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发生于1969年的渤海M_s 7.4地震是渤海海域惟一一个主震被现代仪器记录的大地震,对其发震断层的讨论争议颇多.2005-2008年,我们在主震震区进行了3个航次的高分辨率声学探测,获得了包括浅层单道地震、侧扫声纳以及CHIRP剖面等在内的数百公里综合地球物理观测资料.本文报道了对上述声学资料的研究结果研究表明,在震区海底之下2-3 m发现了一条走向NE30°、长20 km、宽约3 km的微型凹陷带,地质时代为5000 a B.P.,沉积面的最大下凹幅度为1.5 m;在空间位置上微型凹陷带与BZ28断裂、余震活动分布基本一致,因此,微型凹陷带是BZ28断裂活动的结果,而A层底界的下凹变形则是1969年渤海7.4级地震活动的结果,BZ28断裂是该地震的发震断层.BZ28断裂呈NE30°走向,是郯庐断裂带的次级断裂,浅层地震剖面揭示其最新活动时间为全新世中晚期,根据地质时代以及断层的垂向位移量,计算得到BZ28断裂晚更新世以来的垂向活动速率约为0.05 mm/a,全新世为0.3 mm/a,具有活动强度增加的趋势.
The Bohai M_s 7.4 earthquake, which occurred in 1969, was the only major earthquakes recorded by modern instruments in the Bohai Sea Sea. There are many controversies over the seismogenic fault. From 2005 to 2008, we carried out 3 High-resolution acoustic exploration of voyages, obtained hundreds of kilometers of comprehensive geophysical data including shallow single-channel earthquakes, side-scan sonars and CHIRP profiles, etc. The results of research on the above acoustic data are reported in this paper, A subsidence zone of NE30 °, length 20 km and width of 3 km was found within 2-3 m below the seafloor in the earthquake zone, with a geologic age of 5000 a BP and a maximum dipping amplitude of 1.5 m on the sedimentary surface. Therefore, the micro-depression zone is the result of BZ28 fault activity, while the depression deformation of the bottom boundary of layer A is the result of the M7.4 Bohai Rally seismic activity in 1969. The BZ28 fault Is the seismogenic fault of the earthquake.The BZ28 fault is in the NE30 ° direction and is a secondary fault of the Tanlu fault zone. The shallow seismic section reveals that the latest activity time is mid-Holocene. According to the geologic age and the vertical displacement of the fault , Calculate BZ28 to vertical fracture rate since the late activity of about 0.05 mm / a, new world of 0.3 mm / a, having activity tends to increase strength.