Risk to our Cities from Catastrophic Volcanic Eruption
The small but persistent eruption of Raung Volcano, Java, had severe economic consequences throughout July and August 2015, impacting the Balinese travel
industry and disrupting the international movement of goods and services. The highest eruption plumes reported by the Darwin Volcanic Ash Advisory Centre were only between 4.3 and 5.2 km (22-28 July), but ash drifting in various directions for distances of over 400km resulted in the cancellation of numerous flights.
The potential for ongoing impacts and disruptions from volcanic eruptions sets these apart from other natural hazard events, such as earthquakes or floods, where recovery may often begin soon after the hazard peak. Volcanic eruptions are also often characterised by multiple hazards occurring at various scales. Intense localised hazards such as pyroclastic flows, lava flows and lahars (volcanic mud flows) may devastate areas near to source and along the volcano’s drainage networks. In extreme cases, tsunami may originate from the collapse of an edifice or eruption of material into an ocean or lake.
However, the most common wide-reaching hazard is that of tephra (volcanic ash) fall. Even trace amounts of ash in the atmosphere are enough to restrict flight paths and close airports. Road and rail transport may be affected with less than 1 mm of tephra, which obscures road markings and reduces traction. Human respiratory health is adversely affected and eyes and throats can become irritated. 10 cm of ash can weigh over 150 kg per square meter, with rain further increasing this weight. Accumulation of a few centimetres can lead to building collapse with smaller thicknesses damaging gutters, carports, air conditioning units and furnishings. In large eruptions, climate may be affected for several seasons, primarily due to the injection of ash particles into the troposphere and stratosphere, decreasing the amount of sunlight reaching the earth’s surface.
Agriculture is particularly vulnerable to the physical and chemical effects of tephra fall, which may directly impact crops, soil, animal health, farm infrastructure and machinery (Wilson et al., 2011; Magill et al., 2013). Thicknesses of several centimetres can smother crops, change soil properties and damage greenhouses. Even thin dustings of tephra can adhere to plants, blocking photosynthesis and causing acid damage (Figure 1).
Frequency of volcanic eruptions
Volcanic eruptions are classified by a Volcanic Explosivity Index (VEI), a logarithmic semi-quantitative scale incorporating variables such as eruption volume, plume height and duration (Newhall and Self, 1982). While a VEI 0 or 1 eruption may involve only a gentle effusion of lava, eruptions greater than VEI 2 become progressively more voluminous and explosive. Greater than VEI 5, the volume of erupted material may cause large scale disruption and damage while VEI 7 or 8 eruptions are likely to be truly cataclysmic.
The Smithsonian Volcanoes of the World catalogue (http://volcano.si.edu/) includes available information describing volcanic eruptions that have occurred during the last 10,000 years. Data is more complete for recent times and for larger eruptions, which are more likely to be identified in the geological record or have been recorded in historic documents. Mead and Magill (2014) calculated the periods for which data is assumed to be complete for the Smithsonian Volcanoes of the World catalogue. This information was then used to calculate Average Recurrence Intervals for each VEI category for the Asia-Pacific region (volcanoes in the Philippines, Indonesia, Japan, Vanuatu, Papua New Guinea, and between New Zealand and Fiji) (Figure 2).
While eruptions greater than or equal to VEI 2 occur on average more than eight times a year, VEI 6 eruptions such as Pinatubo in 1991 or Krakatau in 1883 occur approximately every 110 years. There have only been three VEI 8 eruptions within the Asia-Pacific Region during the past 254 thousand years (one from Toba, Indonesia, and two from the Taupo Volcanic Zone, New Zealand). A much more pressing worry is a VEI 7 event, similar in magnitude to the 1815 Tambora eruption, Indonesia. Eruptions of this size have an average return period of less than 500 years and tens of millions of people would be affected if such an event were to take place within Indonesia, the Philippines or Japan. Unprecedented losses and casualties would result if the eruption was to occur close to a large population centre.
Another large eruption in the Asia-Pacific region
Although it is impossible to predict the location or nature of the next major eruption, it is interesting to consider the consequences of past events assuming they were to reoccur today given modern societal conditions. We are able to analyse the current areas exposed to tephra fall for four previous eruptions in the Asia-Pacific region (Figure 3 and Table 1). Tephra dispersal is based solely on environmental conditions at the time of each eruption. The same eruption may have vastly different consequences if wind conditions were varied.
A repeat of the immense Toba eruption, approximately 74,000 years ago would severely impact most of the land area of Indonesia, Malaysia, Singapore, Thailand, Laos, Cambodia, Vietnam, Myanmar, Bangladesh, India and Shi Lanka. Tephra would travel as far as Africa and have severe global consequences. A more credible event to consider however is the eruption of Tambora, two hundred years ago. This is the largest eruption in recorded history with ash 1m thick falling 75 km from the volcano; 11-12,000 people were killed directly, while another ~60,000 died from starvation and disease due to crop failure (Oppenheimer, 2003).
Today more than 75 million people would be affected by more than 10 mm of tephra, enough to decimate crops and severely impact infrastructure and urban systems.
Quantifying volcanic risk in Tokyo
A repeat of the 1707 Hoei eruption from Mount Fuji would see an urban area of over 1,500 km2, and approximately 30 million people, impacted by more than 10mm of tephra fall. This includes almost the entire area of Kanagawa and Chiba prefectures, the highly populated eastern area of Tokyo prefecture and parts of Shizuoka, Yamanashi and Saitama. Home to Japan’s national government, this area is also a major national and international hub for business, manufacturing and transport. A similar eruption today, would disrupt these sectors and severely damage buildings and infrastructure. The clean-up requirements would be immense with assistance and resources having to be sourced from outside the affected area.
Although the most visible, Fuji is not the only threat to Tokyo; the city could be affected by any of a large number of volcanoes, with those immediately to the west posing the greatest hazard. As a first step to quantify the impacts of a future eruption impacting Tokyo, the probabilistic KazanRisk model (Figure 4) has been developed. KazanRisk contains a catalogue of 60,000 simulated tephra fall events from six volcanic centres, exposure information and mathematical functions relating the magnitude of tephra fall to damage. By assigning probabilities to each event we are able to calculate likely losses to buildings, the costs associated with the cleanup of roads and public areas, and loss of agricultural productivity due to crop damage, effects to livestock and damage to farm equipment. These methodologies may be extended to consider additional impacts, volcanoes and regions.
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