The oaxaca, Mexico, earthquake of 29 november 1978: a preliminary report on aftershocks.

Aftershocks of the 29 November 1978 Oaxaca, Mexico, earthquake (surface-wave magnitude Ms = 7.8) define a rupture area of about 6000 square kilometers along the boundary of the Cocos sea-plate subduction. This area had not ruptured in a large (Ms >/= 7), shallow earthquake since the years 1928 and 1931 and had been designated a seismic "gap." The region has also been seismically quiet for small to moderate (M >/= 4), shallow (depth </= 60 kilometers) earthquakes since 1966; this quiet zone became about six times larger in 1973. A major earthquake (Ms = 7.5 +/- 0.25) was forecast at this location on the basis of the quiescence that began in 1973. The aftershock data indicate that an area approximately equivalent in size to the seismic gap has now broken.