SEISMIC BEHAVIOR ESTIMATION OF THIN ALLUVIUM LAYERS USING MICROTREMORRECORDINGS

The microtremor spectral ratio proposed by Nakamura (A method for dyna mic characteristics estimation of subsurface using microtremor on grou nd surface. QR of RTRI, 1989, 30(1), 25-33.) developed a new interest on the use of microtremor measurements for the estimation of site effe cts. The Lisbon region is an earthquake-prone area and several descrip tions of destructive earthquakes are reported in the historical data, which suggest the existence of site effects in the town of Lisbon. Sev eral seismic hazard studies were already performed, as well as some mi crozonation studies based on records from blasts. In order to characte rize the seismic behaviour of alluvium layers existing in the town, tw o sets of microtremor measurements were performed. The data were proce ssed according to the Nakamura technique and the results are presented here. These results were correlated with the thickness of the alluviu m deposits and compared with the spectral ratios obtained using record s from blasts, as well as with 1D theoretical models and with 2D theor etical simulations obtained by the use of the Aki-Larner technique. Th ese comparisons seem to prove that the use of microtremor measurements could give a good contribution for the microzonation techniques, spec ially in urban zones, due to the simplicity and rapidity of their acqu isition.