Fig. 20

A conceptual model to explain deepening of seismicity due to pore pressure rise on a Mohr’s diagram. A strike slip stress field in which intermediate principal stress is vertical is assumed here. Before a pore pressurization, optimally oriented faults are assumed to be close to shear failure condition regardless of depth since earthquake swarms are ongoing beneath the volcano (< 6 km in depth). After a pore pressure rise, faults are more randomly oriented and are activated to failure throughout the system and a seismic surge commences. However, the orientation range of activated faults is larger in shallower (θ ₁  > θ ₅ ) regions. Thus, the seismicity is more active where shallower and activation in deeper regions needs further pore pressure rise