Broadband source inversion of Long-Period (LP) events on Turrialba volcano, Costa Rica

Abstract:

A typical moment-tensor (MT) solution of long-period (LP) volcanic signals comprises a tensile crack source mechanism and a pulsing or resonating source-time function (STF). However, due to the small magnitudes of LP events, only the most energetic part of the signals (as seen from velocity waveforms) is used in inversions. Ground displacement periods longer than 5 seconds - if they exist - are very difficult (if not impossible) to recover from such small signals due to the contamination of the signal by high-amplitude microseisms and the velocity signal base-line fluctuations (long-period instrumental noise). Consequently, the source-time functions (STF) obtained in LP inversions are band-limited representations of the true displacements in the source. Here we use a good-quality LP dataset recorded near the summit of Turrialba volcano. Aided by results from step table laboratory experiments we carefully recover the broadband ground motion displacement, containing frequencies much lower than 0.3 Hz observed on the velocity seismograms. The resulting MT solution that we recover is a classical earthquake “ramp” function, rather than a pulsing or oscillating waveform. We suggest that this new observation will contribute to a better understanding of LP seismicity.