1. Behavior of the Sunda Megathrust
in Sumatra
The Sumatran Plate Boundary project is a multi-disciplinary attempt to understand the behavior of the Sumatran section of the Sunda megathrust.
Kerry Sieh
TO Annual Meeting
8 November 2006
Submerging forest on the Mentawai islands, West Sumatra

2. Collaborators Caltech John Galetzka Mohamed Chlieh Don Helmberger
JP Avouac Aron Meltzner Ozgun Konca
Yaru Hsu Mark Simons Andrew Kositsky
Indonesian Institute of Sciences
Danny Natawidjaja Bambang Suwargadi
Nughroho Hanato
Dudi Prayudi
Imam Suprihanto
Scripps
Yehuda Bock et al
Kyoto University
Shigehiro Fujino
USC
Costas Synolakis
Jose Borrero
Here is a list of collaborators and sponsors
Collaborators

3. Segments that produce great earthquakes
I’ll focus on just one aspect of all the things that we are working on. For a more comprehensive view, take a look at the 2006 TO Annual Report. Let’s see what we’ve learned about segmentation of the megathrust. In a nutshell, we have identified both along-strike and down-dip differences in behavior. Principally seismic sections are the Aceh, Nias-Simeulue and Mentawai sections.

4. Segments that are principally aseismic
Largely aseismic sections are the Simeulue saddle, the Equatorial patch and the Enggano section.

5. Down-dip variations
Because we captured the deformations of the Nias-Simeulue earthquake and its aftermath, we also have recognized down-dip variations in behavior there. And because of our long paleogeodetic records, we have uncovered a strange behavior on shallow sections of the Mentawai patch.

6. Segments that produce great earthquakes
1797
Segments that produce great earthquakes
So, let’s look at the segments that produce great earthquakes. The 1797 and 1833 earthquake sources are constrained by uplift known from coral paleogeodesy.
1833
from Natawidjaja et al, JGR, 2006

7. 1797 from Natawidjaja et al, JGR, 2006
Here is one model of the 1797 rupture, in which rupture extends all the way to the trench.
from Natawidjaja et al, JGR, 2006

8. 1833 from Natawidjaja et al, JGR, 2006
In this model, the up-dip limit of rupture is at a depth of 15km.
from Natawidjaja et al, JGR, 2006

9. 1833 from Natawidjaja et al, JGR, 2006
In this model of the 1833 rupture, slip extends all the way up dip to the trench.
from Natawidjaja et al, JGR, 2006

10. 2004 from Subarya et al., Science, 2006
The 2004 Aceh-Andaman earthquake resulted from rupture of a 1600-km long section of the Sunda megathrust that runs from Simeulue island, offshore Aceh province in Sumatra to north of the Andaman Islands.
2004
from Subarya et al., Science, 2006

11. 2004 from Chlieh et al, BSSA, in press
Uplift of the sea floor produced by that rupture was greatest west of the Nicobar islands and offshore Sumatra. These uplifts produced the giant tsunami that killed about a quarter million people.
2004
from Chlieh et al, BSSA, in press

12. 2005 from Briggs et al, Science, 2006
We used coral microatolls, augmented by several cGPS measurements to constrain the slip on the megathrust that produced the great 2005 earthquake. Rupture consists of two patches, the bigger beneath Nias island and the lesser beneath southern Simeulue island.
2005
from Briggs et al, Science, 2006

13. Down-dip variations 2005 from Briggs et al, Science, 2006
The several cGPS stations that we had installed above the 2005 rupture enabled us to study post-seismic deformation associated with the 2005 rupture. Note the location of the coseismic ruptures, principally beneath the islands.
2005
from Briggs et al, Science, 2006
Down-dip variations

14. 2005 afterslip from Hsu et al, Science, 2006
Post-seismic deformation was caused by slip updip and downdip of the coseismic rupture. The rate of afterslip in the filrst 11 months decayed logarithmically. The rate of aftershock occurrence followed the same pattern but account for only a small percentage of the afterslip moment. This relationship shows that the aftershocks were closely related to the afterslip.
2005 afterslip
from Hsu et al, Science, 2006

15. 1907 tsunami ?
The updip section of the megathrust is also a candidate for producing big earthquakes. Our initial paleotsunami work suggests that the 1907 tsunami, which was very destructive on the west coasts of both Simeulue and Nias islands, could have been caused by rapid rupture of this shallow portion of the megathrust.

