A massive earthquake off Indonesia's western coast triggered tsunami fears across the Indian Ocean on Wednesday, sending residents in coastal cities fleeing to high ground in cars and on the backs of motorcycles.

A strong aftershock nearly three hours later sparked a new wave of panic. Indonesia's government responded by issuing a fresh tsunami warning.

Some residents were crying in Aceh, where memories of a 2004 tsunami that killed 170,000 people in the province alone, are still raw. Others screamed "God is great" as they poured from their homes or searched frantically for separated family members.

The U.S. Geological Survey said the first 8.6-magnitude quake was centered 20 miles (33 kilometers) beneath the ocean floor around 269 miles (434 kilometers) from Aceh province.

That prompted the Pacific Tsunami Warning Center in Hawaii to issue a tsunami watch for Indonesia, India, Sri Lanka, Australia, Myanmar, Thailand, the Maldives and other Indian Ocean islands, Malaysia, Pakistan, Somalia, Oman, Iran, Bangladesh, Kenya, South Africa and Singapore.

A wave measuring less than 30 inches (80 centimeters) high, rolled to Indonesia's coast. There were no other signs of serious damage.

But just as the region was sighing relief, an 8.2-magnitude aftershock hit.

"We just issued another tsunami warning," Prih Harjadi, from Indonesia's geophysics agency, told TVOne in a live interview.

People along the western coast of Sumatra island and the Mentawai islands were told to stay clear of coasts.

The Pacific Tsunami Warning Centers watch remained in effect. A tsunami watch means there is the potential for a tsunami, not that one is imminent.

The initial quake was a strike-slip, not a thrust quake, according to experts. In a strike slip quake, the earth moves horizontally rather than vertically and doesn't displace large volumes of water.