R. D. van der Hilst

A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China

On 12 May 2008, a magnitude 7.9 earthquake ruptured the Longmen Shan margin of the eastern Tibetan plateau. This event occurred within the context of long-term uplift and eastward enlargement of the plateau. The area has numerous geological features not typical of active convergent mountain belts, including the presence of a steep mountain front (>4 km relief) but an absence of large-magnitude low-angle thrust faults; young high topography (post ca. 15 Ma) and thickened crust but low global positioning system (GPS) shortening rates (<3 mm/yr); and no coeval foreland subsidence. In our interpretation, crustal thickening beneath the eastern Tibetan plateau occurred without large-scale shortening of the upper crust but instead is caused by ductile thickening of the deep crust in a weak (lowviscosity) layer. Late Cenozoic shortening across the Longmen Shan could be as little as 10–20 km, with folding and faulting mainly accommodating differential surface uplift between the plateau and the Sichuan Basin. The earthquake of 12 May probably reflects long-term uplift, with slow convergence and right-slip, of the eastern plateau relative to the Sichuan Basin. GPS-determined rates in the vicinity of the 12 May event suggest an average recurrence interval of ~2,000–10,000 yr.

Complex morphology of subducted lithosphere in the mantle beneath the Tonga trench

AT the Tonga trench, old Pacific sea floor subducts at a rapid rate below the Indo-Australia plate, generating most of the worlds deep earthquakes (focal depth >300 km)1,2 and producing a deep slab of former oceanic lithosphere. The seismogenic part of the slab has been mapped in detail3,4, but its fate has remained enigmatic. Here I present evidence from seismic tomography that the Pacific plate descends deep into the Earths mantle along a trajectory that is more complex than previously thought. In the north, the slab deflects in the transition zone (between about 400 and 700 km depth) before continuing into the lower mantle (below 700 km). Further south, penetration into the lower mantle occurs without a kink. The slab morphology can be explained in terms of the recent tectonic evolution of the subduction system, and reconciles pre-existing evidence from this region for both local horizontal flow in the transition zone2–8 and slab penetration into the lower mantle9–12.

Seismostratigraphy and Thermal Structure of Earth's Core-Mantle Boundary Region

We used three-dimensional inverse scattering of core-reflected shear waves for large-scale, high-resolution exploration of Earths deep interior (D″) and detected multiple, piecewise continuous interfaces in the lowermost layer (D″) beneath Central and North America. With thermodynamic properties of phase transitions in mantle silicates, we interpret the images and estimate in situ temperatures. A widespread wave-speed increase at 150 to 300 kilometers above the coremantle boundary is consistent with a transition from perovskite to postperovskite. Internal D″ stratification may be due to multiple phase-boundary crossings, and a deep wave-speed reduction may mark the base of a postperovskite lens about 2300 kilometers wide and 250 kilometers thick. The core-mantle boundary temperature is estimated at 3950 ± 200 kelvin. Beneath Central America, a site of deep subduction, the D″ is relatively cold (ΔT = 700 ± 100 kelvin). Accounting for a factor-of-two uncertainty in thermal conductivity, core heat flux is 80 to 160 milliwatts per square meter (mW m–2) into the coldest D″ region and 35 to 70 mW m–2 away from it. Combined with estimates from the central Pacific, this suggests a global average of 50 to 100 mW m–2 and a total heat loss of 7.5 to 15 terawatts.

Joint seismic tomography for bulk sound and shear wave speed in the Earth's mantle

High-quality P and S travel times are now available from careful reprocessing of data reported to international agencies. A restricted data set has been extracted for which comparable ray coverage is achieved for P and S, and used for a joint inversion to produce a three-dimensional model for shear and bulk sound velocities represented in terms of 2° × 2° cells and 18 layers in depth through the mantle. About 106 times for each of P and S are combined to produce 312,549 summary rays for each wave type. Linearizing about the ak135 reference model, 583,200 coupled tomographic equations are solved using an iterative partitioned scheme. Clear high-resolution images are obtained for both bulk-sound speed and shear wavespeed. The bulk and shear moduli have differing sensitivity to temperature and mineral composition, and so the images of the two velocity distributions help to constrain the nature of the processes which produce the variations. Different heterogeneity regimes can be recognised in the upper mantle, the transition zone, most of the lower mantle, and the lowermost mantle. In the upper mantle, many features can be explained by thermal effects; but in some orogenic zones (e.g. western North America), the opposite sense of the bulk-sound and shear wave speed variation requires compositional effects or volatiles to outweigh any thermal effects. In the lower mantle, pronounced narrow structures which may represent remnant subduction are most marked in shear. The level of large-scale variations in bulk sound speed compared to shear diminishes with depth in the lower mantle reaching a minimum near 2000 km. Below this depth, the variability of both wave speeds increases. Near the core-mantle boundary the variations of the two wave speeds show little concordance, suggesting the presence of widespread chemical heterogeneity.

Geodynamics of the southeastern Tibetan Plateau from seismic anisotropy and geodesy

Ongoing plate convergence between India and Eurasia provides a natural laboratory for studying the dynamics of continental collision, a fi rst-order process in the evolution of continents, regional climate, and natural hazards. In southeastern Tibet, the fast directions of seismic anisotropy determined using shear-wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld. These observations are consistent with a clockwise rotation of material around the eastern Himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern Tibet. At the southeastern edge of the Tibetan Plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent E-W direction and a clockwise rotation of the surface velocity, surface strain fi eld, and fault network toward Burma. Around the eastern Himalayan syntaxis, the coincidence between structural crustal features, surface strain, and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust-mantle interface and that the lower crust is suffi ciently strong to transmit stress. At the southeastern margin of the plateau in Yunnan province, a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation. Lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the Himalayan orogen and Tibetan Plateau and require the development of three-dimensional models that incorporate lateral heterogeneity.

