トカラ列島の地震とアカホヤ噴火の記憶Ancient Memories from Tokara's Turmoil

From around June 21, frequent earthquakes have been occurring near the Tokara Islands in southern Kyushu. This region has always been known for its frequent seismic activity, but over the weekend, volcanic eruptions also occurred at Mt. Ontake on Suwanose Island, part of Toshima Village in the Tokara Islands. With these two simultaneous factors—earthquakes and volcanic eruptions—experts are closely monitoring potential impacts on surrounding areas. To those unfamiliar with history, these might seem like distant, isolated events occurring in the southernmost regions of Japan. However, for those knowledgeable about history, these recent events evoke significant past shifts that drastically altered historical trajectories. Thus, attention to these geological disturbances in this region is vital. Living far away in Tokyo doesn’t necessarily mean one is safe.

It goes without saying that Japan frequently experiences earthquakes and volcanic eruptions, and each time these have significantly altered society. Indeed, even before written historical records, these events marked critical turning points. Seeing the current seismic and volcanic activity in the Tokara Islands, I’m reminded of a catastrophic submarine volcanic eruption that occurred about 7,300 years ago.

This dramatic event occurred slightly north of the Tokara Islands at the Kikai Caldera, located about 50-110 kilometers south of present-day Kagoshima Prefecture, around the waters near Yakushima and Tanegashima islands. Even today, this massive submarine volcano lies dormant here, having once profoundly affected the Jomon period. It could even be argued that this eruption shaped much of what we know as the Jomon period itself. This was the Akahoya eruption.

The catastrophic effects of this eruption on early Jomon settlements in southern Kyushu have long been suggested by archaeological investigations. However, last year, a research team from Kobe University published a groundbreaking report clarifying that the Akahoya eruption was far more colossal than previously imagined. According to their findings, the Akahoya eruption from the Kikai Caldera was the largest eruption on Earth during the Holocene epoch (the period from approximately 11,700 years ago to the present), with total volcanic ejecta estimated to have reached between 332 to 457 cubic kilometers, making it the largest eruption worldwide in the last 10,000 years. Significantly, this massive eruption didn’t occur on land, but rather under the ocean—highlighting the scale and unique geological nature of this event.

The volcanic ash ejected reached the stratosphere and was carried by prevailing westerlies, depositing ash not only throughout Kyushu but as far north as the Tohoku region in northeastern Japan. This effectively means that volcanic ash from the Akahoya eruption impacted most parts of the Japanese archipelago. Furthermore, pyroclastic flows surged across the sea surface at incredible speeds, traveling far enough to reach the Satsuma and Osumi peninsulas at the mouth of Kagoshima Bay. Consequently, the landscape and ecosystems of southern Kyushu were devastated and drastically altered. Additionally, it’s highly possible that massive earthquakes and caldera collapses associated with the eruption triggered enormous tsunamis. Recent geological surveys have identified tsunami sediments dating back around 7,300 years in the Miyazaki Plain and even as far away as Sukumo Bay in Shikoku (Kochi Prefecture), strongly suggesting that the coastal areas of the Japanese archipelago during the early Jomon period experienced catastrophic tsunamis linked to the Akahoya eruption.

The simultaneous occurrence of this massive eruption, gigantic earthquakes, and enormous tsunamis would be difficult for contemporary people to comprehend, and it is likely that these disasters inflicted damage on the Jomon people far beyond our current imagination. Indeed, the impact on the Jomon culture flourishing in southern Kyushu was devastating, and some theories suggest that the majority of the Jomon population in this region was completely wiped out.

When discussing the Jomon period, focus often falls primarily on the sophisticated cultures of eastern Japan, especially northern Tohoku. However, such cultures generally emerged from the mid-Jomon period onwards. In fact, southern Kyushu was the earliest region to develop its unique and advanced cultural sphere. The archaeological record of southern Kyushu remains obscured under thick layers of volcanic ash from the Akahoya eruption, significantly limiting discoveries. Another critical factor we must consider is the Jomon transgression, a rapid rise in sea levels following the Last Glacial Period due to global warming. It is highly plausible that the remains of early Jomon tribal groups—who were the architects of early Jomon culture—were submerged under rising seas. Hence, the keys to unraveling the beginning of the Jomon period remain elusive, hidden beneath volcanic ash layers and submerged under the sea.

Indeed, during periods when sea levels were lower, the southern Kyushu peninsula extended further southward than it does today, likely facilitating frequent voyages to the relatively large islands of Tanegashima and Yakushima within the Kikai Caldera region. It is therefore plausible that early Jomon people inhabited these strategic transit points and developed their own distinct maritime culture. Admittedly, this scenario remains merely one hypothesis from archaeological research. Nevertheless, some archaeologists suggest that many defining elements of Jomon culture might have originated among these early southern Kyushu communities. Given the challenges inherent in verifying details of the Jomon period, this remains uncertain. Yet, it is worth considering that the catastrophic destruction caused by the Akahoya eruption may have triggered a significant northward migration of surviving communities to less-affected northern Kyushu, from which cultural elements gradually spread eastward across the archipelago.

