利用反射地震剖面探查隐伏同震破裂——以2018年台湾海峡南部M_(S)6.2地震为例

THE BURIED CO-SEISMIC RUPTURE EXPLORATION USING SEISMIC REFLECTION PROFILE:A CASE STUDY OF THE M_(S)6.2 SOUTH TAIWAN STRAITS EARTHQUAKE IN 2018

调查浅源地震在地表附近产生的同震破裂对研究地震发震构造、防御地震灾害具有重要意义。常规的地形地貌和探槽调查方法易受到盖层复杂多变的影响,在海域更是难以应用。为此,文中提出参考石油工业中时移地震的思路,用分别采集于地震发生前、后的2期反射地震剖面来识别隐伏同震破裂。2018年台湾海峡南部发生了M_(S)6.2地震,文中基于福建省地震局于地震发生前、后(2017年和2019年)在宏观震中西侧分别采集的2期多道地震资料,筛选出噪声残留、照明度、速度场、偏移算法等成像条件完全一致的2期数据体并进行互均化。经对比,同频段(20~50Hz)剖面的主要强反射波组相位相同且基本重合,满足检测地层变化的要求。对比发现,在宏观震中以西约25km、震源机制解反演得到的地震断层与反射地震剖面交叉处,断层F_(1)的形态在地震发生后无显著变化,但400m以下深度与之接触的各反射波组形态发生了显著变化,表现为标志层的极性反转、界面形态变化和基底反射波组移位。结合研究区的地质构造特征,推测此处断层的封闭性在地震发生时发生了快速改变(即同震滑动),孔隙流体沿断层面快速运移导致地层波阻抗剧烈变化。文中研究证明虽然以反射地震剖面的分辨率难以直接识别垂向同震位移,但隐伏同震破裂引起的流体运移可能使断层面附近地层的波阻抗发生显著变化,而在时移地震剖面上可以可靠地识别出这种变化。与传统的地表类调查方法相比,时移地震方法可以克服盖层对同震变形的吸收影响,扩展同震破裂的可识别范围。该方法为判别浅源地震的发震构造、滑动分布提供了新思路,对通常缺少地质调查和地表形变资料的海域地震研究尤为重要。

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.Drawing on the idea of time-lapse seismic techniques in the petroleum industry,we suggest identifying the buried co-seismic ruptures using two reflection seismic data sets acquired before and after earthquake,respectively.In this paper,a case study is presented.On Nov.26^(th),2018,a M_(S)6.2 earthquake occurred in the south Taiwan Straits.The focal mechanism of this event is dextral strike slip with a slight dip-slip component,and the aftershock distribution is E-W oriented.West of the epicenter,multi-channel seismic profiling was carried out twice under the direction of Fujian Earthquake Agency in 2017 and 2019.To avoid the influence of the difference in acquisition conditions,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 20~50Hz shows that the dominant reflecting wave groups are coincident with the time-lapse ones,which means the two sections are in phase.The comparison results show that:at about 25km west of the epicenter,the reflection profile met the earthquake fault inferred by focal mechanism,and the morphology of Fault F_(1) did not change significantly after the earthquake,but at depths greater than 400m,the remarkable reflections near the 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 8milliseconds measured in two-way travel time.The remarkable reflections on the rest of the profile were aligned accurately as the control.These phenomena can be interpreted as a fluid migration through sandstone fractures model perfectly,which is also consistent with the petrological features in the study area.Since the gap between the two acquisition dates is only 26 months and there was no human activity affecting subsurface structure in the vicinity,the pore fluid migration is inferred to be related to the 2018 event.This study demonstrates that although the vertical co-seismic displacement is smaller than the resolution of reflection seismic profiles in most cases,the near-surface fluid migration which accompanied the co-seismic rupture may cause significant impedance changes near the fault plane,and such changes can be reliably identified on time-lapse seismic profiles.Compared with the conventional investigation methods for co-seismic ruptures,the time-lapse seismic method can overcome the displacement absorption of capping formation and expand the identifiable scope of co-seismic ruptures.This method is practicable,especially for marine earthquake researches,because the acquisition repeatability and surface consistence of the marine reflection seismic data are relatively better than that of the land reflection seismic data.This study provides a new idea for recognizing the earthquake fault and slip distribution of shallow source earthquakes,which is especially important for the study of marine earthquakes with fewer geology and geodesy data available.