HR: 09:50h
AN: S51C-08    [Abstracts]
TI: How Large can Mexican Subduction Earthquakes be? Evidence of a Very Large Event in 1787 (M~8.5)
AU: * Suarez, G
EM: gerardo@ollin.igeofcu.unam.mx
AF: Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Mexico, DF 04510, Mexico
AB: A sequence of very strong earthquakes occurred from 28 March to 18 April, 1787. The first earthquake on 28 March, appears to be the largest of the sequence followed by three strong events on 29 and 30 March, and 3 April; strong aftershocks continued to be reported until 18 April. The event of 28 March was strongly felt and caused damage in Mexico City, where several buildings were reported to suffer. The strongest effects, however, were observed on the southeastern coast of Guerrero and Oaxaca. Intensities greater than 8 (MMI) were observed along the coast over a distance of about 400 km. The towns of Ometepec, Jamiltepec and Tehuantepec reported strong damage to local churches and other apparently well-constructed buildings. In contrast to the low intensities observed during the coastal Oaxaca earthquakes of 1965, 1968 and 1978, Oaxaca City reports damage equivalent to intensity 8 to 9 on 28 March, 1787. An unusual effect of this earthquake on the Mexican subduction zone was the presence of a very large tsunami. Three different sources report that in the area known as the Barra de Alotengo (16.2N, 98.2 W) the sea retreated for a distance of about one Spanish league (4.1 km). A large wave came back and invaded land for approximately 1.5 leagues (6.2 km). Several local ranchers were swept away by the coming wave. Along the coast near the town of Tehuantepec, about 400 km to the southeast of Alotengo a tsunami was also reported to have stranded fish and shellfish inland; in this case no description of the distance penetrated by the tsunami is reported. It is also described that in Acapulco, some 200 km to the northwest of Alotengo, a strong wave was observed and that the sea remained agitated for a whole day. Assumming that the subduction zone ruptured from somewhere near Alotengo to the coast Tehuantepec, the resulting fault lenght is about 400 to 450 km. This large fault rupture contrasts with the seismic cycle of the Oaxaca coast observed during this century where apparently earthquakes with magnitudes Mw > 7.4 ruptured asperities with a return period of about 32 to 56 years. The presence of a great earthquake rupturing approximately 450 km of the Mexican subduction zone with an equivalent magnitude of about 8.5 is a phenomenon not observed during instrumental times. The presence of such a large rupture process, as the one suspected on 28 March, 1787, raises the question whether great events like this one may take place in other segments of the Mexican subduction zone.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8104 Continental margins: convergent
DE: 9350 North America
SC: Seismology [S]
MN: 2007 Joint Assembly