Different Radiation Characteristics Between Foreshocks and Aftershocks of the 2016 Mw7.0 Kumamoto Earthquake in Kyushu, Japan: Implication of Pore Pressure, Stress Concentration, and Loading Rate

We estimated the scaled energy () for small earthquakes () in the focal area of the 2016 Mw 7.0 Kumamoto earthquake to examine the underlying physical processes governing the foreshocks‐mainshock‐aftershock sequence. In this sequence, a Mw 6.2 earthquake occurred approximately 28 hr before the mainshock. Many earthquakes occurred in the intervening period, which we refer to as foreshocks. We determined the earthquake source spectra by removing the site and path effects from the observed direct S‐wave spectra and estimated for 10,353 events. The obtained values were distributed around , consistent with previous studies. initially increased immediately after the Mw 6.2 foreshock but decreased rapidly thereafter. After the Mw 7.0 mainshock, increased again and subsequently decreased slowly to the regional average value. The rapid decrease in during the foreshock period differed from the expected trend attributed to rock damage, possibly indicating fault weakening induced by crustal fluids. The increase in immediately after the largest foreshock and mainshock may reflect (a) an increased fault strength owing to the fluid dissipation, (b) aftershock occurrence at fault segments with high‐frictional strength owing to stress concentration, or (c) enhanced rupture complexity and velocity caused by an increased loading rate associated with postseismic slip. The b‐value tended to decrease with increasing , suggesting a positive correlation between and frictional strength. Our results suggest that crustal fluids, stress concentration and increased loading rate play an important role in controlling the earthquake radiation processes.