Hua-wei Zhou

A high‐resolution P wave model for the top 1200 km of the mantle

This work attempts to map aspherical velocities in the top 1200 km of the mantle that are illuminated by the International Seismological Centre P wave data. The processing includes summary ray sorting and hypocentral redetermination. To cope with the extreme variation of ray coverage, a multicell inversion is used to simultaneously constrain anomalies in overlapping cells. The multicell solutions are summed after inversion to form the final model, named P1200, at cell size of 1° × 1° and about 50 km in thickness. The lateral resolution of the model is about 1°–2° in major subduction zones and about 5°–15° in most places. Major features in the model withstood tests on signal coherency and different reference models. The long-wavelength parts of the P1200 model correlate well with previous global models, especially the S12 model of shear wave. Among the large features in the transition zone and below, there are two fast bands, one from Canada to South America and another from Siberia, through East Asia and the Philippine Sea, to western Australia. There are also slow patches under the central Pacific and from India to eastern Africa. Though most subduction zones have slab-like high-velocity anomalies, they are sandwiched by broad slow velocities at shallow depths. Some of the slabs appear to penetrate the 660-km discontinuity, and stagnant slabs are also seen in the transition zone. Plume-like slow anomalies exist below some prominent hot spots like Hawaii, Iceland, Yellowstone, and French Polynesia.

Depth imaging with multiples

Depth imaging with multiples is a prestack depth migration method that uses multiples as the signal for more accurate boundary mapping and amplitude recovery. The idea is partially related to model‐based multiple‐suppression techniques and reverse‐time depth migration. Conventional reverse‐time migration uses the two‐way wave equation for the backward wave propagation of recorded seismic traces and ray tracing or the eikonal equation for the forward traveltime computation (the excitation‐time imaging principle). Consequently, reverse‐time migration differs little from most other one‐way wave equation or ray‐tracing migration methods which expect only primary reflection events. Because it is almost impossible to attenuate multiples without degrading primaries, there has been a compelling need to devise a tool to use multiples constructively in data processing rather than attempting to destroy them. Furthermore, multiples and other nonreflecting wave types can enhance boundary imaging and amplitude recovery...

Multiscale deformable-layer tomography

Traveltime tomography relies on broad ray-angle coverage to constrain the spatial location of velocity anomalies. When ray-angle coverage is narrow, cell or grid tomography can be plagued by smearing artifacts bearing imprints of the raypaths. Multiscale deformable-layer tomography (DLT), which inverts for the geometry of velocity interfaces instead of velocity, can be more effective than grid-based tomography in mitigating such artifacts, especially when velocity values are known for parts of the model. The DLT model consists of geologically sensible layers represented by triangular prisms. In areas of good ray coverage, DLT can be used to invert simultaneously for layer velocities as well as interface geometry. Tests of synthetic models of crosswell refraction, 2D tomostatics, and 3D vertical seismic profiling (VSP) first-arrival data sets show that DLT can produce solutions superior to those produced by cell- or grid-based tomography.

Mapping of P-wave slab anomalies beneath the Tonga, Kermadec and New Hebrides arcs

Abstract P-wave velocity anomalies of several per cent fast relative to the Jeffreys-Bullen (JB) model are seen around subduction zones under the Tonga, Kermadec and New Hebrides trenches from inversion of International Seismological Centre (ISC) travel-time residuals from intermediate and deep-focus earthquakes. Mantle heterogeneities outside the model are corrected using previous velocity models. Ray paths inside the model region are traced allowing two-dimensional velocity variations. The rays of similar source and receiver locations are combined into summary rays, and the hypocenters are relocated as part of the inversion iterations. The resulting images are interpreted in conjunction with resolution and error estimations. The slab-like anomalies are generally continuous in places of high seismicity, and decrease in amplitude with depth. Most coherent slab anomalies are surrounded by large slow patches between the surface and 350-km depth. The fast slab beneath the Tonga arc appears to end at ∼ 550-km depth, before the seismicity ends, but there is a fast band at 750–1000-km depths below the deepest Tongan earthquakes. The fast slab-like anomaly beneath the Kermadec arc tends to flatten to subhorizontal near the bottom of upper mantle, and it occurs aseismically near New Zealand down to 500–550-km depth. Interpretations in the lower mantle in many places, such as Kermadec and New Hebrides arcs, however, are hampered by poor resolution.

Slope of the geoid spectrum and constraints on mantle viscosity stratification

Spectral analysis of a recently obtained high resolution tomographic model, describing the top 1200 km of the mantle, shows a power-law dependence on the degree, for degrees greater than around 10. The spectrum of recent geoid models is also found to decay in a linear fashion with degree on a log-log plot. We have employed the logarithmic slope of the geoid between degrees 10 and 25 as a constraint on the viscosity structure of the top 1200 km of the mantle. The constraint of fitting the geoid slope represents a new and independent approach to the determination of the upper-mantle viscosity structure. From conducting over one million runs in a Monte Carlo inversion, we have found that there are basically three families of viscosity which can fit the geoid slope. They are (1) with a viscosity hill between 660 and 1000 km, (2) with a weak viscosity increase at 660 km, and (3) with a significant viscosity increase at depths between 820 and 1000 km. Below 1000 km the viscosity of the lower-mantle for all 3 families is larger than that in the upper mantle. These results corroborate the complexity of the mantle viscosity profile between 660 and 1200 km, which would have important ramifications on flows between the upper and lower mantle.

