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The Spatial Stability of Rayleigh Wave Amplitudes and Path Dependent Propagation Characteristics of Central Asia,

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We analyzed Rayleigh waves from eleven regional events with pure continental paths recorded by the Kyrghystan broadband network KNET located in the northern Tien Shan mountains of central Asia. We utilized the frequency variable filter technique to isolate fundamental mode Rayleigh waves and extract stable surface wave amplitude spectra in the 10- 40 s period band. We found a strong azimuthal dependence in the amount of variability of observed amplitudes across the 150 km aperture of KNET. Rayleigh waves from northerly azimuths that have propagated across the Kazakh shield show little diiference in amplitudes. At the opposite extreme are Rayleigh waves from nuclear tests at Lop Nor which propagate across KNET from east to west. Lop Nor amplitude spectra vary by approximately a factor of 8 in the 10 to 30 s period band. Other paths generally show small variation in amplitude at periods above 20 s, but variation comparable to the Lop Nor path at periods less than 20 s. We also utilized KNET as an array to construct a suite of beams. We then used the frequency variable filter to extract amplitude spectra from the beam signals as we did for the single station data. Comparison of the results demonstrates that amplitude spectra variability correlates directly with stacking coherence. As a result, the performance of KNET as a long-period array for analyzing Rayleigh waves is highly dependent on azimuth. Events to the north and west can be expected to be stacked reliably to 10 s period or possibly less, while events to the east and south can not be stacked reliably for periods shorter than 20 s. We argue that our observations are explained by radical differences in the Rayleigh wave scattering properties of the crust in the tectonically active region to the south and east compared to the more stable crust of to the north and west.

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

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