Sn Attenuation Model in Tibetan Region and its Geodynamic Implications to Plateau Formation

Abstract:

We collected 23,940 tangential broadband regional seismic records with their ray paths cross the Tibetan plateau. The waveforms are selected based on criteria that these earthquakes were located in the crust, with magnitudes ranging 3.5-6.6, and epicentral distances 200-2000 km. We extract Sn waveforms using a 0.7 km/s group-velocity window around the IASP91 arrival times, and collect the noise in an equal-length time window before the first arriving P wave. By correcting Sn spectra with the noise, we obtain the source-station amplitudes between 0.05 and 10.0 Hz. Both dual- and single-station data are used in the joint inversion for the Sn Q distribution and Sn-wave excitation functions. The wave paths are calculated using the CRUST1.0 model, and the source radiation patterns are obtained from the Harvard CMT. Strong Sn attenuations are observed in the Tibetan plateau, particularly in its northern part, which are correlated with strong Lg wave attenuations in this area. The part of the Tibetan plateau with elevations above 4,000 m has an average Sn Q of 210. On the contrast, regions surrounding the Tibetan plateau are mostly characterized by high Q values except for Yungui plateau, where a relatively low Q of 203 is observed. The dynamic processes of plateau formation are accompanied by strong thermal activities. The temperature variations affect both material rheology and seismic velocity and attenuation. Thus, seismic attenuations may serve as indicators for material flow. We use attenuation data to constrain the flow pattern in the low crust and uppermost mantle. Combining the Lg- and Sn-wave Q models with other geophysical data, a lower-crustal flow channel is found from north Tibet to east and then towards the southeast along the western edge of the rigid Sichuan basin. This work was supported by the National Natural Science Foundation of China (grants 41374065, 41174048).