Figure 6. Seismic gap of earthquakes in the foreshock period projected on the dominant plane. (a ) Comparison of the size of the seismic gap with the estimated sizes of the faults associated with the mainshock. Blue circles indicate the hypocenters of the precursory activity corresponding to the fault sizes by assuming stress drops of 10 MPa. The yellow star indicates the location of the mainshock hypocenter. The red circle represents the size of the fault corresponding to the mainshock estimated based on the model of Sato and Hirasawa (1973). The green circle represents the size of the fault corresponding to the mainshock estimated based on the model of Madariaga (1976). (b ) Moment release amount (color scale) computed for each 0.04 km grid cell. The broken ellipse represents the seismic gap.
The median value of the estimated corner frequencies of the mainshock is 1.9 Hz (Fig. S4). The first and third quartiles are 1.8 and 2.5 Hz, respectively. Based on the median corner frequency and the models proposed by Sato and Hirasawa (1973) and Madariaga (1976), the source radius of the mainshock is 787 and 572 m, respectively. It is much smaller than the foreshock and aftershock regions but comparable to the seismic gap (Fig. 6a).