Acoustic detection of the causative fault of 1969 Ms7.4 earthquake in Bohai Sea
1969年渤海Ms7.4地震发震断层的声学探测

The Ms7.4 earthquake occurred on 18 July 1969 was the only strong earthquake that has been recorded by seismometers in Bohai Sea. Now arguments about this large earthquake are focused on its causative fault. In 2005~2008, we finished three cruises in the epicenter region and collected several hundreds high-resolution acoustic surveying data, including shallow penetrated single channel data, sidescan sonar data and CHIRP data. This paper reports study results of these new acoustic data. A micro depression belt is newly found in Holocene sediments 2~3 m beneath seabed in the epicenter region and it strikes NE30°, is 20 km long and 3 km wide. CHIRP data reveal that depositional interface formed at 5000 a B.P. has a 1.5 m vertical offset in the belt. The depression belt, BZ28 fault and aftershocks distribution of 1969 earthquake coincide in position, so it can be assumed that the depression belt is formed by BZ28 fault and the deformation of the bottom of seismic sequence A is caused by 1969 earthquake. BZ28 fault is the causative fault of this large earthquake and it is a sub-fault in the Tan-Lu Fault Zone and strikes NE30°. Shallow penetrated single channel seismic data show that it is active during the late Holocene. According to sediments age and vertical offset, we calculate the vertical slip rate of BZ28 fault since late Pleistocene. The result shows that activity intensity of BZ28 fault has the trend of increasing from 0.05 mm/a since the late Pleistocene to 0.3 mm/a during the Holocene.