bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of seismicity parameters in different subduction zones and its implications for the Cascadia Subduction Zone
Authors: Acharya Hemendra
Published: February 2008

The Cascadia subduction zone is a young subduction zone with associated volcanic activity but with no history of large thrust earthquakes associated with it as is characteristic of other subduction zones. This has led to questions about the potential for great earthquakes associated with this subduction. The seismic potential associated with this subduction has been investigated in this study by examining and comparing the number of shallow earthquakes with magnitude >6 which have occurred in different subduction zones since 1910. This examination shows that all subduction zones exhibit a higher level of activity than Cascadia. Furthermore, two of these zones with similar rates of plate motion and age at subduction, the Rivera plate and southwest Japan, have not only experienced great earthquakes but show a higher rate of activity than the Cascadia zone. The activity rate in subduction zones with predominantly aseismic slip is also higher than the activity rate in the Cascadia zone. Earthquake activity in the rupture zones of great earthquakes both before and after the occurrence of great earthquakes is higher than the activity rate in the Cascadia zone. These results therefore suggest that the low rate of activity in the Cascadia zone is unusual and supports the idea that the subduction here is different and that perhaps the potential for great earthquakes cannot be extrapolated simply on the basis of certain similarities between the Cascadia zone and other subduction zones with a history of great earthquakes. The low rate of earthquake activity in the Cascadia zone can, however, be explained on the basis of the combined effect of low rate of plate motion, significant aseismic subduction, and young age at subduction. The activity rate in the Cascadia zone is consistent with the activity rates in other subduction zones if it is assumed that much of the plate motion is accommodated aseismically. The seismic slip rate associated with this subduction zone is a small fraction of the rate of plate motion, and therefore even if the potential for great earthquakes may exist, the return period for great earthquakes will be much longer (2 or 3 times) than in other zones with similar characteristics such as the Rivera plate or southwest Japan.
Gravity field approximation using airborne gravity gradiometer data
Authors: Arabelos D. , Tziavos I. N.
Published: February 2008

A set of airborne Gravity Gradiometer Survey System (GGSS) data was used in combination with the OSU89B geopotential model to predict gravity anomalies and deflections of the vertical. The gradiometer data were collected during an airborne survey of the GGSS in the Texas/Oklahoma area. Nineteen flight tracks were in our disposal for further analysis. These tracks resulted from 54 initial distinct tracks from which a processing was performed by the Analytic Sciences Corporation. In order to assess the quality of the GGSS measurements, a first crossover analysis was carried out. The results showed large discrepancies at the crossing points between East‐West (EW) and North‐South (NS) tracks. The least squares collocation method and the fast fourier transform technique were used to produce gravity anomalies and deflections of the vertical referred to geoid surface from second‐order derivatives measured at the flight level. The predicted gravity values were then compared to “truth‐based” point free air gravity anomalies with a standard deviation (s.d.) of the differences at the level of 3 mGal in the best cases. Presumably, improved GGSS data would have given improved predictions of surface gravity values. The predicted deflections of the vertical were compared to “truth‐based” values at a small number of ground stations. The very low accuracy of this prediction (s.d.=5.62 arc sec for ξ and 6.78 for η) should be studied in combination with the inherent characteristics of the gravity field and the validation of GGSS data in the test area.
Seismicity and shape of the subducted Nazca Plate
Authors: Cahill Thomas , Isacks Bryan L.
Published: February 2008

