1983年日本海中部地震の時間依存モーメントテンソル

抄録

A large complex earthquake is modeled as a series of multiple point sources. This is to evaluate the time-dependent moment-tensors of complex earthquakes in a subjective manner using the least-squares criterion. Since the earthquake sources are no longer a point, low pass filterting is applied to both synthetic and observed seismograms before the inversion in order to permit a multiple point-source representation. Numerical tests of inversion schemes and least-squares programings have been done by assuming theoretically the time dependent moment tensors. It is shown that the convergence of the solutions is usually achieved within two to three iterations to the assumed moment tensors.
The theory is applied to the 1983 Japan Sea earthquake. Total seismic moment of 5.4×10²⁷dyne·cm is obtained. The value is consistent with previous estimates from body-waves, surface-waves and tsunamis. It is revealed that the focal mechanism of the earthquake is time-changing. The focal mechanism solution as a function of time is consistent with the inferred fault plane configurations from aftershocks. A low-pass filtered focal-mechanism thus obtained would represent the mechanism for large scale fault segments. Since much more freedom is expected for the mechanisms of small scale fault patches, the mechanism must be highly at random. Such the fault patches are responsible for the short-period excitation of seismic waves due to large earthquakes.