Search

International audience

International audience; Recent research suggests that major meteorite impact events into a Late Hesperian/Early Amazonian ocean likely produced a mega‐tsunami that would have resurfaced coastal areas in northwestern Arabia Terra. The orientations of the associated lobate deposits, a conspicuous type of landforms called Thumbprint Terrain, suggests that if an impact event triggered the mega‐tsunami, the most likely location of the source crater is within the northern plains regions situated north of Arabia Terra. This study focuses on the identification of impact craters that impacted into the ocean and are likely to have produced the tsunami. We selected 10 complex impact craters, based on their diameters, location, and geomorphic characteristics. Of those, the Late Hesperian ~120‐km‐diameter Lomonosov crater exhibits a unique topographic plan view asymmetry (compared to other similar‐sized and similar‐aged craters in the northern plains such as Micoud, Korolev, and Milankovic). We attribute its broad and shallow rim, in part, to an impact into a shallow ocean as well as its subsequent erosion from the collapsing transient water cavity. The likely marine formation of the Lomonosov crater, and the apparent agreement in its age with that of the Thumbprint Terrain unit (~3 Ga), strongly suggests that it was the source crater of the tsunami. These results have implications for the stability of a late northern ocean on Mars.

One of the most powerful eruptions of the Holocene, known as the Samalas eruption, had a VEI of 7 and took place on Lombok, eastern Indonesia in 1257 CE. Thick tephra fall covered the entire island, and pyroclastic density currents (PDCs) up to 50 m thick buried almost half of Lombok. Until now, there has been no detailed study of the geomorphological evolution of Lombok related to this eruption, especially for the eastern part of Lombok. The main goal of this paper is therefore to reconstruct the paleo-topography on the eastern part of Lombok before the eruption of the Samalas volcano in 1257 CE, and to analyse the subsequent landscape evolution following this eruption. Data were collected from more than 1300 points in order to gather geomorphological, geological, and geochemical information for the eastern part of Lombok over the last millennium. Data were obtained from wells (which are widespread in the studied area), natural outcrops (mainly cliffs along the shoreline and the rivers), and human-made outcrops in quarries, as well as from the two-dimensional resistivity profiling (Dipole-dipole array) carried out on eastern Lombok in 2016 and 2017. We estimate that 4435 ± 5.5 × 106 m3 of pumice-rich PDC, over a surface of 171 km2, buried the eastern part of Lombok in the aftermath of the 1257 CE eruption, and as a result of this rapid pumice-rich PDC deposition there has been some aggradation of the shoreline at some points, resulting in most of the pre-1257 coral reef being buried. After several intense rainfall events, the deposited volcanic material was probably eroded during the first decades following the 1257 CE eruption by mountain streams, producing lahars. We estimate that the remaining volume of pumice-rich PDC deposits following this erosion is 625 ± 5.5 × 106 m3 over a surface of 89 km2, which is equivalent to 14% of the initial volume.

Barrier islands are indicators of coastal resilience. Previous studies have proven that barrier islands are surprisingly resilient to extreme storm events. At present, little is known about barrier systems&rsquo, resilience to seismic events triggering tsunamis, co-seismic subsidence, and liquefaction. The objective of this study is, therefore, to investigate the morphological resilience of the barrier islands in responding to those secondary effects of seismic activity of the Sumatra&ndash, Andaman subduction zone and the Great Sumatran Fault system. Spatial analysis in Geographical Information Systems (GIS) was utilized to detect shoreline changes from the multi-source datasets of centennial time scale, including old topographic maps and satellite images from 1898 until 2017. Additionally, the earthquake and tsunami records and established conceptual models of storm effects to barrier systems, are corroborated to support possible forcing factors analysis. Two selected coastal sections possess different geomorphic settings are investigated: (1) Lambadeuk, the coast overlying the Sumatran Fault system, (2) Kuala Gigieng, located in between two segments of the Sumatran Fault System. Seven consecutive pairs of comparable old topographic maps and satellite images reveal remarkable morphological changes in the form of breaching, landward migrating, sinking, and complete disappearing in different periods of observation. While semi-protected embayed Lambadeuk is not resilient to repeated co-seismic land subsidence, the wave-dominated Kuala Gigieng coast is not resilient to the combination of tsunami and liquefaction events. The mega-tsunami triggered by the 2004 earthquake led to irreversible changes in the barrier islands on both coasts.

Tidore and Hiri Islands in North Maluku Province is the result of a complex tectonic setting. In contrast with Ternate Island and its well-known volcano, Gamalama volcano, there is still a lack of research about volcanic information or volcanic landform in Tidore and Hiri Islands. Even though the two islands also have volcanoes, i.e., Hiri and Tidore/Kiematubu volcano. This study aims to provide geomorphological information, especially in Tidore and Hiri Islands, since this information is hard to find whereas it is very important to disaster mitigation and landuse planning. We used remote sensing images, digital elevation models (DEM), and geological maps to classify geomorphological information of the small island of Tidore and Hiri in the North Maluku based on geomorphological aspects, i.e., morphology, morphogenesis, morpho-chronology, and morpho-arrangement. As a volcanic island, the slope in Tidore and Hiri Island is dominated by a slightly steep (8°-16°) and a steep slope (16°-35°), while the relief is dominated by hills and mountainous. Volcanic landforms on Tidore and Hiri Islands are characterized by relatively symmetrical cone-shaped volcanic cones, which are accumulations of falling pyroclastic material and lava ejected from magma vents. Volcanic islands include the case of Tidore and Hiri Island have radial centrifugal river flow patterns. The material on Tidore and Hiri Islands is dominated by Holocene volcanic rocks. In Tidore, there is alluvial material on the west and east coasts of the island. This study result also can be developed into more detailed geomorphological maps, or landscape evolution in a volcanic island, or spatial planning.

Kiematubu volcano is often considered as non-volcanic, even though it consists of basalt material, since it has never erupted before. In fact, that small volcanic islands have a high risk due to their restriction on means and resources. The study aims to assess the social vulnerability of the community in Tidore Island that may be exposed to the eruption of the Kiematubu volcano. There has not been previous research in Tidore Island regarding volcanic vulnerability yet. The social vulnerability is an initial assessment of disaster management, which will affect in optimizing community’s capacity then minimizing the disaster impacts. Social parameter of demographic condition, health facilities, and education facilities was weighted to assess social vulnerability. The result shows that the social vulnerability class of Tidore Island is dominantly low, approximately 80%, the rest is middle and high, with a percentage of 13% and 7%, respectively. Mostly, the low vulnerable villages are due to less population density. However, the highly vulnerable villages, Gamtufkange and Indonesiana, consist of very high and high population density. The southeast part of Tidore Island, where both villages are situated, is the center of human activities, such as governmental, trades, and education.

International audience; This paper discusses the relations between the impacts of volcanic eruptions at multiple-scales and the related-issues of disaster-risk reduction (DRR). The review is structured around local and global impacts of volcanic eruptions, which have not been widely discussed in the literature, in terms of DRR issues. We classify the impacts at local scale on four different geographical features: impacts on the drainage system, on the structural morphology, on the water bodies, and the impact on societies and the environment. It has been demonstrated that information on local impacts can be integrated into four phases of the DRR, i.e., monitoring, mapping, emergency, and recovery. In contrast, information on the global impacts (e.g., global disruption on climate and air traffic) only fits the first DRR phase. We have emphasized the fact that global impacts are almost forgotten in the DRR programs. For this review, we have extracted case studies from Indonesia, and compared them to those of other regions, because Indonesia is home to >130 volcanoes and experienced several latest volcanic eruptions with VEI > 5.

