Thesis on the analysis of Mantle Seismicity and the Subduction Zone in Japan (M/F)

Job Description

Thesis Subject

Subduction zones host large earthquakes, sometimes accompanied by catastrophic tsunamis. Better estimating seismic hazard in these regions is therefore essential for populations. The subduction interface has classically been divided into three main domains: (i) the seismogenic zone (5–35 km depth), where most earthquakes occur and which corresponds to the contact between the upper and subducting plates; (ii) the decoupled zone (greater than 40 km), where the subducting slab can sink freely and which lies beneath the serpentinized mantle wedge, known as the “cold nose”; (iii) the transition zone (30–40 km), where slow slip events (SSEs) may occur.
This fragmented view of subduction zones has been challenged following: (1) the 2004 Sumatra megathrust earthquake (Mw = 9.1), which nucleated on the interface beneath the “cold nose” region; and (2) the 2011 Tohoku-oki earthquake (Mw = 9.0, Japan), which ruptured the interface from the trench down to 50 km depth, also beneath the “cold nose.” Such deep rupture propagation calls into question the mantle serpentinization paradigm, since serpentinite is generally considered aseismic; at these depths, the interface should slip freely according to previous thermo-mechanical models. The rheological and mechanical behavior of the mantle wedge, as well as its potential interactions with the interface, remain largely poorly understood. It is therefore crucial to better constrain the physical processes at play in this key region of subduction.
This PhD project aims to use seismological and statistical tools to better understand the complex feedbacks between the interface and “cold nose” seismicity in subduction environments, across space and time, by addressing three main challenges:
1. The geometry of the mantle wedge and its contact with the interface is poorly constrained.
We aim to better characterize the geometry and structure of the mantle wedge and its contact zone with the interface. This is fundamental to better constrain interface roughness and understand how the mantle wedge may influence interface seismicity.
2. The mechanism(s) behind mantle wedge seismicity remain unclear.
Fluids released at depth and rising along the subducting slab interface may also be diverted into the mantle wedge, as observed in exhumed settings. These fluids could act as a triggering mechanism for mantle wedge seismicity. However, in colder subduction zones, the mantle wedge is only hydrated by about 10–20%, which may not be sufficient to explain the occurrence of earthquakes.
In summary, the objective of this project is to better characterize mantle wedge seismicity in subduction zones and its interactions with the interface. To achieve this, the PhD student will analyze high-precision microseismicity catalogs (earthquakes with magnitude less than 3) as tracers of deformation and apply statistical analyses to these datasets.
Microseismicity is the best indicator of small-scale deformation. However, the smaller the earthquakes, the less likely they are to be detected, as they are often hidden within noise or overshadowed by larger events. We will address this issue by using detection algorithms based on seismic waveform recognition, such as Template Matching and Machine Learning, which are particularly well suited for areas with limited template datasets. The student will then be able to thoroughly analyze the spatio-temporal evolution of earthquakes in order to investigate: (i) the presence or absence of migration patterns indicative of fluid migration and the different forcing mechanisms acting on seismicity; (ii) how an earthquake occurring on the interface can affect the mantle wedge, and up to what magnitude, as well as the reverse interaction; (iii) the behavior of mantle wedge seismicity before and after episodes of tremor (small earthquakes related to fluid presence) and slip.
The expected outcomes are:
- Determine whether mantle wedge seismicity is modulated by interface seismicity
- Improve understanding of the mechanisms behind mantle wedge seismicity by integrating seismic and geodetic observations
- Contribute to broader efforts in earthquake physics and seismic hazard assessment

Your Work Environment

The candidate will work at the ISTerre research laboratory, a joint research unit (UMR) composed of approximately 330 staff members, and part of the Observatory of Sciences of the Universe (OSU) of Université Grenoble Alpes (UGA), which holds national responsibility for numerous observation services.
The student will be supervised by Blandine Gardonio (seismologist at ISTerre Chambéry) and Anne Socquet (geodesist at ISTerre), both specialists in subduction zones. The PhD student will be based at ISTerre Chambéry. The affiliated doctoral school will be ED STEP. The PhD will take place within the “Cycles” research team.
Scientific, material, and financial conditions of the research project:
This PhD is part of an ANR JCJC project called SWAN, led by Blandine Gardonio. The PhD position is fully funded and includes additional financial support to cover participation in international conferences and publication fees.
International environment:
Blandine Gardonio and her collaborators benefit from the exceptional diversity of expertise available within the ISTerre laboratory. The project also relies on collaboration with Japanese seismologists specializing in subduction zones: Naoki Uchida, Associate Professor at Tohoku University, and Seiko Kita, Researcher at the Building Research Institute (Japan).
Additional information:
Applicants must hold a Master's degree (or equivalent) in geophysics or physics and demonstrate a strong interest in geophysical signal processing and statistics.
Disciplinary profile: strong skills in mathematics, physics, and signal processing; autonomy in coding, particularly in Python and Matlab; ability to handle and process data.
Transversal skills: initiative, adaptability, ability to work both independently and as part of a team; good communication and teamwork skills; proficiency in English (reading, writing, and speaking).

Constraints and risks

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Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

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