超导重力仪检测2011年日本东北Mw9.0级地震前的重力扰动信号

Detection of gravity anomalies before the 2011 M_w9.0 Tohoku-Oki earthquake using superconducting gravimeters

对大地震前的扰动现象的研究有助于认识地震孕育的动力学过程,震前重力扰动已成为关注的热点之一.对大地震前的扰动现象的研究有助于认识地震孕育的动力学过程,震前重力扰动已成为关注的热点之一.为检验日本Mw9.0级地震是否存在震前扰动现象,本研究利用全球超导重力仪记录到的地震前后7天内20组秒采样数据进行分析.经潮汐、大气改正等处理去除仪器的漂移及残余潮汐效应,得到非潮汐重力变化曲线.结果表明大部分振幅大于30×10-8 m.s-2的曲线反映了全球Mw≥6级地震引起的高频波动信号,其中11组数据在3月9日Mw7.3级前震之前出现了扰动现象.震前扰动可分解为三个频段,其中,低于0.1Hz和高于0.18Hz的分量分别反映了地震波动信号及非构造信息,中间频段(0.118～0.18Hz)信号能够较大程度地压制地震波动信号、并同时保留异常扰动信息.它的振幅在3月7日10时之前基本保持约1×10-8 m.s-2,之后开始逐渐增大,到3月9日7.3级前震前后达到最大,此后振荡衰减,振幅保持约(5～10)×10-8 m.s-2,直至主震发生.中间频段信号的变化特征与主震前的应力迁移过程以及实验记录到的地震成核过程有许多相似之处;不过,震前重力异常是否与主震前的应力加速积累有关,仍待进一步研究.

The investigation on pre-seismic anomalies prior to big earthquakes will give insights into the dynamic processes of the nucleation/development of the earthquakes;the pre-seismic gravity anomalies have become one of the most focused phenomena.In order to examine if anomalies did present prior to the Mw 9.0 earthquake that occurred on March 11,2011,in Japan,we analyzed 7-day-long 1 Hz sampling records from 20 superconducting gravimeters（SG） worldwide.We obtained the non-tidal gravity change time series by removing the solid tides and the pressure effect as well as the drift.Most of the non-tidal gravity changes with amplitude larger than 30×10-8 m·s-2 were associated with Mw≥6 earthquakes worldwide;pre-seismic anomalies presented in 11 SGs data before the Mw7.3 foreshock.The pre-seismic anomalies can be decomposed into three frequencies,among which the earthquake-related and non-tectonic signals are constrained on the frequencies less than 0.1Hz and larger than 0.18Hz,respectively;whereas the signal of medium frequencies（0.118~0.18 Hz） can maintain the anomalies while suppress the earthquake-related signal.The amplitudes of the medium frequency signal increased gradually from ~1×10-8 m·s-2 at 10：00,7 March,to peaks larger than ~10×10-8 m·s-2 when the largest foreshock（Mw7.3） occurred on 9 March,subsequently the amplitudes fluctuated and decreased to about（5~10）×10-8 m·s-2 and kept the same level until the main shock.Such evolution is similar to the stress migration before the mainshock as well as the nucleation processes recorded in rock rupture experiments;nevertheless it need further study to test if the pre-seismic gravity anomaly is related to the accelerating stress accumulation before the main shock.