Fig. 1. Inset: Location of Etna Volcano is marked by a star. The study area: main structures are indicated. MA: Mascalucia fault, TI: Timpe fault system, BO: Valle del Bove, PE: Pernicana–Provenzana fault, Slf: San Leonardello fault, Rip: Ripa della Naca faults, Pi: Piedimonte fault, Ma: Mascalucia–Tremesieri faults, Tr: Tremestieri fault, Arf: Aci Reale fault, Ti: Timpe faults, Saf: San Alfio faults.

Fig. 2. NDVI-processed ASTER images. The first three pictures (a to c) were (pixel size = 15 m) acquired before the 2002 November eruption, vegetation growth intensity (white pixels) outlines a linear pattern trending E–NE activity on Piano Provenzana (Etna). This trend is similar to the trend of the fracture that produced the March 2003 lava flow. The mean amplitude changes of the NDVI values can be explained by slight variations in photosynthetic activity of the plants over the years. The velocity of the Pernicana fault is 26 mm/yr but its pattern is not clearly defined all along its path. No scarp was visible in the topography before the event.

Fig. 3. Location map of Nyiragongo Volcano (Democratic Republic of Congo) in the western branch of the East African Rift system showing lava flows and fractures formed during the 2002 flank-fissure eruption. The two areas of study are shown by two rectangles related to Fig. 6 and Fig. 7. Modified after [4].

Fig. 4. Detailed map of the central portion of the lava flows and fractures of the Nyiragongo 2002 eruption. Notice the graben structure and the en-echelon geometry of the two-fracture system. They formed as the result of shallow depth dyke wedge emplacement following a tectonically controlled decrease of the minimum horizontal stress below the volcano which triggered sudden drainage of the lava lake and the magma-filled conduit. The boxes correspond to the portion of the new fracture system which showed as a non-fractured linear area (Fig. 5 and Fig. 6) with an elevated NDVI signal on IKONOS satellite imagery typical of enhanced plant growth. We postulate that incremental dyke wedge emplacement lead to a combination of increased heat flow, mild CO₂ degassing and vapor condensation through ground fissures and porous lava flow surfaces and thus boosted photosynthetic processes in overlying vegetation.

Fig. 5. Main new fracture formed during the Nyiragongo January 2002 eruption as viewed from the west. This fracture that cuts through banana fields and villages formed as magma from the summit lava lake drained suddenly triggering the propagation of a dyke wedge within the reactivated southern volcanic rift zone over 16 km and towards the city of Goma (population 400 000) on the right side of the image. The fracture emitted pahoehoe lava flows (smooth grey surface) and scoriaceous aa lava flows (rough dark surface) that reached and partly destroyed the city of Goma causing widespread devastation and the spontaneous exodus of up to 300 000 people [4].

Fig. 6. NDVI signal along the flanks of Nyiragongo 7 km from the summit. The NDVI process (pixel size = 4 m) shows a clear correlation between the fracture-controlled linear patterns of high NDVI values before the eruption (b) and the location of reactivated and new lava-emissive fractures formed during the Nyiragongo 2002 eruption (c). The IR band analysis (a) reveals that the signal is crossing an area (arrow) with growing banana plants. Thus, the signal is not due to a difference of growth of two different vegetal species. The fractures located in this area were observed after the 2002 eruption and identified as newly formed in areas where there was no topographic scarp [4]. Thus the observed signal cannot be linked to a topography effect. The NDVI was processed using IKONOS pictures (Space Imaging Inc.).

Fig. 7. NDVI signal along the lower flanks of Nyiragongo 11 km from the summit. This sequence of IKONOS NDVI-processed images (pixel size = 4 m) is mapping the vegetation activity before (a) on June 23 2001 and after (b) the 2002 Nyiragongo flank-fissure eruption on March 8 2002. It shows a clear correlation between the linear patterns of high NDVI values 7 months before the eruption (2001) and the location of the eruptive fractures developed during the 2002 eruption. Black arrows indicate the presence of well-marked linear pattern in NDVI values 1 yr before the eruption. The dashed line represents the active fracture of the 2002 eruption and its associated lava flows. This data remarkably shows that pre-existing fractures were reactivated in 2002 to feed several lava flows. Non-ortho-rectified images georeferenced in WGS84 geodetic system and projected into UTM35S (in km).

Data set used in this study