Imaging Seismic Velocity Structure Beneath the Iceland Hotspot – A Finite-Frequency Approach - fig. 2
Imaging Seismic Velocity Structure Beneath the Iceland Hotspot – A Finite-Frequency Approach - fig. 2
Credit:
Shu-Huei Hung, Ling-Yun Chiao • National Taiwan University; Yang Shen • University of Rhode Island/IRIS Consortium
Description
Figure 2. Comparison of the tomographic images of (a) P- and (b) S-wavespeed perturbations in the ray-theoretical (left) and finite-frequency kernel (right) models. The contours shape the region in which the P and S velocities are at least 0.5% and 1.0% lower than those in the surrounding mantle. All the models reveal a columnar zone of low-velocity anomaly of ~250-300 km in diameter beneath central Iceland. In the finite-frequency models this low-velocity feature extends to the base of the upper mantle, while the maximum resolvable depth for the ray-based models is ~400 km. Both P- and S-velocity perturbations in the kernel-based models are significantly larger than those in the ray-theoretical models. The station corrections are indicated by squares for the positive or relatively-slow and triangles for the negative or relatively-fast velocity anomaly.
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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