With natural hazard and climate-related disasters on the rise, online tools such as crowdsourced mapping and social media can help people understand and respond to a crisis. They enable people to share their location and contribute information.
But are these tools useful for everyone, or are some people marginalised? It is vital these tools include information provided from all sections of a community at risk.
Current evidence suggests that is not always the case.
Online tools let people help in disasters
Social media played an important role in coordinating response to the 2019 Queensland floods and the 2013 Tasmania bushfires. Community members used Facebook to coordinate sharing of resources such as food and water.
Crowdsourced mapping helped in response to the humanitarian crisis after the 2010 Haiti earthquake. Some of the most useful information came from public contributions.
Twitter provided similar critical insights during Hurricane Irma in South Florida in 2017.
In the rush to develop new disaster mitigation tools, it is important to consider whether they will help or harm the people most vulnerable in a disaster.
Who is vulnerable?
Extreme natural events, such as earthquakes and bushfires, are not considered disasters until vulnerable people are exposed to the hazard.
To determine people’s level of vulnerability we need to know:
- the level of individual and community exposure to a physical threat
- their access to resources that affect their capacity to cope when threats materialise.
Some groups in society will be more vulnerable to disaster than others. This includes people with immobility issues, caring roles, or limited access to resources such as money, information or support networks.
When disaster strikes, the pressure on some groups is often magnified.
The devastating scenes in New Orleans after Hurricane Katrina in 2005 and in Puerto Rico after Hurricane Maria in 2017 revealed the vulnerability of children in such disasters.
Unfortunately, emergency management can exacerbate the vulnerability of marginalised groups. For example, a US study last year showed that in the years after disasters, wealth increased for white people and declined for people of colour. The authors suggest this is linked to inequitable distribution of emergency and redevelopment aid.
We need to ask: do new forms of disaster response help everyone in a community, or do they reproduce existing power imbalances?
Unequal access to digital technologies
These technologies inherently discriminate if access to them discriminates.
Lower digital inclusion is seen in already vulnerable groups, including the unemployed, migrants and the elderly.
Global internet penetration rates show uneven access between economically poorer parts of the world, such as Africa and Asia, and wealthier Western regions.
Representations of communities are skewed on the internet. Particular groups participate with varying degrees on social media and in crowdsourcing activities. For example, some ethnic minorities have poorer internet access than other groups even in the same country.
Research shows participation biases in community mapping activities towards older, more affluent men.
Protect the vulnerable
Persecuted minorities, including LGBTIQ communities and religious minorities, are often more vulnerable in disasters. Digital technologies, which expose people’s identities and fail to protect privacy, might increase that vulnerability.
Unequal participation means those who can participate may become further empowered, with more access to information and resources. As a result, gaps between privileged and marginalised people grow wider.
For example, local Kreyòl-speaking Haitians from poorer neighbourhoods contributed information via SMS for use on crowdsourced maps during the 2010 Haiti earthquake response.
But the information was translated and mapped in English for Western humanitarians. As they didn’t speak English, vulnerable Haitians were further marginalised by being unable to directly use and benefit from maps resulting from their own contributions.
Any power imbalances that come from unequal online participation are pertinent to disaster risk reduction. They can amplify community tensions, social divides and marginalisation, and exacerbate vulnerability and risk.
With greater access to the benefits of online tools, and improved representation of diverse and marginalised people, we can better understand societies and reduce disaster impacts.
We must remain acutely aware of digital divides and participation biases. We must continually consider how these technologies can better include, value and elevate marginalised groups.
Billy Tusker Haworth, Lecturer in GIS and Disaster Management, University of Manchester; Christine Eriksen, Senior Lecturer in Geography and Sustainable Communities, University of Wollongong, and Scott McKinnon, Vice-Chancellor’s Postdoctoral Research Fellow, University of Wollongong
We’re often warned to avoid mosquito bites after major flooding events. With more water around, there are likely to be more mosquitoes.
As flood waters recede around Townsville and clean-up efforts continue, the local population will be faced with this prospect over the coming weeks.
But whether a greater number of mosquitoes is likely to lead to an outbreak of mosquito-borne disease is tricky to predict. It depends on a number of factors, including the fate of other wildlife following a disaster of this kind.