An updated compilation of earthquake locations and focal mechanism solutions from the International Seismological Centre and Preliminary Determination of Earthquakes is the basis of a comprehensive study of the geometry of the Wadati‐Benioff zone beneath western South America. The new data support previous mapping of a sharp flexure rather than a tear in the subducted Nazca plate beneath southern Peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with ∼30° dip at latitude 33°S. In contrast, the transition from 30° slab dip beneath Bolivia to a nearly horizontal dip in the region between 28°S–32°S is more gradual, occurring over several hundred kilometers of along‐strike distance between 20°S and 32°S. This southward flattening corresponds to a broadening of a horizontal, benchlike part of the subducted plate formed between 100 and 125 km depth. The transition in continental tectonic style near 27°S–28°S, from a wide, volcanically active plateau to a narrow, nonvolcanic cordillera, appears not to be associated with the main slab flattening, which begins to the north of these latitudes, but with a more abrupt change in curvature of the subducted slab, from convex upward to concave upward, immediately below the plate boundary interface. The concept of Gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the South American plate. We hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin. Focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses. Results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths.
Possible source models for the 1855 Wairarapa Earthquake, New Zealand
Authors: Darby Desmond J. , Beanland Sarah
Published: February 2008

The magnitude 8 Wairarapa, New Zealand, earthquake of 1855 was associated with surface rupture along the Wairarapa fault and regional uplift of the southwest of the North Island. Forward elastic dislocation modelling shows that movement on a steeply dipping Wairarapa fault alone cannot account for the recorded deformation data. Modelling of movement on the subduction interface that underlies the Wellington region as well as the Wairarapa fault also fails to produce a satisfactory ill to the data. Although a complex Wairarapa fault model may be able to explain the deformation pattern if its location, subsurface geometry, and slip distribution could be independently constrained, the best effort supported by available data, a flexed model incorporating a left side step of 8 km at the surface, incorrectly locates the deformation. The best fit to the data is obtained from a listric Wairarapa fault model involving rupture on 0 to 50 km width of the deeper part of the subduction interface. The shallower pan of the subduction interface, east of the Wairarapa fault, apparently did not rupture in 1855, and the uplift mechanism for the overlying Aorangi Range remains unexplained. Partitioning of strike‐slip and dip‐slip components of the relative plate motions may involve separate earthquakes. Seismological verification of listric fault rupture mechanisms is required to determine the plausibility of the listric model presented here, because its implications arc that the 1855 earthquake did not completely account for the relative plate motion in the region.
The tectonic evolution of the Indian Ocean Triple Junction, anomaly 6 to present
Authors: Mitchell Neil C. , Parson Lindsay M.
Published: February 2008

The tectonic evolution of the Indian Ocean ridge‐ridge‐ridge triple junction, the junction of the Central Indian Ridge (CIR), Southeast Indian Ridge (SEIR) and Southwest Indian Ridge (SWIR), is inferred by mapping structures along its traces on the three plates with a long‐range side scan sonar, Geological Long‐Range Inclined Asdic (GLORIA). The sonar images to the west of the triple junction show two different styles of tectonic evolution. Some sections of the trace on the African plate are marked by a SWIR‐facing scarp containing fine lineaments that are probably normal faults. These correspond with sections of the trace on the Antarctic plate of the same age that have a sharp intersection of SWIR and SEIR abyssal hills that is to be expected from a simple ridge‐ridge‐ridge triple junction evolution (a “herringbone” pattern of isochrons). In contrast there are other sections of the trace on the African plate that have a blocky structure with no definite termination of CIR abyssal hills against those of the SWIR. The corresponding regions of the Antarctic plate show large 10‐ to 30‐km‐long overlapping fault scarps, tapering to the northeast and ending in asymmetric valleys to the southwest. These major normal faults are each offset by 1–6 km, forming an en echelon series. The two different styles of tectonic structures indicate two different modes of evolution, although why the triple junction has evolved with two different modes is unclear. Periods of asymmetric spreading in the CIR magnetic anomaly sequence collected on the African plate may possibly indicate that the CIR and SEIR have been offset by a fracture zone at the triple junction during one of these two modes. Echo sounder traverses on the Antarctic and African plates commonly show a steady shallowing of the CIR and SEIR seafloor toward the traces, a steep scarp facing the SWIR and a broad marginal deep at the base of this scarp. This rift flank shape resembles that of the SWIR valley's rift shoulders close to the present triple junction (Mitchell, 1991a), suggesting that the mechanical evolution has been similar to the present day and the relief due to the rift flank uplift has been preserved by the cooling and strengthening of the lithosphere with age away from the ridge axes. If the shape of the rift shoulder uplift along the trace on the Antarctic plate is interpreted with a simple flexural model involving an end‐loaded elastic plate, the inferred rigidity (1019‐1021 N m or 2–6 km effective elastic thickness) is consistent with the suggestion that the isostatic uplift occurred in young, weak lithosphere, close to the triple junction. Interestingly, the observed ∼1‐km relief of the trace escarpment and the estimated rigidity suggest that the uplift was produced by a load (2–5 × 1011 N/m) that is equivalent to half the present cross‐sectional area of the SWIR valley 35 km from the triple junction.
Ground ruptures of the 1974 and 1983 Kaoiki Earthquakes, Mauna Loa Volcano, Hawaii
Authors: Jackson Marie D. , Endo Elliott T. , Delaney Paul T. , Arnadottir Thora , Rubin Allan M.
Published: February 2008

