Seismic amplification determined from microtremor monitoring at alluvial and rock sites in Newcastle

Abstract

In the magnitude 5.6 Newcastle, N.S.W. earthquake of 28 December 1989, the area with highest seismic intensity (MM VIII) was located some 10 km from the epicentre. In the absence of local instrumental data for the event, various explanations of the intensity distribution have been advanced. A causative relationship has been suggested, with support from wave propagation calculations, in the close spatial correlation between the area with highest seismic intensity and the area with alluvial and/or fill cover. An alternative interpretation correlates intensity primarily with structural vulnerability rather than site geology. New evidence for amplified response of alluvial sites has been obtained from a microtremor monitoring experiment using 1-Hz seismometers.

Amplified response was observed at frequencies ranging from 1.5 Hz to 10 Hz, at sites with alluvial depth ranging from 40m to 5m. Most of these observations have a straightforward interpretation in terms of fundamental-mode (quarter-wavelength) resonance of the alluvium overlying a substratum of much greater rigidity. At some sites there is amplification but the quarter-wavelength resonance is not identifiable, due to a steep interface between the alluvium and the substratum, or perhaps the lack of a sharp rigidity contrast. The microtremor results, while useful for determining site resonance frequencies, are not expected to replicate the degree of amplification under strong earthquake excitation. The microtremor amplification factors are generally higher than the factor 3 ± 1 inferred from the seismic intensity distribution of the 28 December, 1989 earthquake.