However, the situation differs regarding pottery—one of the hallmark features of the Jomon period. This is because currently, the oldest known pottery fragments have been discovered in an entirely different location: the northernmost tip of the Tsugaru Peninsula in northeastern Japan. Thus, while southern Kyushu at the southern extremity and the Tsugaru Peninsula in the far north of the Japanese archipelago appear historically disconnected, each region developed distinctive cultural lineages that later converged under what, since the Meiji era, has been categorized collectively as the Jomon period. This intriguing situation likely reflects different migratory routes—southern and northern routes—that early Jomon peoples took when arriving on the Japanese archipelago. Despite their distinct routes, these early tribal groups may have originally shared similar ancestral roots, resulting in notably similar cultural traits. With this understanding, I suggest that the principal zone of east-west cultural mixing was probably located around the Kansai region and western Tokai.

During the early Jomon period, southern Kyushu likely boasted a notably advanced culture. At a time when the main Jomon populations in Honshu were still using conically pointed-bottom pottery (sentei-doki (尖底土器)), the southern Kyushu people were already utilizing flat-bottomed pottery (hirazoko-doki (平底土器)). This flat-bottomed pottery could be placed directly inside dwellings, serving practical purposes for cooking and storage, indicative of a settled lifestyle. Archaeological evidence also shows sophisticated earplug-shaped ornaments and pot-shaped vessels, clearly demonstrating that southern Kyushu hosted an advanced cultural sphere at this time.

Yet abruptly, following the Akahoya eruption, this sophisticated culture in southern Kyushu disappeared entirely. Post-eruption strata no longer yield advanced pottery styles or refined tools characteristic of the pre-eruption period. Instead, notably older pottery styles—previously in use across Honshu—reappear in the layers immediately following the eruption. Excavations throughout southern Kyushu consistently show that highly sophisticated pottery is found beneath the distinctive orange Akahoya volcanic ash layer, but only older, more rudimentary pottery appears above it. This archaeological pattern provides crucial evidence that southern Kyushu’s advanced Jomon culture was essentially obliterated by the eruption, and survivors or later migrants had no choice but to revert to older cultural practices in reconstructing their lives.

The eruption’s impact was devastating, wiping out the very foundations of daily life for southern Kyushu’s Jomon inhabitants. People in areas directly hit by pyroclastic flows would have perished instantly, engulfed by scorching volcanic debris. Even in surrounding areas untouched by the flows, severe secondary damage from the thick ash fallout undoubtedly occurred. Particularly in Kyushu—a region rich with mythological narratives—the emergence of stories such as the Ama-no-Iwato legend, in which the sun deity hides, plunging the world into darkness, seems like a cultural echo of the real historical period when thick volcanic ash blotted out the sun.

Ash layers from the Akahoya eruption are estimated to have reached depths exceeding 30 centimeters across southern Kyushu. With ashfall this thick, forests would have been buried and perished entirely, likely requiring more than two centuries to fully recover. In reality, the destruction of vegetation rendered the region’s broadleaf evergreen forests incapable of regeneration. Consequently, the large mammals reliant on these forests, such as wild boar and deer—key hunting targets for the Jomon—would have vanished entirely. Moreover, volcanic ash blanketed rivers and coastal waters, causing mass die-offs of benthic organisms like shrimp and crabs, which subsequently led to severe declines in fish populations dependent upon them for food. Such drastic ecological collapse made the continuation of traditional livelihoods impossible, forcing surviving populations to begin migrating in search of new habitable lands.

Thus, as terrestrial and marine food resources simultaneously vanished, southern Kyushu’s Jomon communities—previously living amidst relatively abundant resources—could no longer sustain their customary hunter-gatherer lifestyles. This represents a profound historical turning point, making it extremely difficult for survivors to remain in their homeland. This catastrophe likely sparked a large-scale migration, effectively redistributing populations previously concentrated in southern Kyushu throughout western Japan. While it’s generally agreed that most survivors dispersed across western Japan, I personally speculate that at least a small portion of these survivors may have chosen instead to migrate southward across the sea to the Ryukyu Islands.

In other words, after generations of relatively prosperous coastal living, southern Kyushu’s Jomon inhabitants likely acquired sufficient maritime technology to enable some groups to seek new lands across the sea. While this sounds reminiscent of the “Out-of-Africa” narrative from human evolutionary history, there is a specific reason I propose this hypothesis: the existence of certain ancient legends recorded in the Kojiki, Japan’s oldest chronicle.

The Kojiki is far more than mere fictional storytelling. It preserves ancient memories transmitted orally over thousands of years, and among these collective memories, I strongly believe the Akahoya eruption is encoded. Precisely how and in what form this event appears within the Kojiki’s narratives will be detailed in my future investigations into the Ryukyu Islands.