Spatial location behavior of aftershocks of Wenchuan MS8.0 and Lushan MS 7.0 earthquakes

With the method of GIS, this paper analyzes the point pattern of aftershocks of Wenchuan and Lushan earthquakes, from the perspective about the uncertainty of aftershock distribution and the relationship between seismic intensity region and spatial distance pattern. The result shows that in a smaller scale, the aftershocks are approximately in the random distribution and correlation effect is obvious; while the aftershocks present aggregated distribution in larger scale, but the spatial correlation characteristics are still in power-law relationship, and the upper and lower limits of scale-free area are associated with the major and minor axes of the different intensity seismic region. Wenchuan and Lushan aftershocks form a hot and subhot spot regions, which is like a NE－SW rectangular, the nearest neighbor index for this two regions are 0.99 and 0.76. The nearest neighbor index of Yingxiu Ⅺ degree and Lushan Ⅸ degree was 1.02, 0.95, indicating that the aftershocks tend to distribute randomly in the region with strong intensity and high aggregation. The distance-associated features of the aftershocks indicate that, the aftershocks of Wenchuan earthquake in the intervals of 13.5—20 km, 30—43 km and the aftershocks of Lushan earthquake in the interval of 7—14.5 km are correlated significantly; the aftershocks of Wenchuan earthquake in the intervals of 66—82 km, 225—236 km and 317—321.5 km and the aftershocks of Lushan earthquake in the intervals of 15.5—22 km, 23—32.5 km and 33.5—43.5 km are also have power-law relation, which is coincident with the major and minor axes of Wenchuan earthquake Ⅺ－Ⅸ degree and Lushan earthquake Ⅸ－Ⅶ degree regions, also the 321.5 km and 40 km are consistent with the fracture plane of the two earthquakes. Comparison with the kernel density estimation and the seismic intensity map shows that the consistency between kernel density area and high intensity regions increases with the search radius decreasing; with the expansion of the search radius, the discrepancy between kernel density area and seismic intensity region is more greater.