A magnitude 6.2 earthquake that hit Gansu and Qinghai provinces on Monday night resulted in a large number of casualties because the areas near the epicenter are densely populated and the seismic intensity of the quake was high, experts said.

The earthquake occurred shortly before midnight, with its epicenter in Gansu's Jishishan county, causing maximum damage there and in the city of Haidong in neighboring Qinghai. As of press time, 113 people had died in Gansu, and Qinghai reported 13 deaths.

The quake had a focal depth of 10 kilometers. The average altitude of the areas within a range of 5 km from the epicenter stands at 2,793 meters, according to the Ministry of Emergency Management. The low night temperature in the area around this time of the year would also make survival difficult for those trapped in the rubble.

Chen Yiran, chief engineer at the Sichuan Zhongke Institute of Disaster Prevention and Mitigation, said that based on information he had, there are 10 villages located within 5 km of the epicenter and 10 towns within 20 km.

"The population in the areas close to the epicenter is quite dense," he said.

The seismic intensity of the earthquake near the epicenter is currently estimated to be degree 8, with degree 12 representing the strongest. While buildings in some metropolises such as Beijing are designed and built to resist such high intensity earthquakes, many houses in the affected areas of Gansu and Qinghai were not designed to handle such a strong quake, Chen said.

"The focal depth of the earthquake is shallow. This is also a factor responsible for the extensive damage," he said.

He said the jolt occurred when many people in the affected areas were sleeping. This also contributed to the high casualty figure.

According to the China Earthquake Networks Center, Monday's jolt was a thrust fault earthquake. These usually occur when two slabs of rocks press against each other, but the pressure overcomes the friction holding them in place.

In an interview with China News Service, Gao Mengtan, an expert at the China Earthquake Administration's Institute of Geophysics, said that Monday's earthquake, though not of a very high magnitude, resulted in very intense shocks on the ground and could cause severe mountain collapse and landslides later, which is in line with the characteristics of a thrust fault earthquake.

The China Earthquake Networks Center said that since 1900, three earthquakes above magnitude 6 have occurred within a range of 200 km from the epicenter in Jishishan.

Nine aftershocks of magnitude 3 and above were registered in the first four hours after the Monday earthquake, including two of above magnitude 4, the networks center said.

[email protected]