Imaging Seismic Velocity Structure Beneath the Iceland Hotspot – A Finite-Frequency Approach - fig. 1
Imaging Seismic Velocity Structure Beneath the Iceland Hotspot – A Finite-Frequency Approach - fig. 1
Credit:
Shu-Huei Hung, Ling-Yun Chiao • National Taiwan University; Yang Shen • University of Rhode Island/IRIS Consortium
Description
Figure 1. (Left) Location of study area and seismic stations in Iceland. The data in this study are from two temporary broadband seismic arrays of the ICEMELT and HOTSPOT Experiments and one GSN station, BORG. The outer box spans ~1000 km in both latitude and longitude directions and is divided into equally-spaced grids for model parameterization in travel time tomography. (Right) Azimuthal coverage of the earthquakes with useful P- and S-wave arrivals.
Tomographic models based on hypothetically infinite-frequency ray interpretation of teleseismic travel-time shifts have revealed a region of relatively low P and S wavespeeds extending from shallow mantle to 400 km depth beneath Iceland. In reality, seismic waves have finite-frequency bandwidths and undergo diffractive wavefront healing. The limitation in ray theory leaves large uncertainties in the determinations of the magnitude and shape of the velocity anomaly beneath Iceland and its geodynamic implications. We developed a tomographic method that utilizes the banana-shaped sensitivity of finite-frequency relative travel times from the paraxial kernel theory. Using available seismic data from the ICEMELT and HOTSPOT PASSCAL Experiments (Figure 1), we applied the new method to image subsurface velocity structure beneath Iceland. Given similar fit to data, the kernel-based models yield the root-mean-square amplitudes of P- and S-wavespeed perturbations about 2-2.8 times those from ray tomography in the depths of 150-400 km. The kernel-based images show that a columnar low-velocity region having a lateral dimension of ~250-300 km extends to the base of the upper mantle beneath central Iceland, deeper than that resolved by the ray-based studies (Figure 2). The improved resolution in the upper-mantle transition zone is attributed to the deeper crossing of broad off-path sensitivity of travel-time kernels than in ray approximation and frequency-dependent wavefront healing as an intrinsic measure of the distance from velocity heterogeneity to receivers.
Hung, S.-H., Y. Shen, L.-Y. Chiao, Imaging seismic velocity structure beneath the Iceland hot spot: a finite frequency approach, J. Geophys. Res., 109, 8305, doi: 10.1029/2003JB002889, 2004.
Research was supported by NSC of Taiwan (SHH and LYC) and NSF (SHH and YS) and instruments of the HOTSPOT Experiment provided by IRIS PASSCAL.
Photographer / Contributor: Shu-Huei Hung, Ling-Yun Chiao • National Taiwan University; Yang Shen • University of Rhode Island
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