Mozzies need water
Mosquitoes lay their eggs in and around water bodies. In the initial stages, baby mosquitoes (or “wrigglers”) need the water to complete their development. During the warmer months, it doesn’t take much longer than a week before they are grown and fly off looking for blood.
So the more water, the more mosquito eggs are laid, and the more mosquitoes end up buzzing about.
But outbreaks of disease carried by mosquitoes are dependent on more than just their presence. Mosquitoes rarely emerge from wetlands infected with pathogens. They typically need to pick them up from biting local wildlife, such as birds or mammals, before they can spread disease to people.
Mosquitoes and extreme weather events
Historically, major inland flooding events have triggered significant outbreaks of mosquito-borne disease in Australia. These outbreaks have included epidemics of the potentially fatal Murray Valley encephalitis virus. In recent decades, Ross River virus has more commonly been the culprit.
A focal point of the current floods is the Ross River, which runs through Townsville. The Ross River virus was first identified from mosquitoes collected along this waterway. The disease it causes, known as Ross River fever, is diagnosed in around 5,000 Australians every year. The disease isn’t fatal but it can be seriously debilitating.
In recent years, major outbreaks of Ross River virus have occurred throughout the country. Above average rainfall is likely a driving factor as it boosts both the abundance and diversity of local mosquitoes.
Flooding across Victoria over the 2016-2017 summer produced exceptional increases in mosquitoes and resulted in the state’s largest outbreak of Ross River virus. There were almost 1,700 cases of Ross River virus disease reported there in 2017 compared to an average of around 300 cases annually over the previous 20 years.
Explainer: what is Ross River virus?
Despite plagues of mosquitoes taking advantage of flood waters, outbreaks of disease don’t always follow.
Flooding resulting from hurricanes in North America has been associated with increased mosquito populations. After Hurricane Katrina hit Louisiana and Mississippi in 2005, there was no evidence of increased mosquito-borne disease. The impact of wind and rain is likely to have adversely impacted local mosquitoes and wildlife, subsequently reducing disease outbreak risk.
Australian studies suggest there’s not always an association between flooding and Ross River virus outbreaks. Outbreaks can be triggered by flooding, but this is not always the case. Where and when the flooding occurs probably plays a major role in determining the likelihood of an outbreak.
The difficulty in predicting outbreaks of Ross River virus disease is that there can be complex biological, environmental and climatic drivers at work. Conditions may be conducive for large mosquito populations, but if the extreme weather events have displaced (or decimated) local wildlife populations, there may be a decreased chance of outbreak.
This may be why historically significant outbreaks of mosquito-borne disease have occurred in inland regions. Water can persist in these regions for longer than coastal areas. This provides opportunities not only for multiple mosquito generations, but also for increasing populations of water birds. These birds can be important carriers of pathogens such as the Murray Valley encephalitis virus.
In coastal regions like Townsville, where the main concern would be Ross River virus, flood waters may displace the wildlife that carry the virus, such as kangaroos and wallabies. For that reason, the flood waters may actually reduce the initial risk of outbreak.
There is still much to learn about the ecology of wildlife and their role in driving outbreaks of disease. And with a fear of more frequent and severe extreme weather events in the future, it’s an important area of research.
Although it remains difficult to predict the likelihood of a disease outbreak, there are steps that can be taken to avoid mosquito bites. This will be useful even if just to reduce the nuisance of sustaining bites.
Cover up with long-sleeved shirts and long pants for a physical barrier against mosquito bites and use topical insect repellents containing DEET, picaridin, or oil of lemon eucalyptus. Be sure to apply an even coat on all exposed areas of skin for the longest lasting protection.
The devastating Townsville floods have receded but the clean up is being complicated by the appearance of a serious bacterial infection known as melioidosis. One person has died from melioidosis and nine others have been diagnosed with the disease over the past week.
The bacteria that causes the disease, Burkholderia pseudomallei, is a hardy bug that lives around 30cm deep in clay soil. Events that disturb the soil, such as heavy rains and floods, bring B. pseudomallei to the surface, where it can enter the body through through a small break in the skin (that a person may not even be aware of), or by other means.
