bluuebunny/crossref_metadata_2025 huggingface dataset:
bromine‐containing source gases during EASOE
Authors: Fabian Peter , Borchers Reinhard , Kourtidis Kostas
Published: February 2004

Vertical profiles of CBrClF2 (Halon‐1211), CBrF3 (Halon‐1301) and methyl bromide (CH3Br) were measured during EASOE by means of cryogenic sampling and subsequent GC‐analyses. Three flights of the MPAE balloon‐borne sampler were carried out on January 18, February 6 and March 20, 1992, from Kiruna/Sweden. Stratospheric falloff rates of these substances, which constitute the dominant source of organically bound bromine in the atmosphere, were found, like those of other source gases measured during EASOE, to be considerably larger than those observed at midlatitudes. Total BrOx released from the three source gases was found to increase from about zero at 8 km to 11.4 pptV at 19 km altitude.
A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992
Authors: Dlugokencky E. J. , Masaire K. A. , Lang P. M. , Tans P. P. , Steele L. P. , Nisbet E. G.
Published: February 2004

Global measurements of atmospheric methane have revealed a sharp decrease in the growth rate in the Northern Hemisphere during 1992. The average trend for the Northern Hemisphere during 1983–1991 was (11.6±0.2) ppbv yr−1, but the increase in 1992 was only (1.8±1.6) ppbv. In the Southern Hemisphere, the average increase (1983–1991) was (11.1±0.2) ppbv yr−1, and the 1992 increase was (7.7±1.0) ppbv. Various possibilities for a change in methane sources or sinks are discussed, but the most likely explanation is a change in an anthropogenic source such as fossil fuel exploitation, which can be rapidly and easily affected by man's activities.
Balloon‐borne mass spectrometer measurements of HCL and HF in the winter Arctic stratosphere
Authors: Arnold F. , Spreng S.
Published: February 2004

Altitude profiles of the stratospheric trace gases HCl and HF were simultaneously measured for the first time on 13 February 1992 at ESRANGE (68°N, 20°E, Northen Sweden) with a balloon‐borne CIMS (Chemical Ionization Mass Spectrometry) ‐instrument. The measured maximum volume mixing ratios (VMR's) for HCl and HF are 2 ppbv and 0.6 ppbv, respectively, at 25 km. A comparison of the observed HCl VMR's with data from midlatitudes and model predictions shows, that the present VMR's of HCl are similar to VMR's of out‐of‐vortex situations.
Seismic structure near the inner core‐outer core boundary
Authors: Kaneshima Satoshi , Hirahara Kazuro , Ohtaki Toshiki , Yoshida Yasuhiro
Published: February 2004

A model of P‐wave velocity of the Earth's core 500 km above and below the inner core boundary (ICB) beneath North America is constructed from travel time analysis and broad‐band waveform modeling of core phases at distances from 130° to 160°. Differential travel times between PKPBC (Cdiff) and PKPDF (TBC–TDF) are about 0.5 sec shorter than those computed from PREM at distances from 146° to 152° and increase with distance to nearly the same as those from PREM at 155°. Differential travel times between PKPCD and PKPDF (TCD–TDF) at distances from 130° to 143° are approximately 0.2 sec shorter than those of PREM. Amplitudes of PKPBC (Cdiff) relative to PKPDF (ABC/ADF) at distances 150° to 156° are about 40% larger than those from PREM. These observations require a smaller P‐wave velocity gradient (0.0005 sec−1) in the lowermost 300 km of the outer core than that from PREM (∼0.00065 sec−1), a slower P‐wave velocity (10.96 km/sec) at the top of the inner core, and a larger velocity gradient (0.0005 sec−1) in the uppermost 300 km of the inner core.
SAGE II observations of a previously unreported stratospheric volcanic aerosol cloud in the northern polar summer of 1990
Authors: Yue Glenn K. , Veiga Robert E. , Wang Pi‐Huan
Published: February 2004

Analysis of aerosol extinction profiles obtained by the spaceborne SAGE II sensor reveals that there was an anomalous increase of aerosol extinction below 18.5 km at latitudes poleward of 50°N from July 28 to September 9, 1990. This widespread increase of aerosol extinction in the lower stratosphere was apparently due to a remote high‐latitude volcanic eruption that has not been reported to date. The increase in stratospheric optical depth in the northern polar region was about 50% in August and had diminished by October 1990. This eruption caused an increase in stratospheric aerosol mass of about 0.33 × 105 tons, assuming the aerosol was composed of sulfuric acid and water.
Radar measurements of melt zones on the Greenland Ice Sheet
Authors: Jezek Kenneth C. , Gogineni Prasad , Shanableh M.
Published: February 2004

Surface‐based microwave radar measurements were performed at a location on the western flank of the Greenland Ice Sheet. Here, firn metamorphosis is dominated by seasonal melt, which leads to marked contrasts in the vertical structure of winter and summer firn. This snow regime is also one of the brightest radar targets on Earth with an average backscatter coefficient of 0 dB at 5.3 GHz and an incidence angle of 25°. By combining detailed observations of firn physical properties with ranging radar measurements we find that the glaciological mechanism associated with this strong electromagnetic response is summer ice lens formation within the previous winter's snow pack. This observation has important implications for monitoring and understanding changes in ice sheet volume using space‐borne microwave sensors.
Relaxation of the stressed plasma sheet boundary layer
Authors: Romero H. , Ganguli G.
Published: February 2004

