UC Santa Barbara

Several aspects of the oceanic lithospheric mantle remain unknown, largely due to the lack of dense local seismic instrumentation. In particular, we do not understand the nature of lithosphere-asthenosphere boundary, sparsely observed mid-lithospheric velocity gradients, and dynamic processes below the plate that may play an essential role in controlling Earth surface evolution. The Pacific OBS Research into Convecting Asthenosphere (Pacific ORCA) experiment included a ~1 year deployment of an 500x500 km^2 OBS array consisting of 30 stations in the central Pacific Ocean on ~40Ma seafloor northeast of the Marquesas Islands. Sub-lithospheric small scale convection (SSC) is a promising theory accounting for the observed gravity lineations at this area, although it has not been seismically imaged. Here we present results from the first year of ORCA data. With teleseismic events (30-{95}^\circ), We measured differential travel times for P-waves from both vertical and pressure components using multi-channel cross-correlation. We combined these data and tomographically inverted for 3-D P-wave velocity structure. We find the alternating velocity anomaly bands oriented in WNW-ESE direction, which is nearly parallel to the gravity lineations, offering potential support for the presence of SSC. The depth range of that anomaly is ~150-250km based on a squeezing test. We also used events over Mw6.0 to calculate P-s receiver functions (RF). After conversion multiple/water phase removal using a FIR filter, the RFs were migrated to image deep discontinuities using a CCP stack. The result revealed ~50-60km deep negative velocity gradient (NVG) which still needs to be tested by data from the latest deployment.