European coastal databases contain information on the evolution of European shorelines in the 1990s and the 1980s. We investigate if a shift toward erosion has been observed between these two periods, as it could be expected as a consequence of contemporary sea-level rise or changing coastal management practices. We select comparable European coastal sites, consider their state transitions as the parameters of a discrete-time Markov chain, and analyze their steady states in order to reveal underlying changes in shoreline evolution trends. The results suggest that European coastal wetlands and small beaches have initiated a shift toward erosion, which attenuates previous optimistic statements. Our results should be interpreted with caution due to the limited number of observations and presumed errors in the database. However, they suggest that the impact of contemporary sea-level rise along European coastlines in the 1990s may be more important than previously thought. Our results suggest that more research is needed to quantify the morphodynamics of muddy coasts and to develop data models able to represent coastal morphodynamic changes adequately.

Semeru Volcano is an active stratovolcano located in East Java (Indonesia), where historic lava flows, occasional pyroclastic flows and vulcanian explosions (on average every 5 min to 15 min) generate a stock of material that is remobilized by lahars, mostly occurring during the rainy season between October and March. Every year, several lahars flow down the Curah Lengkong Valley on the South-east flank of the volcano, where numerous lahar studies have been conducted. In the present contribution, the objective was to study the spatial distribution of boulder-size clasts and try to understand how this distribution relates to the valley morphology and to the dynamic and deposition dynamic of lahars. To achieve this objective, the method relies on a combination of (1) aerial photogrammetry-derived geospatial data on boulders' distribution, (2) ground penetrating radar data collected along a 2 km series of transects and (3) a CFD model of flow to analyse the results from the deposits. Results show that

Java Island, the most populated island of Indonesia, is prone to landslide disasters. Their occurrence and impact have increased mainly as the result of natural factors, aggravated by human imprint. This paper is intended to analyse: (1) the spatio-temporal variation of landslides in Java during short term and long-term periods, and (2) their causative factors such as rainfall, topography, geology, earthquakes, and land-use. The evaluation spatially and temporally of historical landslides and consequences were based on the landslide database covering the period of 1981 – 2007 in the GIS environment. Database showed that landslides distributed unevenly between West Java (67 %), Central Java (29 %) and East Java (4 %). Slope failures were most abundant on the very intensively weathered zone of old volcanic materials on slope angles of 30O – 40O. Rainfall threshold analysis showed that shallow landslides and deep-seated landslides were triggered by rainfall events of 300 – 600 mm and > 600 mm respectively of antecedent rainfall during 30 consecutive days, and many cases showed that the landslides were not always initiated by intense rainfall during the landslide day. Human interference plays an important role in landslide occurrence through land conversion from natural forest to dryland agriculture which was the host of most of landslides in Java. These results and methods can be used as valuable information on the spatio-temporal characteristics of landslides in Java and their relationship with causative factors, thereby providing a sound basis for landslide investigation in more detail.

The debris avalanche deposit (DAD) coverage can drastically modify the surrounding landscape of volcanoes. DAD can be distinguished by hummocky irregular surface, jigsaw fissure, and mixing horse-shoe shape. Due to its particular shape of a hummock, the topography can be easily identified using DEM (Digital Elevation Model) and satellite imagery. The aim of this study is to characterize hummocky terrain in Lombok Island, which is located in the Kalibabak formation on the Geological map through the geomorphic approach. Hummocky terrain in this study are analyzed using DEM data from DEMNAS (DEM Nasional: 0.27-arcsecond resolution). Our study of this DAD encompasses seven variables, namely H/L ratio, numbers hummocks, distance to source, slope, area (size), relative height, topographic section, and hummock-spreading shape. A minimum of 756 hills derived from this DEM are considered as hummocks from this DAD, which extends ~18 km (NS) and ~25 km (WE) in the central part of Lombok Island. With an area of ~200 km2 and a volume estimated around 8.8 km3, the Kalibabak DAD is more than three times larger than the one of the Mount St-Helens in 1980. The morphology of hummocky terrain is bounded by a sudden change of slope, which is indicated by a river confluence. Average hummock size is 2.7 ha and average distance between each hummock is 150 m to 300 m. The hummocks are characterized by steep slopes (25 % to 45 %) at the boundary to colluvium plain (debris deposit), which makes them easy to identify using DEM. Hummocks spreading distribution forms a conical-like shape with a H/L ratio of 0.13. The spreading distribution shape, the H/L and V/L ratios are useful as an input for reconstructing the mechanism of debris avalanche emplacement.

As one of the most powerful volcanic events of the Holocene era with a Volcanic Explosivity Index of seven, the 1257 CE eruption of Samalas volcano in the Rinjani Volcanic Complex had large consequences for Lombok Island (Indonesia) and its surrounding area. This huge eruption with an enormous volume of ejected material (4435 ± 5.5 × 106 m3), was recorded in local written sources, called Babad, that were compiled around the sixteenth century and contain oral stories and myths. In this study, we present two written sources, namely Babad Lombok and Babad Suwung, which undoubtedly refer to the 1257 CE ultraplinian eruption of Samalas volcano. In addition to detailed analysis of the volcanic deposits, interviews with old Javanese linguists and museum employees were carried out to clarify the meanings of some words as well as the genesis of the Babads. The results from field observation and interviews were analyzed and compared with previous research. Babad Lombok describes the process of the Samalas eruption, the formation of the Segara Anak caldera, and the impacts of the voluminous ash fall and pyroclastic flows on Lombok Island. Meanwhile, Babad Suwung describes the impacts of the 1257 CE volcanic eruption on Talkuwang (Taliwang), on the western coast of Sumbawa Island. This description encompasses several volcanic processes including fallout and pyroclastic surges. The exegesis of Babad Lombok and Babad Suwung could be the oldest observations of a caldera-forming eruption in Southeast Asia. They also document the oldest pyroclastic surges in the world after those of Vesuvius as described by Pliny the Younger in 79 CE.

Indonesia is exposed to earthquakes, volcanic activities, and associated tsunamis. This is particularly the case for Lombok and Sumbawa Islands in West Nusa Tenggara, where evidence of tsunamis is frequently observed in its coastal sedimentary record. If the 1815 CE Tambora eruption on Sumbawa Island generated a tsunami with well-identified traces on the surrounding islands, little is known about the consequences of the 1257 CE tremendous eruption of Samalas on the neighboring islands, and especially about the possible tsunamis generated in reason of a paucity of research on coastal sedimentary records in this area. However, on Lombok Island, the eruption of the Samalas volcano produced significant volumes of pyroclastic flows that entered the sea in the North and East of the island. These phenomena must have produced a tsunami that left their traces, especially on Sumbawa Island, whose western coastline is only 14 km away from Lombok&rsquo, s eastern shore. Therefore, the main goal of this study is to investigate, find evidence, and determine the age of marine-origin sediments along the shore of the Alas Strait, Indonesia. We collected and analyzed samples of coral and seashells from marine deposits identified along the west coast of Sumbawa, i.e., in Belang Island and abandoned fishponds in Kiantar Village, in order to identify the sources and the occurrence period of these deposits events. Based on the radiocarbon dating of coral and seashell samples, we concluded that none of the identified marine deposits along the western coast of Sumbawa could be related chronologically to the 1257 CE eruption of Samalas. However, possible tsunami deposits located in Belang Island and abandoned fishponds in Kiantar Village yielded 4th century CE, 9th century CE, and 17th century CE. We also conclude that past large earthquakes triggered these tsunamis since no known volcanic eruption occurred near the Alas Strait at that time that may have triggered a tsunami.