16. An aseismic shallow rupture in about 1962 from paleogeodetic data
Furthermore, paleogeodetic data from the Mentawai and Batu islands reveal a rapid subsidence event in about Aseismic slip on the megathrust west of the islands and equivalent to a M 8.4 earthquake could have produced this rapid subsidence.
from Natawidjaja et al, JGR, in press
An aseismic shallow rupture in about 1962 from paleogeodetic data

17. An aseismic shallow rupture in about 1962 from paleogeodetic data

18. Segments that are principally aseismic
Between the three principal seismic sections of the Sunda megathrust in Sumatra are three smaller sections that seem to be less strongly coupled. First, the Simeulue Saddle, under Simeulue island.
Simeulue Saddle
from Briggs et al, 2006, Science

19. The Simeulue Saddle ~ 60 km Misalignment of ridge crests
from Briggs et al, 2006, Science

20. Why? The misalignment suggests a structural cause
from Briggs et al, 2006, Science

21. The Equatorial patch
The Equatorial Patch is another largely aseismic section

22. Large emergence during the Mw 7.7 1935 earthquake
This 4.2-m-diameter microatoll shows that the Equatorial patch has resisted major rupture during three giant earthquakes, in 1797, 1833 and It also was just south of the southern terminus of the 2005 rupture.
Small emergences during four giant earthquakes
from Natawidjaja, 2003, PhD

23. The Enggano patch
The Enggano patch appears to be the southern limit of the 1833 rupture.

24. from Chlieh et al, in preparation
Modeling of the degree of coupling, using SuGAr data recorded since June 2005 shows that this section is largely decoupled. Note that the three southernmost stations are moving southwest at high velocities relative to the Australian plate, while stations on the Mentawai islands are moving at much slower velocities. This indicates that the megathrust beneath the Mentawai islands is highly coupled and the section farther south is not.
from Chlieh et al, in preparation

25. The permanence of patch behavior
The correlation of the rupture areas of great earthquakes with regions of currently high coupling (from GPS and coral paleogeodesy) suggests the variations in coupling are permanent features of the megathrust.
From Chlieh et al, in preparation

26. How often does the Mentawai patch rupture?
Are couplets like 1797 and 1833 typical?
So, how often does the highly coupled patch beneath the Mentawai islands rupture?

27. Paleoseismic records ~1370 ~1600 1797 & 1833 Bulasat Simanganya
Elevation relative to modern sea level
We have not fully answered this question. But these two paleoseismic sites on the Mentawai islands have a record of three episodes of uplift in the past 700 years. It appears that individual great earthquakes or couplets occur about every 230 years.
Simanganya
Years (AD)

28. ~1370 ~1600 1797 & 1833 Bulasat Simanganya Time-predictable?
Elevation relative to modern sea level
These events appear to be approximately time predictable
Simanganya
Years (AD)

29. Toward the application of basic science for human benefit
Tsunami modeling
Educational outreach
What can be done to bring our scientific discoveries to the benefit of people living in coastal Sumatra?
For more information, see Sieh 2006 Proc Royal Soc

30. Tsunami modeling 10-m slip Nias-Simeulue tsunario 20-m slip
We have just begun to attempt forecasts of tsunami characteristics from a future failure of the Mentawai patch
10-m slip
Nias-Simeulue tsunario
20-m slip
Aceh-Andaman tsunario
Tsunami modeling

31. 10-m slip Padang population ~800,000 10-m slip Bengkulu
We have just begun to attempt forecasts of tsunami characteristics from a future failure of the Mentawai patch
10-m slip
Padang
population ~800,000
10-m slip
Bengkulu
population ~350,000

32. Here is a movie of the tsunami caused by 10 m of slip on the megathrust beneath the Mentawai islands on the coastline near Bengkulu, a city with a population of about 350,000 people.

33. Our new educational poster for residents of the Sumatran and Mentawai island coasts
And we have just finished an educational poster aimed at the million+ residents of this coastal region.

34. Tomorrow I’ll discuss possible future directions for the Sumatran project. Thank you.
Where do we go from here?

36. From Chlieh et al, in preparation

37. From Chlieh et al, in preparation

38. From Chlieh et al, in preparation

39. From Chlieh et al, in preparation