Decade Volcano: Sakurajima



Where you live, do the kids have to wear hard hats on their way to school, in case it rains volcanic rocks?

They do on Sakurajima island, in Japan. (EPOD)

But that’s not the only reason why Sakurajima became a Decade Volcano in the 1990s and continues to be one of the most closely studied volcanoes in the world.

It is Japan’s most active fire mountain, as well as the site of that country’s biggest 20th century eruption: the VEI 4 Taisho eruption of 1914-1915.

Casey Lee, CC BY-SA 2.0

Technically, Sakurajima — which rises beautifully out of Kagoshima Bay on southern Kyushu Island — has been a peninsula ever since Taisho lava flows filled in the narrow Osumi Strait on its southeastern flank and welded the volcano to the mainland.

Sakurajima has shown intense violence on other occasions, too, including the An-ei eruption from 1779 to 1781 (VEI 4) and the VEI 5 Bunmei eruption, 1471-1476.

In between these cataclysms come much smaller, more frequent eruptions that make Sakurajima a volcanologist’s ideal field research location.

It’s accessible, too — only a short ferry trip or drive, and close to international airports and the national rail and highway systems.



While Sakurajima is iconic, its closeness poses a threat to people and infrastructure.

Researchers can also take advantage of centuries’ worth of scientific data on this volcano (although much valuable Sakurajima information that had been stored in Tokyo was lost in fires caused by the Great Kanto Earthquake of 1923).

That deep plunge on the graph’s red line is how much Aira deflated after Sakajurima’s 1914-1915 eruption (Image: Figure 1, Hickey et al., https://www.nature.com/articles/srep32691/figures/1 Open access.)

For example, 21st-century volcanologists still use data from leveling surveys done here by the Japanese military in the late 1800s and early 20th-century.

The information helps them investigate a possible magmatic connection between Sakurajima and the giant Aira Caldera (outlined with heavy dashes in the above diagram) that underlies northern Kagoshima Bay.

Sakurajima sits several miles southwest of Aira’s center, on its southern rim.

Those surveys show ground uplift, mostly in the central caldera region, for 20 years before the Taisho eruption started in January 1914.

The surveyors repeated their lines after Sakurajima quieted down and found that the same area had dropped several feet, presumably because magma had exited the underground plumbing system during the eruption. (Aramaki; Hickey et al.; Todde et al.)



Meanwhile, this otherwise good 1914 silent film — made in Hollywood to capitalize on the Taisho eruption — was banned in Japan on grounds that it presented the Japanese as primitive people. They had a point, especially given their work at the time on Sakurajima, one of the first uses ever of modern volcano observation techniques. (Also, the Buddha isn’t into cursing.)


Volcanologists are still trying to understand the apparent connection between Aira’s ground movement and Sakurajima’s eruptions.

The rest of us, though, hear “giant caldera” and immediately think “supereruption?”.

Let’s take a quick look at that, along with Sakurajima’s history.

A fair amount of text is required, but I’ll break it up with some videos of Sakurajima’s recent eruptions — these are low level, compared to what the volcano can do, but still spectacular.



Sakurajima, having a quiet moment earlier this week.


Aira Caldera and Sakurajima in a nutshell

There are differing views on how Aira Caldera formed. For simplicity, I will follow just one of them: a well-cited paper by Shigeo Aramaki (see source list).

Aira is not the only big caldera in town.

Southern Kyushu Island has a line of them, trending roughly north-south from Aso, hundreds of miles north of Aira, down through Kagoshima Bay, to Kikai on the seafloor some 30 miles offshore.

Some of these calderas, including Aira, sit inside a huge rectangular ground depression known as the Kagoshima Graben. All we need to know about that is:

  1. There’s not going to be a simple explanation for Aira and Sakurajima, like there is for Hawaiian (hot spot) and Pacific Northwest (subduction zone) volcanoes. The geology of southern Kyushu Island is insanely complex.
  2. Volcanoes and tectonics interact in ways that aren’t well understood. Nothing like this fire-filled graben exists elsewhere in Japan, so volcanologists have yet another complication to deal with when trying to understand the big picture at Aira and Sakurajima.


