bluuebunny/crossref_metadata_2025 huggingface dataset:
Origin of gabbro sills in the Moho transition zone of the Oman ophiolite: Implications for magma transport in the oceanic lower crust
Authors: Korenaga Jun , Kelemen Peter B.
Published: February 2004

The Moho transition zone (MTZ) of the Oman ophiolite commonly includes a number of gabbro sills surrounded by dunites. The petrology and geochemistry of these sills are investigated to provide constraints on how magma migrates from the subridge mantle to the oceanic crust. The gabbro sills have millimeter‐scale to tens of centimeter‐scale modal layering that closely resembles layering in lower crustal gabbros of the ophiolite. Variations in mineral compositions correlate with the modal layering, but there are no overall trends within the sills. The gabbroic sills and the layered gabbros have clear covariations among mineral compositions, which can be interpreted as a fractional crystallization path from a common parental magma. Together with constraints from mid‐ocean ridge thermal evolution and crustal accretion dynamics, the petrological and geochemical observations on the gabbro sills indicate that they formed from small, open‐system, melt‐filled lenses within the MTZ. The thermal evolution of the MTZ melt lenses, buffered by the ambient mantle, is characterized by a slow cooling rate (<10−3°C/yr) and a small temperature difference (∼0.1–1°C) within lenses. Internal origins for modal layering, such as gravity currents and oscillatory nucleation, are unlikely in such a thermal environment, and we propose that open‐system evolution of the melt lenses is essential to produce the observed layering. The formation of the MTZ melt lenses may be a consequence of porous flow with low Peclet number entering a conductively cooling regime, where porosity becomes “clogged” by crystallized plagioclase. Preservation of fine‐scale vertical variation in mineral composition, together with correlated compositions of different minerals, rules out diffuse porous flow as the primary mechanism of melt transport above these melt lenses. Instead, melt extraction must have been focused into porous channels or melt‐filled fractures. Melt lenses drained by fractures would experience repetitious expulsion with continuous melt replenishment. Modal layering could develop through the expulsion cycles, probably via in situ crystallization at the margins of melt lenses.
Genetic relations between the central and southern Philippine Trench and the Sangihe Trench
Authors: Lallemand Serge E. , Popoff Michel , Cadet Jean‐Paul , Bader Anne‐Gaelle , Pubellier Manuel , Rangin Claude , Deffontaines Benoît
Published: February 2004

We surveyed the junction between the central and southern Philippine Trench and the Sangihe Trench near 6°N using swath bathymetry, gravity, and magnetics. These data, along with seismicity, allow us to discuss the genetic relations between these trenches and the forces acting on converging plates. Our final model favors the northern extension of the Halmahera Arc up to 8°N, with three segments offset left‐laterally along NW–SE transform faults. Accretion of the northern segment to Mindanao Island 4 to 5 m.y. ago resulted in the failure within the Philippine Sea Plate east of the arc. Initiation of the Philippine Trench between 7°N and 10°N agrees with the maximum recorded depth of the Philippine Trench floor (10,000 m below sea‐level) and Philippine Sea slab (200 km). South of 6°N (trench junction), another segment of the arc is being subducted beneath the Sangihe margin, while south of 3°N, the southern segment of the Halmahera Arc is still active. The rapid southward shallowing of the trench floor along the southern Philippine Trench, the type of faulting affecting both sides of the trench, the lack of significant interplate seismicity, and the concentration of the seismicity beneath the Miangas‐Talaud Ridge are interpreted as a slowing down of the subduction along this branch of the Philippine Trench compared with the rest of the subduction zone. The Sangihe deformation front has been recognized up to 7°N but seems active only south of 6°N.
Stressing of the New Madrid Seismic Zone by a lower crust detachment fault
Authors: Stuart William D. , Hildenbrand Thomas G. , Simpson Robert W.
Published: February 2004

A new mechanical model for the cause of the New Madrid seismic zone in the central United States is analyzed. The model contains a subhorizontal detachment fault which is assumed to be near the domed top surface of locally thickened anomalous lower crust (“rift pillow”). Regional horizontal compression induces slip on the fault, and the slip creates a stress concentration in the upper crust above the rift pillow dome. In the coseismic stage of the model earthquake cycle, where the three largest magnitude 7–8 earthquakes in 1811–1812 are represented by a single model mainshock on a vertical northeast trending fault, the model mainshock has a moment equivalent to a magnitude 8 event. During the interseismic stage, corresponding to the present time, slip on the detachment fault exerts a right‐lateral shear stress on the locked vertical fault whose failure produces the model mainshock. The sense of shear is generally consistent with the overall sense of slip of 1811–1812 and later earthquakes. Predicted rates of horizontal strain at the ground surface are about 10−7 year−1 and are comparable to some observed rates. The model implies that rift pillow geometry is a significant influence on the maximum possible earthquake magnitude.
Propagation of rifting along the Arabia‐Somalia Plate Boundary: Into Afar
Authors: Manighetti I. , Tapponnier P. , Gillot P. Y. , Jacques E. , Courtillot V. , Armijo R. , Ruegg J. C. , King G.
Published: February 2004

