Surface-wave Derived Focal Mechanisms in Mid-America R. B. Herrmann 1, C. J. Ammon 2 and H. M. Benz 3 1 Saint Louis University, 2 Pennsylvania State University, 3 U. S. Geological Survey Discussion In regions of low-to-moderate seismic activity, quantitative estimates of source size and faulting mechanism are important for definition of regional tectonics and for notification of the public. This task is challenging because the lack of low- frequency signal content, often due to a low signal-to-noise ratio, precludes the use of standard waveform inversion techniques, which would require detailed path-dependent velocity models to describe the high-frequency waveforms. Surface-wave spectral amplitudes, chosen using an expected Love- and Rayleigh-wave group velocity and spectral amplitude shapes can be used to determine the source depth, seismic moment and to provide four candidate focal mechanisms from which the final solution is selected using the few waveforms with good signal-to-noise. The advantage of fitting the surface-wave spectral amplitudes is that the source parameter estimates are much less sensitive to the details of the Earth model than are time-domain waveforms. We have successfully determined source for moment magnitudes as small as 3.7 for mid-America and 3.4 in Korea. Procedure We use the tools of Computer Programs in Seismology (2003): Use Multiple Filter Analysis, do_mft, to select fundamental mode Love and Rayleigh wave spectra amplitudes, guided by expected spectral shapes (Tsai and Aki, 1971) Search over depth and all focal mechanisms (srfgrd96) to find combination of Mw, depth, strike, dip and rake that fits observed spectra radiation pattern Use first motion data to select one of the 4 possible spectral amplitude solutions (strike, strike + 180, rake, rake + 180) or Compare predicted full waveform synthetics to observed waveforms using wvfmch96 Document and place on the web References Tsai,.Y.B., and L. Aki (1971). Amplitude spectra of surface waves from samll earthquakes and underground nuclear explosions, J. Geophys. Res. 76, Herrmann, R. B., and C. J. Ammon (2002). Computer Programs in Seismology - Source Inversion, uterProg.html Zhu, L., and D. V. Helmberger (1996). Advancement in Source Estimation Techniques Using Broadband Regional Seismograms, Bull. Seism. Soc. Am 86, Zhao, L.-S., and D. V. Helmberger (1994). Source Estimation from Broadband Regional Seismograms Bull. Seism. Soc. Am 84, Introduction During the past year, we have focused on the challenge of obtaining focal mechanisms and moment magnitudes for M < 4 earthquakes in North America and Korea. This task is complicated by low signal-to-noise, station distribution and imprecise velocity models. KOREA UNITED STATES CHALLENGES 2 hour processing time from receipt of waveforms, deconvolution, QC, multiple filter analysis, grid search, mechanism validation, documentation is too long MFT processing time is significant, requires experience, knowledge of theory and must be automated Improve Love and Rayleigh wave group velocity predictions to periods as low as 5.0 sec Use phase match filter to isolate surface- wave signal Combine with pre-P noise to automatically select spectral amplitudes We have had difficulty in selecting fault orientation for events in Utah and Colorado which is related to poor P-wave excitation, and velocity model. Implement improvements to wvfgrd96 to make it less dependent on velocity model following Zhao and Helmebrger (1994) and Luh and Helmberger (1996). Quantify confidence in source depth, Mw and focal mechanism Routine determination of mechanisms with Mw = 3.5 is feasible in central and eastern U.S. Deployment of ANSS is required to permit fast, routine determination of source parameters Detailed surface-wave tomography is required to predict sec period dispersion Korea Earthquakes in Korea, recorded by the modern KMA and KIGAM stations provide the opportunity of testing the limitations of the technique by attempting to use less than 180 degrees of the radiation pattern and spectra amplitudes at distances less than 200 km. We provide two examples: the which barely samples 90° of the radiation pattern and the earthquake which shows the usefulness of multiple filter analysis in extracting spectral amplitudes from noise. Yellow Sea and Korea Focal Mechanisms YRMNDY LAT LON H Mw Stk Dip Rake US Focal Mechanisms YRMNDY LAT LON H Mw Stk Dip Rake GOOD Mw=3.72 NOISY Mw=3.80 Experience from Korea Data Set Sometimes works with < 180° azimuthal coverage Works even for distances as short as 100 km Group velocity measurements are not good at short distance, but group velocites are essentially used as a means of windowing signal of interest base on dispersion curve shape Insight on expected spectral amplitude shapes is required May be slightly automated by use of path specific phase match filter Observed and predicted ground velocities filtered as hp c 0.02 np 3 lp c 0.20 np Missouri Mw=3.71 Internet References Computer Programs in Seismology: Focal mechanisms: Contact