Japan lifts tsunami warning after powerful earthquake off northern coast

What is a thrust-fault earthquake?

Johanna Wagstaff here in Vancouver. I’m a science reporter and my background is in seismology.

Based on seismic data we are already able to tell what type of earthquake it was: a thrust-fault earthquake.

This occurs when a closed section of rock suddenly gives way and an overlapping piece of rock jolts upward. It is that vertical movement that causes tsunamis to be a concern because even a small lift of the sea floor can displace a lot of water.

At a depth of about 50 kilometers, this is not a shallow rift, but still within the range where seafloor deformation can occur.

The earthquake occurred at the boundary where the Pacific Plate dives beneath northern Honshu (Japan’s largest island). A similar system exists in British Columbia where the other end of the same Pacific Plate pushes beneath North America.

These subduction zones store stress for years before releasing it, often in fragments. And because the zone is composed of multiple locked and partially locked sections, a breakage does not reset the entire system. More than 100,000 earthquakes are recorded per year in Japan, mostly small, as the tectonics here are incredibly active and complex.