Indonesian Journal on Geoscience Vol. 2 No. 2 August 2015: 91-99 INDONESIAN JOURNAL ON GEOSCIENCE Geological Agency Ministry of Energy and Mineral Resources Journal homepage: h�p://ijog.bgl.esdm.go.id ISSN 2355-9314 (Print), e-ISSN 2355-9306 (Online) Geothermal System as the Cause of the 1979 Landslide Tsunami in Lembata Island, Indonesia Yudhicara1,2, Phillipson Bani1,3, and Alwin Darmawan1 1Centre for Volcanology and Geological Hazard Mitigation Geological Agency, Jln. Diponegoro No. 57 Bandung, West Java, Indonesia 2Padjadjaran University, Jln. Dipati Ukur No. 35 Bandung, West Java, Indonesia 3Laboratory of Magmas and Volcanoes, Univ. Blaise Pascal-CNRS-IRD-OPGC, 63000, Clermont-Ferrand, France Corresponding author: [email protected] Manuscript received: January 17, 2015; revised: January 18, 2015; approved: June 23, 2015; available online: July 27, 2015 Abstract - A tsunami landslide which caused hundreds casualties and lots of damage took place on Lembata Island in 1979. In order to understand the characteristics of the landslide mechanism, a field survey was conducted in 2013 which sampled both the origin soil and landslide material, and the water from hotspring around the landslide site. The physi- cal properties of the soil obtained show that the original soil has dominantly coarser grain than the landslide material (80.5% coarser grain compared to 11.8% coarse grain respectively) which indicates that the soil has become finer and softer. Hot spring analysis indicated that the mineral content of the water was 99.48% SO4. This shows that magmatism processes are involved which caused the soil to become acidic and may have fragilised the system. Results of X-ray Diffraction Mineralogy Analysis (XRD) show that the original soil is composed of minerals of cristobalite, quartz, and albite, while the landslide material consists of clay minerals such as quartz, saponite, chabazite, silicon oxide, and coesite which are typical minerals in a hydrothermal environment. Based on these results, it can be concluded that the area was influenced by an active geothermal system that could be the main source mechanism behind this disastrous event. Keywords: Lembata Island, landslide, sample analysis, clay minerals, geothermal system How to cite this article: Yudhicara, Bani, P., and Darmawan, A., 2015. Geothermal System as the Cause of the 1979 Landslide Tsunami in Lembata Island, Indonesia. Indonesian Journal on Geoscience, 2 (2) p.91-99. DOI:10.17014/ijog.2.2.91-99 Introduction reported killed by both landslide and tsunami. This event is not well known by most Indonesian Background people, even though newspapers “KOMPAS” of On July 18, 1979,IJOG Lembata Island, located 21 July 1979 and “Herald Tribune” 24 July 1979 in East Nusa Tenggara Province, Indonesia, was reported this event (Lassa, 2009; Arif, 2010). struck by a sudden tsunami, induced by a massive Lembata is an 80 x 30 km island that hosts landslide. Fifty million cubic meters of material numerous massive volcanic edifices, three of was displaced and one third of it tumbled into the which remain active: Ili Lewotolo in the north sea and generated a 7 - 9 m height tsunami which and Ili Labalekan and Ili Werung in the south spread along the Waiteba Bay (Hadian et al., (Figure 1). Most of these edifices extend to the 1979; Lassa, 2009). Four villages were buried by sea, exposing steep slopes and unconsolidated the landslide material and 539 of inhabitant were material. Adding regular heavy rain falls, and the IJOG/JGI (Jurnal Geologi Indonesia) - Acredited by LIPI No. 547/AU2/P2MI-LIPI/06/2013, valid 21 June 2013 - 21 June 2016 91 Indonesian Journal on Geoscience, Vol. 2 No. 2 August 2015: 91-99 123o 15'E 123o 30'E 123o 45'E FLORES SEA o o 8 15'S Ili Lewotolo 8 15'S Ili Boleng Tobiwutun Adonara Island Waicuga Bay Ili Praba Lewoleba Bay Lewoleba Belalarang Peninsula Tsunami devastated zone Waiteba Bay Lembata Island o o SAWU SEA 8 30'S 8 30'S Ili Mingar N Ili Labalekan Ili Werung Labala Bay Hobal Submarine Volcano Atadei Peninsula 0 5 10 km Study Area 123o 15'E 123o 30'E 123o 45'E 95o E 141o E o o 09 S 09 S 0 1.000 km o o 15 S 15 S 95o E 141o E Figure 1. Top: Lembata Island showing active volcanoes (red triangle), study area (red square) and tsunami affected area. Bottom: Location of Lembata Island within Indonesia. intense tectonic activity in that region, Lembata of the landslide materials (soil density, specific Island thus possesses many factors capable in gravity, cohesion, shear friction, and the nature triggering land displacement with a significant of expanded soil) and other factor influencing potential for landslide-tsunami. The 1979 event the system. being the unfortunate example. The landslide