Observations indicate that the plasma sheet boundary layer (PSBL) becomes stressed during active times resulting in enhanced plasma flows, intense wave activity, and its minimum spatial width becoming on the order of the ion Larmor radius. We present simulations showing that the Electron‐Ion‐Hybrid (EIH) instability plays a key role during of the nonlinear relaxation of such a stressed PSBL. The EIH is a long wavelength mode (kyLE ∼1, where ky is the perpendicular wavelength and LE the gradient scale size), its spectrum is broadband (extending from below the lower hybrid frequency to the electron plasma frequency), and its onset can generate substantial perpendicular ion acceleration and anomalous viscosity leading to particle mixing across the PSBL.
Concordant sea‐floor spreading rates obtained from geochronology, astrochronology and space geodesy
Authors: Baksi Ajoy K.
Published: February 2004

Reversals of the Earth's magnetic field, allied to sea‐floor spreading, have left their imprint in the form of a magnetic tape recorder on the sea‐floor. It is demonstrated that the geomagnetic polarity time scale for the past few million years, derived utilizing (a) radiometric dates of magnetic field reversals trapped in subareal, igneous, rocks and (b) as derived from the Milankovitch forcing frequencies and their imprint in the sedimentary environment, are in good agreement. Further, sea‐floor spreading rates calculated from these results, are in substantial agreement with the rate of plate movement averaged over the last decade, as obtained by space geodetic measurements. Since the principles underlying these “dating” techniques are entirely different, it lends credence to the results obtained for sea‐floor spreading, arguably the most fundamental process in plate tectonics.
Strength of cemented grains
Authors: Yin Hezhu , Dvorkin Jack
Published: February 2004

We conducted compaction tests (isotropic drained loading) on randomly packed glass beads that were a) uncemented and b) cemented by epoxy at their contacts. In the latter case, the volume of the epoxy accounted for 10 percent of the pore space. Intensive crushing of grains was observed in the first case at about 50 MPa. In the second case, the cemented grains stayed intact, the failure being localized within the epoxy. Therefore, even small amounts of cement at contacts prevent the failure of grains. Theoretically, this effect follows from our theory of cemented granular materials: stress concentration is high at the contacts of uncemented grains, whereas even small amounts of relatively soft cement result in a more uniform stress distribution over a larger contact area.
A 6‐year time series of hydrothermal plumes over the Cleft segment of the Juan de Fuca Ridge
Authors: Baker Edward T.
Published: February 2004

The causal links between magmatic/tectonic processes and hydrothermal venting along the mid‐ocean ridge axis will be difficult to discern without broadband observations of the temporal variability of hydrothermal activity. I report here on temporal variability in the intensity and distribution of chronic and event plumes generated by continuous and episodic hydrothermal discharge at two vent fields on the Cleft segment of the Juan de Fuca Ridge. Hydrothermal temperature (Δθ) and light attenuation (Δc) anomalies were comprehensively mapped between 1 and 4 times per year during nine cruises from 1986 to 1991. Intensity of Δθ and Δc in the chronic plume overlying the north Cleft vent field showed more variability between years than within a given year. Both Δθ and Δc were exceptionally high in 1986, fell to moderate levels during 1988 to 1990, and declined to minimum values in 1991. The chronic plume over the south Cleft vent field varied similarly. Superimposed on the chronic plumes were brief and intense releases of hydrothermal fluid that created event plumes distinguishable by their symmetry and exceptional rise height. Event plumes at the north Cleft field ranged from the 1986 megaplume, with a rise height exceeding 1000 m, to smaller events in 1987 and 1989 with rise heights of ∼400 m. Chronic hydrothermal heat (660±277 (1σ) MW) and mass (204±86 (1σ) g s−1) fluxes from the north Cleft field were determined by assuming these fluxes were equivalent to the long‐term net advection of a mean cross‐section of the vertically integrated Δθ and Δc in the plume. This 6‐year history of hydrothermal plumes indicates that episodic discharge has been an important component of the recent hydrothermal flux and that the aggregate heat and mass fluxes declined significantly, though not consistently, between 1986 and 1991.
Western U.S. upper mantle structure
Authors: Humphreys Eugene D. , Dueker Kenneth G.
Published: February 2004

Regional three‐dimensional inversions of upper mande P wave velocity structure are created from teleseismic P wave travel time residuals recorded by many of the high‐density high‐frequency regional arrays operated within the western United States. These inversions are adjusted to a global (International Seismic Centre) reference and merged to obtain an image of the upper mantle beneath western United States. The P wave velocities in the upper mantle are slow on average, and the structure is very heterogeneous. Where resolution is good, coherent upper mantle structures are imaged that extend as deep as ∼200 km (the Juan de Fuca and Gorda slabs, which penetrate to greater depths, are exceptions) and deviate from the average velocity (at a given depth) by as much as ±4%. Lateral resolution of these structures usually is very good, although the magnitude of the actual seismic variations is probably greater than that imaged. The long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200–500 km. Within ∼250 km of the Pacific Coast, these structures have a wavelength of ∼250 km and trend parallel to the surface physiography and young tectonic structures. Beneath the continental interior, where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage, structures have a wavelength of ∼500 km and trend northeasterly. This northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity.
An ocean bottom gravity study of the southern Juan de Fuca Ridge
Authors: Stevenson J. Mark , Hildebrand John A. , Zumberge Mark A. , Fox Christopher G.
Published: February 2004