Melioidosis may cause an ulcer at that site, and from there, spread to multiple sites in the body via the bloodstream. Alternatively, the bacterium can be inhaled, after which it travels to the lungs, and again may spread via the bloodstream. Less commonly, it’s ingested.
What are the symptoms?
Melioidosis can cause a variety of symptoms, but often presents as a non-specific flu-like illness with fever, headache, cough, shortness of breath, disorientation, and pain in the stomach, muscles or joints.
People with underlying conditions that impair their immune system – such as diabetes, chronic kidney or lung disease, and alcohol use disorder – are more likely to become sick from the infection.
The majority of healthy people infected by melioidosis won’t have any symptoms, but just because you’re healthy, doesn’t mean you’re immune: around 20% of people who become acutely ill with melioidosis have no identifiable risk factors.
People typically become sick between one and 21 days after being infected. But in a minority of cases, this incubation period can be much longer, with one case occurring after 62 years.
How does it make you sick?
While most people who are sick with melioidosis will have an acute illness, lasting a short time, a small number can have a grumbling infection persisting for months.
One of the most common manifestations of melioidosis is infection of the lungs (pneumonia), which can occur either via infection through the skin, or inhalation of B. pseudomallei.
The challenges in treating this organism, though, arise from its ability to form large pockets of pus (abscesses) in virtually any part of the body. Abscesses can be harder to treat with antibiotics alone and may also require drainage by a surgeon or radiologist.
How is it treated?
Thankfully, a number of antibiotics can kill B. pseudomallei. Those recovering from the infection will need to take antibiotics for at least three months to cure it completely.
If you think you might have melioidosis, seek medical attention immediately. A prompt clinical assessment will determine the level of care you need, and allow antibiotic therapy to be started in a timely manner.
Your blood and any obviously infected body fluids (sputum, pus, and so on) will also be tested for B. pseudomallei or other pathogens that may be causing the illness.
While cleaning up after these floods, make sure you wear gloves and boots to minimise the risk of infection through breaks in the skin. This especially applies to people at highest risk of developing melioidosis, namely those with diabetes, alcohol use disorder, chronic kidney disease, and lung disease.
Many parts of Queensland have been declared disaster zones and thousands of residents evacuated due to a 1-in-100-year flood. Townsville is at the epicentre of the “unprecedented” monsoonal downpour that brought more than a year’s worth of rain in just a few days, and the emergency is far from over with yet more torrential rain expected.
Such monumental disruption calls for emergency work to safeguard crucial infrastructure such as bridges, dams, motorways, railways, power substations, power lines and telecommunications cables. In turn, that requires accurate, timely mapping of flood waters.
For the first time in Australia, our research team has been monitoring the floods closely using a new technique involving European satellites, which allows us to “see” beneath the cloud cover and map developments on the ground.
Given that the flooding currently covers a 700km stretch of coast from Cairns to Mackay, it would take days to piece together the big picture of the flood using airborne mapping. What’s more, conventional optical imaging satellites are easily “blinded” by cloud cover.
But a radar satellite can fly over the entire state in a matter of
seconds, and an accurate and comprehensive flood map can be produced in less than an hour.
Eyes above the skies
Our new method uses an imaging technology called “synthetic aperture radar” (SAR), which can observe the ground day or night, through cloud cover or smoke. By combining and comparing SAR images, we can determine the progress of an unfolding disaster such as a flood.
In simple terms, if an area is not flooded on the first image but is inundated on the second image, the resulting discrepancy between the two images can help to reveal the flood’s extent and identify the advancing flood front.
To automate this process and make it more accurate, we use two pairs of images: a “pre-event pair” taken before the flood, and a “co-event pair” made up of one image before the flood, and another later image during the flooding.
The European satellites have been operated strategically to collect images globally once every 12 days, making it possible for us to test this new technique in Townsville as soon as flooding occurs.
To monitor the current floods in Townsville, we took the pre-event images on January 6 and January 18, 2019. The co-event pair was collected on January 18 and January 30. These sets of images were then used to generate the accurate and detailed flood map shown below.
The image comparisons can all be done algorithmically, without a human having to scrutinise the images themselves. Then we can just look out for image pairs with significant discrepancies, and then concentrate our attention on those.
