Slow Anomalies Research Articles

Crustal heterogeneity in the 2000 western Tottori earthquake region: effect of fluids from slab dehydration

In order to understand the generating mechanism of the western Tottori earthquake ( M 7.3) that occurred on 6 October 2000 in southwest Japan, we determined high-resolution 3D P- and S-wave velocity and Poisson’s ratio structures in the epicentral area. We used 17,542 P- and 13,831 S-wave high-quality arrival times from 721 Tottori aftershocks and other local microearthquakes recorded by the dense High-Sensitivity Seismic Network (Hi-net) installed recently on the Japan Islands. Significant variations of up to 4% for seismic velocity and 9% for Poisson’s ratio are revealed in the aftershock area. Areas with large coseismic slips and high aftershock activity are associated with high P-wave velocity ( V P ), high Poisson’s ratio and high electrical conductivity, which may represent strong and competent parts of the fault zone containing fluids. The western Tottori mainshock hypocenter is located in a boundary zone where both velocity and Poisson’s ratio change drastically. Both P- and S-wave velocities are slower in the lower crust under the mainshock hypocenter, and low-frequency microearthquakes were detected within the slow anomalies around the Moho discontinuity before and after the occurrence of the 2000 western Tottori earthquake. These results and other related evidence suggest that the strong crustal heterogeneity in the western Tottori source area is associated with fluids and arc magma under nearby the Daisen volcano, which may have influenced the nucleation and rupture process of the western Tottori earthquake. The arc magma and fluids are thought to result from the dehydration reactions of the subducting Philippine Sea slab in this region. These results suggest that the nucleation of shallow earthquakes could be controlled by a deep structure and regime and so it is important to study the deeper crust and upper mantle structure and processes for clarifying the generating mechanism of large crustal earthquakes.

Global tomographic images of mantle plumes and subducting slabs: insight into deep Earth dynamics

A new model of whole mantle P-wave tomography is determined with a novel approach. A grid parameterization instead of blocks and spherical harmonics is adopted to express the Earth structure. Depth variations of the Moho, 410 and 660 km discontinuities are taken into account in the inversion. Ray paths and travel times are computed with an efficient 3-D ray tracing scheme. This new approach was applied to a large data set of ISC (International Seismological Center) travel times (P, PP, PcP, pP, Pdiff) to determine a whole mantle P-wave tomography. For the shallow mantle, the new model contains the general features observed in the previous models: a low-velocity ring around the Pacific Ocean basins and high-velocity anomalies under the old and stable continents in the depth range of 0–300 km. One significant difference from the previous models is that stronger and wider high-velocity anomalies are visible in the transition zone depths under the subduction regions, which suggests that most of the slab materials are stagnant in the transition zone before finally collapsing down to the lower mantle as a result of very large gravitational instability from phase transitions. Very slow anomalies exist in the upper mantle right beneath the Wudalianchi and Changbai active volcanoes in Eastern China, right above the stagnant Pacific slab in the transition zone, suggesting that the origin of the intraplate volcanism in East Asia is closely related to the Pacific plate subduction process, such as deep slab dehydration and convective circulation in the mantle wedge. Plume-like slow anomalies are clearly visible under the major hotspot regions in most parts of the mantle, in particular, under Hawaii, Iceland, South Pacific and Africa. The slow anomalies under South Pacific and Africa have lateral extensions of over one thousand kilometers and exist in the entire mantle, representing two superplumes. The Pacific superplume has a larger spatial extent and stronger slow anomalies than that of the Africa superplume. The Hawaiian plume is not part of the Pacific superplume. The slow anomalies under hotspots usually do not show a straight pillar shape, but exhibit winding images, suggesting that plumes are not fixed in the mantle but can be deflected by the mantle flow. As a consequence, hotspots are not really fixed but can wander on the Earth’s surface, as evidenced by the recent paleomagnetic and numeric modeling studies. Wider and more prominent slow anomalies are visible at the core–mantle boundary (CMB) than most of the lower mantle, and there is a good correlation between the distribution of slow anomalies at the CMB and that of hotspots on the surface, suggesting that most of the strong mantle plumes under the hotspots originate from the CMB. However, there are some small-scaled, weak plumes originating from the transition zone or mid mantle depths.

