Fast Anomalies Research Articles

Shear wave speeds at the base of the mantle

We inverted 4864 ScS‐S and 1671 Sdiff‐SKS residual travel times for shear wave speed anomalies at the base of the Earth's mantle. We applied ellipticity corrections, accounted for mantle structure outside of the basal layer using mantle tomography models, and used finite size sensitivity kernels. The basal layer thickness was set to 290 km; however, the data allow thicknesses between 200 and 500 km. The residuals were inverted using a spherical harmonic basis set of degree 30 for a model that both is smooth and has a small Euclidean norm, which limited spectral leakage of higher‐order structures into low‐order wavelengths. Hotspots dominantly overlay slow wave speed regions. Nonsightings of ultralow‐velocity zones (ULVZs) most frequently appear in fast regions, suggesting that slow regions at the base of the mantle are associated with ULVZs. However, ULVZ sightings appear in both slow and fast regions. Recently active subduction zones do not correlate with velocity anomalies; however, the locations of subduction zones active prior to 90 Ma correlate extremely well with fast anomalies, implying that slabs descend as fast as 2 cm yr−1 through the lower mantle. The correlation continues through the historical subduction record to 180 Ma, suggesting that slabs remain in the deep mantle at least 90 Myr. Fast anomalies reach +2%, while slow anomalies extend to −5%. If we assume that the anomalies are thermal and anharmonic in origin and apply a wave speed/thermal anomaly conversion, the temperature deviations would be over −500°K (cold) in fastest regions and over +1000°K (hot) in the slowest regions, which would initiate plumes much hotter than those observed at the surface. Alternative explanations for the large anomalies are widespread partial melt or compositional differences in the lowermost mantle.

Can lower mantle slab‐like seismic anomalies be explained by thermal coupling between the upper and lower mantles?

Below subduction zones, high resolution seismic tomographic models resolve fast anomalies that often extend into the deep lower mantle. These anomalies are generally interpreted as slabs penetrating through the 660‐km seismic discontinuity, evidence in support of whole‐mantle convection. However, thermal coupling between two flow systems separated by an impermeable interface might provide an alternative explanation of the tomographic results. We have tested this hypothesis within the context of an axisymmetric model of mantle convection in which an impermeable boundary is imposed at a depth of 660 km. When an increase in viscosity alone is imposed across the impermeable interface, our results demonstrate the dominant role of mechanical coupling between shells, producing lower mantle upwellings (downwellings) below upper mantle downwellings (upwellings). However, we find that the effect of mechanical coupling can be significantly weakened if a narrow low viscosity zone exists beneath the 660‐km discontinuity. In such a case, both thermally induced ‘slabs’ in the lower mantle and thermally activated plumes that rise from the upper/lower mantle boundary are observed even though mass transfer between the shells does not exist.

The free‐air gravity constraint on subcontinental mantle dynamics

An outstanding geophysical issue concerns the nature, and dynamical role in the mantle general circulation, of the seismically fast body wave anomalies that have been tomographically imaged beneath coatinents. In this paper, we investigate the possibilities that these seismologically imaged “roots” represent either neutrally buoyant, chemically distinct material or cold, negatively buoyant, upper mantle and transition zone downwelling flow. In assessing these alternatives, we first construct disaggregated models of the seismic heterogeneity in which a component associated with subcontinental fast anomalies is isolated from the global tomographic models either by employing the “continent function” or a new “craton function”. We find that the use of the new craton function leads to geophysically more realistic chemical models of subcontinental heterogeneity. The thermal and chemical density fields reconstructed from the disaggregated tomographic models are employed to compute the long‐wavelength nonhydrostatic geoid, the free‐air gravity field and the upper mantle radial flow pattern within the framework of an anelastically compressible internal loading theory. We find that the radial component of flow velocity provides useful insight into the dynamical implications of the alternative density models. However, since this field is not directly observable, we consider the geoid and free‐air gravity anomaly as possible diagnostic discriminants and show that the free‐air gravity anomaly provides a sensitive discriminant of the gravitational differences that characterize the chemical and thermal models, whereas the geoid does not. By focusing on the free‐air gravity low over the Hudson Bay region of Canada, we are able to rule out the hypothesis that positively or neutrally buoyant subcontinental material that is chemically distinct from the surrounding mantle exists below the Laurentian craton. However, when the fast body wave anomaly is mapped into a high‐density downwelling flow beneath this region, we are able to fully explain the fraction of the anomalous free‐air gravity low which is inexplicable as a contribution associated with the existing degree of glacial isostatic disequilibrium due to the disintegration of the Laurentide ice sheet. This conclusion concerning the North American craton may be equally valid for other continental nuclei. We explore the general tectonophysical implications of this dynamical model.

