The spatial amplitude distribution of volcanic tremor at Stromboli volcano (Italy)

A portable seismic station consisting of a three-component seismometer in conjunction with a spectral analyzer was deployed in May and June 1994 to record volcanic tremor in a wide area on Stromboli. For the reduction of path effects, tremor spectra were averaged over 164 observation points. They illustrate smooth and broadband spectral lobes in the frequency range between 1-12 Hz. It is suggestive that these overall spectra represent in a first approximation the spectral radiation caused by source processes. Identical and significant maxima at 2.65 Hz and 3.65 Hz are found in all components. No systematic distinction is found in the amplitude values and spectral forms for either horizontal component. The amplitude of the vertical component presents approximately one third that of the horizontal components. A subclassification of the data according to geological strata shows frequency dependent amplitude amplifications. Thick ash and lapilli beds reach a factor of four within frequencies from 4-6 Hz. The influence of these site effects seems minor below 2 Hz. The paper presents maps for each of the three components showing the distribution of the tremor amplitudes averaged over areas of 150 m by 150 m. Model curves derived from fluid-flow acoustics are compared with the tremor spectra