Abstract:

The Haicheng M_S7.3 earthquake is the first successfully predicted earthquake in China, which saved a large number of lives and avoided property losses. However, the investigation after the earthquake did not find a continuous surface rupture zone, and only some ground fissures and sandblasting were found in the epicenter area. The isoseismal line of this earthquake shows obvious conjugate characteristics. Which fault is the seismogenic structure of the Haicheng earthquake has always been controversial. According to the focal mechanism and distribution of ground fissures, some scholars suggested the seismic structure is the Haicheng River Fault with a strike of NWW. However, other scholars suggested the Jinzhou Fault has a larger scale and controls the geomorphic boundary. Jinzhou Fault is also a major seismic structure distributed in the west of Liaodong Peninsula, with a strike of NENNE and a length of 280km. The north Gaizhou-Anshan segment of the Jinzhou Fault is conjugated with the Haicheng River Fault. Both of them are likely to be the seismogenic structure of the Haicheng earthquake, or both ruptured in the Haicheng earthquake. Based on remote sensing image interpretations, four sites of the fault scarps, including the Yujiagou, Houwudao, Dongjiagou, and Tashan sites, were distinguished and verified in situ. And using micro geomorphology measurement and paleoseismic trench excavation in the Huluyu site of the north Gaizhou-Anshan segment of the Jinzhou Fault which is conjugated with the Haicheng River Fault, this paper obtains the following understandings: The Jinzhou Fault extends from the northeast of the Dashiqiao City to the south of the Anshan City. There are prominent NE-trending fault scarps, which were formed in the late Pleistocene and Holocene, on geomorphic surfaces of the basin mountain transition zone. Due to farming and building, fault scarps are not preserved well, and the distribution of the fault scarp is discontinuous. The height of fault scarps is mostly 1~2m, up to 3m at most. The paleoseismic trench was excavated in the Huluyu village, south of Haicheng City. The paleoseismic trench revealed a ~²0m wide bedrock fracture zone in the north Gaizhou-Anshan City segment of the Jinzhou Fault. Three Late Pleistocene to Holocene strata(U3 to U5)overlie the bedrock fracture zone. Five fault planes(F₁ to F₅)are revealed in the trench. The fault F₁ recorded the newest paleoearthquake event and the Fault F₂ recorded the earlier one. In summary, according to the cover-cut relationship between strata and faults, at least two paleoseismic events occurred from the Late Pleistocene((37.6±2.2)ka)to the Holocene. The newer one occurred in the Holocene(after(11.7±0.8)ka, probably 400~500a before present). However, because of the thin Holocene strata, we cannot distinguish more paleoearthquakes in the trench. Therefore, it is still doubtful whether the north Ganzhou-Anshan segment of the Jinzhou Fault ruptured in the Haicheng earthquake in 1975. However, the confident conclusion is that the north Gaizhou-Anshan City segment of the Jinzhou Fault is an active fault in the latest Late Pleistocene to Holocene.

Key words: Jinzhou Fault, paleoearthquake, fault scarp, Haicheng M_S7.3 earthquake

摘要：

1975年海城 M_S7.3 地震是中国第1次成功预报的7级以上破坏型地震, 避免了大量人员和财产损失。但在地震后的调查中并没有发现较为连续的地表破裂带, 只在零星地点发现了一些地表裂缝和喷砂冒水现象。该地震的等震线表现出较为明显的共轭特征, 因此研究者对于海城地震的发震断层一直存在一定争议。文中对与海城河断裂共轭相交的金州断裂盖州北鞍山段进行了遥感影像解译、 微地貌测量和古地震探槽开挖等工作, 发现金州断裂自大石桥市沿NE向至鞍山市南, 在盆山过渡带的晚更新世和全新世地貌面上存在较为明显的沿NE向展布的断层陡坎。由于人类活动, 断层陡坎展布不连续。断层陡坎的高度多为1~2m, 最大可达3m; 在海城市南葫芦峪村开挖的古地震探槽揭露出盖州北鞍山段具有宽约20m的基岩破碎带, 晚更新世晚期全新世以来(距今(37.6±2.2)ka)至少发生过2次古地震事件。较新的一次地震发生于全新世(距今(11.7±0.8)ka以后, 很可能为距今400~500a)。由于全新世地层太薄所限, 无法识别出更多全新世古地震, 但可以判断金州断裂盖州北鞍山段为晚更新世晚期全新世活动断裂。

关键词: 金州断裂, 古地震, 断层陡坎, 海城地震