On September 19, 2017 an earthquake of magnitude of 7.1 occurred at 120 km south of Mexico City. In that region where it was not considered to happened earthquakes of that scale. The effects in the city affected a zone called transition, between rock and lake zones. The maximum accelerations reached 780 gals. Affecting mainly buildings of 7 to 10 floors. In this paper it is study in detailed a building located in the transition zone. The building is of habitational use, structured with reinforced concrete columns and beams and planar slabs. The main damage was located in the base columns, since that floor was used as parking space (weak floor). An ambient vibration test was done in the roof in 6 different locations. The first three frequencies were identified by the Eigensystem realization algorithm (ERA) along with stabilization diagrams. A mathematical model was constructed with a commercial software. Results shown that a match with an error less of 7% with respect of experimental values.