Tsunami, as a secondary hazard, can be triggered by tectonic earthquakes with its epicenter in the ocean floor, submarine volcanic eruptions, submarine landslides, meteor, or bomb at the ocean. Although volcanic eruption-induced tsunami events rarely happen, however, some events ever had a major impact, e.g., the 1741 CE eruption of Oshima-Oshima Volcano in Japan, the 1792 CE of dome collapse of the Mayuyama volcano in Japan, and the 1883 CE of Krakatoa tsunami in Indonesia. This review represents a brief database of the volcanic eruption-induced tsunami in Indonesia during the Holocene era. The largest historical tsunami due to volcanic activities happened in the 1883 CE in South Lampung. Pyroclastic flows from Krakatoa eruption reached the sea and triggered a tsunami with a run-up of 41.00 m, resulting in 36,000 death tolls. With this review, the purpose is to improve our knowledge of tsunami hazard and risk in Indonesia, especially those caused by volcanic activities, since Indonesia is known as a volcano-rich country, with more than 130 active volcanoes from a total of 400 volcanoes.

Following the 2010 VEI 4 eruption of Merapi volcano, more than 250 lahars were triggered during two rainy seasons from October 2010 to March 2012. This high number of post-eruption lahars mainly occurred in the Kali (valley) Putih watershed and was mostly associated with high-magnitude rainstorms. A lahar occurring on January 8, 2011, caused significant damage to homes in several communities, bridges, sabo dams, and agricultural crops. The aims of this contribution are to document the impacts of lahars on the Kali Putih watershed and specifically (1) to analyze the lahar frequency during the period of 1969–2012 on an inter-annual and intra-annual basis and to determine the link between the volume of tephra and the frequency of lahars; (2) to detail the lahar trajectory and channel evolution following the January 8th lahar; (3) to map the spatial distribution of the thickness and geomorphic effects of the lahar deposit; and (4) to determine the impacts of the lahar on the infrastructure (sabo dams and roads) and settlements in the distal area of the volcano. The Kali Putih watershed has experienced 62 lahars, which represent 22% of all lahars triggered on 17 rivers at Merapi between 2010 and 2012. The main geomorphic impacts are: (1) excessive sedimentation in valleys, settlements and agricultural areas; (2) undercutting of the river banks by as much as 50 m, accompanied by channel widening; and (3) abrupt changes in the river channel direction in the distal area (15–20 km downstream of the volcano). About 19 sabo dams were damaged, and 3 were totally destroyed. Over 307 houses were damaged, and the National Road Yogyakarta–Semarang was regularly cut (18 times during approximately 25 days). Although the sabo dams on Kali Putih were originally constructed to protect distal areas from lahar damage, they had little effect on the 2010–2012 rain-triggered lahars. The underlying design of those dams along this river is one of the main reasons for the major destruction in this sector of the volcano’s lower slope. The catch basin capacity of the sabo dam was only 1.75 × 106 m3, whereas the total volume of the 2010–2011 lahars exceeded 5 × 106 m3. In order to prepare for future lahars, the government has invested in significant mitigation measures, ranging from structural approaches (e.g., building new sabo dams and developing an early warning system) to non-structural approaches (e.g., contingency and preparedness planning and hazard education).

A detailed study had been conducted on the sediment of Mount Samalas’ volcanic eruption in 1257 AD. Using the framework of the reconstruction of the ancient eruption of Mount Samalas, the first step was to map and analyze the deposits of volcanic sediment. Secondly, we analyzed the effect of geomorphology and the distance function to the isopach thickness. The results show that a combination of methods allowed to provide a high resolution map of the distribution of the thickness of the volcanic deposits, both on the slope and in alluvial areas. Geo-electric survey results (both Vertical Electric Sound (VES) and 2D mapping) show consistent changes in the pattern of contrast resistivity layer interface, for all areas. The pattern changes in a row of the top layer, the high resistivity turned into the low. Furthermore, the second and third layer interface changes from low to the high resistivity. High resistivity on the top layer is interpreted as a layer of unconsolidated volcanic sediment. High resistivity values are range from 736 to 2000 Ohm.m on the top layer in the area of the slopes while in the area of alluvial, the resistivity values range from 20 to 958 Ohm.m. Generally, the volcanic deposits in the area of the slopes have a higher value of isopach (>17 m) than in areas of alluvial (

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE; International audience; The eruption of Samalas in Indonesia in 1257 ranks among the largest sulfur-rich eruptions of the Common Era with sulfur deposition in ice cores reaching twice the volume of the Tambora eruption in 1815. Sedimentological analyses of deposits confirm the exceptional size of the event, which had both an eruption magnitude and a volcanic explosivity index of 7. During the Samalas eruption, more than 40 km 3 of dense magma was expelled and the eruption column is estimated to have reached altitudes of 43 km. However, the climatic response to the Samalas event is debated since climate model simulations generally predict a stronger and more prolonged surface air cooling of Northern Hemisphere summers than inferred from tree-ring-based temperature reconstructions. Here, we draw on historical archives, ice-core data and tree-ring records to reconstruct the spatial and temporal climate response to the Samalas eruption. We find that 1258 and 1259 experienced some of the coldest Northern Hemisphere summers of the past millennium. However, cooling across the Northern Hemisphere was spatially heterogeneous. Western Europe, Siberia and Japan experienced strong cooling, coinciding with warmer-than-average conditions over Alaska and northern Canada. We suggest that in North America, volcanic radiative forcing was modulated by a positive phase of the El Niño-Southern Oscillation. Contemporary records attest to severe famines in England and Japan, but these began prior to the eruption. We conclude that the Samalas eruption aggravated existing crises, but did not trigger the famines.

International audience; Polar ice core records attest to a colossal volcanic eruption that took place ca. A. D. 1257 or 1258, most probably in the tropics. Estimates based on sulfate deposition in these records suggest that it yielded the largest volcanic sulfur release to the stratosphere of the past 7,000 y. Tree rings, medieval chronicles, and computational models corroborate the expected worldwide atmospheric and climatic effects of this eruption. However, until now there has been no convincing candidate for the mid-13th century "mystery eruption." Drawing upon compelling evidence from stratigraphic and geomorphic data, physical volcanology, radiocarbon dating, tephra geochemistry, and chronicles, we argue the source of this long-sought eruption is the Samalas volcano, adjacent to Mount Rinjani on Lombok Island, Indonesia. At least 40 km(3) (dense-rock equivalent) of tephra were deposited and the eruption column reached an altitude of up to 43 km. Three principal pumice fallout deposits mantle the region and thick pyroclastic flow deposits are found at the coast, 25 km from source. With an estimated magnitude of 7, this event ranks among the largest Holocene explosive eruptions. Radiocarbon dates on charcoal are consistent with a mid-13th century eruption. In addition, glass geochemistry of the associated pumice deposits matches that of shards found in both Arctic and Antarctic ice cores, providing compelling evidence to link the prominent A. D. 1258/1259 ice core sulfate spike to Samalas. We further constrain the timing of the mystery eruption based on tephra dispersal and historical records, suggesting it occurred between May and October A. D. 1257.

This study examines the fluvial readjustment of a Javanese river impacted by the major eruption of the Merapi volcano (Indonesia) in October and November 2010. The basin of the Opak River, located on the southern flank of the Merapi, was subject to substantial sediment input related to massive pyroclastic deposits that were remobilized by numerous lahars during the year after the eruption. Two study sites were equipped in order to evaluate the morphodynamic evolution of the riverbed of the Opak River. Topographic surveys, bedload particle marking, and suspended sediment sampling revealed an important sediment mobilization during efficient flash floods. Surprisingly, no bed aggradation related to the progradation of a sediment wave was observed. Two years after the eruptive event, marked bed incision was observed. The Opak River readjustment differs from that of other fluvial systems affected by massive eruptions in two ways. Firstly, local population extracted the sand and blocks injected by the eruption as they represent a valuable economic resource. Secondly, several dams trapped the major part of the sediment load remobilized by lahars.

