Pn波的形成及其衰减特征——对加拿大地盾的研究

尽管P_n波品质因子（P_nQ）对上地幔流变性质研究及核试验监测有着深刻含义,迄今它仍然是一个难以准确测定的地震参量.这是因为除了众所周知的影响所有短周期Q的干扰因素外,在所有主要的区域震相中,地震工作者对P_n波的传播模式及几何扩散函数G=d^-k中的扩散率指数k了解最少.不同的研究者任意地假定不同的常数k,因此常算出非常不同的P_nQ值（例如文献1]、2]）.实际上,理论研究已表明,k随频率而变,变化的形式取决于上地幔的速度结构.本文介绍用一推广的逆向双台法同时测定加拿大东部的P_nQ和k值.我们的结果表明:1.加拿大东部的P_n波是由一系列的干涉潜波叠加而形成;2.视P_nQ（散射和固有非弹性的综合效应）随频率而增加,在15Hz处接近2000.

THE ORIGIN AND ATTENUATION CHARACTERISTICS OF P_n WAVE: A CASE STUDY IN THE CANADIAN SHIELD

In spite of its profound implications on the upper mantle rheology and seismic verification of low-threshold nuclear test ban treaties, PnQ, the quality factor for Pn wave, remains an elusive parameter to quantify. This is because in addition to the familiar contaminants affecting all short-period Q measurements, the Pn propagation mode, and hence, the spreading rate coefficient k in its geometrical spreading function PnG=d-k is among the least understood of all major regional phases. Large differences in the estimated PnQ can simply arise from k being arbitrarily assumed to take different (frequency-independent) values (e.g.,1,2]). Theoretical studies 3'4] have in fact shown that k is characterized by a frequency dependence which is a function of the upper mantle velocity structure. We report here the results of a first attempt at a simultaneous determination of the PnQ and the k in eastern Canada using an extended reversed two-station method. The results suggest: a) the Pn waves in eastern Canadian originate from the superposition of a series of interfering diving waves; and b) the apparent PnQ (the net combined effects due to scattering and intrinsic anelasticity) increases very rapidly with frequency, approaching 2000 at 15 Hz.