High-quality lowest-frequency normal mode strain observations at the Black Forest Observatory (SW-Germany) and comparison with horizontal broad-band seismometer data and synthetics
Résumé
We present spectra concentrating on the lowest-frequency normal modes of the Earth obtained from records of the invar-wire strainmeters and STS-1 broad-band seismometers located in the Black Forest Observatory, Germany after the disastrous earthquakes off the NW coast of Sumatra in 2004 and off the coast near Tohoku, Japan in 2011. We compare the spectra to ones obtained from synthetic seismograms computed using a mode summation technique for an anelastic, elliptical, rotating, spherically symmetric Earth model. The synthetics include strain– strain-coupling effects by using coupling coefficients obtained from comparisons between Earth tide signals recorded by the strainmeters and synthetic tidal records. We show that for the low-frequency toroidal and spheroidal modes up to 1 mHz, the strainmeters produce better signal-to-noise ratios than the broad-band horizontal seismometers. Overall, the comparison with the synthetics is satisfactory but not as good as for vertical accelerations. In particular, we demonstrate the high quality of the strainmeter data by showing the Coriolis splitting of toroidal modes for the first time in individual records, the first clear observation of the singlet 2 S 0 1 and the detection of the fundamental radial mode 0 S 0 with good signal-to-noise ratio and with a strain amplitude of 10 −11. We also identify the latter mode in a record of the Isabella strainmeter after the great Chilean quake in 1960, the detection of which was missed by the original studies.
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