Imaggeo on Mondays: Indonesian mangroves and tsunamis

Imaggeo on Mondays: Indonesian mangroves and tsunamis

Pictured here is a solitary mangrove tree, rooted off the northern coast of the Indonesian island Flores. While this tree has the shallow sandy reef to itself, mangroves are often found clumped together in large forests covering tropical and subtropical coastlines. The propped-up roots of mangrove trees often tangle together, creating a dense natural barrier that can weaken the coastal impact of ocean tides, currents and storms. As a consequence, islands with mangrove forests on their coastlines experience less erosion and less damage from storm surges compared to barer shorelines.

Mangroves are also often said to provide protection against tsunami destruction. Indeed, there have been several cases in which mangroves trees were believed to have curtailed the devastating effect of tsunami waves. Recent research suggests that extensive mangrove forests hundreds of metres wide have been able to reduce tsunami wave heights by 5-30 percent.

Unfortunately, over the past decades, these environmental benefits are now under threat due to deforestation. About half of the global mangrove population (32 million hectares) has been wiped out, often to make way for fish farming operations. In Indonesia, mangrove ecosystem decline has been largely attributed to developing shrimp ponds and logging activities. There are now a number of places where mangrove plantations are supported by local individuals and governments.

Jörn Behrens, a professor of numerical methods in Earth sciences at the University of Hamburg in Germany, captured this shot while on a field trip in Indonesia. He and his colleagues were looking for traces of the powerful 1992 tsunami that struck the coast of the Indonesia island of Flores and other nearby smaller islands.

The tsunami, triggered by a magnitude 7.9 earthquake, sent waves reaching 4 to 27-metres high on the island’s northeastern coast, even destroying a whole village situated on the nearby island Babi. About 2,500 residents and tourists died from the event, with hundreds more injured, and thousands more homeless.

The 1992 Flores tsunami was also one of the first such events documented by an international survey that adhered to internationally accepted post-tsunami assessment standards.  On their field trip Behrens and his colleagues revisited some of sites assessed by the 1992 post-tsunami survey, spoke to eye witnesses, learned about the region’s current mitigation measures, and exchanged latest results from modeling and experimental tsunami research.

While on this field trip, Behrens came across this solitary mangrove, surrounded by what appears to be young mangrove propagules growing out from the water.

By Olivia Trani, EGU Communications Officer


Spalding M, McIvor A, Tonneijck FH, Tol S and van Eijk P (2014) Mangroves for coastal defence. Guidelines for coastal managers & policy makers. Wetlands International and The Nature Conservancy.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at

September GeoRoundUp: the best of the Earth sciences from around the web

September GeoRoundUp: the best of the Earth sciences from around the web

Drawing inspiration from popular stories on our social media channels, major geoscience headlines, as well as unique and quirky research, this monthly column aims to bring you the latest Earth and planetary science news from around the web.

Major stories

This month has been a whirlwind of Earth and space science news; the majority focusing on natural hazards. Powerful cyclones, earthquakes, and tsunamis have received significant coverage from the geoscience media. Quickly recap on an action-packed month with our overview:

On 14 September, Hurricane Florence, made landfall in the mid-Atlantic region of the United States, making first contact near Wrightsville Beach in North Carolina then traveling up the East Coast. By the time Florence had reached the US coastline, the cyclone’s sustained wind speed had dropped considerably, downgrading the hurricane from a category 4 to category 1 storm on the Saffir–Simpson scale.

This designation may sound mild, but as many scientists and journalists have pointed out, sluggish hurricanes are especially dangerous, since they are more likely to dump heavy rainfall over a relatively small surface area compared to faster storms that distribute their rainfall over more territory. This proved to be true for Hurricane Harvey, which dumped more than 150 centimetres of rain onto some areas of Houston, Texas.

Hurricane Florence’s record-breaking rainfall forced more than a million people to evacuate their homes, and experts estimate that the storm inflicted damages worth more than $38 billion (USD). The hurricane also produced very concerning environmental damages. In Wilmington, North Carolina, for instance, the the rainfall flooded a pit of coal ash at a power plant, releasing more than 1,530 cubic metres of ash, with much of it likely ending up in a nearby lake.

