One Day To Change Your Life – Mark Sowerby Experienced Many

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It was a long time coming – over 10 years – but Mark Sowerby (52, Australia, MSF bio here) finally achieved the Oceans Seven with unique crossing of the Tsugaru Channel between Honshu and Hokkaido in northern Japan in July 2024.

He not only endured a rapid change in water temperature – from 22°C to 14°C – but also a complete shift in the water flow due to an earthquake. Sowerby literally swam through an underwater tsunami that no one saw coming.

There were two unique happenings during Sowerby’s 10 hour 6 minute crossing that Captain Kawayama, coach Tim Denyer (@redtopperformance), and support crew could not explain. They simply chalked it up to the unique topography, general geomorphology of the area, and the natural phenomenon of the Tsugaru Current.

A 90° Course Shift

Mid-channel at a depth of 200 meters around 4 hours after his start in Aomori Prefecture on Honshu, Sowerby 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. Neither Captain Kawayama nor his coach Tim Denyer specifically remembers making a sudden shift that was identified by his GPS coordinates. Then without another conscious effort, he returned to his intended course and carried on to Hokkaido, his ultimate goal.

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.

What occurred?

An Earthquake Hit 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. Its 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.

It was assumed 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.

Don’t Be Prey

On October 24th, Sowerby’s story will be shown in a 1 hour 34 minute biopic titled Don’t Be Prey (@dontbeprey.film, IMDb here) at its premiere in London.

Based from his farm in the Gold Coast of Australia after a dramatic life-changing event, the 52-year-old Sowerby traveled the world over the 10 years to become the 29th person in history to completed the Oceans Seven. Don’t Be Prey tells of his global journey together with his coach Denyer, wife Heidi and filmmaker Jeff Tseng, and his interactions and relationships with Bengisu Avci (28, Turkey, MSF bio here), Jamie Mackay (43, Great Britain, MSF bio here), Steven Munatones (63, USA, MSF bio here), and shark attack survivors Paul Leonard (52, Great Britain, MSF bio here) and Joanne Norman (55, Australia, MSF bio here).

Don’t Be Prey intersperses the drama of various channel crossings along with the obstacles that stand in front of Sowerby, both on dryland and in the ocean.