Observations of Long-Period Free Oscillations From the 2004 Great Sumatra Earthquake With two Laser Extensometers at Gran Sasso, Italy

Free oscillations excited by the 26 December 2004 Sumatra-Andaman earthquake have been clearly recorded by two laser extensometers operating in the Gran Sasso underground observatory (Central Italy, average 1400 m rock coverage). The two orthogonal baselines are 90-m long. Nominal sensitivity is about 3 × 10-12 and recording rate is 5 Hz. S/N ratio of the shear component of strain, obtained after subtraction of the two strain records, is higher than that of each separate extensometer, since the shear component is poorly affected by changes in environmental parameters (temperature and pressure inside the underground observatory) and unaffected by laser frequency fluctuations. We are improving S/N of each extensometer through correction for changes in environmental parameters to obtain more information from recorded data, but for the moment we have performed a preliminary analysis of the shear component of strain only. Particular attention has been paid to the time history of the clearest oscillations, using several different spectral analysis techniques. Envelopes of sub-millihertz oscillations evidence a delayed ramp-up in modal amplitude, reaching the peak amplitude 30-35 hours after the earthquake and then showing a rapid decrease. Shorter-period modes are not affected by this behavior. We are modelling the splitting of some of the free oscillations to compare the effect of frequency splitting on the envelops of the free oscillations and looking for similar features in seismic records.