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Moment tensor inversion of Explosive Long Period events recorded on Arenal Volcano, Costa Rica, constrained by synthetic tests

We report on investigations of the complexity and limitations related to the inversion of Explosive Long Period events using synthetic and real data from Arenal volcano, Costa Rica. Synthetic data are generated using a 3D model including the real topography of the volcano and a 1D layered velocity model. Solutions for (i) the moment tensor components, and (ii) the moment tensor plus single forces, are analysed. The performed tests show the ability of the inversion to retrieve the correct seismic source mechanism. However under certain conditions, such as noisy data and mislocated source locations, spurious single forces are generated in the solution for the moment tensor components plus single forces. These tests are performed in order to constrain our solutions for real data. An example of an inversion for an explosive event recorded on Arenal in 2005 is presented. The explosive mechanism is retrieved by the inversion procedure in both solutions. However, single forces appear in the solution for moment tensor components plus single forces. Currently the single forces are not fully constrained ; although they may be real, they are likely to be contaminated by spurious forces. Without new constraints, the correct retrieval of these single forces is still problematic. Therefore, with our present knowledge of volcanoes, the interpretation of single forces retrieved from an inversion must be viewed with caution. The ability to recover the explosive mechanism for the 2005 event combined with the interpretative aids from the synthetics tests has allowed us to invert for a higher than normal LP frequency band which will enable us to invert for the large variation of events observed on Arenal.

Arenal volcano; moment tensor inversion; source mechanism

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