SEAmount and FRONTal megathrust Tsunami and Earthquake Risk Assessment. – SEAFRONTTERA

To do so, we will develop a complete multi-scale and multi-disciplinary project. In a first stage, we propose to characterize forearc structures revealing frontal propagation of earthquakes from the mechanical modeling of the South Mentawai segment where a tsunami earthquake occurred. From comparison and estimation of the frictional properties of the megathrust in the gap segment, based on the limit analysis method, we will evaluate if past frontal ruptures have occurred.
Furthermore, dynamic modeling of earthquake cycles will be used to characterize the mechanical requirement for a frontal propagation. We will evaluate if conditions are favorable along the gap segment.
In a second stage, we will address the question of the relation between bathymetric features, in particular seamounts, and earthquake segmentation. From static mechanical modeling, we will study stress perturbation induced by the subduction of such features. The stress perturbations will then be extended to dynamic modeling in order to determine how seamounts can affect earthquake propagation, seismic cycle, and coupling.

The geological study of seimsic profils along the Sumatra subduction zone has allowed to characterized specific structures associated to frontal propagation : landward thrusts. I have developped a mechanical model showing that those structures require specific properties (very high pore pressure along the megathrust and in the upper plate). We are now constraining those properties for future dynamic modelisation.
We have also studied the seismic profile of the gap segment.
The profile shows a great complexity, with at the same time landward structures and duplexes, probably associated to the subduction of a bathymetric feature.
We are now working on the structural analysis and have started the mechanical modelisation that will require a few more months of work.

The mechanical modeling of the North Mentawai profile will allow us to evaluate its seismic and tsunamigenic risks. At the same time, the complexity of the structure seems to be the result of the subduction of a bathymetric feature.Thiss profile will be quite usefull to contrain the deformation and the mechanical properties changes du to the subduction of such feature for the second part of the project.

in preparation : Landward accretionary prism, evidence for frontal propagation?

The two last giant earthquakes of magnitude >9 of the 21st century (2004 Sumatra and 2011 Japan) took both scientists and government agencies by surprise because such large earthquakes were not expected but, most of all, because they ruptured frontals sections of megathrust commonly thought to slip aseismically. The frontal ruptures led to devastating tsunamis with enormous loss of lives, properties and a near-nuclear disaster. In the last decade, the Sumatra subduction zone has experienced the most intense sequence of earthquakes ever recorded on Earth but there is still a large gap of 500 km. From geodetic and paleo-geodetic studies, the gap segment is known to be fully locked and ready to rupture in a very near future. Although the 2004 event ruptured up to 1300 km along the Sumatra-Andaman subduction zone, the southern Mentawai segment located south of the gap has been ruptured by a sequence of earthquakes. Earthquake segmentation has often been related to bathymetric features such as seamounts. Although this relation has been the focus of many studies, only sparse attempts have been made to develop mechanical models, impeding a good understanding of this relationship. However, to assess correctly seismic and tsunami risks, it is of fundamental importance to understand why frontal sections can rupture and produce large tsunamis, and what controls the lateral extent of earthquakes. In this project, we propose to model the physical requirement of frontal rupturing and the effect of subducted bathymetric features on earthquake segmentation. Since high-quality seismic reflections profiles have been acquired along the Sumatra subduction zone and in particular in this gap segment, we thus possess a unique framework to conduct a thorough mechanical analysis.
To do so, we will develop a complete multi-scale and multi-disciplinary project. In a first stage, we propose to characterize forearc structures revealing frontal propagation of earthquakes from the mechanical modeling of the South Mentawai segment where a tsunami earthquake occurred. From comparison and estimation of the frictional properties of the megathrust in the gap segment, based on the limit analysis method, we will evaluate if past frontal ruptures have occurred. Furthermore, dynamic modeling of earthquake cycles will be used to characterize the mechanical requirement for a frontal propagation. We will evaluate if conditions are favorable along the gap segment.
In a second stage, we will address the question of the relation between bathymetric features, in particular seamounts, and earthquake segmentation. From static mechanical modeling, we will study stress perturbation induced by the subduction of such features. The stress perturbations will then be extended to dynamic modeling in order to determine how seamounts can affect earthquake propagation, seismic cycle, and coupling.
From the geometry, stress state and frictional properties of the gap segment and dynamic simulations, we will provide probabilities for frontal rupture, lateral extent, amount of slip, tsunami run-up, as well as earthquake recurrence time.
Although the SEAmount and FRONTal megathrust Tsunami and Earthquake Risk Assessment along the Sumatra subduction is our primary goal, the developed generic tools will then be applicable to any subduction zone.