Topic Area: Natural Hazards
Abstract
Approximately 800 million people reside within a 100 km radius of an active volcano. An estimated 1550 subaerial volcanoes have been active since the Holocene. Pelean to Ultra Plinian eruptions present significant risk to populated areas because of exposed populations. From 10,000 Holocene eruptions, only six eruptions are responsible for < 50 % of the total quantified fatalities. Decade volcanoes are classified as volcanoes that have demonstrated significant destructive power and have close proximity to large populations. One such decade volcano is Sakurajima, in Japan.
Sakurajima is a stratovolcano with a 1117m elevation found in the Aira caldera within the Kagoshima prefecture of Kyushu. Kagoshima is 8km from Sakurajima and has an estimated population of 607 655 in 2018. Arable, fertile land and accessible transportation links have seen Kagoshima’s population increase since its classification from city status on April 1, 1889.
Satellite Image from the ISS of Sakurajima and the surrounding area Jan 10th 2013
Fig A – Image courtesy of NASA, (2013).
This Study will – rather than future consider present or outlining what discussion showed and the effects and risks that you have uncovered from your researches. This helps the reader know what is coming and the depth of information that is in the paper itself discuss Sakurajima and the Aira Caldera’s geological setting, examining four significant plinian eruptions, with specific focus on the 1914 Taisho Eruption, that was the most powerful in twentieth century Japan. Volcanic geohazards, their associated risks and impacts on population and infrastructure will be critically assessed from a number of sources. The validity and reliability of primary and secondary sources is evaluated to determine the most effective mitigation for a large-scale plinian eruption of Sakurajima. This study examines how technological developments in communications has improved past mitigations from the 1914 Taisho eruption to the 2006 Minamidake eruption.
This study indicates disaster reduction measures can limit the effects of large magnitude eruptions. Public awareness, early warning systems, disaster prevention exercises and satellite based remote sensing are all fundamental in reducing potential dangers. Implementing systematic evacuation protocols for Kagoshima is likely to provide the most successful mitigation for VEI 4 or greater eruption of Sakurajima.
You may wish to state what has been found out rather than what you are going to do\find out as this shows to the reader what is actually occurring\being shared with them
Word Count :300
Contents List
Title
Abstract-rework with end findings
1. Introduction
1.1 Geological context.
1.2 Scope and methodology.
1.3 Objectives.
2. Tectonic Setting
2.1 The Aira Caldera and post caldera volcano Sakurajima.
2.2 Japanese Median tectonic line, earthquake magnitude and volcanic activity.
2.3 Sakurajima 1914 eruption and the Osumi Peninsula.
3. Eruptive Activity and Historic eruptions of Sakurajima
3.1 Explosive and Effusive Eruptions.
3.2 Categorized Volcanic Eruptions.
3.3 Eruptive History of Sakurajima.
4. Volcanic Geohazards – each of the subsections could also be subdivided to make them more descriptive of what is making up the section
4.1 Risk, cause and impacts to Kagoshima.
4.2 Past mitigations (Jan 1914 -2006).
4.3 Future mitigation strategies.
5. Discussion
6. Conclusion
6.1 Conclusion
6.2 Recommendations.
References
Introduction
1.1-Geological context
Sakurajima is an andesitic stratovolcano with a 1117m elevation in the Aira Caldera within the Kagoshima prefecture of Kyushu. Formed from pyroclastic injections into the southern rim of the Aira Caldera approximately 29,000 years ago. Frequent but low intensity Vulcanian eruptions (VEI 2) have been observed since 1955, producing over 8000 eruptions. Historically Sakurajima has produced significant eruptions starting in 1471 (VEI 5), 1779 (VEI 4), 1914 (VEI 4) and 1955 (VEI 3).
3D Satellite rendering of terrain around Mt. Sakurajima also displaying the nearby city of Kagoshima to the far left.
Fig 1- Image courtesy of Google Maps (2018)
Presently consisting of two primary adjoining structures, in the north Kitadake 1117m is dormant and in the south Minamidake, 1040m has been active since October 1955. Showa Crater formed on the eastern flank of Minamidake in 1939 and was active until the 1948 eruption that destroyed the village of Kurokami. Dormant for 58 years until resuming in 2006 where it has since displayed increased volcanic activity yearly, (Iguchi et al., 2010).On August 18th 2013, Showa Crater produced an ash plume 5000m high depositing significant ash fall over Kagoshima.
