Category Archives: Tsunami

The Problem of Real and Unreal Tsunamis

Indonesia was beset by disasters in 2018, including two high casualty local tsunamis: in coastal western Sulawesi – impacting the city of Palu, on September 28, and around the Sunda Strait, between Java and Sumatra, on December 22. These events may have appeared unusual, but the great subduction zone tsunamis, like those in the Indian Ocean in 2004 and Japan in 2011, have reset our imagination. Before 2004, forty years had passed without any transoceanic tsunamis. Overall, local tsunamis are more common, presenting many challenges in how they can be anticipated.

The Palu tsunami reminds us how “strike-slip” faults, involving only horizontal displacement can still generate tsunamis, first as a result of vertical displacement at “jogs”, where the fault rupture jumps alignment, as well as from triggered submarine landslides. It seems both factors were important in driving the Sulawesi tsunami that became amplified to more than four meters (13 feet) in the funnel-shaped Palu embayment.

The December 22 Sunda Strait tsunami was caused by a submarine landslide on the erupting Anak Krakatoa volcano and arrived without warning, in the dark of mid-evening. More than 400 people drowned mainly around a series of beach resorts in Banten and Lampung provinces, although water levels in the tsunami only reached a meter or two above sea level. An audience of 200 enjoying a concert at the Tanjung Lesung Beach Resort, staged directly on the beach by Indonesian rock band Seventeen were caught unaware. 29 concertgoers were killed together with four people associated with the band.

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De-risking the City

I am in Wellington, New Zealand, looking out from a rainy hotel window high over the city, admiring the older wooden houses on the forested slopes. Below there are four to eight story office and retail buildings, a number of which are shrouded in scaffolding, still repairing damage from the 2016 Kaikoura earthquake. The earthquake epicenter was some distance from the city, but the pattern of fault ruptures propelled long period ground shaking into the heart of Wellington.

In 1848, only eight years after the city was founded, a Mw7.5 earthquake on the far side of Cook Strait, shattered the town’s brick buildings. The Lieutenant Governor, Edward Eyre, forgetting his official role as colonial booster, declared the “… town of Wellington is in ruins … Terror and despair reign everywhere. Ships now in port … (are) crowded to excess with colonists abandoning the country.” However, the tremors declined, and the town survived.

Many ordinary houses were rebuilt using wood instead of brick. As a result, they suffered far less damage from a larger and closer Mw8.2 earthquake in 1855, that struck at the end of a two-day public holiday to celebrate the fifteenth anniversary of the city’s formation. This ruined all the remaining brick and stone commercial buildings including churches, barracks, the jail, and the colonial hospital. However, the earthquake delivered a tectonic bounty, raising the city by one to two meters (3.2 to 6.5 feet), turning the harbor into new land for development.

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The Lessons From “Last Year’s” Catastrophes

Catastrophe modeling remains work in progress. With each upgrade we aim to build a better model, employing expanded data sets for hazard calibration, longer simulation runs, more detailed exposure data, and higher resolution digital terrain models (DTMs).

Yet the principal way that the catastrophe model “learns” still comes from the experience of actual disasters. What elements, or impacts, were previously not fully appreciated? What loss pattern is new? How do actual claims relate to the severity of the hazard, or change with time through shifts in the claiming process?

After a particularly catastrophic season we give presentations around ”the lessons from last year’s catastrophes.” We should make it a practice, a few years later, to recount how those lessons became implemented in the models.

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How to Maintain Awareness of Tsunami Risk

Today is World Tsunami Awareness Day — designated by the United Nations General Assembly, and according to the United Nations Office for Disaster Risk Reduction (UNISDR), on average, tsunami events have a higher mortality rate than any other hazard. Over the past 20 years (1998-2017) tsunamis have claimed more than 250,000 lives and are also attributable for US$280 billion of the US$661 billion of total recorded economic losses for earthquakes and tsunamis. Between 1978-1997, tsunamis claimed 998 lives, and US$2.7 billion in losses. Overall, tsunamis are rare, but as the UN points out, when they occur they are deadly and hugely damaging. This infrequency makes building awareness and preparedness more of a challenge.

