Nat. Hazards Earth Syst. Sci., 16, 1925–1951, 2016 www.nat-hazards-earth-syst-sci.net/16/1925/2016/ doi:10.5194/nhess-16-1925-2016 © Author(s) 2016. CC Attribution 3.0 License. Lava flow hazard at Fogo Volcano, Cabo Verde, before and after the 2014–2015 eruption Nicole Richter1, Massimiliano Favalli2, Elske de Zeeuw-van Dalfsen1, Alessandro Fornaciai2,3, Rui Manuel da Silva Fernandes4, Nemesio M. Pérez5,6, Judith Levy1, Sónia Silva Victória7, and Thomas R. Walter1 1German Research Centre for Geosciences (GFZ), Potsdam, 14473, Germany 2Istituto Nazionale di Geofisica e Vulcanologia (INGV), Pisa, 56126, Italy 3Dipartimento di Fisica e Astronomia (DIFA), Alma Mater Studiorum – Università di Bologna, Bologna, 40127, Italy 4Instituto D. Luiz, University of Beira Interior, Covilhã, 6201-001, Portugal 5Instituto Volcanológico de Canarias (INVOLCAN), 38400 Puerto de la Cruz, Tenerife, Spain 6Instituto Tecnológico y de Energías Renovables (ITER), 38611 Granadilla de Abona, Tenerife, Spain 7Universidade de Cabo Verde, Praia, Cabo Verde Correspondence to: Nicole Richter ([email protected]) Received: 10 March 2016 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: 30 March 2016 Revised: 18 June 2016 – Accepted: 7 July 2016 – Published: 17 August 2016 Abstract. Lava flow simulations help to better understand of the lava flow model performance. Our results highlight the volcanic hazards and may assist emergency preparedness at fact that lava flow hazards change as a result of modifications active volcanoes. We demonstrate that at Fogo Volcano,Cabo of the local topography due to lava flow emplacement. This Verde, such simulations can explain the 2014–2015 lava flow implies the need for up-to-date topographic information in crisis and therefore provide a valuable base to better prepare order to assess lava flow hazards. We also emphasize that ar- for the next inevitable eruption. We conducted topographic eas that were once overrun by lava flows are not necessarily mapping in the field and a satellite-based remote sensing safer, even if local lava flow thicknesses exceed the average analysis. We produced the first topographic model of the lava flow thickness. Our observations will be important for 2014–2015 lava flow from combined terrestrial laser scanner the next eruption of Fogo Volcano and have implications for (TLS) and photogrammetric data. This high-resolution to- future lava flow crises and disaster response efforts at basaltic pographic information facilitates lava flow volume estimates volcanoes elsewhere in the world. of 43.7 ± 5.2 × 106 m3 from the vertical difference between pre- and posteruptive topographies. Both the pre-eruptive and updated digital elevation models (DEMs) serve as the funda- mental input data for lava flow simulations using the well- 1 Introduction established DOWNFLOW algorithm. Based on thousands of simulations, we assess the lava flow hazard before and after Effusive volcanic eruptions are associated with lava flows the 2014–2015 eruption. We find that, although the lava flow that may cause damage and long-lasting impact on infras- hazard has changed significantly, it remains high at the lo- tructure and economy. The comune San Sebastiano al Vesu- cations of two villages that were destroyed during this erup- vio in Italy was destroyed by Mount Vesuvius’ lava flows tion. This result is of particular importance as villagers have in 1944 for the third time in less than 100 years, yet was already started to rebuild the settlements. We also analysed rebuilt (Kilburn, 2015). In January 2002, lava flows advanc- satellite radar imagery acquired by the German TerraSAR- ing from Nyiragongo Volcano overran the city of Goma in X (TSX) satellite to map lava flow emplacement over time. the Democratic Republic of Congo, which was later rebuilt We obtain the lava flow boundaries every 6 to 11 days during on top of this lava flow (Chirico et al., 2009). Destructive the eruption, which assists the interpretation and evaluation effusive volcanic eruptions also occur frequently at places such as the island of Hawai’i(Kauahikaua and Tilling, 2014; Published by Copernicus Publications on behalf of the European Geosciences Union. 1926 N. Richter et al.: Lava flow hazard at Fogo Volcano, Cabo Verde Poland et al., 2016), or at Mount Etna, Sicily, Italy (Favalli relate to the achievable spatial resolution and coverage, as et al., 2009b). Yet, a common observation in many of these well as information quality and accuracy. The decision of classic examples is that, for various reasons, residents rebuild which method to use highly depends on the specific appli- their houses and return to live in hazardous areas. Studies cation and the user’s needs. Modern high-resolution satellite of effusive eruptions and the mechanisms of lava flow em- systems, such as Pléiades (optical) and TanDEM-X (radar), placement over time, as well as lava flow hazard assessment need to be tasked to acquire topographic data in response to