As I understand Aramaki’s description of the local scene some 22,000 years ago, what’s now northern Kagoshima Bay used to be shallow water or possibly even dry land.

That makes sense — the last ice age was at its worst and global sea level was hundreds of feet lower than it is today.

This quiet coastal landscape then exploded. Multiple times.

The first two eruptions, while bigger than most events that humans have witnessed and documented, were nowhere near “super”-sized. Interestingly, their vent was apparently in the area now occupied by Sakurajima.

Next, what is now Aira’s central vent blasted out chunks of bedrock, as well as ash clouds, widening its maw to accommodate an enormous eruption column that eventually spread ash across Kyushu and neighboring regions in the Ito pyroclastic flow.

At VEI 7, this wasn’t exactly a supereruption, but it was close and it must have devastated human, animal, and plant life in the archipelago and beyond.



VEI 7 eruptions are thousands of times more powerful than this 2015 event. That’s Kagoshima City in the foreground.


Aira’s big show then ended in a caldera collapse and that was that.

While there has been some lesser caldera activity in its northeastern sector, occuring approximately 10,000 years ago, nothing like those enormous eruptions has happened since.

Now for Sakurajima.



Wheee!


About 3,000 years after Aira Caldera foundered, Sakurajima started building itself up from the old south-rim vent, breaking the bay’s surface around 13,000 years ago with what is now its “North Peak” (Kitadake).

Action at Kitadake ceased about five thousand years ago as a new “South Peak” (Minamidake) part of the volcano took over.

These eruptions were mild at first, scattering ash and sandy material in what were probably vulcanian blasts like those shown in these modern videos. But over the last 500 years or so, a tendency for Plinian-type eruptions has appeared (1471-1476, 1779-1782, and 1914-1915) — that’s the phase we’re living with now. (Geological Survey of Japan, 2/e)



Three vulcanian eruptions in a single day: April 24, 2020.


At present, Minamidake’s summit craters just sit there and fume. Eruptions — including the ones shown in this section’s videos — come from Showa crater, an auxiliary vent (Yokoyama, 2013) slightly east of the summit.

Showa, which used to be just a small depression near the summit, opened for business in the 1930s. It coughed up a VEI 2’s worth of lava in 1946, and after dozing for decades, has been the site of increasing numbers of explosive eruptions since 2006.

These fissures on Sakurajima’s western flank opened up first on January 12, 1914. About ten minutes later, fissures on the eastern flank cut loose. (Image: Wikipedia)

Flank vents can open up, too, particularly during major eruptions. Sakurajima actually has the best documented example of rare “twin fissures” that erupt on opposite sides of the volcano.

Magma also vents through the nearby seabed.

Underwater activity in the 1779-1781 An-ei eruption caused deadly tsunamis, created some islets, and left a subsurface plateau, hundreds of feet tall.

So, are Aira Caldera and Sakurajima Volcano connected?

Probably. But how?

The most common explanation I’ve found in the sources listed below is that there might be two magma sources: one deep down underneath Aira and another, much shallower one below Sakurajima that feeds this stratovolcano’s eruptions.

In this hypothesis, magma slowly collects — enough each year to fill more than 220 Epcot centers at Disney World. Only a small fraction of it is erupted at Sakurajima. The ground at Aira moves up as the reservoir fills. When it can’t hold any more, it empties out in one of Sakurajima’s big blasts. (Hickey et al)

Aira then deflates for a while, but the cycle eventually repeats.

What’s happening now?

Around the early 1970s, Aira’s inflation was close to pre-1914 levels. And then, from the mid-1970s to mid-1980s, Sakurajima became very active (see Table 1 in Gomez for a damage summary from those times). Yamasina reports that we may have just avoided another major eruption at that point.

