Session 6.3 - Earthquake Attenuation and Particle Motion

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The Component Attenuation Model for Low and Moderate Seismic Regions

N.T.K. Lam and J.L. Wilson

The Component Attenuation Model (CAM) has been developed over the past five years to model the seismic demand for low and moderate seismic regions. The key objective of this paper is to explain the underlying concept of CAM which is primarily to address problems arising from the paucity of strong motion records. Emphasis is on how to incorporate regional geological and seismological parameters into the modelling. Important features to highlight include the modelling for source effects, long distance attenuation, regional upper crustal effects, and soil resonance with particular reference to velocity and displacement response spectra. CAM is currently used in the assessment of seismic hazard for Australia, Southern and Eastern China, Singapore and Indo-China.

Paper 099: [Read][Print]

Keywords: response spectrum, intraplate, low and moderate seismicity, component attenuation model

Average Response Spectra from Some Australian Earthquakes

T.I. Allen, G. Gibson and J.P. Cull

Response spectra have been calculated for three moderate magnitude southeastern Australian earthquakes (from MW 4.1 to MW 4.4) with epicentral locations within Palaeozoic terranes. Average 5 percent damped spectral accelerations and spectral pseudo-velocities are compared with expected results from published attenuation functions using Californian data. Results indicate that for Australian earthquakes, a single-degree-of-freedom system will tend to approach a peak ground acceleration (PGA) or peak ground pseudo-velocity (PGV) at higher frequencies than their Californian counterparts, particularly in the near-field. This can be attributed to the shorter duration and unusually high stress drops typically observed for Australian earthquakes. In far-field events, attenuation of the high frequency content yields equivalent responses for both regions. Moreover, low frequency ground motion is comparable in the near-field, however, at distance the long period motion is lower for Australian events.

More data are required to produce a comprehensive local spectral attenuation function. These results, however, take a constructive step towards the development of Australian response attenuation functions derived entirely from spectral amplitude data. In the absence of quality Australian strong motion recordings from very large earthquakes (necessary to describe engineering design spectra), these data may assist in choosing between published attenuation functions for future earthquake hazard studies.

Paper 095: [Read][Print]

Keywords: Australian earthquakes, response spectra, attenuation functions, high frequency PGA

Factors Bounding Prograde Rayleigh-wave Particle Motion in a Soft-soil Layer

W.R. Stephenson

The ellipticity of particle orbits is calculated for Rayleigh waves travelling in a soil layer lying upon a rock half-space, for a selection of different combinations of Poisson ratio in the soil, and shear wave impedance contrast between the soil and the rock. If either or both the Poisson ratio of the soil and the shear wave impedance contrast between the soil and the rock, are high, there is a range of frequencies for which the particle orbit becomes prograde. In transitions between prograde and retrograde motion, the Horizontal-to-Vertical Spectral Ratio (HVSR) of the waves becomes in turn very large and then very small with increasing frequency. It follows that any site with an HVSR that has the typical structure of a high peak followed by a low trough must be soft, must trap energy by reflection, and will be particularly prone to amplify earthquake shaking.

Paper 056: [Read][Print]

Keywords: Rayleigh wave, prograde, Nakamura, amplification

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