Time variations of the stresses in the source region of the Tohoku earthquake of March 11, 2011 (M = 9) from tidal response data

Abstract

The idea of forecasting the earthquakes by continuous monitoring the time variations of tidal responses was suggested by E. Nishimura in 1950. However, the implementation of this idea has only become possible with the deployment of the GSN and F-net global seismic networks. Below, we present the new method for identifying the time variations of tidal response of the medium in seismically active regions using the data from these networks. We show that our approach significantly increases the sensitivity and time resolution of the standard methods of moving spectral analysis. The analysis of the data from the horizontal pendulums which record the northward tilts demonstrates the slow growth of the tidal tilts during six years before the earthquake, which is followed by the instantaneous drop of the amplitudes at the moment of the earthquake. During four years after this drop, the tidal amplitudes remained at a noticeably lower level than their average preearthquake values. These changes in the tidal amplitudes indicate the nonlinear response of the medium in the presence of large tectonic