Borobudur basin is located in Central Java (Indonesia), 30 km to the North of Yogyakarta City. The basin is famous for its UNESCO world heritage temple and for one of the world's most active volcanoes, Merapi, located to the East of Borobudur basin. Merapi is one of the three andesitic volcanoes that surround the basin: Merapi, Merbabu and Sumbing volcanoes. Therefore, volcanic activity has strongly influenced the evolution of Borobudur basin. The object of this contribution is to present the evolution of Borobudur basin over the last 161,000 years in the light of volcanic influence. The methodology and tools developed for this research span over different areas of expertise, from geochemistry, geology and geomorphology to remote sensing, GIS and archeology. Results highlight the following points: 1. Two major volcanic events deposited volcaniclastic materials up to tens of meters thick ~ 119,000 years BP and ~ 31,000 years BP. in the Southern part of the Borobudur basin. The second volcanic event could correspond to the collapse of the older Ancient Merapi ( Camus et al., 2000 ) or Proto-Merapi Stage ( Newhall et al., 2000 ). 2. There is no trace in the Borobudur basin of a large debris avalanche 3. There are several generations of paleolakes in the Borobudur basin. The latest one has shrunk until historical times, corroborating the theory of Newhall et al. (2000) and Murwanto et al. (2004) that Borobudur Temple was standing by a water body. Most of these paleolakes were impounded following volcanic events. Paleolakes most probably originated from the blockage of the hydrographic network by volcanic material. 4. Borobudur temple was never buried under volcanic material during historic times.

Semeru Volcano is the highest mountain of Java (Indonesia), and a vulcanian explosion occurs every 15 minutes on average, since 1967. Thus a constantly renewed stock of material and the heavy monsoon rainfall [3700 mm yr−1 at 1500 m above sea level (a.s.l.)] provide a perfect setting for the study of lahars and their deposits. Hence, we examined the architecture of lahars' terraces 9·5 km from the summit in the Curah Lengkong Valley. We first used ground penetrating radar (GPR) over vertical exposures of the lahars cut-bank terraces. This allowed us to better understand transversal radargrams across terraces, which are not visually accessible in the field. Preliminary results from a single radargram are very instructive, since (1) they prove that the lateral architecture does not correspond to that observed from banks only; (2) we could observe the presence of lenses and stratigraphic discontinuities; (3) the setting of the various units can also help reconstruct deposition processes and the chronology of different units. In order to finalize these preliminary results, we however need to perform multiple GPR radargrams and provide a complete set of results. Copyright © 2010 John Wiley & Sons, Ltd.

Large explosive eruptions inject volcanic gases and fine ash to stratospheric altitudes, contributing to global cooling at the Earth’s surface and occasionally to ozone depletion. The modelling of the climate response to these strong injections of volatiles commonly relies on ice-core records of volcanic sulphate aerosols. Here we use an independent geochemical approach which demonstrates that the great 1257 eruption of Samalas (Lombok, Indonesia) released enough sulphur and halogen gases into the stratosphere to produce the reported global cooling during the second half of the 13th century, as well as potential substantial ozone destruction. Major, trace and volatile element compositions of eruptive products recording the magmatic differentiation processes leading to the 1257 eruption indicate that Mt Samalas released 158 ± 12 Tg of sulphur dioxide, 227 ± 18 Tg of chlorine and a maximum of 1.3 ± 0.3 Tg of bromine. These emissions stand as the greatest volcanogenic gas injection of the Common Era. Our findings not only provide robust constraints for the modelling of the combined impact of sulphur and halogens on stratosphere chemistry of the largest eruption of the last millennium, but also develop a methodology to better quantify the degassing budgets of explosive eruptions of all magnitudes.

The 1257 A.D. caldera-forming eruption of Samalas (Lombok, Indonesia) was recently associated with the largest sulphate spike of the last 2 ky recorded in polar ice cores. It is suspected to have impacted climate both locally and at a global scale. Extensive fieldwork coupled with sedimentological, geochemical and physical analyses of eruptive products enabled us to provide new constraints on the stratigraphy and eruptive dynamics. This four-phase continuous eruption produced a total of 33–40 km3 dense rock equivalent (DRE) of deposits, consisting of (i) 7–9 km3 DRE of pumiceous plinian fall products, (ii) 16 km3 DRE of pyroclastic density current deposits (PDC) and (iii) 8–9 km3 DRE of co-PDC ash that settled over the surrounding islands and was identified as far as 660 km from the source on the flanks of Merapi volcano (Central Java). Widespread accretionary lapilli-rich deposits provide evidence of the occurrence of a violent phreatomagmatic phase during the eruption. With a peak mass eruption rate of 4.6 × 108 kg/s, a maximum plume height of 43 km and a dispersal index of 110,500 km2, the 1257 A.D. eruption stands as the most powerful eruption of the last millennium. Eruption dynamics are consistent with an efficient dispersal of sulphur-rich aerosols across the globe. Remarkable reproducibility of trace element analysis on a few milligrammes of pumiceous tephra provides unequivocal evidence for the geochemical correlation of 1257 A.D. proximal reference products with distal tephra identified on surrounding islands. Hence, we identify and characterise a new prominent inter-regional chronostratigraphic tephra marker.

Yogyakarta urban area (500,000 inhab.) is located in Central Java on the fluvio-volcanic plain beside Merapi volcano, one of the most active of the world. Since the last eruption of Merapi in November 1994, the Code river, which goes across this city, is particularly threatened by lahars (volcanic debris flows). Until now, no accurate hazard map exists and no risk assessment has been done. Therefore, we drew a detailed hazard map (1/2,000 scale), based on morphometric surveys of the Code channel and on four scenarios of discharge. An additional risk assessment revealed that about 13,000 people live at risk along this river, and that the approximate value of likely loss is US $ 52 millions. However, the risk level varies between the urban suburbs.

International audience; The well-documented 1883 eruption of Krakatau volcano (Indonesia) offers an opportunity to couple the eruption’s history with the tsunami record. The aim of this paper is not to re-analyse the scenario for the 1883 eruption but to demonstrate that the study of tsunami deposits provides information for reconstructing past eruptions. Indeed, though the characteristics of volcanogenic tsunami deposits are similar to those of other tsunami deposits, they may include juvenile material (e.g. fresh pumice) or be interbedded with distal pyroclastic deposits (ash fall, surges), due to their simultaneity with the eruption. Five kinds of sedimentary and volcanic facies related to the 1883 events were identified along the coasts of Java and Sumatra: (1) bioclastic tsunami sands and (2) pumiceous tsunami sands, deposited respectively before and during the Plinian phase (26–27 August); (3) rounded pumice lapilli reworked by tsunami; (4) pumiceous ash fall deposits and (5) pyroclastic surge deposits (only in Sumatra). The stratigraphic record on the coasts of Java and Sumatra, which agrees particularly well with observations of the 1883 events, is tentatively linked to the proximal stratigraphy of the eruption.

