Recovery of Stress During the Interseismic Period Around the Seismogenic Fault of the 1995 M w 6.9 Kobe Earthquake, Japan

Abstract

We analyzed the current stress state on the Nojima Fault, the source of the 1995 M w 6.9 Kobe (Japan) earthquake, using breakouts in a borehole through the fault to a depth of ~1,000 m in 2017. The main fault was found at a depth of 529.3 m, with a damage zone ~60 m thick. Statistical analysis shows that the maximum horizontal stress (σ Hmax ) in the depths shallower than 500 m rotates counterclockwise with depth toward the fault and reaches 138° at a depth of 500 m approximately perpendicular to the fault, coinciding with that measured in 1997. In the depths of ~650–1,000 m, the σ Hmax is oriented 241°, closely matching the regional tectonic stress direction. Our results reveal an accumulation of compressive stress, the driving force of faulting in the deep seismogenic zone since the 1995 earthquake, implying the fault zone has been healing continuously throughout the 22-year postseismic period.