New constraints on source processes of volcanic tremor at Arenal Volcano, Costa Rica, using broadband seismic data, Geophys

Abstract. Broadband seismic data recorded 2.3 km from the active vent of Arenal Volcano, Costa Rica, provide new constraints on tremor source processes. Arenal's tremor contains as many as seven harmonics, whose frequencies vary temporally by up to 75 percent, from initial values of 1.9 Hz for the first peak immediately following explosive eruptions to 3.2-3.5 Hz several minutes later. We infer that gas bubble concentration is variable within the conduit and also changes as a function of time, thereby changing the acoustic velocity. We infer that the source is a shallow, 200-660 m-long, vertically oriented 1-D resonator with matched boundary conditions, radiating seismic energy from a dis-placement antinode. Polarization analyses show that particle motion azimuths abruptly rotate, which may be explained by a decrease of the incidence angle. We suggest that energy i