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

Abstract

S U M M A R YStrain 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.

The geodetic laser interferometers at Gran Sasso, Italy: Recent modifications and correction for local effects

“…Strain distortions can be estimated under the assumption that the scale of these effects is much smaller than the scale of the measured stress and strain perturbations (e. g. Park et al 2008). For example, this assumption holds for observations of Earth tides.…”

“…Global phenomena include the study of the free oscillations of the Earth (the capability of strainmeters of observing fundamental toroidal modes is much higher than that of horizontal seismometers, e. g. Park et al, 2008) and strain tides (as regards the diurnal resonance due to the free core nutation, see e. g. Mukai et al, 2004).…”

Earth's free oscillations excited by the 2011 TohokuMw9.0 earthquake detected with a groundwater level array in mainland China

“…Ferreira et al (2006) suggested that long-base fluid tube tiltmeters could potentially contribute to obtaining high-quality measurements 1458 R. Yan et al of the long-period seismic surface waves and free oscillations. Park et al (2008) reported long-period toroidal free oscillations from the great Sumatra-Andaman earthquake observed by paired laser extensometers in Gran Sasso, Italy. Recently, Mitsui & Heki (2012) observed Earth's free oscillation excited by the 2011 Tohoku earthquake by a dense GPS array of more than 300 stations in Japan, and Zürn et al (2015) showed high-quality low frequency normal mode strain observations for the 2011 Tohoku earthquake.…”

Normal mode coupling observations with a rotation sensor

“…One rare example of improvement of toroidal motion observations was achieved on data recorded by paired laser extensometers in Gran Sasso, Italy, which were used for the evaluation of the composite centroid moment tensor (CMT) source model of the 2004 Sumatra-Andaman earthquake (Park et al 2008).…”

“…On the other hand, seismic waves are not exclusively recorded by seismometers. Different instruments, such as superconducting gravimeters (Camp 1999), laser extensometers (Park et al 2008), ring lasers (Igel et al 2011;Nader et al 2012) and fluid tiltmeters (Ferreira & D'Oreye 2006) for example, are also sensitive to long-period ground motions. Such instruments enable alternative observations of high-quality Earth's free oscillations, potentially providing complementary information to traditional seismometer data (Park et al 2005;Ferreira & D'Oreye 2006).…”