Across the planet, just one day following Hurricane Florence’s landfall, Super Typhoon Mangkhut wreaked havoc on southeast Asia, pounding the Mariana Islands, the Philippines, China, Taiwan, and Vietnam with strong wind and rain. Reaching wind speed over 240 kilometres per hour, Mangkhut is the most intense storm of the year so far. The New York Times created an interesting three-dimensional visual of the storm’s intensity, using NASA satellite data.

In addition to unleashing incredibly strong winds, the typhoon’s rainfall also triggered deadly landslides. Just outside of the city Baguio, which recorded more than 75 centimetres of rain, more than 40 gold miners were buried under a landslide that hit their bunkhouse.

Big storms like Hurricane Florence and Typhoon Mangkhut are expected to be more frequent in the future as our climate changes. And this stems from many factors; a recent article from the New York Times explains that, due to climate change, the world’s oceans are warming (fueling more hurricane formation), the atmosphere is holding more moisture (leading to wetter storms), hurricane wind speeds are slowing down (causing more concentrated rainfall), and Earth’s sea levels are rising (increasing the risk of flooding).

Last week, a 7.5-magnitude earthquake struck the Indonesian island of Sulawesi, sending a massive tsunami, with waves up to 6 metres high, into Palu Bay, causing massive devastation in the regional capital Palu and surrounding areas. Officials report that nearly 1,350 people have died from the earthquake and tsunami, and the death toll is expected to rise as rescue workers make their way towards more remote places. Scientists told BBC that “a combination of geography, timing and inadequate warnings meant that what happened in Palu was a worst case scenario.”

Map of the September 28, 2018 Palu, Indonesia Earthquake. Credit: USGS.

Indonesian aid workers and humanitarian relief envoys are currently working to provide supplies and assistance to the affected communities. At the same time, scientists are still puzzling over the tsunami’s strength, which caught many experts by surprise. This is because the earthquake’s behavior isn’t known for generating catastrophic tsunamis.

Powerful tsunamis are typically caused by earthquakes with vertical motion, where part of the seafloor juts forward, disturbing the water column and consequently sending massive waves to the coast. The 2004 Indian Ocean tsunami, for example, was caused by a 9.1 magnitude megathrust earthquake. On the other hand, last week’s quake is known as a ‘strike-slip earthquake,’ where the ground shifts horizontally. This kind of movement doesn’t move ocean water as dramatically.

“Some early calculations suggest a floor displacement of perhaps half a metre. Significant but generally insufficient to produce the waves that were recorded,” reported the BBC.

While it is too early to tell what exactly happened, scientists suspect that a number of factors could have played part in helping the tsunami gather strength. For example, underwater landslides have been known to trigger tsunamis of similar strength. Additionally Palu Bay’s narrow geometry could have amplified the waves’ height.

The underlying factors that contributed to the event will hopefully become more clear as scientists analyse the series of events in more detail.

What you might have missed

This month, the Japanase spacecraft Hayabusa 2 has sent three robots to the rocky surface of an asteroid near Earth, known as Ryugu. The spacecraft had successfully reached the asteroid this June, after travelling for more than three years. The craft first released two small devices, no bigger than frying pans, which tumbled around the rock’s surface and even sent digital postcards and a short video back home. A few days ago, Hayabusa 2 released a third rover, which will use a suite of different scientific instruments to collect data on the asteroid. “Hayabusa2 itself is likely to make the first of three touchdowns on the asteroid to collect samples later this month,” said Science Magazine.

Links we liked

  • StarTrek creators once said that Spock’s fictional home planet Vulcan orbited the 40 Eridani A star. Now scientists have found an exoplanet that fits the description.
  • Rediscovered: the 19th century geological drawings of Orra White Hitchcock, a pioneering female scientific illustrator
  • Researchers discover that kidney stones grow and dissolve much like geological crystals
  • We all know about lava volcanoes, but have you heard of ice volcanoes? New study suggests that cryovolcanoes have likely been erupting for billions of years on Ceres.
  • This new map of Antarctica is like ‘putting on glasses for the first time and seeing 20/20’

The EGU story

Last week, the EGU hosted its first science-policy dinner debate in Brussels. The event, ‘Horizon Geoscience: overcoming societal challenges, creating change’, was organised in collaboration with the European Federation of Geologists (EFG) and brought together geoscientists, policymakers and industry representatives. On the EGU website, we report on the outcome of the discussion and publish the key findings from the Horizon 2020 Geoscience Survey conducted earlier this year.