Comparing P and S wave heterogeneity in the mantle

From the reprocessed data set of Engdahl and co-workers we have carefully selected matching P and S data for tomographic imaging. We assess data and model error and conclude that our S model uncertainty is twice that of the P model. We account for this in our comparison of the perturbations in P and S-wavespeed. In accord with previous studies we find that P and S perturbations are positively correlated at all depths. However, in the deep mantle systematic differences occur between regions that have undergone subduction in the last 120 million years and those that have not. In particular, below 1500 km depth ∂ln Vs/∂ln Vp is significantly larger in mantle regions away from subduction than in mantle beneath convergent margins. This inference is substantiated by wavespeed analyses with random realizations of the slab/non-slab distribution. Through much of the mantle there is no significant correlation between bulk sound and S-wave perturbations, but they appear to be negatively correlated between 1700 and 2100 km depth, which is also where the largest differences in ∂ln Vs/∂ln Vp occur. This finding supports convection models with compositional heterogeneity in the lowermost mantle.

Imaging of subducted lithosphere beneath South America

Tomographic images are produced for the deep structure of the Andean subduction zone beneath western South America. The data used in the imaging are the delay times of P, pP and pwP phases from relocated teleseismic earthquakes in the region. Regionally, structural features larger than about 150 km are resolved by the data. Presentations of layer anomaly maps and cross sections reveal: (1) The Nazca slab is probably continuous laterally and at depth over most regions; (2) The offset between the north and south deep earthquake zones, containing the 1994 deep Bolivia main shock and its aftershocks, can be modelled by a northwest striking and steeply northeast dipping slab structure; and (3) The Nazca slab clearly penetrates the lower mantle beneath central South America, but is partly deflected in the southern deep zone.

Seismic Imaging of Transition Zone Discontinuities Suggests Hot Mantle West of Hawaii

Hot material upwelling from deep below Hawaii may pool up far below and to the west of the islands. The Hawaiian hotspot is often attributed to hot material rising from depth in the mantle, but efforts to detect a thermal plume seismically have been inconclusive. To investigate pertinent thermal anomalies, we imaged with inverse scattering of SS waves the depths to seismic discontinuities below the Central Pacific, which we explain with olivine and garnet transitions in a pyrolitic mantle. The presence of an 800- to 2000-kilometer-wide thermal anomaly (ΔTmax ~300 to 400 kelvin) deep in the transition zone west of Hawaii suggests that hot material does not rise from the lower mantle through a narrow vertical plume but accumulates near the base of the transition zone before being entrained in flow toward Hawaii and, perhaps, other islands. This implies that geochemical trends in Hawaiian lavas cannot constrain lower mantle domains directly.

Seismic imaging with the generalized Radon transform: a curvelet transform perspective*

A key challenge in the seismic imaging of reflectors using surface reflection data is the subsurface illumination produced by a given data set and for a given complexity of the background model (of wave speeds). The imaging is described here by the generalized Radon transform. To address the illumination challenge and enable (accurate) local parameter estimation, we develop a method for partial reconstruction. We make use of the curvelet transform, the structure of the associated matrix representation of the generalized Radon transform, which needs to be extended in the presence of caustics and phase linearization. We pair an image target with partial waveform reflection data, and develop a way to solve the matrix normal equations that connect their curvelet coefficients via diagonal approximation. Moreover, we develop an approximation, reminiscent of Gaussian beams, for the computation of the generalized Radon transform matrix elements only making use of multiplications and convolutions, given the underlying ray geometry; this leads to computational efficiency. Throughout, we exploit the (wave number) multi-scale features of the dyadic parabolic decomposition underlying the curvelet transform and establish approximations that are accurate for sufficiently fine scales. The analysis we develop here has its roots in and represents a unified framework for (double) beamforming and beam-stack imaging, parsimonious pre-stack Kirchhoff migration, pre-stack plane-wave (Kirchhoff) migration and delayed-shot pre-stack migration.

Correlation between the shear-speed structure and thickness of the mantle transition zone

The 410 and 660 km seismic discontinuities that bound the mantle transition zone (TZ) are attributed to phase transformations in olivine structure. This implies that variations in TZ thickness ( HTZ) should correlate with those in TZ temperature. Pertinent seismic evidence has so far been ambiguous, however. We measure converted-wave (P ds) differential times tdiff = tP 660s − tP 410s in SE Asia and Australia and compare them with S-velocity (βTZ) estimates from regional tomographic models. Both tdiff and βTZ vary on a scale of a few hundred kilometers. Inferred variations in HTZ are up to ±30 km over length scales larger than 500 km, implying ±200 K thermal heterogeneity if the effect of composition can be neglected. tdiff and βTZ correlate strongly; the linear dependence of HTZ on the average temperature within the TZ is consistent with olivine Clapeyron slopes. We also show that this relationship holds on a global-scale as well, provided that the scalelengths and uncertainties of the variations in tdiff and βTZ are taken into account. These results confirm that the transformations in olivine structure give rise to the 410 and 660 km discontinuities globally.