محاسبات سرعت فاز و سرعت گروه با استفاده از نگاشت هاي دوره بلند

Phase and group velocity dispersions using long-period records

In this investigation long-period records of earthquake data were used for separation of surface waves and calculation of phase and group velocity dispersion curves. Kazeroun main shock (3 May, 1995) recorded by Iranian Long Period Array (ILPA) at station 1, 2, and 7; Sant-Hellana main shock (27 April, 1986) recorded by two very long-period European stations (KON in Spain and TOL in Norway) have been processed. First, we have corrected all records by special techniques for the arm and tilting of instrumentation. Then, we processed the earthquake data for separation of surface waves (Love and Rayleigh waves) by two dimensional frequency-time algorithms. After separation of surface waves from main shocks full automotive intelligent programs have been used for detection and computation of phase and group velocity dispersion curves. Results show that phase velocities calculated from three-components of ILPA data at stations 1, 2, and 7 are less than the group velocities for the same stations (inverse dispersion). Vertical components of the three stations show that the phase velocity with respect to frequency is increased, but for the horizontal components (N-S and E-W directions) the phase velocities with respect to frequency are decreased. This behavior shows that the phase velocity of shear waves is reduced with depth of penetration. Besides, the higher modes of surface waves are mimicked as they pass through the Zagros belt from the earthquake epicenter to ILPA recording stations. Similar computations of phase and group velocities from the TOL and the KON data stations show that the phase velocities are greater than the group velocities (forward dispersion).