Excitation and Attention of Regional Waves, and Magnitude Dependance of PN/LG Ratios in Eastern Eurasia

摘要

In the past one and half years I have been processing large amount of regional/teleseismic data from IRIS, CDSN, GBA, Kyrghizstan and Kazakhstan network stations in eastern Eurasia. Fourier spectra of Pn, Lg and Pg waves were computed for many events/paths to study characteristics of seismic sources and path attenuations. Estimations of source spectral parameters and path attenuation were difficult for all wave types. I have used various methods to tackle the difficulties, including the standard two-station method for Q measurement, the reversed two-station method of Chun et al. (1987) for Q measurement, the empirical Green's function method for measuring source rise times, and the simultaneous Bayesian inversion method for sourcepath spectral parameters by Xie (1993, 1998, 1999). It is well known that seismic source parameters, such as magnitude, seismic moment and corner frequency tend to scale with one another. Xie and Patton (1999) obtained details of these scalings for each wave types (Pn and Lg), and source types (underground nuclear explosions and earthquakes) in central Asia. These scalings, and an spectral overshoot in explosion excited Pn and Lg waves, make the Pn/Lg spectral ratios from explosions differ from those from earthquakes in a frequency-dependent manner. For a given event size, there is a frequence band in which the Pn/Lg spectral ratios from both source types differ the most, resulting in an optimal frequency band for the ratios to be used for a discrimination purpose. This optimal frequency band is dependent on the corner frequencies derived using Pn and Lg. For events with smaller magnitudes or seismic moments, the Pn and Lg corner frequencies become higher, causing the optimal frequency band to shift higher. Until recently, the analyses of explosions in central Asia were conducted only for a limited magnitude range (about 5.0 to 6.0).