[S-IT16] Deep Earth Sciences

[SIT16-01] Seismic velocity structure beneath South America from finite-frequency tomography

*Yuko Kondo¹, Masayuki Obayashi², Hiroko Sugioka¹, Hajime Shiobara³, Aki Ito², Masanao Shinohara³, Hikaru Iwamori³, Masataka Kinoshita³, Miller Matthew⁴, Tassara Carlos⁵, Ojeda Javier^5,6 (1.Kobe University, Japan, 2.Japan Agency for Marine-Earth Science and Technology, Japan, 3.Earthquake Research Institute, University of Tokyo, Japan, 4.Departamento de Geofísica, Universidad de Concepcion, Chile, 5.Facultad de Ciencias, Universidad de Arturo Prat, Iquique, Chile, 6.Departamento de Geofísica, Universidad de Chile, Chile)

We present a new tomographic image beneath South America using finite frequency P-wave traveltime tomography. Combining the global traveltime data set and the regional traveltime data, which are measured from the broadband ocean-bottom seismic stations around the Chile Triple Junction (CTJ), we conducted a tomographic inversion to estimate the 3D seismic structure. In our tomography method, we employed ray-theoretical kernels (Inoue et al., 1990) for the onset times and ray-theoretical finite frequency kernels (Hung et al., 2000; Nolet et al., 2000) for regional differential travel times measured by cross-correlation. For tomographic inversion, we employed a conjugate-gradient method with a first-order smoothness constraint to obtain the 3-D P velocity perturbation with respect to an average one-dimensional structure.
The new tomographic image highlights the continuous Nazca slab geometry and broad extension of the fast anomaly beneath the slab at around 26-35°S. In addition, a prominent slow anomaly is observed to the east of the CTJ in the upper mantle. Careful resolution tests and synthetic recovery tests confirmed the robustness of these features. The fast anomaly beneath the Nazca slab (F1 in Figure 1) has a complex geometry that is highly variable from north to south and extends over a broad area from 200-900 km in depth. Based on the strong anomaly amplitude and spatial coincidence with the current Pampean flat slab segment and the past Payenia flat-slab segment, this was interpreted as a relic Nazca slab segment. The remarkable slow anomaly in the vicinity of the CTJ (S1 in Figure 1) is notably consistent with the previously inferred extent of the shallow Patagonian slab window from kinematic reconstruction and body wave seismic imaging. It was also coincident with the occurrence of active adakitic volcanoes and the Plateau basalts. Considering the extent of this anomaly, we propose that the root of upwelling, associated with the slab window, is at around 250 km.