site is located at 08o28’44”S - IJOGo To gain further insights into the origin of the 123 33’34.3”E, a hill ca. 200 m above sea level, disastrous 1979 event, a field investigation was while the landslide material reach the sea of carried out in April 2013, observing landslide 08o28’30.8”S - 123o33’24.7”E (Figure 1). material and the surrounding edifice morphology, According to the reports published in 1979 evaluating potential links to volcanic activities and compiled in Lassa (2009), the Volcanologi- and determining tectonic structures in the study cal team from Bandung reported that there was area. Soil and water samples were taken for labo- no volcano activity during the two days preced- ratory analysis to constrain the physical properties ing the landslide (16 - 17 July 1979), while 92 Geothermal System as the Cause of the 1979 Landslide Tsunami in Lembata Island, Indonesia (Yudhicara et al.) meteorological and geophysical teams noted no Ili Boleng, Ili Lewotolo, Ili Werung, Ili Topaki/ earthquakes during that time either. According to Sirung, Batutara, and Komba (Figure 2). Lamanepa (2013), in the month of July 1979 the There are two major geological structures in Lembata Island and its surroundings were in a Lembata Island, which have trends of southwest- rainy season and had heavy rainfall in some areas northeast and northwest-southeast. One of them is of East Nusa Tenggara. The study is intended to a normal faulting in a northwest-southeast direction understand what caused this landslide given that from the Plio-Pleistocene age which has produced lack of an obvious triggering mechanism. a scattered hotsprings near to the landslide site that may influence the soil condition (Figure 2). Scientific Review There is also a lineament showing a volcanic Geologically, the region where the landslide chain which has moved to the southwestward, and site is located consists of a Tertiary lithological a submarine volcano named Ili Hobal at the end unit called Kiro Formation, lying beneath the of faulting which has been observed since 1972. Quaternary old volcanic rocks and Quaternary A dense distribution of epicentres around young volcanic products (Koesoemadinata and Lembata Island indicates that the area is seismi- Noya, 1990). Old volcanic rock units consist of cally very active. One earthquake which occurred lava, breccia, agglomerate, volcanic sandy tuff, in 1977 (USGS, 2013) and located nearby the and pumiceous sandy tuff as products of volca- landslide site resulted in a surface rupture. This noes which are no longer active, such as Wikiri- may have fragilised soil at the site, although the wak, Ili Lewung, Ili Minggar, Ili Ujolewung, earthquake magnitude was less than five (Figure 3). Pura, Ternate, and Treweg. These volcanoes According to the eruptive history around the form high mountains along the southern Lembata Lembata Island, the last eruption occurred on Ili Island. There are also locally younger volcanic Hobal started in 1972 and ended in 1974. The Ili rocks, consisting of lava, agglomerates, bombs, Hobal is a volcano located at the southern end sands, and volcanic ashes which are products below the sea (CVGHM, 2013 in http://www. of younger active volcanoes, such as Watuomi, volcano.si.edu/). 123o 12'E 123o 56'E o o 8 9'S N 8 9'S FLORES SEA ILI LEWOTOLO Tmpw Qtvw Ql D Tmk Ili Watulolo U Tmn Tmn Ql Ql Tobiwutun Qhvl Tmk Qv Tmn ADORA ISLAND Ili Praba Qal Lewoleba Bay Qal Alluvium Waicuga Bay Ql Qhv Young volcanic products Tmk Lewoleba Tmn Qct Terraces Tmk Qal L EMBA T AI S L AND Belarang Ql Coralline limestone Peninsula Qtv Old volcanic products IJOGTmpw Tmpw Waihekang Formation D Tml Tmk Kiro Formation U Waiteba Bay Tmn Nangapanda Formation Ili Mingar Faults Qtvb Qct SAWU SEA Qtvm Qtvm Ql Qtve Labala Bay Ili Labalekang Ili Werung 0 5 10 km Atadei Peninsula o o 8 36'S 8 36'S 123o 12'E 123o 56'E Figure 2. Geological Map of Lembata Island and location of hotsprings (vents) (Koesoemadinata and Nova, 1990). 93 Indonesian Journal on Geoscience, Vol. 2 No. 2 August 2015: 91-99 123o 15'E 123o 30'E 123o 45'E Legend: Magnitude 0 - 4.0 F L O R E S S E A 4.1 - 5.0 >5.0 ILI WATULOLO o 8 15'S o 8 15'S ILI BOLENG Tobiwutun Year 1961 - 1970 1971 - 1980 1981 - 1990 ADONARA ISLAND Waicuga Bay ILI PRABA Lewoleba Bay 1991 - 2000 2001 - 2010 LEWOLEBA >2010 Belalarang Peninsula LEMBATA ISLAND Rusa Island D U Waiteba Bay ILI MINGAR 1977 SA W U S EA o o N 8 30'S 8 30'S ILI WERUNG ILI LABALEKANG Labala Bay 0 5 10 km Atadei Peninsula Location of Landslide 123o 15'E 123o 30'E 123o 45'E Figure 3. Seismicity in Lembata Island and its surroundings (USGS, 2013). Red arrow indicates the epicentre of the earth- quake that occurred in 1977 close to the landslide site.