Ambient noise tomography of the Tainan deformation front in southwestern Taiwan

The Tainan deformation front is an anticline located at the Chinese continental shelf-slope break in the transition from subduction to collision in southwestern Taiwan. Several NE-SW trending mud diapirs and mud volcanoes have been identified by marine geophysical survey offshore southwest of Tainan, where one submarine-diapirs is spatially closely linked to the onshore Tainan anticline. In contrast, distinct fold-and-thrust belts occur east of the Tainan anticline, where thrust faulting on the detachment folds is often considered the dominant mechanism for the Tainan anticline. Whether a frontal thrust or diapirism formed the Tainan anticline relates to the earthquake potential of the ~1.8 million populated Tainan city. We selected 2-3 months-long continuous seismic recordings of 33 seismic stations covering a 15 by 10 km² area in the Tainan deformation front of southwestern Taiwan from February to June 2021 to conduct ambient noise tomography. With an inter-station distance of about 1 km, the seismic array spanned from west to east across four major tectonic regimes: Anping Plain, Tainan Tableland, Dawan Lowland, and Chungchou Tableland. Two to three months of ambient noise were cross-correlated between each station pair, and clear fundamental-mode Rayleigh waves were observed between 1 and 3 seconds. For the Eikonal tomography analysis, five BATS (Broadband Array in Taiwan for Seismology) stations and six CWASN (Central Weather Administration Seismographic Network) stations were chosen at distances ranging from 40 to 80 kilometers away from the Tainan array, with azimuths between 45° to 140° and 270° to 360°. We then calculated the cross-correlation function (CCF) between 33 seismometers and these stations, and we measured the relative surface wave arrival times using the beamforming method. To perform Eikonal tomography, we calculated the surface wave propagation of the ambient noise and the shallow velocity structure for each period between 4 and 9 seconds. The ambient noise tomography and the Eikonal tomography results were combined to construct the three-dimensional shear wave velocity model of the Tainan anticline. Our result shows that the velocity at 1-3 seconds under the study area is almost uniform, while the velocity at 4-9 seconds shows up to +18% velocity perturbations beneath the Tainan anticline. This suggests that there are no significant lateral variations in lithology at a shallow level (1-3 seconds) and a high-density body, the mud-diapir extruded upward beneath the Tainan anticline at a deeper level (4-9 seconds).