A possible factor which triggers shallow intra-plate earthquakes

Taking the focal mechanisms into consideration, strain perturbations caused by preceding earthquakes are computed at the foci of 24 shallow intra-plate earthquakes (M ? 6) which occurred in the Japan area in the last 30 years. Significant accumulation of differential strain (namely, shear strain on the fault plane or earthquake-generating stress) preceded the occurrence of these major shocks in many cases (16 out of 24). The duration of strain accumulation amounted to several or ten years. Quantitatively, these effects were about 10^?8-10^?6 in strain and 1–100 mbar in stress. It is questionable whether small disturbances of this magnitude always constitute a definitive trigger. In some critical cases, however, a fractional change in strain (and stress) will probably play an essential role in initiating the precursory process which results in the occurrence of earthquakes.