Balance of seismic moment in the Songpan-Ganze region, eastern Tibet: Implications for the 2008 Great Wenchuan earthquake

The 12 May 2008 Wenchuan earthquake (Ms 8.0) occurred on the Longmen Shan fault zone, where slip rates are low and earthquakes are infrequent in comparison with other major fault zones in the Songpan-Ganze region, eastern Tibet. We have investigated the evolution of strain energy along major faults in this region by comparing the accumulation and release of seismic moment. First, we calculated the slip rates on the Longmen Shan and other major faults in the region using a three-dimensional regional-scale block model, constrained by the latest GPS data. On the Longmen Shan fault, the predicted right-lateral and dip-slip rates are respectively 1.7 ± 0.8 mm/year and 1.2 ± 1.0 mm/year along the southwestern segments, and 1.4 ± 1.1 mm/year and 3.3 ± 1.3 mm/year on the northeastern segments. These slip rates are one order of magnitude lower than those on the Xianshuihe fault and other faults in the region. Second, using the earthquake catalog, we estimated the scalar moment released on major faults in the Songpan-Ganze region between 1879 and 2007. The released seismic moment was ~ 63% of the regional scalar moment accumulated during this period. The moment deficits were found mainly on the western Xianshuihe and eastern Kunlun faults. The eastern Xianshuihe fault has been a focus of studies because of the high slip rates and frequent earthquakes, but a sequence of major quakes there since 1893 has reduced the moment deficit to 3.68 × 1019 N m, barely enough for a Mw 7.0 event. It takes the Longmen Shan fault more than 1000 years to accumulate the seismic moment (1.15 × 1021 N m) released during the 2008 Great Wenchuan earthquake, hence we conclude that a repeat of great earthquakes on the ruptured segment of the Longmen Shan fault is unlikely in the next few hundred years, but the unruptured southwestern segment of the Longmen Shan fault is capable of producing a Mw 7.7 earthquake in the next 50 years.