A large (Mw 7.7) intermediate-depth earthquake occurred on 13 June 2005 in the Tarapac¿ region of the northern Chile seismic gap. Source parameters are inferred from teleseismic broadbands, strong motions, GPS and InSAR data. Relocated hypocenter is found at ~98 km depth within the subducting slab. The 21-days aftershock distribution, constrained by a postseismic temporary array, indicates a sub-horizontal fault plane lying between the planes of the double seismic zone and an upper bound of the rupture area of 60 km ¿ 30 km. Teleseismic inversion shows a slab-pull down dip extension mechanism on a nearly horizontal plane. Total seismic and geodetic moments are ~5.5 ¿ 1020 N.m, with an averaged slip of 6.5 m from geodesy. The earthquake rupture is peculiar in that the effective velocity is slow, 3.5 Km.s-1 for a high stress-drop, 21--30 MPa. We propose that rupture was due to the reactivation by hydraulic embrittlement of a inherited major lithospheric fault within the subducting plate. The stress-drop suggests that the region of the slab between planes of the double seismic zone can sustain high stresses.