SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Ivan Lokmer Early Stage Researcher Host Institution: UCD Dublin Place of Origin: Zagreb Appointment Time: January 2004 Project: Long-period Seismic Activity at Mt. Etna: Wavefield Analysis and Moment-tensor Inversion Task Groups: TG Small Scale Cooperation: University of Hamburg, OV-INGV Naples, INGV Catania Ivan Lokmer Early Stage Researcher Host Institution: UCD Dublin Place of Origin: Zagreb Appointment Time: January 2004 Project: Long-period Seismic Activity at Mt. Etna: Wavefield Analysis and Moment-tensor Inversion Task Groups: TG Small Scale Cooperation: University of Hamburg, OV-INGV Naples, INGV Catania Seismic wave Propagation and Imaging in Complex media: a European network

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Project Scope Better understanding of LP events at Mt. Etna A large number of LP events (~ 56,000) recorded at the Mt. Etna from November September 2004 enables us to get better insight into processes within the volcano by performing detailed wavefield analysis and a robust moment-tensor inversion. What is the source mechanism of LP activity?

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Location of events Similarity of events enabled us to stack them and so increase signal-to-noise ratio Events were located using non-linear probabilistic approach (Lomax) and 3D velocity model of Patane et al. (2002) VLP component points to the LP source volume

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland An example of recorded events – similarity of waveforms The same spectral peak observed at all sites Non-destructive repetitive source process Resonance as the most likely source process

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Source damping factor – SOMPI method Source Q can tell us about the physical properties of the fluid at the source SOMPI: AR model of a linear dynamic system  signal as a sum of exponentially decaying/growing harmonics Effective method for spectral analysis of volcanic LP signals

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Temporal change of frequency and Q-factor Two most energetic components considered for each event Small variations in both frequency and Q Almost equally-spaced spectral bands  constraint for a possible source geometry

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Moment-tensor inversion Volcanic processes may require three additional single-force components for their total description

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Scattering of the wavefield on Etna topography

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland 3D Elastic-Lattice Method (ELM) Particles connected by springs Discrete approach suitable for parallel programming Free surface is implicit and does not require a correction Tests for topography, absorbing boundaries and reciprocity performed

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Test for absorbing boundaries Simple AB – exponentially decaying wavefield Good agreement with analytical solution Near-field effect observed  important for volcanic studies Z - component Z-force

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Green’s functions computation 27 Green’s functions required for each station  (3 single forces x 3 components) + (6 MT components x 3 components)  minimum 3 simulations for each source position Search for the best MT solutionover the extended source volume  500m x 500m x 2000m (3000 sources)  Reciprocity required

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Reciprocity Aki and Richards (1980): Source x 1, F j  Receiver x 2, u i  = Receiver x 1, u j  Source x 2, F i  /            + … + =    3 times number of receivers computations needed  considerably reduced computational cost

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Test for reciprocity Excellent agreement Major problem for reciprocity is the free-surface correction  not present in the ELM method

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Green’s functions Vertical force at the source Significant amount of transversal motion generated at some stations  importance of topography inclusion to the simulations related to volcanoes

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Summary and Outlook Green’s functions are calculated using topography and heterogeneous velocity model Non-destructive LP source at Etna Moment tensor inversion is going to be performed in the frequency domain (the code will previously be tested against the code from the Hamburg group) This will lead to a better understanding of source processes

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Comparison between ELM and staggered FD scheme Good fit between seismograms obtained by both methods Minimum 25 points per minimum wavelength required

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland Topography effect – snapshot of the wavefield

SPICE Research and Training Workshop III, July 22-28, Kinsale, Ireland