A Japanese researcher says seismic activity has heightened in an extensive area around the focal zone of the major quake that devastated many parts of the Noto Peninsula in central Japan on New Year's Day.

He warns that quakes and tsunami could hit places not only near the focal zone but also in a broad area beyond it.

Tohoku University Professor Toda Shinji compared the daily number of quakes observed on and around the peninsula for about two years before the January 1 quake and the corresponding figure recorded for 28 days after that. The analyzed quakes included jolts imperceptible to humans.

The focal zone of the powerful quake is estimated to have spread across a wide area from the west to the northeast of the peninsula.

Toda says the frequency of quakes jumped more than 100 times on the peninsula and in waters to the west of the focal zone.

He says the frequency rose several times in some parts of Sado Island and in waters close to it. The island is situated east of the focal zone.

Toda says the frequency grew dozens of times in Toyama Bay, which is south of the focal zone.

The professor says the frequency also multiplied roughly 10 times in the cities of Kanazawa and Toyama, which sit near the base of the peninsula.

Toda says seismic activity around the focal zone is believed to have increased because crustal strain built up after fault sections running as much as more than 100 kilometers in total shifted at the time of the major quake.

Toda says large quakes are more likely to occur when small quakes are frequently happening. He urges people to make sufficient preparations, saying strong quakes focused underwater could generate a tsunami.