Surface-wave tomography of the South-East Asia by joint inversion of Rayleigh and Love phase velocities from seismic ambient noise and teleseismic earthquakes

South-East Asia hosts the largest and most complicated subduction system of our planet, associated with extensive volcanism and seismicity. Obtaining high-resolution seismic images of South-East Asia can provide important constraints on the lateral variations of physical parameters such as density, composition, temperature, and viscosity of this dynamic patchwork. In turn, this has relevant implications on our ability to forecast its geodynamic evolution by numerical modeling. In this study, we join all the publicly-available seismic data distributed across the Malay Peninsula, Sumatra, Java, Sulawesi, South Borneo, and North Australia (amounting of 468 broad-band seismic receivers) with the continuous seismograms from 70 receivers recently installed in North Borneo, resulting in an unprecedented seismic coverage of the region.
We first use such data to extract Rayleigh and Love phase velocities based on both seismic ambient noise and teleseismic earthquakes. Overall, we retrieve 14,036 Rayleigh- and 12,005 Love-wave dispersion curves, covering surface-wave periods between 3 and 150 s and sensitive to both the shallow crust and the upper mantle. We then invert the dispersion curves for phase-velocity maps at different periods, using a linearized-inversion algorithm based on the ray theory with a roughness damping constraint. In doing so, we adopt an adaptive parameterization, allowing for a finer resolution of the resulting maps in the areas characterized by a relatively high density of measurements. At relatively short periods (<20 s), the phase-velocity maps are characterized by strong lateral heterogeneities. We find, for example, relatively low velocities in correspondence of the Central- and South-Sumatra Basin, ascribed to thick sedimentary layers, and higher velocities in the (adjacent) Barisan Mountains. Low velocities also characterize a large region approximately centered onto the Merapi volcano (Central Java), the Mentawai islands (in correspondence of the Mentawai Fault System), the Sahul Shelf (including the East Timor island), and the marine region between east Borneo and Sulawesi. Relatively high velocities are found below the Banda Sea. The amplitude of such lateral variations quickly decreases at larger periods and, among the most pronounced features, we observe relatively low velocities in the north-east of Borneo (as opposed to its south-western part), and high velocities in the Celebes Sea (north of the North-Sulawesi Trench).
At the time of writing, we are planning to translate the phase-velocity maps thus retrieved into shear-wave velocity (Vs) as a function of depth. Specifically, we plan to extract one Rayleigh- and one Love-wave phase-velocity profile for each grid cell constituting our phase-velocity maps, and (non-linearly) jointly invert them for Vs using the neighbourhood algorithm. The resulting 3-D tomographic model will thus be interpreted in light of the existing literature on the study area, involving (but not limited to) geodynamic and geologic models, geophysical, geochemical, and geodetic observations.