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Wavefield Synthetics in 3D and Fully Automatic Event Locations,

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The research under this contract have mainly been along two directions 1 three-dimensional 3D finite difference FD modeling of scattering from free surface topography, and 2 near real time event location using only seismogram envelopes from local networks. In 1 we have simulated scattering from teleseismic P-waves using a plan vertical incident P-wave and real topography from a 40 x 40 km area centered at the NORESS array in south-eastern Norway. Snapshots and synthetic seismograms of the wavefield show clear conversion from P to Rg short period fundamental mode Rayleigh waves in an area of rough topography approximately 10 km east of NORESS. This result is consistent with numerous observations. In 2 a new method is described for automatic epicenter locations in near real time using short period z-component data from local seismograph networks. The original waveform data is bandpass filtered, STA envelope parameterized and then resampled at a rate of 2 Hz. The physical principle invoked for epicenter determinations is that of constructing the spacetime image of a source in the gridded network area on the basis of P- and S-wavelet intensities. Since such intensities for a stratified half space is almost model independent we did not bother to use local travel time tables nor crustal models. The latter information is naturally needed for precise origin time estimates. The method is robust because and to our surprise, P- and S-intensities low frequency wavelets vary smoothly 1D with distances at least out to 700 km. Compared to conventional event location approaches, we bypassed tasks like signal detection, phase identification and phase association - in essence we aim for joint event detection and location.

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  • Government and Political Science
  • Seismology
  • Seismic Detection and Detectors
  • Nuclear Weapons

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