2008年汶川MS8.0地震前地震活动异常特征

分析了汶川地震前地震活动时空演化特征．结果表明:①汶川地震前38a龙门山断裂带及其附近形成5级地震背景空区，震前6.5a形成ML4.0地震孕震空区，震前1a孕震空区内部及其两端相继发生多次ML4.0—5.0地震，空区缩小；②中国大陆西部及邻区2001年以来处于大震活跃时段，而中国大陆内部地震活动水平非常低，出现非常显著的7级、6级和5级地震平静；③南北地震带7级以上地震在时间上具有准周期特征，空间上存在由南向北迁移的特点，汶川地震的发生符合这一规律；④1998年以来南北地震带中段为7级地震空段，汶川地震就发生该空段内；⑤2003年云南大姚地震后，南北地震带地震活动显著增强，且在中、南段形成4.6级以上地震环形分布，四川及其附近表现为异常平静，同时震群活动显著，且在4.6级地震平静区内形成震群空区，汶川地震就发生震群空区的边缘，震前8个月，震群频度出现高值异常；⑥汶川地震前7个月，青藏块体大范围ML≥4.0地震平静103d，2008年1月13日以后平静区逐渐解体，至汶川地震前４级地震平静区缩小到巴颜喀拉地块，汶川地震就发生在巴颜喀拉地块的东边界带上，汶川地震前3 个多月，孕震空区内部出现NW走向的3级地震条带，与龙门山断裂带斜交． 

Characteristics of seismic activity before 2008 Wenchuan M_S8.0 earthquake

The spatio-temporal evolution characteristics of seismic activity before 2008 Wenchuan M_S8. 0 earthquake are studied. Some results are as follows：①A background gap of M≥5. 0 earthquakes formed in the area of Longmenshan fault zone and its vicinity 38 years before the 2008 Wenchuan earthquake. A seismogenic gap of M_L≥4. 0 earthquakes appeared in the same area 6. 5 years prior to the Wenchuan earthquake. It is noticed that one year before the Wenchuan earthquake some particular earthquakes with M_L4. 0-5. 0 occurred successively in Longmenshan fault zone and its two end regions，and the original gap was reduced. ②Western China mainland and its adjacent area has been in the seismically active period since 2001, while the inner land of China is seismically inactive，with remarkable quiescence of M≥7. 0，M≥6. 0 and M≥5. 0earthquakes. ③The activity of M≥7. 0 earthquakes in the South-North Seismic Zone showed temporally quasi-cyclic variation and spatial migration from south to north. The occurrence of the Wenchuan earthquake itself followed this feature. ④A quiescence area of M≥7. 0 earthquakes formed in the middle and northern part of the South-North Seismic Zone since 1988 and the Wenchuan earthquake occurred in it. ⑤The seismic activity obviously strengthened and a doughnut-shape pattern of M≥4. 6 earthquakes appeared in the middle and southern part of the South-North Seismic Zone after the 2003 Dayao，Yunnan，earthquake. Sichuan province and its vicinity in the middle of the doughnutshape pattern were obviously quiescent. In the same time，earthquake swarms were often seen and showed heterogeneous temporal and spatial distribution. A swarm gap appeared in the quiet area of M4. 6 earthquakes and the Wenchuan earthquake occurred just on the margin of the gap. Earthquake swarms occurred quite frequently during the 8 months before Wenchuan earthquake. ⑥Quiescence of M_L≥4. 0 earthquakes appeared in Qinghai-Xizang block 7 months before Wenchuan earthquake and the quiescence state gradually disappeared since 13 January 2008. Before the Wenchuan earthquake the quiescence area reduced to the region of Bayankala block and the Wenchuan mainshock occurred just in eastern boundary zone of the block. A belt of ME≥3. 0 earthquakes striking in NW-SE direction，which obliquely crosses to the Longmenshan fault zone，formed 3 months before the Wenchuan earthquake in the seismogenic gap.