High-resolution seismic velocity structure and azimuthal anisotropy around the 2010 Ms = 7.1 Yushu earthquake, Qinghai, China from 2D tomography

To investigate upper crust anisotropy, a tomography algorithm was reformulated to include lateral variations in both velocity and azimuthal anisotropy, and it was applied to Pg travel time data around the epicenter of the 14 April 2010 Ms = 7.1 Yushu earthquake. A high-resolution two-dimensional (horizontal) seismic velocity and azimuthal anisotropy model was obtained using this simple tomography method including both station and event depth corrections. The results show clear anisotropy in the upper crust and it is as important as the velocity variation in explaining the travel time residuals. The most striking result is a high velocity anomaly with large anisotropy at the epicenter on the Yushu–Garze fault. The main rupture originated within this high velocity anomaly and propagated southeastward into a low velocity anomaly with small anisotropy at Yushu. The azimuthal anisotropy shows fast velocity direction along Yushu–Garze fault and larger anisotropy exists in the epicenter than surrounding regions. These results demonstrate a clear example that lateral variation and anisotropy in seismic structures of the upper crust controlled the origination (stress accumulation) and rupture propagation of the 2010 Yushu earthquake and distribution of aftershocks as well. The simple 2D Pg-wave travel time tomography method presented here introduces a new approach utilizing abundant aftershocks data for investigating the rupturing process of a major earthquake and possible fracture distribution around seismogenic zone.

► A new simple tomography method including anisotropy is presented.
► High resolution Pg velocity and anisotropy model is obtained near Yushu earthquake region.
► Seismic structures control the earthquake origination and rupture propagation.