Local Elastic Effects in Low-Frequency Spectra of Earth's Free Oscillations

 Observed Earth tide tilts and strains are known to be strongly affected by local distortions of the large scale deformation field by local heterogeneities (cavities, topography,

 and geology). It was predicted that these distortions will also occur in other phenomena, like seismic free oscillations and/or long period surface waves.

 The megaquakes near Sumatra 2004 and Japan 2011 excited the normal modes to unprecendented levels since 1964 and high-quality spectra could be obtained. Two

 observations at the Black Forest Observatory provided more or less direct evidence for the effects of local distorsions which will be described.

 Firstly, spectra from the seismograms of the Sumatra quake from three different accelerometers with two components each showed significantly different spectral line shapes

 for several low-frequency modes, especially in the NS-components. Secondly, taking a local (probably topographic) effect into account in synthetic seismograms improved

 their fit in spectra of the observed strains for a wire strainmeter significantly for both quakes. This local effect was incorporated into the synthetics using coupling

 coefficients derived from comparison of Earth tide records of the same instrument with synthetic tides.

 These local effects could well be one of the reasons why synthetic seismograms for current heterogeneous Earth models provide excellent fits for vertical but not for

 horizontal records.