It is generally accepted that the Aden ridge has propagated westward from ∼58°E to the western tip of the Gulf of Aden/Tadjoura, at the edge of Afar. Here, we use new tectonic and geochronological data to examine the geometry and kinematics of deformation related to the penetration of that ridge on dry land in the Republic of Djibouti. We show that it veers northward, forming a narrow zone of dense faulting along the northeastern edge of the Afar depression. The zone includes two volcanic rifts (Asal‐Ghoubbet and Manda Inakir), connected to one another and to the submarine part of the ridge by transfer zones. Both rifts are composite, divided into two or three disconnected, parallel, NW‐SE striking subrifts, all of which appear to have propagated northwestward. In Asal‐Ghoubbet as in Manda Inakir, the subrifts appear to have formed in succession, through north directed jumps from subrifts more farther south. At present, the northernmost subrifts (Manda and Dirko Koma) of the Manda Inakir rift, form the current tip of the northward propagating Arabia‐Somalia plate boundary in Afar. We account for most observations by a mechanical model similar to that previously inferred for the Gulf of Aden, in which propagation is governed by the intensity and direction of the minimum horizontal principal stress, σ3. We interpret the northward propagation on land, almost orthogonal to that in the gulf, to be related to necking of the Central Afar lithosphere where it is thinnest. Such necking may be a consequence of differential magmatic thickening, greater in the center of the Afar depression where the Ethiopian hot spot enhanced profuse basaltic effusion and underplating than along the edges of the depression. The model explains why the Aden ridge foregoes its WSW propagation direction, constant from ∼58°E to Asal‐Ghoubbet. At a smaller scale, individual rifts and subrifts keep opening perpendicular to the Arabia‐Somalia (or Danakil‐Somalia) motion vector and propagate northwestward. Concurrently, such lithospheric cracks are forced to jump northward, such that the plate boundary remains inside the regional N‐S necking zone. Changes of obliquity between the directions of overall and local propagation may account for different segmentation patterns, a small angle promoting long, en échelon subrifts, and a high‐angle, smaller, nested, “subrifts within subrifts.” The propagation mechanism is thus similar, whether in oceanic or continental lithosphere, the principal change being the overall propagation path, here governed by thickness changes rather than by the geometry in map view as previously inferred for the rest of the Aden ridge. Finally, because the same mechanism has led rifting along the Red Sea to propagate southward and jump to the western edge of Afar, the Arabia‐Somalia and Arabia‐Nubia plate boundaries tips have missed each other and keep overlapping further, leading to strain transfer by large‐scale bookshelf faulting.
Model quakes in the two‐dimensional wave equation
Authors: Shaw Bruce E.
Published: February 2004

This paper presents a new two‐dimensional wave equation model of an earthquake fault. The model generates a complex sequence of slip events on a fault with uniform properties when there is a frictional weakening instability. Previous models of long faults in one and two dimensions had the driving in the bulk, giving the Klein‐Gordon equation in the bulk. Here, I place the driving on the boundary, giving the wave equation in the bulk. The different models are, however, shown to behave similarly. I examine a whole range of frictions, with slip weakening as one end‐member case and velocity weakening as the other end‐member case, and show that they display a generic type of slip complexity: there is an exponential distribution of the largest events and, for sufficient weakening, a power law distribution of small events. With the addition of a viscous‐type friction term on the fault, I show that the results are independent of grid resolution, indicating that continuum limit complexity is achieved.
Morphology of the transition from an axial high to a rift valley at the Southeast Indian Ridge and the relation to variations in mantle temperature
Authors: Shah Anjana K. , Sempéré Jean‐Christophe
Published: February 2004

The Southeast Indian Ridge exhibits a transition in axial morphology from an East Pacific Rise‐like axial high near 100°E to a Mid‐Atlantic Ridge‐like rift valley near 116°E but spreads at a nearly constant rate of 74–76 mm/yr. Assuming that the source of this transition lies in variations in mantle temperature, we use shipboard gravity‐derived crustal thickness and ridge flank depth to estimate the variations in temperature associated with the changes in morphological style. Within the transitional region, SeaBeam 2000 bathymetry shows scattered instances of highs, valleys, and split volcanic ridges at the axis. A comparison of axial morphology to abyssal hill shapes and symmetry properties suggests that this unorganized distribution is due to the ridge axis episodically alternating between an axial valley and a volcanic ridge. Axial morphology can then be divided into three classes, with distinct geographic borders: axial highs and rifted highs are observed west of a transform fault at 102°45′E; rift valleys are observed east of a transform fault at 114°E; and an intermediate‐style morphology which alternates between a volcanic ridge and a shallow axial valley is observed between the two. One segment, between 107° and 108°30′E, forms an exception to the geographical boundaries. Gravity‐derived crustal thickness and flank depth generally vary monotonically over the region, with the exception of the segment between 107°E and 108°30′E. The long‐wavelength variations in these properties correlate with the above morphological classification. Gravity‐derived crustal thickness varies on average ∼2 km between the axial high and rift valley regions. The application of previous models relating crustal thickness and mantle temperature places the corresponding temperature variation at 25°C–50°C, depending on the model used. The average depth of ridge flanks varies by ∼550 m over the study area. For a variation of 25°–50°C, thermal models of the mantle predict depth variations of 75–150 m. These values are consistent with observations when the combined contributions of crustal thickness and mantle density to ridge flank depth are considered, assuming Airy isostasy. Crustal thickness variations differ at the two transitions described above: A difference of 750 m in crustal thickness is observed at the rift valley/intermediate‐style transition, suggesting small variations in crustal thickness and mantle temperature drive this transition. At the axial high‐rifted high/intermediate‐style transition, crustal thickness variations are not resolvable, suggesting that this transition is controlled by threshold values of crustal thickness and mantle temperature, and is perhaps related to the presence of a steady state magma chamber.
A generalized law for brittle deformation of Westerly granite
Authors: Lockner David A.
Published: February 2004