An 11-minute sequence of laterally-directed explosions and retrogressive collapses on 5 November 2010 at Merapi (Indonesia) destroyed a rapidly-growing dome and generated high-energy pyroclastic density currents (PDCs) spreading over 22 km2 with a runout of 8.4 km while contemporaneous co-genetic valley-confined PDCs reached 15.5 km. This event formed Stage 4 of the multi-stage 2010 eruption, the most intense eruptive episode at Merapi since 1872. The deposits and the widespread devastating impact of associated high-energy PDCs on trees and buildings show striking similarities with those from historical volcanic blasts (Montagne Pelee, Martinique, Bezymianny, Russia, Mount St. Helens, USA, Soufriere Hills, Montserrat). We provide data from stratigraphic and sedimentologic analyses of 62 sections of the first unequivocal blast-like deposits in Merapi's recent history. We used high resolution satellite imagery to map eruptive units and flow direction from the pattern of extensive tree blowdown. The stratigraphy of Stage 4 consists of three depositional units (U0, U1, U2) that we correlate to the second, third and fourth explosions of the seismic record. Both U1 and U2 show a bi-partite layer stratigraphy consisting each of a lower L1 layer and an upper L2 layer. The lower L1 layer is typically very coarse-grained, fines-poor, poorly-sorted and massive, and was deposited by the erosive waxing flow head. The overlying L2 layer is much finer grained, fines-rich, moderately to well-sorted, with laminar to wavy stratification. L2 was deposited from the waning upper part and wake of the PDC. Field observations indicate that PDC height reached ~ 330 m with an internal velocity of ~ 100 m s− 1 within 3 km from the source. The summit's geometry and the terrain morphology formed by a major transversal ridge and a funneling deep canyon strongly focused PDC mass towards a major constriction, thereby limiting the loss of kinetic energy. This favored elevated PDC velocities and high particle concentration, promoted overspilling of PDCs across high ridges into other river valleys, and generated significant dynamic pressures to distances of 6 km that caused total destruction of buildings and the forest. The Merapi 2010 eruption highlights that explosive and gravitational disintegration of a rapidly growing dome can generate devastating high-energy, high-velocity PDCs. This constitutes a credible high impact scenario for future multi-stage eruptions at Merapi and at other volcanoes that pose particular monitoring, crisis response, and risk reduction challenges.

The 2010 Merapi eruption was exceptional on several levels (intensity of the eruption, destructions, casualties…) and for the first time, created major air traffic disruptions in Yogyakarta, leading to the closure of the airport. Some companies suspended their flights, others adapted to the crisis by transferring their flights to other airports, and some companies even continued to fly despite the risks involved. Four major phases emerged; first, a few days corresponding to the rise of the activity of the eruption, a second corresponding to the start of the eruption and first ash emissions. Then, a third peak marked by the eruption which led to the closure of the Yogyakarta airport for 15 days and finally, a fourth one-month-long phase where airport activity returned to normal. We studied the evolution of disturbances on the field and the correlation between volcanic activity and flight cancelations. Adaptations between airports were observed, Adisucipto Airport (Yogyakarta) transferred several of its flights to the Adi Soemarmo of Surakarta airport and it transferred its flights to Ahmad Yani Airport in Semarang and Juanda in Surabaya. Moreover, the eruption disrupted the pilgrimage to Mecca for thousands of Muslims who had waited and saved for years to be able to go. Nevertheless, the organizers coped with the crisis by changing departure airports for the pilgrimage. This study allowed us to understand the impacts of a major Merapi eruption on air transport, from the onset of ash emissions until the late disturbances.

The 2010 VEI 4 eruption of Merapi volcano deposited roughly ten times the volume of pyroclastic materials of the 1994 and 2006 eruptions, and is recognized as one of the most intense eruption since 1872. However, as the eruptive phase is now over, another threat endangers local communities: rain-triggered lahars. Previous papers on lahars at Merapi presented lahar-related risk following small-scale dome-collapse PDCs. Thus the aim of this study is to provide new insights on lahar-related risk following a large scale VEI 4 eruption. The paper highlights the high number of events (240) during the 2010–2011 rainy season (October 2010–May 2011). The frequency of the 2010–2011 lahars is also the most important ever recorded at Merapi. Lahars occurred in almost all drainages located under the active cone, with runout distances exceeding 15 km. The geomorphic impacts of lahars on the distal slope of the volcano are then explained as they directly threaten houses and infrastructures: creation of large corridors, avulsions, riverbank erosion and riverbed downcutting are detailed through local scale examples. Related damage is also studied: 860 houses damaged, 14 sabo-dams and 21 bridges destroyed. Sedimentological characteristics of volcaniclastic sediments in lahar corridors are presented, with emphasis on the resource in building material that they represent for local communities. Risk studies should not forget that thousands of people are exposing themselves to lahar hazard when they quarry volcaniclastic sediment on lahar corridors. Finally, the efficient community-based crisis management is explained, and shows how local people organize themselves to manage the risk: 3 fatalities were reported, although lahars reached densely populated areas. To summarize, this study provides an update of lahar risk issues at Merapi, with emphasis on the distal slope of the volcano where lahars had not occurred for 40 years, and where lahar corridors were rapidly formed.

The 2010 pyroclastic density currents (PDC) at Merapi have presented a rare opportunity to collect a uniquely detailed dataset of the source, extent, lateral variations and impact of various PDC deposits on a densely populated area. Using traditional volcanological field-based methods and a multi-temporal dataset of high-resolution satellite imagery, a total of 23 PDC events have been recognized, including 5 main channeled flows, 15 overbank flows derived from overspill and re-channelization of the main PDCs into adjacent tributaries and two main surge events. The 2010 PDC deposits covered an area of ~ 22.3 km2, unequally distributed between valley-filling (6.9%), overbank (22.4%) and surge and associated fallout deposits (71.7%). Their total estimated non-DRE volume is ~ 36.3 × 106 m3, with 50.2% of this volume accounting for valley-filling deposits, 39.3% for overbank deposits and 10.5% for surge and associated fallout deposits. More than 70% of the total volume was deposited during the third eruptive phase (4–5 November), and only 16.6%, 11.5% and 0.9% during the first (26–29 October), second (30 October – 3 November) and fourth phase (6–23 November), respectively. The internal architecture and lithofacies variations of the 2010 PDC deposits were investigated using data collected from 30 stratigraphic sections measured after one rainy season of erosion. The results show that complex, local-scale variations in flow dynamics and deposit architectures are apparent and that the major factors controlling the propagation of the main flows and their potential hazards for overbanking were driven by: (1) the rapid emplacement of several voluminous PDCs, associated with the steady infilling of the receiving landscape after the two first phases of the eruption; (2) longitudinal changes in channel capacity following increased sinuosity in the valley and decreased containment space; and (3) the effects of varying source mechanisms (gravitational dome collapse, vertical or lateral dome explosions and column-collapse) and source materials involved during individual PDC-forming events. Integration of these data into numerical simulations of the channeled and overbank PDCs of the third eruptive phase (4–5 November) using two well-established geophysical mass flow models, Titan2D and VolcFlow, allow us to evaluate the ability of these models to reproduce the main features of the natural deposits and some of the flow overbanking processes observed in the field. Using such a multi-technique approach, the dataset obtained in this study not only characterizes the PDCs and related hazards at Merapi, but will allow comparisons with similar events at other volcanoes around the globe.

The hazardous and unpredictable nature of lahars makes them challenging to study, yet the in-flow processes characterizing these events are important to understand. As a result, much of the previous research on lahar sedimentation and flow processes has been derived from experimental flows or stratigraphic surveys of post-event deposits. By comparison, little is known on the time-dependent sediment and flow dynamics of lahars in natural environments. Using video-footage of seven lahars on the flanks of Semeru Volcano (East Java, Indonesia), the present study offers new insights on the in-flow evolution of sediment in natural lahars. Video analysis revealed several distinctive patterns of sediment entrainment and deposition that varied with time-related fluctuations in flow. These patterns were used to generate a conceptual framework describing possible processes of formation for subsurface architectural features identified in an earlier lateral survey of lahar deposits on Semeru Volcano (Gomez and Lavigne, 2010a). The formation of lateral discontinuities was related to the partial erosion of transitional bank deposits followed by fresh deposition along the erosional contact. This pattern was observed over the course of several lahar events and within individual flows. Observations similarly offer potential explanations for the formation of lenticular features. Depending on flow characteristics, these features appeared to form by preferential erosion or deposition around large stationary blocks, and by deposition along channel banks during episodes of channel migration or channel constriction. Finally, conditions conducive to the deposition of fine laminated beds were observed during periods of attenuating and surging flow. These results emphasize the difficulties associated with identifying process–structure relationships solely from post-event deposit interpretation and illustrate that an improved understanding of the time-dependent sediment dynamics in lahars may be advantageous when interpreting post-event structural features.

