Oaxaca, México, earthquake of 29 noviember 1978: a preliminary report on spatio-temporal pattern oe preceding seismic activity and mainshock relocation.

Abstract

An analysis of earthquake activity (m≥2.8) recorded by a local field array of 6 seismographs during 3 week prior to a large earthquake (29 November 1978,Ms=7.8) in Oaxaca, Mexico reveals the following: (1) Spatio-temporal patterns suggest a lineation in the preceding seismic activity which trends N66°±6°W and possibly 2 other parallel lineations oriended N39°±6°E and crossing the western and eastern flanks of the aftershock area. (2) Seismic quiescence prevailed (for earthquakes m ≥ 2.8) within an area of 3000 km2 surrounding the mainshock except for two periods of foreshock (within 24 km of the mainshock): the first, 2 weeks before the mainshock the second 21 hours before the mainshock and then activity subsided for 17 hours until failure. The last temporal pattern of activity observed immediately preceding the mainshock is similar to foreshock patterns observed world-wide. While foreshocks occurred in the northern half of a circle with radius of 24 km centered at the mainshock epicenter, 5 of 12 largest aftershocks (mb > 4.0. 30 Nov - 8 Dec) occurred within this circle but south of the foreshock activiy. Also 8 of these aftershocks, the mainshocks, foreshocks and 2 of the largest preceding earthquakes (m= 3.9 and 3.4) are elustered along a N - S trend. An acceptable correlation between lineations defined by seismic data and active faults delineated from geomorpho-structural analysis is found. A significant result of this study is the documentation of preceding activity at energy levels below standard defection threshold (mb≈ 4). Relocation of the mainshock yields a new epicenter about 50 km to the southwest of the published NEIS location; relocated aftershocks fell from 40-100 km from the published locations.