Anisotropy in central part of inner core

The anisotropy in the central part of the inner core has been difficult to study because of the paucity of PKP data at antipodal distances, especially for those paths running nearly north‐south (polar paths). In this paper, we systematically search through the International Seismological Center (ISC) picks of PKIKP travel times for polar paths at near‐antipodal distances reported recently by Su and Dziewonski [1995]. Corresponding seismograms were obtained from the Worldwide Standardized Seismographs Network (WWSSN) film chip library at Lamont‐Doherty Earth Observatory. By using seismograms, we can impose quality control on the data, measure differential times (AB‐DF in this case) by cross correlation and use long‐period waveforms if available. Nonpolar paths at near‐antipodal distances are assembled as well, to address biases from lateral variation in the lowermost mantle on AB‐DF times. Our results show large AB‐DF residuals of 2 to 8 s for the polar paths at near‐antipodal distances. Moreover, the absolute DF travel times for the polar paths exhibit fast anomalies of the same magnitude relative to nonpolar paths, while AB times show no obvious difference between polar and nonpolar paths. These observations are consistent with an axisymmetric anisotropy of ∼3% around the spin axis throughout the bulk of the inner core, with the possibility of even stronger anisotropy near Earth's center. This degree of anisotropy is still greater than what the most recent models based on normal modes would indicate.

Global mapping of upper mantle reflectors from long-period SS precursors

SUMMARY Long-period precursors to SS resulting from underside reflections off upper mantle discontinuities (SdS where d is the discontinuity depth) can be used to map the global distribution and depth of these reflectors. We analyse 5,884 long-period seismograms from the Global Digital Seismograph Network (1976-1987, shallow sources, transverse component) in order to identify SdS arrivals. Corrections for velocity dispersion, topography and crustal thickness at the SS bounce point, and lateral variation in mantle velocity are critical for obtaining accurate estimates of discontinuity depths. The 410 and 660 km discontinuities are observed at average depths of 413 and 653 km, and exhibit large-scale coherent patterns of topography with depth variations up to 40 km. These patterns are roughly correlated with recent tomographic models, with fast anomalies in the transition zone associated with highs in the 410 km discontinuity and lows in the 660 km discontinuity, a result consistent with laboratory measurements of Clapeyron slopes for the appropriate phase changes. The best resolved feature in these maps is a trough in the 660km discontinuity in the northwest Pacific, which appears to be associated with the subduction zones in this region. Amplitude variations in SdS arrivals are not correlated with discontinuity depths and probably result from focusing and defocusing effects along the ray paths. The SdS arrivals suggest the presence of regional reflectors in the upper mantle above 400 km. However, only the strongest of these features are above probable noise levels due to sampling inadequacies.

