## 5 Conclusions  
We measured the long-term displacement in recent lava fields at Piton de la Fournaise volcano using TSX/TDX InSAR data between 2009 and 2014. The spatial pattern and temporal behavior of the displacement of each lava flow, and the relationship between displacement, lava thickness and age are investigated by a detailed statistical analysis. The subsidence behavior exhibits a significant difference between the lava fields outside and inside of the EFA (Eastern Flank Area). The former shows a clear linear relationship with lava thickness and it decays with lava age, which is attributed to the post-lava emplacement processes. The latter however displays an irregularity in the subsidence and the relationship between subsidence and thickness and age, which confirms that, in addition to the post-lava emplacement processes, other processes must have played a role in the observed subsidence on the Eastern Flank. Qualitative analysis show that the contribution of other processes to the flank downward motion could reach up to 78%. A detailed analysis of the displacement behavior in lava fields inside the EFA favors the interpretation that two pre-existing normal faults could have been activated during the March-April 2007 eruption and have resulted in a slip along a pre-existing structural discontinuity beneath the Eastern Flank, though the existence of the discontinuity is still an issue of debate. This paper presents for the first time a spatial and temporal analysis of ground displacement in multiple lava fields at Piton de la Fournaise, by characterizing the correlations between lava subsidence, lava thickness, and age. Although more studies are required to determine the dynamic process driving the instability of the Eastern Flank, the results from this study add new knowledge and essential insight for understanding and monitoring potentially hazardous flank instabilities.  
**Acknowledgments:** The authors greatly appreciate the financial support from the National Natural Science Foundations of China (No. 41730109, 41471356 and 41804013). We would like to acknowledge the support of the Natural Science Foundation of Jiangsu Province, China (No. BK20180666), and the Jiangsu dual creative talents and Jiangsu dual creative teams program projects of Jiangsu Province, China, awarded in 2017. We would like to thank the DLR for providing TSX/TDX images through the LAN 0237 project, and CNES (TOSCA) and INSU (SNOV) for supporting InSAR data acquisition at Piton de la Fournaise. The TSX/TDX images have been acquired within the framework of the French National Service OI2. Part of the work exposed in this paper was also supported by the ANR-16-CE04-0004-01 (SlideVOLC) project funded by the Agence Nationale de la Recherche. The GNSS data used in this paper were collected by Observatoire Volcanologique du Piton de la Fournaise/ Institut de Physique du Globe de Paris (OVPF/IFGP).  
**Author Contributions:** [PERSON] conceived and designed the experiments; [PERSON] performed the experiments and analyzed the results. [PERSON], [PERSON], [PERSON] contributed to the logic development of the scientific presentation, the structure, the interpretation of the results and the review of the manuscript. [PERSON], [PERSON], [PERSON], [PERSON] contributed to the review of the paper. [PERSON] provided the SAR data. [PERSON] provided the pre-processed GNSS data. [PERSON] and [PERSON] contributed to the English writing and polishing work. [PERSON] and [PERSON] sponsored the publishing charge. [PERSON] provided the DEM and the lava flow contours. [PERSON] wrote the paper.  
**Conflicts of Interest:** The authors declare no conflict of interest.