The US Geological Survey (USGS), confirmed that the first Aceh quake measured 8.6 and was centred at a depth of 33km (20 miles), about 495km from Banda Aceh, the provincial capital.
A second major aftershock, measuring 8.3, struck shortly after.
A Tsunami warning was issued by the Pacific Tsunami Warning Centre following the quake. An animated explanation of how tsunamis occur can be viewed here
With memories of the events that followed the 2004 earthquake still fresh in their minds, it was reported that people in Banda Aceh on the Indonesian island of Sumatra jumped into cars and the backs of motorcycles, clogging streets as they fled to high ground.
Tsunami warning sirens were heard as people struggled to contact friends and family when the electricity failed and telephone lines failed after the ground shook for 5 minutes.
People near the coast in six Thai provinces were ordered to move to higher ground and stay as far away as possible from the sea. Phuket airport, located on the coastline, was closed for a time.
Later, this warning was cancelled as the risk receded.
Despite such a large earthquake, there was no significant tsunami wave on this occasion. This was due to the type of earthquake that occurred – The 2004 earthquake which led to the devastating tsunami was a subduction earthquake - meaning one plate went underneath the other plate - which then pushed a lot of the water up and out from the epicentre. In this case, the two plates are slipping side by side, so not as much water is going to be displaced.
A brief explanation is given in the video below:
Sources – BBC News; AJE Live; CNNI