The Upper Mantle Velocity Structure Beneath the Sea of Okhotsk

We use Rayleigh-wave tomography to explore the velocity structure of the upper mantle beneath the Sea of Okhotsk. The Sea of Okhotsk is a complex geologic region, frequently defined as an independent plate, with large variations in crustal structure and an unresolved tectonic history. Upper mantle tomography provides constraints on the tectonic evolution of a region by imaging fast and slow velocity anomalies associated with structures that may result from recent tectonic activity (i.e. remnant slabs). Using long-period waveforms recorded by IRIS, Michigan State University, and the Northeast Interdisciplinary Scientific Research Institute, we find the best-fitting multi-mode synthetics along a variety of regional source-receiver paths. The multi-mode synthetics produce path-averaged velocity profiles that act as model parameter constraints when the paths are combined and inverted to produce a 3-D tomographic model. Our model shows high and low-velocity anomalies, primarily associated with the modern slab and mantle wedge. Both anomalies vary vertically and laterally as the angle of the Wadati-Benioff zone changes. In general, the depth to the Moho discontinuity in our model agrees well with the interpretation of refraction and reflection surveys in this region (e.g. Pavlenkova, 1996). Thicker crustal structures and a deeper Moho boundary are present in the shallow central and northeastern portions of the Okhotsk Sea, while the Moho is shallower in the Kuril Basin. Measurements of residual travel times for multiple ScS waves agree with the velocity structure of the 3-D model. We find positive (slow) residuals in the central portion of the Okhotsk Sea, and negative (fast) residuals in regions where the modern slab is present. The most negative (fastest) ScS residual is located north of Sakhalin Island. As revealed by the 3-D model, this fast residual appears to be the result of a shallower Moho depth and a high-velocity zone extending down from approximately 150 km depth. This high velocity anomaly corresponds with similar anomalies interpreted as remnant slabs from earlier subduction of the Okhotsk plate west of the modern subduction zone (Gorbatov et al., 2000). A high spectral ratio measurement of Q_ScS further supports the possible existence of a remnant slab in the upper mantle of this region.