宝坻-苗庄凸起新生断裂脆韧性转换深度研究

Brittleness ductility transition depth of new fracture in the Baodi Miaozhuang Uplift

现今地震是最新构造活动的最直接表现,是认识大陆内部非刚性变形特征的最有力工具。尤其是对晚更新世以来开始活动的新生断裂来说,由于发育成熟度低,几何构造上贯通性差且不同区段走向/不同深度倾向差异明显,地震能够揭示其三维空间的变化,尤其是脆韧性转换深度特征,促进对新生活动断裂孕育、发震过程的认识。渤海湾盆地新生走滑活动断裂曾多次发生强震,如唐山M_(S)7.8、三次宁河M_(S)≥6.2地震,这些地震揭示的地壳脆-韧性转换深度对认识该地区地震发震机理和活动性具有重要的现实意义。本文以渤海湾盆地内宝坻-苗庄凸起新生断裂为目标断裂,以发生于宝坻凸起的2012年宝坻M_(S)4.0、M_(S)3.5地震以及发生于苗庄凸起的1976年宁河M_(S)6.2、M_(S)6.9、1977年宁河M_(S)6.2地震为研究对象,基于首都圈数字地震台网的波形资料,采用CAP方法研究宝坻M_(S)4.0、M_(S)3.5地震震源机制解和震源深度,利用近震转换波Sp精确确定震源深度,采用双差方法定位两个地震序列的震源位置,并结合其他资料探讨该地区地震的发震机理以及新生断裂脆韧性转换深度。结果显示:(1)宝坻M_(S)4.0、M_(S)3.5地震震源性质与宝坻-苗庄凸起出露地表的已知断裂不尽相符,结合宁河三次MS≥6.2地震震源参数的有关研究成果,推测它们均发震于宝坻-宁河深大断裂;(2)结合震源区电性结构以及流变结构等模型,宝坻-苗庄凸起这五次显著地震揭示出宝坻-宁河深大断裂的脆韧性转换深度为15km左右;(3)结合渤海湾盆地动力演化过程以及深部地球物理探测等相关资料,推测该地区地震活动主要是宝坻-宁河深大断裂与深部流体作用的结果。

Today’s earthquakes are the most direct manifestation of the latest tectonic activity and are the most powerful tool to understand the characteristics of nonrigid deformation within a continent.Particularly,the newly formed faults that have been active since the Late Pleistocene are characterized to have low development maturity,poor connectivity in geometric structure,and obvious difference in strike/depth dip in different sections.Therefore,earthquakes can reveal their three-dimensional space changes,especially the characteristics of brittle–ductile transition depth.Earthquakes can also promote the understanding of the preparation and earthquake generation process of newly formed active faults.The newly formed strike-slip active faults in the Bohai Bay Basin have experienced many strong earthquakes,including the Tangshan M_(S)7.8 and the three Ninghe M_(S)≥6.2 earthquakes.The crustal brittle–ductile transition depth revealed by these earthquakes has an important practical significance to understanding the seismogenic mechanism and activity of the region.In this paper,the new fault in the Baodi Miaozhuang uplift in the Bohai Bay Basin is taken as the target fault.In addition,the 2012 Baodi M_(S)4.0 and M_(S)3.5 earthquakes in the Baodi uplift and the 1976 Ninghe M_(S)6.2 and M_(S)6.9 and the 1977 Ninghe M_(S)6.2 earthquakes in the Miaozhuang uplift are taken as the research objects.Finally,the CAP method is used to study the source mechanism solution and the source depth of the Baodi M_(S)4.0 and M_(S)3.5 earthquakes based on the waveform data from the Capital Circle Digital Seismic Network.The double difference method is used to locate the source positions of two earthquake sequences by employing the Sp converted wave of near earthquakes to accurately determine the source depth.Moreover,combined with other data,the seismogenic mechanism of earthquakes in this area and the brittle–ductile transition depth of new fractures are discussed.The following results were found.(1)The source properties of the Baodi M_(S)4.0 and M_(S)3.5 earthquakes are not completely consistent with the known faults on the exposed surface of the Baodi Miaozhuang uplift.Combining the research results of the source parameters of the three Ninghe earthquakes having an M_(S)≥6.2,it is inferred that they all occurred in the Baodi–Ninghe deep fault.(2)Based on the models of electrical structure and rheological structure in the focal area,the five significant earthquakes in Baodi Miaozhuang uplift reveal that the brittle–ductile transition depth of the Baodi–Ninghe deep fault is about 15 km.(3)Based on the dynamic evolution process of the Bohai Bay Basin and relevant deep geophysical exploration data,it is hypothesized that the seismic activity in this area is mainly the result of the Baodi–Ninghe deep fault and deep fluid actions.