A semiempirical constitutive law is presented for the brittle deformation of intact Westerly granite. The law can be extended to larger displacements, dominated by localized deformation, by including a displacement‐weakening break‐down region terminating in a frictional sliding regime often described by a rate‐ and state‐dependent constitutive law. The intact deformation law, based on an Arrhenius type rate equation, relates inelastic strain rate to confining pressure Pc, differential stress σΔ, inelastic strain εi and temperature T. The basic form of the law for deformation prior to fault nucleation is
where σo and εo are normalization constants (dependent on confining pressure), a is rate sensitivity of stress, and α is a shape parameter. At room temperature, eight experimentally determined coefficients are needed to fully describe the stress‐strain‐strain rate response for Westerly granite from initial loading to failure. Temperature dependence requires apparent activation energy (E* ∼ 90 kJ/mol) and one additional experimentally determined coefficient. The similarity between the prefailure constitutive law for intact rock and the rate‐ and state‐dependent friction laws for frictional sliding on fracture surfaces suggests a close connection between these brittle phenomena.
Detrital zircon U‐Pb geochronology of Cambrian to Triassic miogeoclinal and eugeoclinal strata of Sonora, Mexico
Authors: Gehrels George E. , Stewart John H.
Published: February 2004

One hundred and eighty two individual detrital zircon grains from Cambrian through Permian miogeoclinal strata, Ordovician eugeoclinal rocks, and Triassic post‐orogenic sediments in northwestern Sonora have been analyzed. During Cambrian, Devonian, Permian, and Triassic time, most zircons accumulating along this part of the Cordilleran margin were shed from 1.40–1.45 and 1.62–1.78 Ga igneous rocks that are widespread in the southwestern United States and northwestern Mexico. Zircons with ages of approximately 1.11 Ga are common in Cambrian strata and were apparently shed from granite bodies near the sample site. The sources of 225–280 Ma zircons in our Triassic sample are more problematic, as few igneous rocks of these ages are recognized in northwestern Mexico. Such sources may be present but unrecognized, or the grains could have been derived from igneous rocks of the appropriate ages to the northwest in the Mojave Desert region, to the east in Chihuahua and Coahuila, or to the south in accreted(?) arc‐type terranes. Because the zircon grains in our Cambrian and Devonian to Triassic samples could have accumulated in proximity to basement rocks near their present position or in the Death Valley region of southern California, our data do not support or refute the existence of the Mojave‐Sonora megashear. Ordovician strata of both miogeoclinal and eugeoclinal affinity are dominated by >1.77 Ga detrital zircons, which are considerably older than most basement rocks in the region. Zircon grains in the miogeoclinal sample were apparently derived from the Peace River arch area of northwestern Canada and transported southward by longshore currents. The eugeoclinal grains may also have come from the Peace River arch region, with southward transport by either sedimentary or tectonic processes, or they may have been shed from off‐shelf slivers of continents (perhaps Antarctica?) removed from the Cordilleran margin during Neoproterozoic rifting. It is also possible that the Ordovician eugeoclinal strata are far traveled and exotic to North America.
Cellular convection models of mid‐ocean ridge hydrothermal circulation and the temperatures of black smoker fluids
Authors: Wilcock William S. D.
Published: February 2004

Mid‐ocean ridge hydrothermal vent fields are characterized by maximum sustained venting temperatures of 320°–380°C, irrespective of the spreading rate and the inferred depth of circulation, Metamorphic assemblages, fluid inclusions, oxygen isotope data, and the salinity of vent fluids have all been used to infer maximum circulation temperatures of up to 500°–700°C. In this paper I investigate the pattern of circulation and the relationship between venting and basal temperatures using simple models of steady open‐top porous convection in a rectangular medium heated from below. Solutions obtained with variable fluid properties which approximate seawater include much more recirculation than solutions obtained with uniform fluid properties, and the temperatures of upwelling fluids are greater. For a uniform permeability, models obtained with seawater properties yield a ratio of the venting temperature to the bottom temperature in the range 0.5–0.65, values which are reasonably consistent with the observations. However, a surficial layer of high permeability significantly reduces vent temperatures. Since extrusive basalts are almost certainly very permeable, this result seems incompatible with the existence of black smoker vents. It has been suggested previously that the precipitation of hydrothermal minerals where hot and cold fluids meet will lead to the formation of an impermeable shell around upflow zones. Such a shell may provide a means to tap hot fluids from depth.
Geothermics and climate change: 1. Analysis of borehole temperatures with emphasis on resolving power
Authors: Harris Robert N. , Chapman David S.
Published: February 2004

