The southern Chile earthquake of 14 June 1970: Not an interplate thrust event!

While mega-earthquakes (e.g., the great Chilean event of 1960) are known to occur north of the Nazca-South America-Antarctic triple junction, the largest thrust event documented south of it had a moment of only 2*10²⁴ dyn*cm . Yet global plate motions predict a convergence rate of 17 mm/yr between the Antarctic and South America plates at latitude 53°S, suggesting that large interplate thrust events may have occurred in historical times. We present a reassessment, including focal mechanism determination using WWSSN records, of the 14 June 1970 earthquake (latitude 51.85°S, longitude 73.84°W; m_b = 6.-; M_s = 6.6). It is the largest event occurring south of the triple junction during the era of instrumental records. Using first-motion data we determined a strike-slip focal mechanism (φ = 323° δ = 83° λ = 338° ). By relocating 6 aftershocks, we tentatively place the rupture on the NW-trending fault plane (striking 323°), which matches the structural orientation of islands and fjords in this region. Spectral amplitudes of surface waves with periods between 50-250 s suggest a preliminary moment of 1.5*10²⁶ dyn*cm. Our preferred mechanism, which cannot be reconciled with subduction between the Antarctic and South America plates, indicates that no such motion was taken up in a large earthquake since the beginning of instrumental seismology.