RESUMO
Between December 1st, 1994, and January 30, 1995, 5 digital, Three-component seismographs were installed to record seismic activity associated with Popocatépetl volcano. We describe the records obtained at PPC before, simultaneous with, and after the crisis of December 21, 1994. Site effect amplification at PPC occurs at frequencies larger tham 15 Hz. Therefore, differences in wavw shape and dominant frequencies between Type A and Type-B events may be due to source or path effects. We analyzed the spectra of volcanic tremors and Type-B events before the crisis (AU)
Assuntos
Terremotos , Estudos de Casos e Controles , Erupções Vulcânicas , Medição de RiscoRESUMO
The objetive of this paper is to evaluate the utility of microtremor measurements to estimate site effects in earthquake engineering. To this end we use microtremor records obtained in three different cities in Mexico: Mexico City, Oaxaca, Oax. and Acapulco, Gro. These data are analyzed following three different techniques: direct computation of amplitude spectra, computation of spectral ratios relative to a firm soil, reference station, and finally, computation of spectral ratios between horizontal components of motion relative to the vertical (Nakamura's method). In order to evaluate the performance of these techniques, the results obtained with microtremors were compared with spectral ratios of weak and strong motion records. Our results show that microtremor measurements allow to determine dominant periods with Nakamura's technique. Results from simple numerical simulations indicate that microtremors consist of a high percentage of Rayleigh waves (AU)