Panel members during the Horizon Geoscience dinner debate. From Left to right: Jonathan Bamber, John Ludden Lieve Weirinck, Jean-Eric Paquet and Vitor Correia

In the past few weeks, we have also issued three press releases highlighting research published in some of EGU’s open access journals. Follow the links to find out how bombing raids in the Second World War impacted the ionosphere, how glacial geoengineering could help limit sea-level rise, and what the point of no return for climate action might be.

And don’t forget! To stay abreast of all the EGU’s events and activities, from highlighting papers published in our open access journals to providing news relating to EGU’s scientific divisions and meetings, including the General Assembly, subscribe to receive our monthly newsletter.

Geosciences Column: How El Niño triggered Indonesia corals die-off

Geosciences Column: How El Niño triggered Indonesia corals die-off

In the glistening waters of Indonesia, shallow corals – the rain forests of the sea – teem with life.  Or at least they did once. Towards the end of 2015 the corals started to die, leaving a bleak landscape behind. An international team of researchers investigated the causes of the die-off. Their findings, published recently in the EGU’s open access journal, Biogeosciences, are rather surprising.

Globally, corals face tough times. Increasing ocean-water temperatures (driven by a warming climate) are disrupting the symbiotic relationship between corals and the algae that live on (and in) them.

The algae, known as zooxanthellae, provide a food source for corals and give them their colour. Changing water temperatures and/or levels, the presence of contaminants or overexposure to sunlight, put corals under stress, forcing the algae to leave. If that happens, the corals turn white – they become bleached – and are highly susceptible to disease and death.

Triggered by the 2015-2016 El Niño, water temperatures in many coral reef regions across the globe have risen, causing the National Oceanic and Atmospheric Administration (NOAA) to declare the longest and most widespread coral bleaching event in recorded history. Now into its third year, the mass bleaching event is anticipated to cause major coral die-off in Australia’s Great Barrier Reef for the second consecutive year.

The team of researchers studying the Indonesian corals found that, unlike most corals globally, it’s not rising water temperatures which caused the recent die-off, but rather decreasing sea level.

While conducting a census of coral biodiversity in the Bunaken National Park, located in the northwest tip of Sulawesi (Indonesia), in late February 2016, the researchers noticed widespread occurrences of dead massive corals. Similar surveys, carried out in the springs of 2014 and 2015 revealed the corals to be alive and thriving.

In 2016, all the dying corals were found to have a sharp horizontal limit above which dead tissue was present and below which the coral was, seemingly, healthy. Up to 30% of the reef was affected by some degree of die-off.

Bunaken reef flats. (a)Close-up of one Heliopora coerula colony with clear tissue mortality on the upper part of the colonies; (b)same for a Porites lutea colony; (c) reef flat Porites colonies observed at low spring tide in May 2014. Even partially above water a few hours per month in similar conditions, the entire colonies were alive. (d) A living Heliopora coerula (blue coral) community in 2015 in a keep-up position relative to mean low sea level, with almost all the space occupied by corals. In that case, a 15 cm sea level fall will impact most of the reef flat. (e–h) Before–after comparison of coral status for colonies visible in (c). In (e), healthy Poritea lutea (yellow and pink massive corals) reef flat colonies in May 2014, observed at low spring tide. The upper part of colonies is above water, yet healthy; (f) same colonies in February 2016. The white lines visualize tissue mortality limit. Large Porites colonies (P1, P2) at low tide levels in 2014 are affected, while lower colonies (P3) are not. (g) P1 colony in 2014. (h) Viewed from another angle, the P1 colony in February 2016. (i) Reef flat community with scattered Heliopora colonies in February 2016, with tissue mortality and algal turf overgrowth. Taken from E. E. Ampou et al. 2016.

