2D long-streamer seismic waveform tomography imaging: contribution to the study of the Sanak forearc Basin, Alaska Subduction Zone

En salle Froidevaux - E314, 24 rue Lhomond et online : https://global.gotomeeting.com/join/710993933 Soutenance de Thèse d'Amin Kahrizi (LGENS) "2D long-streamer seismic waveform tomography imaging: contribution to the study of the Sanak forearc Basin, Alaska Subduction Zone" Abstract: This thesis discusses the application of full waveform inversion (FWI) to two 45-km-long 2D seismic sections crossing the Sanak forearc basin in Alaska, where evaluating the recent activity of a previously imaged deep landward-dipping normal fault is crucial to understand the creeping Shumagin segment of the subduction zone. Conventional processing shows that the extension in part of the basin is mainly localized on seaward-dipping faults rather than landward-dipping fault. We also show that bottom currents affect the sedimentary record of the faults activity. As a result the nature and timing of the extension is unclear in the basin. After streamer traveltime tomography, FWI is first tried in frequency domain but muting of shelf guided waves led to time-domain FWI imaging. Variable water velocity in the water column also had to be dealt with. The final high resolution velocity models confirm the effect of bottom currents in one of the profile. FWI also image a velocity inversion from a bottom simulating reflector (BSR), explaining previously unexpected amplitude variations that were troublesome for seismic interpretation. Low velocity anomalies above several well defined basement highs are imaged that could indicate fluid escapes, but there are no major velocity discontinuities along the faults. All our results are then compiled with legacy data in order to redraw a structural map of the forearc around the Shumagin segment. We suggest that the oblique Central Sanak basin is a pull-apart basin resulting from the onset of the partitioning of the Aleutian oblique convergence. In this new context, it is possible that recent activity on landward-dipping sedimentary faults is not necessarily related to motion on the associated deep reflector and we conclude on the usefulness of FWI images to locate sampling of fluids in future investigations. Jury: Sylvie LEROY (ISTeP-Sorbonne Université) : Rapportrice / Alessandra RIBODETTI (Geoazur-Université de Nice) : Rapportrice / Hélène CARTON (Geosciences Marines IPGP) : Examinatrice / Jacques DEVERCHERE (IUEM-Université de Bretagne Occidentale) : Examinateur / Javier ESCARTIN (LG-École Normale Supérieure) : Examinateur / Matthias DELESCLUSE (LG-École Normale Supérieure) : Directeur / Manuel PUBELLIER (LG-École Normale Supérieure) : Invité / Anne BÉCEL (LDEO-Columbia University) : Invitée