High Resolution of Crustal Seismic Wave Attenuation Tomography in Eastern Tibetan Plateau

We investigate the frequency dependent attenuation tomography of regional seismic phases Lg and Pg. The intrinsic attenuation of Lg and Pg is employed as an approximation of Qs and Qp in the crust and used as a constraint in interpretation of crustal geothermy, rheology and tectonics. We have generated tomographic images with the best resolution to observe structures as small as 100km2. We applied waveform data from 769 regional events and 222 stations of permanent or temporary networks including CDSN, INDEPTH-IV-ASCENT, NETS, Namche Barwa, and MIT-China within this region. We used a Reverse Two-station/event Method (RTM) to measure inter-station Q; this method theoretically eliminates any contributions from source excitation and site amplification from the estimation of path-based Q. The tomographic images with significant lateral variations in Q suggest a strong lateral variation in the geothermal and rheological properties of the Tibetan crust. The disadvantage of the Two-Station Method (TSM) is that the measurements is contaminated by site amplification terms, thus the RTM is a significant improvement in the methodology of measuring Q. Large-scale scattering is a significant contributor to Lg and Pg attenuation however, we suggest the patterns in our tomographic images suggest that the intrinsic attenuation is the dominant factor causing the observed Q anomalies. The most remarkable results in this study include that (1) a high Q zone bands around the eastern Himalayan syntaxis and even spreads to the entire three-river zone tectonically between the Indus-Yalu suture and the Bangong-Nujiang suture in the southeastern TP; (2) the TP has widespread low to middle Q values, except the mid-eastern Qiangtang terrane, east of the INDEPTH-III profile, with relatively middle to high Q values; (3) approximately along the Kunlun Fault system there is a nearly 1000km E-W very low Q band; (4) high Q values are observed widely in the Qaidam Basin, Tarim Basin, Sichuan Basin, and Ordos Block; (5) the Qilian Shan-Nan Shan thrust belt has low to middle Q values, lower than all of its surrounding areas; and (6) a nearly 400km very low Q zone is exactly consistent with the western Longmenshan thrust belt. Based on the estimation of Qs, Qp, their frequency dependence η, and Qp/Qs in this study, we deduce various possible interpretations on these anomalies. For example, most of the low Q zones are probably due to fluid content within active fault systems or high temperatures in middle to lower crust, and the high Q anomalies seem to correlate well with tectonically stable and aseismic regions within TP and surrounding area.