More magnitude-5 or stronger earthquakes have rocked China this year than in any other year, a senior seismologist said on Wednesday.

An average of 18 such quakes strike China each year. But 21 quakes, including the one that devastated Sichuan Province on May 12, have struck in just eight months of this year, Yin Chaomin, deputy director of China Earthquake Administration (CEA), said.

Speaking at a joint briefing of the CEA and the State Bureau of Surveying and Mapping (SBSM), he said the tremors that rocked Xinjiang, Tibet and the Sichuan-Yunnan border region were caused partly by the May 12 quake. Part of the cause, of course, is their geographical locations on seismic belts.

The Wenchuan quake, however, had nothing to do with the tremor that jolted Panzhihua in southern Sichuan on Saturday, said Liu Jie, China National Seismological Network director in charge of earthquake forecasting.

Addressing another briefing, Liu said the epicenters of the two quakes, which occurred in different fault zones, were 550km apart.

The Wenchuan quake caused a horizontal displacement of 238cm and ground shifts of up to 70cm, Li Weisen, SBSM deputy director, said.

The displacements were observed in southern Shaanxi and Longnan region of Gansu Province during a CEA-SBSM joint field study between late May and mid-July, Li said.

Two- to 3-cm horizontal and vertical displacements were also observed on the southwestern foot of Mt Qomolangma in Nepal, he said.

"Results showed the May 12 quake had a considerable impact on the terrain of the quake zones, but not much in the areas beyond. Surveying and mapping infrastructure in the quake zones was severely impaired, whereas those outside the area suffered only minor damage."

Li rubbished some reports that said the Qinghai-Tibet Plateau had shifted several meters. "Media reports that claim the plateau shifted several meters because of the Wenchuan earthquake lack scientific evidence," he said.

The CEA is positioning more than 50 GPS monitoring stations in the quake zones where earlier surveying and mapping results are being re-examined.

China has suffered eight magnitude-6 or above aftershocks since the May 12 quake, Yin said. The number of magnitude-5 or stronger tremors is more than 30.

(China Daily September 4, 2008)