High‐resolution analysis of the gravest seismic normal modes after the 2004 M_w = 9 Sumatra earthquake using superconducting gravimeter data

Abstract

[1] The M w > 9 Sumatra-Andaman earthquake on December 26, 2004 has strongly excited the lowfrequency seismic modes and is a unique opportunity to improve the frequency and damping measurements of the gravest seismic modes. The precise estimation of the frequency splitting of the seismic modes below 1 mHz is needed to improve 1D-density models of the Earth. From this event, 2 S 1 is observed for the first time without any stacking on both SG (Superconducting Gravimeter) and seismic records. We report here analyses of SG data obtained from 11 sites. The spectra of SG records clearly show the splitting of 0 S 2 into 5 completely resolved singlets and the splitting of 0 S 3 into 7 singlets at individual stations. The present results demonstrate that SGs provide high quality data for a precise analysis of the low-frequency seismic modes.

Geographical variations of the 0S0 normal mode amplitude: predictions and observations after the Sumatra-Andaman earthquake

“…2(a), example at Canberra SG site), and its amplitude decay could be observed up to the second Sumatra event in March 2005 (compare Fig. 2(b), example at Canberra SG site) at most SG sites that have a noise level of a few 10 −2 nm/s 2 at 0 S 0 frequency (Rosat et al, 2005). The datasets from 13 GGP sites-Canberra (CB, Australia); Sutherland (SU, South Africa); Ny-Alesund (NY, Norway); Medicina (MC, Italy); Membach (MB, Belgium); Strasbourg (ST, France); Vienna (VI, Austria); Syowa (SY, Antarctica); Wettzell (WE), Bad-Homburg (BH) and Moxa (M1) (all in Germany); Kamioka (KA) and Matsushiro (MA) (both in Japan)-have been corrected for the local tides (solid tides and oceanic loading) and atmospheric pressure effect using a nominal admittance factor of −3 nm/s 2 /hPa.…”

“…The 34-day record length is close to the 0.5-Q cycle for 0 S 0 which is the optimal data length to measure the normal mode amplitude (Dahlen, 1982). The analysis method is similar to that used in Rosat et al (2005). At each SG site, the frequency of 0 S 0 has been estimated by fitting a Lorentzian function (Dahlen and Tromp, 1998) to the Hanning tapered FFT spectrum.…”

Steady-frequency waves at intradiurnal periods from simultaneous co-located microbarometer and seismometer measurements: a case study

“…As noted in the above discussion about the phase shift ϕ, the penetration of waves from the solid Earth into the atmosphere is possible. It is known that the Earth's free oscillations are considerably enhanced after large earthquakes (e.g., Rosat et al, 2005). However, we have not revealed any correlation between earthquake occurrence and detection of oscillations falling within and near the frequency ranges of the 0 S 2 , 2 S 1 , and 0 S 3 multiplets.…”

“…In the following, we consider possible origins of seismic waves at steady frequencies in the ∼0.5-2 h period range (∼150-550 µHz). The Earth's free spheroidal oscillations of 0 S 2 , 2 S 1 , and 0 S 3 fall in the high-frequency part of this period region (e.g., Rosat et al, 2005). The Earth's free oscillations are shown to occur not only after large earthquakes, but also on seismically quiet days (e.g., Lin'kov et al, 1991;Davydov and Dolgikh, 1997;Nawa et al, 1998).…”

Review of the source characteristics of the Great Sumatra–Andaman Islands earthquake of 2004

“…The shaking registered clearly on seismometers worldwide (Park et al 2005a, b). The earthquake strongly excited low degree free oscillations of the earth, so that the globe rang like a bell for several days afterward (Park et al 2005a, b;Rosat et al 2005). Static deformation, as determined by the Global Position System (GPS), exceeded 0.1 m for hundreds of kilometers around the epicenter (Catherine et al 2005;Khan and Gudmundsson 2005).…”