Temperature‐depth data from six boreholes in western Utah and nine boreholes in southeastern Utah are reanalyzed for evidence of ground surface temperature (GST) histories. We invert the temperature‐depth data using the functional space inverse algorithm of Shen and Beck [1991, 1992] which we prefer over previous inversions of these data because of its greater sophistication and flexibility in suppressing noise. GST histories for western and southeastern Utah are generally consistent and suggest that temperatures in the mid‐1800s are, on average, cooler than previous centuries, followed by about 0.6°C of warming in this century. Attention is given to the temporal resolution of our GST solutions showing the time‐smearing effects of heat conduction on the solutions. GST solutions represent an average ground temperature over a time window that expands as we look farther into the past. The size of the time window is a function of measurement and geologic noise and limits the ultimate resolution of GST reconstructions.
A theory of intraplate tectonics
Authors: Wdowinski Shimon
Published: February 2004

The theory of plate tectonics assumes a rigid behavior of tectonic plates and therefore fails to account for observed intraplate deformation. A new theory of intraplate tectonics is developed to calculate the first‐order intraplate deformation induced by horizontal displacement of deformable plate boundaries. It is based on simple assumptions that link the well‐established directions of relative plate motion to the displacement and deformation fields within a plate interior adjacent to three types of deformable plate boundaries: inward‐, outward‐, and tangential‐displaced boundaries. The theory predicts the direction of intraplate displacement, displacement rate, strain, and stress fields in terms of small circles, great circles, and 45° loxodromes around the pole of rotation of two adjacent plates. The 45° loxodromes are two orthogonal sets of directions, clockwise and counterclockwise, that intersect both small and great circles at 45°. The principal axis of the maximum horizontal stress follows small circles for inward‐displaced boundaries, great circles for out ward‐displaced boundaries, and loxodromes for tangential‐displaced boundaries. The theoretical predictions are systematically compared with more than 4000 reliable observed directions of maximum horizontal stress provided by the world stress map project [Zoback, 1992]. The theory indicates that the first‐order intraplate deformation is predominantly induced by horizontal forces acting on plate boundaries and by buoyancy forces that arise from lateral density variations between mid‐ocean ridges and plate interiors (ridge push). The simplicity of the predictions and their good agreement with observations suggests that intraplate deformation should be investigated in the pole of rotation spherical coordinate system.
Single‐domain‐like behavior in a 3‐mm natural single crystal of magnetite
Authors: Özdemir Özden , Dunlop David J.
Published: February 2004

We have observed single‐domain (SD) like behavior in a 3‐mm natural single crystal of magnetite following low‐temperature demagnetization (LTD), which consists of zero‐field cycling through the Verwey transition to erase remanence carried by pinned domain walls. We compared stepwise alternating field (AF) and thermal demagnetization curves of 1‐mT total thermoremanent magnetization (TRM), 1‐mT partial TRM (pTRM) acquired between the Curie point (TC = 575°C) and 565°C, and saturation isothermal remanent magnetization (SIRM) measured with and without prior LTD. AF demagnetization curves of untreated TRM and SIRM decreased exponentially with increasing AF. SIRM was more resistant to demagnetization than was TRM, a multidomain (MD) result of the Lowrie‐Fuller test. After LTD the TRM and SIRM memories had AF demagnetization curves with sigmoid shapes and initial plateaus below 8 mT in which little or no demagnetization occurred. Both features are reminiscent of SD behavior. During thermal demagnetization, untreated TRM and SIRM decreased almost linearly with increasing temperature up to ≈500°C. Such distributed unblocking temperatures TUB are expected for pinned walls in MD grains. The remaining 60% of TRM and 40% of SIRM were lost over a narrow temperature interval concentrated between 560°C and TC. The TRM memory after LTD was very stable against thermal demagnetization. There was no decrease in remanence below 550°C and very little change until 565°C, only 10°C below TC. This very high TUB fraction of TRM seems to have SD‐like character. SIRM has similar behavior. In the case of pTRM, both the untreated remanence and the memory after LTD have almost entirely high TUBs, identical to the range of pTRM blocking temperatures TB, 565°C–TC. TUB = TB is a basic property of SD partial TRM. The pTRM memory fraction is also larger than that of total TRM or SIRM. These observations suggest that pTRM(TC, 565°C) isolates an SD‐like fraction of remanence similar to that constituting TRM and SIRM memory. The high AF coercivities of the TRM and SIRM memories indicate that the source of SD‐like behavior may be unusually strong domain wall pinning by crystal defects formed at monoclinic twin boundaries as a result of crystal distortion from cubic to monoclinic structure in passing through the Verwey transition.
Chemical‐ and Clapeyron‐induced buoyancy at the 660 km discontinuity
Authors: Weidner Donald J. , Wang Yanbin
Published: February 2004

