Press Release 05-079
Analysis of the Sumatra-Andaman Earthquake Reveals
Longest Fault Rupture Ever
Global seismic recording network made first-time
measurements possible
The December 2004
tsunami-producing earthquake sent reverberations around the world.
The shock heard 'round the
world.' The December 2004 tsunami-producing earthquake sent reverberations
around the world.
Credit: California Institute
of Technology
May 19, 2005
Three papers published in the
May 20 issue of the journal Science by an international group of seismologists
provide a comprehensive scientific analysis of the December 2004 and March 2005
earthquakes, and discuss future earthquake potential near Sumatra and the
Andaman Islands. The data raise new
concerns about the possibility of earthquakes in a region to the southeast of
the great Sumatra-Andaman earthquake of
Dec. 26, 2004, which last experienced a great earthquake in 1833.
The Incorporated Research Institutions for Seismology
(IRIS), a university research consortium, played a central role in these
reports. IRIS, funded by the National Science Foundation (NSF), operates a
global network of seismic monitoring stations that provided much of the data
for the analysis.
"The extensive
seismological data that underlies the scientific results in these papers is a
result of the 20-year investment by NSF and the U.S. Geological Survey to
develop the Global Seismic Network," said James Whitcomb, section head in
NSF's earth sciences division.
"This far-flung network of more than 130 seismic sensors, on every
continent and all major oceanic islands, provides essential information for
earthquake warnings and for research on earthquakes and deep-Earth
structure."
The great Sumatra-Andaman
earthquake was an event of stunning proportions, both in its human
dimensions--nearly 300,000 lives lost--and as a geological phenomenon. The
sudden rupture of a huge fault beneath the Indian Ocean unleashed a devastating
tsunami. It was the largest earthquake in the past 40 years and was followed by
the second largest just three months later on March 28, 2005.
The new analysis gives the
Sumatra-Andaman earthquake a seismic magnitude of at least 9.1, and possibly as
high as 9.3. Earlier estimates had put it at magnitude 9.0. By comparison, the
1960 earthquake in Chile was magnitude 9.5, and the 1964 Alaska earthquake was
magnitude 9.2.
Record-setting features of
the Sumatra-Andaman earthquake of Dec. 26, 2004, include the longest fault
rupture ever observed (720 to 780 miles or 1,200
to 1,300 kilometers) and the longest duration of faulting (at least 10 minutes). The aftershocks included
the most energetic earthquake swarm ever observed.
Said Thorne Lay, a geologist
at the University of California at Santa Cruz, "We've never had such
comprehensive data for a great earthquake, because we didn't have the
instrumentation to gather it 40 years ago."
Lay is lead author of one of
the reports, which provides an overview of the two earthquakes, and is a
co-author of a second report, which focuses on the processes involved in the
rupture of the fault. A third article describes how the earthquakes caused the
whole planet to vibrate with "free oscillations," like the ringing of
a bell.
"The research results
presented in these papers are major steps forward in our understanding of how
great earthquakes happen," said David Simpson, president of IRIS. "With 40 authors from seven countries,
the papers are wonderful examples of international scientific
collaboration. These results were
possible only with the open sharing of data from multi-national sensor
networks, and close collaboration among seismologists worldwide."
The two earthquakes are the
largest since the global deployment of highly sensitive digital broadband
seismometers. These instruments, deployed around the world by IRIS and other
organizations, recorded both the huge ground motions from the mainshocks and
the tiny motions from small aftershocks and free oscillations of the planet.
The ground motions during the prolonged, intense shaking of the main shock were greater than in any earthquake previously recorded by global broadband seismometers. As far away as Sri Lanka, a thousand miles from the epicenter, the ground moved up and down by more than 3.6 inches (9 centimeters). Ground motions greater than one-half inch, but too gradual to be felt, occurred everywhere on Earth's surface as seismic waves from the event spread around the globe.
During the Sumatra-Andaman earthquake, the ground shook more than 100 times harder than it did during the 1989 California Loma Prieta earthquake--a magnitude 6.9 event that caused major destruction from Santa Cruz to the San Francisco Bay.
"Even among
seismologists, we call this a monster earthquake," Lay said.
The earthquake took place
along the curving boundary between major plates of the Earth's crust, where the
Indo-Australian plate plunges beneath the southeastern Eurasian plate in what
is called a subduction zone. Before the
fault ruptured, the edge of the Eurasian plate was being dragged downward by
the descending Indo-Australian plate.
Released by the rupture of the fault, the edge of the plate sprang back up, lifting the ocean floor and setting off the tsunami that inundated coastal areas throughout the Indian Ocean. The fault slipped by as much as 50 feet (15 meters) in places, averaging about 33 feet (10 meters) of displacement along the segment off the northwestern tip of Sumatra where the quake was centered.
"The earthquake rupture
ran a distance equivalent to the area from Jacksonville, Fla., to Boston,
Mass.," said Charles Ammon, a geoscientist at Penn State University and lead
author of one of the reports. "This
earthquake lasted just under 10
minutes, while most large earthquakes take only a few seconds."
From the epicenter, the
rupture expanded along the fault at a speed of about 1.5 miles (2.5 kilometers) per second toward the north-northwest. But the initial
movement of the fault was much less along the northern segment than in the
south. This was fortunate, said Lay, because it spared much of the coastline in
the north from the massive tsunami waves that caused the destruction farther
south.
Eventually the northern part
of the fault slipped about as much as the southern part, uplifting and tilting
the Andaman Islands. The tilting of the islands shows that the northern part
must have slipped about 33 feet, but much of that slip occurred gradually,
without generating seismic waves.
"We think that slip was occurring in the northern part for about an
hour, well after the 10 minutes of rapid motions were over," Lay said.