The confinement of the dead tissue to the tops and flanks of the corals, lead the scientists to think that the deaths must be linked to variations in sea level rather than temperature, which would affect the organisms ubiquitously. To confirm the theory the researchers had to establish that there had indeed been fluctuations in sea level across the region between the springs of 2015 and 2016.

To do so they consulted data from regional tide-gauges. Though not located exactly on Bunaken, they provided a good first-order measure of sea levels over the period of time in question. To bolster their results, the team also used sea level height data acquired by satellites, known as altimetry data, which had sampling points just off Bunaken Island. When compared, the sea level data acquired by the tidal gauges and satellites correlated well.

Sea-level data from the Bitung (east North Sulawesi) tide-gauge, referenced against Bako GPS station. On top, sea level anomalies measured by the Bitung tide-gauge station (low-quality data), and overlaid on altimetry ADT anomaly data for the 1993– 2016 period. Note the gaps in the tide-gauge time series. Middle: Bitung tide-gauge sea level variations (high-quality data, shown here from 1986 till early 2015) with daily mean and daily lowest values. Bottom, a close-up for the 2008–2015 period. Taken from E. E. Ampou et al. 2016.

The data showed that prior to the 2015-2016 El Niño, fluctuations in sea levels could be attributed to the normal ebb and flow of the tides. Crucially, between August and September 2015, they also showed a sharp decrease in sea level: in the region of 15cm (compared to the 1993-2016 mean). Though short-lived (probably a few weeks only), the period was long enough that the corals sustain tissue damage due to exposure to excessive UV light and air.

NOAA provides real-time Sea Surface Temperatures which identify areas at risk for coral bleaching. The Bunaken region was only put on alert in June 2016, long after the coral die-off started, therefore supporting the crucial role sea level fall played in coral mortality in Indonesia.

The link between falling sea level and El Niño events is not limited to Indonesia and the 2015-2016 event. When the researchers studied Absolute Dynamic Topography (ADT) data, which provides a measure of how sea level has change from 1992 to 2016, they found sea level falls matched with El Niño years.

The results of the study highlight that while all eyes are focused on the consequences of rising ocean temperatures and levels triggered by El Niño events, falling sea levels (also triggered by El Niño) could be having a, largely unquantified, harmful effect on corals globally.

By Laura Roberts Artal, EGU Communications Officer

References and resources

Ampou, E. E., Johan, O., Menkes, C. E., Niño, F., Birol, F., Ouillon, S., and Andréfouët, S.: Coral mortality induced by the 2015–2016 El-Niño in Indonesia: the effect of rapid sea level fall, Biogeosciences, 14, 817-826, doi:10.5194/bg-14-817-2017, 2017

Varotsos, C. A., Tzanis, C. G., and Sarlis, N. V.: On the progress of the 2015–2016 El Niño event, Atmos. Chem. Phys., 16, 2007-2011, doi:10.5194/acp-16-2007-2016, 2016.

What are El Niño and La Niña? – a video explainer by NOAA

Coral Reef Watch Satellite Monitoring by NOAA

Global sea level time series – global estimates of sea level rise based on measurements from satellite radar altimeters (NOAA/NESDIS/STAR, Laboratory for Satellite Altimetry)

El Niño prolongs longest global coral bleaching event – a NOAA News item

NOAA declares third ever global coral bleaching event – a NOAA active weather alert (Oct. 2015)

The 3rd Global Coral Bleaching Event – 2014/2017 – free resources for media and educators

What is coral bleaching? – an infographic by NOAA
The ENSO (El Niño–Southern Oscillation) Blog by (a NOAA resource)

Geosciences Column: The calamity of eruptions, or an eruption of benefits?

Geosciences Column: The calamity of eruptions, or an eruption of benefits?

