Ling-Yun Chiao

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;

Seismic Quiescence before the 1999 Chi-Chi, Taiwan, Mw 7.6 Earthquake

The Chi-Chi, Taiwan, earthquake of 20 September 1999 was preceded by a notable decrease of the regional seismicity rate. The anomalous period started in January 1999 and lasted about 9 months, up to the occurrence of the mainshock, as revealed from analyzing the Taiwan catalog data from 1 January 1994 to 20 September 1999. Our results indicate that the mean seismicity rate is 435 events per month with a standard deviation of 78 events for earthquakes with magnitude above 2.0. During the anomalous period, seismicity rates fell outside the range of one standard deviation with a mean value of 314 events per month. There was also a consistent trend of decreasing regional b -value as well as notable increases of seismic activity in areas surrounding the Chi-Chi earthquake source region via the Z-test map (zmap) analysis.

Multiresolution parameterization for geophysical inverse problems

A model parameterization based on a multiresolution wavelet representation is proposed for geophysical inverse problems in this study. The matrix equation for the wavelet representation of the model is constructed by dual wavelet transforms of a readily built matrix based on the pixel parameterization. We demonstrate the merits of the proposed parameterization using the simple examples of geophysical data gridding and 3D seismic tomography, and comparing the results with those obtained by the conventional pixel parameterization. It is shown that for the conventional damped least-squares solutions of pixel-parameterized models, regularization by norm damping deteriorates the underlying model correlation due to sparse and heterogeneous sampling, whereas roughness damping schemes impose a priori arbitrary correlation length irrespective of the heterogeneous sampling. The scale hierarchy embedded in the proposed multiresolution parameterization facilitates the compromise between the dual spatial and scale resolution. Depending on the local richness of the data constraints across different scales at a site, model variation of various scale spectra can be robustly resolved through the scale hierarchy. There is thus no need to invoke additional smoothness regularization.

Membrane strain rates in the subducting plate beneath South America

Geometric constraints associated with the concave oceanward bend in the trench and Andes Mountains, known as the Bolivian Orocline, force along-strike compression in the subducting slab. Our calculations of membrane strain rates, in an assumed continuous slab, demonstrate that this can be accommodated by either 10% along-strike compressive strain, or by geometric buckling. In nearly all other subduction zones exhibiting a similar bend in the trench, the compression is accommodated by slab-arching landward of the trench bend. The slab geometry is markedly different in South America. Two regions of nearly flat-lying slab north and south of the Bolivian Orocline result in the 150-km slab depth contour being much straighter than the trench profile. Our calculations show that this geometry helps to relieve along-strike compression. Along the deeper part of the subduction zone, the along-strike compression seems to be accommodated by pronounced along-strike buckling. We infer the approximate geometry of this buckle, in an otherwise aseismic region, from the great Mw 8.3 Bolivian deep earthquake on June 9, 1994, and its aftershocks. The deep slab appears to have a nearly east-west strike in the vicinity of this earthquake, whereas the strike is more nearly north-south to the north and south. The focal mechanisms of deep earthquakes exhibit consistent down-dip compression and along-strike null axes, tracking the contortions in slab geometry. However, our calculations indicate that the particle paths themselves are not down dip everywhere. Instead, where the deep-slab strike is oriented nearly east-west, the particle flow is more nearly along strike than down dip. In general this is not a major problem, but if the slab is anchored in the lower mantle, and moving very slowly, this produces large deformation rates near the base of the upper mantle where the slab is assumed to move at near plate rates. This increased, complex deformation in the deep slab may contribute to the conditions for very large earthquakes in two ways. First, this may dramatically increase deformation rates, and secondly, the increased deformation may lead to anomalous advective thickening of a proposed overdriven olivine wedge. This provides a mechanism for locally thickening the olivine wedge and allowing transformational faulting to occur over a much larger volume than allowed for normal subduction.

Layered deformation in the Taiwan orogen

Lower crustal deformation takes a turn Collisions creating mountain belts frequently involve a tectonic plate plunging into the mantle. Huang et al. connect the deformation of rock from the subducting plate to the surface topography in Taiwan (see the Perspective by Long). Subsurface deformation mapping required interpreting certain seismic wave velocities as they travel through the crust. The subsequent images of Taiwans deep crust show two distinct layers of deformation. The bottom layer comprises the subducting slab, which is being pulled into the mantle. This mechanically couples with the upper layer of crust, compressing it into a mountain range. Science, this issue p. 720; see also p. 687 A change in the direction of deformation with depth helps explain subduction-driven uplift. [Also see Perspective by Long] The underthrusting of continental crust during mountain building is an issue of debate for orogens at convergent continental margins. We report three-dimensional seismic anisotropic tomography of Taiwan that shows a nearly 90° rotation of anisotropic fabrics across a 10- to 20-kilometer depth, consistent with the presence of two layers of deformation. The upper crust is dominated by collision-related compressional deformation, whereas the lower crust of Taiwan, mostly the crust of the subducted Eurasian plate, is dominated by convergence-parallel shear deformation. We interpret this lower crustal shearing as driven by the continuous sinking of the Eurasian mantle lithosphere when the surface of the subducted plate is coupled with the orogen. The two-layer deformation clearly defines the role of subduction in the formation of the Taiwan mountain belt.