The UN has promoted World Tsunami Awareness Day since 2015, and the UN Secretary-General’s Special Representative for Disaster Risk Reduction, Mami Mizutori, stated that “…it is an occasion to promote greater understanding of tsunami risk to avoid future loss of life. This year we also want to bring attention to the economic losses tsunamis can inflict as a result of damage to critical infrastructure located along vulnerable, densely populated coastlines.”

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Ultra-liquefaction Changes Everything

It turns out the biggest killer in the Palu earthquake on the island of Sulawesi, Indonesia, may not have been the tsunami after all — but liquefaction. Two thousand victims of the earthquake and tsunami are confirmed but 5,000 people remain missing, many of them presumed swallowed up in extraordinary ground deformation and mudflows, which took off when the underlying solid ground liquefied. Some buildings were transported hundreds of meters, others were ripped apart, many collapsed into fragments that then became absorbed into the mud. Media reports state that in Balaroa, just a few kilometers from Palu City, many of the 1,747 houses in the village appear to have sunk into the earth. In Petobo, a village to the east of Palu, many of the village’s 744 houses have disappeared.

What we have witnessed at Palu merits the term “ultra-liquefaction”, as witnessed in the 2011 Christchurch, New Zealand earthquake when perhaps half the total insurance loss costs were a consequence of liquefaction. For Christchurch, in the eastern suburbs it was single storey houses, ripped apart by the ground movements. In the Central Business District (CBD), many mid-rise buildings had to be demolished because underlying liquefaction had led to one corner of the structure sinking by ten or twenty centimeters (four to eight inches).

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The Tragedy at Palu

A version of this article was originally published in Insurance Day

The Mw7.5 earthquake in Sulawesi, Indonesia on September 28 reminds us that fourteen years after the terrible Indian Ocean tsunami, and despite significant investment in systems intended to provide tsunami warnings, the risk to life and property is not going away. To understand why the destruction and loss of life in the city of Palu, with a population of 350,000, is so great (1,300 and rising) we need to understand why this location has proved such a nexus of vulnerabilities.

First, Palu is located less than one degree south of the equator. That means it is in the “shadow zone” for tropical cyclones. In most of the world’s oceans, no tropical cyclone can exist within ten degrees of the equator, although in the western Pacific the typhoon exclusion zone can narrow down to six to eight degrees from the equator. The lack of Coriolis force at the equator prevents a collection of thunderstorms gaining a structured rotation (and tropical cyclones spin in opposite directions in the northern and southern hemispheres).

The lack of tropical cyclones means there are no significant storm surges, or even much in the way of significant wind-driven waves, and as a result people build their houses right down to sea level. This means, in comparison even with a coastal city in Philippines or China, there were many more seafront buildings exposed to a tsunami that reached no more than three to five meters above sea level.

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Sulawesi Earthquake and Tsunami: The Deadliest Earthquake of 2018

The earthquake and subsequent tsunami that struck the Indonesian island of Sulawesi on Friday, September 28, has already claimed the sinister accolade of being the deadliest earthquake in the world this year.

According to local authorities, there have so far been 1,374 reported fatalities, but this figure is set to rise as rescue efforts spread out from the main cities. At this stage, thousands of people are believed to still be trapped under the rubble of collapsed buildings, and at least 60,000 people are displaced with limited food and water supplies.

The 7.5 magnitude earthquake struck the island of Sulawesi on Friday, September 28, approximately 48 miles (78 kilometers) north of Palu, a coastal city with around 330,000 residents. The earthquake triggered a ten foot (three meter) high tsunami, that impacted the coastal areas of western Central Sulawesi, including Palu City and Donggala, a regency with a population of around 275,000.

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The Ever-present Threat of Tsunami: Are We Prepared?

Last week’s Mw8.3 earthquake offshore the Coquimbo region of central Chile served as a reminder that many coastal regions are exposed to earthquake and subsequent tsunami hazard.

While the extent of damage and loss of life from the recent Chile earthquake and tsunami continues to emerge and is tragic in itself, it is safe to say that things could have been much worse. After all, this is the same subduction zone that produced the 1960 Valdivia earthquake (or “Great Chilean earthquake”) 320 miles further to the south—the most powerful earthquake in recorded history.

The 1960 Valdivia earthquake had a magnitude of Mw9.6 and triggered a localized tsunami that battered the Chilean coast with waves in excess of 20 meters as well as far-field tsunami around the Pacific Ocean. Many events of M8.5+ produce tsunami that are truly global in nature and waves of several meters height can even reach coast lines more than 10,000 kilometers away from the event source, highlighting the need for international tsunami warning systems and awareness of population, city planners, and engineers in coastal areas.