The impact of phase equilibria and equation of state measurements on defining the buoyancy associated with the 660 km discontinuity is examined. Aluminum has a striking effect on the sequence of phase transitions expected at 660 km. With a pyrolite chemistry the sign of the phase transition contribution to buoyancy depends on temperature, with convection‐assisting forces being victorious in hotter regions. Lower Al and/or temperature result in the perovskite‐forming transformation resisting convection but with a much reduced magnitude compared with a pure olivine mantle. Density considerations suggest that there is no positive evidence for chemical‐induced buoyancy across the 660 km discontinuity. The surface processing which forms a basaltic crust will cause a density contrast that assists convection in the transition zone but resists convection through the expanded garnet stability field of mid‐ocean ridge basalt. The detailed seismic character of the 660 km depth region and its lateral variations hold the key to defining the location of the mantle state relative to the parameter space defined by Al and temperature in this study.
Dike‐induced earthquakes: Theoretical considerations
Authors: Rubin Allan M. , Gillard Dominique
Published: February 2004

Earthquakes of magnitude 1 and greater seem to be ubiquitous features of dike propagation, but their origin is not well understood. We examine the elastic stress field surrounding propagating fluid‐filled cracks, with an emphasis on assessing the ambient stress required to produce earthquakes with linear dimensions of ∼100 m near dikes with linear dimensions of a few kilometers. An important feature of the solutions is the dike “tip cavity,” a low‐pressure region where magma cannot penetrate and where the stress field differs most from the classical near‐tip stress field. Two regions are considered: near the dike tip but away from the tip cavity and near the tip cavity. The stress state most conducive to failure occurs near the tip cavity when the cavity pressure is maintained by influx of host rock pore fluids rather than by exsolution of magmatic volatiles. Even in this case, however, shear fracture of previously intact rock seems unlikely. Thus most dike‐induced seismicity with a frequency content typical of “tectonic” earthquakes should be interpreted as resulting from slip along suitably aligned existing fractures. Production of magnitude 1 earthquakes appears to require either large ambient differential stresses or low ambient confining pressures; in the latter case, the effective normal stress on prospective faults may be low enough for slip to be aseismic. We conclude that the distribution of (recorded) dike‐induced seismicity reflects the distribution of ambient stresses that are near to failure and does not necessarily reflect the extent of the dike. This result is consistent with recent images of the seismicity associated with the 1983 dike intrusion at Kilauea.
Effect of the Pacific Ocean on the Earth's seasonal wobble inferred from National Center for Environmental Prediction ocean analysis data
Authors: Furuya Masato , Hamano Yozo
Published: February 2004

In order to assess oceanic influence on the excitation of the Earth's seasonal wobble, which is not totally explained by atmospheric angular momentum (AAM) we employed Pacific Ocean monthly analysis data from the U.S. National Center for Environmental Prediction for the period from 1980 to 1994 to compute the Pacific Ocean angular momentum (POAM) around two axes on the equatorial plane. These were X1 and X2 with respect to axis 1 and axis 2 which corresponded to motion along the Greenwich meridian and 90°E, respectively. In particular, we paid attention to the X1 component, which was barely influenced by AAM. The variance in the AAM plus POAM X1 was almost 10 times that of the AAM alone, while that of AAM plus POAM X2 decreased to ∼60% of that of AAM alone. Thus the mean seasonal cycle of POAM plus AAM roughly accounted for the observed wobble, and its year‐to‐year fluctuations overlapped with those of seasonal wobble excitation. We also found that the seasonal X1 POAM mostly consisted of the motion (current) term and that the mass term played a secondary role; this is in contrast to AAM's contribution to X2. Although intensive seasonal fluctuations in the X1 motion term were localized around the western Pacific region, we found that middle‐to‐higher‐ and lower‐latitude regions were inphase and out of phase with the basin‐wide contribution. However, the influence of the Pacific Ocean may be reduced by other unaccounted for sources such as effects from other oceanic basins and land water.
Three‐dimensional seismic refraction tomography: A comparison of two methods applied to data from the Faeroe Basin
Authors: Zelt Colin A. , Barton Penny J.
Published: February 2004

This paper presents a comparison of two tomographic methods for determining three‐dimensional (3‐D) velocity structure from first‐arrival travel time data. The first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays. The second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors. Both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration. Travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing. Both inverse methods are applied to 3‐D ocean bottom seismometer (OBS) data collected in 1993 over the Faeroe Basin, consisting of 53,479 travel times recorded at 29 OBSs. This is one of the most densely spaced, large‐scale, 3‐D seismic refraction experiments to date. Different starting models and values for the free parameters of each tomographic method are tested. A new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion, but the latter method provides a simpler model for little additional computational expense when applied to the Faeroe Basin data. Bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints. An inversion of synthetic data corresponding to 100% data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3‐D refraction experiments to provide well‐resolved, long‐wavelength velocity models. The similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion.
Relative motion between the Caribbean and North American plates and related boundary zone deformation from a decade of GPS observations
Authors: Dixon Timothy H. , Farina Frederic , DeMets Charles , Jansma Pamela , Mann Paul , Calais Eric
Published: February 2004

