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In order to make a comparison with standard attenuation laws based on peak ground acceleration (PGA), the intensity attenuation equations are converted into PGA attenuation relationship using standard intensity PGA relationships. The following intensity PGA relationships have been used:
log (PGA) = 0.14 + 0.300I (Trifunac and Brady, 1975) (4)
log (PGA) = -0.430 + 0.350I (Murphy & O’brien, 1977) (5)
log (PGA) = 0.453 + 0.273I (Wald et.al., 1999) (6)
Figure 5: Comparison of developed PGA attenuation models with other attenuation laws.
(a) Model-1(using epicentral distance), (b) Model-2 (using hypocentral distance).
Three equations representing horizontal PGA as a function of R and M are obtained for Attenuation Model-1 using Eq.(4), Eq.(5) and Eq.(6). In Fig.5a, these three equations of Attenuation Model-1 are compared with well-established attenuation laws of Abrahamson & Silva (1997), Ambraseys (1996) and Chapman (1999) for a M=7 magnitude earthquake. All three equations of Model-1 appear to be representing smaller attenuation of PGA, in other words greater PGA value compared to other laws. However Murphy and O’Brien (1977) relationship place Model-1 quite close to these attenuation laws. Wald et al. (1999) relationship places Model-1 far above these attenuation laws which appear to be very unrealistic. Intensity Attenuation Model-2 is also converted to PGA attenuation relationships for an assumed M=7 magnitude earthquake with a focal depth of 60 km. Fig.5b presents comparison of Model-2 PGA Attenuation using Eq.(4) and Eq.(5) with attenuation laws proposed by Atkinson & Boore (1995), Zare and Bard (1999) and Sabri (2001). It is observed that attenuation relationship of Sabri (2001) for NE India yield much higher PGA, representing lower attenuation. The Model-2 PGA Attenuation based on Murphy and O’Brien (1977) relationship falls between curves of Atkinson & Boore (1995) and Zare and Bard (1999). Although the developed laws need further verification from measured strong motion data in the country, in the absence of such data, both Attenuation Model-1 and Model-2 based on Murphy and O’Brien (1977) relationship may be used as an approximate first hand estimate of PGA based attenuation law for probabilistic hazard assessment studies in Bangladesh.
Posted in: Comparison of Attenuation
