A seismologist with the US Geological Survey warned that aftershocks caused by the 7.5 magnitude earthquake that struck western Japan on Monday could last for months.

Susan Hough said people living in that part of the country have felt earthquakes before, but she believes this is “the biggest earthquake by far” — which means most residents likely don’t have experience with a seismic event of this scale.

“An earthquake this big is going to continue to have aftershocks. It could easily have aftershocks bigger than magnitude 6, so that is going to be a hazard in its own right,” Hough said.

The initial earthquake collapsed buildings, caused fires and triggered tsunami alerts as far away as eastern Russia, prompting orders for residents to evacuate affected coastal areas of Japan.

In 2011, a 9.1 magnitude earthquake in eastern Japan caused a tsunami with 30-foot waves that damaged several nuclear reactors. Hough said, comparatively, while the risk of a tsunami of that 2011 size is less likely in the case of Monday’s earthquake, people living near the coast should still evacuate. 

Some of the first reports came from the city of Wajima in Ishikawa prefecture, which saw tsunami waves of around 1.2 meters (3.9 feet) around 4:21 p.m. local time, according to Japanese public broadcaster NHK.

Ultimately, location plays a major part in how big of an impact an earthquake can have, Hough added.

Hough said this earthquake was shallow. "Where the fault was moving was very close to where people were living, and that can concentrate shaking close to where the earthquake happened. The energy just doesn't have a lot of time to travel and spread out before it gets close to where people live," she explained.