JAKARTA, Indonesia (CNN) -- A strong earthquake with a preliminary magnitude of 6.4 struck off the southwestern coast of Sumatra Friday in the same area shaken by a major 8.4-magnitude temblor that killed 13 people earlier in the week.
A woman salvages items from her newly built house at Air Besi in North Bengkulu Thursday.
The Pacific Tsunami Warning Center initially issued an alert but lifted it later after no problems were reported.
The quake struck at 6:01 a.m. (2300 GMT Thursday), 120 km (75 miles) west-southwest of Sumatra's Bengkulu province and 650 km (405 miles) west-northwest of Jakarta.
The Bureau of Meteorology and Geophysics agency reported the epicenter was located 10 km (6 miles) underwater 153 km (95 miles) west of Lais, Bengkulu.
The region has been wracked by quakes and aftershocks for the past three days.
Earlier Friday, the USGS reported four more earthquakes ranging between magnitude 5.0 and 5.5 had rattled Sumatra.
Seven hours ahead of Friday morning's quake, at 11:09 p.m. (1609 GMT) a 6.2 quake hit 110 kilometers (65 miles) west-northwest of Bengkulu province at a depth of only 3 km (2 miles), the USGS said.
There were no immediate reports of injuries or damage.
A quake with the same magnitude struck the region several hours earlier, at 5:48 p.m. (1348 GMT). The temblor vibrated under the Celebes Sea at a depth of about 21 km (13 miles).
It was centered about 290 km (180 miles) northeast of Bitung, a city on the northern coast of Sulawesi, and the same distance south-southeast of General Santos, Mindanao, Philippines.
Wednesday's quake generated a series of aftershocks, including two major ones early Thursday measuring 7.8 and 8.1, said David Applegate, senior science adviser at the U.S. Geological Survey.
"It's been an incredible number of years for Indonesia and particularly for Sumatra" in terms of earthquakes, Applegate said on CNN's "American Morning" on Thursday.
"What we have here is a subduction zone, where one of the Earth's plates is moving down beneath the other," he said. "In this case, the Indian Ocean and the Australian Plate are moving beneath the Eurasian Plate.
"In this kind of a situation you're going to get earthquakes as the strain builds up, but what we're seeing now is almost every segment of this plate has ruptured just in the last several years," Applegate said.
"In each case, it relieves pressure in one area but then that increases the pressure somewhere else. And so, for example, what we saw yesterday was the magnitude-8.4 quake ruptured to the north along this boundary. This 7.8 was at the northern end of that."
People in the Indian Ocean region have been extremely skittish about the possibility of earthquake-induced tsunamis since December 2004, when gigantic waves triggered by a 9.1-magnitude quake killed more than 200,000 people in seven countries.
Since the December 2004 tsunami, almost 8,000 people have died in 15 earthquakes of magnitude 6.3 or higher, according to the USGS. E-mail to a friend
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