Our technique potentially avoids the need to monitor floods from airborne reconnaissance planes – a dangerous or even impossible task amid heavy rains, strong wind, thick cloud and lightning.
This timely flood intelligence from satellites can be used to switch off critical infrastructure such as power substations before flood water reaches them.
A strong low-pressure system has meant severe thunderstorm and hail warnings are in effect for much of the New South Wales South Coast. At the same time, very dry conditions, strong winds and high temperatures are fuelling dozens of bushfires across Queensland.
The two events are actually influencing each other. As the low-pressure system moves over the Greater Sydney area, a connected wind change is pushing warm air (and stronger winds) to Queensland, worsening the fire conditions.
These lows over NSW are the kind we might see a couple of times a year – they’re not just regular weather systems, but neither are they massively out of the ordinary.
However, when combined with the current record-breaking heat in Queensland, the extra wind is creating exceptionally dangerous fire conditions. Queensland’s emergency services minister, Craig Crawford, has warned Queenslanders:
We are expecting a firestorm. We are expecting it to be so severe that it won’t even be safe on the beach […] The only thing to do is to go now.
Conditions in Queensland
At least 80 bushfires were burning in Queensland on Wednesday, with more than a dozen fire warnings issued to communities near the Deepwater blaze. Queensland Police Deputy Commissioner Bob Gee said that “people will burn to death” unless they evacuate the area.
These fires have come during a record-breaking heatwave. On Tuesday Cooktown recorded 43.9℃, beating the previous November high set 70 years ago by more than two degrees. Cairns has broken its November heatwave record by five whole degrees.
Grasslands and forests are very dry after very little rain over the past two years. Adding to these conditions are strong winds, which make the fires hotter, faster and harder to predict. This is where the storm conditions in NSW come in: they are affecting air movements across both states.
NSW low is driving winds over Queensland
A large low-pressure system, currently over the Hunter Valley area, is causing the NSW storms. As it moves, it’s pushing a mass of warm air ahead of it, bringing both higher temperatures and stronger winds across the Queensland border.
Once the low-pressure system moves across the Hunter area to the Tasman Sea east of Sydney, it will drag what we call a “wind change” across Queensland. This will increase wind speeds through Queensland and temperatures, making the fire situation even worse.
This is why emergency services are keeping watch for “fire tornado” conditions. When very hot air from large fires rises rapidly into a turbulent atmosphere, it can create fire storms – thunderstorms containing lightning or burning embers. Strong wind changes can also mean fire tornadoes form, sucking up burning material. Both of these events spread fires quickly and unpredictably.
What does this mean for the drought
Unfortunately, it’s not likely the heavy rains over NSW will have a long-term effect on the drought gripping much of the state. While very heavy rains have fallen over 24 hours, the drought conditions have persisted for years.
The wet weather may bring some temporary relief, but NSW will need much more rain over a longer period to truly alleviate the drought.
In the meantime, the Bureau of Meteorology will be monitoring the Queensland situation closely. You can check weather warnings for your area on the bureau’s website.
California is burning, again. Dozens of peoples have been killed and thousands of buildings destroyed in several fires, the most destructive in the state’s history.
Why do wildfires seem to be escalating? Despite president Donald Trump’s tweet that the California fires were caused by “gross mismanagement” of forests, the answer is more complex, nuanced, and alarming.
What caused the California fires?
The current California fires reflect a complex mix of climate, social, and ecological factors. Fuels across California are currently highly combustible due to a prolonged drought and associated low humidity and high air temperatures. Indeed, it is so dry fires burn freely through the night. Such extreme weather conditions have the fingerprints of climate change.
Compounding the desiccated fuels are the seasonally predictable strong desert winds (the Diablo and Santa Ana) that help fires spread rapidly towards the coast.
Low density housing embedded in flammable vegetation has created an ideal fuel mix for these destructive fires. Having people scattered across the landscape ensures a steady source of ignitions, ranging from powerline faults to carelessness and arson, making fires a near certainty when dangerous weather conditions arise.
