Funasa-Yamauchi Reverse Fault Zone

Funasa and Yamauchi Reverse Fault Zone: Reading Two Million Years of Crustal Movement in the Hills of Hiroshima

Deep in the inland mountains of Hiroshima Prefecture, where rice terraces drape the hillsides and the Gōnokawa River carves its way toward the Sea of Japan, lies one of Japan's most remarkable natural archives. The Funasa and Yamauchi Reverse Fault Zone (船佐・山内逆断層帯, Funasa-Yamauchi Gyakudansōtai) is not a single dramatic cliff or a single famous fault line, but a string of geological outcrops scattered across three municipalities—Akitakata, Miyoshi, and Shōbara—that together tell the story of how the Chūgoku Mountains and the Seto Inland Sea were born.

Designated a National Natural Monument by the Japanese government on May 6, 1961, this fault zone offers an unusually clear glimpse of geologically recent crustal upheaval. The deformation preserved in its rocks dates from the Quaternary Period—the last two million years or so—making it a precious natural laboratory for understanding the violent forces that shaped this part of Japan within a timeframe almost contemporary in geological terms.

Geological Background

To appreciate what makes this site special, it helps to understand a curious anomaly preserved in the rocks. In an ordinary geological sequence, older rocks lie at the bottom and younger rocks rest on top, like layers of sediment quietly accumulating over time. At Funasa and Yamauchi, this natural order has been turned upside down. Older basement rocks—Cretaceous granite roughly 100 million years old, and Miocene diorite-like intrusives—have been thrust upward and pushed over the top of much younger Neogene and Quaternary sediments along low-angle reverse faults.

The Funasa Fault is located about ten kilometers west of central Miyoshi City. Along its exposed face, granite has migrated from north to south, climbing over Neogene strata and the Pliocene–Pleistocene gravels (the Kōtachi Gravel Formation) that rest on top of them. The fault plane strikes roughly 70 to 80 degrees west of north and dips around 30 to 50 degrees toward the north. Because the displaced gravel layer is so geologically young, scientists can be confident that the fault movement itself occurred in extremely recent times.

The Yamauchi Fault, traceable along about sixteen kilometers of mountain footslope from the northern edge of the Miyoshi Basin to the town of Yamauchi in Shōbara City, tells a closely related story. Here a reverse fault has pushed older porphyrite and the basal conglomerate of the Miocene Bihoku Group up onto younger sandstones of the same group. In some places the lower beds have been so violently disturbed that nearly horizontal strata are now folded or even overturned. The fault movement traveled from north to south, leaving behind a south-facing escarpment that, while modest in height, marks the surface trace of the deformation.

Why It Was Designated a National Natural Monument

Japan is one of the most tectonically active countries on Earth, and outcrops of faults are not rare. What sets the Funasa and Yamauchi fault zone apart is the rare combination of three things in a single location: clearly exposed fault planes, well-preserved evidence of geologically very recent movement, and a direct connection between that movement and the larger-scale formation of the Chūgoku Mountains and the Seto Inland Sea.

The presence of Quaternary gravels caught beneath the overthrust granite is particularly important. It demonstrates that the crustal compression responsible for raising the Chūgoku Mountains was still active in extremely recent geological time, not just in some distant prehistoric era. This makes the site a foundational reference for any study of the landscape evolution of western Japan and helps researchers reconstruct the sequence of events that shaped the gentle hills, basins, and inland seas of the region.

The site is also valued as an educational and scientific resource. The clarity of the exposures—where one can literally see older rock sitting on top of younger rock along a tilted fault plane—makes it an outstanding teaching example, both for university geology students and for anyone curious about the deep history beneath their feet.

Highlights and What to Look For

The fault zone is not a single attraction but a collection of separate outcrops spread across roughly sixteen kilometers of countryside. Each section has its own character.

Visitors should bear in mind that this is a working agricultural landscape rather than a manicured park. Many outcrops sit on the edges of fields, roadside cuttings, or wooded slopes. A sturdy pair of shoes, a small bottle of water, and a willingness to peer closely at rock faces will be rewarded.

Surrounding Attractions

The triangle of cities that share the fault zone offers a rewarding rural travel circuit beyond the geology itself.

• Miyoshi City Sea of Clouds (Miyoshi Unkai): From autumn through early spring, the Miyoshi Basin frequently fills with a thick sea of clouds at dawn, viewable from observation points such as Mount Takatani. The fault zone helped create the very basin in which this phenomenon occurs.
• Okuizumo Orochi Loop and the Gōnokawa River Valley: The dramatic river landscapes of the Gōnokawa, Japan's largest river west of Honshū's spine, cut through the geological formations the fault zone helped shape.
• Tojō Plateau and Taishakukyō Gorge: A short drive east of Shōbara, the limestone landscapes of Taishakukyō, with their natural bridges and cool gorges, provide a striking geological contrast to the granite of the fault zone.
• Mōri Clan Historic Sites in Akitakata: The former town of Yoshida-Kōriyama, near Akitakata, was the heartland of the medieval Mōri clan. Visitors interested in samurai history can climb to the ruins of Yoshida-Kōriyama Castle and visit the related shrines and museums.
• Local Hot Springs: Modest but welcoming onsen facilities in Shōbara, Takano, and other parts of the region offer a soak at the end of a day of countryside exploration.