An earthquake, also known as a quake, tremor or temblor, is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity.
Hong Kong feels another quake from Guangdong's shaky city, Heyuan
Thousands of Hongkongers on Friday called the Hong Kong Observatory after a magnitude 4.8 earthquake 180 kilometres away was felt in the city.
Anxious residents reported by phone and online that they felt buildings shaking, objects swinging and chairs moving for a few seconds.
The quake struck Dongyuan county in Heyuan, northeast of Hong Kong, at about 11.30am.
There were no reports by last night of casualties or damage even though the quake was felt in many cities across Guangdong and in neighbouring Fujian province .
In Hong Kong the tremor registered 3 on the 12-level modified Mercali scale. At that intensity, it would feel like vibrations from a small truck passing nearby.
Observatory scientific officer Leung Yin-kong said the public need not worry about quakes of that magnitude, which were not uncommon in the region.
Guangdong seismologists said the quake was quite shallow, at a depth of about 11 kilometres, which explained why it was felt across a wide region.
It was the fourth tremor originating in Heyuan, a city of more than 200,000 residents covering an area of 15,500 square kilometres, in the past 12 months. The last one, of magnitude 3.7, was on September 2.
Professor Chan Lung-sang, from the University of Hong Kong's department of earth sciences, said the recent quakes in Heyuan were not a sign of increasing seismic activity. He said all of southern China was influenced by tectonic plate movements in Taiwan and Philippines, and Heyuan was more prone to quakes because of its proximity to the huge Xinfengjiang freshwater reservoir
Chan said such a large man-made body of water would cause underground pressure that might lead to occasional quakes.
"The public needn't worry as mild quakes are quite common in southern China," Chan said.
Hong Kong has felt 62 tremors, including yesterday's, Since 1979. Of these, 12 originated in Heyuan.