Long-Term Subsidence in Lava Fields at Piton de la Fournaise Volcano Measured by InSAR: New Insights for Interpretation of the Eastern Flank Motion

Abstract

Long-term deformation often occurs in lava fields at volcanoes after flow emplacements. The investigation and interpretation of deformation in lava fields is one of the key factors for the assessment of volcanic hazards. As a typical Hawaiian volcano, Piton de la Fournaise volcano’s (La Réunion Island, France) main eruptive production is lava. Characteristics of the lava flows at Piton de la Fournaise, including the geometric parameters, location, and elevation, have been investigated by previous studies. However, no analysis focusing on the long-term post-emplacement deformation in its lava fields at a large spatial extent has yet been performed. One of the previous studies revealed that the post-emplacement lava subsidence played a role in the observed Eastern Flank motion by conducting a preliminary investigation. In this paper, an InSAR time series analysis is performed to characterize the long-term deformation in lava fields emplaced between 1998 and 2007 at Piton de la Fournaise, and to conduct an in-depth investigation over the influence of post-emplacement lava subsidence processes on the instability of the Eastern Flank. Results reveal an important regional difference in the subsidence behavior between the lava fields inside and outside of the Eastern Flank Area (EFA), which confirms that, in addition to the post-lava emplacement processes, other processes must have played a role in the observed subsidence in the EFA. The contribution of other processes is estimated to be up to ~78%. The spatial variation of the observed displacement in the EFA suggests that a set of active structures (like normal faults) could control a slip along a pre-existing structural discontinuity beneath the volcano flank. This study provides essential insights for the interpretation of the Eastern Flank motion of Piton de la Fournaise.

The role of Interferometric Synthetic Aperture Radar in Detecting, Mapping, Monitoring, and Modelling the Volcanic Activity of Piton de la Fournaise, La Réunion: A Review

“…Due to the volcano's high level of activity and frequent eruptions, these processes often occur simultaneously or in rapid succession, so that related surface deformation phenomena can be difficult to discriminate. For example, the same location on the flank of the volcano might be affected by both seaward flank sliding, which is characteristically observed as subsidence and motion towards the unsupported flank [26,27], and lava flow compaction, a process that adds another subsidence component to the total surface deformation observed at this location [19]. In order to study deformation source processes, to quantify involved physics, and to understand associated volcanic hazards, however, it is important to be able to unequivocally attribute surface deformation components to their corresponding sources.…”

“…Additional knowledge on the timing and temporal evolution of events, the location and extent of the affected area, and the expected long-term deformation behavior can assist the deformation signal decomposition. For instance, the timing of lava flow emplacement depends on the onset of the corresponding eruption and its effusion rate, the area covered by the lava flow can (under certain circumstances) be retrieved from co-eruptive interferometric coherence or other sources (e.g., aerial photography or infrared observations), and the expected long-term displacement behavior could be described as exponentially decaying subsidence where the magnitude positively correlates with the thickness of the lava flow [19,[28][29][30].…”

“…For example, overlapping lava flows of successive eruptions can hamper lava flow characterization using InSAR. Chen et al [19] investigated the long-term post-emplacement deformation of lava flows that were emplaced between March 1998 and April 2007 along the eastern flank of Piton de la Fournaise volcano. The authors succeeded in telling apart the proportion of the displacement signal related to the thermal contraction and compaction of the newly emplaced lava flows and the depression of the underlying substratum from the proportion of the movement due to the slow subsidence and seaward motion of the volcano's mobile eastern flank.…”

“…Chen et al [59] found seaward motion of the volcano's eastern flank also during the 2009-2014 period (a portion of the data used by Chen et al [59] which are covering the mid-2011 to mid-2014 period are shown in Figure 5), which the authors interpreted as the continuity of the post-2007 displacement in the Grandes Pentes due to the fact that about the same area was affected by the detected motion, although in a more subtle manner ( Figures 5 and 9). The same authors then retrospectively analyzed long-term displacements of areas covered by the 1998-2007 lava flows and found an additional subsidence component to detected lava flow compaction that the authors attributed to flank sliding prior to the major 2007 eruption [19]. Now that flank motion has been proven a fact at Piton de la Fournaise volcano, its detailed mechanism and characterization remains challenging and the subject of scientific investigation and debate.…”

“…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].…”