36. Theoretical Seismograms and Earthquake Mechanism : Part 3. Azimuthal Variation of the Initial Phase of Surface Waves

Displacements of the spheroidal and torsional oscillations excited by a double couple point source of dip-slip type located in a radially heterogeneous earth are calculated at various stations on the free surface. Calculations are performed using fundamental modes with colatitudinal order number n=0～2000, namely, those with period larger than about 6 seconds. The azimuthal variation of the initial phase of surface waves is investigated by comparing the values obtained by Fourier analysis of synthetic seismograms with the ones defined theoretically by excitation functions of the free oscillation. Coincidence of the two patterns of the azimuthal. variation of the initial phase is. excellent. This suggests the possibility of estimating a source time function from the analysis of the phase angle of observed seismograms.|(I)で述べた方法に基づいてグーテソベルグ・ブレソーA'地球モデル中に,dip-slip型点震源(dip anerle 60°,深さ5.35km)を仮定した場合の変位についての理論地震記象を,赤道上の方位角O°(30°)330°の各点を含む合計17点で計算した.震源での時間関数はランプ型,たち上がり時間約45秒を仮定した.自由振動の基準モードのみを採用したので理論地震記象は主に表面波,特に半径方向変位はレーリー波の鉛直成分を,方位角方向変位はラブ波の方位角成分を表わしている.これらの理論地震記象からここで仮定した震源模型についての,振巾と位相の方位角分布の様子が知られる.振巾のそれはレーリー波では二象限型,ラブ波では四象限型に近い型をそれぞれ示すが,位相角は必らずしもそれらの様子とは一致しない