The Shimabara Disaster
In August 1791, the twelfth California Mission was established in my town of Santa Cruz. Why bring this up? Well, the very next year hosts today’s topic, The Shimabara Disaster. Let’s set the date on the “way back machine” at 1792; the place – Shimabara Peninsula, southern Kyushu, Japan.
While Santa Cruz was a dusty backwater speck in 1792, the lands around Ariake Bay Japan had long been organized under the Tokugawa Shogunate and offered a fertile and thriving home to many tens of thousands of people. The region’s showcase, Shimabara Castle, sat near the foot of the Fugen/Heisei volcanic complex that peaks to 1400 m elevation.
Although no stranger to the volcano’s whims, inhabitants began to take serious notice in February 1792 when several big earthquakes shook, accompanied by ash cloud emission from one of the complex’s side vents. February through April witnessed lava flows and steam eruptions from various locations. On April 29 a large slope failed on Mt. Mayuyama, east of the high peaks and about 4 km west of Shimabara Castle.
Events culminated on May 21 when an earthquake-induced landslide dropped 440 million cubic meters of rock from the 760 m elevation of Mt. Mayuyama’s flank. On impact with Ariake Bay, the landslide debris generated a towering tsunami that crossed 20 km of Bay and inundated the flatter lands to the east. Extant tsunami-dome Ishi stones still mark the limit of the wave that reached 22 m elevation and ran in as far as 6 km from the coast. It is estimated that 15,000 people perished in the landslide and tsunami. To this day the Shimabara Disaster ranks as the worst volcanic disaster in Japanese history.
Although they did not know it, compared to the inhabitants of southern Kyushu that May 21-st, folks in the dusty backwater of 1792 Santa Cruz could count their blessings.
Steven N. Ward Santa Cruz
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John Rundle is a Distinguished Professor of Physics and Geology at UC Davis and the Executive Director of the APEC Collaboration for Earthquake Simulations. He chaired the Board of Advisors for the Southern California Earthquake Center from 1994 to 1996. Read John's blog.