Global Positioning System (GPS) measurements in 1986, 1994, and 1995 at sites in Dominican Republic, Puerto Rico, Cuba, and Grand Turk define the velocity of the Caribbean plate relative to North America. The data show eastward motion of the Caribbean plate at a rate of 21 ± 1 mm/yr (1 standard error ) in the vicinity of southern Dominican Republic, a factor of 2 higher than the NUVEL‐1A plate motion model prediction of 11 ± 3 mm/yr. Independent measurements on San Andres Island, and an Euler vector derived from these data, also suggest a rate that is much higher than the NUVEL‐1A model. Available data, combined with simple elastic strain models, give the following slip rate estimates for major left‐lateral faults in Hispaniola: (1) the North Hispaniola fault offshore the north coast of Hispaniola, 4 ± 3 mm/yr; (2) the Septentrional fault in northern Dominican Republic, 8 ± 3 mm/yr; and (3) the Enriquillo fault in southern Dominican Republic and Haiti, 8 ± 4 mm yr. The relatively high plate motion rate and fault slip rates suggested by our study, combined with evidence for strain accumulation and historical seismicity, imply that seismic risk in the region may be higher than previous estimates based on low plate rate/low fault slip rate models and the relatively low rate of seismicity over the last century.
Crustal velocity structure across the eastern Snake River Plain and the Yellowstone swell
Authors: Peng Xiaohua , Humphreys Eugene D.
Published: February 2004

Teleseismic receiver functions are used to estimate the crustal structure beneath a 36‐station, 500‐km‐long, NW oriented linear array centered on the eastern Snake River Plain and crossing the Yellowstone hotspot swell 250 km SW of Yellowstone. Structure derived previously for this region from wide‐angle reflection data is used as an initial model, and this structure explains most features observed in our receiver functions. Based on a combination of forward and inverse modeling, our data require several modifications to the initial structure: (1) Moho depth is ∼42 km beneath most of the Snake River Plain, shallows to ∼37 km to either side, and thickens abruptly to ∼47 km beneath SW Wyoming; (2) a midcrustal layer interpreted previously as a ∼9‐km‐thick gabbroic sill is flat topped across the entire ∼90 km width of the Snake River Plain; and (3) a low‐velocity layer is found beneath and southeast of the Snake River Plain, which probably is partially molten lower‐most crust. Using the seismic structure of the crust to estimate the crustal load upon the mantle, and assuming local isostasy, we calculate that mantle beneath the Yellowstone swell is approximately uniformly as buoyant as 12‐million‐year‐old ocean mantle, and more buoyant than the adjacent Wyoming mantle by an amount equivalent of ∼1.5 km of elevation. The transition between these regions of greatly different mantle occurs across a major Paleozoic boundary that now separates the Basin and Range from the Rocky Mountains.
Anomalous rotation of the inner core and the toroidal magnetic field
Authors: Aurnou Jonathan , Brito Daniel , Olson Peter
Published: February 2004

We use numerical calculations of magnetic induction by axisymmetric motions in a spherical shell of conducting fluid to investigate the relationship between the Earth's toroidal magnetic field and the time‐dependent anomalous rotation of the solid inner core. We compute the induced toroidal magnetic field and inner core rotation maintained by the interaction of time‐independent, axisymmetric outer core fluid flow with models of the poloidal magnetic field. Three possible models of the azimuthal flow in the outer core are investigated: two thermal wind flows inside the tangent cylinder (predicted by some numerical models of the geodynamo) and a columnar flow outside the inner core tangent cylinder inferred from the geomagnetic westward drift. Results indicate that electromagnetic torques tightly couple the inner core rotation to the fluid motion. Electromagnetic spin‐up of the inner core occurs through damped torsional oscillations with periodicity near 4 years depending on the strength of the poloidal magnetic field. In steady state the thermal winds inside the tangent cylinder generate a peak toroidal field of 25 mT accompanying a prograde inner core rotation rate of 1°/yr. In contrast, the columnar westward drift model generates toroidal field with peak intensity near 4 mT and a small, retrograde anomalous inner core rotation of −0.013°/yr. The weak retrograde motion of the inner core produced by electromagnetic coupling to the westward drift cannot explain the seismically inferred prograde anomalous rotation.
The use of high‐resolution seismic reflection profiles for fault analysis in the near‐shore environment, Weymouth Bay, Dorset, England, United Kingdom
Authors: Hunsdale Robert , Bull Jon M. , Dix Justin K. , Sanderson David J.
Published: February 2004

