Postdoc position in seismology at Volcanological and Seismological Observatoriy of Martinique January 23 2014
Institution and location: IPGP, Volcanological and Seismological Observatoriy of Martinique, F.W.I.
Duration: 1 year, position available immediately.
Subject: Stresses and mechanics of the Caribbean subduction from seismic analysis.
The subduction of the American plate under the Caribbean plate in the Lesser Antilles, with a rate of 2cm/an, creates a deformation zone of 100 to 200 km wide underlined by shallow seismicity from the trench to the Leeward Islands, where the volcanic chain is located. Deep seismicity within the slab is observed down to 200 km. Major thrusting earthquakes potentially tsunamigenic (M>8) have occurred in the past centuries. Owing to this geodynamic and tectonic context, the eastern Caribbean arc is an area of major scientific interest.
The French Volcanological and Seismological Observatories (OVS), depending of the Institut de Physique du Globe de Paris (IPGP), are responsible for Martinique and Guadeloupe seismic monitoring. In an effort to establish a tsunami detection network for the Caribbean region and participate to the implementation of the Caribbean Tsunami Warning System, the OVS have recently undertaken a major evolution in the geometry of their networks, which have evolved from networks focusing on each French island to a virtual network deployed all along the arc (i.e. spatial extension ≈ 600 km), in particular thanks to the Tsuareg project, co-fund by Interreg (European). Through collaborative projects between the different actors of the monitoring, a catalogue of seismicity between 1972 and 2012 at the scale of the subduction has been recently produced. It is the most complete (more than 15 000 events) and most reliable document on the zero-order characteristics (location, duration magnitude) of the arc seismicity ranging from the Virgin Islands to Trinidad and Tobago. However, in order to progress in our understanding of the present dynamics of the Eastern Caribbean subduction, a better view of the seismicity is required: relocations with double differences techniques, seismic moment, source dimensions (corner frequencies), focal mechanisms.
The objective of the work is to contribute to this effort with the determination of the focal mechanisms for magnitudes as low as possible. Several techniques might be used, depending on the magnitude and distance range of the event. For the smallest events, P polarities and S to P amplitude ratios can be used, as in HASH (USGS) method, and for slightly larger events (above magnitude 3.5-4), the complete waveform can be modelled and inverted (e.g Mercier de Lépinay et al., 2011, and Dreger et al., 2000). Both methods may be used in parallel for the same event. Note that since 2004 (beginning of the broad band records), 276 events with M>4, 106 with M>4.5, and 24 with M>5 occurred. For magnitudes above 4.5 to 5, teleseismic data can be used, with P polarities or even amplitudes, and for magnitudes above 5.5, the waveforms themselves can be analysed, as in the SCARDEC method (Vallée et al., 2011). Another methodology, which might be useful, would be the inversion of the stress tensor in a given region, (e.g. Rivera and Cisternas, 1991) using the recorded P polarities.
The results will be interpreted in the various mechanical contexts of the region: interplate thrust, normal faulting and strike-slip within the Caribbean plate, slab-pull and slab-push events in the subducting slab. In particular, this should better document the partitioning between normal and strike slip deformation, when going to the north of the arc. Also, this will be of particular interest to understand the interaction between subducted ridges and the Caribbean plate or the generation of tsunami.
The work will be fund by the Tsuareg project. It will be based in Martinique, directed at OVSM by Valérie Clouard, in collaboration with Frédérick Massin at OVSG and the seismology team of IPGP (Marie-Paule Bouin, Martin Vallée, Alex Nercessian, Pascal Bernard, and others). Fundings include travel expenses for one conference and for a 2-week workshop at IPGP. This work should lead to one scientific paper, to be published in an international journal.
- PhD in geophysics, and expertise in seismology requested
- Ability to work with local and remote teams
- Knowledge in programming appreciated whatever the language
- Experience in seismological data formats
Application including must be sent to Valerie Clouard (email@example.com), including a cover letter, a CV and a list of publications.