For the study of macroscopic characteristics of earthquake, it is necessary to investigate both historical and modern earthquakes from the same point of view. Historical earthquakes present informations on damages only. Modern earthquakes have various kind of data obtained from instrumental observations. Intensity data is common to both old and modern earthquakes. As the first step to study historical earthquakes, we studied the seismic intensity distribution of recent earthquakes, in the Tohoku district, which occurred in the interval from 1926 to 1990. Results are summarized as follows.
(1) The isoseismal contours for many earthquakes off the Pacific coast elongate to north and south, and the seismic intensity decays steeply across the line which is almost parallel to the volcanic front. We call this line “Steeply Decay Line of Seismic Intensity (SDLSI)”. On the contrary, the distribution of seismic intensity for shallow inland earthquakes doesn't show this tendency.
(2) We investigated the seismic intensity attenuation in the Tohoku district and obtained the next formula,
I=A-BX
where, I is the seismic intensity, X the hypocentral distance. Coefficients A and B are expressed as follows:
for earthquakes of G1, G2 and G3 (see Fig. 2)
A=0.198+0.679M, A/B=-1332+299.9M (east side of SDLSI),
A=0.944+0.589M, A/B=-1329+279.5M (west side of SDLSI).
for earthquakes of G4
A=-1.315+0.912M, A/B=-410+134.9M
(3) For the earthquakes along the plate boundary, we defined relatively low- and high-frequency earthquakes according to the value of MJ-MI, where MJ is JMA magnitude and MI the one determined by comparing observed intensity attenuation data of each earthquake with a curve calculated from formulas in (2). The epicenteral distribution of low- and high-frequency earthquakes does not show clear characteristics. But, off Fukushima Prefecture region, there seems to be a boundary of the high-and low-frequency earthquake's distribution along the plate boundary of about 40km depth. The high-frequency earthquakes are found in the west side of the boundary.
