Subduction zone segmentation and controls on earthquake rupture: The 2004 and 2005 Sumatra earthquakes

Lead Research Organisation: University of Liverpool
Department Name: Earth Surface Dynamics

Abstract

The Sumatran earthquake of December 26th 2004 was the second-largest earthquake on record. The growing concentrations of population in regions prone to great earthquakes makes it a matter of urgency to study the processes that control these earthquakes. The Sumatran earthquake is the first to which modern geophysical tools can be applied, so offers a unique opportunity for such study. Most great earthquakes (magnitude 8 and larger) take place where two plates converge; such regions lie mostly under water, which makes them difficult to investigate, and which means that the hazard of tsunamis is added to the dangers of ground shaking. In the December 26th 2004 earthquake, the Australian and Eurasian plates slipped towards each other by up to 25 metres, on a fault that runs for 1200 km along the earth's surface. An obvious question is: Why was this earthquake so large? But perhaps the question should be: Why wasn't this earthquake larger? because, in March 2005 an adjacent 400 km of the plate boundary slipped in a second huge earthquake. All plate boundaries are divided into segments - pieces of fault that are distinct from one another, either separated by gaps or with different orientations. The boundaries between segments provide barriers that limit how far an earthquake can spread. A large earthquake may rupture a whole segment of plate boundary, but a great earthquake usually ruptures more than one segment at once. However, we do not know what determines whether an earthquake stays within one segment of plate boundary (and remains relatively small), or jumps across barriers between segments (to become a great earthquake). The Sumatran earthquakes give a unique framework to attack this problem. Detailed analyses of the seismic waves radiated by the Sumatran earthquakes give accurate locations for the barriers controlling the sizes of the earthquakes. The December 2004 earthquake started close to Banda Aceh and spread almost entirely northwards. Although 25 metres of slip occurred near the southern end of the rupture, almost no slip spread to the south; clearly, an important barrier here prevented the earthquake spreading to the next fault segment. In March 2005 another great earthquake occurred within this next segment, and spread southwards until it was stopped by a barrier at its southern end. We will conduct detailed geophysical surveys of the plate boundary to determine the nature of these two barriers. Large-scale (1-20km resolution) images of the plate boundary will be obtained in a combined land-sea experiment using air-gun explosions to bounce seismic waves off structures inside the plate boundary. In a longer-term experiment, seismometers left on land and on the sea-bed for several months will pick up the seismic signals from distant earthquakes. These waves, travelling upwards through the earth to the array of seismometers, can be used (in a fashion similar to CAT scanning) to form 3-dimensional images of the deeper parts of the crust and upper mantle. At the same time, new techniques will be developed to give more precise pictures of the distribution of slip in the two earthquakes, in order to link the static structure of the plate boundary to the dynamics of the earthquakes. Cores from the seabed will show when large earthquakes have occurred on these faults in the past, and whether the segmentation seen in 2004-5 was similar in the past events. In addition, we will collect evidence for the ways in which fault slip affects the seabed and generates a tsunami. The results will be significant both locally and globally. It is important to compare the barrier between the 2004 and 2005 earthquakes with the barrier at the south of the 2005 earthquake, because the plate boundary immediately to the south slipped in 1833, causing a devastating tsunami. More generally, the results will have implications for other convergent boundaries, such as those beneath Japan, and on land associated with the Himalayas.

Publications

10 25 50
 
Description This project studied a part of the Sumatra subduction zone where major earthquakes on the plate boundary occurred in 2004 and 2005.

Key findings are:

1. Slip during the 2004 Boxing Day earthquake reached very close to the seabed, much closer than in the 2005 earthquake which was more like typical subduction earthquakes. This shallow slip was an important contributor to the size of the resulting tsunami. The region where shallow slip occurred has thick sediments on the incoming plate with a high amplitude reflection near the base; we suggest that the thick sediments are probably unusually hot and have become stronger as a result, an idea which will be tested by ocean drilling in 2016.

2. Topography on the subducting plate within depth range of the plate boundary where earthquakes occur can be a primary cause of segmentation, the observation that subduction plate boundary earthquakes each break only a part of the length of the subduction zone. We have been able to show that the southern end of the 2005 earthquake coincides with a feature on the down-going plate that is 15kmx10km laterally and 3km high. We have also shown that an isolated seamount further to the south may have a similar role in controlling earthquake slip in a part of the plate boundary that has not experienced a major recent event.

3. Based on work from some other subduction zones it has been suggested that major earthquakes always produce turbidity flow deposits and that determining the age and extent of these deposits can produce a history of previous large earthquakes. We were able to show that offshore Sumatra this idea does not hold: Neither the 2004 nor 2005 earthquakes left turbidite deposits in basins within the accretionary prism, even though they did leave deposits within the trench. More broadly even sites that are close together showed very different histories of turbidite deposits that are generally not correlated with each other or with the known earthquake history of the region.

4. The Sumatra subduction zone has some extremely unusual characteristics within the accretionary prism, the sediments that are scraped off the downgoing plate. In many subduction zones faulting within the prism is relatively simple with more or less parallel faults that dip towards the land. Offshore Sumatra significant faults within the prism dip away from the land as well as towards it. Some areas have a primary direction of dip, but others show dips which vary rapidly along the subduction zone, and show different dip directions active at different times. This complexity may be in part due to subduction of topographic features disrupting the usual fault development.
Exploitation Route Our findings show that the Sumatra subduction zone has many differences from other subduction zones that have been studied around the world. The reasons for these differences are not always clear at present, although we have suggested some hypotheses, and will be testing some later in 2016.

The findings on segmentation and on how close to the seabed slip during major earthquakes may extend are very relevant to both the direct earthquake hazard (they control the earthquake magnitude and likely degree of shaking) and also the characteristics of tsunami that may be produced.
Sectors Education,Environment,Government, Democracy and Justice

 
Title IRIS 
Description Temporary seismic station time series recordings available from IRIS-DMC 
Type Of Material Database/Collection of data 
Year Produced 2012 
Provided To Others? Yes  
Impact n/a 
 
Title SO198-1 
Description Marine Geophysical Data from research cruises SO198-1 
Type Of Material Database/Collection of data 
Year Produced 2010 
Provided To Others? Yes  
Impact n/a 
URL https://www.bodc.ac.uk/data/information_and_inventories/cruise_inventory/report/8532/
 
Title so200-1 
Description Marine geophysical data from cruise SO200-1 
Type Of Material Database/Collection of data 
Year Produced 2010 
Provided To Others? Yes  
Impact n/a 
URL https://www.bodc.ac.uk/data/information_and_inventories/cruise_inventory/report/9461/