Array analysis of the seismic wavefield of long-period events and volcanic tremor at Arenal volcano, Costa Rica

International audienceWe use wavefield decomposition methods (time-domain crosscorrelationand frequency-domain MUSIC) to analyze seismic data recordedby a dense, small-aperture array located 2 km West of Arenal volcano, CostaRica, and operated during 2.5 days. The recorded wavefield is dominated byharmonic tremor and includes also spasmodic tremor and long-period (LP)events. We find that the initial stages of LP events are characterized by threedifferent wave arrivals. These arrivals propagate with similar back-azimuthspointing to the volcano summit (~80◦N) and increasing apparent slownessesof 0.4, 1.1, and 1.7 s/km. Spasmodic tremors can not be regarded as coherentsignals. On the contrary, harmonic tremors are highly coherent, characterizedby the stability of the apparent slowness vector estimates. Apparentslownesses lay in the range 1-2 s/km. Back-azimuths point in the general directionof the volcano, but with a large variability (40-120◦N). Nevertheless,there are long-term variations and evidences of multiple simultaneous componentsin the harmonic tremor wavefield. These observations suggest thatLP events and tremor are generated in a shallow source area near the volcanosummit, although they do not share exactly the same source region orsource processes. The tremor source is located in the shallowest part of theplumbing system, beneath the lava crust. This dynamic region is subject tocomplex fluctuations of the physical conditions. Degassing events at differentlocations of this region might generate variable seismic radiation patterns. The effects of topography and heterogeneous shallow structure of the volcanomay amplify these variations and produce the wide directional span observedfor volcanic tremor. On the other hand, the LP source seems to bemore repeatable. LP events are likely triggered by fragmentation of the fluidflow in a slightly deeper portion of the volcanic conduits