# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 2 Martinique and Study Sites
### Study Sites: Presentation and Previous Works
#### 2.2.2 Morne-Figue Area  
The Morne-Figue area is located in the commune of La Trinite on the east coast of the island (Figure 3b). The study site covers an area of 0.36 km\({}^{2}\) and is limited to the north and west by the Morne-Figue (129 m a.s.l.) and the Morne-Congo (232 m a.s.l.) areas, respectively, which are carved in compact andesite, to the south by the Gue stream and to the east by Crosmy Bay. The site is hilly with elevations varying between 10 m and 100 m a.s.l. and both steep and gentle slopes varying between 5\({}^{\circ}\) and 45\({}^{\circ}\). The site, which is very anthropized (approximately thirty houses), is bordered to the west by national road RN1 and is crossed by a small road connecting the neighborhood to the center of the municipality (Figure 2c,d). Since 1977, the east-facing slope has been subject to landslides (rotational and shallow translational landslides). The main phenomena is located below RN1. This landslide, which is approximately 2.99 ha and was triggered in 1988 [64], is associated with rotational failure in the upper part and a translational component in the lower part ([64]; Figure 3d). This phenomenon has two very active periods: from 1987 to 1988 and in 2004. Since 2004, new geomorphological features such as cracks (i.e., traction and compression), small scarps and new tension cracks on the road were observed intermittently (Figure 3). One drilling campaign [64] and geophysical investigations allowed the involved materials and their thicknesses to be defined (Figure S2). Four types of formations from the \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**La Médaille Landslide**} & \multicolumn{2}{c}{**Morne-Figue Area**} \\ \cline{2-5}  & **Number (n)** & **Year** & **Number (n)** & **Year** \\ \hline \multirow{2}{*}{Boreholes} & \multirow{2}{*}{15} & 1966, 1968, 1974, 1981, & \multirow{2}{*}{9} & 1981, 1982 1989, 1997 \\  & & 1986, 1996 & & \\ \hline Geotechnical characterization & \multirow{2}{*}{2 campaigns} & 1981, 1983 & \multirow{2}{*}{1 campaign} & 1989 \\ \hline Geophysical investigations & \multirow{2}{*}{1 campaign (AEM)} & \multirow{2}{*}{2013} & 2 campaigns & \multirow{2}{*}{2007, 2013} \\  & & & (electrical and AEM) & \\ \hline Hydrogeological investigations & \multirow{2}{*}{2 periods} & Precipitation= since 1969 & \multirow{2}{*}{1 period} & Precipitation 1988–1989 \\  & & PiezoT = 2000 and 2002 & & Piezometer = 1988–1989 \\ \hline \multirow{2}{*}{Field investigations} & \multirow{2}{*}{9 campaigns} & 1966, 1968, 1974, 1981, 1986, & \multirow{2}{*}{7 campaigns} & 1981, 1982 1989, 1997, \\  & & 1996, 2002, 2014, 2016 & & 2007, 2016, 2017 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Investigations and monitoring of the two sites since the 1960s. topographic surface were defined: (i) an upper layer composed of clays or backfill with a thickness between 1 m and 3 m; (ii) a second layer with a thickness from 8 m to 10 m and represented by weathered very clayey materials; (iii) a third layer with a thickness between 10 m and 12 m composed of weathered basalts (Pre-vert basalts); and (iv) a weak horizon of basalt whose thickness is not known. A piezometer was implemented for the period of 1987-1988. During the humid period (from May to October), the GWL lies between 1.4 m and 0.5 m below the topography from upstream to downstream. In the dry season, the GWL is approximately 8 to 9 m below the topography. The piezometric levels and displacement measurements were implemented for a short period of one year, showing a correlation between rainfall and landslide activity [64]. Consequently, the landslide was more active during the humid period, with a maximum displacement of approximately 1.4 cm for the period of 1987-1988. In 2016, the geological knowledge of the surroundings was refined by field observations along cross-sections. Beyond the acquisition of new geological information, two units within the landslide were delineated with a very active unit upstream and a latent unit downstream.