We use seafloor and sea surface gravity data to model density structure along the southern Juan de Fuca Ridge. We determine the average density of the shallow oceanic crust at the ridge using seafloor gravity measurements and then use these data in conjunction with sea surface gravity observations to construct density structure models. Of 63 seafloor gravity measurements obtained, 42 observations were distributed over the 3‐km left‐lateral overlapping rift zone (ORZ), located at latitude 45°03′N, and separating the Juan de Fuca ridge into the Cleft and Vance segments. A 21‐measurement seafloor gravity transect was made perpendicular to the ridge strike at latitude 44°52′N, a region of linear ridge geometry on the Cleft segment. These seafloor gravity measurements, which are sensitive to shallow crustal density variations, were augmented by roughly 800 km of sea surface gravity measurements. Using a bathymetry‐Bouguer anomaly analysis of the seafloor gravity data, we determined the average density of the shallow (roughly upper 2 km) oceanic crust at these two locations to be 2630 kg m−3±50 kg m−3. Within the uncertainties of the density determinations, there is no difference between the average shallow oceanic crustal density at the linear Cleft segment and at the Cleft‐Vance ORZ. Using the seafloor measurements, we modeled the study area's underlying density to provide constraints on its fine‐scale structure. Localized porosity of up to 17% within the upper 500 m of oceanic crust (layer 2A) provide a possible explanation for the observed gravity anomalies at the Cleft segment. Two‐dimensional density models of the crust underlying the linear Cleft segment (44°52′N) show that no deep source is required to explain the topographical asymmetry observed between the east and west sides of the ridge axis. For the Cleft‐Vance ORZ, three‐dimensional modeling suggests low‐density material between the rifts, in agreement with thickened layer 2A from seismic observations. In contrast, layer 2A variations do not explain the low density inferred at the northern end of the Cleft segment since seismic observations suggest a thin layer 2A. The magmatic activity recently observed in this region may have a distinct, deeper source.
Comparative volcanology of small axial eruptive centers in the MARK area
Authors: Bryan Wilfred B. , Humphris Susan E. , Thompson Geoffrey , Casey John F.
Published: February 2004

We describe selected examples of small volcanic features identified during Ocean Drilling Program (ODP) site surveys in the MARK area for which additional observations and sample data have been obtained by manned submersible and/or drilling. Based on both morphologic relations and similarities in chemistry and petrography, Serocki Volcano, the site of bare rock drilling on ODP legs 106 and 109, appears to be a flat‐topped megatumulus, a large rootless vent developed on a lava flow originating from an adjacent cone located above the true eruptive fissure. A linear set of five apparent volcanic centers on the east side of the median valley (the EastMARK cones) differ from one another in morphology and degree of tectonic degradation. They formed at different times and from different vents, but all were localized by damming of the flows against the steep walls of the linear valley containing them. The two northernmost features are probably megatumuli at the terminal ends of lava flows descending the valley walls. The other three vents are cones with breached craters and probably were more recently erupted in situ. The Snakepit hydrothermal area is located near the highest and widest part of a 35 km‐long ridge that appears to be a major, relatively young fissure eruption. The associated lavas are among the geochemically most primitive in the MARK area but have their counterparts in several subsets of lavas associated with Serocki Volcano in spite of the morphologic contrasts and large geographic separation between the two features. We conclude that a volcano like form, geographic proximity, and conformance to a linear geometry do not guarantee compositional or temporal coherence, nor is the alignment of apparent volcanic centers a reliable guide to underlying tectonic patterns. The chemistry and petrography of the Serocki and EastMARK basalts resemble older adjacent lavas and also conform to subtle along‐ridge variations. This and the relatively old appearance of their flow surfaces indicate that these lavas represent the waning stage of an older magmatic event. The more recent eruptions in the Snakepit area seem to reflect a renewed influx of the same relatively primitive (and presumably hotter) magma that initiated the Serocki eruptions. Residual pockets of this hotter magma in the shallow crust may provide the heat that drives the present hydrothermal system.
Composition and sedimentation of hydrothermal plume particles from North Cleft segment, Juan de Fuca Ridge
Authors: Feely Richard A. , Massoth Gary J. , Trefry John H. , Baker Edward T. , Paulson Anthony J. , Lebon Geoffrey T.
Published: February 2004

