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.
The people open their hearts
They were not invited to any official memorial ceremonies, but ordinary people across China still sought their own way to commemorate the first anniversary of a catastrophe that shocked the nation to its core.
In Tiananmen Square, a small group of people gathered at 2.28pm - the moment the earthquake struck - and began chanting slogans. 'Go China,' they cried. 'We support you, Sichuan.' Passers-by joined in, and the crowd quickly grew to several hundred. Police kept a close watch but did not intervene, and the chanting lasted about 30 minutes.
The short rally was a rare, unfettered public gathering in the capital's iconic plaza. But yesterday's demonstration was just one of many spontaneous acts of remembrance across the country, which were in stark contrast to the staid formality of official memorial services held by the central and local governments.
In Shanghai, main streets erupted into a cacophony of car horns at 2.28pm. At a kindergarten in Chengdu, the Sichuan capital, students decorated paper hearts and hung them from trees in a nearby park, China News Service reported.
Students in primary and secondary schools nationwide carried out fire and earthquake drills and received first-aid training on the first national 'disaster prevention, disaster reduction' day.
Several popular mainland news websites - including 163.com.cn and oeee.com.cn - changed their colour schemes to black and white to mark the solemn anniversary. But the feeling on the streets remained muted compared with the mass outpourings of grief last year.
In most parts of the country, memorial events were dominated by Communist Party officials, with little chance for individual participation.
Sichuan-based blogger Ran Yunfei said the public needed to be given an outlet for their feelings about the disaster.
'The government should not take charge of everything, and should let the people have more spontaneous mourning events,' Mr Ran said. 'To this day, we still don't know many quake victims' names and identities. We don't even have a museum or building dedicated to them.'
Mr Ran's comments were echoed by many contributors to online forums and message boards, who made use of anonymous postings to voice their frustrations.
One wrote: 'We hope today it really is about mourning and remembering the quake victims, not just another opportunity for the government to showcase itself.'