Seismic velocities and thermal structure in uppermost mantle across the Mendocino Fracture Zone, Pacific ocean

Cooling of the oceanic lithosphere over time increases the lithospheric thickness and seismic velocities. Compressional waves propagating in uppermost oceanic mantle (Pn waves) generally travel at velocities between 8.0-8.2 km/s. However, there are Pn velocity variations in different ocean basins, perhaps due to different spreading rates. Using hydrophone records, previous authors have shown estimations of Pn velocities ranging 7.5-8.0 km/s in the Pacific, and an average from 7.8 to 8.5 km/s in the cold upper mantle of the equatorial Atlantic ocean. Therefore, the study of variations in seismic wave velocities is crucially important to understanding the effect of spreading rate and crustal age on the velocity structure of the lithosphere in different ocean basins. Here, we analyze estimates of Pn wave velocities, teleseismic P waves delays, and thermal structure models of the oceanic lithosphere beneath Mendocino Fracture Zone (MFZ). We used records of 16 regional earthquakes identified with a set of 20 sea-bottom instruments deployed in a 200 km long N-S array across the MFZ having an age offset of 26 Ma. We also use cross-correlation to analyze the variations in P wave arrival times from 20 teleseismic earthquakes (Mw>7) recorded by sea-bottom instruments of the USArray. Preliminary results obtained show Pn velocity with variations ±0.3 km/s between the lithosphere in basins on the two sides of MDF. We observe a partial concordance of the Pn velocities with lithospheric age, as also with the modeled uppermost mantle temperatures in MFZ. The north/young lithosphere has an average Pn velocity ∼8.08 km/s. In comparison, higher velocities were identified in south/older lithosphere with average of ∼8.41 km/s. These velocities are lower than previous results in the equatorial Atlantic. We also observed that Pn velocities show azimuthal variations, which should be a derivation of the seismic anisotropy in the uppermost mantle beneath MFZ. First results in teleseismic P waves travel times shows us delays since 0.17 to 0.5 s for the two sides of MFZ. Therefore, these results in MFZ indicates that younger/older sides of the fracture zones have a complex geodynamical mechanism beneath the lithosphere, in which the behavior of the seismic wave propagation can vary depending on the settings in uppermost mantle of each ocean basin.