In 1990 and 1991, particles from buoyant and neutrally buoyant hydrothermal plumes above hydrothermal vents at the North Cleft segment of the Juan de Fuca Ridge were sampled to study their changing composition and fluxes away from the vent field. In the rising buoyant plume, >75% of the P, V, Cr, and As scavenging from seawater by hydrothermal precipitates occurs in the first 50 m above the vent. Cu and Zn are most enriched in buoyant plume particles collected from the first few meters above the vent. However, the degree of enrichment decreases very rapidly with increased height above the vents due to sedimentation of the more dense Cu‐ and Zn‐rich sulfide phases. Using the plume data, coupled with the results of our analysis of sediment trap samples, we estimated that more than 99% and 99.9%, respectively, of the total hydrothermal Fe and Mn produced at the vent field are transported beyond the vent field and dispersed in the open ocean.
Southeastern boundary of the Juan Fernandez microplate: Braking microplate rotation and deforming the Antarctic plate
Authors: Kleinrock Martin C. , Bird Robert T.
Published: February 2004

Data from a 1991 multibeam, sidescan sonar, and geophysical survey of the Juan Fernandez microplate at the Pacific‐Nazca‐Antarctic triple junction strongly support a model of edge‐driven microplate rotation. They reveal the nature of the microplate's southeastern boundary with the Antarctic plate and show that plate boundary interactions are capable of significantly altering plate motions. Microplate rotation slowed progressively from >30°/m.y. to <10°/m.y. between Anomaly 2 time (∼1.9 Ma) and the early Brunhes (∼0.6 Ma). Magnetics, bathymetry, and morphology suggest that this deceleration resulted from coupling across a rapidly evolving Juan Fernandez‐Antarctic (JF‐A) plate boundary. Over the past ∼2 m.y., the JF‐A plate boundary has lengthened, changed orientation, and migrated to the southeast, into the Antarctic plate. This process deformed and transferred to the Juan Fernandez plate about 3000 km2 of the Antarctic plate. Kinematic and morphologic observations support interpreting curved lineaments within this deformed zone as originally N‐S abyssal hills generated at the Pacific‐Antarctic Ridge that were later sheared by dextral slip bookshelf faulting within the migrating sinistral shear zone of the JF‐A boundary. Deformation within this system documents that shear zone migration, not rift propagation, is the key process associated with development of curved lineaments and transferred lithosphere. The JF‐A plate‐boundary shear‐zone may have been quite narrow (∼10–20 km) despite its migration through the Antarctic plate. Narrow migrating plate boundaries are also observed at other systems, such as the Galapagos 95.5°W migrating offset which also has a width of ∼20 km. Such narrow deformation zones may be common along migrating plate boundaries in various tectonic environments. Triple junction systems in this area appear to represent a suite of plates and triple junctions crossing a range of scales. The observations here support models for triple junctions having complex evolutionary histories involving rapid changes in plate boundary configuration and stress field.
Wide‐angle elastic wave one‐way propagation in heterogeneous media and an elastic wave complex‐screen method
Authors: Wu Ru‐Shan
Published: February 2004

In this paper a system of equations for wide‐angle one‐way elastic wave propagation in arbitrarily heterogeneous media is formulated in both the space and wavenumber domains using elastic Rayleigh integrals and local elastic Born scattering theory. The wavenumber domain formulation leads to compact solutions to one‐way propagation and scattering problems. It is shown that wide‐angle scattering in heterogeneous elastic media cannot be formulated as passage through regular phase‐screens, since the interaction between the incident wavefield and the heterogeneities is not local in both the space domain and the wavenumber domain. Our more generally valid formulation is called the “thin‐slab” formulation. After applying the small‐angle approximation, the thin‐slab effect degenerates to that of an elastic complex‐screen (or “generalized phase‐screen”). Compared with scalar phase‐screen, the elastic complex‐screen has the following features. (1) For P‐P scattering and S‐S in‐plane scattering, the elastic complex‐screen acts as two separate scalar phase‐screens for P and S waves respectively. The phase distortions are determined by the P and S wave velocity perturbations respectively. (2) For P‐S and S‐P conversions, the screen is no longer a pure phase‐screen and becomes complex (with both phase and amplitude terms); both conversions are determined by the shear wave velocity perturbation and the shear modulus perturbation. For Poisson solids the S wave velocity perturbation plays a major role. In the special case of α0 = 2β0, S wave velocity perturbation becomes the only factor for both conversions. (3) For the cross‐coupling between in‐plane S waves and off‐plane S waves, only the shear modulus perturbation δμ has influence in the thin‐slab formulation. For the complex‐screen method the cross‐coupling term is neglected because it is a higher order small quantity for small‐angle scattering. Relative to prior derivations of vector phase‐screen method, our method can correctly treat the conversion between P and S waves and the cross‐coupling between differently polarized S waves. A comparison with solutions from three‐dimensional finite difference and exact solutions using eigenfunction expansion is made for two special cases. One is for a solid sphere with only P velocity perturbation; the other is with only S velocity perturbation. The Elastic complex‐screen method generally agrees well with the three‐dimensional finite difference method and the exact solutions. In the limiting case of scalar waves, the derivation in this paper leads to a more generally valid new method, namely, a scalar thin‐slab method. When making the small‐angle approximation to the interaction term while keeping the propagation term unchanged, the thin‐slab method approaches the currently available scalar wide‐angle phase‐screen method.
Separation of lithospheric, external, and core components of the south polar geomagnetic field at satellite altitudes
Authors: Alsdorf Douglas E. , von Frese Ralph R. B. , Arkani‐Hamed Jafar , Noltimier Hallan C.
Published: February 2004