Instead, Aira began to deflate, and Sakurajima explosions decreased.

But in 1993 the caldera center began to rise again. This continues today.



Understandably, few people stop to wonder what’s under the bay. Scientists, however, are watching Aira and Sakajurima 24/7/365.


The good news: Aira seems to be working in a low-key style now. But the relationship between its deformation and Sakurajima’s eruptions needs further research.

Given the intricate geology here, volcanologists have their cut out for them!

In the meantime, the caldera and Sakurajima are both monitored carefully.

The bad news? On June 30th, a panel of experts announced that a major eruption of Sakurajima Volcano, while not necessarily imminent, probably is in the works.

Location:

31.593° N, 130.657° E, Kagoshima Prefecture, Kyushu, Japan. The GVP Volcano Number is 282080.

Nearby Population:

Per the Global Volcanism Program, and not counting tourists, including the two million who visit Sakurajima every year:

  • Within 5 km (3 miles): 4,918
  • Within 10 km (6 miles): 113,874
  • Within 30 km (19 miles): 905,254
  • Within 100 km (62 miles): 2,610,033

Current Status:

Level 3 on a five-point system, with 5 the highest; don’t get closer than 2 km (1.3 miles) to the summit.

Eruptions:

  • Eruption styles: For many decades, Sakurajima has had frequent explosions and vulcanian eruptions that produce plumes and small pyroclastic flows. The most powerful ones are accompanied by shock waves that occasionally break windows as far away as Kagoshima City and Tarumuzu. The last three major eruptions followed an unusual pattern, starting with a Plinian-style eruption that was followed by pyroclastic flows and then large lava flows. Volcanoes typically have either explosive or effusive lava eruptions, not both. And then there are those twin-fissure flank vents, described by Yokoyama as “mechanically normal but empirically odd.”
  • Kagoshima City, shown here covered in ash from the 1914-1915 eruption, also experienced a massive earthquake. (Image: Wikimedia

  • Biggest recorded event: 1914-15’s Teisho VEI 4 event was the largest 20th-century eruption in Japan, and some local jurisdictions, like Kagoshima City, use it for modeling emergency plans. However, the Bunmei eruption in 1471-1476 was ten times larger (VEI 5). That is based on geological evidence; the sequence of events and resulting damage, only described in historic records as “huge,” aren’t clear. Nevertheless, hazard maps for the broad region of Kagoshima Prefecture and environs are based on the distribution of Bunmei ashfall. (Biass et al.; Geological Society of Japan, 2/e)
  • Most recent eruption: Ongoing explosive events The most recent lava effusion came from Showa Crater in 1946.
  • Past history: See the GVP for details. Table 1 in the Gomez paper referenced in the source list is also helpful because it breaks down phases of the 1955-2016 period that GVP describes as a single VEI 3 eruption.

    Monitoring:

    Sakurajima is monitored through the combined resources of several Japanese universities and government agencies.

    Status updates and warnings are issued by the Japanese Meteorological Agency (Japanese).

    Here is the Sakurajima observation page and some webcams (both in Japanese).

    Tokyo VAAC issues air traffic advisories on ash emissions.


    Epilogue:

    This is a good place to include something that puts a human face on eruptions at Sakurajima and elsewhere.

    There’s a “monument to distrust of science” in Kagoshima, based on an incident that happened during the start of Sakurajima’s 1914-1915 eruption.

    I have only found mention of it in the Japanese Wikipedia, though it must be common knowledge locally.

    A little background, first.

    As we’ve seen, Sakurajima was one of the first volcanoes to be monitored with modern techniques. The first seismometer was installed there in 1888, and there were also those levelling surveys mentioned above, plus other state-of-the-art fieldwork.

    Then, as now, public warnings and other volcano information were issued by official weather offices. So people turned to the local weather office when multiple low-level earthquakes, as well as well water level changes, gas surges, marine creature die-offs, ground temperature rises, and other precursors started happening.

