Kerry Sieh

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Segmentation of the Sunda megathrust
Hints from giant earthquakes
Kerry Sieh WGCEP, Caltech 15 March 2006
Submerging forest on the east coast of South
Pagai island, West Sumatra
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Collaborators
Caltech John Galetzka Mohamed
Chlieh JP Avouac Aron
Meltzner Yaru Hsu Mark
Simons
Scripps Yehuda Bock Linette
Prawirodirdjo
Indonesian Institute of Sciences Danny
Natawidjaja Bambang Suwargadi Nughroho
Hanato Dudi Prayudi Imam Suprihanto
Submerging forest on the east coast of South
Pagai island, West Sumatra
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From Subarya et al. 2006 and Briggs et al. in
press
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The March 2005 event also resulted in impressive
uplift
Old high tide
Old low tide
Examples from Nias, Simeulue and the Banyak
islands
Old high tide
New high tide
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and submergence
Submergence in the Banyak islands
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A
A
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March 28,2005 Vertical deformation
up
down
Briggs et al. in press
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March 28,2005 Slip on the megathrust
Hsu et al. submitted
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Two discreet patches
Hsu et al. submitted
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Batee fault
Hsu et al. submitted
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Contours follow topography
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The 2005 rupture occurred in Sumatras central
domain characterized by a disrupted forearc
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Simeulue saddle
2004
2005
Northern limit to rupture
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The Simeulue Saddle
60 km
Misalignment of ridge crests
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The saddle exists in slip on the megathrust, too
Subarya et al. 2006
Hsu et al. in review
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The Simeulue Saddle
Uplift during the November 2002 foreshock
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Post-seismic deformation
GPS data vs. Model prediction
Hsu et al. in review
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cm
Hsu et al. in review
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cm
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cm
1935 M 7.7
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what has happened at the Equator over the past
several hundred years?
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1935 coseismic uplift and subsidence
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Pre-, co- and post-seismic behavior
Steve Wards modeling of Dannys and my coral
data 1999
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Behavior plotted orthogonal to trench at the
Equator
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Four giant earthquake ruptures have failed to
rupture beneath this patch
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Large emergence during the Mw 7.7 1935 earthquake
Small emergences during four giant earthquakes
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The Mentawai islands patch has had giant
earthquakes about every 2 centuries and is
currently accumulating strain
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Strain still accumulating
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GPS data (horizontal)
Coral data (vertical)
Chlieh et al. in preparation
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15 years of GPS and 50 years of corals show the
shape of the locked megathrust
Chlieh et al. in preparation
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The historical ruptures of 1797 and 1833 (from
coral data) coincide with current locked patches
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Evidence for the giant earthquakes of 1797 and
1833
Fossil microatolls in the intertidal zone, east
coast of North Pagai island
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Microatoll sampling sites
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Fossil microatoll (si94a6)
Modern microatoll (srm00A1)
Siruamata site
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1.85 m
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Uplift in 1833
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Uplift in 1797
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Magnitude 8.5 - 8.7
Magnitude 8.6 - 8.9
HOW OFTEN DO GREAT EVENTS OCCUR?
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1797
1360
1590
1833
centimeters
Sea level

• Two cycles, each about 2 centuries long

• Roughly predictable?

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What can we do scientifically to forecast future
events?
The Sumatran GPS Array (SuGAr)
27 continuously recording stations currently
telemetered via satellite
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Order is the dream of man Chaos is the law of
nature
Henry Adams
Thank you
New life starting on an emerged beach on Simeulue
island, Aceh