Photo/Illutration The Asahi Shimbun

Scientists are piecing together data from "silent earthquakes," known as slow-slip events, detected in the Nankai Trough to better understand the mechanisms behind massive temblors and promote research on quake forecasting. 

A joint research team of the Institute of Industrial Science at the University of Tokyo and the Japan Coast Guard published the results in the U.S. journal Science Advances on Jan. 15.

The researchers said they detected slow-slip activity, in which a plate boundary moves without causing tremors, in assumed seismogenic zones of the Nankai Trough off the Pacific coast of western Japan. 

"It's the first time for slow-slip events to be detected on the offshore side of the (Nankai Trough)," Yusuke Yokota, a lecturer at the institute who specializes in underwater information systems, said, adding that conducting research offshore and beneath the seafloor has been difficult. 

"The impact of slow-slip events on asperity (a vulnerable area of a fault considered "locked" and potentially the site of an earthquake) is still unknown," said Yokota, who led the research team. "But once we understand the impact, the findings will be helpful in giving a fuller picture of an earthquake."

During a slow-slip event, a fault moves gently, releasing energy accumulated over time, without releasing a seismic wave. Such phenomena have been regularly detected in plate boundary regions and other areas, and researchers have studied the correlation between such activity and regular quakes.

As part of its research, the team analyzed data on crustal deformations observed from 2006 to 2018.

The data was collected at 15 seafloor locations within the assumed seismogenic zones of the trough, which is 50 kilometers or more off the coast, using a JCG survey ship and satellite positioning system.

They found that slow slips of about 5 to 8 centimeters occurred at seven locations during the period from 2009 to 2018. 

All were beneath the seafloor, 10 to 20 km from the surface of the sea. The areas were on the offshore side of the asperity, where subducting plate boundaries are believed to be pressed together firmly.

The Japan Meteorological Agency has also monitored slow-slip events in certain areas of the zones concerned in the Nankai Trough. 

If a slow slip with unusual speed and size is detected, the agency will issue an "incidental information" warning to the public that there is a heightened risk of an earthquake. 

As the research team’s observations were made at different locations from those that the agency has set up, the findings were not used to issue a warning.