El terremoto de Michoacán de septiembre de 1985: efectos de fuente, trayecto y sitio.

Abstract

The September 19. 1985. Michoacán earthquake produced uíiprecedented human and material losses in Mexico City. The seismie waves generated by the sonree at almost 400km of distance were greatly amplified at periods around 3 sec. In order to explain this phenomenon. simultaneous consideration is made of saurce. path, and site effects. Analysis of teleseismic P wave records suggests that Michoacán earthquake was exceptionally energetic in the frequency band 0.3 to 0.7 Hz. Instrumental rccordings In the epicentral zone show that the rupture process consisted of the growth of a smooth crack. Near source recordings also shaw conspicuous ripples with a penad of about 3 sec. Numerical simulation shows that thesc ripples can be explained by a series of changes of the rupture front velocity. Vertical displacemenr seismograms in Mexico City are almost identical tndependently of whether the recording site is on soft clay sedimenis or on hard ground. The same is true in the low frequency range for horizontal motions. These records are interpreted as the superposition of long period Rayleigh wave and short penad Lg. It has been shown that waves propagating inland undergo signiíicant less attenuation than waves propagating along the coast. Damage in Mexico City was confined to thc ancient lake bed zane, where the thickness of clay ranges between 20 and 40 m. This aud the typieal values of shear wave propagation velocities suggest that a goad deal of the observations can be explained in terms of the usual ID shear model. Nevertheless, the recorded ground motion shows a large duration that cannot be explained with this model. Under the light shed by recorded strong motion data since 1985, different proposais to explain these observations are examined. 1.