| Abstract |
Earth's free oscillations are normal modes of the earth. Waveforms of elastic waves excited by a seismic source can be expressed by superposition of the normal modes. Recently the author extended the method to a system of planets having atmosphere. At a supposed upper boundary of the model atmosphere, elastic or acoustic wave energy can be escaped into the space. Such a system with the radiative or open boundary condition, eigenfrequencies are complex numbers and we are forced to search complex eigenvalues in the complex plane. We developed an efficient method to solve the eigenvalue problem. Using the newly developed method, we calculated a million of acoustic normal modes with periods > 10 sec which have their energy predominantly in the atmosphere but also in the solid earth. Hence we can directly calculate seismic acoustic waveforms by the same manner as seismic waveforms. We applied the method to Iwate-Miyagi Nairiku earthquake and compared the synthetic waveforms with observed waveforms at a CTBTO station, Isumi. The calculated waveforms can successfully explain the observation, and a tuning process of synthetics provides us with a device to find seismic source parameters of the shallow earthquake since excitation of acoustic waves are strongly dependent on the source depth. |
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