6.8 earthquake traps shoppers as mall collapses in Japan —‬ here's why the seismic event was unusual

A powerful earthquake struck southwestern Japan Tuesday afternoon local time, collapsing part of a shopping mall, toppling a stone wall at the centuries-old Kumamoto Castle, and sending more than 150,000 people to evacuation shelters, the BBC reported.

The quake, which the U.S. Geological Survey (USGS) measured as a magnitude 6.8, hit at 4:27 p.m. local time near Kumamoto, a city of more than 700,000 people located on the island of Kyushu in southern Japan.

Thousands of homes lost power, and roads buckled across the region. The second floor of a shopping center collapsed, trapping shoppers inside, according to the BBC report; some of these people were evacuated and later hospitalized. A brief tsunami warning was issued and lifted within an hour.

The USGS measurement is different from the magnitude 7.1 reported by the Japan Meteorological Agency, "primarily because the agencies are reporting different magnitude types," the USGS reported. While Japan is looking at local magnitude, the USGS measured the moment magnitude, which is derived from the physical size of the fault rupture and gives the "most reliable estimate of earthquake size" for large earthquakes, since local-magnitude scales can lose accuracy once a rupture grows big enough, the USGS reported.

These measurements are helpful, as "different magnitude types capture different characteristics of earthquake source processes," the USGS noted. Local magnitude, for example, reflects how strongly the ground shook nearby, while moment magnitude reflects the total energy released across the fault.

Why this quake is scientifically interesting

Japan sits atop one of the most active plate boundaries on Earth, where the Philippine Sea Plate dives beneath the Eurasia Plate. That process, known as subduction, is responsible for the massive, tsunami-generating earthquakes that periodically strike offshore Japan, and it's been studied intensively for decades.

But Tuesday's earthquake didn't occur at the boundary itself. Instead, the rupture happened within the Eurasia Plate, more than 60 miles (100 kilometers) inland from where the plates meet, Jeffrey Park, a theoretical seismologist at Yale University, told Live Science. Roughly 90% of earthquakes happen at plate boundaries, where the constant grinding of two plates builds up stress until a fault gives way. Quakes that strike farther inside a plate — sometimes called intraplate earthquakes — are rarer and less understood, though scientists suspect that stress transmitted through the plate can reactivate old, dormant faults far from the boundary that produced it.

According to Park, the fault in Tuesday's earthquake moved in a way that suggests the plate tore horizontally as it was twisted by subduction occurring at an angle.

The USGS describes this event as shallow strike-slip faulting: the two sides of the fracture slid past each other horizontally, rather than one side thrusting up and over the other. This faulting style, combined with the shallow depth of just 6 miles (10 kilometers) down, points to the same type of rupture behind Kyushu’s deadly 2016 Kumamoto earthquake sequence.

That inland location is also why the quake didn't generate a damaging tsunami, despite the brief precautionary warning. Tsunamis are produced when a big event ‪—‬ such as an earthquake, volcanic eruption or landslide ‪—‬ displaces water; the force from that displacement then propagates as a wave.

Tuesday's rupture stayed entirely on land, so it never pushed the seafloor up the way offshore quakes do. That's why the tsunami risk stayed low, even with the brief precautionary warning. Still, an inland quake isn't automatically off the hook: if the shaking triggers a landslide that spills into the sea, that alone can generate a tsunami.

The depth mattered just as much as the location. The quake was shallow enough to concentrate its energy near the surface, which Park said explains the structural damage.

"Because it is shallow it will cause more structural damage than a similar-sized event that might occur deeper and offshore," he told Live Science in an email.

Earthquake country

Several earthquakes have already clustered in the area, and numerous aftershocks are expected over the coming days. Japan experiences roughly 1,500 earthquakes annually, making it one of the most seismically active countries on the planet — a byproduct of sitting at the junction of several tectonic plates.

To protect itself, Japan has spent decades — and learned from painful lessons from disasters like the 2011 Tohoku earthquake and tsunami — engineering a response system built for exactly this scenario.

Bullet trains are programmed to halt automatically the instant seismic sensors detect a major earthquake, a safeguard Park pointed to as a defining feature of the country's earthquake readiness. That automatic shutdown extends to other transit and utility systems, part of a broader early-warning network that alerts the public within seconds of a quake's detection, giving people crucial time to take cover before shaking arrives.

Japan's investment in its infrastructure may help to explain why, even after a magnitude 7 event near a city of 700,000, Japan's death toll and damage have historically been far lower than those from comparable quakes elsewhere in the world.

Search-and-rescue efforts are continuing in Kumamoto, with CNN reporting that between 20 and 30 employees remained unaccounted for at the Aeon Mall in Kashima Town, where the building’s second floor had collapsed. Firefighters were also working to reach the site and contain nearby fires.