Keywords: earthquake, building monitoring, accelerometer, GPS code.

Two tall buildings originally instrumented to study the characteristics and effects of wind loading have provided much useful information about seismic effects of buildings in Singapore resulting from earthquakes with epicentres hundreds of km distant in neighboring Indonesia. In particular, the performance of the 280m Republic Plaza office tower was tracked for almost ten years using a simple monitoring system built around pairs of high-resolution accelerometers installed in the basement and in the highest floor. During the monitoring it became clear that the largest acceleration responses should result from ground motions due to earthquakes having magnitudes between 6 and 9 and epicentres at least 350 km distant. The paper describes the strategy for identifying and capturing the signals from distant tremors, which depends on tracking the RMS response levels in the second vibration mode. Characteristics of the recorded signals are given and a composite response spectrum generated. The reliability of using acceleration data from the basement to represent the true local ground motion is confirmed first by modal analysis of sample seismic data and then by examination of GPS data for the 2004 Aceh Boxing Day earthquake. While response levels in Republic Plaza are generally small and resulting base shears well within the provision of local (British) building codes, largest response spectra values occur at natural frequencies likely to match those of high-rise residential buildings in Singapore which would experience significantly higher shear forces.

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