BEIJING - The seismic intensity scale in the severely quake-hit area in Southwest China's Sichuan province is measured at the IX degree, said the Chinese Academy of Sciences (CAS).

The China Seismic Intensity Scale is classified into 12 degrees, from I for insensible to XII for landscape reshaping.

The quake-hit area is about 15,720 square kilometers and a total of 1.85 million people have been affected, according to the CAS's quick assessment on the scale of the disaster using high-resolution remote sensing images.

Experts with China Earthquake Networks Center (CENC) told Xinhua that the 7.0-magnitude quake on Saturday was an isolated disaster,  not an aftershock of the Wenchuan 8.0-magnitude quake that jolted Sichuan and other southwest regions in May 2008.

The quake on Saturday occurred on the south part of Longmenshan fracture zone and the Wenchuan quake was on the middle part of it. But the two quakes have something in common.

This quake and the Wenchuan earthquake both occurred on the Longmenshan fracture zone and were both triggered by crustal movement of Bayan Har block of the Qinghai-Tibet Plateau, said Zhou Bengang, a researcher with the CENC.

Zhou said earthquake activities on the south part of Longmenshan fracture zone had a comparatively low strength in historical records. There were three quakes above 6.0 degrees on Richter scale in record in the mountainous Ya'an.

According to the CENC, there have been 12 earthquakes at or above 7.0 degrees on Richter scale occurred with China's territory since the beginning of this year.

Jiang Haikun, another researcher with the CENC, believed that the Earth is experiencing a seismic active period since there were 12 quakes at or above 7.0 degrees on Richter scale on average per year around the world in recent years.

The earthquake hit Lushan County of Ya'an City in the province at 8:02 a.m. Saturday Beijing Time, according to the China Earthquake Networks Center.