The last eruption of Merapi volcano occurred from October until December 2010. The existing contingency and evacuation plans were only able to handle the crisis from 26 October, 2010 until 3 November, 2010. By 3 November, 2010, the contingency and evacuation plans were no longer used. The authorities decided to move the refugee camps 15 km away from the summit instead of the initial 10 km. The largest and unexpected eruption occurred on 4 November, 2010 and the safety zone was raised to a radius of 20 km. This paper aims to analyse the difficulties during the evacuation and to examine the key issues that must be overcome in the evacuation and contingency plans in volcanic areas. The results show that contingency and evacuation plans should not be based on a single scenario. In the high volcanic risk areas such as Merapi's region, a multiscale and multiscenario evacuation plan is crucial.

SUMMARY The Mw 7.8 2006 July 17 earthquake off the southern coast of Java, Indonesia, has been responsible for a very large tsunami causing more than 700 casualties. The tsunami has been observed on at least 200 km of coastline in the region of Pangandaran (West Java), with run-up heights from 5 to more than 20 m. Such a large tsunami, with respect to the source magnitude, has been attributed to the slow character of the seismic rupture, defining the event as a so-called tsunami earthquake, but it has also been suggested that the largest run-up heights are actually the result of a second local landslide source. Here we test whether a single slow earthquake source can explain the tsunami run-up, using a combination of new detailed data in the region of the largest run-ups and comparison with modelled run-ups for a range of plausible earthquake source models. Using high-resolution satellite imagery (SPOT 5 and Quickbird), the coastal impact of the tsunami is refined in the surroundings of the high-security Permisan prison on Nusa Kambangan island, where 20 m run-up had been recorded directly after the event. These data confirm the extreme inundation lengths close to the prison, and extend the area of maximum impact further along the Nusa Kambangan island (about 20 km of shoreline), where inundation lengths reach several hundreds of metres, suggesting run-up as high as 10–15 m. Tsunami modelling has been conducted in detail for the high run-up Permisan area (Nusa Kambangan) and the PLTU power plant about 25 km eastwards, where run-up reached only 4–6 m and a video recording of the tsunami arrival is available. For the Permisan prison a high-resolution DEM was built from stereoscopic satellite imagery. The regular basin of the PLTU plant was designed using photographs and direct observations. For the earthquake's mechanism, both static (infinite) and finite (kinematic) ruptures are investigated using two published source models. The models account rather well for the sea level variation at PLTU, showing a better agreement in arrival times with the finite rupture, and predict the Permisan area to be one of the regions where tsunami waves would have focussed. However, the earthquake models that match the data at PTLU do not predict that the wave heights at Permisan are an overall maximum, and do not predict there more than 10 m of the 21 observed. Hence, our results confirm that an additional localized tsunami source off Nusa Kambangan island, such as a submarine landslide, may have increased the tsunami impact for the Permisan site. This reinforces the importance for hazard assessment of further mapping and understanding local potential for submarine sliding, as a tsunami source added to usual earthquake sources.

We focus in this paper on the processes and consequences of an unusual volcanic eruption at Kelut volcano, East Java. In November 2007, after two months of worrying precursor signs, Kelut volcano erupted. But neither explosions nor the usual hazards observed during the historic eruptions happened (e.g. ash falls, volcanic bombs and pyroclastic flows). Instead of an explosive eruption, the 2007 eruption was extrusive. Given than such an eruption could not be predicted, the authorities had to manage a new situation. We conducted interviews with nine stakeholders of the crisis management team, and undertook a questionnaire-based survey in the settlement nearest to the crater, in order to understand how the authorities managed the crisis, and how people reacted. Inquiries and questionnaires were carried out shortly after the end of the evacuation process, when the volcano was still under surveillance for fear of an explosive phase. The results display a real gap in what it takes to manage a crisis or live through a crisis. This suggests that the “unusual” eruption pattern of Kelut volcano was not the only factor of the misunderstanding between the authorities and the population. These problems stem from more structural causes such as the lack of communication and information when there is a need to adapt to a new scenario. In such a situation, the inability of the crisis management system to take decisions underscored the intrinsic vulnerability of the population despite a hierarchical and strategic top-down crisis management approach.

International audience; The December 26th tsunami of 2004 caused an unprecedented disaster in the Indian Ocean. In Sumatra, a third of the Banda Aceh area was destroyed and 70,000 people died. The Tsunarisque Program e a FrencheIndonesian research project e brings new considerations to tsunami dynamics and damage intensity in this urban area: An original method of damage spatial analysis is based on field surveys, photo interpretations and GIS. The first result is a very accurate cartography of the tsunami breaking zone that is shown by a steep drop in the damaging gradient around 2.7 km from the coast. The second is a new “macro-tsunamic” intensity scale based on special typologies of buildings and damages. This analysis is complemented by fragility curves that give the statistical relationships between mean damage intensities and wave heights. These results will allow developing application in tsunami potential losses modelling.

Estimating the magnitude of a past tsunami from its deposits is one of the major topics to be developed in future studies on tsunami hazard assessment. Main limitations are (1) the great variability of tsunami sandy sheets deposited on land (the sediment source and the topography controlling many aspects of the deposition), (2) the preservation of these soft sediments, and (3) the controversial interpretations of coastal boulder accumulations. In this paper, we investigate sediment transport and deposition during the December 26, 2004 tsunami inflow and outflow in the Lhok Nga Bay, located 10 km west of the city of Banda Aceh (northwest Sumatra, Indonesia). Side-scan sonar data of the near shore area are used to study the morphometry and distribution of boulders offshore. Entrainment of finer sediments offshore is inferred by estimating the movable grain sizes based upon the simulated current velocities of the tsunami waves. Results demonstrate that the tsunami waves raised the bed shear velocities to levels above critical values for the entrainment of coarse sands on the continental shelf. Most of the sediments deposited on land came from offshore, from fine sands to coral boulders. With very high values of u ⁎ (> 30 cm/s), the outflow (backwash) reworked and re-deposited large volume of sediments offshore. All rocky outcrops offshore were affected by the tsunami (down to 25 m deep). The fraction of boulders transported from offshore and deposited inland represents only 7% of the total number of boulders moved during the tsunami. Characteristics of the boulders can help to estimate flow velocities required for detaching them, and their imbrication (if any) to infer flow directions. However, calculations of flow depth and transport distance do not provide convincing results. Future studies coupling offshore–onshore mapping of boulder accumulations with reconstitutions of the morphological history (sea-level variations, coastal sediment discharge and landform evolution) may allow distinguishing storms and tsunami deposits.

In 2006, a series of block-and-ash flows swept the southwestern and southern flanks of Merapi Volcano, Java, Indonesia. In the K. Gendol valley, near the village of Kaliadem, we conducted a GPR survey on the most distal lobe of the June 14th second block-and-ash flow deposit. For this 100 m-long transect, we used a commercial GPR RAMAC© mounted with 100 MHz antennas. We measured the topography with a synchronized GPS and a laser rangefinder. Back at the laboratory, we processed the dataset with the software REFLEX®. Data of the subsurface reveals a series of layers, separated by strong reflective horizons. These horizons are the manifestation of intercalations of fine materials in between more coarse layers. The architecture of these layers presents progradation, retrogradation and aggradation patterns that we relate to the block-and-ash flow deposition process. Based on these observations we proposed a relative chronology of the deposition and a simple conceptual model of the deposition. The model show that the block-and-ash flow can deposit either long, close to horizontal single layers, or shorter layers that imbricate themselves, following different patterns (progradation, retrogradation or aggradation). Nevertheless we remained cautious, since we only studied a very short portion of the deposit, and similar experiences need to be repeated. Moreover there are reflections in the radargram that we could not identify, and further studies need to be conducted.

