Abstract
The seismograms obtained by long-period seismographs in Tokyo at the time of the Sanriku earthquake of March 3, 1933 are analysed. As a first step, we examine the differences of the near-field properties of theoretical seismograms for a semi-infinite medium from those for an infinite medium. The wave forms of the horizontal components of displacements in the two media do not show much difference, while those of the vertical components are considerably different.
In the next step, an attempt is made to estimate the focal parameters, such as the rise time and the rupture velocity, by comparing wave forms of direct P and SP waves. A good agreement of wave forms of the first half cycle on EW components of the synthesized and observed seismograms yields the following conclusions: (1) the rise time of the source time function is about 5 sec, (2) the average dislocation over the fault is 2-5m, (3) the particle velocity of the fault motion is 20-50cm/sec, (4) the rupture velocity is about 3.0km/sec, (5) the effective stress is 40-140 bar, and (6) the rupture began at the northern part of the fault area and propagated to the south.
