Explosion mechanisms at Arenal volcano, Costa Rica: An interpretation from integration of seismic and Doppler radar data

Abstract

We execute an integrated analysis of broadband seismic and Doppler radar data to gain insights into the subsurface mechanisms that drive repetitive, mildly explosive activity of Arenal volcano (Costa Rica). We find large variability of both seismic and radar waveforms, and nonsystematic relationships between the two. Seismic recordings display long-lasting tremor sequences and numerous explosion quakes. Radar measurements show that tephra emissions are poorly correlated, in both time and energy, to the seismic activity. Tephra emissions were found in association with explosion quakes but also during episodes of tremor and seismic quiescence. Moreover, the exit velocity, mass loading, and kinetic energy of the emissions show no clear relationship with the coeval seismic amplitude and frequency content. We propose a conceptual source model whereby degasing is controlled by opening and closing of fractures that crosscut a rigid cap atop the conduit. When the fracture's strength is overcome by the gas pressure building up below, it suddenly opens and high-velocity gas escapes, producing high-frequency elastic waves typical of explosion quakes. Gas release also occurs in relation to periodic opening and closure of the fractures to produce repetitive pressure pulses, this being the source of tremor. In both cases, varying quantities of fragmented material may be carried by the gas, which can be detected by the radar if their concentration is high enough. Moreover, the highly variable, constantly changing state of lava cap (e.g., thickness, fracture network and gas permeability) results in nonrepeatable source conditions and explains the complex relationship between tephra emissions and associated seismic signals.