Time-space Evolution Pattern and Numerical Analysis of Wave Velocity Ratios Before Lijiang and Ninglang Earthquakes

Dynamic tracing of space-time evolution pattern of wave velocity ratio before the Lijiang earthquake with M=7.0 occurred on February 3, 1996 in northwestern Yunnan. We compared the Ninglang earthquakes with M=6.7 and M=6.4 which occurred on November 7 and December 13. 1976, 90 km away from the Lijiang earthquake. We found that the space-time evolution patterns of velocity that various authors' studied at different times are very similar.Anomaly areas of wave velocity ratio with high values appeared in the seismogenic areas 5 -7 years before the strong earthquake. Anomalies with low values in large areas appeared 3-4 years before the earthquake. Once again the anomaly areas of wave velocity ratio with high values appear in a lower range 1 ~2 years before. The strong earthquake occurred in the overlapping area of two high value anomaly areas, surrounded by the anomaly areas with low values. The monthly mean values of wave velocity ratio before the two strong earthquakes had maintained low value anomalies for several years, and obviously deviated from mean values about 1 year before. The seismogenesis of strong earthquake has entered a short-term and impending stage when the high and low value anomalies tend to be normal.Further study of the numerical distribution of high and low anomaly areas of wave velocity ratio shows that the earthquake with the largest wave velocity ratio in the first high value anomaly area occurs on the edge of the anomaly area, and the earthquakes with the lowest and the largest wave velocity ratio in the low value anomaly area and the second highest value anomaly area occurred within the anomaly area. The distances of these earthquakes with the largest and the lowest wave velocity ratios from the main earthquake are not greater than 60km.