首次直接观测到与理论预测一致的同震静态应力偏量变化——2016年4月7日山西原平M_(L)4.7地震的钻孔应变观测

The variation of coseismic static stress deviation consistent with theoretical prediction was observed for the first time—Observation of borehole strain of the Yuanping M_(L)4.7 earthquake in Shanxi on April 7,2016

地震会引起同震位移和应力场变化,造成现存各断层上的库仑应力变化量的增加或减小并影响后续地震风险,对于地震预测具有重要意义.现有的大量地震实例中,理论计算的同震位移场与GPS、InSAR等实测的同震位移吻合,间接证实了基于均匀理想弹性假定下可以使用半无限空间Okada公式计算与位移微分相关的同震应力变化.然而,由于地壳并非严格的均匀理想弹性介质,理论计算和直接测量的同震应力变化进行比较仍是一个重要的研究课题.中国对四分量钻孔应变仪的数百个地震同震阶变的研究,发现记录绝大多数阶变不能满足自检条件;美国板块边界观测(PBO)中数百个同震钻孔应变记录同样发现观测幅度与理论预期值相差甚多,可达一两个数量级,这成为国际钻孔应变观测中的一个未解难题.2016年4月7日山西原平发生M_(L)4.7地震,该地震虽然不大,但原平钻孔应力台站距离震中仅19 km,得到了优质的同震阶变记录.本文以此展开研究,结果表明台站观测到的主应力方向和应力偏量大小,均与基于震源机制解的破裂模型的理论预计值很好吻合,这是首次实际观测到与理论计算预测基本一致的同震水平应力偏量变化,为利用库仑应力概念估计后续地震活动性提供了观测基础.然而,观测到的应力变化与理论计算值相差109 Pa的围压,本文也对其可能原因进行了探索,认为在探头、固结水泥和围岩三个可能影响观测的因素中,固结水泥影响可能性最大,并对今后工作提出了建议.

Earthquakes can cause coseismic displacement and stress changes,increasing or decreasing the Coulomb stress on existing faults and affect subsequent earthquake risks,which is of great significance for earthquake prediction.Observed coseismic displacements by GPS,InSAR,etc.are in good agreement with the calculations,indirectly confirming that using Okada formula for semi-infinite space,calculated coseismic stress changes should also be true,since the uniform half space elastic model works well for displacements.However,due to the earth′s crust is not an ideal uniform elastic medium,the comparison between theoretical calculations and directly measurements of coseismic stress changes is still an important research topic.Hundreds records of coseismic off-sets from four-component borehole strain gauges have been obtained in China,but most of them cannot pass the self-check conditions;hundreds of coseismic off-sets in borehole strain records have also been obtained by the PBO(plate boundary observation)observations in the United States,the observed amplitude are significantly different from the theoretical expectations,with huge deviation up to one or two orders of magnitude.It has become an international unsolved problem in borehole strain observation.An earthquake of magnitude M_(L)4.7 occurred in Yuanping,Shanxi on April 7,2016.Although the earthquake was not large,the Yuanping borehole stress station was only 19 km away from the epicenter,and a high-quality coseismic off-set was recorded.Our study shows that the direction of principal stress and the magnitude of the deviatoric stress observed at the station are in good agreement with the theoretical predictions.This is the first time to show such consistency,which provides an observation basis for the use of the Coulomb stress concept to estimate the subsequent seismic activity.However,there is still a 109 Pa difference in confining pressure between the observed and the theoretical values.We also try to explore its possible causes.There are three affecting factors:the sensor,the cement and the surrounding rocks.It is indicated that,among them,the cement for consolidation is the most possible source for erroneous measurement,and improvement measures for future work are suggested.