Mapping lateral variations of intrinsic attenuation of the crust from the coda of the noise cross correlations in Taiwan.

The lapse-time dependence of the quality factor Qc measured from the S-wave codaof local earthquakes at frequencies above 1 Hz has long been proven effective toinvestigate separately the scattering and absorption properties of the crust. On the otherhand, recent studies demonstrate that the Green’s function retrieved from the noisecross-correlation (NCF) between two stations contains a tail of multiply-scattered waves afterballistic waves like the coda of earthquakes, the decay rate of which can thereforebe also exploited for mapping of subsurface attenuation structures. In this study,we estimate frequency-dependent Qc of the coda in the dominant period range ofsecondary microseisms (3-9 s) derived from the yearly-stacked NCFs between allthe available station pairs separated by less than 80 km apart in Taiwan, which isknown as one of the most seismically hazardous and geologically complex regions inthe world. To ensure the Qc measured by the coda at sufficiently long lapse timeto be representative of the physically meaningful intrinsic quality factor (Qi) assuggested by Calvet and Margerin (2013), we choose different onset times (tw)of the coda at 10 or 20 s after the direct surface wave arrivals calculated by theinter-station distance divided by the average group velocity and measure Qc as afunction of the length of the coda window (Lw). The Qc values at periods of 3-6 sand 4.5-9 s determined with different choices of tw and the fixed Lw reveal noobvious trend with the inter-station distance. However, similar to the Qc observationsfrom the earthquake coda, they generally follow a transient increase with Lw atrelatively short lapse times and then approach a stabilized plateau value at Lw ∼75-100 s considered as an optimal approximation of Qi. We preliminarily map thelateral variations of intrinsic attenuation by simply distributing the determined Qcuniformly along the corresponding inter-station path and taking the mean values ofQc within individual 0.2∘× 0.2∘ cells. The results show that the Qc variation atthe shorter periods of 3-6 s is more pronounced and the lowest (∼50), implyingthe highest intrinsic attenuation, in the Coastal Plain of SW Taiwan covered withthick sedimentary deposits. It gradually increases toward the Western Foothillsfold and thrust belt in south central Taiwan and reaches the highest (∼100) in theHsuehshan Range in north central Taiwan, which seems to be correlated with theprogressive increase of the metamorphic grade in the Taiwan orogen as a result of thesouthward-propagating arc-continent collision. Further attenuation tomographyusing the proper sensitivity kernels will be conducted to reveal both lateral anddepth variations of the absorption properties across diverse litho-geological units inTaiwan. [ABSTRACT FROM AUTHOR]