Besides, there have been a growing number of observations and numerical simulation studies on asperity 7, 19, 20, 21, 22. Various studies have been conducted to explore the crustal structure of the large earthquake source regions and link the observations to mechanism of earthquake generation 16, 17, 18. Therefore, studying the fine velocity structure around the source regions could shed light on the relationship between the velocity features and large earthquakes and furtherly the seismogenic mechanism of large earthquakes. Seismological studies suggest the occurrence of strong earthquakes is closely related to the abnormal distribution of crustal velocity 12, 13, 14, 15. Faults that are fully or partially locked by strong asperities breed great seismic hazard because the accumulated stress on the asperities are prone to be released through large earthquakes, contrasting sharply with the faults which are characterized by creeping deformation 9, 10, 11. Moreover, earthquake processes are significantly affected by the heterogeneity of mechanical properties of the fault zone, often represented conceptually as asperities 6, 7, 8. The diversity of earthquake source is generally associated with the geometrical complexities of the fault systems, which are attributed to the heterogeneity of the dynamic rupture process 2, 3, 4, 5. There has been an obvious surge of great earthquakes with magnitudes ≥ 8.0 during the past decade with great diversity at various aspects 1. Large earthquakes usually cause great hazards on natural environment and/or humans. If the close relationship between the aperity and high-velocity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies.Įarthquakes occur when the stored energy in the Earth’s lithosphere is suddenly released. Asperities are strong in mechanical strength and could accumulate tectonic stress more easily in long frictional locking periods, large earthquakes are therefore prone to generate in these areas. High-velocity anomalies encompassing the seismogenic faults are observed to extend to depths of 15 km, suggesting the asperity (high-velocity area) plays an important role in the preparation process of large earthquakes.
In this paper, we obtained a high-resolution three-dimensional crustal velocity model around the source regions of these two large earthquakes using an improved double-difference seismic tomography method. Numerous large earthquakes have occurred in the NE Tibetan Plateau, including the 1920 Haiyuan M8. Detailed crustal structure of large earthquake source regions is of great significance for understanding the earthquake generation mechanism.