Abstract
SUMMARYWe use seismic noise cross-correlations to obtain a 3-D tomography model of SV-wave velocities beneath the western Indian Ocean, in the depth range of the oceanic crust and uppermost mantle. The study area covers 2000 × 2000 km2 between Madagascar and the three spreading ridges of the Indian Ocean, centred on the volcanic hotspot of La Réunion. We use seismograms from 38 ocean bottom seismometers (OBSs) deployed by the RHUM-RUM project and 10 island stations on La Réunion, Madagascar, Mauritius, Rodrigues, and Tromelin. Phase cross-correlations are calculated for 1119 OBS-to-OBS, land-to-OBS, and land-to-land station pairs, and a phase-weighted stacking algorithm yields robust group velocity measurements in the period range of 3–50 s. We demonstrate that OBS correlations across large interstation distances of >2000 km are of sufficiently high quality for large-scale tomography of ocean basins. Many OBSs yielded similarly good group velocity measurements as land stations. Besides Rayleigh waves, the noise correlations contain a low-velocity wave type propagating at 0.8–1.5 km s−1 over distances exceeding 1000 km, presumably Scholte waves travelling through seafloor sediments. The 100 highest-quality group velocity curves are selected for tomographic inversion at crustal and lithospheric depths. The inversion is executed jointly with a data set of longer-period, Rayleigh-wave phase and group velocity measurements from earthquakes, which had previously yielded a 3-D model of Indian Ocean lithosphere and asthenosphere. Robust resolution tests and plausible structural findings in the upper 30 km validate the use of noise-derived OBS correlations for adding crustal structure to earthquake-derived tomography of the oceanic mantle. Relative to crustal reference model CRUST1.0, our new shear-velocity model tends to enhance both slow and fast anomalies. It reveals slow anomalies at 20 km depth beneath La Réunion, Mauritius, Rodrigues Ridge, Madagascar Rise, and beneath the Central Indian spreading ridge. These structures can clearly be associated with increased crustal thickness and/or volcanic activity. Locally thickened crust beneath La Réunion and Mauritius is probably related to magmatic underplating by the hotspot. In addition, these islands are characterized by a thickened lithosphere that may reflect the depleted, dehydrated mantle regions from which the crustal melts where sourced. Our tomography model is available as electronic supplement.
Highlights
Cross-correlations of ambient seismic noise have become widely used to constrain crustal structure
Subsurface structure is constrained by Western Indian Ocean noise tomography 925 group velocity measurements, which are derived from the PHASE CROSS-CORRELATION (PCC)
MAID, RUM1 and TROM were part of the temporary RHUM-RUM network 2011–2015 (Barruol et al 2017); LAHA and ANLA were part of the temporary MACOMO experiment on Madagascar (Wysession et al 2011); FOMA, RER and ROCAM are permanent GEOSCOPE stations (IPGP & EOST 1982); PRO is a permanent station of Observatoire Volcanologique du Piton de la Fournaise (OVPF); and MRIV is a permanent station of the Mauritius Seismic Network
Summary
Cross-correlations of ambient seismic noise have become widely used to constrain crustal structure. Nishida et al 2009; Haned et al 2016) The majority of these studies used cross-correlations between land stations to investigate continental crust and/or mantle. Mazzullo et al (2017) used earthquake data recorded by the RHUM-RUM stations to retrieve Rayleigh-wave phase velocities for periods of 30–300 s and group velocities for 16–250 s. The scope of our study is to tomographically image the crust and lithosphere beneath the RHUM-RUM deployment, using crosscorrelations of ambient seismic noise recorded by RHUM-RUM OBSs and island stations. Subsurface structure is constrained by Western Indian Ocean noise tomography 925 group velocity measurements, which are derived from the PCCs. our group velocity values are inverted jointly with the earthquake-generated Rayleigh-wave data previously measured and inverted by Mazzullo et al (2017).
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