Shu-Huei Hung

Imaging seismic velocity structure beneath the Iceland hot spot: A finite frequency approach

[1] Tomographic models based on hypothetically infinite frequency ray interpretation of teleseismic travel time shifts have revealed a region of relatively low P and S wave speeds extending from shallow mantle to 400 km depth beneath Iceland. In reality, seismic waves have finite frequency bandwidths and undergo diffractive wave front 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 experiments, we applied the new method to image subsurface velocity structure beneath Iceland. Taking advantage that the sensitivity volume of broadband waveforms varies with frequency, we measured relative delay times in three frequency ranges from 0.03 to 2 Hz for P and 0.02 to 0.5 Hz for S waves. Given similar fit to data, the kernel-based models yield the root-mean-square amplitudes of P and S wave speed 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. 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 wave front healing as an intrinsic measure of the distance from velocity heterogeneity to receivers. INDEX TERMS: 8180 Tectonophysics: Tomography; 7203 Seismology: Body wave propagation; 7218 Seismology: Lithosphere and upper mantle; 3260 Mathematical Geophysics: Inverse theory; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes;

Anomalous seafloor spreading of the Southeast Indian Ridge near the Amsterdam-St. Paul Plateau

The Amsterdam-St. Paul Plateau is bisected by the intermediate-rate spreading Southeast Indian Ridge, and numerous geophysical and tectonic anomalies arise from the interactions of the Amsterdam-St. Paul hotspot and the spreading center. The plate boundary geometry on the hotspot platform evolves rapidly (on timescales <1 Myr), off-axis volcanism is abundant, the seafloor does not deepen away from the axis, and transform faults do not have fracture zone extensions. Away from the hotspot platform the ridge-transform geometry is typical of mid-ocean ridges globally. In contrast, the Amsterdam-St. Paul Plateau spreading segments are shorter, they often overlap each other significantly, and the intervening discontinuities are smaller, more ephemeral, and more migratory. Abyssal hills are smaller and less uniform on the hotspot platform than on neighboring spreading segments. From gravity and isostasy analysis the average thickness of the platform crust is ∼10 km, approximately 50% thicker than that of typical oceanic crust. Most of the isostatic compensation of the hotspot plateau occurs at the Moho or within the lower crust, and the effective elastic thickness of the plateau lithosphere is ∼1.6 km, less than half that of adjacent spreading segments. Away from the platform some transform faults contain intratransform spreading centers; on the platform the two transform faults have valleys which may be depocenters for abundant axial or off-axis volcanism and mass wasting. Although not wellconstrained by magnetics coverage, the Amsterdam-St. Paul hotspot appears to have been “captured” by the Southeast Indian Ridge, enhancing crustal production at the ridge since about 3.5 Ma. Prior to this time the hotspot formed a line of smaller, isolated volcanoes on older Australian plate. The underlying cause for the present-day crustal accretion anomalies is the effect of melt generation from separate sources of mantle upwelling (due to plate spreading and the hotspot) which has a consequent effect of weakening the lithosphere.

Can a narrow, melt‐rich, low‐velocity zone of mantle upwelling be hidden beneath the East Pacific Rise? Limits from waveform modeling and the MELT Experiment

One of the goals of the Mantle Electromagnetic and Tomography (MELT) Experiment is to determine whether a narrow zone of enhanced melt concentration consistent with focused upwelling exists beneath the East Pacific Rise. Using SKKS, sScS, and S phases from two intermediate-depth earthquakes in the Banda Sea and the Tonga-Kermadec region, we demonstrate that there is no positive evidence for the existence of such a zone and that travel time delays for shear waves traveling through it must be <0.5 s. To test whether diffraction and wave front healing could obscure evidence for its existence, we employ a pseudospectral method to simulate finite frequency teleseismic waves propagating through narrow, vertical low-velocity zones. A rich set of reflected, diffracted, and guided waves is generated when S waves encounter such a low-velocity channel, particularly at high frequencies. Limiting the frequency content to the lower-frequency bands with good signal-to-noise ratios in the observed phases obscures these waveform complexities. The travel time anomaly is broadened and reduced in amplitude but remains detectable unless the low-velocity zone is very narrow or has only modest velocity contrast. The lower limit of detectability corresponds to a 5-km-wide channel of partial melt extending from 10 to 60 km below the seafloor at the ridge axis with a shear velocity contrast of 0.5 km/s. Although these limits are severe, 3 to 4% melt retention might cause a large enough viscosity reduction and anomalous buoyancy to dynamically focus upwelling into a 5-km-wide channel that falls within the limits. Strongly focused, dynamic upwelling beneath the ridge, however, is probably not compatible with the existence of a broad region of very low shear velocities in the surrounding mantle.

Dynamic ray tracing and traveltime corrections for global seismic tomography

We present a dynamic ray tracing program for a spherically symmetric Earth that may be used to compute Frechet kernels for traveltime and amplitude anomalies at finite frequency. The program works for arbitrarily defined phases and background models. The numerical precisions of kinematic and dynamic ray tracing are optimized to produce traveltime errors under 0.1 s, which is well below the data uncertainty in global seismology. This tolerance level is obtained for an integration step size of about 20 km for the most common seismic phases. We also give software to compute ellipticity, crustal and topographic corrections and attenuation.

