Investigating Northern Ninety East Ridge (NER) using high-resolution Multichannel Seismic Reflection Data

The Ninety East Ridge (NER) is a prominent linear intraplate volcanic ridge in the eastern Indian Ocean. Seismicity has long been recognized to occur along it in the context of the Indian Ocean intraplate deformation zone. At the latitude of northern Sumatra, the NER hosted moment release during the great Mw 8.6 Wharton Basin earthquake rupture, and several aftershocks with dominantly strike-slip mechanisms. However, its crustal architecture in that area, especially the extent of active faults at depth, is poorly known owing to a scarcity of active-source seismic data. In this study, we utilize a two-dimensional marine multichannel seismic (MCS) dataset acquired by the R/V Marion Dufresne (MIRAGE experiment) in the region of the 2012 rupture zone over northern NER (1.5-3N). An airgun source of 2750 in³ volume was fired at 50 m interval, and a streamer equipped with 720 hydrophone groups spaced at 6.25 m interval was used to record seismic data. We processed the MCS data using a conventional marine seismic processing workflow using successively a band-pass filter, an FK filter for coherent linear noise attenuation, several passes of velocity analyses, normal move-out correction, stacking, and finally post-stack time migration. We focus here on a 176-km long profile oriented N-S direction, along which the water depth varies between ~ 2 km in the north and 3 km in the south. The preliminary interpretation of the migrated image reveals several structural features indicating active deformation and segmentation along the profile. We find a veneer (200-300 m thick) of pelagic sediments underlain by 300-500 m thick volcanoclastic deposits over the acoustic basement. In the crust below the top of basement, we observed a low-frequency event, which could be due to the Layer 2A/2B boundary. The seafloor and sediments show signs of active deformation, possibly associated with NW-SE trending fault planes of strike-slip earthquakes. The presence of a couple of negative flower structures further supports strike-slip deformation on the NER. In this presentation, we will present results linking seismic reflection images with drilling results (DSDP Hole 216) and earthquakes on the NER to shed light on active deformation along the NER in this region.