V. Kostoglodov, R. W. Valenzuela, A. Gorbatov, J. Mimiaga, S. I. Franco, J. A. Alvarado, R. Peláez, Instituto de Geofísica, UNAM, México, D.F., MEXICO

A study of the vertical surface deformation (uplift) along profiles perpendicular to the coast provides important data for understanding the long-term interseismic process of elastic strain accumulation. Four leveling lines were installed in the states of Guerrero and Michoacán to measure interseismic uplift at the "Guerrero seismic gap". These profiles are 10-15 km long and have the following locations: (i) south of Acapulco, (ii) in the middle of the seismic gap (Atoyac), (iii) in the northern part of the gap (Barra de Potosí), and (iv) near Playa Azul (the central part of the rupture zone of the 1981 and 1985 Michoacán earthquakes). Up to date, several, subsequent, one-year period, high-accuracy leveling surveys have been carried out on these profiles. The vertical uplift rate across the Guerrero coastal region was determined through the changes in repeated leveling surveys. The relative rate of coastal uplift is 7-10 mm/yr measured on baselines ~15 km-long at the Barra de Potosí, Atoyac and Acapulco profiles. An important finding shows that the tilt on the Atoyac line is opposite (tilting off land) to the one observed on the other lines (inland tilt). These results are compared with the uplift rate distribution calculated from elastic dislocation models for different subduction, thrust fault angles and various widths and locations of the locked zone. A model with a thrust fault angle of 12^o and a completely locked zone of ~40-45 km width starting at ~55 km from the trench axis gives a good fit to the data in the Acapulco and Barra de Potosí lines. Nevertheless, the observed uplift rate on the Atoyac profile is in contradiction with a reasonable location of the "interseismically coupled" zone. The latter observation could be explained by the occurrence of mostly aseismic slip in the gap or even by an unusual, extremely slow rupture. The GPS data collected in 1996 and 1998 in Guerrero should provide a more definitive conclusion to these hypotheses.