Seismic Attenuation of Sn phase beneath the Ordos Plateau

Abstract:

We have used attenuation tomography of the regional seismic phase Sn to characterize the uppermost mantle shear wave Q (Qs) over a large part of northern China. The Sn phase is often a difficult phase to identify for continental paths since it usually has a relatively small amplitude compared to the regional phase Lg. Also Sn is often a high frequency phase and thus it is often blocked for paths that cross tectonically active regions. We have used the unprecedented amount of national network and temporary stations that were deployed across China over the last five years to be able to successfully identify Sn phases and use them to measure Sn Q using a reverse two station method. The initial waveforms was filtered with the frequency band of 0.5-3 Hz, and Sn time window was computed using velocities range of 4.3-4.7 km/s. Sn waveforms from 43 earthquakes recorded by 63 stations were manually picked out in order to obtain the ratio of Sn amplitude from each two-station pair. Those ratios describe Sn attenuation along each inter-station path. We have used to approaches: the two-station method was used to isolate factors, such as source, and earth response, and calculate inter-station Q value. And LSQR algorithm was used to obtain tomographically map lateral variations in Sn Q.

We find relatively low uppermost mantle Q anomaly is consistent with the Weihe graben, a young active rifting system with hot uppermantle. Low Q value also appears in the southern part of the Ordos plateau, which shows the opposite result to the characteristics of lithospheric mantle in a craton. This may be a result of scattering attenuation of Sn or possible thermal erosion of the lithospheric root beneath the southern Ordos.