Aibing Li

Array Analysis of Two‐Dimensional Variations in Surface Wave Phase Velocity and Azimuthal Anisotropy in the Presence of Multipathing Interference

Multipath propagation of surface waves introduces distortions in waveforms that can bias array measurements of phase velocities. We present a method for array analysis of laterally and azimuthally varying phase velocities that represents the incoming wavefield from each earthquake as the sum of two interfering plane waves. This simple approximation successfully represents the amplitude and phase variations for most earthquakes recorded in the MELT Experiment on the East Pacific Rise in the period range from 16 to 67 s. The inversion for velocities automatically reduces the importance of data from earthquakes or periods that are not described well by this approximation. Each iteration in the inversion involves two stages: a simulated annealing inversion for the best wave parameter description of each event, and a linearized inversion for velocities and changes in the wave parameters. At 29 s period, the two-plane-wave solutions indicate that nearly every signal is significantly affected by multipathing. The larger of the two plane waves typically has an apparent azimuth of propagation that is within a few degrees of the great circle path. The smaller wave is more scattered, differing in apparent azimuth from the larger wave by an average of about 13° at 29 s. Both lateral and azimuthal variations in Rayleigh wave phase velocity in the study area are significant, although it is possible to trade off azimuthal anisotropy with rapid and probably unrealistic lateral variations in velocity. Apparent azimuthal anisotropy reaches 5 to 6%, with the fast direction approximately perpendicular to the ridge.

Mantle discontinuities and temperature under the North American continental keel

A ubiquitous feature of upper-mantle seismic velocity models has been the presence of high-velocity ‘keels’ beneath stable continental interiors. Uncertainty remains, however, regarding the maximum depth to which continental keels extend, the degree to which they have cooled the mantle that surrounds them and their role in mantle flow. Here we investigate thermal anomalies across the eastern margin of the North American continental keel by imaging the seismic discontinuities at depths of 410 and 660 km with compressional-to-shear converted waves recorded by a 1,500-km-long seismometer deployment in the eastern United States. The thickness of the transition zone (the region nominally between depths of 410 and 660 km) and the depth to the ‘410-km’ discontinuity indicate that cold keel material and sub-keel downwellings must be largely confined to the upper mantle and may impinge on the transition zone only in localized regions and with thermal anomalies of less than ∼150 K. A 20-km depression of the ‘660-km’ discontinuity to the south of the westernmost stations coincides with a region of fast velocity in the deep transition zone and may be associated with the remnants of the subducted Farallon plate,,.

Azimuthal anisotropy and phase velocity beneath Iceland: implication for plume^ridge interaction

Abstract We have determined the pattern of azimuthal anisotropy beneath Iceland from shear-wave splitting and Rayleigh wave tomography using seismic data recorded during the ICEMELT and HOTSPOT experiments. The fast directions of shear-wave splitting are roughly N–S in western Iceland and NNW–SSE in eastern Iceland. In western Iceland azimuthal variations in Rayleigh wave phase velocity show that fast directions are close to the plate spreading direction at short periods of 25–40 s and roughly parallel to the Mid-Atlantic Ridge at longer periods of 50–67 s. Beneath the rift zones in central Iceland, we find a ridge-parallel fast direction at periods of 25–40 s and significantly weaker azimuthal anisotropy at periods of 50–67 s. The 2-D variation of isotropic phase velocity at periods of 33–67 s indicates that the lowest velocities are beneath the rift zones in central Iceland rather than above the plume conduit in southeast Iceland. While ridge-parallel alignment of melt films might contribute to anisotropy above 50 km depth beneath the rift zones, the overall observations are consistent with a model in which plume-influenced, hot, buoyant mantle rises beneath the Mid-Atlantic Ridge at depths greater than 50 km and is preferentially channeled along the ridge axis at the base of the lithosphere beneath Iceland. The shear-wave splitting results are attributed primarily to a N–S mantle flow at depths greater than 100 km.

