1998 年 51 巻 2 号 p. 211-228
According to SHIMAZAKI (1986), the large and small Japanese intraplate earthquakes obey the different scaling laws: M0∝L3 for small events but M0∝L2 for large events, where L and M0 are fault length and seismic moment, respectively. This is caused by the fact that the fault widths W for the large events are bounded by the thickness of the seismogenic layer in the crust. We examined the relations among source parameters for 33 Japanese intraplate earthquakes from 1885 to 1995 and confirmed the validity of the results obtained by SHIMAZAKI (1986) for the events of M=5 to 8, where M is the magnitude in the scale of the Japan Meteorological Agency (JMA). Relations between source parameters and JMA magnitude M were also derived from the relations among source parameters, using the M0-M relation by TAKEMURA (1990).
SHIMAZAKI (1986) also indicated the offset of the L-M0 relation at the transition between small and large earthquakes, and suggested that the offset appeared to be due to the difference in boundary conditions between buried and surface faults. We found an offset from 6.5 to 6.8 in the JMA magnitude M, as well as the offsets of a factor of about 2 in D and M0, but no offset in L and W, where D is the average slip of the fault. Also we found that almost all events with M≥6.8 accompanied the surface faults, while most of the events with M≤6.5 did not accompany any surface fault. These results strongly supported that the offsets in D, M0, and M were caused by the surface fault breaks for the large earthquakes. Furthermore, we examined the relation between the damages from the Japanese intraplate earthquake and its JMA magnitude M. The damages suddenly increased from M=6.5 to M=6.8. The scaling law obtained above gave the large earthquake a strip fault whose location was very shallow. Because of these conditions, the intraplate earthquakes with M≥6.8 bring about strong ground motions in the wide area.