10:45 AM - 12:15 PM
[SSS07-P05] Seismic attenuation with amplitude dependent
Keywords:seismic attenuation, intrinsic attenuation, Q
While the Earth can be approximated as an elastic body, it also has anelasticity (Q-1) that causes energy dissipation (intrinsic attenuation) during the deformation. Theoretical models of grain-boundary processes predict that attenuation depends on amplitude, but deformation experiments have shown amplitude independent attenuation at strain comparable to seismic wave propagation. Therefore, because all seismological analyses assume that attenuation is independent of amplitude, the purpose of this study is to clarify whether seismic wave attenuation is truly independent of amplitude.
Here we analyzed 458 aftershocks (M3-5) of the 2003 Miyagi-oki intraslab earthquake (Mjma7.1) that occurred in a depth range of 60-80 km during the period from May 27, 2003 to December 2007. First, we calculated the spectral amplitudes using the vertical component of P waves and took the spectral ratio of any pairwise aftershocks in inter-event distances within 5 km at stations located within 200 km. Then, we divided the spectral ratio by the travel times to remove the effect of hypocentral distance. Finally, we calculated the observed spectral ratios by stacking all pairwise earthquakes whose observed amplitude ratios fell within a certain range (every 0.2 in the ordinary logarithm of the amplitude ratio) and fitting the theoretical spectral ratios to them to determine the Q-1 difference (ΔQ-1) between earthquakes with different observed amplitude ratios.
As a result, we obtained that ΔQ-1 is zero for earthquake pairs with no difference in observation amplitude, whereas ΔQ-1 monotonically increases as the amplitude ratio increases. This positive correlation between the observed amplitude ratio and ΔQ-1 suggests that seismic waves are amplitude dependent for intrinsic attenuation. Moreover, our model calculation suggests that Q-1 is proportional to amplitude (A) with Q-1 ∝ A0.1-0.2, which introduces attenuation variation along a ray path with significant enhancement of attenuation near the hypocenter.
Here we analyzed 458 aftershocks (M3-5) of the 2003 Miyagi-oki intraslab earthquake (Mjma7.1) that occurred in a depth range of 60-80 km during the period from May 27, 2003 to December 2007. First, we calculated the spectral amplitudes using the vertical component of P waves and took the spectral ratio of any pairwise aftershocks in inter-event distances within 5 km at stations located within 200 km. Then, we divided the spectral ratio by the travel times to remove the effect of hypocentral distance. Finally, we calculated the observed spectral ratios by stacking all pairwise earthquakes whose observed amplitude ratios fell within a certain range (every 0.2 in the ordinary logarithm of the amplitude ratio) and fitting the theoretical spectral ratios to them to determine the Q-1 difference (ΔQ-1) between earthquakes with different observed amplitude ratios.
As a result, we obtained that ΔQ-1 is zero for earthquake pairs with no difference in observation amplitude, whereas ΔQ-1 monotonically increases as the amplitude ratio increases. This positive correlation between the observed amplitude ratio and ΔQ-1 suggests that seismic waves are amplitude dependent for intrinsic attenuation. Moreover, our model calculation suggests that Q-1 is proportional to amplitude (A) with Q-1 ∝ A0.1-0.2, which introduces attenuation variation along a ray path with significant enhancement of attenuation near the hypocenter.