Abstract: Enumerating algorithm is introduced into the fitting procedure of accelerating strain release (ASR) model. Based on the detailed study of 21 large earthquakes with M≥6.8 in mainland China, the statistical features of seismic strain release before large earthquakes have been summarized. On the whole, the strain release mode could be divided into five types. The first is DA mode, in which the accelerating and decelerating strain release has been checked respectively in broad and in small region around the epicenter, about 38% earthquake samples belongs to this type. The second is AD mode, in which the decelerating and accelerating strain release has been checked respectively in broad and in small region around the epicenter, accounting about 19%. The third is ASR, in which only accelerating strain release could be checked, it is about 14%. The forth is DSR, also only decelerating strain release could be checked, occupying ratio is about 24%. There is only one earthquake sample belongs to the fifth type (LSR), which shows a linear strain release. There is a difference of 3 to 6 years between the accelerating and decelerating duration of preshock sequence, which means that seismic quiescence after the activity generally is a typical feature of small earthquake activity before the large earthquakes. For DA mode, the average size of critical regions for steady accelerating and decelerating strain release are about 260 to 400 km and 100 to 200 km, which is 3 to 5 times and 1 to 2 times of fracture size of an earthquake with about M7, respectively. The AD mode is just opposite to the DA mode. The model parameter, m value, has a good stability, the ratio of ASR is about the same for accelerating seismic strain release phenomena, whether what the strain release modes are, or how large the strain release quantity is. Seeing from decelerating seismic strain release phenomena, the characteristic of decelerating strain release is the most obvious for DA mode, which shows a typical feature of “decelerating in-accelerating out seismic strain”.

Key words: Accelerating strain release, Decelerating strain release, Size of critical region, Duration of preshock sequence