We present a new approach to producing scalar Magsat magnetic anomaly maps based on correlation coefficient filtering and the use of almost all of the available orbits. Our method differs from earlier techniques with respect to the following: (1) Passes are selected based on their variance properties rather than planetary indices such as Kp. (2) The core field model is least squares fit to individual passes and subsequently removed instead of subtracting the model directly. This technique replaces band pass filtering or polynomial trend removal methods. (3) Each selected pass is sorted geographically and by local time, placed into one of four different altitude bands, and correlation coefficient filtered with the two adjacent passes. The filtering is the second step toward isolating the static lithospheric signal from the more dynamic external field signals. (4) Least squares collocation is used to grid the correlated passes; subsequently, the dawn and dusk maps are also correlation filtered providing another step toward removal of external fields. (5) The four resultant total field maps are continued to a common altitude and again correlation filtered for the static lithospheric anomalies. (6) The filtered results are then averaged together to provide a new total field map of the lithosphere south of 40°S latitude. Our total field map differs from previous efforts over the crustal blocks of West Antarctica. We obtained a positive anomaly over Edward VII Peninsula, extending into the Byrd subglacial basin and obtained a negative anomaly over the Ellsworth Mountains and parts of the Byrd subglacial basin. Also, a positive anomaly extending from the Ross Sea to offshore Wilkes Land is present in our map; however, this feature is absent in other maps. Positive anomalies marking the Weddell Sea in previous efforts are not present in our map. Prominent external field anomalies in the quadrant offshore of Wilkes Land are present in all previous efforts; however, these signatures are reduced in our total field map. This map significantly advances efforts for the identification and interpretation of magnetic anomalies of the south polar lithosphere.
A constitutive law for rate of earthquake production and its application to earthquake clustering
Authors: Dieterich James
Published: February 2004

Seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stressing history control the timing of earthquakes. The model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate‐ and state‐dependent fault properties. This yields a general state‐variable constitutive formulation for rate of earthquake production resulting from an applied stressing history. To illustrate and test the model some characteristics of seismicity following a stress step have been explored. It is proposed that various features of earthquake clustering arise from sensitivity of nucleation times to the stress changes induced by prior earthquakes. The model gives the characteristic Omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate. Earthquake data appear to support a model prediction that aftershock duration, defined as the time for rates to return to the back‐ground seismicity rate, is proportional to mainshock recurrence time. Observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial dependence of stress changes of the first event of the pair and stress‐sensitive time‐dependent nucleation. Applications of the constitutive formulation are not restricted to the simple stress step models investigated here. It may be applied to stressing histories of arbitrary complexity. The apparent success at modeling clustering phenomena suggests the possibility of using the formulation to estimate short‐ to intermediate‐term earthquake probabilities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress.
Three‐dimensional micromagnetic analysis of stability in fine magnetic grains
Authors: Enkin Randolph J. , Williams Wyn
Published: February 2004

We examine the stability of magnetic remanences in grains of magnetite near the critical single‐domain grain size (d0). The magnetic structure of a grain is followed during a 180° magnetization reversal, and it is shown that noncoherent reversals can occur even in grains smaller than d0. The energy barrier to noncoherent revesal is much lower than that of coherent rotation, so that pseudo‐single‐domain grain behavior will be exhibited by grains significantly smaller than d0.
Theoretical study of nonlinear elastic wave propagation
Authors: McCall K. R.
Published: February 2004

A theoretical study of the propagation of a plane wave in a material with nonlinear response is presented. We start with the wave equation for an isotropic, homogeneous, elastic solid with cubic anharmonicity in the moduli, accounting for attenuation by introducing complex linear and nonlinear moduli. A hierarchy of equations, ordered in powers of the displacement field, is developed. Using a Green function technique, we solve this set of equations systematically for the displacement field at distance x from the source. We examine the influence of propagation distance, source frequency spectrum, source displacement amplitude, attenuation, and nonlinear coefficient on the spectrum of a propagating wave. The displacement field for various source functions is calculated using parameters typical of rocks.
The short‐term prediction of universal time and length of day using atmospheric angular momentum
Authors: Freedman A. P. , Steppe J. A. , Dickey J. O. , Eubanks T. M. , Sung L.‐Y.
Published: February 2004

