Abstract
Available mechanism solutions of most intermediate depth earthquakes in northern Chile have T axes aligned subparallel to the direction of plate subduction (down-dip tension). However, the mechanism of the 1977 January 17 earthquake (24.9°S, 68.7°W) is quite different from this general pattern and shows an almost horizontal compression in east-west direction. The focal depth was reexamined by two independent methods (master event method, and travel-time difference between direct and surface-reflected waves), and it was concluded that this earthquake occurred at about 120 km depth within the subducting slab. The systematic discrepancies observed in the focal depths of intermediate earthquakes in northern Chile between those obtained by the least squares method and those by travel-time differences can be ascribed to the effect of a higher P velocity within the subducting plate. There is a suggestion of a two-layer like structure in the E-W cross section at about 24°S if we only plot the earthquake foci determined by pP-P time difference. The 1977 earthquake either 1.(1) is an event in the compressional part of the double seismic zone, or2.(2) shows an anomalous stress orientation because it occurred near the break in the dip angle of the subducting slab.
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