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The largest aftershock ever recorded ?
Jean-Robert Grasso, Christophe Voisin
To cite this version:
Jean-Robert Grasso, Christophe Voisin. The largest aftershock ever recorded ?. Eos, Transactions
American Geophysical Union, American Geophysical Union (AGU), 2005, 86 (22). �hal-00109906�
Eos, Vol. 86, No. 22,31 May 2005
SECTION NEWS
S E I S M O L O G Y
Looking at this event from another per spective helps to explain its unusually huge size. Its epicenter, located 2 5 0 km south of the M,n 9.3 event, lies on the northern edge ofthe Mw 8.5 1861 rupture zone. The 2 8 March
rupture length, whether estimated by its own aftershock distribution or by the seismic wave forms'inversion [U.S. Geological Survey, 2005] (www.gps.caltech.edu/~jichen), reproduces in size and s p a c e the Mw 8.5 1861 event rupture
area. From such an analysis, it emerges that the Mw 8.7 2005 event that overwhelms the
1861 main s h o c k is a main s h o c k itself. This raises the question of the possible relationship between the D e c e m b e r 2004 and March 2005 events.
In considering this question, first sup p o s e that the D e c e m b e r 2004 event did not happen. Would the 2 8 March event have o c curred anyway? T h e return period of large earthquakes along the S u n d a trench, the s u b d u c t i o n z o n e all along the Sumatra Island, c a n b e estimated by 7r = D / c K w i t h coupling coefficient c, average slip D, and m e a n plate velocity V. Assuming ( 1 ) a perfect coupling b e t w e e n the India and B u r m a plates ( t h e coupling coefficient for s u b d u c t i o n z o n e s is estimated in the 2 8 - 1 0 0 % range, with a 7 0 % m e a n value [Bird and Kagan, 2 0 0 4 ] ) , ( 2 ) an estimate of 10 ± 2 m slip for an Mw 8.5 earth
quake, and ( 3 ) a l o c a l plate velocity of 5 ± 0.5 c m / y r ( B o c k , personal c o m m u n i c a t i o n , 2 0 0 5 ) , a return period of 2 1 0 ± 60 years ( 2 5 0 ± 60 years for an Mw 8.7 event) is o b t a i n e d .
The previous Mw 8.5 rupture o c c u r r e d in
1861, about 150 years ago. The 2 8 March event that would have o c c u r r e d eventually around 2111 ± 60 is clock-advanced by the megarup-ture of D e c e m b e r 2004. Using the Gutenberg-Richter law for the Andaman-Sumatra region
[Kagan, 1997], a return period of 150 years is
independently obtained, which is still consis tent with the occurrence of the 28 March event.
Therefore, the Mw 8.7 event of 2 8 March
2004 is an aftershock triggered by the Mw 9.3
rupture of late D e c e m b e r 2004. This regular aftershock takes advantage of the vicinity of the former 1861 main s h o c k area to grow to a huge Mw 8.7 earthquake,by reactivating the
former Mw 8.5 1861 rupture zone.
What is the greatest immediate threat? T h e empirical Bath law predicts a 1.2 lower mag nitude, on average, for the largest aftershock. This leads to an 8.1 largest magnitude for an aftershock, with respect to the Mw 9.3 main
s h o c k of D e c e m b e r 2004. The t e c t o n i c setting superimposes another constraint: A patch that ruptured in 1833 in a major Mw 8.7 event is lo
cated farther south along the Sunda trench. If the s a m e m e c h a n i s m that is proposed for the 28 March event applies farther south, the prob ability of another big quake of magnitude 8.7 or greater occurring along the 1833 rupture area is increased.
In favor of such a prediction are ( 1 ) the increase in seismicity rate after the 28 March event due to its own aftershock s e q u e n c e en h a n c e s the probability for another big quake to occur, according to the Gutenberg-Richter relationship; and ( 2 ) elastic and viscoelastic stress transfers e n h a n c e the triggering of a ma jor rupture along the megathrust zone [Freed
and Lin, 2001; McCloskey et al., 2 0 0 5 ] . If the
same mechanism that triggered the 28 March event, i.e.,an aftershock that grows to the size of a large main shock, were to apply farther south on the former 1833 Mw 8.7 event area, the next
Indonesian Big One may b e expected to o c c u r there within a few months.