Large tsunamis are major geomorphic crises, since they imply extensive erosion, sediment transport and deposition in a few minutes and over hundreds of kilometres of coast. Nevertheless, little is known about their geomorphologic imprints. The December 26, 2004 tsunami in Sumatra (Indonesia) was one of the largest and deadliest tsunamis in recorded human history. We present a description of the coastal erosion and boulder deposition induced by the 2004 tsunami in the Lhok Nga Bay, located to the West of Banda Aceh (northwest Sumatra). The geomorphological impact of the tsunami is evidenced by: beach erosion (some beaches have almost disappeared); destruction of sand barriers protecting the lagoons or at river mouths; numerous erosion escarpments typically in the order of 0.5–1.5 m when capped by soil and more than 2 m in dunes; bank erosion in the river beds (the retreat along the main river is in the order of 5–15 m, with local retreats exceeding 30 m); large scars typically 20–50 cm deep on slopes; dislodgement of blocks along fractures and structural ramps on cliffs. The upper limit of erosion appears as a continuous trimline at 20–30 m a.s.l., locally reaching 50 m. The erosional imprints of the tsunami extend to 500 m from the shoreline and exceed 2 km along riverbeds. The overall coastal retreat from Lampuuk to Leupung was 60 m (550,000 m 2 ) and locally exceeded 150 m. Over 276,000 m 3 of coastal sediments were eroded by the tsunami along the 9.2 km of sandy coast. The mean erosion rate of the beaches was ~ 30 m 3 /m of coast and locally exceeded 80 m 3 /m. The most eroded coasts were tangent to the tsunami wave train, which was coming from the southwest. The fringing reefs were not efficient in reducing the erosional impact of the tsunami. The 220 boulders measured range from 0.3 to 7.2 m large (typically 0.7–1.5 m), with weights from over 50 kg up to 85 t. We found one boulder, less than 1 m large, at 1 km from the coastline, but all the others were transported less than 450 m ( 30 m) and the moderate tsunamis (height

In 2006 Merapi volcano, Indonesia, erupted for a few months, producing several block-and-ash flows reaching a maximum distance of 7.5 km from the main vent. During the eruption, we conducted a survey on those flow deposits in the Gendol Valley at Kaliadem village, about 4.5 km from the Merapi submit, using a Ground Penetrating Radar (GPR). The upper deposit was studied in its distal reaches, whereas the one below was studied in its medial reaches. The field study was carried out with a commercial RAMAC® GPR coupled with 100 MHz antennas, and the data treatment conducted with Reflex™ software. From this survey, we determined both deposits' local (1) thickness – reaching a maximum of 15 m – and (2) internal architecture. This last one is governed by long reflecting horizons extending over 20 to 30 m that delimit layers showing progradation patterns in their distal reaches. Within these layers we could also observe an internal architecture of still unknown origin. The layers are interpreted as the result of the flow pulses that progressively deposited downstream-ward by progradation. However the interpretation of those GPR profiles is a bit hazardous, because of the absence of outcrops, and we can only proceed by analogy with other studies. Nevertheless, despite numerous limitations, GPR is a helpful tool to understand pyroclastic deposits' structure when no visual observations are available.

International audience; Mt. Semeru, the highest mountain in Java (3,676 m), is one of the few persistently active composite volcanoes on Earth, with a plain supporting about 1 million people. We present the geology of the edifice, review its historical eruptive activity, and assess hazards posed by the current activity, highlighting the lahar threat. The composite andesite cone of Semeru results from the growth of two edifices: the Mahameru ‘old' Semeru and the Seloko ‘young' Semeru. On the SE flank of the summit cone, a N130-trending scar, branched on the active Jonggring- Seloko vent, is the current pathway for rockslides and pyroclastic flows produced by dome growth. The eruptive activity, recorded since 1818, shows three styles: (1) The persistent vulcanian and phreatomagmatic regime consists of short-lived eruption columns several times a day; (2) increase in activity every 5 to 7 years produces several kilometer-high eruption columns, ballistic bombs and thick tephra fall around the vent, and ash fall 40 km downwind. Dome extrusion in the vent and subsequent collapses produce block-and-ash flows that travel toward the SE as far as 11 km from the summit; and (3) flank lava flows erupted on the lower SE and E flanks in 1895 and in 1941– 1942. Pyroclastic flows recur every 5 years on average while large-scale lahars exceeding 5 million m3 each have occurred at least five times since 1884. Lumajang, a city home to 85,000 people located 35 km E of the summit, was devastated by lahars in 1909. In 2000, the catchment of the Curah Lengkong River on the ESE flank shows an annual sediment yield of 2.7×105 m3 km−2 and a denudation rate of 4 105 t km−2 yr−1, comparable with values reported at other active composite cones in wet environment. Unlike catchments affected by high magnitude eruptions, sediment yield at Mt. Semeru, however, does not decline drastically within the first post-eruption years. This is due to the daily supply of pyroclastic debris shed over the summit cone, which is remobilised by runoff during the rainy season. Three hazard-prone areas are delineated at Mt. Semeru: (1) a triangle-shaped area open toward the SE has been frequently swept by dome-collapse avalanches and pyroclastic flows; (2) the S and SE valleys convey tens of rain-triggered lahars each year within a distance of 20 km toward the ring plain; (3) valleys 25 km S, SE, and the ring plain 35 km E toward Lumajang can be affected by debris avalanches and debris flows if the steep-sided summit cone fails.

The Papandayan volcano in Indonesia erupted in November 2002, and expelled more than one million cubic meters of volcanic ash. Although the eruption magnitude was low, the direct financial loss amounted to 1.5 x 106 US$. The whole drainage basin was disrupted through several geomorphic processes which followed one another over time and space., Le volcan indonésien Papandayan est entré en éruption en novembre 2002, éjectant plus d'un million de mètres cubes de cendres volcaniques. Malgré la faible magnitude de l'éruption, les pertes directes ont été considérables (1,5 M US$) et l'ensemble du bassin versant a été bouleversé par plusieurs processus morphodynamiques qui se sont succédés dans le temps et dans l'espace., Lavigne Franck, Hadisantono Rudy, Surmayadi Mamay, Flohic François, Geyer Florian. L'éruption du volcan Papandayan (Indonésie) en novembre 2002: aléas directs et induits (The november 2002 eruption of the Papandayan volcano (Indonesia) : direct and induced hazards). In: Bulletin de l'Association de géographes français, 81e année, 2004-1 ( mars). Proximités spatiales dans la ville. Risques naturels, risques de sociétés. pp. 84-92.

The ninth-century world-heritage Buddhist monument of Borobudur (Java, Indonesia) stands above the floor of a dried-out palaeolake, but it remains uncertain as to whether it was ever constructed on a lake shore. Here we reveal through new chronological and palaeoenvironmental data on the extant sediment record of the area that Borobudur intentionally stood by an existing lake. For the first time, evidence of this conjunction validates quite literally the debated cosmological interpretation of the edifice as an aquatic lotus symbol upon which Buddha is seated. The fluctuating life history of the lake spanned at least 20 000 years.

