A tsunami warning is currently in effect (11:58 GMT) for the western coast of Indonesia and the islands of Sumatra and Mentawai after a strong earthquake and equally strong aftershock were felt in the populous Aceh province.

The magnitude of the first earthquake was 8.6 on the Richter scale, and its epicenter was more than 400 kilometers away from the city of Banda Aceh. Residents still scarred by the memory of the horrific 2004 tsunami, which left a death toll of over 200,000, scrambled for higher ground as soon as the first earthquake hit. The initial tsunami warning was lifted hours later, but many people still crowded the top floors of many commercial and residential structures.

Seismologists noticed an 80 centimeter wave cresting, which caused no damage. The quake was felt in several countries, prompting evacuations in Thailand and the Maldives. A strong aftershock was detected hours later, and once again Indonesia issued a tsunami warning. People in Aceh who sought higher ground after the first quake refused to come down.

The Associated Press spoke to a scientist at the British geological survey who explained that this particular seismic movement moved the Earth in a horizontal direction, which typically does not cause tsunamis. This phenomenon is known as a strike-slip. The devastating 2004 tsunami was caused by a vertical thrust quake.

Tsunami alerts and warnings are determined, among many other causes, by changes in sea level. Water receding by about ten meters was reported by residents of islands near Sumatra and other Indonesian locations, but the report coincided with tidal activity. No reports of casualties or severe damage have emerged, but electricity in Aceh had not returned after 12:16 GMT.

Source: BBC News, Twitter