High attenuation in the lower crust in Kinki region, Japan

Seismic attenuation is important for predictions of strong earthquake ground motions, and also it gives additional constraints on the physical properties of deep medium. In (Petukhin et al., 2003) the high-frequency Q-value, parameter of seismic attenuation, was studied in Kinki region. In that work effect of geometrical spreading, which is necessary to remove before inversion for Q-value, was calculated numerically using realistic 3D velocity model and ray approximation. Generally, estimated total Q-values agree well with the results of other studies and with common expectations based on the tectonic structure, except of one striking result: Q-value for the lower crust become extremely low, Q_total ~ 20f^0.9. To interpret this result we compiled phenomena, related to attenuation, that were observed in the studied region. They are: (1) seismogenic upper crust, depth 0-17km; (2) aseismic lower crust; (3) reflective lower crust (RLC) in depth range 17-35km revealed by the deep seismic exploration studies in the studied region; (4) belt-like zone of the deep low-frequency tremor generation (LFT), parallel to the slab, which was observed using the high-sensitivity borehole Hi-net stations (Obara, 2002); (5) a few deep low-frequency earthquakes (LFE) near Moho boundary were observed in the central part of Kinki region, far from volcanic centers. Generation of the LFT and LFE is usually explained by the presence of liquid phase in the crust near/above Moho boundary, which in turn can be explained by the dehydration and/or partial melting in the depth range 30-50km. Analysis shows, that anomalously low Q-value in lower crust can be explained by two processes: (1) high intrinsic attenuation (low Q_in value) due to presence of fluids in the lower crust, which are indicated by the LFE and LFT phenomena; (2) high scattering attenuation (low Q_sc value) due to scattering on the lower crust heterogeneities, indicated by the RLC. To separate these two cases we analyzed HF envelopes: in first case envelopes should have pulse-like shape with short duration and low coda, in second case envelopes should have intensive scattered part (coda) and long duration respectively. HF average envelopes of records of small earthquakes are analyzed for two cases: for shallow earthquakes with rays covering mostly upper crust and for deep earthquakes with rays covering mostly lower crust. The results indicate that the case of low Q_in value and/or low Q_sc value for the case of small-scale heterogeneities, is valid for the lower crust in Kinki region.