So here is a question: why would anyone want to live in the vicinity of an active volcano? The risks are well known, with hazards arising from lava flows, lahars, ash falls, debris avalanches, and pyroclastic density currents, with many often having deadly consequences. But despite the danger, more than half a billion people live in the direct vicinity of volcanoes. Could it be that communities proactively choose to settle in areas surrounding volcanoes; and if so, why? That is the very question research published earlier this year in the EGU open access Journal, Natural Hazards and Earth System Science (NHESS), seeks to address.

The team of scientists, led by Syamsul Bachri, a researcher at the University of Innsbruck, approach the question from a novel angle. Often, when studying hazards and risk management strategies associated with volcanoes, the focus is on the volcano itself, with researchers commonly taking a very scientific approach to the problem. Instead of focusing on how people are able to adapt to living near the constant threat of an erupting volcano, the new study takes a more holistic view: perhaps a volcano presents opportunities as well as hazards, and society and nature are complexly interlinked?

Previous studies of a similar nature found that people often live in hazardous regions due to a lack of hazard knowledge, a lack of alternatives and/or because they are forced to due to a marginalised social status. However, the new study considered a new option: ‘upside risks’, or opportunities, may offset some of the downsides of living in hazardous areas and should be taken into account in disaster risk reduction and management strategies.

In order to fully understand the complex interaction between humans and volcanoes the researchers used an approach which bridges social and natural sciences. They conducted a series of interviews and focus groups with communities living around Mt. Bromo in Java, Indonesia.

(Left) Bromo Volcano and its landforms: (1) Gunung Bromo and its crater; (2) a Strombolian cone, Gunung Batok; (3) complex of rest volcanic cone (G. Kursi); (4) complex of rest volcanic cone (G.Widodaren) and SegaraWedi; (5) Sand of Sea ; (6) Tengger caldera formation (upper and middle slope); (7) foot slope of Tengger caldera (Sukapura Barranco); (8) Sapi Kerep outlet valley (interpretation from SRTM Image and field survey.) (Right) Human–volcano system at Bromo Volcano. (1) Mt. Batok, (2) Bromo Volcano, (3) Mt. Kursi, (a) Ngadas Village, (b) Ranupane Village, (c) Ngadirejo Village, (d) Sumber Village, (e) Ngadisari Village, (f) Wonokitri Village, (g) Tosari Village, (h) Wringinanom Village. From Bachri et al., 2015.

(Left) Bromo Volcano and its landforms: (1) Gunung Bromo and its crater; (2) a Strombolian cone, Gunung Batok; (3) complex of rest volcanic cone (G. Kursi); (4) complex of rest volcanic cone (G.Widodaren) and SegaraWedi; (5) Sand of Sea ; (6) Tengger caldera formation (upper and middle slope); (7) foot slope of Tengger caldera (Sukapura Barranco); (8) Sapi Kerep outlet valley (interpretation from SRTM
Image and field survey.) (Right) Human–volcano system at Bromo Volcano. (1) Mt. Batok, (2) Bromo Volcano, (3) Mt. Kursi, (a) Ngadas Village, (b) Ranupane Village, (c) Ngadirejo Village, (d) Sumber Village, (e) Ngadisari Village, (f) Wonokitri Village, (g) Tosari Village, (h) Wringinanom Village. From Bachri et al., 2015. (Click to enlarge).


The volcano has erupted 56 times since 1804 and continues to be active today. The most recent eruption took place in 2010, and was sustained over a period of nine months. The estimated total economic loss was valued at USD ~15.5, affecting agriculture and the tourism industry, as well as causing significant loss of property. Disruption caused to the electricity supply, transport and water availability is more difficult to quantify. In total, 70,000 people, across 33 villages were affected by the eruption.

The communities living around the volcano are known as the Tenggerese, a Javanese ethnic minority, counting a population of about 600,000. The Tenggerese consider Mt. Bromo a deity and symbol of their culture. At lower altitudes, on the flanks of the volcano, they cultivate the fertile volcanic soils and raise livestock, while at higher altitudes they live as nomadic herds.

Despite the significant disruption caused to the Tenggerese by the 2010 eruption, the interviews conducted by the researchers revealed that the local communities benefited from the main resulting hazards: tephra fall, lahars and landslides. They found that the communities felt the effects of the eruption were negative whilst the eruption was ongoing and for a short period after. However, once the short-term disruption ended, the overall perception was one where the hazards presented opportunity.

