Review of the source characteristics of the Great Sumatra–Andaman Islands earthquake of 2004
William Menke
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Hannah Abend
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Dalia Bach
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Kori Newman
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Vadim Levin
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V. Levin Department of Geology, Wright Geological Laboratory, Rutgers University
, 610 Taylor Road, Busch Campus, Piscataway,
NJ 08854, USA
The December 26, 2004 Sumatra-Andaman Island earthquake, which ruptured the Sunda Trench subduction zone, is one of the three largest earthquakes to occur since global monitoring began in the 1890s. Its seismic moment was M0 = 1.00 1023-1.15 1023 Nm, corresponding to a moment-magnitude of Mw = 9.3. The rupture propagated from south to north, with the southerly part of fault rupturing at a speed of 2.8 km/s. Rupture propagation appears to have slowed in the northern section, possibly to ~2.1 km/s, although published estimates have considerable scatter. The average slip is ~5 m along a shallowly dipping (8 ), N31 W striking thrust fault. The majority of slip and moment release appears to have been concentrated in the southern part of the rupture zone, where slip locally exceeded 30 m. Stress loading from this earthquake caused the section of the plate boundary immediately to the south to rupture in a second, somewhat smaller earthquake. This second earthquake occurred on March 28, 2005 and had a moment-magnitude of Mw = 8.5.
1 Introduction
The Mw = 9.3 December 26, 2004 SumatraAndaman Island earthquake is the
largest earthquake since the moment-magnitude Mw = 9.6 1960 Chile and the
Mw = 9.4 1964 Alaska earthquakes occurred more than 30 years ago (Stein and Okal
2005; Tsai et al. 2005; E. Okal, personal communication, 2005). The earthquake
occurred in a complex tectonic region, along the boundaries of the Indo-Australian
and Eurasian plates, the Sunda and Burma microplates and the Andaman subplate
(Fig. 1). It ruptured the subduction zone megathrust plate boundary on the Sunda
Trench (Bird 2003).
The December earthquake and its tsunami caused tremendous devastation to the
Indian Ocean region. An accounting by the United Nations estimates that 229,866
persons were lost, including 186,983 dead and 42,883 missing, with an additional
1,127,000 people displaced (United Nations Office of the Special Envoy for Tsunami
Recovery 2006). The shaking registered clearly on seismometers worldwide (Park
et al. 2005a, b). The earthquake strongly excited low degree free oscillations of the
earth, so that the globe rang like a bell for several days afterward (Park et al. 2005a,
b; Rosat et al. 2005). Static deformation, as determined by the Global Position
System (GPS), exceeded 0.1 m for hundreds of kilometers around the epicenter
(Catherine et al. 2005; Khan and Gudmundsson 2005). The amplitude of its Rayleigh
wave exceeded 0.1 m at Diego Garcia (2,900 km distant), and 0.006 m in New York
(15,000 km distant). Its effects were felt around the world, triggering seismicity at
Mount Wrangell, a volcano in Alaska (West et al. 2005). Acoustic vibrations
traversed the world oceans, and were recorded on several hydroacoustic arrays (Garce s
et al. 2005). Seismic intensities near the rupture zone were, however, surprisingly
small for such a large event, with northern Sumatra experiencing only intensity VIII
on the EMS-98 scale (Martin 2005).
The first and larger mainshock was due to low angle thrust faulting with a
nucleation point (hypocenter) at latitude 3.3 N, longitude 96.0 E with an origin (start time)
of 00:58:53.5 UTC (Figs. 1, 2, 3a) (Nettles and Ekstro m 2004). Its hypocentral depth,
28 km, was shallow (Harvard CMT). The faulting propagated 1,2001,300 km
northeastward along the Sunda Trench (Ammon et al. 2005; Ni et al. 2005; Vigny et al.
2005) with a downdip width of ~200 km (Ammon et al. 2005). The mainshock was
followed by over 2,500 aftershocks with magnitudes greater than 3.8. In the several
months following the mainshock, these aftershocks mostly occurred in a region
northward of the nucleation point. However, a second large earthquake of
momentmagnitude Mw = 8.5 occurred on March 28, 2005. This second mainshock nucleated
~170 km south of the first, at latitude 2.1 N, longitude 97.0 E at 16:09:36 UTC, with
the faulting propagating southeastward along the plate boundary for ~300 km
(Bilham 2005). This event was followed by aftershocks as well. The two regions of
aftershocks delineate the respective rupture zones of the two mainshocks (Fig. 1).
Although the immediate area of the December 26, 2004 mainshock had been
previously active, only a few aftershocks occurred there. One of the most notable
aftershock features is the swarm of strike-slip and normal faulting events that
occurred between 7.58.5 N and 9495 E involving more than 150 M 5 earthquakes
that occurred from January 2730 (Lay et al. 2005).
2 Tectonic setting The tectonics of the SumatraAndaman Island region is controlled by the boundaries between the Indo-Australian plate and by two segments of the southeastern section of the Eurasian plate, the Burma and Sunda subplates (Fig. 1) (Bird 2003).
Fig. 1 Top Seismicity of the SumatraAndaman Isla (...truncated)