Kagoshima is 8km from Sakurajima and has an estimated population of 607 655 in 2018, (Population city, 2015). Population estimates for Kagoshima in 1650,1750,1850 and 1873 were 50 000, 58 000, 42 000 and 89 374 respectively. Sakurajima is classified as one of the sixteen Decade volcanoes identified by the (IAVCEI). Close proximity to a Decade volcano exhibiting substantial destructive power creates a real risk and significant threat to Kagoshima’s population
1.2-Scope and Methodology
This study will critically evaluate literature from a variety of sources. Primarily but not exclusively peer reviewed articles and scientific journals from Google scholar and the OU library resources. Part of the criteria is having an awareness of unsubstantiated inferences, conclusions and biases.
Questioning the reliability of the source material and critically assessing the language used where terms such as can, could or may are used to infer an outcome but do not claim it with certainty will be critically reviewed.
The aim of this paper is to ascertain the potential risk to Kagoshima from volcanic geohazards associated with a major eruption from Sakurajima. This study will demonstrate the geological background of the Aira Caldera, define geohazards, risk and evaluate proposed mitigation strategies.
1.3-Objectives
Expanding upon this the following objectives will be to:
• Summarize the tectonic setting of the Aira caldera in Kagoshima Bay containing the post-caldera Sakurajima volcano with specific focus on the 1914 eruption and the Osumi Peninsula. Describe the relationship between the Japanese Median tectonic line,earthquake magnitude and volcanic eruptions.
• Define VEI and quantify 54 Holocene eruptive periods.
• Explain how particular types of eruptions at Sakurajima, are formed from different geological settings.
• Define geohazards, risks, causes, impacts/effects on economy, infrastructure and how relevant organizations can mitigate the severity of natural disasters, specifically a VEI 4 eruption or greater from Sakurajima.
• Explain the effects of eruptions on Kagoshima’s increasing population.
• Discuss how improved technology and communications has improved past mitigations (Jan 1914 eruption), (2006 eruption).
• Critically assess the proposed mitigation strategies of future Sakurajima eruptions.
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2. Tectonic Setting
2.1 The Aira Caldera and post caldera volcano Sakurajima.
Calderas are sub-circular subsidence basins created by magma withdrawal from magma reservoirs, typically during large volcanic eruptions.
Variations in the Calderas age range between approximately 22 000 and 29 000 years old dependent on how current the research is. Recent scientific papers have a tendency to be more reliable and accurate because of technological advances, dating techniques and understanding in field geophysics. “Increase in magma flux as the result of conduit enlargement is one of the key process that triggered caldera collapse and eruption of the Ito ignimbrite from Aira Calder at ~29 Ka.” (Geshi et al, 2016). In contrast to “About 22,000 years ago a series of large‐scale pyroclastic eruptions produced the Aira caldera 20 km×20 km wide” (Aramaki, 1984). Indirect methods are generally employed to ascertain the calderas age as subsidence of the caldera block occurs below the surface of the eruption and is inaccessible. Pumice and minor ignimbrite deposits rest below the lag breccia in caldera-forming eruptions. The 2016 paper used 16 samples for upper and lower units of the maximum grain size of pumice and lithic fragments and concluded a coarsening of these fragments suggesting an increase in eruption rate approximately 29 000 Ka. The 1984 paper determined twelve radio carbon dates between 16 300 BP and 34 500 BP and took the mode to give an approximation of 22 000 years for the Caldera’s age. BP denotes Before Present and the origin year is generally regarded as 1950 for use with radiocarbon dating.
