Mainshock and aftershock analysis of the June 17, 1996, deep Flores Sea earthquake sequence: Implications for the mechanism of deep earthquakes and the tectonics of the Banda Sea

One of the largest (M_w = 7.8) deep earthquakes (595 km) in recorded history occurred beneath the Flores Sea on June 17, 1996. We analyze broadband body waveforms to determine the seismic source characteristics of this important earthquake. The source time function has a small initial subevent (5 s duration) followed by three major subevents with a total source duration of 29 s. Directivity analysis indicates an unilateral rupture to the east-southeast with a subhorizontal rupture length of 75 km. A time-independent moment tensor (TIMT) inversion gives a total seismic moment of 5.4 × 10²⁰ N m (M_w = 7.8). A time-dependent moment tensor (TDMT) inversion indicates rupture on a near-vertical fault (strike of 130°, dip of 80°SW, rake of -166°) during the first 5 s, releasing 8% of the seismic moment. The major part of the moment was released along a shallower dipping plane (strike of ∼100°, dip of ∼55°, rake of ∼-45°). This change of faulting geometry may imply that the rupture initiated at a structural, thermal, or phase boundary of high stress concentration and then triggered the major moment release along a regional weak zone. The mainshock produced at least 23 (m_b > 2.9) aftershocks within the first 3 days. Seventeen of the largest aftershocks were located using the Joint Hypocenter Determination algorithm. The aftershocks define a plane dipping antithetically to the dip of the subducting slab with lateral and depth dimensions of 145 km and 76 km, respectively. The aftershock locations correlate with the south-east striking nodal plane of the moment tensor mechanism.