 Coastlines At Risk of Tsunami

Tsunami and their deadly consequences have been with us since the beginning of mankind. What’s new, however, is the increasing awareness of the economic and insured losses that tsunami can cause. There are several mega cities in developed and emerging nations that are in the path of a future mega-tsunami, as reported by Dr. Robert Muir-Wood in his report Coastlines at Risk of Giant Earthquakes & Their Mega-Tsunami.

The 2011 earthquake and tsunami off the Pacific coast of Tohoku, Japan acted as a wake-up call to the insurance industry moving tsunami out of its quasi-niche status. With more than 15,000 lives lost, more than USD 300 billion in economic losses, and roughly USD 40 billion in insured losses, clients wanted to know where other similar high magnitude earthquakes and subsequent tsunami could occur, and what they would look like.

In response, RMS studied a multitude of high magnitude (Mw8.9-Mw9.6) event sources around the world and modeled the potential resulting tsunami scenarios. The scenarios are included in the RMS® Global Tsunami Scenario Catalog and include both historical and potential high-magnitude tsunami events that can be used to identify loss accumulations and guide underwriting decisions.

For example, below is an example output, showing the potential impact of a recurrence of the 1877 Chile Mw9.1 Earthquake (Fig 1a) and the impact of a potential future M9 scenario (Fig 1b) stemming from the Nankai Trough on the coast of Toyohashi, Japan.

Fig 1a: Re-simulation of the 1877 Chile Mw9.1 Earthquake. Coquimbo area shown. The inundation from this event would impact the entire Chilean coastline and exceed 9 meters inundation depth (further to the North). Fig 1b: M9 scenario originating on the Nankai Trough south of Japan, impacting the city of Toyohashi (population ~376 thousand), with inundation going far inland and exceeding 6 meters in height.

With rapid advances in science and engineering enabling a deeper understanding of tsunami risk, the insurance industry, city planners and local communities can better prepare for devastating tsunami, implementing appropriate risk mitigation strategies to reduce fatalities and the financial shocks that could be triggered by the next “big one.”

The 1960 Tele-tsunami: Don’t Forget the Far Field

On May 22, 1960 the most powerful earthquake ever recorded struck approximately 100 miles off the coast of southern Chile. The 9.5 Mw event released the energy equivalent to 2.67 gigatones of TNT (178,000 times the energy yielded from the atomic bomb dropped on Hiroshima) leading to extreme ground shaking in cities such as Valdivia and Puerto Montt, triggering landslides and rockfalls in the Andes as well as resulting in a Pacific basin wide tsunami. In Chile, 58,622 houses were completely destroyed with damages totalling $550 million (~$4 billion today adjusted for inflation).

However, the effects in the far field were also significant. While the majority of the damage and approximately 1,380 fatalities occurred in close proximity to the earthquake, a proportion of the tsunami death toll and damage occurred over 5,000 miles away from the epicentre and reached as far away as Japan and the Philippines.

Such tsunamis with the potential to cause damage and fatalities at locations distant from their source are known as tele-tsunamis or far-field tsunamis and require a large magnitude earthquake (>7.5) on a subduction zone to be triggered. Recent events, such as the 2011 Tohoku and 2010 Maule earthquakes, demonstrated that even if these criteria are met, the effects of any resulting tsunami may not be felt significantly beyond the immediate coastline. As such, it can be easy to forget the risks at potential far field sites. However, the 55th anniversary of the 1960 Chilean earthquake and tsunami provides a useful reminder that megathrust earthquakes can have far reaching consequences.

Across the Pacific, the 1960 tsunami caused 61 deaths and $75 million damage (~$600 million today) in Hawaii, 138 deaths and $50 million damage (~$400 million today) in Japan, and left 32 dead or missing in the Philippines.

Hilo Bay, on the big island of Hawaii, was particularly hard hit with wave heights reaching 35 feet (~11 meters), compared to only 3-17 feet or 1-5 meters elsewhere in Hawaii. Approximately 540 homes and businesses were destroyed or severely damaged, wiping out much of downtown Hilo.