Decades of wildfire suppression have created fuel loads that sustain intense fires. That these fuels are burning in late autumn is even more alarming. Under severe fire weather forest fires can engulf entire communities, with fires spreading from house to house, and human communities turning into a unique wildfire “fuel”. Suburbs can burn at the rate of one house per minute .
The standard response to wildfires is to fight them aggressively, using a military-style approach involving small armies of fire fighters combined with aircraft that spread fire retardant and saturate fire-fronts with water. Such approaches are extraordinarily costly. Annual spending on fire fighting has been steadily rising. In the US, annual fire-fighting costs now exceed several billion dollars, with individual fire campaigns costing ten to over a hundred million dollars.
Although industrial fire-fighting approaches currently enjoy political and social support, the strategy is economically unsustainable. And they are impotent in the face of climate change driven fire disasters such as those currently occurring in California.
A human disaster
Across the fire science community there is growing recognition this “total war” on fire approach has failed. The key to sustainable co-existence with flammable landscapes is instead managing fuels around settlements, and stopping wildfires from starting in the first place.
Spain and Portugal are good examples of why this is so important. In these Mediterranean lands, humans have sustainably co-existed with flammable landscapes for thousands of year. However, the near ubiquitous depopulation of rural lands following the second world war has led to the proliferation of flammable vegetation that had previously been held in check by intensive small-scale subsistence agriculture.
With the loss of this traditional agriculture Mediterranean countries are now experiencing regular fire disasters (such as the 2018 Greek fires and the 2017 Portuguese and Spanish fires). These are equivalent to fires in more recently settled flammable landscapes in the Americas and Australia.
This seems to be the story in most flammable landscapes on earth: the removal of traditional landscape management by colonisation and globalisation has combined with climate change to turn these landscapes into tinderboxes.
But just as it is unrealistic for Australia to faithfully restore Indigenous fire management practices, expecting a return to historical practices in the Mediterranean is not realistic. There is little economic or social reason for people to return to traditional rural lifestyles, and the gravitational pull of the social and economic advantages in urban areas is too great to stem rural depopulation.
Living with fire
But we can adapt traditional practices to help us live with fire. In the Mediterranean, people are already experimenting with different ways to manage landscapes, such as managing forests for cork and bioenergy, combined with prescribed burning and grazing.
This can create picturesque landscapes that are fire-resistant and easy to defend. Similarly, in Australia, the Victorian government has created parkland-like green fire breaks that were used for back burning operations to protect communities during 2009 Black Saturday wildfires.
The Hobart City Council is planning to use similar fire breaks to protect its outer suburbs with dense bushland. Such management could be used on a larger scale to substantially reduce fire risk. The challenge for landscape fuel management is providing financial and regulatory incentives for citizens and local communities to reduce fuel.
Currently, no society is sustainably co-existing with wildfire. Globally, the situation will worsen under a rapidly-warming climate with ballooning firefighting costs, and huge loss of life and destruction of property. This is the bitter lesson of the Californian fires.
Sulawesi’s recent tsunami is a striking reminder of the devastating, deadly effects that the sudden arrival of a large volume of water can have.
Published today, our new research shows what might happen if a tsunami hit Sydney Harbour. A large tsunami could cause significant flooding in Manly. Even very small waves might result in dangerous currents in the entrance of the Harbour and in narrow channels such as at the Spit Bridge.
Beyond Sydney, large areas of the east coast of Australia would also be affected.
Making waves: the tsunami risk in Australia
Our study considered a range of tsunamis, with heights ranging from just 5cm to nearly 1.5m when measured outside the Heads of Sydney Harbour. These wave heights sound small, but because the wavelengths of tsunami are so long (tens to hundreds of kilometres), these waves contain a very large mass of water and can be incredibly powerful and destructive. Wave heights also increase as the tsunami encounters shallower water.
How a tsunami might happen
Most tsunamis are caused by earthquakes at sea, where a shift in the sea floor creates the sudden movement of a large volume of water.
Our study approach involved modelling the likely effects of different-sized tsunamis generated by earthquakes on the New Hebrides trench to the northeast (in line with the Vanuatu islands) and the Puysegur trench (south of New Zealand).
For each event we assigned Average Recurrence Intervals (ARI), which provide an average indication of how often tsunamis of different sizes are likely to occur.