Upper mantle convection beneath the central Rio Grande rift imaged by P and S wave tomography

We present models for upper mantle P and S velocity structure beneath a southwestern United States transect extending from near the center of the Colorado Plateau across the Rio Grande rift to the Great Plains. The models were derived from travel times of compressional and shear seismic phases recorded by the La Ristra passive seismic array deployed from July 1999 to May 2001. Large variations in seismic velocity (up to 8% in S and 5% in P) are seen across the transect in the upper 200 km of the mantle. Seismically slow mantle underlies the Rio Grande rift and Jemez lineament and relatively slow mantle is seen beneath the Navajo volcanic field within the Colorado Plateau. The relative variations of P and S velocity imply that high temperatures are the primary cause of the slow mantle although a small amount of partial melting or hydration cannot be ruled out. Sharp boundaries in mantle seismic velocity are coincident with boundaries of Proterozoic structural trends implying that ancient lithospheric structure exerts a control on the tectonic and magmatic activity in the region. Weaker seismic variations are imaged below 200 km depth with three southeastward dipping structures in our images. Two of the structures have fast seismic anomalies, beneath the central Colorado Plateau and the Great Plains respectively, and the third has anomalously slow seismic wave speed. We interpret the western deep seismic anomaly to be foundering Farallon slab and the slow anomaly just to the east as upwelling mantle possibly associated with volatile release from the sinking Farallon slab. Beneath the Great Plains, there is also downwelling in the upper mantle. The combination of upwelling in the west and downwelling in the east are likely part of an upper mantle convection cell that may include entrained lithosphere from beneath the rift.

PKP travel times at near antipodal distances: implications for inner core anisotropy and lowermost mantle structure

Previous studies from PKP(AB–DF) differential travel times at large distances suggest that the central part of the inner core is very anisotropic. These differential times, however, can be affected greatly by strong heterogeneity in the lowermost mantle. Here we examine a unique data set of PKP travel times from global digital and analog stations at near antipodal distances, where the effects of both inner core anisotropy and mantle heterogeneity are the greatest. Our results show that the AB–DF residuals for the polar paths are consistently larger than those of the equatorial paths by over 3–4 standard deviations. We also measured DF and AB absolute times, and found that the DF residuals are negatively correlated with the AB–DF residuals while the AB residuals have a much weaker correlation with the AB–DF residuals. We compare several mantle models with the data. Our results suggest that the mantle structure can explain part of the residuals of the equatorials paths, but cannot explain the polar path anomalies. These results strongly suggest that most of the AB–DF anomalies for the polar paths are likely from the inner core anisotropy and not from mantle heterogeneity. Assuming a uniform cylindrical anisotropy model, the average inner core anisotropy amplitude is about 2.5%. On the other hand, equatorial paths from events in the west Pacific and recorded at South America show a steep azimuthal change in AB–DF times (about 4 s over a 60° azimuthal range). The sharp change is well predicted by existing P and S tomographic models: the azimuthal change occurs as the AB paths sweep across the great slow anomaly in the central Pacific. The high correlation between the P and S velocities suggests that the central Pacific slow anomaly may be of thermal origin.

Seismic structure and origin of hotspots and mantle plumes

A new global tomography approach has been used to study the deep structure and dynamics of hotspots and mantle plumes. In this approach, depth variations of the Moho, 410 and 660 km discontinuities are considered, the Earth structure is expressed by seismic velocities at grid nodes instead of blocks, and ray paths and travel times are calculated with a 3-D ray tracing technique. Prominent plume-like slow anomalies are detected under major hotspot regions from the crust down to the core–mantle boundary (CMB). Hotspots in Hawaii, Iceland, South Pacific and East Africa are located above slow anomalies in the lower mantle down to the CMB, suggesting that the mantle plumes under those hotspots may originate from the CMB. However, there may be some small-scaled, weak plumes originating from the transition zone or middle mantle depths. The slow anomalies under hotspots usually do not show a vertical pillar shape, which suggests that plumes are not fixed in the mantle but can be deflected by the mantle flow. As a consequence, hotspots are not fixed but can wander on the Earth’s surface, as evidenced by recent geomagnetic and numerical modeling studies.

The three-dimensional shear velocity structure of the mantle from the inversion of body, surface and higher-mode waveforms