Crustal magnetization equivalent source model of Mars constructed from a hierarchical multiresolution inversion of the Mars Global Surveyor data

[1] Several magnetic field models of Mars have been constructed since the Mars Global Surveyor data became available. Three distinct schemes formulated through spherical harmonic functions, discrete equivalent dipoles, and the continuous magnetic field kernels have yielded results that are grossly compatible but with very different details. Models of internal potential function in terms of spherical harmonics tend to yield divergent high-degree Mauersberger-Lowes spectra, whereas crustal magnetization models exhibit flat but still significant spectra up to high degrees. To have a better fitting to the observed data seems to have dominated previous efforts that have yielded fine details with wavelengths shorter than the lateral track spacing. The variance-reduction versus model-variance tradeoff analysis is invoked in this study for the determination of the appropriate regularization. Taking advantage of the recently developed multiscale inversion, we are able to conservatively retain only the model components that are robustly constrained by the data rather than unilaterally pushing for a higher degree of fitting. With the variance reduction around 82%, we find that to reach a reasonably fair data fitting without high model variance, the high-degree power spectra of our preferred model exhibit an obvious decaying trend, implying that a lot of the short-wavelength energy embedded within established models is either not robustly resolvable or is of external origin or is simply reflecting the nonuniform distribution of sampling at short scales. The reason that models based on spherical harmonics have greater high-degree power is attributed to the spectral leakage due to the truncated representation. Citation: Chiao, L.-Y., J.-R. Lin, and Y.-C. Gung (2006), Crustal magnetization equivalent source model of Mars constructed from a hierarchical multiresolution inversion of the Mars Global Surveyor data, J. Geophys. Res., 111, E12010, doi:10.1029/ 2006JE002725.

Dynamic interaction of cold anomalies with the mid-ocean ridge flow field and its implications for the Australian–Antarctic Discordance

Abstract Negative thermal anomalies beneath a mid-ocean ridge are dynamically isolated from the ambient upwelling and diverging flow field in the asthenosphere whose viscosity is on the order of 5×10 19 Pa s or less. This study examines on what condition a near-ridge cold anomaly ascends with the upwelling passive flow and spread off-axis. Three-dimensional numerical modeling demonstrates that, for a given magnitude of the cold anomaly, the viscosity of the asthenosphere, the spreading rate and the interference from continental rifting are the dominant controlling factors to the ascent/descent of the anomaly. To overcome the weight of such an anomaly and couple it with the upwelling, either the spreading rate or the asthenospheric viscosity has to be high. In a low viscosity asthenosphere, the cold anomaly also ascends during the early stage of continental rifting due to the enhanced upwelling induced by the thick continental lithosphere. The dynamic interaction between the cold anomaly and the ambient flow renders a transient nature of the subsidence of the seafloor, which may lead to exaggerated temperature variation estimated by using a conduction model alone. The scenarios examined are employed to place a constraint on dynamic models recently proposed for the Australian–Antarctic Discordance, in which the source of the negative anomaly is hypothesized to be deeply rooted in the upper mantle. With the asthenospheric viscosity less than 10 20 Pa s, the upwelling of the cooler material from great depths, which causes a significant topographic low at the Discordance, is made possible only by rifting of the Australian continent off the Gondwanaland.

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.

An alternative interpretation for slip vector residuals of subduction interface earthquakes: a case study in the westernmost Ryukyu slab

Abstract Slip partitioning along oblique subduction zones has conventionally been examined by slip vector residuals determined from interface earthquakes. It is measured through contrasting the slip vector with respect to the local plate convergence direction defined on the surface of the Earth by the Euler vector. Interpretation of regional plate kinematics based on slip vector residuals thus defined may be misleading. Besides the apparent discrepancy attributable to the surface projection of vectors on an obliquely plunging surface, a more fundamental problem is the construction of the reference subduction flow field for the subducted slab based upon the known plate convergence. It can be shown that for complicated slab geometry, the conventional practice of intuitively rotating the Euler kinematics onto the subducted slab leads to unrealistic intraplate deformation that invalidates the mechanical integrity of the oceanic lithosphere. Alternatively, we propose to calculate the velocity field for the specific slab geometry based on the rationale that the subduction flow field should be the one that endures the least amount of intraplate deformation. The calculated flow field for the westernmost Ryukyu slab, that is in transition to the Philippine Sea plate (PSP)–Eurasian Plate (EP) collision near Taiwan, reveals quite distinct subduction flow field. The particle paths associated with the calculated flow field are in better accordance with the observed slip vectors while the corresponding strain-rate field is consistent with the previously reported lateral compression seismogenic zone. Furthermore, it is speculated that the plate kinematics of the northwestern corner of PSP that is impinging the vicinity of the collision might have also undergone significant readjustment to accommodate the potential intraplate deformation. It is noted that the westernmost subducted Ryukyu slab and the northwestern corner of PSP belong to a mechanically coherent tectonic unit. The 3D geometric constraints set up by the subducted slab and the “unfriendly” kinematic constraints imposed by the collision boundary both play important roles in configuring the intraplate deformation within PSP. Manifestation in the local kinematics might indicate deviation from the general PSP–EP convergence, and is thus of crucial importance to tectonic interpretations of geophysical observations in this area.

Multiresolution Interpolation and Detiding of the ADCP Data

Abstract To account for the uneven sampling of current measurements collected by acoustic Doppler current profilers (ADCPs), a robust, three-dimensional interpolation scheme based on the multiresolution representation of the regional mean and tidal current fields is proposed. Instead of reconstructing the tidal field by getting bin-averaged time series that rely on heavy sampling or invoking radial basis function expansions, such as using the biharmonic splines with subjectively selected knots, the resolving capability of the proposed scheme relies fundamentally on the scale hierarchy of the resolvable local information constrained by the data. It is demonstrated that the proposed scheme flexibly incorporates the merits of the two conventional techniques. It enforces the resolution of model information while accommodating the local sampling density. Since it is based on a knots network defined by regular grids, attempts at experimentally and subjectively constructing the proper number and locations of con...