Upper-mantle shear velocity structure beneath the equatorial East Pacific Rise from array-based teleseismic surface wave dispersion analysis

The Discovery/Gofar transform fault system is associated with a fast-spreading centre on the equatorial East Pacific Rise. Most of the previous studies focus on its regular seismic cycle and crustal fault zone structure, but the characteristics of the upper-mantle structure beneath this mid-ocean ridge system are not well known. Here we invert upper-mantle shear velocity structure in this region using both teleseismic surface waves and ambient seismic noise from 24 ocean bottom seismometers (OBSs) deployed in this region in 2008. We develop an array analysis method with multidimensional stacking and tracing to determine the average fundamental mode Rayleigh-wave phase-velocity dispersion curve (period band 20-100 s) for 94 teleseismic events distributed along the E-W array direction. Then, we combine with the previously measured Rayleigh-wave phase-velocity dispersion (period band 2-25 s) from ambient seismic noise to obtain the average fundamental mode (period band 2-100 s) and the first higher mode (period band 3-7 s) Rayleigh-wave phase-velocity dispersion. The average dispersion data are inverted for the 1-D average shear wave velocity (Vs) structure from crust to 200-km depth in the upper mantle beneath our study region. The average Vs between the Moho and the 200-km depth of the final model is about 4.18 km s(-1). There exists an similar to 5-km thickness high-velocity lid (LID) beneath the Moho with the maximum Vs of 4.37 km s(-1). Below the LID, the Vs of a pronounced low-velocity zone (LVZ) in the uppermost mantle (15-60-km depth) is 4.03-4.23 km s(-1) (similar to 10 per cent lower than the global average). This pronounced LVZ is thinner and shallower than the LVZs beneath other oceanic areas with older lithospheric ages. We infer that partial melting (0.5-5 per cent) mainly occurs in the shallow upper-mantle zone beneath this young (0-2 Myr) oceanic region. In the deeper portion (60-200-km depth), the Vs of a weak LVZ is 4.15-4.27 km s(-1) (similar to 5 per cent lower than the global average). Furthermore, the inferred lithosphere-asthenosphere boundary with similar to 15-km thickness can fit well with the conductive cooling model. These results are useful for understanding the depth distribution and melting characteristics of the upper-mantle lithosphere and asthenosphere in this active ridge-transform fault region.