Tracking volcanic stress and strength changes using the seismic moment ratio (Mstk/M0) at Kirishima volcano, Kyushu, Japan

Seismic activity in volcanic regions is strongly influenced by spatio- temporal changes in stress and crustal strength associated with magma intrusion and fluid migration. We investigate to capture these processes using the seismic moment ratio, Mstk/M0, defined as the ratio of the norm of a stacked seismic moment tensor to the sum of scalar seismic moments of individual earthquakes. This parameter provides a quantitative measure of crustal criticality, approaching unity for optimally oriented slip under high stress and decreasing under reduced strength or heterogeneous stress conditions.

We apply this approach to the Kirishima volcanic area, Kyushu, Japan, where volcanic activity has repeatedly intensified and declined over the past two decades. Focal mechanism solutions derived from waveform data recorded by permanent and temporary seismic networks between 2000 and early 2025 were analyzed. Seismic moment tensors were estimated from focal mechanisms and magnitudes and stacked within spatial blocks containing at least 20 events.

The inferred stress field indicates a strike-slip to normal-faulting regime around Shinmoe-dake, with the minimum principal stress axis oriented northwest–southeast, consistent with regional vent alignment. Spatially, Mstk/M0 values are systematically lower near Shinmoe-dake than in surrounding regions, suggesting locally reduced crustal strength and/or short-wavelength stress heterogeneity. Temporally, Mstk/M0 exhibits large fluctuations near the volcanic center, whereas values remain consistently high in distal areas. Comparison with focal mechanism misfit angles indicates that these variations are primarily controlled by temporal changes in medium strength, likely driven by magmatic fluids. Our results demonstrate that Mstk/M0 is a useful proxy for monitoring evolving stress–strength conditions in active volcanic systems.