Chiba quakes: Experts expect slow slip event to trigger more temblors

A series of earthquakes have recently occurred off Chiba Prefecture's eastern coast. Experts say a phenomenon known as a "slow slip event" appears to be taking place where tectonic plates meet. They are cautioning people to expect more seismic activity.

Japan's Meteorological Agency says seismic activity has been increasing in the area near Tokyo since February 26. As of 6 p.m. on Sunday, 31 earthquakes with an intensity of one or higher on the Japanese scale of zero to seven have been observed.

Three of the quakes were four or more. A magnitude 4.9 earthquake hit Thursday evening, a magnitude 5.3 quake struck Friday morning and a magnitude 5.0 quake took place around 2 a.m. Saturday.

The earthquakes occurred not only off Chiba's eastern coast but also in its southern inland area.

What are slow slip events?

Slow slip events are caused by tectonic plates moving against each other at speeds slower than a regular quake.

Oceanic and land tectonic plates overlap off the east coast of Chiba Prefecture. The normal movement of the oceanic plate causes the ground to creep northward.

After seismic activity intensified in February, an expert at Kyoto University's Disaster Prevention Research Institute analyzed GPS data from Japan's Geospatial Information Authority and saw the ground was moving southeast in almost the opposite direction.

Nishimura Takuya says, "It is highly likely that slow slip is occurring."

A Geospatial Information Authority analysis estimates the amount of southeast movement to be around 2 centimeters.

Normally the tectonic movement off Chiba shifts northward (yellow). In February, however, an expert observed movement in a southeast direction (red), suggesting slow slip is taking place.

Slow slip events have been observed six times off the east coast of Chiba between 1996 and 2018 — a time marked by increased seismic activity.

The Meteorological Agency and other experts are calling on people to exercise caution.

Nishimura says, "It's better to think that there's a possibility of seismic activity of lower 5 or so for the next month or so."

Nishimura Takuya of Kyoto University's Disaster Prevention Research Institute is a crustal movement expert.