April 28, 2016

Japan's San Andreas

The 7.3 magnitude mainshock that struck Kumamoto prefecture on 16 April (Japan time) was preceded by a 6.5 magnitude earthquake and followed by more than 1000 aftershocks. Less a devastating head-on collision than the wheels falling off a truck at 60 MPH and the axles dragging along a road filled with potholes.

Unlike the colossal magnitude 9 quake that devastated the Tohoku region of Northern Japan in 2011, the Kumamoto earthquake did not trigger a tsunami. Occurring in a largely rural part of Japan on the southern island of Kyushu, it has caused only 49 deaths to date. The most severe damage was from landslides.

Not the edge of a cliff, a landslide (courtesy Japan Times).

The earthquake struck along a shallow inland slip fault, specifically the Futagawa-Hinagu fault link, at the western edge of the Japan Median Tectonic Line.

A slip fault vividly illustrated (courtesy Japan Times).

Like the San Andreas, the Japan Median Tectonic Line is clearly visible on topographical maps. It begins its journey off the western coast of Kyushu, heads east-northeast, hugs the northern coast of Shikoku, bisects the Kii Peninsula, doglegs around Mt. Fuji, and plunges back into the ocean fifty miles east of Tokyo.

To the north, the Philippine Sea Plate is colliding with the Pacific Plate and the Okhotsk Plate. Moving at a brisk 48-84 mm/year to the west-northwest, the Pacific Plate rams the southern edge of the Okhotsk Plate and the northern edge of the Philippine Sea Plate, crumpling the bedrock of central Japan into the Japan Alps.

The collision between the Philippine Sea Plate and the Pacific Plate forms the Ogasawara Islands, that start at the Izu Peninsula and include Iwo Jima at its southern end. The Izu Peninsula sits at the feet of Mt. Fuji and is home to Hakone National Park and its countless hot springsā€”a mere 50 miles south of Tokyo.

Sitting on top of the convergence of four massive tectonic plates, perhaps the most amazing thing about Tokyo is that it continues to exist at all.

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