In opposition to this prediction, i.e. the reactivation of the 1833 rupture area, are the localization and o c c u r r e n c e of a major quake along any regional tectonic feature, including the major strike-slip fault of Sumatra. According to static stress transfer theory, this strike-slip fault underwent a 9-bar stress increase due to the D e c e m b e r 2004 event [McCloskey et al, 2 0 0 5 ] . The circumstances have worsened since then with the o c c u r r e n c e of the Mw 8.7 event.
References
Bird, P, and YY Kagan (2004), Plate-tectonic analysis of shallow seismicity: Apparent boundary width, beta, corner magnitude, coupled lithosphere thickness, and coupling in seven tectonic settings,
Bull. Seismol. Soc. Am., 94(6), 2380-2399.
Freed, A. M.,and J. Lin (2001), Delayed triggering of the 1999 Hector Mine earthquake by visco-elastic stress transfer, Nature, 411, 180-183.
Gutenberg, B., and C. F Richter (1954),Seismicity of
the Earth and Associated Phenomena, Princeton
Univ. Press, Princeton, N. J.
Kagan,Y Y (1997), Seismic moment-frequency rela tion for shallow earthquakes: Regional compari son, J. Geophys. Res., 102, 2835-2852.
McCloskey, J.,S. S. Nalbant,and S. Steacy (2005), Earth quake risk from co-seismic stress, Nature, 434, 291. Omori,E (1894),The aftershock of earthquakes,
J. Coll. Sci. Imp. Univ. Tokyo, 7, 111-120.
Richter, C. F (1958),Elementary Seismology, 768 pp., W E Freeman, New York.
U.S. Geological Survey (2005), Magnitude 8.7—Northern Sumatra, Indonesia, preliminary earthquake report, Earthquake Hazards Program, Reston,Va. (Available at http://earthquake.usgs. gov/eqinthenews/2005/usweax/)
Utsu,T.,Y Ogata, and S. Matsu'ura (1995),The cen tenary of the Omori formula for a decay law of aftershock activity, J. Phys. Earth, 43, 1-33.
— J . R. GRASSO and C. VoiSIN, Laboratoire de Geophysique Interne et Tectonophysique, Grenoble, France
For additional information, contact J. R. Grasso; E-mail: [email protected].
Section President, Paul G. Silver, Section S e c r e t a r y , John Ebel
The Largest Aftershock
Ever Recorded?
PAGE 2 1 1
The largest aftershock recorded so far of the
Mw 9.3 Sumatra earthquake of 26 D e c e m b e r
2004 might b e assumed to b e the 28 March off-Sumatra event (Mw 8.7), which occurred
three months later, 160 km away, and with a 0.6 magnitude deficit. B e c a u s e the 28 March event reproduces in size and location the Mw 8.5 main
shock of 1861, it could be argued that it should rank as among Earth's top 10 largest main shocks since 1900. Whether the 28 March event was an aftershock or compound earthquake, what does it imply for the occurrence of the next Indonesian "Big One"?
What is known about fault interactions and earthquake triggering primarily derives from shallow earthquakes in the brittle crust. This knowledge reduces to distribution laws in size and time: a power law for the frequency size distribution of earthquakes [Gutenberg
and Richter, 1954]; a power law for the d e c a y
of seismicity rate after any earthquake [Omo
ri, 1894; Utsu et al, 1995]; and the Bath law
for the average size of the largest aftershock
[Richter, 1 9 5 8 ] .
Evidence that the Mw 8.7 event is an after
shock are ( 1 ) in time, the regional seismicity rate had not yet returned to the pre-December 2004 background level; (2) in space, it is located within 0.15 times the rupture length of the D e c e m b e r event, i.e., it is smaller than the one-rupture-length distance classically considered as a maximum for the aftershock-main shock distance; and ( 3 ) in size, its magnitude is small er than that of the D e c e m b e r event.
Therefore, the Mw 8.7 event takes on all the
characteristics of a genuine aftershock, so far being the largest aftershock ever recorded on Earth. However, the fact that the Mw 8.7 event
is an aftershock is tempered by the moderate 0.6 difference between the observed main s h o c k and aftershock magnitudes as c o m pared with the 1.2 average value predicted by the Bath law.