Effects of volcano topography on seismic broad-band waveforms

Volcano seismology often deals with rather shallow seismic sources and seis mic stations deployed in their near field. The complex stratigraphy on volc anoes and near-field source effects have a strong impact on the seismic wav efield, complicating the interpretation techniques that are usually employe d in earthquake seismology. In addition, as most volcanoes have a pronounce d topography, the interference of the seismic wavefield with the stress-fre e surface results in severe waveform perturbations that affect seismic inte rpretation methods. In this study we deal predominantly with the surface ef fects, but take into account the impact of a typical volcano stratigraphy a s well as near-field source effects. We derive a correction term for plane seismic waves and a plane-free surface such that for smooth topographies th e effect of the free surface can be totally removed. Seismo-volcanic source s radiate energy in a broad frequency range with a correspondingly wide ran ge of different Fresnel zones. A 2-D boundary element method is employed to study how the size of the Fresnel zone is dependent on source depth, domin ant wavelength and topography in order to estimate the limits of the plane wave approximation. This approximation remains valid if the dominant wavele ngth does not exceed twice the source depth. Further aspects of this study concern particle motion analysis to locate point sources and the influence of the stratigraphy on particle motions. Furthermore, the deployment strate gy of seismic instruments on volcanoes, as well as the direct interpretatio n of the broad-band waveforms in terms of pressure fluctuations in the volc anic plumbing system, are discussed.