ON THE FREQUENCY CONTENTS OF LOCAL EVENTS - SOURCE OR PATH EFFECT

Local events which occurred in the Timanfaya volcanic field are analys ed and their spectral contents are interpreted as being due to the eff ect of a finely layered medium with large impedance contrast, the pres ence of which is well known from well-logging data. By considering the layered medium as a filter, its spectral response (amplitude and phas e) is computed from different randomly generated values of its paramet ers (total thickness, mean thickness of the individual layers and impe dance contrast), showing that this stratigraphic filter acts as a low- pass filter. This effect can be correctly modelled by means of an expo nential correlation function. However, in some cases resonant peaks ap pear in the spectrum super-imposed on the low-pass filter effect, whic h can only be attributed to resonant scattering. An analysis of the ph ase and group delay associated with the amplitudes is made and strongl y suggests that it is necessary to incorporate the theory of resonant scattering for the retrieval of medium properties. From the point of v iew of the amplitudes, the superposition of both effects, low-pass fil ter and resonant peaks, allows us to reproduce the gross features that are usually attributed to source effects. The need for a careful anal ysis of the local site response previous to any interpretation of the spectral features in terms of source characteristics is thus emphasize d.