Updip limit of the seismogenic zone beneath the accretionary prism of southwest Japan: An effect of diagenetic to low-grade metamorphic processes andincreasing effective stress

Off southwest Japan the seaward limit of coseismic displacement (or updip l imit of the seismogenic zone) of the 1946 M-w 8.3 thrust earthquake reaches to 4 km depth and similar to 40 km landward of the trench. This limit coin cides with the estimated location of the 150 degreesC isotherm, and has bee n linked to changes in physical properties associated with the smectite to illite clay-mineral transition. Here we show that this limit correlates wit h a suite of diagenetic to low-grade metamorphic processes characterized by (1) declining fluid production and decreasing fluid pressure ratio (lambda *) and (2) active clay, carbonate, and zeolite cementation and the transiti on to pressure solution and quartz cementation. These diagenetic to low-gra de metamorphic changes cause the onset of velocity weakening during thrust faulting, an increase in effective stress, and strengthening of the hanging wall, which together combine to produce recordable earthquakes.