Japan downgrades tsunami warning, but warns of potential for stronger quakes in coming days

Here's how earthquakes tin do tsunamis

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Scientists will now focus on how large the rupture patch was and how much stress was transferred to neighbouring segments. That area has a complicated seismic history, and even moderate-sized quakes like this one can reshape the stress landscape for years.

Thanks to Japan's dense offshore seismic network, researchers will quickly piece together a high-resolution image of the rupture. In fact, every aftershock helps outline the edges of the section that actually moved, giving us a clearer picture of how this event fits into the broader earthquake cycle.

Johanna Wagstaffe here in Vancouver. I'm a science reporter and I have a background in seismology.

We are already able to tell based on the seismic data what kind of earthquake this was: a thrust-fault quake.

These happen when a locked section of rock suddenly gives way and the overlapping piece of rock jolts upward. That vertical motion is the reason a tsunami is a concern because even a small lift of the sea floor can displace a lot of water.

At roughly 50 kilometres deep, it's not a shallow rupture, but is still well within the range where seafloor deformation can happen.

This earthquake struck on the boundary where the Pacific plate dives beneath northern Honshu (Japan's largest island). British Columbia has a similar set-up where the other end of the same Pacific plate pushes under North America.

These subduction zones spend years storing stress before releasing it, often in patches. And because the zone is made up of many locked and partially locked sections, one rupture doesn’t reset the whole system. Japan records more than 100,000 earthquakes a year, mostly tiny ones, because the tectonics here are incredibly active and complex.

There are three levels of emergency tsunami alerts in Japan, the highest being a major tsunami warning and second highest a tsunami warning. A tsunami advisory is the lowest of the three.

A major warning is for tsunamis expected to be higher than three metres, while a warning — like the one at first issued today — is for tsunami waves of up to three metres.

An advisory is when there are forecasts for tsunami waves of at least one metre. It calls for people to "get out of the water and leave coastal areas immediately," according to the JMA's website.

"Anybody exposed will be caught in strong tsunami currents in the sea. Fish farming facilities will be washed away and small vessels may capsize," it says.

The magnitude of the earthquake that took place at around 11:15 p.m. Local time has also been updated from a 7.6 to 7.5, according to Reuters.

Prime Minister Sanae Takaichi told reporters early on Tuesday morning local time that she had been told of seven injuries so far.

The JMA's advisory applies to a wide region from the northernmost island of Hokkaido down to Chiba prefecture, east of Tokyo, calling on residents to be on alert for the possibility of a powerful earthquake hitting again within a week.

The Japanese government has just downgraded the tsunami warning to a tsunami advisory.

The International Atomic Energy Agency (IAEA) says it hasn't observed any abnormalities at the Fukushima nuclear power plant today, Reuters reports.

The northeastern region of Japan is regularly at risk of serious earthquakes — and about 15 years ago, it experienced one of the most devastating ones in the world to date.

On March 11, 2011, a 9.0 magnitude earthquake struck under the Pacific Ocean off the coast of the northern city of Sendai.

It was the most powerful ever recorded in Japan, according to Reuters, and set off a series of massive tsunamis that killed nearly 20,000 people.

That tsunami also damaged the Fukushima Daiichi nuclear plant, leading to a series of explosions and meltdowns in the world's worst nuclear disaster since the Chornobyl disaster in 1986.

"There is a possibility that further powerful and stronger earthquakes could occur over the next several days," a JMA official said at a briefing. The warning for a possible tsunami up to three metres high is still in place.

About 90,000 people are under an evacuation order in the region.

The waves observed at several ports were as high as 70 centimetres, according to Reuters.

The number of people without power has been lowered from thousands to hundreds, Tohoku Electric now says.

In addition to the injuries reported at a hotel in the Aomori town of Hachinohe, a man in the town of Tohoku was slightly hurt when his car fell into a hole, public broadcaster NHK reported.

The number of people hurt at the hotel and the extent of their injuries are unclear.

The JMA says a tsunami of 50 centimetres was measured in Kuji port in Iwate prefecture, just south of Aomori.

Tsunami levels of 40 to 50 centimetres struck other coastal communities in the region, the agency said.

Chief Cabinet Secretary Minoru Kihara urged residents to immediately head to higher ground or take shelter inside buildings or evacuation centres until the alert is lifted, The Associated Press reported.

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