Man standing on a road damaged by an earthquake

The 2011 Tohoku earthquake, which was centred at a fault in the Pacific Ocean, killed more than 18,000 people in Japan. Credit: Hitoshi Yamada/SIPA/REX/Shutterstock

Geophysics

Gentle ‘slow slip’ earthquakes belie hidden danger

Fluid build-up after a slow quake raises the risk of massive rupture.

Seemingly mild slow-motion earthquakes can raise the risk of more dangerous quakes by shifting fluids along geological faults.

‘Slow-slip’ quakes release their energy over weeks or months, rather than seconds. Many such quakes occur at subduction zones, where one plate of Earth’s crust dives beneath another.

Junichi Nakajima at the Tokyo Institute of Technology and Naoki Uchida at Tohoku University in Sendai, Japan, analysed earthquakes off the Pacific coast of Japan, a subduction region prone to both slow-slip quakes and sudden big ones such as the devastating Tohoku quake of 2011. The scientists found that several months after a slow-slip event, the number of shallow earthquakes temporarily increased.

The authors say that a slow earthquake can cause fluids such as water to migrate upward within Earth’s crust and trigger small quakes. If these fluids are unable to drain properly, this could raise the risk of a big quake.