Seismicity characteristics in and around Shikoku, southwest Japan are investigated using recent data from microearthquake observations. Earthquakes occurring in the regions are distinctly divided into crustal and mantle earthquakes according to about 22 km in focal depth. Crustal and mantle earthquakes have predominantly the strike-slip type with P axes in the east-west and north-south directions, respectively. Although the Philippine Sea plate is subducting under Shikoku in the northwest direction from the Nankai trough, the P axis of mantle earthquakes in eastern Shikoku is inferred to be stable in the N-S direction during the period before the 1946 Nankai earthquake until today. On the other hand, earthquakes with normal fault type occur infrequently in the aseismic portion deeper than the north dipping mantle earthquake zone.
A gradual lowering of crustal earthquake activity has continued since 1968 in which our microearthquake observation started in central Shikoku. Taking the abrupt decreasing of events in Shikoku before the 1946 Nankai earthquake by JMA data into consideration, the present activity is expected to become lower extremely just before the next Nankai earthquake.
