Seismotectonic Changes Before the M = 9.1 Sumatra Earthquake of December 26, 2004

Records of vertical surface displacement velocities in the vicinity of broadband seismic stations located around the epicenter of the 9.1 magnitude Sumatra earthquake on December 26, 2004 are analyzed. During five years from 1996 to 2000, the COCO station, nearest to the epicenter and located at a distance of 1700 km from it, has recorded a steady daily behavior of seismic noise. In 2001, step-like distortions of the level of the recorded seismic noise appeared at this station. These distortions continued up to the time of the earthquake. The station also detected pulses above the diurnal behavior, with a gradual increase and subsequent decrease in the amplitude of the oscillations which lasted a few minutes. The pulses occurred under quiet weather conditions and geomagnetic activity. No such pulses were observed at stations more than 2000 km from the epicenter. It is hypothesized that before the earthquake, there had been a slip on the geological faults in the lithosphere of the Indian Ocean.