Year and duration (days) of Mt. Bromo eruption in a 200-year period (for 1804–2010, CVGHM 2010; and for 2011–2012, Field survey, 2012).

Year and duration (days) of Mt. Bromo eruption in a 200-year period (for 1804–2010, CVGHM 2010; and for 2011–2012, Field
survey, 2012). From Bachri et al., 2015. (Click to enlarge).

Areas covered by volcanic ash and fine rock material could not be planted for two years following the eruption, but areas covered only by fine volcanic ash became more fertile. The Tenggerese farmers referred to this as Berkah Bromo (Bromo’s opportunity), and stated that Mt.Bromo provided benefits for the continuity of their livelihood.

The eruption also caused a number of lahars – volcanic mud flows known locally as lahar hujan – which destroyed some 20 houses. Despite the short-term negative effects of the lahars, agricultural productivity in the affected region was increased and is already being exploited by the local farmers. Bapak Kirno* (the head of Wrininganom village) stated during the interviews:

“Areas which are affected by lahar hujan from Bromo will be more fertile after some period if they are not dominated by sand materials.”

Landslides caused significant disruptions, in particular causing road accessibility problems, but at the same time, transferred fertile materials to new areas, contributing positively to soil quality.

(Top) Tenggerese priests during Dutch East Indies era which lasted from 1800 to 1949. (Bottom left) Tenggerese woman with two children. (Bottom right) Tenggerese priest holding a dedication ceremony of a new build house. (images provided to Wikimedia Commons by the Tropenmuseum, author unknown).

(Top) Tenggerese priests during Dutch East Indies era which lasted from 1800 to 1949. (Bottom left) Tenggerese woman with two children. (Bottom right) Tenggerese priest holding a dedication ceremony of a new build house. (images provided to Wikimedia Commons by the Tropenmuseum, author unknown). Click to englarge.

The research also found that volcanoes are a powerful force in shaping cultural identity. The essence of who the Tenggerese are is intrinsically linked to Mt. Bromo and they have a deep spiritual connection with the volcano. The Tenggerese believe that the attitude they have towards the mountain will play a role in how Mt. Bromo behaves towards them. Bapak Wahyu*, a participant of a focus group discussion, considers that the unusually sever 2010 eruption was a result of the abandonment of old customs. The younger generations used the benefits from plentiful agricultural yields, not to save in the traditional fashion, but to purchase unnecessary consumer gadgets. He argues this left villager’s ill prepared, with insufficient resources to survive the sustained eruption.

The interviews and focus groups allowed Bachri and his team to identify five ways in which the Tenggerese have culturally adapted to living in the shadow of Mt. Bromo and which have also enhanced their life. They have, not only a heightened resilience to hazards, but a greater capacity to recover from them, too. The limited and unique extent of the territory they inhabit gives the Tenggerese a strong local attachment and a deep knowledge of the hazard posed by the volcano. At the same time, their proximity to the volcano instils a sense of social and moral order and allows them to frame and voice dissent in a larger cosmological setting. Finally, volcanic eruptions are often the catalysts for change and this is largely viewed positively by the local inhabitants.

 “I am never scared of Bromo’s eruption because I always believe that this is temporary. Bromo’s eruption always benefit us. We believe that Bromo always gives us what we need to live here,” says Bapak Rudi*, Ngadirejo village official.

*All names used in the study were changed to protect the identity of the informants.

By Laura Roberts Artal, EGU Communications Officer


Bachri, S., Stötter, J., Monreal, M., and Sartohadi, J.: The calamity of eruptions, or an eruption of benefits? Mt. Bromo human–volcano system a case study of an open-risk perception, Nat. Hazards Earth Syst. Sci., 15, 277-290, doi:10.5194/nhess-15-277-2015, 2015.

Tilling, R.I.: Volcano hazard, in: Volcanoes and the Environment, edited by: Mart, J. and Ernst, G., Cambridge University Press, United States of America, 55-90, 2005.