Breaking News: Seismological Shift Sends Swimmer Shifting Along the Surface of Tsugaru

On Saturday, June 29th, Australian Mark Sowerby achieved the Oceans Seven with a 19.5 km crossing of the Tsugaru Channel between Honshu and Hokkaido in northern Japan.

In particular, there were two happenings during his 10 hour 6 minute crossing that Captain Kawayama, coach Tim Denyer, and support crew could not quite explain. They simply chalked it up to the unique topography, general geomorphology of the area, and the natural phenomenon of the Tsugaru Current.

Phenomenon 1: A Massive Course Shift

Mid-channel at a depth of 200 meters around 4 hours after his start in Aomori Prefecture on Honshu, Sowerby suddenly took an inexplicable sharp turn towards the east (see below). There was no conscious effort to make a dramatic – or even modest – change in his swimming orientation or direction. No one on his escort boat remembers specifically making a sudden shift that was identified by his GPS coordinates. And then without another conscious effort, he returned to his intended course and carried on to Hokkaido, his ultimate goal.

Phenomenon 2: A Massive Water Temperature Shift

Approximately two hours from his finish, about 8 hours into his swim, Sowerby suddenly stopped because he felt a dramatic drop in the water temperature. The water temperature within a 100-meter area at a depth of between 50-70 meters decreased from 20°C to 14.5°C. The tough Aussie was hardened, but the water temperature shift was a kick to the gut and made him focus.

Sowerby did not specifically see anything particular during either the course shift or the temperature shift. “As far as the sunlight reached the depths, I could see deep below in the clear waters of Tsugaru,” he recalls.

So what might have occurred?

Earthquake Hits Northern Japan

A magnitude 2.5 earthquake hit the Sea of Japan at 8:14 am – which was 4 hours 8 minutes after Sowerby started on the Kodomari Cape.

The earthquake had a very shallow depth of 7.2 km and was not felt on dryland in the area.

The earthquake epicenter was 40.633°N / 139.12°E while Sowerby was approximately at 41.1243°N / 140.2479°E or about 513 km to the east.

Tsunamis can move at great speeds and have tremendous energy, but its speed depends on the depth of the water it is traveling through (see here). In deeper water, the water moves faster. At a depth of 200 meters, the speed of a tsunami is 159 kph. The ocean at the epicenter was greater than 200 meters, but where Sowerby was swimming, the depth was 200 meters.

So it is reasonable to assume that the displacement of the water at the epicenter could have impacted the surface waters across the Tsugaru Channel and direction and speed of the Tsugaru Current four hours after the earthquake hit.

It is also reasonable to assume that the displacement of the water could have impacted the water temperatures at a later time as the cold water from below gradually mixed with warmer water at the surface – resulting in a 6.5°C temperature drop.

Conjecture

Sowerby’s crossing started with very light winds and unexpectedly warm (22°C) water temperatures at the start in Kodomari on Aomori Prefecture.

Could the magnitude 2.5 earthquake impact the flow of water – that always flows in the eastward direction across the Tsugaru Channel – and the surface water temperatures?

Quite possibly.

Cameraman Jeff Tseng who was filming throughout the crossing is reviewing the drone footage taken during Sowerby’s swim to see if there are any dramatic changes in water color, texture, speed, conditions, or whitecaps during these times.

More research will be conducted, but if there are seismologists and oceanographers with experience and interest in researching and analyzing the possibility of an undersea earthquake impacting a channel crossing, please contact Steven Munaton e s via email at steven.munatones@kaatsu.com.