The Effect of Recent Volcanic Activity on the Seismic Structure of Madagascar

Wednesday, 17 December 2014: 11:20 AM
Michael Edward Wysession1, Ghassan I Aleqabi2, Martin J Pratt2, Patrick Shore3, Douglas A Wiens2, Andrew Nyblade4, Gérard Rambolamanana5, Fenitra Sy Tanjona Andriampenomanana Ny Ony5 and Rakotondraibe Tsiriandrimanana5, (1)Washington Univ, Saint Louis, MO, United States, (2)Washington University in St Louis, Department of Earth and Planetary Sciences, St. Louis, MO, United States, (3)Washington University in St Louis, St. Louis, MO, United States, (4)Penn St Univ, University Park, PA, United States, (5)Institute and Observatory of Geophysics Antananarivo-University of Antananarivo, Physics, Antananarivo, Madagascar
The seismic structure of Madagascar is determined using ambient-noise and two-plane-wave earthquake surface waves analyses. A deep low-velocity anomaly is seen in regions of recent volcanic activity in the central and northern regions of the island. The primary data used are from the 2011-2013 MACOMO (Madagascar, the Comoros, and Mozambique) broadband seismic array from the PASSCAL program of IRIS (Incorporated Research Institutions for Seismology), funded by the NSF. Additional data came from the RHUM-RUM project (led by G. Barruol and K. Sigloch), the Madagascar Seismic Profile (led by F. Tilmann), and the GSN. For the ambient-noise study, Rayleigh wave green’s functions for all interstation paths are extracted from the broadband seismic data recorded from August 2011 until October 2013. Rayleigh wave group and phase velocity dispersion curves are extracted in the 8 – 50 s period range, identifying shallow crustal structure. For deeper structure, the two-plane-wave method is used on teleseismic earthquake data to obtain surface wave phase velocities in the 20 – 182 s period range. In the inversion, a finite-frequency kernel is used for each period, and a 1-D shear velocity structure is determined at each location. A three-dimensional S-wave velocity model of the crust and upper mantle is obtained from assembling the 1-D models. Preliminary results show a good correlation between the Rayleigh wave velocities and the geology of Madagascar, which includes areas of ancient Archaean craton. The slowest seismic velocities are associated with known volcanic regions in both the central and northern regions, which have experienced volcanic activity within the past million years.