Properties of the near-field term and its effect on polarisation analysis and source locations of long-period (LP) and very-long-period (VLP) seismic events at volcanoes

Seismicity that originates within volcanic magmatic and hydrothermal plumbing systems is characterised by wavelengths that are often comparable to or longer than the source–receiver distance. The effect of such a near-field configuration must be explored when analysing these signals. Herein, we summarise properties of near-field observations for both a single force and moment-tensor seismic sources. We show radiation patterns of the near-, intermediate- and far-field terms for the source types that are most likely candidates for long- (LP) and very-long-period (VLP) volcanic seismicity, including: a single force, compensated linear vector dipole (CLVD), a tensile crack and a pipe-like source. We find that the deviation of the first motion polarisation from the radial direction is significant in all planes except one whose normal is parallel to the symmetry axis (if there is one) of the source mechanism. However, this deviation is less pronounced (or even negligible), when there is a considerable volumetric component in the source (as in the case of a tensile crack or pipe). Our location test shows that the accuracy of locations obtained using the semblance or cross-correlation techniques is very significantly affected by the near-field geometry. This effect is especially pronounced for shallow sources, such as often encountered on volcanoes, and decreases with increasing source depth. Hence, in practical applications on volcanoes, 3D full waveform numerical simulation (including topography and structural heterogeneities) should be used in order to both validate location techniques and as an interpretational aid to reduce misinterpretations of location results.