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.

This study uses monocular digital photography and a measurement robot (MDP&MR) to monitor the sluice vibration deformation in the gate lift test and in the sluicing test. The photographing scale transformation and time baseline parallax (PST-TBP) method is proposed in the field of monocular digital photography. Results show that the PST-TBP method improves measurement accuracy slightly. Measurement accuracies on the reference plane were 0.62 pixels (0.40 mm), 0.76 pixels (0.49 mm), and 1.10 pixels (0.71 mm) in the X, Z, and comprehensive-directions, respectively. The PST-TBP method overcomes the limitation that the photographing direction must be perpendicular to the sluice plane when monocular digital photography is used to monitor the sluice. MDP&MR proves an effective method in monitoring the sluice vibration deformation as MDP can be used to monitor the trend of the sluice vibration deformation and the MR allows high-accuracy monitoring of the short periodic deformation of the sluice. They have complementary advantages to achieve better results. Thus, MDP&MR can provide a new way to assess sluice health in vibration.