3D Shallow crustal structure of Madeira island revealed from ambient noise tomography

Madeira is an intraplate volcanic island, located at the eastern North Atlantic Ocean, in front of the Moroccan cost, with an emerged area of 737 km2 and maximum altitude of 1861 m. Madeira shows an E-W-oriented elongation, which reflects the orientation of its rift zone. Rift zones play a fundamental role in the constitution and evolution of volcanic islands and it is important to image their internal structure as a function of depth. Constrains like source-receiver geometry, irregular seismicity distribution or, for some methods, low seismicity occurrence did not allow to obtain high-resolution models of the Madeira crustal structure using traditional passive seismology. Seismic interferometry/ambient noise surface-waves tomography allows imaging regions with a resolution that mainly depends on the seismic network coverage. From May 2011 to September 2012, a temporary pool of 23 seismometers has been continuously recording at Madeira Island. This deployment was complemented with other local permanent stations. The ambient noise data was processed following five main steps: (1) Data quality control; (2) Cross-correlation of 1 hour time windows between each station pair and subsequent stacking for the entire recording period; (3) Time-frequency analysis to measure group-velocity dispersion curves between 0.5 and 6 seconds; (4) 2D inversion to obtain lateral variations of the Rayleigh-wave group-velocities as function of the period; (5) Group velocity inversion as a function of depth to map the 3D structure beneath Madeira. From the surface to 4 km depth, the edge of the rift, along which the island possibly grow, is well correlated with a strong positive anomaly on our maps. This anomaly seems to be perturbed by the presence of low velocities at a depth of 2 km. After 5 km the rift signature is no longer visible. This work is supported by project QUAKELOC Reference: PTDC/GEO-FIQ/3522/2012