Abstract In November 2003, 15 ocean bottom seismometers were deployed in the southwestern Okinawa Trough. More than 3300 microearthquakes were located during the 10-days passive seismic experiment. The earthquake activity is characterized by the ceaseless occurrence of small earthquakes in the vicinity of all the instruments. The seismicity is essentially restricted to the central part of the Southwestern Okinawa Trough, except for one cluster of events situated in the southern part of it (cluster 2). The seismic activity terminates abruptly against the NE–SW trending prolongation of the Lishan fault. Most of the microearthquakes are aligned along the E–W trending normal faults, showing where the present-day active normal faulting occurs and how it accounts for the N–S extension in the Okinawa Trough. According to the P-wave velocity spectra estimated from some deep earthquakes located beneath the cross backarc volcanic trail area, the existence of a lower crustal/upper mantle magma chamber is confirmed by the presence of low frequency earthquakes in the 3–10 Hz bandwidth.