The magnitude-7.3 earthquake that pounded Kumamoto Prefecture early Saturday was more powerful than a foreshock that struck two days earlier.

The seismic activity then moved northeast to reach areas in Oita Prefecture about 100 kilometers away from the focus of the magnitude-7.3 quake. Experts are concerned that quake activities could spread to a major active fault that lies further ahead in that direction.

On Saturday morning, Yomiuri Shimbun reporters entered the town of Mashiki, Kumamoto Prefecture, where aftershocks continue to rattle the area. Many houses in the town’s Dozono district have collapsed. A large crack ran through a field, leaving a line of what looked like overturned earth. A road crossing the field was buckled out of alignment.

According to a calculation by Masashi Omata of the Japanese Society for Active Fault Studies, the ground here moved about 20 centimeters vertically, and about two meters horizontally. This measurement was taken in a northeastern section of the Futagawa fault zone, which extends for at least about 64 kilometers. “Part of the Futagawa fault zone shifted and caused the magnitude-7.3 temblor,” Omata said.

Another fault zone, the Hinagu fault zone, lies south of the quake’s focus. The Hinagu zone is at least about 81 kilometers long, and a magnitude-6.5 quake that struck on Thursday evening was centered in its northeastern section. Nagoya University Prof. Yasuhiro Suzuki, an expert on active faults, said, “The faults are connected, so the earthquakes on Thursday and Saturday occurred as a series of movements.”

The Japan Meteorological Agency initially thought Thursday’s temblor was the main quake. However, it turned out to be a smaller foreshock that was a precursor to the main magnitude-7.3 quake. “When a magnitude-6 level quake happens, it’s difficult to predict whether an even bigger earthquake will occur,” said Gen Aoki, head of the agency’s earthquake and tsunami monitoring section.

Activity moving northeast

The quake activity that began on Thursday became even more heightened following Saturday’s main quake. Furthermore, the location of this activity, which started in areas around Kumamoto city, moved northeast and expanded to include the Aso region of Kumamoto Prefecture and central Oita Prefecture.

These three areas where large quakes have recently occurred are in or near what is called the Beppu-Shimabara rift zone. The ground structure in the Kyushu region is subject to forces that pull it north-south. Many active fault lines, including the Futagawa and Hinagu fault zones and the Beppu-Haneyama fault zone in central Oita Prefecture, have developed where the ground has been warped in parts of the rift zone.

According to the agency, whenever a fault slips, it can place additional stress on the ground further along the line. “Earthquakes in the three areas are likely affecting each other,” an agency official said.

Yasuhiro Umeda, a professor emeritus at Kyoto University and a seismology expert, was surprised by the distance between the epicenters of the quakes. “I didn’t anticipate that an earthquake might be set off so far away,” Umeda said.