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Studies on attenuation of seismic waves are helpful to constrain tectonic models. S-wave attenuation in some cases is more sensitive to cracks and fluids than seismic velocity. Specifically, analysis of a frequency dependent Q-value can help to resolve the source of attenuation in the subduction zone: Is it mostly intrinsic attenuation, caused by the presence of fluids and/or melts? Alternatively, is it scattering attenuation, caused by the heterogeneous velocity structure? Q-value is expected to approach frequency independence in the first case, and will develop strong frequency dependence in the second case. In this study, we use original method of attenuation tomography inversion and estimated attenuation structure in Kinki region on this base. To estimate Q-value, parameter of attenuation, at first step it is necessary to eliminate effect of elastic attenuation (mainly geometrical spreading). Result for Q-value depends on the method of elimination used. For high-frequency range (f>1Hz) we employ the ray approximation and calculated elastic attenuation using the ray-theory. Another feature of the developed method is the using of the double-spectral ratio method in tomography inversion to reduce trade-off between source (and/or site) effect and Q-value.
The studied region was the Kii Peninsula segment of the Philippine Sea subduction zone. Many low-frequency earthquakes, which indicate the presence of fluids in the Earth?fs crust, were observed here. 3-D velocity model for studied region was compiled from available information: seismicity data, travel-time tomography results, seismic exploration results, gravity data and borehole measurements. Data used in the inversion are high quality data of CEORKA and Hi-net networks. Generally, estimated total Q-values agree well with results of other studies and with common expectations based on the tectonic structure, except of one striking result: Q-values for the lower crust and the subducting oceanic crust become extremely low, Qtotal ∼ 20-30f 0.9. In order to interpret this result we compiled attenuation-related phenomena that were observed in the studied region: reflective lower crust (RLC), belt-like zone of the deep low-frequency tremor generation (LFT), that is parallel to the slab, low-frequency earthquakes (LFE), and reflective subducting oceanic crust (SOC). Analysis of ray coverage reveals that anomalously low Q-value in RLC and SOC can be explained mostly by high scattering attenuation in the reflective layers. The frequency dependence of the Q-value is also an important observation to support the scattering model of loss.
Good correlation with zones in SOC, having high Vp/Vs ratio (which usually used in seismic tomography as an indicator of the presence of fluids), also was found. The presence of the fluids in SOC can be explained by dehydration of minerals in the oceanic crust, such as chlorite in altered basalts and serpentin in altered dunites.
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