Abstract: In this study, we utilized the adjoint-state traveltime tomography based on eikonal equation to invert the surrounding area of M_S7.3 Landers earthquake by time-evolving method. The inversion obtained high-resolution three-dimensional P-wave and S-wave velocity structures for different time windows. The correlation among the geological structures and velocity structures of the study area and seismic activity were analyzed. The results indicated that the velocity structure in the Landers earthquake source area and its surrounding area exhibited significant lateral heterogeneities, which was well consistent with the geological structure near surface, basins and faults showing relatively low velocity anomalies, and major mountain ranges showing relatively high velocity anomalies. The occurrence and distribution of earthquakes in the region were closely related to the lateral heterogeneity of the velocity structure, with most earthquakes occurring within high-velocity anomalies or transition zones between high- and low-velocity anomalies. There are significant differences for the velocity structure near large earthquakes in different time windows. For example, in the second time window, the velocities near the Landers and Anza earthquakes became slow down, possibly due to the changes in the geological structure nearby after the large earthquakes, which in turn caused changes in velocity.

Key words: Seismic tomography, Time-evolving method, Landers earthquake, Velocity structures