Marina Corradini and Manon Bickert winners of the 2018 AGU Outstanding Student Presentation Award
Publication date: 22/02/2019
Awards and Distinctions, Institute Life, Press, Research
Related teams :
Marine Geosciences
The OSPA (Outstanding Student Presentation Award) competition, organised at the AGU (American Geophysical Union) autumn conference, gives doctoral students the opportunity to present their work in a poster or oral presentation. Only 5% of participating students are awarded a prize each year.
Two IPGP students won this prestigious prize at the 2018 conference in Washington D.C.: Manon Bickert, working in the marine geosciences team, and Marina Corradini, from the seismology team. The award was all the more significant this year as the AGU celebrated its 100th anniversary.
Summary of Marina’s work
High-frequency seismic radiation and the complexity of seismic rupture as seen by the backpropagation technique
High-frequency (HF) seismic radiation is associated with abrupt changes in rupture velocity and slip rate during the seismic rupture process
seismic rupture process (Madariaga, 1983). Many studies have attempted to shed light on the rupture heterogeneities of large earthquakes through the use of coherent imaging techniques such as back-projection (BP) (Satriano et al., 2014, Lay et al.,2012).
Recently, Fukhata et al (2014) showed that, from a theoretical point of view, the BP image of a seismic rupture is related to variations in the amount of slip on the fault. However, the quantitative interpretation of BP images in the light of physical parameters and properties of the rupture process still remains difficult. In this work, we seek to clarify the influence of spatial heterogeneities on rupture velocity and slip rate, using BP techniques. Our results show that the HF emissions extracted from the BP analysis are in fact associated with spatio-temporal variations in slip acceleration.
Summary of Manon’s work
How can detachment faults be created on mid-ocean ridges with a low melting rate and very thick axial lithosphere?
Oceanic accretion at the axis of slow ridges is characterised by the exhumation of mantle material by high-replay normal faults, known as detachment faults. Successive detachments with opposite vergence (flip-flop) have been observed along amagmatic corridors in the eastern part of the South-West Indian Ridge (Sauter et al., 2013). The depth of the microearthquake foci recorded in this zone implies the existence of a brittle lithosphere around twenty kilometres thick (Schlindwein and Schmid, Nature, 2016).
This hypothesis is consistent with petrological observations made on serpentinised peridotites dredged along these corridors, revealing heterogeneous deformation at high stress and high temperature in the spinel stability domain (absence of plagioclase, dynamic recrystallisation of primary minerals including spinel in micro shears, deviatoric stresses > 200 MPa estimated by piezometry on recrystallised olivines).
These observations contradict the thermomechanical models of Lavier and Buck (2002), which predict the development of flip-flop detachments for a brittle lithosphere thickness of less than 15 km. These models use cohesion loss as the main weakening mechanism. In this presentation we explore two additional weakening mechanisms observed in the samples: serpentinisation (T<350°C) and grain size reduction (T ~ 800-1000°C). These two mechanisms are initially studied separately: serpentinisation alone does not allow the formation of successive detachments with opposite vergence, whereas grain size reduction does, albeit with unrealistic reliefs. The combination of these two weakening mechanisms in a model leads to the formation of four different accretion modes: Horst, spider, long detachment and flip-flop detachments. The transition between these different modes depends on the proportion of serpentine formed along these detachments. The flip-flop mode, once initiated, develops successive detachments at opposite vergence in a stable manner, with a relief and duration of fault activity consistent with those of the amagmatic corridors of the eastern part of the South-West Indian Ridge.