The tsunamis we studied range from an ARI of 25 years to 4,700 years. The tsunami with an ARI of 4,700 had a wave height of 1.4m outside the Heads and is the largest tsunami we could reasonably expect in Sydney Harbour. An event with an ARI of 4,700 can also be considered as an event with a 1.5% chance of occurring over a 70-year lifetime.
What would the tsunami look like?
The tsunamis we’d expect to see in Sydney Harbour would be a sequence of waves with about 15-40 minutes on average between each peak. Some waves might break, and others might appear as a rapid rising and falling of the water level.
The highest water levels would depend on the tide and the size of the event – the largest events could raise the water level up to several metres higher than the predicted tide levels.
The visualisation below represents a tsunami in a fictional location, and shows the rise and fall of water levels (with time sped up).
What area is at highest risk?
A tsunami is not just one single wave, but generally a sequence of waves, lasting hours to days. Within the Harbour, larger waves are most likely to breach land, and high tide increases the risk.
The narrow part of Manly – where The Corso part-pedestrian mall is located – is one of the most exposed locations. The largest tsunamis we could expect may flood the entire stretch of The Corso between the open ocean and the Harbour.
The low-lying bays on the southern side of the Harbour could also be affected. A tsunami large enough to flood right across Manly is estimated to have a minimum ARI of 550 years, or at most a 12% chance of occurring over an average lifetime.
Examining these worst-case scenarios over time shows how this flooding across Manly may occur from both the ocean side and the harbour side, isolating North Head.
How fast would a tsunami move?
Even though the smaller tsunamis may not flood the land, they could be very destructive within the Harbour itself. Our modelling shows the current speeds caused by smaller tsunamis have the potential to be both damaging and dangerous.
The map below shows the maximum tsunami current speeds that could occur within the Harbour for the largest event we could reasonably expect.
Areas exposed to the open ocean and locations with a narrow, shallow channel – such as those near the Spit Bridge or Anzac Bridge – would experience the fastest current speeds. A closer look at the area around the Spit Bridge, shows how even smaller tsunamis could cause high current speeds.
The animation below shows a comparison between the current speeds experienced during a regular spring high tide and those that may occur if a tsunami generated by a 8.5 magnitude earthquake on the New Hebrides trench coincided with a spring high tide. A tsunami of this size (0.5m when outside the Harbour) has been estimated to occur once, on average, every 110 years (a 47% chance of occurring over a lifetime).
This video below shows similar current speeds (7m/s based on video analysis) when the Japanese tsunami of 2011 arrived in the marina in Santa Cruz, California, and caused US$28 million of damage.
Historical records show us what happened when a tsunami generated by an earthquake off Chile reached Sydney Harbour in 1960. We didn’t have any instruments measuring current speeds then, but we have witness accounts and we know that many ships were ripped from their moorings.
A whirlpool and significant erosion was also reported in the Spit Bridge area. Photographs from the time show just how much sand was washed away at Clontarf Beach.
How to stay safe
A large tsunami affecting Australia is unlikely but possible. Remember that tsunamis are a sequence of waves that may occur over hours to days, and the biggest wave in the sequence could occur at any time.
The Joint Australian Tsunami Warning Centre (JATWC), jointly operated by Geoscience Australian and the Bureau of Meteorology, provides a tsunami warning system for all of Australia.
The bathymetry compilations used by this research are publicly available and can be viewed as a publication with links for free download.
The magnitude 7.5 earthquake, and subsequent tsunami, that struck Indonesia days ago has resulted in at least 1,200 deaths.
Authorities are still gauging the extent of the damage, but it’s clear the earthquake and tsunami had a devastating effect on the Sulawesi region, particularly the city of Palu.
It’s not the first time earthquakes have caused mass destruction and death in Indonesia. The tsunamis that follow are particularly damaging. But why?
A combination of plate tectonic in the region, the shape of the coastline, vulnerable communities and a less-than-robust early warning system all combine to make Indonesian tsunamis especially dangerous.
Indonesia covers many complex tectonic environments. Many details of these are still poorly understood, which hampers our ability to predict earthquake and tsunami risks.
The biggest earthquakes on Earth are “subduction zone” earthquakes, which occur where two tectonic plates meet.
