16:00 〜 16:15
[SSS03-08] Lithosphere and Asthenosphere Seismic Structure Beneath the Hawaiian Swell from Rayleigh Wave Dispersion
キーワード：surface wave, shear velocity structure , Hawaii swell, Lithosphere and asthenosphere, PLUME OBS project
In this study, we re-examine the shear velocity structure in the vicinity of the Hawaiian hotspot and Hawaiian swell using Rayleigh wave data from the PLUME project in the 20-125 s period range. We improve resolution and stability of the tomographic imaging by: removing tilt and compliance noise from the vertical component; using a higher sampling rate in the time series; equalizing resolution of phase velocity across all periods; beginning with an a priori crustal model based on receiver function analysis and topography; employing finite frequency kernels that use both phase and amplitude data; and iterating progressively from coarse resolution to finer resolution with the previous coarser model serving as a new starting model. With the noise removal, we have approximately 3600 seismograms at 100 s with good signal-to-noise compared to ~ 5200 at 50 s. In the phase velocity inversion with the multiple plane wave technique, we solve simultaneously for azimuthally averaged phase velocities, azimuthal anisotropy, coefficients describing the incoming wavefield, station amplitude corrections and attenuation.
In the 30-70 km depth range, the shear-wave velocity structure is largely featureless with anomalies typically less than +/- 1% from the mean. The lack of features beneath the swell in this depth range shows that the excess elevation is not created by reheating or replacement of the upper lithosphere. At depths shallower than about 70 km, dehydration of the mantle inherited from formation at mid-ocean ridges may make the lithosphere more resistant to tectonic erosion by underlying asthenospheric convection.
At 80-150 km depth, a pronounced region of anomalously low velocities is well-resolved, with average shear velocities varying by as much as 8% from the slow region to the surrounding faster anomalies. The lowest velocities are found beneath the Hawaii-Maui-Molokai part of the island chain, where they reach values ~4.0 km/s. Vertically, the lowest velocities are in the 110-130 km depth range, just beneath the ~100 km vertical extent of cooling beneath normal oceanic lithosphere. This pattern strongly suggests that hot, damp, low-viscosity, buoyant mantle from the plume source spreads out laterally near the top of the pre-existing, low-viscosity channel in the normal oceanic asthenosphere. We demonstrate that the anomalous elevation of the Hawaiian swell is largely explained by the uplift of a 30-km-thick elastic plate from below by this buoyant, low-seismic-velocity layer in the asthenosphere.
In the 30-70 km depth range, the shear-wave velocity structure is largely featureless with anomalies typically less than +/- 1% from the mean. The lack of features beneath the swell in this depth range shows that the excess elevation is not created by reheating or replacement of the upper lithosphere. At depths shallower than about 70 km, dehydration of the mantle inherited from formation at mid-ocean ridges may make the lithosphere more resistant to tectonic erosion by underlying asthenospheric convection.
At 80-150 km depth, a pronounced region of anomalously low velocities is well-resolved, with average shear velocities varying by as much as 8% from the slow region to the surrounding faster anomalies. The lowest velocities are found beneath the Hawaii-Maui-Molokai part of the island chain, where they reach values ~4.0 km/s. Vertically, the lowest velocities are in the 110-130 km depth range, just beneath the ~100 km vertical extent of cooling beneath normal oceanic lithosphere. This pattern strongly suggests that hot, damp, low-viscosity, buoyant mantle from the plume source spreads out laterally near the top of the pre-existing, low-viscosity channel in the normal oceanic asthenosphere. We demonstrate that the anomalous elevation of the Hawaiian swell is largely explained by the uplift of a 30-km-thick elastic plate from below by this buoyant, low-seismic-velocity layer in the asthenosphere.