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Adjoint Tomography of South America and Geometry of the Nazca Slab

Session: Subduction Processes Along Latin America Subduction Zones II

Type: Oral

Date: 4/19/2021

Presentation Time: 02:15 PM Pacific

Description: 

We use full-waveform adjoint tomography with 3D spectral-element and continental-scale seismic wave simulations (Komatitsch & Tromp, 2002). 112 earthquakes recorded by 1311 stations were used. We detect and remove noisy and problematic data using our multi-stage algorithm before the time-window selection, reducing the likelihood of discarding useful data or assimilating bad-quality waveforms. Our misfit function is a complex-exponentiated instantaneous phase, which optimizes the information from each time series without the need for short time windows. We fine-tuned our window-selection algorithm to select long time windows as much as the data quality permits. We use a preconditioner based on the pseudo-Hessian kernel and weigh our misfit function to balance the source-receiver distribution for faster convergence. We performed 23 iterations, gradually increasing the frequency content of the data to avoid local minima. Our final model (SAAM23) shows a significant decrease in the misfit with periods down to 17s. The robustness of the model is confirmed by the misfit reduction of 53 independent earthquakes, not included in the inversion. At long wavelengths, SAAM23 is compatible with previous models, such SA2019 (Celli et al.,2020), and GLADM25 (Lei et al.,2020). SAAM23 consistently maps the thick, fast lithospheric velocities of the Amazon craton, the São Francisco craton, and the Paranapanema block, as well as the Andean subduction.

The Nazca slab is well imaged down to the transition zone. A continuous slab is seen at 300-400 km depth after the Peruvian flat segment, where a gap had been suggested previously. Beneath the Amazon, the slab crosses the transition zone and plunges directly into the lower mantle. In the South, the slab stagnates near the 660 km discontinuity for longer distances. A vertical, cylindrical low-velocity zone from 1100 to 200 km depth is confirmed, perhaps due to mantle upwards flow from beneath the slab or water from slab dehydration (Rodríguez et al., 2021).

Presenting Author: Caio Ciardelli

Student Presenter: Yes

Authors