Insights into the magmatic system from seismic signal at Mayon Volcano, Philippines

• Main Author: Yeo, Shu Hui
• Other Authors: Benoit Taisne
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2019
• Subjects: DRNTU::Science::Geology::Volcanoes and earthquakes
• Online Access: http://hdl.handle.net/10356/77273
• Institution: Nanyang Technological University

Located on the Bicol peninsula in South-East Luzon of the Philippines, the Mayon Volcano is a symmetrical basaltic-andesitic stratovolcano, characterized by an openly-degassing mode. Being one of the most active volcanoes in the Philippines, it is critical to establish a better understanding of its magmatic system for further volcanic hazard assessment and risk management. One of the prominent tools that can be used to routinely observe changes in real-time is volcano seismology. Volcano seismology, where seismic signals can be linked to the direct and indirect physical processes, offers a glimpse into the complex internal dynamics of the volcanic system. However, as Mayon Volcano is an open system, generally continuously degassing, inhibiting pressure from building up, it can be difficult to observe precursory deformation before an eruption. Hence, in our study, we will explore the potential of volcano seismology in providing tell-tale signs of the internal dynamics. We will first look at separating the different types of volcanogenic seismic signals, then associate the dominant signals with the volcanic activities and identify possible source mechanisms. This way, we infer a seismic signature corresponding to each phase of the eruption, and offer insights into the processes within the edifice. Results from a frequency analysis show the dominance of long-period signals (1 to 5 Hz) during the second phase of intense, sporadic lava fountaining, as well as later phases of continuous and weak fountaining. These phases are characterised by harmonic tremors and even exhibit cyclicity of so-called banded tremor. It is worthwhile to note that the harmonic tremor in the seismic signals during phases of continuous and weak lava fountaining are accompanied by harmonic tremor in the infrasound signal. However, it is not the case during the phase of intense sporadic fountaining, possibly pointing to a slightly different source mechanisms triggering the harmonic tremor, which we explore further in our study. As the spectral lines of the harmonic tremor in the later phase wavered, we attributed that to stick-slip motion of rising magma affecting its ascent velocity. The cyclicity of banded tremor is postulated to be due to changes in the elastic response of the conduit walls, storing magma and building up pressure before releasing the magma in a pulse of extrusion. A family of low-frequency events persisted throughout the eruption, pointing to repeated excitation of a fluid-filled conduit at the same source location by a non-destructive mechanism. Through particle motion analysis, the source location for the family of low-frequency events is estimated to be at a shallow depth of 1 to 1.5 km beneath the epicenter. Two very-long-period events after the growth of a new lava dome further suggest a pulsating stationary crack above a shallow magma storage region being excited repeatedly. This illustrates the potential of volcano seismology as a forecasting tool to enhance the hazard assessment and risk management at Mayon.