Long period or very long period seismic signals (LP-events or VLP-events respectively) may be important indicators of fluid or magma movement within a volcano. Their analysis is vital for the monitoring of an active volcanic region towards the estimation of the eruption hazard or volcanic unrest.  We have built a tool to include such an analysis in Exupéry.

Suiting the requirements of an early warning system, the method has to be operating fast, reliably, and automatically. Based on experiences with fast analysis applications for tectonic events, which have already proven to be functional, we have set up an algorithm, which is optimised with regard to the work on local volcanic seismic data.

In the case an lp-event is identified by the detection and classification package,  an algebraic inversion combined with a grid search  over possible source points is carried out, yielding a full centroid moment tensor solution with source location and time.

For the inversion part  one needs a set of Green’s functions (GFs), which take into account both the local or regional seismic velocity model and information about the topography. The GFs are calculated in advance and are stored within the database, allowing a very fast data processing.

Theoretical background

In the beginning of the setup, a geometrical grid is set up to cover all possible points of seismic sources. Additionally the locations of the stations are known; so a set of Green's functions can be calculated.

Fig.: Example for a geometrical setup (Mt. Erebus)

In the case of a detected LP-event/VLP-event, for every single possible source point the seismic data traces are bandpass filtered and inverted for a source mechanism.  The residuals with respect to a forward modelling are calculated and the grid point with the lowest residuum is taken as the source point.  

IMAGE/Formula: Theory

Focussed on LP-events/VLP-events, the wavelength of the seismic signal, analysed here, are in such a range that several approximations are justified.

Fig.: Example data set for a typical (V)LP event at Mt. Erebus

Not only in space but also in time, the source mechanism may be assumed to be singular. So the time evolution of the source brakes down to a delta distribution and the result is a centroid moment tensor solution.

Literature

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