Wednesday’s 6.6-Magnitude Quake Thought to Have Different Mechanism than Expected Nankai Trough Quake

The Yomiuri Shimbun

18:16 JST, April 18, 2024

A massive earthquake of magnitude 8 to 9 with a focus in the Nankai Trough, which stretches from Suruga Bay in Shizuoka Prefecture to the coast of eastern Kyushu, is estimated to have a 70% to 80% chance of occurring within 30 years. The quake is expected to leave as many as 320,000 people dead or missing.

The 6.6-magnitude earthquake on Wednesday night occurred within the expected hypocenter area of the anticipated Nankai Trough earthquake, but Meteorological Agency officials say it is not considered a precursor to a massive earthquake.

One of the reasons cited by the officials was that the difference in the rupture mechanism between the expected massive quake and the latest tremor. In the Nankai Trough, an oceanic sea plate subducts a few centimeters per year under a continental plate. In this region, the plate boundary moves significantly and massive earthquakes have been documented with an average recurrence time of about 100 to 150 years. The next great earthquake is expected to occur in a similar manner.

On the other hand, the focus of Wednesday’s quake was at a depth of 39 kilometers, a location deeper underground than the plate boundary at a depth of about 30 kilometers, and that the earthquake occurred within the oceanic plate that is subducting beneath the continental plate. Officials say that the movement of the plate is believed to have caused tensions to pull east and west, breaking a bedrock.

In addition, experts point out that the size of the latest quake is relatively too small to trigger a massive earthquake. The area around the latest focus has been hit by several quakes in the past, including a magnitude 6.6 quake in August 1968 and a magnitude 5.7 quake in July 2015.

“The scale of the latest quake doesn’t seem to be large enough to trigger a Nankai Trough earthquake,” said Prof. Junichi Nakajima of the Tokyo Institute of Technology.