Ban-Yuan Kuo

Gravity anomalies of the ridge-transform system in the South Atlantic between 31 and 34.5° S: Upwelling centers and variations in crustal thickness

To decipher the distribution of mass anomalies near the earths surface and their relation to the major tectonic elements of a spreading plate boundary, we have analyzed shipboard gravity data in the vicinity of the southern Mid-Atlantic Ridge at 31–34.5° S. The area of study covers six ridge segments, two major transforms, the Cox and Meteor, and three small offsets or discordant zones. One of these small offsets is an elongate, deep basin at 33.5° S that strikes at about 45° to the adjoining ridge axes.By subtracting from the free-air anomaly the three-dimensional (3-D) effects of the seafloor topography and Moho relief, assuming constant densities of the crust and mantle and constant crustal thickness, we generate the mantle Bouguer anomaly. The mantle Bouguer anomaly is caused by variations in crustal thickness and the temperature and density structure of the mantle. By subtracting from the mantle Bouguer anomaly the effects of the density variations due to the 3-D thermal structure predicted by a simple model of passive flow in the mantle, we calculate the residual gravity anomalies. We interpret residual gravity anomalies in terms of anomalous crustal thickness variations and/or mantle thermal structures that are not considered in the forward model. As inferred from the residual map, the deep, major fracture zone valleys and the median, rift valleys are not isostatically compensated by thin crust. Thin crust may be associated with the broad, inactive segment of the Meteor fracture zone but is not clearly detected in the narrow, active transform zone. On the other hand, the presence of high residual anomalies along the relict trace of the oblique offset at 33.5° S suggests that thin crust may have been generated at an oblique spreading center which has experienced a restricted magma supply. The two smaller offsets at 31.3° S and 32.5° S also show residual anomalies suggesting thin crust but the anomalies are less pronounced than that at the 33.5° S oblique offset. There is a distinct, circular-shaped mantle Bouguer low centered on the shallowest portion of the ridge segment at about 33° S, which may represent upwelling in the form of a mantle plume beneath this ridge, or the progressive, along-axis crustal thinning caused by a centered, localized magma supply zone. Both mantle Bouguer and residual anomalies show a distinct, local low to the west of the ridge south of the 33.5° S oblique offset and relatively high values at and to the east of this ridge segment. We interpret this pattern as an indication that the upwelling center in the mantle for this ridge is off-axis to the west of the ridge.

Tomographic inversion of S–SKS times for shear velocity heterogeneity in D″: Degree 12 and hybrid models

Differential travel times between S (or diffracted S) and SKS were measured to study the global distribution of shear wave velocity heterogeneity in the lowermost 250 km of the mantle (the D″ region). Commencing with ∼3000 S-SKS times with variable qualities, we minimize uneven path coverage by thinning redundantly sampled regions (e.g., the Fiji-Tonga to North America corridor) and collect additional data for sparsely sampled areas, especially in the Southern Hemisphere. About 1500 paths were retained, distributed to reconcile both high-density and homogeneous sampling. We compare (1) spherical harmonic and (2) equal-area block parameterizations of D″ shear velocity heterogeneity for identical minimum resolving lengths and mean model errors. We show that the two parameterizations result in indistinguishable patterns of heterogeneity and power distribution for resolution down to a block size of 4.5°×4.5° and maximum spherical harmonic degree of 40. We demonstrate synthetically that a high-degree (L) inversion followed by a lower-degree (Ls) spherical harmonic synthesis effectively circumvents model contamination from expansion truncation effects. Tomographic inversion of this data set yields a global distribution of D″ heterogeneity robust up to degree 12. In our preferred model (L = 40, Ls = 12, rms ∼ 1%), surface hotspots and estimated lower mantle plume roots are located in or at edges of low-velocity regions (δVs< −2%), for both the Atlantic-Indian low-velocity corridor as well as the low velocities beneath the central Pacific. Although the above parameterizations entail no intrinsic difference in resolvability, higher resolution is regionally achieved by removing the derived degree 12 model from the data then inverting the corrected residuals using 4.5°×4.5° blocks. This hybrid method prevents power loss at intermediate to long wavelengths from otherwise severe damping and recovers smaller-scale structure where constraints are better than the global average, such as beneath the Pacific and Eurasia. A synthetic recovery test yields resolvability maps that reflect both the path geometry and the quality of data, which help to identify robust features in both the degree 12 and the hybrid D″ models.