    Per Japanese Wikimedia, on January 11, 1914, a small landslide occurred near the summit and “thin white smoke rose on the mountainside.”

    The next day, from 8 to 10 a.m., “a mushroom cloud-like white smoke was seen boiling from the middle of Sakurajima.”

    The local weather office reportedly said this cloud was a meteorological phenomenon. Civic leaders on the western side of the volcano believed them, staying put while other residents fled the scene.

    At about 10 a.m., the western flank opened up with a big explosion, followed about ten minutes later by a similar blast from a twin fissure on the eastern flank. The eruption escalated to Plinian-style levels.

    As pumice poured down onto the bay, limiting the movement of rescue ships, people who had stayed, in the translated words of Japanese Wikimedia contributors, “rushed immediately after the eruption started on January 12 and it became a big mess . . . in Higashi-Sakurajima village, there were a number of people who fell off the coast due to confusion and people who froze to death or drowned in an attempt to swim across to the other side [a 2.5-mile swim across open water. In January].”

    Major eruptions, from eyewitness descriptions ranging from Pliny the Younger at Vesuvius to camera operators who filmed parts of Nova’s “In the Path of Killer Volcanoes” at Pinatubo in 1991, are terrifying, disorienting experiences.

    There can be no blame for the mistaken interpretation of that white cloud early on the morning of the 12th. The experts made their best call, based on what little they knew.

    Perhaps they relied too much on instruments, which can only show what we want to see, not always the things we need to see.

    Science always walks the fence between objectivity and common sense. And scientists are only human, while volcanoes have many ways to surprise us.

    The civic leaders weren’t wrong, either, in following the official information.

    Nevertheless, the mayor of Higashi-Sakurajima felt so bad about the subsequent loss of life that he wanted to put up a memorial to warn future generations.

    He couldn’t manage it, but his successor did, erecting the monument on the tenth anniversary of the eruption’s start.

    Even without that, no one would ever forget the 1914-1915 eruption of Sakurajima.

    Today they conduct evacuation drills on that day, January 12th, while researchers like Biass et al. model what a similar eruption would do to the region today.

    Still, nothing beats advice from those who have actually been through it. Here’s an excerpt from the inscription as given on this page via Google Translate, emphasis added:

    . . . the whole island was covered in fierce fire, volcanic blocks fell, and ash fall covered the heavens and earth, and the sight was extremely devastating. Annihilated eight villages and injured about 140. Earthquakes occurred frequently several days before this explosion, some collapse was observed at the summit, hot water spewed on the coast, old eruption The village mayor repeatedly asked the weather station to make a judgment because there was a phenomenon such as white smoke rising from the mouth that was not easy to engrave, but because Sakurajima said that there was no eruption, the village mayor asked the remaining residents. I was told that I wouldn’t be upset and evacuated, but soon after a big explosion occurred, a knowledgeable person who trusted the weather station got into a disaster, and the village chiefs had no place to avoid the trouble, and they were jumping into their own sea and drifting, Yamashita’s income earner, Oyama secretary, etc., finally reached a tragic end. The explosion on the main island would inevitably be escaped later in the history of ancient times. When recognizing, the most important thing is to prepare for evacuation in advance, save and save money from the plain, and be prepared not to get lost in the road at any time of disaster. Record [“Remember” ?].


    Source



    Featured image: Sean Pavone/Shutterstock


    Sources:

    Some are in Japanese and were translated using either the browser or Google Translate online.

    Aramaki, S. 1984. Formation of the Aira caldera, southern Kyushu,∼ 22,000 years ago. Journal of Geophysical Research: Solid Earth, 89(B10): 8485-8501.

    Biass, S.; Todde, A.; Cioni, R.; Pistolesi, M.; and others. 2017. Potential impacts of tephra fallout from a large-scale explosive eruption at Sakurajima volcano, Japan. Bulletin of Volcanology, 79(10): 73.

