The 1940 Shakotan earthquake occurred on August 2 at 00:08:22 JST with a moment magnitude (Mw ) of 7.5 and maximum JMA seismic intensity of Shindo 4.

Along the island arc's east and southeast coast, subduction of the Pacific and Philippine Sea Plates occur at the Japan Trench and Nankai Trough, respectively.

East–west compression resulted in the formation of fold and thrust belts along its eastern margin during the late Pliocene.

[2] Seismicity along the eastern margin of the Sea of Japan extends from the west coast of Honshu and Hokkaido to Sakhalin in the north.

A focal mechanism solution obtained for the 1940 shock corresponded to pure dip-slip (reverse) faulting on a near north–south striking plane.

[8] These reverse fault planes run parallel to the eastern margin of the Sea of Japan and are steeply-dipping.

A dive survey around the earthquake source area in the 2000s revealed anticlines of the fold and thrust belt were affected by recent seismic shaking.

[9] The tsunami was generally moderate near the source area (Hokkaido) but surprisingly large at distant places (Korea and Russia).

On the Shakotan Peninsula, the waves killed 10 people, destroyed 20 homes, swept away 644 boats, and damaged a further 612.

Seven hundred and twenty one fishing boats were damaged or destroyed; including a motor ship which sunk.

It carried three motor torpedo boats inland, where they were dumped on peatland between the Oprichninka and Pryamaya Pad rivers.