Characterizing and better understanding the subsurface processes in terms of their spatio-temporal dynamics therefore requires natural study sites that are (a) highly active and (b) well-monitored over long time periods by a dense network of instrumentation. Frequently erupting and closely monitored volcanic systems serve as natural laboratories for studying subsurface magmatic transport and storage systems, identifying and better understanding long-term and short-term eruption precursors, and constraining related volcanic hazards. With one eruption every eight months on average over a \(\sim\)50 year period [1; 2], and operational monitoring employed since late 1979, Piton de la Fournaise, La Reunion, is undoubtedly one of the most active and best-studied basaltic hot-spot ocean island volcanoes on Earth [3]. Compared to other volcanic settings, Piton de la Fournaise seems a fairly intelligible volcanic system. The volcano formed on the eastern flank of its larger neighbor Piton des Neiges (3069 m), a piggyback set-up that gives direction of gravitational spreading of the Piton de la Fournaise volcanic edifice towards the unsupported eastern flank [4]. Measurable surface deformation and/or (sometimes catastrophic) topographic changes in the volcano's past provide evidence of internal structural adjustments that occur as a result of the edifice's growth. The most characteristic topographic feature of Piton de la Fournaise volcano is a prominent east-facing, u-shaped, and steep Enclos Grand Brule collapse structure (extending \(\sim\)8 km N-S and \(\sim\)13 km W-E, cf. Figure 1), the upper part of which is referred to as the Enclos Fouque caldera, while the steep upper slopes are known as the Grand Pentes, and the lower slopes as the Grand Brule (Figure 1). While the true origin of the collapsed structure remains controversial, the Enclos Fouque seems to at least partially result from vertical collapse(s), while the Grandes Pentes and the Grand Brule have been affected by lateral flank failure that occurred about 4500 years ago [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. The Enclos Fouque caldera hosts the currently active volcanic center, a 400-m-high central cone with its two deep summit craters, Bory and Dolomieu. The volcano's interior has been imaged down to a depth of about 10 km by means of geophysical data (Figure 1), and it can be roughly described as a pile of poorly consolidated eruption deposits that host a shallow plumbing system consisting of several intermittently connected magma reservoirs, the shallowest of which is located at \(\sim\)2-2.5 km depth below the summit craters [1; 16]. This is the central magmatic storage from which most of the laterally propagating dikes feeding recent eruptions are initiating or passing through. Three major rift zones are radiating North-East (NE), South-East (SE), and in 120\({}^{\circ}\) from the central cone, and function as preferred zones of magmatic intrusions (Figures 1 and 2) [3]. Seismicity related to volcanic activity at Piton de la Fournaise is generally small in magnitude (below magnitude 3, cf. Figure 1) [17]. Underneath the eastern flank of the volcano, seismic events are sparse compared to the number of volcano tectonic (VT) events recorded below the central cone, and they occur in a few seismic clusters (Figure 1). Several tectonic structures have been identified from seismicity and deformation [13; 18; 19], but not much structural evidence of compression exists along the volcano's spreading flank [3]. The volcano has erupted frequently throughout the past 20 years. Following a rest period of more than five years, the eruptive activity of Piton de la Fournaise resumed on 9 March 1998 [20]. From then until 2007 continuous refilling of the plumbing system had been observed, and short-term eruptive cycles pre-dominated the volcano's eruptive pattern and culminated in more important distal eruptions.