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.

Split S waveforms observed in northern Taiwan: Implications for crustal anisotropy

Short-period stations ENT and NSK in northern Taiwan have frequently recorded split S waveforms from earthquakes in the underlying subduction zone. To determine splitting parameters together with objective, straightforward error estimations, we employ a waveform cross-correlation method. The error for each measurement is estimated by translating the lower limit of the 95% confidence interval of the cross-correlation coefficient into azimuth and time. Only measurements with 95% confidence regions excluding zero delay time are accepted as splitting data. We also use an energy minimization method to constrain the solutions with minor splitting. We interpret the splitting as caused by stress-aligned cracks in the upper crust. The average delay times are 0.03–0.04 s for ENT and 0.07 s for NSK. The fast directions at the two stations are predominantly N-S, which roughly match the stress trajectories predicted by a model simulating the arc-continent collision in the Taiwan area. However, at ENT there is a subset of data with fast directions oriented in roughly E-W. The nearly 90° rotation of the inferred crack orientation occurs in a horizontal distance of 10–15 km in the upper crust. Both the predicted heterogeneous stress regime and the local structural setting in northeastern Taiwan favor a lateral variation with a similar length scale.

Global shear velocity heterogeneities in the D″ layer: Inversion from Sd‐SKS differential travel times

A global map of shear velocity in the D″ layer results from the inversion of 340 differential travel times of diffracted S(SH) minus SKS(SV) (Sd-SKS), from long-period records of global seismic networks. The two-phase design reduces contamination from upper mantle heterogeneities and errors in location and origin time of the events. Additional corrections are made for (1) azimuthal anisotropy at stations where shear wave splitting parameters are available and for (2) travel time perturbations due to lower mantle asphericity, although both effects are minor compared with the observed residuals with respect to the preliminary reference Earth model (PREM) [Dziewonski and Anderson, 1981]. The corrected residuals, ranging from −16 to 18 s, are attributed to anomalies in D″ sampled by both phases. Taking these residuals as data and assuming a constant, 250-km-thick D″ layer, we invert for a lateral velocity variation model of D″ using spherical harmonics. In parameterizing D″ velocities, a high degree expansion (L=14) avoids aliasing, but only the reliably determined, low degree components (LI<L) produce optimal models. The optimal model LI=6 achieves a variance reduction of 56% and mean error 0.019 km/s, and the velocity is insensitive to the thickness of the D″ layer assumed. A baseline shift of ∼0.022 km/s from PREM is resolved. The magnitude of variation (∼0.2 km/s or 3%) is nearly twice as large as that of Tanimoto [1990] and Su et al. [1994], while comparable to that of Liu and Dziewonski [1994] which includes S-SKS data and Li and Romanowicz [1996]. Degree 2 variation dominates the D″ with positive anomalies beneath the Asian, Australian, and American continents and southernmost Pacific and negative anomalies beneath the Pacific, western Africa, Atlantic, and the southern Indian Ocean. This pattern is similar to that found in previous tomograpic models; significant discrepancies in nondegree 2 features are discussed in terms of the constraints from Sd. The fast and slow regions of D″ correlate with the inferred paleoslab positions and the major hotspot distribution, respectively, both at the 99% confidence level. Implications of these correlations for mantle dynamics are suggested. The global model agrees reasonably well with the velocities determined regionally from the Sd slowness measurement and the ScS-S residuals in other studies. The latest radial models of D″ constrained from S triplications at distances of 75°–90° provide a completely independent benchmark; models for Eurasia, Alaska, and the Caribbean predict Sd travel times corresponding to 0.03–0.08 km/s anomalies, consistent with our lateral variation model in respective regions. These vertical structures are also consistent with this study in ScS times, indicating that they agree in a vertically averaged sense. Major inconsistency exists for the India model SYLl [Young and Lay, 1987b], which predicts Sd and ScS arrivals much slower than those expected from the 0.1 km/s fast anomaly we found, suggesting either the presence of strong lateral heterogeneities or the need for further mapping in this area.

