BASIN EFFECTS ANALYSIS FROM A DENSE STRONG-MOTION OBSERVATION NETWORK

The strong ground motion observation network began to operate in the T aipei Basin area as part of the Taiwan Strong Motion Instrumentation P rogram (TSMIP). It is operated by the Seismological Observation Centre of the Central Weather Bureau. Forty-three free-field accelerometers have been installed in the Taipei Basin, and several earthquakes have been recorded by this network since its installation. Three events wit h magnitudes of 5.7, 5.4 and 6.2 that triggered more than ten stations are used here to study the characteristics of the basin effects on gr ound motions both in the time and frequency domains. From this study, it is clear that vibrations of different frequencies have different am plifications in the Taipei Basin. In fact, the frequency responses sho w the variation can even be up to about 7 times. The horizontal peak g round acceleration (PGA) and spectral ratio contours in the low-freque ncy band are closely correlated with the geological structure in the T aipei Basin. The analysis of the response spectra also shows this corr elation phenomenon and the relation between the damage from an earthqu ake and the basin effects.