Residual r w - 0.4 - 0. 2 0 . 0 0 . 2 0 . 4 0. 6 9 0 1 2 1 5 0 1 8 0 2 1 0 2 4 0 27 0 Period (seconds) 6 0 0 Harvard M w 6.0 6. 4 6. 8 7. 2 7. 6 8. 0 8. 4 "Other" Instruments Three-dimensional plots of the residuals r w as a function of period and seismic moment (expressed as Mw published in the Harvard Catalogue). For each of the standard frequencies, values of rw are regrouped in magnitude bins of width 0.1 Mw−unit, and an average value computed in each bin. The resulting function is then plotted in three dimensions and color-coded. Separate plots are made for the various types of instruments. 9 0 9 0 2 1 0 1 5 1 8 0 2 1 0 2 4 0 27 0 Period (seconds) 2 6.0 6. 4 6. 8 7. 2 7. 6 8. 0 8. 4 Harvard M w 1 0 Period 1 5 1 8 0 2 1 0 2 4 0 27 0 (seconds) 6.0 6. 4 6. 8 7. 2 7. 6 8. 0 8. 4 Harvard M w 62
2006 IRIS 5-YEAR PROPOSAL SIGNALS AND SYSTEMS SAC Availability for the IRIS Community Peter Goldstein • Lawrence Livermore National Laboratory Arthur Snoke • Virginia Polytechnic Institute SAC (also known as SAC2000) is a signal processing and analysis code that has been developed by Lawrence Livermore National Laboratory (LLNL) over the past 20+ years for a variety of seismic and geophysical research projects. SAC has evolved into a general purpose interactive program designed for the study of sequential signals, especially time-series data. Emphasis has been placed on analysis tools used by research seismologists in the detailed study of seismic events. Analysis capabilities include general arithmetic operations, Fourier transforms, three spectral estimation techniques, IIR and FIR filtering, signal stacking, decimation, interpolation, correlation, and seismic phase picking. SAC also contains an extensive graphics capability. SAC is used extensively by the seismic community because: 1) it has a broad range of well-tested, efficient data analysis capabilities (examples include: data inspection, phase picking, signal correction, quality control, unary and binary data operations, traveltime analysis, spectral analysis including highresolution spectral estimation, spectrograms and binary sonograms, and array and three-component analysis); 2) it is easy to use and reliable; 3) it has a macro programming language that allows users to develop innovative new analysis techniques; 4) it has interfaces to the Unix operating system, Matlab , and the Generic Mapping Tools (GMT) software that make it very flexible, allowing researchers to solve many research problems innovatively with minimal programming effort; and 5) the suite of analysis capabilities are integrated so that innovative processing schemes are easily implemented. SAC is also widely used because of its user-oriented development philosophy, which has led to consistent, easy to use capabilities that are backward compatible. IRIS and LLNL have recently signed a contract to provide IRIS with a license for SAC that would allow the sharing of the SAC/SAC2000 source code with the IRIS community, and provide limited support to facilitate a community development effort. The first of the above objectives has been met: as of March 1, 2005, the source code can now be downloaded for members of IRIS from Web site . SAC, and its auxiliary graphics conversion program sgftops, have been built successfully on three operating systems: Sun Solaris (2.9), Mac OS X (10.3), and Linux (Redhat 8.0 and Debian). Pre-built binary executables for those operating systems can be downloaded from the same site. Links are also given to a SAC tutorial and usersʼ manual. (Much of the contents of the usersʼ manual can be accessed from within SAC using the help utility.) Goldstein, P., D. Dodge, M. Firpo, and Lee Minner, SAC2000: Signal processing and analysis tools for seismologists and engineers, Invited contribution to The IASPEI International Handbook of Earthquake and Engineering Seismology, Edited by WHK Lee, H. Kanamori, P.C. Jennings, and C. Kisslinger, Academic Press, London, 2003. Goldstein, P. and A. Snoke, SAC Availability for the IRIS Community, DMS Electronic Newsletter, Vol. VII, No. 1 , 2005. 63
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