Detection of gravity anomalies before the 2011 Mw9.0 Tohoku-Oki earthquake using superconducting gravimeters

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 M-w 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 X 10(-8) m . s(-2) were associated with M-w >= 6 earthquakes worldwide; pre-seismic anomalies presented in 11 SGs data before the M(w)7.3 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 similar to 0.18 Hz) can maintain the anomalies while suppress the earthquake-related signal. The amplitudes of the medium frequency signal increased gradually from similar to 1 X 10(-8) m . s(-2) at 10:00, 7 March, to peaks larger than similar to 10 X 10(-8) m . s(-2) when the largest foreshock (M(w)7.3) occurred on 9 March, subsequently the amplitudes fluctuated and decreased to about (5 similar to 10) X 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.