The ability to predict short‐term variations in the Earth's rotation has gained importance in recent years owing to more precise spacecraft tracking requirements. Universal time (UT1), that component of the Earth's orientation corresponding to the rotation angle, can be measured by a number of high‐precision space geodetic techniques. A Kalman filter developed at the Jet Propulsion Laboratory (JPL) optimally combines these different data sets and generates a smoothed time series and a set of predictions for UT1, as well as for additional Earth orientation components. These UT1 predictions utilize an empirically derived random walk stochastic model for the length of the day (LOD) and require frequent and up‐to‐date measurements of either UT1 or LOD to keep errors from quickly accumulating. Recent studies have shown that LOD variations are correlated with changes in the Earth's axial atmospheric angular momentum (AAM) over timescales of several years down to as little as 8 days. AAM estimates and forecasts out to 10 days are routinely available from meteorological analysis centers; these data can supplement geodetic measurements to improve the short‐term prediction of LOD and have therefore been incorporated as independent data types in the JPL Kaiman filter. We find that AAM and, to a lesser extent, AAM forecast data are extremely helpful in generating accurate near‐real‐time estimates of UT1 and LOD and in improving short‐term predictions of these quantities out to about 10 days.
Physical modeling of slow seafloor spreading
Authors: Shemenda Alexander I. , Grocholsky Andrey L.
Published: February 2004

A properly scaled thermo‐mechanical experimental model of slow seafloor spreading is developed to investigate the mechanism of lithosphere accretion. The melt of a specially fabricated hydrocarbon compositional system is cooled from above in a tank. The crystallizing upper layer (the lithosphere) possesses semiplastic‐semibrittle properties. Horizontal tension of the layer is produced by a paddle drawn at a constant rate. Spreading in the model is very unstable, asymmetric, and involves regular jumps of the spreading center. Scaling up the experimental results, to match seafloor spreading, gives the length of the jumps of the order of 10 km and the period of the order of 105–106 years. The jumps normally occur in the wall of a rift valley resulting in quasiplastic stretching of the lithosphere at this place and then, in the formation of two intersecting major normal fault zones crossing the plate from some shallow depth to the base. The wedge separated by these zones rises up symmetrically prior to the cessation of movement along one zone, with motion only continuing along the other. From this moment the deformation becomes asymmetrical. The material crystallized at the base of the lithosphere is dragged along the active zone to the upper horizons of the plate and to the surface. This process leads to both the nonisostatic uplifting of the central part of the pulled block and subsidence near the inlet on the surface of the fault (detachment) zone which forms the center of this asymmetric lithospheric accretion. At a specific stage, under the action of both horizontal tension and vertical nonisostatic forces the strength of the pulling block is exceeded within the rift valley. The result is the next jump of the spreading center to the point of breakage and a repetition of the whole cycle (the jump can also occur at other places weakened during previous cycles). Normally, the asymmetry of the spreading alternates at a new center. The dimensions of the valley, as well as the overall seafloor relief, are strongly dependent on the spreading rate V; the lower the V value, the rougher the relief. The jumps of the spreading centers are not synchronous along the plate boundary. Moreover, they can be of different types, with varying distances and in opposite directions on adjacent segments of the spreading axis, leading to the occurrence of transfer or accommodation zones.
Body wave and surface wave analysis of large and great earthquakes along the Eastern Aleutian Arc, 1923–1993: Implications for future events
Authors: Estabrook Charles H. , Jacob Klaus H. , Sykes Lynn R.
Published: February 2004

The 500‐km‐long Alaska Peninsula and Shumagin Islands segments of the Aleutian arc have a moderate to high probability of rupturing in one or more large or great earthquakes in the next few decades. To understand the likely modes of rupture in the next sequence of large and great events and to delineate the current geometry of the plate interface, we determine focal mechanisms, depths, and source time functions from seismic records for the largest events since 1917: the great earthquake of November 10,1938, and seven events with surface wave magnitude (Ms) of 6.9 to 7.5. Teleseismic body waves and surface waves are used to estimate the seismic moment and gross rupture characteristics of the great earthquake of November 10,1938, along the Alaska Peninsula. Body wave inversion of five P and four SH waves gives a duration of about 110 s in which moment was released in two episodes, each of about 50 s duration, with the second being larger than the first. The first source was located in the general vicinity of the epicenter of the mainshock, and the second, which occurred about 60 s later, was centered about 180 km to the northeast. The body‐wave‐derived seismic moment is 3.7×1021 N m (Mw 8.3). We corroborate the body wave results by calculating surface waves by normal‐mode summation and comparing them with data at periods greater than 50 s. An adequate fit to observed seismograms is obtained for either a single point source or two point sources, with one located about 180 km northeast of the mainshock epicenter. Rupture in 1938 appears to have been confined to the Alaska Peninsula segment; uniform rupture into the Shumagin region is not supported by the data. The Ms 6.9 earthquake of May 13,1993, ruptured a small portion of the Shumagin gap. The earthquake of May 14,1948, (Ms 7.5) occurred on a shallow dipping thrust fault with a depth of about 31 km, not 60 km as originally suggested. Five Ms ∼ 7 events in the 1938 rupture zone have locations, depths, and mechanisms that define a shallow dipping (16°–19°) plate interface which shallows to 8° under Kodiak Island. Subduction of seamounts of the Gulf of Alaska seamount province may explain the location, moment, focal mechanism, and depth of the five Ms ∼ 7 events. The plate interface dips nearly uniformly between the Alaska Peninsula and Shumagin segments, indicating that segmentation, if any, is not controlled by the orientation of the plate interface. A slight warp of the plate interface may form the boundary between rupture zones of great earthquakes along the Kodiak and Alaska Peninsula segments of the arc and may explain why rupture zones of large to great earthquakes rarely cross this tectonic boundary.
Paleomagnetism of the Brushy Basin Member of the Morrison Formation: Implications for Jurassic apparent polar wander
Authors: Bazard David R. , Butler Robert F.
Published: February 2004