    Brown, S.K.; Jenkins, S.F.; Sparks, R.S.J.; Odbert, H.; and Auker, M. R. 2017. Volcanic fatalities database: analysis of volcanic threat with distance and victim classification. Journal of Applied Volcanology, 6: 15.

    Geological Survey of Japan. n. d. Active Volcanoes of Japan. Sakurajima, 2/e https://gbank.gsj.jp/volcano/Act_Vol/sakurajima/text/eng/exp01-1e.html Last accessed July 2, 2020.

    Global Volcanism Program, 2020. Report on Aira (Japan). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 24 June-30 June 2020. Smithsonian Institution and US Geological Survey. https://volcano.si.edu/showreport.cfm?doi=GVP.WVAR20200624-282080

    Hickey, J.; Gottsmann, J.; Nakamichi, H.; and Iguchi, M. 2016. Thermomechanical controls on magma supply and volcanic deformation: application to Aira caldera, Japan. Scientific Reports, 6: 32691.

    Japanese Meteorological Agency. 2013. National Catalogue of the Active Volcanoes in Japan (4/e, English edition). https://www.data.jma.go.jp/svd/vois/data/tokyo/STOCK/souran_eng/volcanoes/090_sakurajima.pdf Last accessed July 2, 2020.

    ___. 2020. Sakurajima. http://www.data.jma.go.jp/svd/vois/data/fukuoka/506_Sakurajima/506_index.html Last accessed July 2, 2020.

    EPOD. 2015. Sakurajima in 2013. https://epod.usra.edu/blog/2015/07/sakurajima-in-2013.html

    Gomez, C. 2014. Digital photogrammetry and GIS-based analysis of the bio-geomorphological evolution of Sakurajima Volcano, diachronic analysis from 1947 to 2006. Journal of Volcanology and Geothermal Research, 280: 1-13.

    Kriswati, E., and Iguchi, M. 2003. Inflation of the Aira Caldera prior to the 1999 Eruptive Activity at Sakurajima Volcano detected by GPS network in South Kyushu.
    https://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/129106/1/a46b0t76.pdf Last accessed July 2, 2020.

    McCurry, J. 2016 New data points to major eruption of Japanese volcano. https://www.theguardian.com/world/2016/sep/13/new-data-points-to-major-eruption-of-japanese-volcano Last accessed July 2, 2020.

    Photovolcanica. n.d. Sakurajima. http://www.photovolcanica.com/VolcanoInfo/Sakurajima/Sakurajima.html Last accessed July 2, 2020.

    Todde, A.; Cioni, R.; Pistolesi, M.; Geshi, N.; and Bonadonna, C. 2017. The 1914 Taisho eruption of Sakurajima volcano: stratigraphy and dynamics of the largest explosive event in Japan during the twentieth century. Bulletin of Volcanology, 79(10): 72.

    Wikipedia. 2020. Sakurajima. https://en.wikipedia.org/wiki/Sakurajima Last accessed July 2, 2020.

    ___. 2020. The Wrath of the Gods (1914 film) https://en.wikipedia.org/wiki/The_Wrath_of_the_Gods_(1914_film) Last accessed July 2, 2020.

    Wikipedia (Japanese). 2020. Sakurajima. https://ja.wikipedia.org/wiki/桜島 Last accessed July 2, 2020.

    Yamashina, K. I. 2013. The Inflation-deflation History of Aira Caldera for the Last 120 Years and the Possibility of a Forthcoming Large Eruption at Sakurajima Volcano (“Special Section” Sakurajima Special Issue). Bulletin of the Volcanological Society of Japan, 58(1): 103-114.

    Yokoyama, I. 2013. Parasitic eruptions on Sakurajima Volcano (“Special Section” Sakurajima Special Issue). Bulletin of the Volcanological Society of Japan, 58(1): 91-102.

    ___. 2015. Eruption patterns of parasitic volcanoes. Annals of Geophysics, 58(3): 0327.



Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.