A 3-D spectral-element and frequency-wave number hybrid method for high-resolution seismic array imaging

We present a three-dimensional (3-D) hybrid method that interfaces the spectral-element method (SEM) with the frequency-wave number (FK) technique to model the propagation of teleseismic plane waves beneath seismic arrays. The accuracy of the resulting 3-D SEM-FK hybrid method is benchmarked against semianalytical FK solutions for 1-D models. The accuracy of 2.5-D modeling based on 2-D SEM-FK hybrid method is also investigated through comparisons to this 3-D hybrid method. Synthetic examples for structural models of the Alaska subduction zone and the central Tibet crust show that this method is capable of accurately capturing interactions between incident plane waves and local heterogeneities. This hybrid method presents an essential tool for the receiver function and scattering imaging community to verify and further improve their techniques. These numerical examples also show the promising future of the 3-D SEM-FK hybrid method in high-resolution regional seismic imaging based on waveform inversions of converted/scattered waves recorded by seismic array.

Characteristics of short period secondary microseisms (SPSM) in Taiwan: The influence of shallow ocean strait on SPSM

Taking advantage of a unique opportunity provided by a dense array of coastal short-period seismic stations and the diverse bathymetry around Taiwan, we examine how the long-range coherent ambient noises are influenced by surrounding ocean settings using the cross-correlation functions (CCFs) between pairs of stations. The effective energy of the CCFs derived from three components of short-period seismometer data falls within the frequency range of the short period secondary microseism (SPSM). The spatial variations mapped from the amplitude asymmetry of CCFs and source migration images evidently demonstrate that the SPSM strengths are closely linked to the drastic changes in offshore ocean characteristics and result in much stronger SPSM in the shallow and narrow Taiwan Strait than in deep open seas of eastern Taiwan. The temporal variations of the CCF strengths exhibit very good correlations with the wind speeds and wave heights, explicitly indicating the observed SPSM is dominated by local sources generated from wind-driven ocean waves around offshore Taiwan.

Full-Wave Effects on Shear-Wave Splitting

In this chapter, we develop an approach to the inversion of 3D anisotropy structure using the sensitivity (Frechet) kernels calculated by an efficient and flexible full-wave algorithm based on the normal-mode theory.

Anisotropy in the oceanic lithosphere from the study of local intraplate earthquakes on the west flank of the southern East Pacific Rise : Shear wave splitting and waveform modeling

Shear wave splitting is observed on ocean bottom seismometer records from local, intraplate microearthquakes on the west flank of the East Pacific Rise at 18°S. Split times reach a maximum of about 0.2 s. For most of the incoming waves with long mantle paths (∼20 km), the polarization direction of the fast arrival is subparallel to the spreading direction, which we attribute to anisotropy caused by the strain-induced preferential orientation of olivine. In contrast, for some records with short paths or shallower sources or propagation along the spreading direction, the fast direction is nearly parallel to the ridge axis. These polarizations are probably caused by seismic anisotropy from aligned cracks in the uppermost crust. In addition, for some events, the apparent splitting is frequency dependent. To explore the pattern of shear wave splitting that would be expected for nonvertical paths in an oceanic lithosphere with two distinct anisotropic layers, we generate synthetic seismograms for a variety of source depths and mechanisms. We employ a multidomain, pseudospectral method to simulate the elastic wave fields from point sources in an inhomogeneous, anisotropic medium. Splitting parameters measured from synthetic S waves demonstrate that the apparent fast direction is not always parallel to the symmetry axes and that, in some cases, fast directions at higher frequencies will be more characteristic of the shallower crustal anisotropy while fast directions at lower frequencies are dominated by the mantle portion of the path. Most of the observed characteristics of splitting can be reproduced if there is approximately 8% S wave anisotropy in the mantle and an average of about 6% S wave anisotropy in the upper crustal, seismic layer 2.

Non‐double‐couple focal mechanisms in an oceanic, intraplate earthquake swarm: Application of an improved method for comparative event, moment tensor determination

Between February 1991 and May 1992, an earthquake swarm including 33 teleseismically recorded events occured about 300 km west of the axis of the southern East Pacific Rise. Joint hypocentral relocations show that the best located events were in a 10-km-wide band between and parallel to two seamount chains. New, simulated-annealing inversion procedures were developed to find the moment tensors of three of the largest events that then served as master events for focal mechanism determination of the smaller events. Path/station corrections were applied to the spectra of surface waves from the smaller events, and the corrected spectra were linearly inverted for the moment tensors. The new procedures made it possible to derive focal mechanisms for 32 of the 33 earthquakes, ranging in size from Mw 4.2 to 5.9. All of the events have double-couple components that are nearly pure normal faulting with a wide range of strikes of the nodal planes. Most have a non-double-couple component that may have been caused by simultaneous slip on nearly randomly oriented fault planes. The individual mechanisms and net horizontal extension that is nearly equal in all directions are consistent with the release of thermal stresses in the cooling oceanic lithosphere.

Seismological evidence for a non-monotonic velocity gradient in the topmost outer core.

The Earths core is mostly an Fe-Ni alloy with a fraction of light elements (~10 wt%, mainly O, S and Si). Accumulation of these light elements under the core-mantle boundary (CMB) may lead to chemical stratification. Seismic observations have been presented both for and against the stratification in the topmost region of the outer core. Here we investigate the structure under the CMB using differential travel times between SKKS and S3KS waves. We obtain 606 high-quality S3KS-SKKS differential travel times with global path coverage. Result from a Bayesian inversion of these differential times indicates that the seismic velocity in the top 800 km of the outer core is ~0.07% on average lower than that in model PREM. The depth-dependent velocity profile, in particular a low-velocity zone of up to ~0.25% lower than PREM at ~80 km below the CMB, strongly favors the existence of stratification at the top of the outer core.