THE
GREAT EARTHQUAKE AND TSUNAMI OF 1833 OFF THE COAST OF CENTRAL
SUMATRA IN INDONESIA
George
Pararas-Carayannis
INTRODUCTION
In 1833 a great earthquake
off the coast of Central Sumatra in Indonesia generated a destructive
tsunami that must have affected Sumatra and the entire Indian
Ocean basin and western coast of Australia.
 THE EARTHQUAKE
Magnitude
- The Moment Magnitude
of the 1833 earthquake has been estimated to have been in the
order of Mw= 8.7 to Mw= 8.8. However, based on field studies
of coral growth rings in the region (Zachariasen et al.), it
was postulated that the moment magnitude may have been as great
as Mw=9.2 - almost as much as that of the Great Earthquake of
December 26, 2004 (Mw=9.3) and much greater than the 1861 earthquake
( for which estimates range from Mw= 8.3 to Mw=8.5).
Tectonic
Setting
Active subduction
of the Indian and Australian oceanic plates beneath the Sunda
Plate - which is part of the Eurasian plate - has shaped the
young (40 million years) Sunda Arc and Trench system along its
entire length. All segments of the great subduction zone off
Sumatra have been extremely active and have produced numerous
major and great earthquakes in recorded history from as early
as 1797, 1833 and 1861 and as recently as 2004, 2005 and 2007
(Newcomb et al. 1987; Pararas-Carayannis 2005a,b and 2007).
The earthquake of
1833 occurred along the active Central/Souhern Sumatra segment
of the Sunda Subduction Zone (SSZ), (known as the Mentawai Fault)
where the Australian plate subducts beneath the Burma microplate.
The subduction rate in the Mentawai Segment of this system is
about 60 mm per year.The earliest known great earthquake along
this segment occurred in 1797. The latest great earthquake (Mw=8.4)
on this same segment occurred on September 12, 2007 (Pararas-Carayannis,
2007). Actually, three other major earthquakes occurred in the
same segment shortly after the great 2007 earthquake.
Vertical
Deformations Caused by the 1833 Earthquake - Comparison with
the Earthquake of 1797.
The vertical deformations
caused by the 1833 and the 1797 earthquakes in the outer-arc
islands off Central/Southern Sumatra were determined by extensive
field studies which correlated patterns of uplift and U-Th dating
of coral microatolls (Zachariasen et al., 1999). Based on patterns
of deformation and on constrain models of slip on the subduction
interface, detailled maps of uplift were prepared for these earthquakes.
Accordingly, Sipora, North Pagai and South Pagai Islands, which
span a distance of about 160-km in length along the outer-arc
ridge of the tectonic boundary, have corals that display evidence
of uplift associated with the 1797 earthquake. This uplift ranges
from zero to 70 cm, and has a distinct northeastward tilt, away
from the trench.
Also, these islands
have coral growth rings that recorded the vertical deformations
associated with the 1833 earthquake. The uplift values for this
earthquake range from 1 meter up to 2.30 meters. The corals that
were studied show a pronounced tilt away from the trench as with
the 1797 event. However, no evidence of uplift or submergence
was found from either the 1797 or the 1833 earthquakes on neighboring
Siberut Island, farther northwest (Zachariasen et al., 1999).
Slips, Orientation
and Estimated Moment Magnitudes of the 1833 and 1797 Earthquakes.
On the basis of the
above described field measurements (Zachariasen et al., 1999)
it was concluded that the 1797 earthquake had slips of about
3 meters and the 1833 earthquake had slips of about 12 meters.
Also, the dips associated with these earthquakes were estimated
to be about 12° in a northeastward direction at about 25
km below the outer-arc islands.
 Estimates of Moment Magnitudes for
the 1797 and 1833 Earthquakes
Based on deformations,
slip estimates and extend of ruptures the Moment Magnitude of
the 1797 event was estimated at about 8.4 while that of the 1833
was estimated at about 8.7.