Upper mantle structure beneath the Caribbean‐South American plate boundary from surface wave tomography

[1] We have measured shear wave velocity structure of the crust and upper mantle of the Caribbean-South American boundary region by analysis of fundamental mode Rayleigh waves in the 20- to 100-s period band recorded at the BOLIVAR/GEODINOS stations from 2003 to 2005. The model shows lateral variations that primarily correspond to tectonic provinces and boundaries. A clear linear velocity change parallels the plate bounding dextral strike-slip fault system along the northern coast of Venezuela, illustrating the differences between the South American continental lithosphere, the Venezuelan archipelago, and the Caribbean oceanic lithosphere. At depths up to 120 km beneath the Venezuelan Andes and the Maracaibo block, there is evidence of underthrusting of the Caribbean plate, but there is no other evidence of subduction of the Caribbean plate beneath the South American plate. In eastern Venezuela, linear crustal low velocities are associated with the fold and thrust belts whereas as higher crustal velocities are imaged in the Guayana shield lithosphere. The subducting oceanic part of the South American plate is imaged beneath the Antilles arc. The surface wave images combined with seismicity data suggest shear tearing of the oceanic lithosphere away from the buoyant continental South American plate offshore of northeastern Venezuela. The continental lithosphere south of the slab tear is bent down toward the plate boundary in response to the propagating tear in the lithosphere. We interpret a nearly vertical low-velocity ‘‘column’’ west of the tear centered beneath the Cariaco Basin, with three-dimensional asthenospheric flow around the southern edge of the subducting oceanic lithosphere, with the asthenosphere escaping from beneath continental South America and rising into the plate boundary zone. The complex plate boundary structure is best examined in three dimensions. We discuss the new surface wave tomographic inversion in the context of results from other researchers including local seismicity, teleseismic shear wave splits, and interpretations from active source profiling.

The distribution of the mid‐to‐lower crustal low‐velocity zone beneath the northeastern Tibetan Plateau revealed from ambient noise tomography

We collected continuous seismic data recorded between 2007 and 2010 by 208 broadband stations from the Chinese Provincial Digital Seismic Networks, A Seismic Collaborative Experiment of Northern Tibet, and the Northeastern Tibet Seismic experiment. Cross correlations of vertical component records are computed to extract the Rayleigh wave empirical Greens functions. Group and phase velocities are then constructed from the empirical Greens functions in 8 to 50 s period. At periods ≤25 s, more than 10% lower velocities are imaged beneath the Qaidam Basin, and high velocities are observed beneath the nonbasin regions. At periods ≥30 s, up to 10% lower velocities are imaged in the Qiangtang and Songpan-Ganze Terranes. From these group and phase velocity maps, a three-dimensional (3-D) Vsv model of the crust is derived. The model shows that the Qiangtang and Songpan-Ganze Terranes have a very thick crust with a prominent low-velocity zone (LVZ) in the middle crust. The LVZ thins out in the vicinity of the eastern Kunlun Mountains, providing a new constraint on the mode of deformation across the Tibetan Plateau. The northwestern Qilian Orogen, where receiver functions reveal a Moho deeper than the surrounding areas, also features a relatively weak midcrustal LVZ, which we interpret as an intracrustal response associated with the shortening between the North China Craton and the Tibetan Plateau.

Evidence for shallow isostatic compensation of the southern Rocky Mountains from Rayleigh wave tomography

In the past decade, seismologists have found that the topography in Colorado correlates with seismic velocities in the upper mantle, not with crustal thickness, and postulated that isostatic compensation of the southern Rocky Mountains takes place largely in the mantle. To test the validity of this hypothesis, we use Rayleigh wave phase and amplitude data recorded in the Rocky Mountain Front PASSCAL Experiment to obtain crustal thickness and shear-wave structure in the crust and upper mantle. The thickest crust in the model is beneath one of the two most elevated regions, the San Juan volcanic field. The other most elevated region in central Colorado, the Sawatch Range, is underlain by crust of anomalously low velocity. The seismically defined mantle lithosphere is thicker beneath the Great Plains than beneath the Colorado Plateau and is largely absent beneath the southern Rocky Mountains, but the correlation of topography with mantle velocities is weaker than the correlation with crustal anomalies. We construct a model that matches the observed Bouguer gravity anomaly primarily with variations in crustal thickness and density by assuming that density anomalies are proportional to the observed velocity anomalies. The anomalous mantle may also contribute to maintaining regional isostasy in Colorado, but its contribution may be much less important than the crust.

