Abstract: Choosing the Longmenshan region as the study area, making use of the historical earthquake data from the year 1931 to 2010, taking 10 years as the time interval, this paper establishes a generalized extreme value model of seismicity, and it estimates the magnitude upper bound and the strong earthquake return level associated with the Longmenshan region. The results indicate that the model follows the Weibull distribution with a finite upper bound; the magnitude upper bound is 8.3; and the strong earthquake return level in 20, 50, 100 future years are 7.9, 8.1, 8.1, respectively. Moving the starting year from 1930 to 1933 year by year, remaining the time interval constantly, we find that the difference of the results of the maximal magnitude and of the strong earthquake return level are less than 0.2, which means that the generalized extreme value model for the Longmenshan region has some degree of stability; moreover, the results can be a reference for earthquake zoning and seismic hazard analysis research.

Key words: Seismic hazard, Magnitude upper bound, Strong earthquake return level, The Longmenshan Region