Skip to main content

Secondary navigation:

Office of News & Public Affairs

Aftershocks Hamper Rescue and Recovery Following Quakes in Samoa and Sumatra

seismology expert John Ebel directs BC's Weston Observatory

CHESTNUT HILL, MA (September 30, 2009) – The 8.0-magnitude earthquake that shook the South Pacific region and launched a deadly tsunami upon the islands of Samoa and a second quake that struck the Indonesian island of Sumatra already have and will continue to be followed by a series of devastating aftershocks that will pose risks to rescue workers and survivors, according to Boston College Professor of Geology and Geophysics John Ebel.

“Both Samoa and Sumatra are likely to experience aftershock activity over the next several days and weeks, and strong aftershocks could contribute more damage to those areas already devastated by the main earthquakes,” said Ebel, who is the director of Boston College’s Weston Observatory, which houses seismic instruments for the World-Wide Standardized Seismic Network and the New England Seismic Network.

“Rescue workers will need to be ready to temporarily suspend operations on a moment's notice if a strong aftershock hits,” said Ebel, co-author of the recent book Earthquakes and Tsunamis in the Past (Cambridge University Press, 2009).

Ebel, an expert on earthquakes, seismology, climate change and the environment, said the Samoa quake, which struck at approximately 6 p.m. local time Tuesday, is associated with the Pacific Ocean plate beneath the Samoa Islands.

“The earthquake itself was caused by bending of the Pacific Ocean plate where it begins subducting back into the Earth,” said Ebel. “The focus of the earthquake faulting was only about 6 miles below the surface of the Earth, and it displaced the ocean floor enough to cause a locally damaging tsunami on the islands of Samoa.  This tectonic plate boundary has seen numerous devastating earthquakes and tsunamis, particularly to the south of yesterday's earthquakes.”

Just hours later, at approximately 10 a.m. local time Wednesday, the Sumatra quake struck with a magnitude of 7.6. The quake is associated with the subduction of the Indian Ocean plate beneath Sumatra.

“The earthquake apparently did not cause a tsunami, likely due to its focal depth of about 50 miles beneath the Earth’s surface,” Ebel said. “This earthquake occurred on the same zone as the major earthquakes and tsunamis took place in 2004 (magnitude 9.2) and 2005 (magnitude 8.6).  The entire length of the subduction zone beneath Indonesia has been rocked by a number of damaging earthquakes during the past decade.”

Ebel said the twin temblors within a 24-hour span are uncommon.

"On the other hand, there have been exceptions, most notably in 2004 when the great Sumatra magnitude 9.2 earthquake and subsequent tsunami was preceded by two and a half days by a major earthquake of magnitude 8.1 south of New Zealand.  While we don't understand the long-range, short-term connections between such major earthquakes, they deserve more study as to their causes."

Details about Boston College’s Weston Observatory are available here.

For more information, contact Ed Hayward of the Boston College Office of News & Public Affairs at 617-552-4826, or by email at