The three‐dimensional structure of Kilauea Volcano, Hawaii, from travel time tomography

A linear, travel time tomography study of the most active shield volcano of the world, Kilauea Volcano, Hawaii, was undertaken to determine the lateral heterogeneities produced by its intricate magmatic and tectonic environment. Kilauea provides an ideal setting to do tomography because of its dense seismograph array and many local earthquakes that allow excellent ray coverage of complex subsurface features. Local P wave data from ∼ 12,295 events were inverted using a one‐dimensional layered velocity model. Inversions were done for two cell sizes (5×5×5 km and 1×1×1 km) to resolve structural regions on different length scales. This study provided a view of the average velocity variations relative to a one‐dimensional velocity model. Analysis and interpretation of the tomographic images allowed us to infer the following model. The main shallow magma reservoir is delineated by a slow velocity region southeast of the summit from 0 to 2 km depth. There is a distinct high velocity region centered northwest of the summit from 0 to 2 km depth that represents a cap of dense, intrusive dikes surrounding the magma chamber. We suggest that the shallow reservoir is a narrow, compartmentalized region of sills and dikes, centered just south‐southeast of Halemaumau caldera. Below the main reservoir, the summit is imaged as a slightly fast region from 5 to 10 km in the coarse model indicating that the main conduit is structurally defined by an intrusive dike complex until about 10 km. The rift zones of Kilauea are imaged as major, high velocity entities, widening to the south with depth until 6 km. These fast anomalies are related to the sheeted dike complexes along the rifts. On a finer scale, slow anomalies suggest the presence of magma pockets centered at 0–2 km depth beneath Mauna Ulu, Makaopuhi and Puu Oo, along the east rift zone (ERZ). Two significant high velocity regions along the lower ERZ near Kalalua and Kaliu are inferred to represent intrusive barriers to magma injection along the shallow (0–4 km) ERZ conduit. The southwest rift zone may have an intrusive barrier related to a high velocity region just southwest of Mauna Iki. The Hilina and Kaoiki fault zones are imaged as slow features at shallow depths (< 5 km) related to the open fractures and scarps along the normal faults. The Koae fault system is imaged as a slightly fast shallow structure (< 6 km) possibly related to intrusive diking from the adjacent rift zones that fill and may even induce the extensional structures associated with this complex fault zone. Continued inversions with the immense amount of seismic data collected for Hawaiian events will allow the detailed development of a three‐dimensional structural model for Kilauea. Such a model will be extremely useful to seismologists and petrologists alike for understanding the tectonic growth and magmatic evolution of this dynamic shield volcano.

Tomographic inversions for mantle P wave velocity structure based on the minimization of l2 and l1 norms of International Seismological Centre Travel Time Residuals

We use International Seismological Centre (ISC) P arrival data (1964–1987) and iterative algorithms which minimize the l1 and l2 residual norms to solve simultaneously for three‐dimensional P velocity variations in Earth's mantle, source mislocations, and station corrections. We find that the maximum velocity perturbations produced by the l1 minimization (approximately ±4% in our final model) are relatively insensitive to smoothing and damping constraints. Therefore, using an l1 norm criterion allows us to keep the bias introduced to the inversion to a minimum. Among the well‐resolved features contained in both the l1 and the l2 velocity models are a fast anomaly in the lower mantle beneath the Tonga‐New Hebrides subduction zone to a depth of 1670 km and another fast anomaly beneath the Japanese island arc and eastern Asia. Continuity between these anomalies and shallower fast anomalies is not clear. A fast anomaly extending from 670 km to 2070 km depth appears beneath eastern North America, the Caribbean, and north central South America. A broad, fast anomaly appears beneath eastern Asia just above the core‐mantle boundary as well as several slow anomalies under the Pacific basin of comparable size. Both models contain a circum‐Pacific ring of 2% lower velocities in the depth range 0–200 km, associated with back arc basins. High velocities (over 2%) associated with the continental shields tend to disappear below 400 km, though a significant region of high velocity remains beneath the Siberian platform in the 400 to 670 km depth interval.

