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The 2021 deep volcano-tectonic swarms of Pinatubo volcano: A case of magmatic intrusion in a deep plumbing system

Lois Abigail Jumawan , Winchelle Ian Sevilla, Christian Joseph Clarito, Kenneth Jhon Remo, Rey Lumbang, Jun Bantigue, Joel Marilla, Francis Joseph Ticsay, April Angelique Dominguiano, Ma. Antonia Bornas

Abstract

Deep volcanic earthquake swarms can be caused by various geotectonic processes, ranging from external tectonic activity to inner volcano dynamics. Seismic swarms of volcano-tectonic (VT) earthquakes occurred in Pinatubo Volcano, Philippines in 2021, three decades after its cataclysmic eruption in 1991. These earthquakes originated at depths far below the assumed shallow magma reservoir. The precise location and analysis of these earthquakes play a critical role in volcano monitoring and determining the processes involved in generating these earthquakes. In this study, a minimum 1D velocity model for Pinatubo Volcano was estimated using the 2021 local VT swarms and simultaneous inversion approach, which served as the basis for precise location using double-difference method. The resulting earthquake distributions were spherical or cylindrical beneath the volcano's edifice. Cross-correlation analysis indicated that most events were non-repeating, with short-lived families of repeating events. Focal mechanisms obtained using first-motion polarity and full waveform inversion indicated primarily normal faulting with strike-slip components, although there were differences in the fault plane orientations. These results suggest that the 2021 VT swarms of Pinatubo were driven by volcanic processes originating from a deep-seated intrusion or fluid migration beneath the volcanic edifice around 35km to 40km deep. While this period of unrest did not culminate in an eruption, the resulting data will be crucial for developing a conceptual model of the magmatic system of Pinatubo. These findings also have broad implications for more accurate hazard assessments and eruption forecasts, as well as providing an analog for other volcanoes exhibiting similar earthquake characteristics.