In December 2004 and March 2005, there were a pair of subduction zone earthquakes along the Sunda Trench offshore of the west coast of Sumatra. In particular, the magnitude-9.1 quake in December 2004 generated a devastating tsunami that killed almost a quarter of a million people in countries and islands surrounding the Indian Ocean.
But only looking out for these kinds of earthquakes can blind us to other dangers. Eastern Indonesia has many small microplates, which are jostled around by the motion of the large Australia, Sunda, Pacific and Philippine Sea plates.
The September quake was caused by what’s called a “strike-slip” fault in the interior of one of these small plates. It is rare – although not unknown – for these kinds of quakes to create tsunamis.
The fault systems are rather large, and through erosion processes have created broad river valleys and estuaries. The valley of the Palu river, and its estuary in which the regional capital Palu is located, have been formed by this complex fault system. Studies of prehistoric earthquakes along this fault system suggests this fault produces magnitude 7-8 earthquakes roughly every 700 years.
The sea floor shapes the wave
Another important factor for tsunamis is the depth and shape of the sea floor. This determines the speed of the initial waves. Strong subduction zone earthquakes on the ocean floor can cause the entire ocean water column to lift, then plunge back down. As the water has momentum, it may fall below sea level and create strong oscillations.
The bulge of water moving outward from the centre of a earthquake maybe of limited height (rarely much more than a metre), but the mass of water is extremely large (depending on the surface area moved by the earthquake).
Tsunami waves can travel very fast, reaching the speed of a jet. In water 2km deep they can travel at 700km per hour, and over very deep ocean can hit 1,000km per hour.
When the wave approaches the shallower coast, its speed decreases and the height increases. A tsunami may be 1m high in the open ocean, but rise to 5-10m at the coast. If the approach to the shoreline is steep, this effect is exaggerated and can create waves tens of metres high.
Despite the fact that the waves slow down near the coast, their immense starting speeds mean flat areas can be inundated for kilometres inland. The ocean floor topography affects the speed of tsunami waves, meaning they move faster over deep areas and slow down over submarine banks. Very steep land, above or below water, can even bend and reflect waves.
The coastlines of the Indonesian archipelago are accentuated, in particular in the eastern part and especially at Sulawesi. Palu has a narrow, deep and long bay: perfectly designed to make tsunamis more intense, and more deadly.
This complex configuration also makes it very difficult to model potential tsunamis, so it’s hard to issue timely and accurate warnings to people who may be affected.
Get to high ground
The safest and simplest advice for people in coastal areas that have been affected by an earthquake is to get to higher ground immediately, and stay there for a couple of hours. In reality, this is a rather complex problem.
Hawaii and Japan have sophisticated and efficient early warning systems. Replicating these in Indonesia is challenging, given the lack of communications infrastructure and the wide variety of languages spoken throughout the vast island archipelago.
After the 2004 Indian Ocean disaster, international efforts were made to improve tsunami warning networks in the region. Today, Indonesia’s tsunami warning system operates a network of 134 tidal gauge stations, 22 buoys connected to seafloor sensors to transmit advance warnings, land-based seismographs, sirens in about 55 locations, and a system to disseminate warnings by text message.
However, financing and supporting the early warning system in the long term is a considerable problem. The buoys alone cost around US$250,000 each to install and US$50,000 annually for maintenance.
The three major Indonesian agencies for responsible for earthquake and tsunami disaster mitigation have suffered from budget cuts and internal struggles to define roles and responsibilities.
Lastly, the Palu tsunami event has highlighted that our current tsunami models are insufficient. They do not properly consider multiple earthquake events, or the underwater landslides potentially caused by such quakes.
No early warning system can prevent strong earthquakes. Tsunamis, and the resulting infrastructure damage and fatalities, will most certainly occur in the future. But with a well-developed and reliable early warning system, and better communication and public awareness, we can minimise the tragic consequences.
With earthquakes that occur very close to the beach – often the case in Indonesia – even an ideal system could not disseminate the necessary information quickly enough. Indonesia’s geography and vulnerable coastal settlements makes tsunamis more dangerous, so we need more and concerted efforts to create earthquake and tsunami resilient communities.