The paleomagnetism of the ∼147 Ma (Tithonian) Brushy Basin Member of the Morrison Formation was analyzed to obtain a Late Jurassic paleomagnetic pole for North America. A total of 200 samples were collected from 25 sedimentary horizons (sites) at Norwood Hill in southwest Colorado. At Montezuma Creek in southeast Utah, 184 samples were collected from 26 sites. Detailed thermal demagnetization (up to nine temperature steps between 600°C and 680°C) and principal component analysis were required to confidently isolate characteristic remanent magnetization (ChRM) directions carried by hematite. Demagnetization behavior for many horizons is erratic and does not allow isolation of a high unblocking‐temperature ChRM. Data selection criteria required sample ChRM directions to be defined by three or more thennal demagnetization steps and maximum angular deviations of sample ChRM directions to be ≤20°. Eight sites from the Norwood Hill location and 10 sites from the Montezuma Creek location passed these criteria. The 18 site‐mean virtual geomagnetic poles yield a paleomagnetic pole position from the Brushy Basin Member of 68.3°N, 156.2°E (A95 = 4.8°, K = 53). This pole position is within 2° of the paleomagnetic pole which Steiner and Helsley (1975a) reported for the “upper” Morrison Formation at Norwood Hill, Colorado. A second paleomagnetic pole was calculated after excluding sites with site‐mean α95 > 20° and sites with fewer than three samples that passed the above selection criteria. This additional editing did not significantly change the paleomagnetic pole position at the 95% confidence level. Along with other paleomagnetic poles from the continental interior the paleomagnetic data from the Brushy Basin Member of the Morrison Formation are interpreted to indicate that the Late Jurassic part of the North American apparent polar wander path progresses from a late Middle Jurassic (∼160 Ma) position at ∼60°N, 135°E toward the mid‐Cretaceous pole position at 72°N, 191°E.
Seismic constraints on a model of partial melts under ridge axes
Authors: Tanimoto Toshiro , Stevenson David J.
Published: February 2004

In a recent global scale seismic study, the correlation between S wave velocity under ridge axes and spreading rate was pointed out. The correlation is strong for depths to about 70 km, but it diminishes below this depth. We present the correlation plots at four depths, 38, 66, 90, and 110 km, for which correlation is strong at 38 and 66 km but is weak at 90 km and is almost nonexistent at 110 km. We present a model to explain this behavior, which includes a thermal conduction model for the development of lithosphere and a simple melt percolation. Thermal effects on S wave velocity are assumed to be accounted for entirely by the plate cooling (thermal conduction) model. We point out that the thermal model under this assumption predicts asymptotically no correlation between S wave velocity and spreading rate, specifically for spreading rate larger than about 3 cm yr−1. This contradicts the correlation observed in the data at shallow depths. The existence of partial melt is thus required to explain the observed behavior at 38 and 66 km depths. We start from four basic equations that govern the distribution of partial melt and derive the relation between the amount of partial melt and the spreading rate. We adopt a simple power law relation between permeability (k) and porosity (ƒ) by k(ƒ) = k0ƒn, where k0 and n are constants and assume that pores are filled with melt. We then set up an integral relation between S wave velocity and spreading rate. The final formula indicates that the gradient in the correlation plots is the inverse of the power (1/n) in the permeability‐porosity relation, thus enabling us to constrain n as well as k0 from seismic data. The data also have some sensitivity to the depth to solidus. We show that (1) the depth to solidus is probably within the range 60–100 km and (2) if the power n is n = 2–3, then k0 = 10−8 ‐ 10−10 m2. These parameters predict that porosity and fluid velocity are 1–2% and about 0.5 m yr−1, respectively. The depth to solidus is consistent with previous estimates by petrological data but is perhaps the first and direct seismological evidence of partial melt from surface wave data. Analytical forms for the dependence on depth and spreading rate of porosity, fluid velocity within permeable rocks, and ascent times of magma are also obtained.
Tomographic inversion for the seismic velocity structure beneath northern New England using seismic refraction data
Authors: Zhu Hong , Ebel John E.
Published: February 2004