P and S wave travel time inversions for subducting slab under the island arcs of the northwest Pacific

We have observed slablike high P and S velocity anomalies around the Wadati‐Benioff zone under island arcs of the northwest Pacific through travel time tomographic inversions. Nineteen years of International Seismological Centre travel time residuals for events and stations in this large region are used. Analyses of resolution and noise show that the images are generally well resolved. The images illustrate that slab anomalies are continuous along strike in most parts of the upper mantle of the region and become contorted and generally broadened with depth. Near the bottom of the upper mantle, fingering of the slabs, including segmenting and spreading, is indicated. The fast anomalies associated with the Japan, Izu‐Bonin, and Mariana subduction zones tend to flatten to subhorizontal at depth, while downward spreading may occur under parts of the Mariana and Kurile arcs. The fast anomalies below 700 km are not in the shape of a single coherent sheet. The principal compressional axes of focal mechanisms in the region consistently follow the downdip direction of the high‐velocity slab, even when it bends to subhorizontal at depth. The depth at which compression begins to dominate the downdip stress regime in the slab apparently depends on bending of the slab and its dip. Slab fingering and intense deep seismicity probably are the consequence of the slab encountering a barrier of some form around the “670‐km” discontinuity.

Whole mantle P-wave travel time tomography

A method of tomographic inversion to obtain three-dimensional velocity perturbations in the Earth's whole mantle has been developed, and applied to more than two million P-wave arrival time data reported by International Seismology Center (ISC). The model is parameterized with 32,768 blocks; the divisions in latitude, longitude, and radius are 32, 64, and 16, respectively. Horizontal cell size is 5.6° × 5.6°. The layer thicknesses vary with depth; 29 km just below the surface and 334 km just above the core-mantle boundary. Starting from a spherically symmetric Earth model, we obtained a three-dimensional model using the following iterative procedures. First, we relocated all the events; second, we backprojected the residual into the whole mantle; third, we refined the spherically symmetric Earth model. The solutions have been converged in five iterations. We adopted the following techniques in the backprojection procedure. The first order smoothness was introduced as a damping, which makes the solution independent of the starting model and its apparent fluctuation minimal. The basic equations for delay times and smoothness are solved using the conjugate gradient method, an iterative method which guarantees the convergence of solution into the exact least squares solution. The weight on the smoothness, i.e., the damping factor, was objectively determined by a simplified cross validation technique. The final solution was obtained as an average of the ten solutions, each of which was derived from one tenth of the total data set. The reliability of the solution is examined in two ways: (1) mapping the resolution given by the reconstruction of checkerboard patterns, and (2) mapping the variance given from the gaussian noise input. This is the first result which delineates the whole mantle structure with its reliability mapping. The longest wavelength anomalies of the lower mantle are similar to those of previous studies by Dziewonski (1984) and Hager and Clayton (1989). However, the shorter wavelength patterns have many discrepancies. The fast anomalies of the uppermost mantle beneath the shield and the slow anomalies beneath the active tectonic regions are clearly seen. We also revealed the l = 2 pattern at the transition zone reported by the previous surface wave and free oscillation studies. The most striking feature is that of the fast anomalies lying along the extension of the large subducting plate up to depths of about 1000 km or more. The resolving powers in these regions are generally good.

Three dimensional reconstruction of Pangea

The three dimensional seismic velocity structure from the surface of the Earth to a depth of about 900 km was determined from fundamental and higher mode surface waves. Velocity anomalies near the surface correlate well with the age of the surface rocks. Reconstruction of the ancient continent Pangea connects all fast velocity anomalies near the center of Pangea. Fast velocity anomalies extend to 200‐400 km. Considering the resolving power of long period surface waves, this indicates that Pangea was a continent with a lithospheric thickness of about 200 (±100) km or larger, which broke up right in the middle (Central Atlantic). Examinations of seismic velocity sections with respect to depth show different depth penetration of fast velocity anomalies under various continents such as North America, South America, Eurasia and Africa. We speculate that continents possessed different thicknesses before Pangea was formed and had moved over the surface of the Earth independently.