Seismic hazard analysis of the Sichuan-Yunnan region, western China

As the southeastern margin of the Tibetan Plateau, the Sichuan-Yunnan region is carved by ample active faults, such as the Xianshuihe, Anninghe, Zemuhe, Xiaojiang, Honghe, Lancangjiang, Jiali, and Longmenshan fault systems. Many strong earthquakes have taken place in the past, and according to the Chinese catalog more than 283 M>5.5 events occurred in the region since 1970, including the 2008 Mw7.9 Wenchuan earthquake. Some of the strong earthquakes have resulted in great damages and losses of human lives, and assessment of seismic hazard potential in the region is vitally important. In this study we attempt to estimate the seismic hazard potential using three different approaches. Our first approach is to use secular geodetic strain rate deduced from GPS velocity data to study the earthquake activity. Assuming that earthquake numbers and magnitudes obey the Gutenberg-Richter law and its intensity distribution is proportional to the geodetic strain rate, we devise a model of earthquake probability for the region. Retrospective test of the probability model with earthquake occurrence of the past 30 years shows that the model 'forecasted' poorly. However, the model seems to 'forecast' spatial intensity of earthquakes for the past 500 years reasonably well, suggesting that the geodetic strain rate obtained at the decade scale may still be a good indicator of long term earthquake activity in the region, but only at a time scale of hundreds of years. Our second approach is to take the same assumptions as the first one, but using a regional earthquake catalog to constrain the probability model. Retrospective test shows that the 'forecasts' have some predictive power for strong events occurred on fault segments with shorter earthquake recurrence time, but not for that with long recurrence time such as the Longmenshan fault. Our third approach is to use GPS velocity data to determine the seismic moment accumulation rates on major faults, and use a historical earthquake catalog to estimate seismic moments released in the past. Comparison of the two yields estimates of present day seismic moments accumulated on major faults, and a retrospective test shows some predictive power of the method. Our result suggests that some of the faults have accumulated seismic moments capable of producing M>7.5 earthquakes, which are the Xiaojiang, Daliangshan, Jiali, Nujiang, Yongsheng-binchuan, and junction between the Xianshuihe and Ganzi-Yushu faults.