SUMMARY We present a 3-D model of shear heterogeneity (Vsh) in the whole mantle, derived from the inversion of hand-picked body, surface and higher-mode waveforms. The forward and inverse problems are formulated using the non-linear asymptotic coupling theory, the zeroth-order asymptotic expansion of a Born seismogram computed by normal mode summation and including coupling across mode branches. We invert iteratively for mantle heterogeneity and for centroid moment tensors until convergence. Our model, SAW24B16, is parametrized laterally in spherical harmonics up to degree 24 and radially in 16 cubic b-splines with knots spaced to reflect the data sampling with depth. The power spectrum of the model is dominated by heterogeneity in the boundary layers at low degrees: degrees 5 and 6 near the surface and degree 2 near the core–mantle boundary. The rest of the lower mantle is dominated by degree 3. We find significant heterogeneity up to spherical harmonic degree 24 in the transition zone and the uppermost part of the lower mantle. Our model displays three slow domains extending continuously between uppermost and lowermost mantle, one originating under Africa and two in the Pacific. The results of our modelling also suggest an interaction near the surface between the anomalies in the Pacific and the network of mid-ocean ridges in the East Pacific, and between the African anomaly and the low-velocity zone in the Red Sea, the East African Rift, the Mid-Indian Ridge and the Mid-Atlantic Ridge. These anomalies appear to ascend into the upper mantle without altering their shape near the 670 km discontinuity. Fast anomalies accumulate near the 670 km discontinuity under the Southern Kurile, the Japan trench and the Izu arc, with little or no penetration into the lower mantle, in agreement with results from a variety of regional studies. Cross-sections through subduction zones such as the Marianas, Tonga–Kermadec, Java, the palaeosubduction zone beneath North America and the Peru–Chile trench show continuous fast structures penetrating into the lower mantle, with the depth of penetration varying from region to region. The distribution of both fast and slow anomalies near 670 km thus suggests that the discontinuity does not act as a strong barrier to upgoing and downgoing flow between the upper and the lower mantle.

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.

Detection of mantle plumes in the lower mantle by diffraction tomography: Hawaii

A new method is proposed to detect mantle plumes in the lower mantle. The method is akin to diffraction tomography, and relies on the scattering of long-period seismic body waves by nearly vertical heterogeneities. The theory is described in a companion paper [Ying Ji, H.-C. Nataf, Earth Planet. Sci. Lett., this issue]. Here we apply this method to an actual set of long-period digital seismograms recorded on the world-wide network of seismic stations between 1980 and 1994. We select seismograms that `illuminate' the lower mantle in a 20°×20° region around Hawaii. We construct an image of `plume-like' heterogeneities, using a cell size of 1°×1°, by a LSQR inversion of the scattered waves. This image shows a strong slow anomaly about 200 km northwest of Hawaii. Although the image is somewhat noisy, resolution tests indicate that this feature is fairly robust. The amplitude of the anomaly is between 30 and 60 times larger than what we predict from a simple thermal plume model, built with a 600 K maximum temperature excess, and a gaussian horizontal cross-section with an 1/ e diameter of 250 km. It is the first time that such a method is applied to the mantle. While we think that the existence of this anomaly is real, we do not know how to explain such a large amplitude. If real, this observation has a number of important geodynamical consequences. It indicates that a mantle plume is indeed responsible for the Hawaii hotspot, as speculated by Morgan [W.J. Morgan, Nature 230 (1971) 42–43]. The plume is nearly vertical. It originates from the D″ region, at the base of the lower mantle. The amplitude of the anomaly suggests that partial melt or/and a chemical anomaly must be present. The plume source is to the northwest of its surface expression, as predicted by some models of plume advection in the `mantle wind'.

Test of tomographic models of D″ using differential travel time data

We compare local measurements of SH‐velocity in D″ under the Pacific Ocean with four recent S‐velocity models derived with different techniques. From the local measurements, we find evidence for both strong fast and slow anomalies with an amplitude sometimes exceeding 10%, as well as strong lateral velocity gradients. The tomographic models underestimate the magnitude of the observed anomalies by roughly a factor of 3. The model that best matches both the sign and the localisation of the anomalous structures is exclusively an SH model. This indicates, in particular, that the presence of anisotropy in D″ may not be ignored and that it is important to separate SH and SV contributions in tomographic studies of the lowermost mantle.

Depth Extent of the Lau Back-Arc Spreading Center and Its Relation to Subduction Processes

Seismic tomography and wave form inversion revealed that very slow velocity anomalies (5 to 7 percent) beneath the active Lau spreading center extend to 100-kilometer depth and are connected to moderately slow anomalies (2 to 4 percent) in the mantle wedge to 400-kilometer depth. These results indicate that geodynamic systems associated with back-arc spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab. The slow regions associated with the Tonga arc and the Lau back arc are separated at shallow levels but merge at depths greater than 100 kilometers, suggesting that slab components of back-arc magmas occur through mixing at these depths.

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.