Boundary element modeling of nondissipative and dissipative waves

A boundary element (BE) algorithm is developed to compute acoustic or SH-waves in models consisting of limited or unlimited volumes and irregular interfaces. We solve the BE system in the frequency domain so that anelasticity can be easily represented by different viscoelastic models, such as the Kelvin-Voigt type. Three illustrative computations are shown. The waveform given by the BE method for a circular inclusion model agrees well with that given by the finite-difference (FD) method. Dissipation of waves at high frequency caused by the presence of multi-cracks in an elastic medium resembles the masking effect of anelasticity. The waveforms for nondissipative and dissipative models containing hexagonal inclusions illustrate some interesting characteristics of the composite media.

Blind Test of Methods for Obtaining 2-D Near-Surface Seismic Velocity Models from First-Arrival Traveltimes

ABSTRACT Seismic refraction methods are used in environmental and engineering studies to image the shallow subsurface. We present a blind test of inversion and tomographic refraction analysis methods using a synthetic first-arrival-time dataset that was made available to the community in 2010. The data are realistic in terms of the near-surface velocity model, shot-receiver geometry and the datas frequency and added noise. Fourteen estimated models were determined by ten participants using eight different inversion algorithms, with the true model unknown to the participants until it was revealed at a session at the 2011 SAGEEP meeting. The estimated models are generally consistent in terms of their large-scale features, demonstrating the robustness of refraction data inversion in general, and the eight inversion algorithms in particular. When compared to the true model, all of the estimated models contain a smooth expression of its two main features: a large offset in the bedrock and the top of a steeply...

Constrained deformable layer tomostatics

Although first-arrival tomography provides an effective waytoestimatenear-surfacevelocitiesandstaticcorrections, the undulation of velocity interfaces such as the base of the weatheredzonemaynotbeeasilydeterminedbythismethod. The main reason is that first arrivals are insensitive to small geometric changes in velocity interfaces because their raypaths tend to traverse along those interfaces. To improve the solution of interface geometry, we developed a deformable layer tomostatics method that approximates the near-surface velocity field as several layers of constant velocity and variablethicknessthatcanbeinvertedforthegeometryofthevelocityinterfaces.Weuseamultiscalemodelparameterization in the inversion for interface geometry. Synthetic and field datatestsshowedthatthemethodcandeterminetheinterface geometry. Constraining the depth range of the basal boundary of the weathered zone increases the convergence rate of the iterative inversion process. Tests on field data showed greater reflection coherency in a stacked section based on constrainedstaticcorrectionsthaninonefromunconstrained static corrections. The method yielded a better match with statics computed from sand-dune curves than does a match obtained by using two commercial grid tomography packages.

Joint VSP And Surface Seismic Tomography

VSP data provide depth control and increased resolution in subsurface imaging. However, VSP tomography suffers from problems of non-uniqueness due to lack of ray crossing. In order to resolve this problem, we suggest simultaneous use of VSP and surface seismic data. We used tomographic method, which relies on the shortest path ray tracing in blocky parameterized model. The analogue of the conjugate gradient method was implemented for the inversion.

Miocene rifting in the Los Angeles basin: Evidence from the Puente Hills half-graben, volcanic rocks, and P-wave tomography

Formation of the Puente Hills half-graben in the northeastern Los Angeles basin and eruption of the Glendora and El Modeno Volcanics (16–14 Ma) help to define the timing of extension in the basin. Normal faulting on the proto-Whittier fault ca. 14 Ma established the Puente Hills half-graben, in which sedimentary strata accumulated between ca. 14 and 10 Ma and into which diabase sills intruded. North-South contraction began to invert the Puente Hills half-graben ca. 7 Ma, leading to formation of the Puente Hills anticline and the Whittier fault. Our high-resolution three-dimensional P-wave velocity model shows two anomalous higher velocity (6.63 km/s) bodies at depths between 9 and 18 km, which we attribute to dioritic plutons named here for Whittier Narrows and El Modeno. The stocklike Whittier Narrows pluton could have been a source for the Glendora Volcanics and the diabase sills in the Puente Hills half-graben. The sill-shaped El Modeno pluton was a likely source for the El Modeno Volcanics. The northwesterly alignment of the plutons may mark the location of the northeastern Los Angeles basin rift boundary, which is associated with the clockwise rotation of the western Transverse Ranges. Three active faults, the Elysian Park blind thrust, the Puente Hills blind thrust, and the Whittier fault, converge on the Whittier Narrows pluton, which may have played a role in their location and segmentation.