High‐resolution seismic reflection profiles using a Chirp source image a north‐south extensional fault set, which cuts rocks of Upper Jurassic age, cropping out on the seafloor of Weymouth Bay, Dorset, England, United Kingdom. The same fault set cuts rocks of similar age along the adjacent coast, and field mapping can be compared directly with the Chirp profiles. Survey lines were shot perpendicular to the fault strike to produce dip sections from which displacements could be measured. One hundred and fifty‐three faults were picked on a 15 km line, yielding a fault density of ∼10 km−1, similar to that measured in the coastal section. Chirp‐resolved fault displacements as small as 0.5 m and a maximum fault displacement of 221 m could be inferred from the data. Distribution analysis of offshore fault data indicated that fault displacement is power law with a well constrained exponent of −0.9. This value is consistent with the power law exponent estimate for fault displacement, over the scale range 2–8 m, onshore. Thus Chirp near‐shore seismic reflection profiles can infill a data gap for fault size‐frequency relationships that commonly occurs when combining data from outcrops/cores and conventional seismic reflection profiles.
Relaxation of transient ionization in the lower ionosphere
Authors: Rodger Craig J. , Molchanov Oleg A. , Thomson Neil R.
Published: February 2004

This paper presents a set of basic expressions and parameters to describe the relaxation of ionization in the upper atmosphere and lower ionosphere. Both spatial (diffusion) and chemical loss are included, along with the varying electrical conductivity caused by changing electron density and temperature. In particular, reasonable values for the attachment rates and recombination coefficients at these high altitudes are discussed. Example calculations are given involving studies into red sprites. However, the modeling and parameters are not restricted to these events, and might well be useful in a number of other modeling problems.
On the origin of the ion‐electron temperature difference in the plasma sheet
Authors: Schriver D. , Ashour‐Abdalla M. , Richard R. L.
Published: February 2004

The results of a study of proton and electron acceleration in the Earth's magnetotail are presented. By following the trajectories of thousands of charged particles launched from mantle and tail lobe source regions, distribution functions are calculated at different locations in a model magnetotail. The magnetic field is based on the Tsyganenko [1989] model combined with a constant cross‐tail convection electric field. Despite the simplicity of the model and the lack of self‐consistent fields, a qualitative picture of the proton/electron plasma sheet emerges including an ion to electron temperature ratio Ti/Te ranging from about 4 to 6 in the magnetotail, in approximate agreement with plasma sheet observations. To explain this result, an analytic expression for Ti/Te is derived based on the particle motion of ions and electrons in a current sheet configuration with a varying normal magnetic field component. The derived expression depends on the ion to electron mass ratio to the one‐third power (mi/me)1/3 and a factor that takes the local field gradient into account. Using numerical values from the Tsyganenko [1989] field model in the derived equation gives Ti/Te ∼ 5. Another result is that the heated electron distribution functions formed in the plasma sheet are not Maxwellian but instead have power law high‐energy tails much like the so‐called “kappa” distributions reported by Christon et al. [1989]. At the edge of the plasma sheet, the calculations show the electron plasma sheet boundary layer extends further towards the lobe than the ion plasma sheet boundary layer, also in agreement with observations [Takahashi and Hones, 1988]. Nonisotropic distribution functions form at different locations in the plasma sheet, including electron beams streaming along field lines just inside the separatrix in the deep magnetotail. Electron distributions that are highly skewed in velocity space are found very near the magnetic null point. The nonisotropic distributions suggest that plasma instabilities and wave‐particle interactions could occur in those regions. That such a simple model should reproduce many of the features of the observed plasma sheet indicates that adiabatic and nonadiabatic single‐particle motion play important roles in the quiet time magnetotail and suggests that ion and electron plasma sheet formation is a natural consequence of single‐particle motion in an X line type magnetotail geometry.
Two‐fluid theory of drift‐kink instability in a one‐dimensional neutral sheet
Authors: Yoon Peter H. , Lui A. T. Y. , Wong H. K.
Published: February 2004

A thin current sheet with thickness comparable to the thermal ion gyroradius possesses free energy capable of driving a number of plasma instabilities for its destabilization. Among these instabilities is the drift‐kink instability (DKI) unveiled through previous numerical simulations. To advance our theoretical understanding of DKI, we formulate a two‐fluid theory with finite ion and electron temperatures in a one‐dimensional (Harris) current sheet to examine the linear properties of DKI. We reduce the stability analysis to solving a nonlocal eigenvalue equation. We find that the eigenvalue equation gives a number of growing modes with both symmetric and antisymmetric magnetic perturbations with respect to the neutral sheet. These solutions have high growth rates, generally of the same order as their real frequencies, which are sizable fractions of the ion gyrofrequencies evaluated outside the current sheet. Near the center of the current sheet, the eigenfunction exhibits fine spatial structures with dimensions much smaller than the ion inertial length. These structures become progressively broadened as the ion to electron mass ratio is arbitrarily reduced. This theoretical result is potentially useful in assessing the impact of adopting unrealistic ion to electron mass ratio in numerical simulations of DKI or thin current sheet stability.
Observations of black auroral patches and of their relationship to other types of aurora
Authors: Kimball J. , Hallinan T. J.
Published: February 2004

A morphological study of black patches and newly reported black rings (black patches with islands of embedded diffuse aurora) was conducted using an archive of low‐light level all‐sky and narrow‐field television camera data. The overwhelming majority of black patches and rings drifted eastward, poleward of westward drifting pulsating aurora in the early recovery phase of the auroral substorm. Occasionally, black aurora would overlap with pulsating aurora. Sometimes the pulsations would add to the intensity in the black aurora more than in the surrounding diffuse aurora (called veiling). One example was documented where the pulsation was purely additive, adding to the intensity of the black aurora and the surrounding diffuse aurora equally. Cases of total veiling suggest an intensity limit, possibly associated with the strong pitch angle diffusion limit.
Effect of magnetic turbulence on the ion dynamics in the distant magnetotail
Authors: Veltri P. , Zimbardo G. , Taktakishvili A. L. , Zelenyi L. M.
Published: February 2004

