Long‐period toroidal earth free oscillations from the great Sumatra–Andaman earthquake observed by paired laser extensometers in Gran Sasso, Italy

SUMMARY Strain data recorded by two crossed laser extensometers operating in the Gran Sasso underground observatory recorded seismic free oscillations excited by the 2004 December 26 Sumatra–Andaman earthquake. The main source of instrumental noise can be mitigated by differencing data from the crossed strainmeter arms, so that the resulting differential strain data set offers an unprecedented resolution of the seismic toroidal free oscillations with periods T > 1000 s. We reconstruct the time evolution of selected free‐oscillations for comparison with synthetic seismograms that include normal‐mode coupling effects from Coriolis force, attenuation and ellipticity. Envelopes estimated for the Gran Sasso differential data set for free oscillations with period T < 1000 s (frequencies f > 1 mHz) are approximated adequately by a composite Centroid–Moment–Tensor (CMT) source with five subevents and an aggregate M w = 9.3 moment‐magnitude. Envelopes for several toroidal free oscillations with T > 1000 s are predicted less well. The amplitude of the rarely observed mode 0 T 2 is overpredicted at Gran Sasso by roughly a factor of two, and other modes are underpredicted. The amplitude discrepancy for 0 T 2 is confirmed at selected exceptionally low‐noise seismic stations. Hypothetical explanations include a slow‐slip component of the seismic moment release, errors in the composite‐CMT source model, unmodelled coupling effects to Earth's secular modes and feedback from the Sumatra–Andaman tsunami on Indian Ocean coastlines. Of these hypotheses, either an extended‐duration strain release or tsunami feedback seem most plausible. The viability of the tsunami‐feedback mechansim deserves further investigation.