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.

A fast velocity anomaly to the west of the Australian‐Antarctic discordance

To examine the hypothesis that the Australian-Antarctic Discordance overlays an anomalously cold upper mantle, we have measured Rayleigh wave group velocities along paths between the Southeast Indian Ridge and the Australian station NWAO. The group velocity peaks for events in the vicinity of 117°E, rather than for those within the discordance zone between 120 and 127°E. This variation is resolved at least for periods of 35–45 s. To model the cause, assumed velocity anomalies with simple geometry were tested using the surface wave Gaussian beam method in a forward sense. The preferred model consists of an elongated structure centered 300–500 km west of the discordance and stretching northward for at least 1000 km. The position of the anomalous structure is consistent with recent global tomographic models in this region.

Marine Cable Hosted Observatory (MACHO) Project in Taiwan

Taiwan is located in a junction corner between the Philippine sea plate and Eurasian plate. Because of active convergence, numerous earthquakes have taken place in and around Taiwan. On average, there are about two earthquakes greater than magnitude 6 each year and over 70% of earthquakes occurred in the offshore area. Because of the subduction of Philippine Sea Plate beneath the western end of the Ryukyu Arc and northern Taiwan, both the tectonics and seismic activity are intensive. The 2004 Sumatra earthquake has induced giant tsunami attacking coastal countries of South Asia. In a similar geodynamic context, the Sumatra event has aroused the attention of Taiwan government. Specialists from Taiwan earth scientists and ocean engineers have quickly teamed up to discuss the potential and mitigation of natural hazards from the western end of the Ryukyu subduction zone. To construct a submarine cable observatory off eastern Taiwan (MACHO project) was proposed. MACHO means a sea goddess who protects people at sea. The purpose of MACHO project has several folds. Firstly, the extension of seismic stations on land to offshore area can increase the resolution of earthquake relocating. Secondly, the extension of seismic stations may obtain tens of second before the destructing seismic waves arrive on land or tens of minute before the arrival of giant tsunami, which is helpful for earthquake or tsunami warning. Thirdly, the seafloor scientific station can monitor the active volcanoes in the Okinawa Trough, which is directly adjacent to the Ilan plain in northeastern Taiwan. Fourthly, the seafloor observatory can be used to continuously study the Kurosho current, off eastern Taiwan. The MACHO project has been granted for the fiscal year of 2007. The MACHO project is expected to be fulfilled in 2009.

Low radiation efficiency of the intermediate‐depth earthquakes in the Japan subduction zone

Robust determination of earthquake source parameters over a continuous depth range is central to inferring rupture mechanisms dominant at different depths. We employed a cluster-event method to constrain the source parameters as well as along-path attenuation for earthquakes over 0–150 km depths and 4 orders of seismic moments in the Japan subduction zone. We found that corner frequency and stress drop increase with depth, whereas the radiated energy scaled by seismic moment declines with depth slightly. As a result, the radiation efficiency exhibits a notable deficit for events deeper than 60 km. Together these suggest an increased energy dissipation during faulting in ductile deformation regime, consistent with shear heating instability as an important faulting mechanism for intermediate-depth earthquakes.

Faulting and hydration of the upper crust of the SW Okinawa Trough during continental rifting: Evidence from seafloor compliance inversion

The elastic response of seafloor to ocean gravity wave loading, or seafloor compliance, provides a constraint on the elastic properties of the crust. We measured seafloor compliance at three ocean bottom seismometer (OBS) sites around Taiwan—two in the southwestern (SW) Okinawa Trough and one on the Ryukyu arc—and performed inversion for crustal structures beneath them. Models best fitting the data demonstrate a decrease in upper crustal shear velocity and an increase in the compressional/shear velocity ratio from the arc site to the trough sites with increasing amount of back-arc extension. This variation suggests that the upper continental crust is highly faulted and hydrated during rifting of the Eurasian lithosphere.