Historical Constructions, P.B. Lourenço, P. Roca (Eds.), Guimarães, 2001 877 ABSTRACT: Results from 16 seismic events, recorded in the first three years of operation of a strong motion instrumentation network, provide a significant insight about the seismic performance of the Mexico City Cathedral and other similar buildings. Due to soil-interaction effects, and to great differences between dominant frequencies of the induced motion and fundamental frequencies of the building vibration; seismic effects on this and similar buildings are much smaller than those induced in most modern constructions. On the other hand, several parts of the building show important local vibrations, particularly the main façade and the bell towers. Results indicate that in its present state the building has an acceptable level of seismic safety. 1 INTRODUCTION 1.1 Seismic performance of monuments Mexico possesses both a large wealth of extremely valuable historical buildings, and a long history of damage and destruction of them, by severe earthquakes frequently affecting most of the country. The most recent experience comes from the June 1999, Tehuacan earthquake (Magnitude, M=7.0), that damaged about 1800 historic buildings, most of them being colonial temples and convents of the central states of Puebla and Oaxaca. Sources of weakness of historic buildings, typically heavy masonry structures, are rather well known, nevertheless, procedures for evaluating their seismic safety are not well established, and quantitative information about their behaviour and response is very scarce. Seismic response of historic buildings is significantly different from that of common modern structures. Differences come from their form and structure, and from their basic constituting materials, which are weak in tension, thus making it almost impossible to provide continuity within and between structural members, and lending to specific mechanisms for resisting seismic actions, different from those of modern construction. Despite of their seemingly inherent weakness to earthquakes, historic buildings in Mexico City have shown a rather good performance under the many earthquakes that have shaken the city during their existence; specifically, in the great 1985 Mexico earthquake (M=8.1), they performed much better than most, supposedly more resistant, modern structures. In particular, the Mexico City Cathedral has successfully survived earthquakes until now, nevertheless it shows some signs of distress that arise suspicion about its safety under future events. Measured seismic response of the Mexico City Cathedral Roberto Meli, Darío Rivera National University of Mexico (UNAM), Institute of Engineering, Mexico City Eduardo Miranda Stanford University, John A. Blume Earthquake Engineering Center, Stanford, California