The seismic velocity structure of the crust and the upper mantle beneath northern New England was investigated from the travel time data of the 1984 Maine Seismic Refraction Profile and the 1988 Ontario‐New York‐New England Seismic Refraction Profile seismic refraction experiments by using tomographic inversion techniques. Strong lateral seismic velocity variations were found in the upper 15 km of the crust, and these variations are correlated with regional geologic and tectonic structures. The Grenville basement in the Adirondack Mountains of eastern New York is characterized by an unusually high P wave velocity of 6.7 km/s in the upper 15 km of the crust. This high‐velocity basement extends eastward below western Vermont to a depth between 15 and 20 km from the surface. More typical upper crustal P wave velocities of 5.6 to 6.4 km/s were generally found in central and southern Maine and in New Hampshire, and the anisotropic upper crustal zone west of Penobscot Bay in Maine was imaged by locally slower velocities. The correlation of the inversion velocity structure with the tectonic features of northern New England shows that the Appalachian mountains in northern New England may extend, tectonically, to at least 20 km deep in the crust. The velocity distribution at 20 km depth shows three northeast elongate juxtaposed belts which correspond to the Connecticut Valley‐Gaspe Synclinorium, the Bronson Hill Anticlinorium and the Kearsarge‐Central Maine Synclinorium, respectively. The Connecticut Valley‐Gaspe and Kearsarge‐Central Maine Synclinoria have relatively high velocities from 6.94 to 7.12 km/s while the Bronson Hill Anticlinorium has relatively low velocities from 6.52 to 6.75 km/s in the lower crust. Another low‐velocity area of 6.52 km/s at 20 km depth is imaged beneath the White Mountain Batholith in central New Hampshire and may be a lower crustal expression of this batholith. Upper mantle velocity structures were obtained for the Grenville basement, the Appalachians in northern New England and the Avalon terrane in southeastern Maine from the long‐range seismic refraction data. Generally, the older Grenville lithosphere (about 1100 Ma) in Quebec has higher velocity in the upper mantle than does the younger Avalon terrane (no older than 650 Ma) in the southeastern part of northern New England. The data suggest that the velocity of the upper mantle beneath the old North American craton increases stepwise with observable velocity discontinuities in depth, while that beneath the Avalon terrane does not increase through the first 40 km of the upper mantle. The data analysis also shows that the upper mantle suture between the Grenville basement and the Avalon terrane occurs in a narrow belt beneath the Appalachian mountains of central Maine and New Hampshire. The did of the Moho discontinuity beneath northern New England is modeled as nearly flat as 0.7° in the N57°W direction and as steep as 1.2° beneath the Grenville basement.
Volatile fluxes integrated over four decades at Grímsvötn volcano, Iceland
Authors: Ágústsdóttir Anna María , Brantley Susan L.
Published: February 2004

Samples of the subglacial lake in the crater of the tholeiitic basaltic caldera Grímsvötn in Iceland were obtained by using a hot‐water drill to sink two boreholes through the 250‐m‐thick ice shelf covering the lake. The lake generally shows an increase in solutes with increased depth, as solutes are added from the lake's bottom and dilute glacial meltwater is added continuously from above. The crater lake temperature ranges from 0°C in the upper part to temperatures of 1°C to 4°C near the bottom of the lake. The lake water pH ranges between 7.0 and 5.7. The crater lake is assumed to be closed, with respect to volatile components released from subsurface magma, except for periodic draining by jökulhlaups. From the periodicity and water chemistry of the jökulhlaups, we have estimated the volcano's average release rates of carbon, sulfur, chlorine and fluorine between 1954 and 1991 and corrected these rates for dissolution of bedrock into the lake water and seepage of solutes to groundwater. The corrected mean release rates are 5.3×107 kg C yr−1, 5.3×106 kg S yr−1, 6.6×105 kg Cl yr−1, and 1.5×105 kg F yr−1. The emission rate estimates for Grímsvötn, one of the most active volcanoes in Iceland, are the longest integrated estimates obtained for an active volcano and are equal to or lower than those of other major active volcanoes worldwide. This difference may imply that published release rates for other volcanoes are overestimated, because they are usually not integrated over time. The values of the S/Cl and F/Cl ratios for noneruptive periods are 0.53±0.20 and 0.013±0.003, and for the two eruptive events are 0.69 and 2.14, and 0.034 and 0.041, respectively. The response of the elemental ratios to eruptive events, followed by the return to lower ratios, supports the assumption of steady state, because no long‐term accumulation of volatiles occurs. The energy output from the volcano, estimated from the amount of ice melted by hydrothermal heat, is 4250 MW over the last four decades. Using the energy output to calculate magma solidification rates and maximum possible volatile release rates, we observe that emissions of F and S are strongly suppressed at Grímsvötn, while a significant portion of available Cl and C are released. These calculations also reveal that coverage of a volcano by a glacier, and subsequent raising of the water table, may cause significant scrubbing of magmatic gases so that these gases do not reach the atmosphere.
Real aperture radar imaging of ocean waves during SAXON‐FPN: A case of azimuth‐traveling waves
Authors: Askari F. , Keller W. C.
Published: February 2004

This paper investigates the radar imaging of ocean waves under two separate wind speed and sea state conditions using an X band real aperture radar (RAR). It is shown experimentally that for the case of higher sea state and wind speed (9.1 m/s), the RAR is capable of imaging the waves from all directions including the azimuth. The case for low sea state and wind speed (4.5 m/s), however, shows somewhat stronger imaging contrast in the range direction with reduced contrast for the azimuth‐traveling waves. The results of the aircraft observations are compared with numerical simulations of a RAR imaging model. Simulations show that cross tilt is a viable mechanism for enhancing the azimuthal modulations, but the results are sensitive to the level of upwind/cross‐wind anisotropy of the shortwave spectrum. Simulations also show that the variations in incidence angle create irregularities in the azimuthal angle distribution.