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Variations in Pn seismic velocities and lithospheric thermal structure across Mendocino fracture zone, eastern Pacific ocean


Oceanic lithosphere varies considerably around the globe, with different geodynamic settings that contribute to various models of seismotectonic activity over the tectonic plate boundaries. Lithospheric age, temperature, and thickness are three fundamental features to understand the geodynamic behavior. However, the model in which they influence the seismic wave propagation is poorly understood over different oceanic areas. Here, we analyze the behavior of seismic velocities and lithospheric thickness from measurements of uppermost mantle-Moho Pn velocities and thermal structure modeling over the Mendocino fracture zone (MFZ), eastern Pacific ocean. We use data obtained by 21 ocean-bottom instruments deployed across the MFZ in October-December of 2017. We present results obtained from 19 regional earthquakes of global catalogs with magnitudes $\geq$2.5, with their epicenters raging from 311 to 588 km away. We obtained 205 Pn ray paths with Pn velocities ranging from 8.01$\pm$0.1 to 8.48$\pm$0.2 km/s. The results presented correlations between the increase in Pn velocities with the lithosphere aging and the velocity decrease in parallel to the increase of temperatures. Also, the highest velocities are directed near parallel to the fracture zone and perpendicular to the ridge axis. The lithospheric thermal model show that the down limitation bounding with the asthenosphere is placed at $\sim$29-74 km depth comparing the young (8 Ma) and older (33 Ma) sides of the fracture zone. Therefore, our results demonstrate the MFZ particular area is fundamental to the understanding of seismic wave propagation over the uppermost mantle-Moho zone and the lithosphere thickness dependence over geodynamic features.

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