Rayleigh Wave Constraints on Shear-Wave Structure and Azimuthal Anisotropy Beneath the Colorado Rocky Mountains

We inverted Rayleigh wave data recorded in the Rocky Mountain Front Broadband Seismic Experiment for shear-wave velocity structure and azimuthal anisotropy. Distinctive structures are imaged beneath the southern Rocky Mountains, the western Great Plains, and the eastern Colorado Plateau. Beneath the southern Rockies, shear velocities are anomalously low from the Moho to depths of 150 km or more, suggesting replacement or delamination of the mantle lithosphere. The lowest velocities are beneath the extension of the Rio Grande rift into southern Colorado and are probably associated with partial melt. Beneath the Colorado Plateau, a thin, high-velocity lid is underlain by a low velocity layer to a depth of at least 160 km. Under the high plains, the velocities are above average down to ∼150 km depth, but not as fast as beneath the cratonic core of the continent. A crustal, low-velocity anomaly is observed beneath the high elevations of central Colorado. Elsewhere, inferred crustal thickness correlates with elevation, with the thickest crust beneath the San Juan Mountains in southwestern Colorado. These crustal anomalies suggest that much of the isostatic compensation for the high topography takes place within the crust. We observe a simple pattern of azimuthal anisotropy in the Rocky Mountain region with fast directions rotated slightly counterclockwise from the absolute plate motion of the North America plate and strength increasing with period. The observed anisotropy can be explained by deep asthenospheric flow dominated by current plate motion and shallower and perhaps laterally variable anisotropy in the upper lithosphere.

SLICING INTO THE EARTH

Regional arrays of seismometers provide a powerful means of mapping the details of deep-Earth structure. Our understanding of the geological processes at work within our planet depends on our ability to examine them; seismic techniques remain the best tool available. However, spatial aliasing due to the less-than-optimal distribution of global seismometers has long made it difficult to determine deep-Earth structure from teleseismic waves. The temporary deployment of portable broadband seismometers can help by providing high-resolution windows into the Earth. Patterns of global mantle convection create seismically observable features such as anisotropy at the top and bottom of the mantle, topography of upper mantle discontinuities, and heterogeneous structure at the core-mantle boundary.

Crustal shear wave velocity and radial anisotropy beneath the Rio Grande rift from ambient noise tomography

Shear wave velocity and radial anisotropy beneath New Mexico are obtained from ambient seismic noise tomography using data from the Transportable Array. Besides the distinct seismic structure imaged across the Rio Grande rift from the Colorado Plateau to the Great Plains, both velocity and anisotropy models also reveal significant variations along the rift. The rift at Albuquerque is characterized by remarkably low velocity in the shallow crust, high velocity and strong positive anisotropy in the middle and lower crust, and low velocity in the upper mantle. These observations can be interpreted as magma accumulation in the shallow crust and significant mafic underplating in the lower crust with abundant melt supply from the hot mantle. We propose that the Albuquerque region has recently been experiencing the most vigorous extensional deformation in the rift. Positive anisotropy with Vsh > Vsv appears in the central and southern rifts with a stronger anisotropy beneath younger volcanoes, reflecting layering of magma intrusion due to past and recent rifting activities. The low velocities in the uppermost mantle are observed under high-elevation places, the Jemez Lineament, northern rift, and east rift boundary, implying that the buoyancy of hot mantle largely compensates the local high topography. Low mantle velocities appear at the boundary of the southern rift, corresponding to the large lithosphere thickness change, instead of the rift center, consistent with the prediction from the small-scale, edge-driven mantle convection model. We conclude that the edge-driven upper mantle convection is probably the dominant mechanism for the recent and current rifting and uplift in the Rio Grande rift.

Destruction of the Wyoming craton: Seismic evidence and geodynamic processes

Cratons are old and strong continental cores where the lithosphere is thick and remains largely undeformed for 2–3 b.y. Unlike typical cratons, the Wyoming craton underwent pervasive deformation ca. 80–55 Ma during the Laramide orogeny in the west-central United States, and has been subsequently encroached upon by the Yellowstone hotspot since 2.0 Ma. However, the mechanism for the deformation and the craton-hotspot interaction are not well understood. We present here a three-dimensional shear wave velocity model beneath the Wyoming craton constrained from Rayleigh wave data, which reveal new details about the cratonic lithosphere. The average lithosphere thickness beneath the craton is ∼150 km, significantly thinner than a normal cratonic root (>200 km). Continuous low velocities are observed beneath the Yellowstone hotspot and the Cheyenne belt. A low-velocity column is also present in the central-eastern craton at depths of 115–250 km. These low velocities can be explained by hot temperature and partial melting, implying mantle upwelling. A high-velocity anomaly with a dripping shape in central Wyoming extends to 200–250 km depth, indicating mantle downwelling and lithosphere erosion. Our model provides the first seismic evidence for complex small-scale mantle convection beneath the Wyoming craton. The convection probably developed during the subduction of the Farallon plate and has been reinforced by the Yellowstone hotspot. We propose that the combination of flat-slab subduction, small-scale convection, and hotspot activity can lead to massive destruction of a cratonic lithosphere.