关键词: 同震破裂, 时移地震, 浅源海域地震, 流体运移

Abstract: Co-seismic near-surface rupture is one of the keys to the recognition of earthquake fault and the defense to seismic hazard. However, conventional investigation methods such as outcrop mapping and trenching, are often disturbed by the variation of capping formation. Besides, it’s difficult to apply these methods under the sea water. To this end, we suggest that to explore the buried co-seismic rupture using seismic reflection profiles. In this paper, a case study is presented. In the west of the epicenter of 2018 South Taiwan Strait MS 6.2 earthquake, a multi-channel seismic profile was carried out twice under the direction of Fujian Earthquake Agency, in 2017 and 2019. To avoid the pseudo phenomena and the effects of the variation in data characteristics, before comparing we reprocessed and cross equalized the two data sets with the same de-noise method, illumination, migration algorithm and velocity field. The profile correlation in 20~50 Hz show that the dominant reflecting wave groups coincident with the time-lapse ones, which means the two sections are in phase. Located about 25 km west of the epicenter, where the profile met the earthquake fault inferred by focal mechanism, at depths greater than 400 m, the remarkable reflections near a strike-slip fault plane changed significantly. From 2017 to 2019, the strongest reflection in the hanging wall reduced in amplitude and shifted from near horizontal to a jagged fold in shape, besides the polarity of two remarkable reflections reversed, and a piece of the basement reflection in the heading wall close to the fault plane subsided about 8 milliseconds measured in two-way travel time. The remarkable reflections in the rest of the profile were aligned accurately as the control. On these grounds, it is determined that these variations with time imaged the fluid migrations caused by co-seismic fault slip, which the site and the displacement of formations coincident with the focal mechanism. Furthermore, the result indicates that co-seismic near-surface ruptures could be identified with the reservoir changes measured in P-wave impedance which revealed by time-lapse seismic profiles. This method is practicable, especially for shallow-focus marine earthquakes. This study is of great significance for improve the recognition of earthquake faults and the slip distribution.

Key words: Co-seismic Rupture, Time-lapse Seismic, Shallow-focus Marine Earthquake, Fluid Migration