Tsunami Boulders

Ishigaki Photo Diary: 8th Feb 2010.For anybody who's every gazed out across one of Ishigaki's calm and tranquil coastlines it's hard to imagine the hell which was unleashed on the island one Thursday morning 239 years ago. On the 24th of April 1771 Ishigaki was hit by one of the largest tsunamis in recent natural history, the source of which is thought to be an earthquake with a magnitude of around 7.5 which took place to the east of the island.

I'd heard of the tsunami before I moved to Ishigaki but it wasn't until recently that I learned that it had deposited some huge boulders inland which would later provide evidence of the sheer size of the tidal wave which battered Ishigaki. The best example is probably the huge boulder which can be found in Ohama's park, about a 10 minute drive from the city center.

Ohama park is home to a huge boulder which was carried inland by the 1771 tsunami. The tsunami may claim one more victim when those stairs finally collapse.

Although a flight of stairs has been erected next to the boulder I was surprised by the absence of any memorial or information describing the origin of the huge rock. It was carried over 150 meters from the coast to a height of around 6m above sea-level and remains of dead coral which coat the rock leave little doubt that it originated from the ocean.

The boulder was carried over 150m inland

Gazing down the road towards the ocean it's hard to imagine the size of wave which would have been required to carry the boulder to its current resting place (right of picture - covered by trees)

Coral remains which coat the boulder leave no doubt that it originated in the ocean

The run-up height of the tidal waves which struck Ishigaki's east coast has been the subject of much debate. Even conservative estimates indicate that it must have been at least 15m high but many scientists believe that boulders such as the one found at Ohama provide evidence that the largest waves may have reached a height of 30m.

Whatever the height the undisputed fact is that the tsunami was utterly devastating with most villages from Ohama all the way up to Ibaruma being completely destroyed. Around 13,500 people were killed on Ishigaki alone and many other islands in the Yaeyama archipelago also suffered considerable losses.

Although Ohama may be home to the largest rock which has been desposited inland the east coast of Ishigaki island is littered with evidence of the tsunami with one coral boulder having been carried to a height of 17m above sea level. Boulders are found all the way up to Ishigaki's Hirakubo peninsula and coastlines such as those found at Miyara and Ibaruma are surrounded by large coral rocks.

Miyara's coastline is also littered with large boulders

What if it happened again?

Just this weekend I experienced something new on Ishigaki island - my first tsunami warning, complete with sirens and announcements telling everybody to leave coastal areas immediately. What interested me is that the vast majority of people seemed to pay no notice whatsoever and the general attitude seemed to be one of 'oh, it probably won't arrive' or 'tsunamis don't tend to be that big nowadays'. This made me question the value of having an early-warning system at all. I fear that if Ishigaki was to suffer another large tsunami along the lines of the one which hit the island in 1771 then the outcome in terms of loss of human life may once again be devastating.