The outline of the Aira Caldera was heavily influenced by the faults bounding the volcano‐tectonic graben forming Kagoshima Bay. The available evidence suggests that The Aira Caldera, southern Kyushu, Japan, was formed by magma withdrawal circa 29,000 BP after an ignimbritic eruption ( Kobayashi et al. 2013) . This eruption resulted in a crater of over 20 km diameter, in excess of 300 km3 of pyroclastic material being ejected and fine ash deposits have been found over 1000 Km form the central vent. The eruption was a combination of Plinian pumice eruption and pyroclastic flow. The vent for these are at the present site of Mt Sakurajima (Aramaki, 1984). Subsequent activity within the Caldera approximately 13 000 years ago, formed the volcano Sakurajima.
Sakurajima is an andesitic stratovolcano containing three main peaks named Kitadake with (1117 m), Minamidake (1040 m) and Nakadake (1060 m) located in the northern, southern and central regions. Kitadake has been approximately dormant for the last 4900 years. Showa Crater with an 800 m elevation found on the eastern flank of Minamidake formed in 1939 and was active until 1948, became dormant until 2006 where it has subsequently demonstrated increased activity most notably in 2006, 2013 and 2016. Since 1955 Sakurajima entered a new phase of activity, producing hundreds of small eruptions each year totaling over 8000 in the last 63 years.
Geological Map of Sakurajima with labelled eruptive dates
Fig 2 – Courtesy of Fukuyama and Ono (1981) and Kobayashi (1988). The Geological survey of Japan
Key
K- Kitidake M-Minimadake N-Nikadake S-Showa crater
subm-submarine volcanic and intrusive deposits
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2.2 Japanese Median tectonic line, earthquake magnitude and volcanic activity.
The Japanese Median tectonic line (MTL) is the longest fault system within Japan extending for over 800 km. It is a “tectonic discontinuity that separates the low-P/high-T Ryoke metamorphic terrain from the high-P/low-T Sanbagawa terrain in SW Japan” (Shigematsu et al., 2016). The fault runs parallel to Japans volcanic arc beginning in the Ibaraki Prefecture and ending through the Hoyo Strait to Kyushu.
Tectonic map of Southwest Japan showing Japanese Median Tectonic Line (MTL)
Fig 3 – Image Courtesy of Wikimedia Commons-
Key
Orange Area – Fossa Magna region
Blue Line – Itoigawa-Shizuoka Tectonic Line
Red Line – Median Tectonic Line
“The 1914 Taisho eruption of Sakurajima volcano was Japan’s highest intensity and magnitude eruption of the twentieth century”. (Todde et al.,2017) Figure 3 shows the tectonic environment associated with the Japanese Median Tectonic Line. Within Kyushu, the MTL travels between the Kunisaki and Saganoseki peninsula. The MTL fault zone is divided into six regions, categorized from recent seismic events, typically earthquakes with magnitudes 6-7 have occurred in all six regions.
Word Count-109
2.3 Sakurajima 1914 eruption and the Osumi Peninsula.
Taisho Eruption (1914 to 1915 AD)
Southern Kyushu experienced seismic and volcanic activity before and after the Taisho Eruption of 1914.On January 10th, 1914, five strong earthquakes were observed near Sakurajima, the following day over 250 were felt. Public awareness had been elevated from other eruptions within the past five years that led to an effective evacuation of over 23 000 people. Hundreds were injured but fatalities were minimized to 29.
On the 12th January a 7.1 magnitude earthquake was recorded in Kagoshima city killing 35 people. Notably the earthquake had higher mortalities than the eruption of Sakurajima due to heightened public awareness.
The Sakurajima eruption consisted of two stages, the first was the sub plinian explosive phase approximately lasting 48 hours that produced widespread tephra. It began on the western flank and was shortly proceeded by an eruption on the eastern flank. The secondary phase produced greater ash emissions and less energetic effusive eruptions that lasted until April. Lava flowed from both vents in east and westerly directions into Kagoshima Bay. On the eastern side lava had crossed the sea lane, and what was previously a volcanic island became the Osumi Peninsula. “About 0.6 km3 of pumice showered over the eastern foot and 1.6 km3 of andesitic lava flowed from the lateral vents of Sakurajima” (Koto, 1916; Omori,1916).
The volume of magma from the Taisho eruption was estimated to be 1.5 km3 that corresponds to a Pelean type eruption with VEI 4.A large scale circular subsidence occurred approximately 2 m at the center due to the evacuation of the magma.