The crustal magma chamber of the Katla volcano in south Iceland revealed by 2-D seismic undershooting

SUMMARY Results of a 2-D, seismic undershooting experiment on the Katla central volcano in south Iceland are reported. Large localized traveltime anomalies (0.4 s) are observed on an array within the Katla caldera. The traveltimes are forward modelled using a wavefront tracker developed in Appendix A. Thus, non-linear effects encountered in traveltime tomography are avoided as well as common problems with ray tracing in the presence of strong lateral heterogeneity. An extreme variation in compressional velocity is required to extend over a significant volume in order to model the data. The resulting model is not unique, but constraints on the allowable range of velocities (2.5-6.0 km s-') render the basic features well constrained. A clear S-wave shadow is closely associated with delays in traveltime due to a shallow slow anomaly. Low-amplitude P waves go hand in hand with early arrivals due to thin structural features flanking the slow anomaly. The model is interpreted in terms of a magma chamber containing extensively molten rock. The magma chamber is shallow, with a bottom at a depth of about 1.5 km below sea-level (3.0 km below surface), and measures about 5 km across. The depth of the chamber is roughly at the level of buoyant equilibrium for basaltic melt in the crust. Owing to poor vertical resolution at shallow depths in the undershooting geometry the top of this shallow magma chamber is not well resolved. On the other hand, the bottom of the chamber is well resolved. The chamber is underlain by rocks of average or high velocity for that depth. The magma chamber is a persistent feature, big enough (10 km3) to supply magma for large eruptions and to supply heat to permit remelting of hydrated basaltic crust to produce silicic magmas at shallow levels. The chamber is fed by magma fracturing from below. The model agrees with phenomenological models of magma chambers in Iceland based on geological observations and provides a quantification of those models in terms of depth and size. On the other hand, it is fundamentally different from recent models of magma chambers at mid-ocean ridges which may be more akin to the pervasive region of partial melt at depth beneath Iceland. This underlines the important effect of the Icelandic hotspot on tectonics and volcanism in Iceland and implies a substantially different crustal and thermal structure in Iceland from that of 'normal' mid-ocean ridges.

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.

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.

Aspherical heterogeneity of the mantle from phase velocities of mantle waves

Long-period surface waves are used to map the lateral heterogeneity of the upper 100–600 km of the mantle. There is good correlation of velocity with surface tectonics and heat flow. Convergence regions are generally slow for Love waves and fast for Rayleigh waves. Back arc basins have slower than average shallow mantle. Some island arcs show evidence of fast material at greater depth. Deep-seated slow anomalies underlie the Red Sea–Afar region of north-east Africa, western North American–northern East Pacific Rise, Indian Ocean triple junction and the Tasman Sea–Campbell Plateau regions. The fastest regions are in the north central Australia–New Guinea and the South Atlantic.

Experimental Kuru in the rhesus monkey: A study of EEG modifications in the waking state and during sleep

EEG patterns recorded in the waking state and during sleep were studied in 6 rhesus monkeys inoculated with a strain of Kuru previously passaged in rhesus monkey (ENAGE strain, rhesus L6 56). The onset of the disease was confirmed by the appearance of various clinical signs in 4 monkeys 15 months after inoculation. At the 16th month, the first EEG modifications appeared during sleep, which became lighter. The waking EEG was abnormal during the mature phase of the disease; it was characterized by slow anomalies and scattered spikes. The sleep EEG still presented 3 stages of Slow Wave Sleep which, however, were totally unlike the physiological stages. REM sleep rapidly disappeared, as did the cyclic organization pattern. Irritative phenomena became very significant and, in particular, very frequent 'tonic seizures' were observed. Experimental Kuru thus appears, in the rhesus monkey, as an epileptogenic encephalopathy, which is differentiated from both the human disease and the experimental disease in the chimpanzee.

The isotopic composition of tellurium

The isotopic composition of tellurium in meteoritic and terrestrial materials was measured using a solid source mass spectrometer. The relative isotope abundances for terrestrial tellurium are 0.00284, 0.07701, 0.02687, 0.14250, 0.21122, 0.56073 and 0.93753 for 120Te, 122Te, 123Te, 124Te, 125Te, 126Te, 128Te relative to 130Te, respectively. Apart from linear mass fractionation these data are in agreement with other published data. Tellurium is unique in that it is the only element which possesses three s-only process nuclides. A comparison of the isotopic composition of 8 meteorites with the terrestrial data did not reveal any evidence of rapid and slow neutron capture process anomalies.

The transfluxor as an accurate analog magnetic memory

The transfluxor can serve as the analog storage element which is needed to couple the digital process control computer to the analog process. It stores an analog command in a few milliseconds, and the value is sensed nondestructively for an indefinitely long period. In this system, a closed loop is used to set the analog level, with proportional and integral compensation to modulate the size of setting pulses. The accuracy obtained is typically 0.1 percent at all scale positions in a time of 20 ms. The characteristics of the transfluxor which make setting accuracy difficult are its variable gain, variable time lag in response to a series of equal pulses, dead zone with a spurious output, and slow magnetic anomaly effect.