a and the proposal of risk mitigation strategies, therefore, have volcanic crisis. Updates of pre-existing topographic informa- developed into fundamental branches of volcano sciences. tion can also be achieved using ground-based technologies, Recent crises, such as the 2014–2015 Pahoa lava flow cri- such as terrestrial laser scanning and camera- or drone-based sis at K¯ılauea Volcano, Hawai’i (Poland et al., 2016), and photogrammetry. These methods are often more flexible than the highly destructive 2014–2015 eruption of Fogo Volcano, satellite observations with respect to the acquisition time and Cabo Verde (González et al., 2015; Cappello et al., 2016; date. For instance, we produced a posteruptive DEM from Bagnardi et al., 2016), have again shown that up-to-date lava ground-based data in January 2015, while the next (and only flow hazard information is needed in inhabited volcanic envi- other) available posteruptive DEM data were acquired more ronments and that this information has to be effectively com- than 5 months after the end of the 2014–2015 eruption (on 20 municated to the officials in charge of public safety. June 2015) by the Pléiades satellite (Bagnardi et al., 2016). A variety of algorithms have been developed with the com- TanDEM-X bistatic data are not available for Fogo after mon aim of understanding the dynamics of lava flow em- the 2014–2015 eruption. Ground-based techniques are espe- placement, forecasting lava flow paths, and constructing lava cially effective for effusive volcanic eruptions, where only flow hazard maps (e.g. Favalli et al., 2005; Harris and Row- the directly affected areas need to be updated. The potential land, 2015; Del Negro et al., 2008). These algorithms have of very long-range terrestrial laser scanner (TLS) instruments been applied to numerous volcanoes, including but not lim- to survey the dynamics of active lava flow fields and to map ited to Nyiragongo Volcano, Mount Cameroon, and Mount the topographic changes associated with the emplacement Etna (Favalli et al., 2009a, 2011b; Tarquini and Favalli, of new flows was shown at Mount Etna, Italy (James et al., 2011). Modelling techniques follow either the probabilistic 2009). We produced the first posteruptive topographic map of or the deterministic approach. The MAGFLOW simulation the 2014–2015 Fogo lava flow using TLS and ground-based code (Del Negro et al., 2008) is a deterministic approach photogrammetric data in order to update a pre-eruptive pho- that relies on pre-existing knowledge or at least simplified togrammetric DEM of Fogo Island, both featuring a 5 m spa- assumptions about the physical and rheological character- tial resolution. We estimated lava flow characteristics, such istics of flowing lava (Cappello et al., 2015; Tarquini and as lava flow thickness and volume. We also generated and Favalli, 2015). The FLOWGO model by Harris and Row- compared pre- and posteruptive lava flow hazard maps. land (2015), also a deterministic model, allows for the sim- ulation of changing physical properties, e.g. changes in ve- locity or thermorheology of flowing basaltic lava following a 2 Geologic setting and eruptive history predefined channel downslope (Harris et al., 2015). Here we use the DOWNFLOW probabilistic code (Favalli et al., 2005) Fogo Island is one of the youngest volcanic islands of the to create lava flow hazard maps. Based on the law of gravita- Cabo Verde archipelago in the Atlantic Ocean and is built tion, DOWNFLOW follows the simple assumption that lava up from the remnants of one single giant volcano, known flows downhill from an eruption site. One main advantage of as the Monte Amarelo Volcano (Day et al., 1999). The east- this code over deterministic models is that only basic physics ern coastline reflects a catastrophic flank collapse event in applies, therefore no pre-existing knowledge or assumptions the island’s geologic history (Day et al., 1999; Ramalho et on physical properties of the lava flows are needed. The most al., 2015), which is thought to date back ∼ 73 ka (Ramalho important input for the DOWNFLOW simulation is an accu- et al., 2015). This event left a prominent east facing, ex- rate and up-to-date digital elevation model (DEM) and the tremely steep collapse structure, the Monte Amarelo escarp- location of the eruptive vent. ment or Bordeira. It reaches a height of up to 1000 m above High-resolution topographic information does not only the relatively flat, 9 km-wide, N–S elongated, caldera-like serve as an essential prerequisite for lava flow simulations, plain, called the Chã das Caldeiras (Chã). The Chã is located it is also one of the first requirements for any effusive erup- at an average elevation of ∼ 1700 m and covers an area of tion as it allows for lava flow thickness and volume estimates. ∼ 35 km2 (Fig. 1). To the east the Chã is bound by the high- Modern remote sensing techniques, such as photogramme- est point of the island, the Pico do Fogo stratocone (2829 m).