An attenuation model for distant earthquakes

Abstract

SUMMARYLarge magnitude earthquakes generated at source-site distances exceeding 100 km are typiÿed by lowfrequency (long-period) seismic waves. Such induced ground shaking can be disproportionately destructive due to its high displacement, and possibly high velocity, shaking characteristics. Distant earthquakes represent a potentially signiÿcant safety hazard in certain low and moderate seismic regions where seismic activity is governed by major distant sources as opposed to nearby (regional) background sources. Examples are parts of the Indian sub-continent, Eastern China and Indo-China. The majority of ground motion attenuation relationships currently available for applications in active seismic regions may not be suitable for handling long-distance attenuation, since the signiÿcance of distant earthquakes is mainly conÿned to certain low to moderate seismicity regions. Thus, the e ects of distant earthquakes are often not accurately represented by conventional empirical models which were typically developed from curve-ÿtting earthquake strong-motion data from active seismic regions. Numerous well-known existing attenuation relationships are evaluated in this paper, to highlight their limitations in long-distance applications. In contrast, basic seismological parameters such as the Quality factor (Q-factor) could provide a far more accurate representation for the distant attenuation behaviour of a region, but such information is seldom used by engineers in any direct manner. The aim of this paper is to develop a set of relationships that provide a convenient link between the seismological Q-factor (amongst other factors) and response spectrum attenuation. The use of Q as an input parameter to the proposed model enables valuable local seismological information to be incorporated directly into response spectrum predictions. The application of this new modelling approach is demonstrated by examples based on the Chi-Chi earthquake (Taiwan and South China), Gujarat earthquake (Northwest India), Nisqually earthquake (region surrounding Seattle) and Sumatran-fault earthquake (recorded in Singapore). Field recordings have been obtained from these events for comparison with the proposed model. The accuracy of the stochastic simulations and the regression analysis have been conÿrmed by comparisons between the model calculations and the actual ÿeld observations. It is emphasized that obtaining representative estimates for Q for input into the model is equally important. Thus, this paper forms part of the long-term objective of the authors to develop more e ective communications across the engineering and seismological disciplines.

Peak displacement demand of small to moderate magnitude earthquakes in stable continental regions

“…More elaborate expressions have been developed by the authors [39] for earthquakes with larger source-site distances. Details are not given herein since this paper is concerned with small and moderate magnitude earthquakes, which typically have engineering signiÿcance only in the near-ÿeld.…”

“…• Factor for mid-crustal ampliÿcation ( mc ) [18,37] • Factor for upper crustal modiÿcation ( uc ) [18,38] • Factors for whole path attenuation (G and ÿ) [18,39] • Factor for soil ampliÿcation arising from resonance (S) [40][41][42] The PDD of the ground shaking may be obtained by taking the product of the listed path and site factors with the source factor (U max ) developed in the earlier part of this paper [Equation (17)]. Thus PDD = U max :( mc ):( uc ):G:ÿ:S…”

Modelling Earthquake Ground Motions by Stochastic Method

“…(Lam & Chandler, 2004); (2) and (6-8) are for predicting the values of the V max and D max parameters at a reference distance of 30km (Lam et al, 2000b). These equations were obtained from ensemble average response spectra that were simulated in accordance with the seismological source model of Atkinson (1993).…”

“…This reference distance value is unique to CAM and is based on conditions of low and moderate seismicity which is characterised by moderate ground shaking with return periods of 500 -2500 years. In a study by Chandler and Lam (2004) on the attenuation of long distance earthquakes, expressions defining the value of  V and  D (ie. rate of decrease in the values of the V max and D max parameters) as function of Q 0 and R have been derived from stochastic simulations of the seismological model.…”

“…The development and application of seismological modelling technique as applied to different countries, which forms the basis of CAM, has been reported in a range of journals spanning a period of ten years since 2000 (eg. Lam et al, 2000a-c;Chandler & Lam, 2004;Lam & Chandler, 2005;Hutchinson et al, 2003 ;Wilson et al, 2003). The writing of this book chapter enables CAM to be presented in a coherent, compact, and complete manner.…”