2011年3月11日,一个Mw9.1地震袭击了日本本州地区,为了分析这次地震前后主震破裂区内应力时空变化,我们选取1996年1月～2016年6月期间发生在破裂区内的563个5.0≤Ms≤6.9地震,研究了视应力随时间的变化和空间分布.日本Mw9.1地震前从2002年中起视应力开始呈趋势性上升变化,到2009年初以0.18 MPa/a的速率从0.6 MPa上升到1.76 MPa,相差约3倍,直到地震发生前夕一直保持在1.5 MPa之上.地震发生之后,直到2016年6月在破裂区内视应力呈缓慢下降变化,但仍保持在1.5 MPa之上较高水平.视应力在地面上和断层面上的分布显示,1996-2005年间破裂区仅存在个别视应力高值,从2006年到2011年2月,破裂区大面积出现视应力高值.在日本Mw9.1地震发生之后的近3个月内,破裂区视应力整体处于高值水平,之后在较高的水平上缓慢减弱.视应力是地震断层面上平均应力的下限,视应力的高低在一定程度上反映的是震源断层面上平均应力的高低.在日本Mw9.1地震前,发生在破裂区内的地震,其断层面上的平均应力经历了大约8.5年的趋势上升变化过程.这次大地震前破裂区所在的地壳应力逐渐增加,最后导致断层面错动发生日本Mw9.1地震.%On 11 March 2011,the Mw9.1 earthquake struck Tohoku,Japan.In order to investigate the temporal and special changes of apparent stress in the rupture volume of the mainshock,we selected 563 earthquakes (5.0≤Ms≤6.9) located within the rupture volume which occurred between Jan.,1996 and Jun.,2016 to analyze the temporal variation and spatial distribution of apparent stress.It was found that the apparent stress started to increase at the middle of 2002,and reached to the highest value of 1.76 MPa from 0.6 MPa at the beginning of 2009,which increased about 3 times.Until when the 2011Tohoku Mw9.1 earthquake was drawing closer,the apparent stress kept above 1.5 MPa.From the occurrence of the Tohoku Mw9.1 earthquake to Jun.2016,the apparent stress manifested a gradually dropping but kept above 1.5 MPa with the higher values.It showed that there were only individual higher values of apparent stress on the ground or the focal fault plane from 1996 to 2005,while there were lots of higher values of apparent stress in the rupture zone,from 2006 to Feb,2011.In the three months after the occurrence of the Tohoku Mw9.1 earthquake,the apparent stress was at a high value,and then,gradually dropped with a slow rate.Since the apparent stress is a lower bound for the average stress on the fault plane,higher values of apparent stress could show higher values of average stress on the fault plane.Therefore,it can be said that,before the Tohoku Mw9.1 earthquake,the average stresses on fault planes of the rupture zone had been increasing for about 8.5 years.As a result,the stress in the rock of the rupture zone increased gradually before the Tohoku Mw9.1 earthquake,which cause faulting and occurrence of the earthquake.
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