AGU Abstract

August 31st, 2010

A Joint Rayleigh and Love Wave Analysis for the Hawaiian PLUME Project

During the two-phase Hawaiian PLUME (Plume-Lithosphere Undersea Mantle Experiment) seismic deployment from January 2005 through June 2007, we collected continuous seismic data at thirteen (ten temporary and three observatory) land stations and nearly 70 ocean bottom sites. Most of these sites were occupied with broad-band 3-component seismometers (Laske et al., 2009). This provides the ideal basis to analyze both Rayleigh and Love waves across a broad frequency band, between 10 and 50 mHz. Our analysis explores the radially anisotropic shear velocity structure of the region through comparison of long-period teleseismic Rayleigh and Love waves recorded from phase 2 of the PLUME project which spans the time interval of May 2006 through June 2007.  The collection of earthquakes suitable for a fundamental mode surface wave analysis consists of 190 shallow events.

Using the two-station method, we determine the path-averaged phase velocity for over 60 paths from 1322 Rayleigh wave and 749 Love wave phase measurements. Most of the high quality Love wave data collected exists between 2 land station paths: BIG2-KIP and KIP-POHA. Here, we present a detailed comparison of the vertical shear velocity, $V_{SV}$, and the horizontal shear velocity, $V_{SH}$, between these 2 two-station paths. From the path-averaged dispersion curves, we use the weighted average, where the weight is determined by individual error bars, to perform 1D inversions. We determine the percentage radial anisotropy as $((V_{SH}-V_{SV})/V_{SV})\times100$, as a function of depth.

In order to distinguish between effects caused by mantle anomalies and those caused by variations in bathymetry and crustal thickness, we explore the changes in phase velocity through forward modeling. Compared to predictions for 52-100 Ma old lithosphere, our dispersion data reveals high phase velocities for Love waves and relatively low phase velocities for Rayleigh waves, along both station paths. This suggests that $V_{SH}$ in the lithosphere and possibly the asthenosphere must be higher than $V_{SV}$. Our inversions for shear velocities as functions of depth confirm this. We observe radial anisotropy mainly in the lithosphere to be up to 10\%. We infer from this rather high value that radial anisotropy beneath the island chain reflects mostly horizontal fabric and flow in the shallow mantle.

References:
Laske, G., Collins, J.A., Wolfe, C.J., Solomon, S.C., Detrick, R.S., Orcutt, J.A., Bercovici, D. and Hauri, E.H., 2009. Probing The Hawaiian Hot Spot With New Ocean Bottom Instruments, EOS Trans. AGU, 90, 362-363.