The ion dynamics in the distant Earth's magnetotail is studied in the case that a cross tail electric field E0 and reconnection‐driven magnetic turbulence are present in the neutral sheet. The magnetic turbulence observed by the Geotail spacecraft is modeled numerically by a power law magnetic fluctuation spectrum. The magnetic fluctuations have the tearing mode parity with respect to the neutral sheet and are superimposed on a modified Harris sheet. A test particle simulation is performed for the ions, and the particle density, current density, bulk velocity, temperature, pressure, and heat flux are obtained for every point in the distant tail and as a function of the magnetic fluctuation level, δB/B0. It appears that the magnetic turbulence is very effective in maintaining the stationary structure of the current sheet and in changing the ion acceleration due to the electric field to thermal motion. Also, magnetic turbulence can inflate the current carrying region up to a thick current sheet, in contrast with the often assumed thin current sheet. The values obtained for the ion temperature are consistent with those observed in the distant tail by the Geotail spacecraft. The main results are the following: (1) the thickness of the current sheet increases with the level of fluctuations. The thickness λ corresponding to the average magnetic field current profile is obtained for δB/B0 ≃ 0.3. (2) The magnetic pressure outside the current sheet is balanced by particle pressure for δB/B0 ≃ 0.3. This is obtained mostly by an increase in the temperature, while the density profile is not much peaked in the neutral sheet. (3) For low fluctuation levels, heating is anisotropic, most heating going into the y direction; increasing δB/B0 and making reference to the local magnetic field, more heating goes in the parallel rather than in the perpendicular direction, in agreement with part of the observations. (4) A possible splitting of the bulk velocity and of the current density in two sheets is obtained for δB/B0 ≥ 0.2. In general, a relevant level of magnetic turbulence, like δB/B0 ≃ 0.3, appears to be a basic ingredient of the distant magnetotail equilibrium structure.
Observations of mesoscale auroral plasma cavity crossings with the Freja satellite
Authors: Mäkelä Jakke S. , Mälkki Anssi , Koskinen Hannu , Boehm Manfred , Holback Bengt , Eliasson Lars
Published: February 2004

We have used data from the Freja satellite, with an apogee of 1700 km over the auroral oval, to make a statistical analysis of mesoscale auroral plasma density cavities. Such cavities are defined to be regions in the auroral zone with size scales between discrete arcs and inverted‐V events in which the plasma density abruptly drops to less than 50% of the ambient plasma density. We present occurrence distributions by magnetic activity, local time, and magnetic latitude. It is found that cavity occurrence does not depend on magnetic activity. In contrast to earlier studies, we find cavities at all local times; however, a dawn‐dusk asymmetry is found in the distribution. There is likewise a weak dawn‐dusk asymmetry in the latitudinal distribution. Since our data cover a full year, seasonal dependence of cavity occurrence can also be investigated. We find a small seasonal trend, with cavities found more commonly during the dark season, but the trend is not quite statistically significant. We also study several plasma depletion processes that could be responsible for cavities at this altitude. The results suggest that more than one process is compatible with the distributions we find.
A globally integrated substorm model: Tail reconnection and magnetosphere‐ionosphere coupling
Authors: Kan J. R.
Published: February 2004

We present a globally integrated substorm (GIS) model in which the patchybursty tail reconnection process at ∼20–35 RE and the magnetosphere‐ionosphere (M‐I) coupling process form a global feedback system to cause substorms. We identify the necessary and sufficient conditions for the expansion onset in the near‐Earth plasma sheet. The necessary condition for substorm onset is an abrupt convection braking in the near‐Earth plasma sheet at a rate ≥∼3 (mV/m)/RE, which is equivalent to dipolarizing the taillike field at a rate ≥∼0.5 nT/s. The sufficient condition requires the disrupted tail current to be carried away by the Alfvén wave to form the substorm current wedge. These conditions must be satisfied by any onset mechanism. The Alfvén wave launched by the dipolarization‐induced electric field must carry away the disrupted tail current from the dipolarization region. As an example, we show that substorm expansion onset can be triggered by abruptly braking a bursty bulk flow of ∼3 mV/m within ∼1 RE. The disrupted tail current at the expansion onset is ∼105 A/RE carried away by the Alfvén wave launched by the dipolarization‐induced electric field of ∼14 mV/m. Substorm expansion onset in the ionosphere occurs ∼1 min later when the Alfvén wave is reflected at the ionosphere. The dipolarization‐induced tail reconnection and the M‐I coupling form a feedback system to power the ongoing expansion phase which typically lasts ∼30 min. As the dipolarizing region expands tailward, the expansion phase turns into recovery phase when the local tail current becomes too weak to sustain the expansion phase and the aurora activities start to retreat equatorward.