汶川地震前地震活动特征的普遍性及其机理探讨

汶川地震前地震活动较为显著的异常是：1970—2008年汶川地震前，从云南北部至甘青川交界形成规模巨大的5.5级以上地震活动增强区（或称环形分布）；1970—1999年围绕龙门山断裂带形成5级以上地震背景空区，汶川地震发生在增强区内的背景地震空区里；2001—2007年形成M_L4.0以上地震孕震空区，震前1年孕震空区内部及其两端相继发生多次M_L 4.0~5.0地震，空区打破.上述地震活动增强区、背景空区和孕震空区是大地震前普遍出现的现象.为对比分析，本文系统研究了2001年以来我国大陆及邻区4次M_S≥7.8级地震和全球10次M_W≥8.0级地震前类似地震活动异常，并给出统计特征.结果显示：地震增强区规模为850~2700 km，持续时间13—38年，增强区长轴对数与主震震级呈正相关关系.增强区与余震区规模之比为2.3~7.7，其对数与主震震级呈负相关.背景空区长轴300~1100 km，持续时间10—32年，其长轴对数与主震震级呈正相关关系.孕震空区长轴为370~780 km，持续时间1—7年，孕震空区长轴对数与主震震级呈正相关关系.对于板内地震，构成增强区的最低震级为5.0级或5.5级，构成背景空区和孕震空区的最低震级分别为5.0级和4.0级.而对于板间地震，构成增强区和背景空区的最低震级为6.0级或6.5级，构成孕震空区的最低震级为5.0级或5.5级.基于坚固体地震孕育模型，认为地震活动增强区的环形分布是由于震源区的破裂强度高于周围介质造成的，地震孕育过程中体应变的范围和强度存在逐渐增大和变小的过程，这是地震活动增强区出现三阶段特征的原因.从包体弹性理论可以推导出增强区尺度的对数与主震震级、增强区与震源体比值的对数与主震震级存在线性关系.

On the universality and mechanism interpretation of the seismicity characteristics before the 2008 Wenchuan MS8.0 earthquake

The remarkable seismicity anomalies before the 2008 Wenchuan M_S8.0 earthquake are as follows. An active strengthening area, namely a ring distribution with M≥5.5 events appeared from the northern part of Yunnan to the border area of Gansu, Qinghai and Sichuan provinces during the time of 1970 to 2008. A background gap with M≥5.0 earthquakes was formed around the Longmen Shan fault zone from 1970 to 1999. The Wenchuan earthquake occurred in the background gap which was in the active strengthening area. From 2001 to 2007, the seismogenic gap with M_L≥4.0 shocks was formed. While one year prior to the Wenchuan earthquake, several shocks with M_L4.0~5.0 occurred in the seismogenic gap and its two ends, and the gap was broken. The above seismicity patterns, such as the active strengthening area, background gap and seimogenic gap, appeared before the Whenchuan earthquake are a universal phenomenon. For contrastive analysis, we study the seismicity anomalies of 14 great earthquakes in a global area since 2001, in which 4 of them are M_S≥7.8 events in China mainland and adjacent areas, and the other 10 are M_W≥8.0 shocks. The statistical characteristics are stated below. The length of the active strengthening area is 850~1800 km and its duration time is 13-38 years. The logarithm of the long axis of the active strengthening area is correlated with the magnitude of the main shocks in a positive way. The ratio of the scale of the strengthening area to the aftershock area is 2.3~7.7 and the logarithm of the ratio is correlated with the main shock in a negative way. The long axis of the background gaps is 300~1100 km and the duration time is 10-32 years. There is a positive correlation between the logarithm of the long axis of the background gaps and the magnitude of the main shock. The length of the seismogenic gaps is 370~780 km, and the duration time of it is 1.2-7.0 years. The logarithm of the long axis of seismogenic gaps is correlated with the magnitude of the main shock in a positive way. For intraplate earthquakes, the minimum magnitude of the strengthening area is M5.0 or M5.5, the minimum magnitudes of the background gap and the seimogenic gap are M5.0 and M4.0, respectively. While for interplate earthquakes, the minimum magnitudes of the strengthening area and the background gap are M6.0 or M6.5, respectively, and the minimum magnitude of the seismogenic gap is M5.0 or M5.5. Based on the strong body seimogenic model, the ring distribution of the strengthening area is due to that the rupture strength of the source body is higher than that of the surrounding medium. During the earthquake preparing process, the range and strength of the body strain are gradually increasing and decreasing. This is the reason for the evolution of the three stages of the active strengthening area. The linear relation between the logarithm of the long axis of the strengthening area and the main shock magnitude, and that between the logarithm of the ratio of the length of the strengthening area to aftershock area can be deduced from the elastic body source theory.