AGU Abstract

Rayleigh-wave Group Velocity Tomography in the Vicinity of the Hawaiian Hotspot

Anne E Strader¹, Gabi Laske², John A Orcutt², Cecily J Wolfe⁴, John A Collins³, Sean C Solomon⁵, Robert S Detrick³, David Bercovici⁶, Erik H Hauri⁵

1. Bucknell University, Lewisburg, PA, USA.
2. Scripps Institution of Oceanography, La Jolla, CA, USA.
3. Woods Hole Oceanographic Institution, Woods Hole, MA, USA.
4. University of Hawaii at Manoa, Honolulu, HI, USA.
5. Carnegie Institute of Washington, Washington, DC, USA.
6. Yale University, New Haven, CT, USA. 

We present maps of long-period Rayleigh wave group velocity maps for the area spanned by the Hawaiian PLUME (Plume-Lithosphere Undersea Mantle Experiment) project. Specifically, we used observations from the second deployment of ocean-bottom and land broadband instruments that operated from April 2006 through May 2007. The recording network consisted of13 land stations with ten temporary and three observatory instruments and 38 ocean bottom sites that were equipped with 4-component broad-band instruments. With an average station spacing of approximately 200 km, this network had an aperture of nearly 1300 km.

For this study, we used an efficient interactive screen tool that employs a multiple filtering technique to measure the frequency-dependent group velocity. The spectra are pre-whitened to reduce biasing effects at frequencies with strong dispersion. We established that the technique provides reliable results for the two-station approach used here, at frequencies between 7 and 60 mHz. Our analysis includes records from 182 shallow earthquakes with focal depth h₀<200 km, and a scalar seismic moment M₀>0.01×10²⁰ Nm, and surface wave magnitudes M_S≥5.6. Six smaller events also have signal levels suitable for analysis.

For initial dispersion quality and consistency checks, we inspected local group velocity maps obtained from 555 path-averaged group velocity curves for paths that cross the PLUME network. Occam-smoothed matrix inversions are performed for maps with 1° in latitude and longitude. The data are highly consistent at frequencies above 10 mHz. At frequencies below 25 mHz, there is an anomaly downstream of the island of Hawaii that intensifies with decreasing frequency. This result suggests a deep-seated structural anomaly. Group velocities at frequencies above 40 mHz also map with high fidelity. However, in an initial inversion for three-dimensional mantle shear velocity structure we discarded such data, as they are highly sensitive to bathymetry (which is well known and can be corrected for) and crustal thickness (which is less well known). The dominant feature in the resulting model is a strong low-velocity anomaly in the lower lithosphere and asthenosphere downstream of the island of Hawaii. This outcome is broadly consistent with earlier results obtained from the analysis of Rayleigh wave phase data.