Photo/Illutration The Asahi Shimbun

Tokyo and other areas in the Kanto region have been rattled by a host of earthquakes in recent months that originated in the same area, prompting questions about what has been occurring beneath the surface.

Officials recorded a tremor of up to 3 on the Japanese seismic intensity scale of 7 in Tokyo’s 23 wards, as well as in nearby Ibaraki, Saitama, Chiba and Kanagawa prefectures, when an earthquake struck at 10:29 p.m. on April 4.

Two minutes later, another quake registering a maximum intensity of 2 was felt across the 23 wards and Yokohama.

The two quakes originated in northwestern Chiba Prefecture, according to the Japan Meteorological Agency.

The northwestern portion of the prefecture was also jolted by a temblor at 8:52 p.m. on March 31. The focus of the quake was located in Tokyo Bay off the prefecture, and officials observed a maximum intensity of 4 in Chiba’s Hanamigawa Ward.

The focuses of these quakes had a depth of 60-73 kilometers, not far from the ground’s surface.

The temblor that registered a maximum intensity of 5 in Tokyo’s Adachi Ward and Kawaguchi, Saitama Prefecture, on Oct. 7 was similar to the recent quakes in terms of the location of its focus. The Oct. 7 quake derailed cars of the Nippori-Toneri Liner train in the capital.

These temblors all hit around the same area, raising questions of whether they are associated with each other.

An official with the Meteorological Agency said that remains “unclear,” but added that the area from which the recent spell of earthquakes originated has historically been home to a slew of temblors.

For instance, three powerful earthquakes that measured a magnitude 6.0 or more shook the area in the 1950s. And similar quakes struck there in 1980 and 2005.

“People may have the impression that quakes are occurring more frequently than before because the recent quakes hit in rapid succession,” the official said.

The Kanto region is prone to earthquakes, as the boundaries of the tectonic plates underneath it have a complex structure.

The Philippine Sea Plate sinks under the North American Plate. The Pacific Plate subducts under the Philippine Sea Plate.

Even though the focuses of the temblors last fall and in recent weeks are located close to each other, how they occurred are dissimilar, according to the agency.

Agency officials believe that the two temblors this month occurred in the Philippine Sea Plate while the one in October and another in late March originated between the boundaries of the Philippine Sea Plate and the Pacific Plate.

In addition, the Kanto region can be shaken by an earthquake that strikes outside the region.

For example, Tokyo’s 23 wards recorded an intensity of 4 when a quake struck off Fukushima Prefecture on March 16. It was a temblor originating in the Pacific Plate. An upper 6 was observed in Fukushima and Miyagi prefectures from the quake.

The Great Kanto Earthquake of 1923, a magnitude-7.9 quake that left 105,000 people dead, struck between the boundaries of the Pacific Plate and the Philippine Sea Plate.

The phenomenon of these boundaries slowly sliding was observed in 2018, with noticeable tremors felt across the Kanto region.

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