Merapi volcano, in Central Java, is one of the most active volcanoes in the world. At least 23 of the 61 reported eruptions since the mid-1500s have produced source deposits for lahars. The combined lahar deposits cover about 286 km 2 on the flanks and the surrounding piedmonts of the volcano. At Merapi, lahars are commonly rain-triggered by rainfalls having an average intensity of about 40 mm in 2 h. Most occur during the rainy season from November to April, and have average velocities of 5–7 m/s at 1000 m in elevation. A wide range of facies may be generated from a single flow, which may transform downvalley from debris flow to hyperconcentrated streamflow. Because of the high frequency and magnitude of the lahar events, lahar-related hazards are high below about 450–600 m elevation in each of the 13 rivers which drain the volcano. Hazard-zone maps for lahar were produced by Pardyanto et al. (Volcanic hazard map, Merapi volcano, Central Java (1/100,000). Geol. Surv. of Indonesia, Bandung, II, 4, 1978) and the Japanese–Indonesian Cooperation Agency (Master plan for land conservation and volcanic debris control in the area of Mt Merapi, Jakarta, 1980), but these maps are of a very small scale to meet modern zoning requirements. More recently, a few large-scale maps (1/10,000- and 1/2000-scale) and risk assessments have been completed for a few critical river systems.

More than 50 volcanic debris flows or lahars were generated around Mt Merapi during the first rainy season following the nuees ardentes of 22 November 1994. The rainfalls that triggered the lahars were analyzed, using such instruments as weather radar and telemetered rain gauges. Lahar dynamics were also monitored, using new non-contact detection instrumentation installed on the slopes of the volcano. These devices include real-time seismic amplitude measurement (RSAM), seismic spectral amplitude measurement (SSAM) and acoustic flow monitoring (AFM) systems. Calibration of the various systems was accomplished by field measurements of flow velocities and discharge, contemporaneously with instrumental monitoring. The 1994–1995 lahars were relatively short events, their duration in the Boyong river commonly ranging between 30 min and 1 h 30 min. The great majority (90%) of the lahars was recognized at Kaliurang village between 13:00 and 17:30 h, due to the predominance of afternoon rainfalls. The observed mean velocity of lahar fronts ranged between 1.1 and 3.4 m/s, whereas the peak velocity of the flows varied from 11 to 15 m/s, under the Gardu Pandang viewpoint location at Kaliurang, to 8–10 m/s at a section 500 m downstream from this site. River slopes vary from 28 to 22 m/km at the two sites. Peak discharges recorded in various events ranged from 33 to 360 m3/s, with the maximum value of peak discharge 360 m3/s, on 20 May 1995. To improve the lahar warning system along Boyong river, some instrumental thresholds were proposed: large and potentially hazardous lahars may be detected by RSAM units exceeding 400, SSAM units exceeding 80 on the highest frequency band, or AFM values greater than 1500 mV on the low-gain, broad-band setting.

This paper présents a method of volcanic risk assessment and mapping, based on a topologic and vector-mode GIS (Geographic Information System). Risk zo nation is obtained in three steps : (1) a mapping of hazards, based on four scenarios of eruptions recognized in the past at Merapi ; (2) a semi-quantitative appraisal of factors of people's vulnerability ; (3) a vulnerability assessment of goods and economic estates, which aims to estimate the amount of potential losses in each scenario. GIS is a powerful tool for risk zonation because it allows the integration of a lot of data from various origin, the regular updating of data, the modulation and the simulation of different scenarios for prevention planning management, and the interrogation of the cartographic database., Résumé. - Cet article présente une méthode d'évaluation et de zonage des risques volcaniques appuyée par un Système d'Informations Géographiques (SIG) topologique en mode vecteur. La méthode repose sur une triple démarche : une approche des menaces fondée sur quatre scénarios d'éruptions identifiées dans le passé au Merapi, une évaluation semi-quantitative des facteurs de vulnérabilité des populations et un constat d'endommagement, visant à estimer le coût des pertes matérielles directes. Etant multiscalaire, évolutif, opérationnel et didactique, le SIG MERAPI ouvre de larges horizons sur le plan de la recherche fondamentale et appliquée sur les risques volcaniques., Lavigne Franck. SIG et zonage des risques volcaniques : application au volcan Merapi, Java, Indonésie (GIS and zonation of the volcanic hazards : application to Merapi volcano, Java, Indonesia). In: Bulletin de l'Association de géographes français, 76e année, 1999-4 ( décembre). L'homme et le volcanisme. Les eurorégions, sous la direction de Gérard Mottet et Henri Rougier. pp. 371-382.

We studied emplacement dynamics of the December 26, 2004 tsunami deposits in artificial shallow ponds and in shallow lagoons, along the northern coast of Sumatra in Banda Aceh area. Successive tsunami waves inundated topographic lows, which acted as sediment traps and favored preservation of deposits. Individual waves led to the deposition of fining upwards sedimentary sequences. This multiple normal grading deposit with a thickness ranging from 2 to 80 cm constitutes a site of major interest for the study of the deposits emplaced by a tsunami wave train. The main interest lies in the characteristics of the deposits that can be correlated to the behavior of the waves attested by numerous testimonies. In this setting, we aimed at testing the applicability of the AMS (Anisotropy of Magnetic Susceptibility) technique as a proxy of unconsolidated sediment fabric. The combination of this proxy and traditional sedimentary analysis helped reconstructing the hydrodynamic conditions — particularly the flow direction, prevailing during sedimentation. We applied the AMS technique on eight sections stretching inland from 1.5 km to 2.2 km from the pre-tsunami coastline. Grain-size and AMS data from these deposits provide similar and partially complementary information. Traction dominates in the basal parts of the deposits and occurs mainly for the basal layers of the sequences whereas settling dominates for the upper layers of these sequences. The vertical evolution of the AMS parameters within the deposits is related to the arrival of the successive waves with variations in the traction and decantation processes during deposition. Variations of current strength during sedimentation are related to the wave's momentum and to the progressive infill of topographic depressions. The vertical change of the fifth coarsest percentile vs. the median grain sizes of the deposits suggests strong variations of the energy of the currents, and confirms the evolution of the AMS parameters. In the present study, the fabric of the deposits, which developed exclusively during the uprush phase of the cycle, also allows the reconstruction of flood directions that are both normal and parallel to the shoreline, with variations related to flow dynamics, mean grain size and local topographic control. Due to the absence of backwash in between the waves and low velocity of the final backwash, information about the sedimentation conditions of the tsunami deposits appears to be better preserved in topographic depressions in areas located remote from the seashore. The AMS proxy appears to be a promising tool for further studies of recent- and paleo-tsunami deposits.

The 17 July 2006, a tsunami struck the southern coast of Java, Indonesia, causing over 730 casualties. The triggering earthquake located 225 km off the coast of Pangandaran (9.222° S, 107.320° E), occurred at 15:19 LT (UTC +7) with a 7.7 magnitude on the Richter scale (Harward Center and CEA/DAM). In order to calibrate numerical models and understand the phenomenon, we conducted a 6-weeks field survey in July and August 2006 from Cimerak district in West Java to Gunung Kidul district in Central Java. Data collection involved measurements of wave height before its breaking, flow depth, run-up height, inundation depth, flow directions and a detailed chronology of the tsunami. Eyewitnesses accounted for three main waves. The maximum height of the second wave ranged from 4.2 to 8.6 m before its breaking. Maximum flow depth after the wave's breaking reached 5 m, and maximum runup heights reached 15.7 m. Our run-up values are about 1.5 higher than those obtained by the other field surveys carried out until present. They are also higher than the values computed through preliminary models. The 17 July 2006 tsunami has been generated by a "tsunami earthquake", i.e. an earthquake of low or medium scale that triggers a tsunami of high magnitude. The run-up heights progressively decreased eastwards, which is consistent with a tsunami triggered by fault dislocation, as the one that hit the Nicaragua's coast with similar run-up heights on the 2 September 1992. An earthquake with associated landslides could also have generated the 17 July 2006 tsunami, as ever observed in Papua-New-Guinea in 1998.