中国大陆7级强震前中小地震应变释放特征

给出加速模型拟合的穷举方法,以中国大陆21次6.8级以上地震为例,在单个震例详细研究的基础上,总结中国大陆7级左右地震前地震应变释放的群体特征。 总的来看,7级左右地震前中小地震应变释放模式可分为5类,第Ⅰ类是震前震中周围较大范围应变释放加速、震中附近较小范围应变释放减速(DA),约占38%; 第Ⅱ类是震中附近较大范围应变释放减速、较小范围应变释放加速(AD),约占19%; 第Ⅲ类仅能检测出应变释放的加速特征(ASR),约占14%; 第Ⅳ类仅能检测出应变释放的减速特征(DSR),约占24%; 第Ⅴ类仅1例,各种尺度的时、空范围内,基本上呈现应变匀速释放特征(LSR)。 前兆性地震序列应变加、减速释放的持续时间一般相差3~6年,这表明增强基础上的平静往往是强震前中小地震活动典型的应变释放特征。 DA模式中震前具有稳定的应变加、减速释放特征的临界区域范围平均来看大约分别是震中附近260~400 km及100~200 km,大约分别是7级左右地震平均破裂尺度的3~5倍及1~2倍; AD模式与此相反。 模型参数m值具有较好的稳定性,就应变释放加速现象而言,无论何种应变释放模式、无论应变释放的绝对量如何,其加速释放速率大体相当。 而从应变释放减速情形来看,“外围增强、内部减弱”的DA模式,其应变减速释放特征最为明显。

Characteristics of Strain Release before M7 Earthquakes in Mainland China

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”.