The magnitude-6.7 earthquake that struck Japan’s Hokkaido island on 6 September had an outsize impact on the landscape: hundreds of landslides that scarred hillsides, decapitated ridges, and caused most of the 41 deaths attributed to the earthquake. Now, some scientists are saying the island was primed for the landslides when heavy rains soaked subsurface deposits of volcanic soil in the region, turning them into a geologic grease layer. Others, however, aren’t convinced yet.
Like much of Japan, Hokkaido hosts numerous volcanoes, both active and dormant. Kyoji Sassa, a landslide expert and professor emeritus at Kyoto University in Japan, believes eruptions over the ages have left layers of volcanic material such as pumice draping the hilly landscape beneath sediments deposited later, as is seen in many other volcanic areas in Japan. The porous volcanic material readily soaks up water and becomes slippery, Sassa says. And Hokkaido had just been hit by drenching rains from Typhoon Jebi, the most powerful storm to strike Japan in 25 years. It pummeled the Osaka area to the south on 4 September, then traveled up the archipelago, delivering heavy rains to Hokkaido.
When the earthquake shook the water-logged soils a few days later, Sassa says the shear forces easily ruptured the weak pumice strata, allowing tons of heavy, wet soil to slide downhill. The landslides, concentrated in an area near the quake epicenter on the southern side of the island, “moved very rapidly,” says Sassa, founder and secretary general of the International Consortium on Landslides, because wet pumice is particularly slippery.
Shin-ichiro Hayashi, a landslide scientist at Hokkaido University in Sapporo, Japan, who has been out to investigate the landslides, agrees that volcanic material is laced through the soils. But he says “further investigation is required” to determine whether those deposits really set the stage for the recent landslides. Hayashi notes that Jebi’s rains were modest on Hokkaido. Instead, he says, a simpler explanation is that the rare inland earthquake along a previously unknown fault gave the unstable landscape near the epicenter an unprecedented shaking.
But Sassa thinks the landslides defacing the wooded hills deliver a clear message: “An earthquake after rainfall is very dangerous; if one occurs, people living near cliffs should evacuate rapidly.”