For both events, rupture
beneath all three of the outer-arc islands (Sipora, North Pagai
and South Pagai) was required to produce the uplift that was
observed. The northwestern limit of both 1797 and 1833 ruptures
must be no more than 40 km northwest of Sipora (the northernmost
of the three islands). The southeastern limit of the 1797 rupture
is well constrained to beneath the southern part of South Pagai
(the southernmost of the three). The southeastern limit of the
1833 rupture is poorly constrained, but extended well beyond
South Pagai Island. The downdip limit of rupture in 1833 is appreciably
deeper (farther northeast) than that of the 1797 event.
The adjacent map shows
the estimated dimensions of the tsunami generating source areas
off the coast of the island of Sumatra associated with the earthquakes
of 1833, 1861, 2004, 2005 and 2007.
The giant 1797 and
1833 earthquakes involved rupture of the same patch of the subduction
interface beneath the islands, but slip extended farther downdip
and southeast in 1833. Slip on the patch beneath the islands
was also several times larger in the latter event, far more than
could have accumulated in the 36 years between events. Thus it
appears that slip magnitudes can vary by a factor of four or
so on the same fault patch and that not all accumulated strain
need be relieved in a giant earthquake.
Prior to that, in
1797, another great earthquake occurred roughly along the same
segment. However the displacements it caused were not as great
as those of the 1833 event and did not cause as destructive of
a tsunami. Apparently, major and great earthquakes in this segment
exhibit great variabilities in the ground displacements and the
lengths of ruptures. Often, tectonic stress is dissipated along
this segment by two or more events
Paleoseismic evidence
indicates that over the past millennium, the islands have risen
during giant earthquakes or earthquake couplets about every 230
years. At least three of the episodes appear to have been couplets
separated by just a few decades.
THE TSUNAMI
OF 1833 IN INDONESIA
There is not much
data on the effects of the 1833 tsunami in Sumatra, Australia,
Sri Lanka, India, the Maldives or on any of the islands or countries
bordering the Indian Ocean. Undoubtedly the tsunami must have
been devastating on the adjacent coastline of Sumatra. However,
because of the orientation of the generating area, most of the
tsunami energy radiated primarily towards unpopulated coastal
areas of the Southwest Indian Ocean.
The tsunami must have
had an impact on Sri Lanka, the Maldives, Madagascar and other
islands and coastlines of Africa. Not much of its energy radiated
towards the Bay of Bengal, so India - with the exception perhaps
of the Tamil Nadu State - was not greatly affected nor was the
coastlines of Bangladesh, Burma or Thailand. Also because of
the tsunami's source area orientation, western Australia was
not impacted in any significant way.
Tsunami
Generating Area
The rupture of the
1833 earthquake is estimated to have been between 300 to 350
km. It was considerably less than the 1,200 km rupture of the
December 26, 2004. Although the magnitude of the 1833 earthquake
was great, the tsunami generating area was much smaller than
that of 2004 - the latter extending along two segments of the
great Sunda tectonic boundary in northern Sumatra and the Andaman
Sea.
Engraving
of the 1833 Tsunami (by Edouard Riou, 1833-1900; National Information
Service for Earthquake Engineering, University of California,
Berkeley).
REFERENCES
and ADDITIONAL BIBLIOGRAPHY
Newcomb KR &
McCann WR. 1987. Seismic history and seismotectonics of the Sunda
Arc. Journal of Geophysical Research; 92:421-439.
Pararas-Carayannis,
G. 2005, The
Great Earthquake and Tsunami of 28 March 2005 in Sumatra, Indonesia
Pararas-Carayannis, G. 2005, The
Great Earthquake and Tsunami of 26 December 2004 in Southeast
Asia and the Indian Ocean
Pararas-Carayannis, G. 2007, The
Earthquakes and Tsunami of September 12, 2007 in Indonesia
Zachariasen M, Sieh
K, Taylor FW, Edwards RL & Hantoro WS. 1999. Submergence
and uplift associated with the giant 1833
Sumatran subduction earthquake: Evidence from coral micro-atolls.
Journal of Geophysical Research; 104:895-919.
Nott J & Bryant
E. Extreme marine inundations (Tsunamis?) of coastal Western
Australia. Geology Journal; 11:691-706.
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