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Along and across strike variations in the structure, material and fault properties of the Sumatran Subduction Zone

Along and across strike variations in the structure, material and fault properties of the Sumatran Subduction Zone
Along and across strike variations in the structure, material and fault properties of the Sumatran Subduction Zone
Earthquake ruptures offshore of north-central Sumatra in 2004 (Sumatra-Andaman segment) and 2005 (Nias segment) exhibited variable shallow slip behaviour that is well resolved by tsunami, seismologic, and geodetic observations. Closely spaced 2D MCS profiles collected covering these rupture segments, the segment to the south (Mentawai segment), and the segment boundaries allow an unprecedented opportunity to study the detailed along- and across-strike variation in the structure and properties of the wedge and plate boundary that may be related to the variable shallow slip behaviour. The large tsunami generated by the 2004 earthquake is thought to have been the result of unexpected shallow slip on the shallow plate boundary of the southern Sumatra-Andaman segment; whereas in the Nias segment the shallow plate boundary showed evidence of significant afterslip and the 2005 earthquake did not generate a significant tsunami. In the northern Mentawai segment, Batu Segment Boundary Zone, and southern Nias segment the shallow plate boundary is relatively weak and the weakened plate boundary is likely related to dehydration reaction occurring below the frontal prism and potential fluid flow focussing on basement topographic highs. In these areas, imbricate faulting in the frontal prism occurs along landward and/or seaward dipping faults. At the mid-slope break, the slip rate along these imbricate faults, formed in the frontal prism, decreases and they are potentially crosscut by out-of-sequence faults. In the southern Sumatra-Andaman segment, our findings of a relatively strong plate boundary fault are consistent with increased compaction and early dehydration of material within the thick incoming section which strengthens the incoming section and results in a potentially seismogenic shallow plate boundary. The accretionary prism here is characterised by an unusual prism profile with a narrow steep toe and broad flat plateau. We propose that the steep prism toe is built up by coeval faulting on both seaward and landward dipping faults. At the break in slope, underplating becomes the dominant prism and the transition from frontal accretion to underplating contributes to the development of the unusual prism geometry. Within our study area, we find that there is significant variability in the frictional strength of the shallow plate boundary and mechanisms of accretion, which can be correlated with the geometry and faulting within the prism. We show that these changes are related to a combination of subducting plate topography, trench sedimentation rates, overall sediment thickness and sediment/fault permeability.
Cook, Becky J.
c114ede4-47e6-450b-a754-71097473245e
Cook, Becky J.
c114ede4-47e6-450b-a754-71097473245e
Henstock, Timothy
27c450a4-3e6b-41f8-97f9-4e0e181400bb

Cook, Becky J. (2014) Along and across strike variations in the structure, material and fault properties of the Sumatran Subduction Zone. University of Southampton, Ocean and Earth Science, Doctoral Thesis, 113pp.

Record type: Thesis (Doctoral)

Abstract

Earthquake ruptures offshore of north-central Sumatra in 2004 (Sumatra-Andaman segment) and 2005 (Nias segment) exhibited variable shallow slip behaviour that is well resolved by tsunami, seismologic, and geodetic observations. Closely spaced 2D MCS profiles collected covering these rupture segments, the segment to the south (Mentawai segment), and the segment boundaries allow an unprecedented opportunity to study the detailed along- and across-strike variation in the structure and properties of the wedge and plate boundary that may be related to the variable shallow slip behaviour. The large tsunami generated by the 2004 earthquake is thought to have been the result of unexpected shallow slip on the shallow plate boundary of the southern Sumatra-Andaman segment; whereas in the Nias segment the shallow plate boundary showed evidence of significant afterslip and the 2005 earthquake did not generate a significant tsunami. In the northern Mentawai segment, Batu Segment Boundary Zone, and southern Nias segment the shallow plate boundary is relatively weak and the weakened plate boundary is likely related to dehydration reaction occurring below the frontal prism and potential fluid flow focussing on basement topographic highs. In these areas, imbricate faulting in the frontal prism occurs along landward and/or seaward dipping faults. At the mid-slope break, the slip rate along these imbricate faults, formed in the frontal prism, decreases and they are potentially crosscut by out-of-sequence faults. In the southern Sumatra-Andaman segment, our findings of a relatively strong plate boundary fault are consistent with increased compaction and early dehydration of material within the thick incoming section which strengthens the incoming section and results in a potentially seismogenic shallow plate boundary. The accretionary prism here is characterised by an unusual prism profile with a narrow steep toe and broad flat plateau. We propose that the steep prism toe is built up by coeval faulting on both seaward and landward dipping faults. At the break in slope, underplating becomes the dominant prism and the transition from frontal accretion to underplating contributes to the development of the unusual prism geometry. Within our study area, we find that there is significant variability in the frictional strength of the shallow plate boundary and mechanisms of accretion, which can be correlated with the geometry and faulting within the prism. We show that these changes are related to a combination of subducting plate topography, trench sedimentation rates, overall sediment thickness and sediment/fault permeability.

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Published date: 19 September 2014
Organisations: University of Southampton, Ocean and Earth Science

Identifiers

Local EPrints ID: 374820
URI: http://eprints.soton.ac.uk/id/eprint/374820
PURE UUID: 6e9a1fbd-0e9f-47ec-b92d-58c666b7a6ae
ORCID for Timothy Henstock: ORCID iD orcid.org/0000-0002-2132-2514

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Date deposited: 05 Mar 2015 14:42
Last modified: 06 Jun 2018 12:52

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