SPATIOTEMPORAL PATTERN OF b - VALUE BEFORE MAJOR EARTHQUAKES IN THE SICHUAN-YUNNAN REGION

Because b-value can be used as an indicator to measure the level of crustal stress,the seismicity pattern recognition based on b-value is considered as one of the most intuitive methods for earthquake forecast research and experiments. The intense seismicity in the Sichuan-Yunnan region makes the ar- ea to be an ideal field for earthquake forecast experiment. We investigated the spatial and temporal variations of b-values before nineteen earthquakes with M≥6. 5 in the Sichuan-Yunnan region from 1981 to 2008 using regional small earthquake data. The results show that spatial b-value variations portending M≥6. 5 earthquakes in the Sichuan-Yunnan region are insignificant. Less than 50% of the major earthquakes occurred in the regions with low b-values relative to the entire study area. In terms of temporal evolutions,the b-values in seismic source regions showed a systematic drop before the oc- currence of earthquake. They showed an obvious decreasing trend during the period one year prior to strong earthquakes in 58% of the seismic source regions. Combined with other geological and seismo- logical data,the short-term seismicity data can provide certain enlightenment for the estimation of po- tential seismic hazard in specific regions.