Hilo aftermath copy   hilo tsunami copy
                          Aftermath of the event in Hilo (USGS)                                               Inundation extent of the 1960 tsunami in Hilo (USGS)

Despite an official warning from the U.S. Coast and Geodetic Survey and the sounding of coastal sirens, 61 people in Hilo died as a result of the tsunami and an additional 282 were badly injured. The majority of these casualties occurred because people did not evacuate, either due to misunderstanding or not taking the warnings seriously. Many remained in the Waiakea peninsula area, which was perceived to be safe due to the minimal damage experienced there during the event triggered by the 1946 Aleutian Islands earthquake.

Others initially evacuated to higher ground but returned before the event had finished. A series of waves is a common feature of far field tsunamis, with the first wave typically not being the largest. This was the case with the 1960 event with a series of 8 waves striking Hawaii. Thethird of these was most damaging, killing many of those who returned prematurely.

These avoidable casualties highlight the need for adequate tsunami mitigation measures, including education to ensure that people understand the warnings and the correct actions to take in the event of a tsunami. This is particularly important in areas exposed to far field tsunami hazard, where people may be less aware of the risk and there is enough time to evacuate. The introduction of a Pacific Tsunami Warning System in 1968 as a consequence of the event was a big step forward in improving such measures, the presence of which would no doubt substantially reduce the death toll were the event to reoccur today.

Mitigation efforts can also be supported by tools like the RMS Global Tsunami Scenario Catalog, which provides information on the inundation extent and maximum inundation depth for numerous potential tsunami scenarios around the globe. This can be used to identify areas at risk to far-field tsunami events, including those with no historical precedent, enabling the quantification of exposures likely to be worst impacted by such events.

A Decade Later – Reconsidering The Indian Ocean Earthquake and Tsunami

This December marks the 10-year anniversary of the Indian Ocean earthquake and tsunami, a disaster that killed more than 230,000 people in 14 countries. The disaster hit Thailand and Indonesia especially hard and is considered one of the ten worst earthquakes in recorded history based on damages.

Click here for full size image

In advance of the anniversary on December 26, 2014, Dr. Robert Muir-Wood, RMS chief research officer, and Dr. Patricia Grossi, RMS senior director of global earthquake modeling, hosted their second Reddit Science AMA (Ask Me Anything). Back in October, Muir-Wood and Grossi hosted another AMA on the 25th anniversary of the Loma Prieta earthquake in the San Francisco Bay Area.

The latest Reddit thread generated almost 300 comments. Muir-Wood and Grossi discussed topics including: early warning systems for disasters like tsunamis, what variables are considered in catastrophe models, and if better building design can protect against natural disasters – particularly tsunamis. Highlights of the chat follow:

What kind of structural elements or configurations are best to combat or defend against these disasters?

Muir-Wood: There have been research studies on buildings best able to survive tsunamis. The key is to make them strong (from well engineered reinforced concrete) but with ground floor walls running parallel with the shoreline that are weak, so that the walls can be overwhelmed without threatening the whole building.

The 2004 Indian Ocean tsunami took a lot people by surprise due to the lack of a tsunami warning system even though there was a gap between the earthquake and the tsunami. If there was a tsunami warning system in place at the time would that have decreased the death toll by a lot, or not make too much of a difference considering how strong the tsunami was.

Grossi: Early warning systems are excellent tools for reducing the loss of life during an earthquake-induced tsunami event. But education is one of the easiest ways to reduce tsunami life loss. Such education needs to include knowledge of the cause of a tsunami and its association with the largest earthquakes to help individuals understand how their own observations can help them take appropriate action (e.g., seeing the water recede from the coastline). In essence, official warning systems can provide only part of the solution, as information can never be effectively disseminated to everyone along a coastline. With only 10 to 30 minutes warning in the nearfield of major tsunamis, it is imperative that people are taught to take their own action rather than wait for official instruction.

Show me the coolest tsunami video.

Muir-Wood: There are amazing videos of the Japan 2011 tsunami. I wouldn’t pick just one of them – but recommend you watch quite a few – because they are interestingly different. The most amazing feature of the tsunami is the way the water can continue to rise and rise, for five or ten minutes, apparently without end. And then how the people watching the tsunami, climb to higher locations and then realize that if it keeps rising there will be nowhere for them to go.

Was there anything we missed you wanted to discuss? Please let us know in the comments.