News from the Earth Sciences
UCL Earth Sciences

Sediments tell a tsunami story.

26 May 2017

Results of the Sumatra Subduction Zone IODP Expedition (2016) have been published in the journal Science.

Plate-boundary fault rupture during the 2004 Sumatra-Andaman subduction earthquake extended closer to the trench than expected, increasing earthquake and tsunami size. International Ocean Discovery Program Expedition 362 sampled incoming sediments offshore northern Sumatra, revealing recent release of fresh water within the deep sediments. Thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial-induced temperature increases in the past 9 million years. Complete dehydration of silicates is expected before plate subduction, contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction. Shallow slip offshore Sumatra appears driven by diagenetic strengthening of deeply buried fault-forming sediments, contrasting with weakening proposed for the shallow Tohoku-Oki 2011 rupture, but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records.

Kevin Pickering (UCL Earth Sciences) is co-author of a paper in the journal "Science" published on 26 May 2017. In 2016, Pickering was a shipboard scientist and lead sedimentologist on the International Ocean Drilling Project, Expedition 362 (Subduction Inputs to the Sumatra Subduction Zone). He led the team of 6 sedimentologists who helped establish the stratigraphy and sedimentology of the Indian Ocean drill sites in which the chemical and mineralogical anomaly was discovered and upon which this Science paper is based.

Sumatra - IODP map

Image: Map of IODP Expedition 362 sites (red dots) and 2004 and 2005 rupture zones. Red arrows and numbers show convergence vectors (cm/year). White arrows and numbers indicate subduction velocities accounting for fore arc motion. Yellow lines mark seismic profiles where a HANP (high-amplitude normal polarity) reflector is imaged or reported (yellow area). Dashed yellow line shows estimated extent of the HANP reflector. NER, Ninetyeast Ridge.

The 2004 ~Mw 9.2 Sumatra-Andaman earthquake and tsunami devastated coasts throughout the Indian Ocean, killed more than 250,000 people, and made scientists reassess our understanding of great earthquakes. This unexpected and high moment release event was centered beneath a wide forearc plateau, and slip approached the trench. This earthquake was the first great earthquake to be recorded and analysed with modern geophysical techniques, and rapid-response surveys collected high quality multibeam and seismic data to help characterize the geology post-earthquake. Based on these data and analyses, many questions were raised, including: What controls the geometry of the wide forearc plateau? What enabled extensive slip along this plate boundary and to shallow depths? And what are the material properties and pressure and stress states along this margin? Expedition 362 approached these questions by investigating the input sediments to the Sumatra subduction zone because input sediments are the building blocks for the forearc plateau and influence the location and conditions of the décollement and its slip behavior during earthquakes.

The primary objectives of Expedition 362 were to: (1) determine the lithology, sedimentation rates, and physical, chemical, and thermal properties of the input section, and determine how temporal changes in sedimentation rate and lithology influence the physical properties; (2) assess compaction, induration, and diagenetic state of the deep input sediments that eventually form the interior and base of the accretionary prism and develop into the décollement; and (3) determine similarities and differences in lithology and physical properties within the stratigraphic section, in particular between the Bengal-Nicobar Fan sediments and the more slowly deposited pelagic sediments.

Expedition 362 addressed these objectives by drilling Sites U1480 and U1481 ~ 225 km seaward of the deformation front (Figure 1). Riserless drilling at these sites provided access to the complete stratigraphic section and upper oceanic crust prior to entering the subduction zone.