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.
Scramble to reach quake survivors in Aceh
Toll rises to 24, with panicked Acehnese forced to endure more widespread devastation
Rescuers battled through landslides and blocked roads yesterday to reach survivors from an earthquake in Indonesia's Aceh province that killed at least 24 people, including several children who died when a mosque collapsed.
Almost 250 people were also injured in Aceh's remote, mountainous interior when the strong 6.1-magnitude quake struck on Tuesday, flattening buildings and triggering landslides.
The quake has sparked panic in the natural disaster-prone region where more than 170,000 people were killed by the quake-triggered tsunami of 2004.
In Blang Mancung village, Central Aceh district, there was widespread devastation, with many homes reduced to rubble and at least six children killed when a mosque collapsed during a Koran reading session.
Rescuers dug all night with an excavator through the rubble of the mosque looking for more children but a local disaster agency official said he did not believe anyone else was buried.
As 16 aftershocks rocked Aceh late on Tuesday, about 700 people from the village and its surrounding areas took refuge in makeshift shelters.
Those who remained dug through the rubble of their collapsed houses with bare hands to search for their belongings.
Bodies of the dead were laid out and covered in blankets at a makeshift emergency health post in the village.
"This is the biggest earthquake we've ever had here," Subhan Sahara, head of the district's disaster agency, said.
"People are still frightened, especially after the aftershocks last night. Nobody dared to stay at home. Everyone slept on the roads or in car parks. The quake triggered many landslides. People could not get out of the area because of fallen trees and mounds of earth blocking roads."
The main hospital in the district was overwhelmed and tents had been set up outside to treat the flood of patients, he said, adding that food and water were in short supply.
Military, police and local government officials were trying to get affected areas, but some roads were blocked by landslips.
"Bad phone communications, damage to several roads, and landslides are making rescue efforts difficult," national disaster agency spokesman Sutopo Purwo Nugroho said.
The agency dispatched a helicopter from neighbouring Riau province, while an air force plane was also deployed to assess the damage.
"We have recorded 24 people dead and 249 people injured," said Nugroho, adding that 375 buildings had been destroyed or damaged.
The casualties were spread over the two worst-hit districts of Central Aceh and Bener Meriah.
Aceh, on the northern tip of Sumatra, is regularly hit by quakes. The huge quake-triggered tsunami of 2004 not only killed tens of thousands in the province, but also many in countries around the Indian Ocean.
In April last year an 8.6-magnitude quake struck 431 kilometres off Banda Aceh, leaving five dead in the province and prompting an Indian Ocean-wide tsunami alert.