Why was the Kumamoto earthquake different? Scientists explain a rare mechanism behind its activity

 

Scientists have announced that the devastating earthquake that struck Kumamoto Prefecture in southwestern Japan on Tuesday, killing 13 people and injuring hundreds, was caused by a relatively rare type of internal seismic activity.

The powerful earthquake, measuring 6.8 on the Richter scale, struck at approximately 4:27 p.m. local time on the southern Japanese island of Kyushu, with its epicenter in Kumamoto Prefecture, according to local authorities. 

Japan experiences frequent earthquakes due to its location at the intersection of several major tectonic plates, and its land is crisscrossed by numerous shallow faults. However, scientists explained that this particular earthquake was caused by a relatively rare type of internal seismic activity.

According to the U.S. Geological Survey (USGS), the earthquake was caused by a shallow strike-slip fault. Unlike most major earthquakes in Japan, which occur off its east coast due to tectonic plate collisions and one plate sliding beneath another (known as subduction), Tuesday's earthquake occurred inland, as a result of so-called thrust events, where one layer of rock is forced upwards, overriding the layer beneath it.

In their newsletter "Earthquake Insights," prominent seismologists Jody Hubbard and Kyle Bradley explained that these internal earthquakes are usually smaller than major subduction earthquakes, but they are closer to population centers, making their tremors more impactful in populated areas.

Scientists found that the earthquake occurred several hundred kilometers northwest of the Ryukyu Trench, where the Philippine Sea Plate beneath Japan and the Eurasian Plate are sliding northwest at a rate of 58 millimeters per year. However, the U.S. Geological Survey confirmed that the shallow depth and fault pattern indicate that the earthquake originated within the upper Eurasian Plate, not at a subduction zone.

Scientists warn that Japan's internal fault network is extremely complex, and large ruptures often activate multiple fault threads over wide areas. Therefore, determining the precise cause of the earthquake and its exact depth will take longer, relying on field observations and satellite radar surveys to be conducted in the coming period.