Then there were the
aftershocks, including the most energetic earthquake swarm ever observed. More
than 150 earthquakes of magnitude 5 and greater occurred over a four-day period
in late January on faults beneath the Andaman Sea. These faults were activated by the rupture of
the main fault along the plate boundary to the west. There were also numerous
aftershocks of magnitude 6 and greater throughout the fault zone.
"It's an incredible
aftershock series," Lay said. "It is hard to get a feeling for the
scale of it. If you take the aftershock zone and superimpose it on California,
it completely covers the state."
The Sumatra-Andaman
earthquake produced the best documentation of Earth's free oscillations ever
recorded, said geologist Jeffrey Park of Yale University, lead author of one of the reports. The free oscillations have natural periods
that depend on the composition and temperature of Earth's deep interior. "Just like grocery shoppers thump a
watermelon to find out whether it's ripe, seismologists measure the natural
tones from earthquakes to detect properties of Earth’s deep mantle and
core," said Park. "This huge
earthquake may resolve several controversies, such as whether a heavy slagheap
of old tectonic plates is stuck near the core-mantle boundary beneath
Africa."
The March 28, 2005, earthquake struck with a magnitude of 8.6 on an adjacent portion of the plate boundary to the southeast. This was not an aftershock, but a new rupture of an adjacent segment of the fault.There is the potential for future major earthquakes not only on the thrust fault along the plate boundary, said Lay, but also on a related fault system beneath the island of Sumatra that involves horizontal shearing, the same type of faulting that occurs on the San Andreas Fault.
"The Sumatra Fault runs
right down the length of the island. Because it is close to major population
centers, the seismic hazard is significant even for a smaller event," Lay
said.
Major faults elsewhere in the
world--in northern Turkey, for example--have experienced sequences of
earthquakes moving progressively along a fault line.
"When one part of the
fault slides, that loads up the adjacent region and transfers stress. So you
have a heightened potential for earthquakes on the adjacent section. The
concern is that something like this could happen in Sumatra," Lay said.
The three papers published
this week in Science and their authors are as follows:
"The Great Sumatra-Andaman Earthquake of 26
December 2004"
Thorne Lay (UC Santa Cruz),
Hiroo Kanamori (California Insitute of Technology), Charles Ammon (Pennsylvania
State University), Meredith Nettles (Harvard University), Steven Ward (UCSC),
Richard Aster (New Mexico Institute of Mining and Technology), Susan Beck
(University of Arizona), Susan Bilek (New Mexico Tech), Michael Brudzinski
(University of Wisconsin-Madison), Rhett Butler (IRIS), Heather DeShon
(UW-Madison), Goran Ekstrom (Harvard), Kenji Satake (Geological Survey of
Japan), Stuart Sipkin (U.S. Geological Survey)
"Rupture Process of the Great 2004
Sumatra-Andaman Earthquake"
Charles Ammon (Penn State),
Ji Chen (Caltech), Hong-Kie Thio (URS Corp.), David Robinson (University of
Oxford), Sidao Ni (University of Science and Technology of China), Vala
Hjorleifsdottir (Caltech), Hiroo Kanamori (Caltech), Thorne Lay (UCSC), Samita
Das (Oxford), Don Helmberger (Caltech), Gene Ichinose (URS Corp.), Jascha Polet
(UC Santa Barbara), David Wald (USGS)
"Excitation of Earth's Free Oscillations by the
26 December 2004 Sumatra-Andaman Earthquake"
All Images
Press Release 05-079
Analysis of the
Sumatra-Andaman Earthquake Reveals Longest Fault Rupture Ever
Global seismic recording
network made first-time measurements possible
The Global Seismic Network
provides earth scientists with information on seismic activity around the
world.
Credit: IRIS
Regional map showing
earthquakes with magnitudes greater than 5.0 from 1965 to Dec. 25, 2004.
Locations of previous large earthquake ruptures along the Sunda-Andaman trench
system are shown in pink. Green stars show the epicenters of the two recent
great events; the green diamond shows the central location for the Dec. 26,
2004, event. The thick red arrows indicate relative tectonic plate motions
between the Indo-Australian and Eurasian plates.
Credit: National Earthquake
Information Center
Looking towards the ocean
from the bridge (Sumatra)
Credit: Jose C. Borrero,
University of Southern California Tsunami Research Group
Uplifting of ground by the
earthquake on the western tip of Simeuleu Island thrust coral from underwater.
Credit: Kerry Sieh, Tectonics
Observatory, Caltech
Jeffrey Park (Yale
University), Teh-Ru Alex Song (Caltech), Jeroen Tromp (Caltech), Emile Okal
(Northwestern University), Seth Stein (Northwestern), Genevieve Roult (IPGP,
Paris), Eric Clevede (IPGP), Gabi Laske (UC San Diego), Hiroo Kanamori
(Caltech), Peter Davis (UCSD), Jon Berger (UCSD), Carla Braitenberg (University
of Trieste), Michel Van Camp (Royal Observatory of Belgium), Xiang'e Lei,
Heping Sun, and Houze Xu (Chinese Academy of Sciences), Severine Rosat
(National Astronomical Observatory of Japan)
-NSF-
Media Contacts
Cheryl L. Dybas, NSF, (703)
292-7734, cdybas@nsf.gov
The National Science Foundation (NSF) is an independent federal agency that supports
fundamental research and education across all fields of science and
engineering. In fiscal year (FY) 2016, its budget is $7.5 billion. NSF funds
reach all 50 states through grants to nearly 2,000 colleges, universities and
other institutions. Each year, NSF receives more than 48,000 competitive
proposals for funding and makes about 12,000 new funding awards. NSF also
awards about $626 million in professional and service contracts yearly.
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