Stress State Inferred from b Value and Focal Mechanism Distributions in the Aftershock Area of the 2005 West Off Fukuoka Prefecture Earthquake

摘要

The spatiotemporal stress states in the aftershock region of the 2005 West Off Fukuoka Prefecture Earthquake are examined via an analysis of the b values and focal mechanism solutions. The aftershocks are aligned roughly NW-SE, with the southeastern part of the aftershock region believed to correspond to the Kego Fault, which extends beneath the Fukuoka metropolitan area. This study reveals depth-dependent b values in the focal region, where the b values (b = 0.7-1.4) are generally higher above the mainshock depth (9.5 km) and lower (b = 0.5-1.0) at greater depths. The shallower region possesses a significant temporal increase in b values, whereas a lateral b value heterogeneity is observed in the deeper region. The b values (b similar to 1.0) near the mainshock are relatively high, whereas the northwestern and southeastern edges of the deep region have lower b values (b = 0.5-0.7). On the other hand, many of the focal mechanisms for the M >= 3.5 events are located in the low b value area of the deep region. The stress tensor inversion results reveal a change in stress state from strike-slip to strike-slip/normal faulting. These findings imply that the stress state remains high and/or slightly decreased in the northwestern and southeastern parts of the deep region. These results and the findings of previous research on this earthquake sequence suggest that the likelihood of future large earthquakes along the southeastern part of the aftershock region should be considered relatively high.