The Great Sumatra-Andaman Earthquake of December 26, 2004: New Insights That Will Change the Next 40 Years and the Plate Tectonic Paradigm
by Arthur E. Berman
Nearly one year ago, the Great Sumatra-AndamanEarthquake produced the most destructive tsunami in history, with 283,000 people dead or missing in the Indian Ocean region. Modern geophysical data recorded during the earthquake and tsunami revealed unprecedented complexity in the mechanics of plate boundary rupture. No inversion model successfully accounts for the slip or rupture pattern produced by this event. New understanding of plate boundary behavior and mechanics that arises from the Great Sumatra-Andaman Earthquake may ultimately modify the existing plate tectonic paradigm.
The earthquake initiated just before 8:00 a.m. local time at the epicenter on the morning of December 26, 2004, approximately 255 km (158 mi) south-southwest of Banda Aceh off the western coast of northern Sumatra (Figure 1). There was no warning for the earthquake or the ensuing tsunamithat rapidly reached the shores of the Indian Ocean, devastating coastal areas of Indonesia, Thailand,Myanmar, India and Sri Lanka.
An Exceptional Earthquake
The Great Sumatra-Andaman Earthquake was exceptional in every way. Its 1300-km (807-mi) rupture length is the longest of any known earthquake (Hanson, 2005). The rupture zone was as much as 240 km (149 mi) wide (Lay and others,2005). The rupture lasted over an hour, making it the longest known rupture period of any earth- quake (Hanson, 2005). It deformed an entire hemisphere, moving global positioning system (GPS) stations in southern India 4 m (13 ft) with peak-to-peak ground motion over 9 cm (3.5 in) in Sri Lanka (Bilham, 2005). No point on Earth was undisturbed, with peak ground motion greater than 1 cm (0.4 in) everywhere (Park and others,2005).
The Great Sumatra-Andaman Earthquake was the second largest earthquake in instrumental history releasing 4.3 × 1018 Joules (Bilham, 2005). This is approximately equivalent to a 100-gigaton nuclear explosion or the total energy used in the United States in 6 months. The tsunamiproduced by the earthquake displaced 30 km3 (7.2 mi3) of sea water (Bilham,2005). The 9.3 moment magnitude (Mw)of the earthquake was equal to the sum of all moment magnitudes of earthquakes during the decade that preceded it (Lay and others, 2005). The earthquake andtsunami killed more people than any othernatural disaster in history.
The Great Sumatra-Andaman Earthquake was also exceptional as the first very large earthquake to be recorded and measured by a spectrum of digital technologies that were not available during other large earthquakes of the 20th century. Previous great earthquakes—the 1960 Chile Earthquake(Mw = 9.5) and the 1964 Alaska Earthquake (Mw = 9.1)—saturated existing analog measurement and recording equipment. The Sumatra-Andaman event was recorded by a global network of broadband,high dynamic range, digital seismometers. It was the first major earthquake to be monitored by the GPS. In addition, it was thefirst application of digitally recorded, long-period, free-oscillation modal geophysics.
A moment magnitude of 9.0 was initially calculated for the Great Sumatra-Andaman Earthquake. In March 2005, researchers at Northwestern University determined that the true magnitude was 9.3—approximately 3 times the energy released by a 9.0 event—after taking into account the full length and slip of the rupture,particularly the “slow slip” of the rupture’s northern portion (Fellman, 2005).
Rupture Segmentation and Slow Slip
The December 2004 earthquake trajectory may be divided into three segments, along with a fourth segment created by the contiguous March 2005 Nias Earthquake (Figure 2). Rupture initiated at a depth of about 30 km (18.7 mi) within the Sumatra segment. Rupture speed was slow and slip was small for the first 50 seconds. Rupture then expanded over the approximate 420 km (261 mi) of the Sumatra segment at an average speed of 2.7 km/s(1.7 mi/s) and rapid slip of between 5 and 20 m (16.4-65.6 ft) (Lay and others, 2005).
From 230 to 350 seconds, rupture progressed along the 325-km (202-mi) Nicobar segment of the earthquake with an average 5 m (16.4 ft) of rapid slip and an average rupture speed of 1.1 km/s (0.7 mi/s). In the Nicobar segment, an additional 5 m of slow slip proceeded up to 3500 seconds after rupture initiation (Lay and others, 2005).
The Andaman segment of the December rupture was characterized by less than 2 m (6.6 ft) of slip from 350 to 600 seconds after rupture initiation. An additional 5 m of slow slip occurred from 600 seconds to more than 3500 seconds. Rupture speeds in the northern segment were only about 0.3 km/s (0.2 mi/s) (Lay and others, 2005). The most remarkable aspect of the Great Sumatra-Andaman Earthquake was the slow slip that followed the initial, rather characteristic rupture and unzipping of a plate boundary. Slow slip tripled the earthquake’s energy release and accounts for the revision of its moment magnitude from 9.0 to 9.3. Slip along the northern segments of the rupture zone occurred too slowly to generate tsunami waves. Had slip been as rapid on the Nicobar and Andaman segments of the rupture as on the Sumatra portion, the resulting tsunami would have been many times more devastating than what actually occurred.
The March 28, 2005, Nias Earthquake may be thought of as a large, late aftershock. It had a moment magnitude of 8.7 and an average 8 m (26.3 ft) slip along a 300-km (186.5-mi) segment. It was probably produced by plate boundary failure because of stress changes that resulted from the December 26 rupture. No significant tsunami was produced by this rupture.
Slow slip is poorly understood and its causes are largely empirical at present. Slow slip occurred where plate convergence became increasingly oblique. It also coincided with a change in age of the subducting lithospheric plate from 60 Ma to 90 Ma between the Sumatra and Andaman rupture segments. Age and accompanying textural differences may have resulted in changes in mechanical coupling along the fault plane: subduction of younger lithosphere may have provided a broader contact area (Lay and others, 2005). Subduction of older lithosphere to the north may have contributed to displacement transfer across zones of previous back-arc spreading and associated dispersion of rupture energy (Figure 2).
Slow slip excited several of Earth’s fundamental resonances called free vibrational oscillations. Free oscillations were first reported after Fourier analysis of digitized analog seismic records from the 1960 Chilean earthquake. When seismic