Wavelet filter analysis of splitting and coupling of seismic normal modes below 1.5 mHz with superconducting gravimeter records after the December 26,2004 Sumatra earthquake

摘要

New generation superconducting gravimeters (SGs),which have been demonstrated to be better than the best seismometers STS-1 at frequencies below 1 mHz,can be accepted as the quietest vertical seismometers for observation of long-period earth free oscillations.Wavelet filtering with narrow band-pass frequency response as shown in this paper is very helpful in removing atmospheric pressure effects from on gravity records in long-period seismic mode frequency bands.The processing of high quality SG records after the great Sumatra earthquake (Dec.26,2004) with wavelet filtering leads to clear observations of all coupled toroidal modes below 1.5 mHz except these for 0T5,0T7 and 1T1; moreover 1T2 and 1T3 are,for the first time,unambiguously revealed in the vertical components of the free oscillations.The three well-resolved splitting singlets of overtones 2S1 are observed from a single SG record for the first time.