The November 30, 1974, ML = 5.5 and November 16, 1983, ML = 6.6 earthquakes generated left‐stepping, en echelon ground cracks within the Kaoiki seismic zone, on the southeast flank of Mauna Loa volcano, Hawaii. The general trend of the ruptures, N48°–55°E, parallels a nodal plane of the main shocks' focal mechanisms. The ruptures themselves consist of short, predominantly extension cracks, which are up to 20 m long and strike roughly E‐W, 30°–50° clockwise from the overall trend of the zones. Some of the cracks are linked by secondary fractures and rubble breccia to form left‐stepping crack arrays, which are themselves linked to form longer en echelon systems of ground rupture. Geologic maps and field observations indicate that these features emerge from an underlying strike‐slip fault, and they form a “fracture‐process zone” above its tip. The maximum displacement measured across cracks in the 1983 rupture zone is 0.5 m. Trilateration data, however, suggests that the overall shear displacement was about 1.5 m at depth. Elastic solutions indicate that a region of significant tensile stress can exist above buried strike‐slip faults. We suggest that these stresses generated the extensional ground cracks and that shear displacements were transmitted to the Earth's surface by subsequent growth and linkage of these cracks into the observed arrays. We infer that the crack arrays accommodate increased displacement with depth and they merge downward into the “parent” strike‐slip fault at an estimated 1–2 km depth, where strike‐slip displacement was probably more or less continuous along the ∼7 km length of the rupture. In the Kaoiki region, only three major ground ruptures traverse a series of basaltic lava flows that date back 1500 years. This suggests that the recent ∼10‐year periodicity of moderate‐magnitude Kaoiki strike‐slip events may not have extended far into the past. The tectonic significance of strike‐slip faulting on Mauna Loa volcano remains enigmatic.
Degree of melting and source composition of Cenozoic basalts in southwest Japan: Evidence for mantle upwelling by flux melting
Authors: Iwamori Hikaru
Published: February 2008

Based on a study of Cenozoic basalts in southwest Japan where mantle upwelling is considered to have occurred, the degree of melting for primary magmas and weight fractions of residual phases are quantitatively estimated using mass balances for major and compatible components among magma, residual phases and source. Assemblage and composition of the residual phases are obtained from melting experiments of primary magmas, and various source compositions were assumed. The calculation shows an increase of degree of melting toward the upwelling center from approximately 4 to 20%. This is consistent with a change in the residue from a spinel Iherzolite to a harzburgite, which was determined by melting experiments. We also obtain constraints on the source composition. Peridotite with more than 3.0 wt % Al2O3 and 2.0 wt % CaO and a high Al2O3/CaO ratio of approximately 1.5 is suitable as a source. Incompatible element concentrations of the source for Cenozoic basalts in southwest Japan are calculated assuming a batch melting model. At the center of upwelling, the source is anomalously enriched in incompatible elements (up to 200 times compared with primitive mantle model) excepting Nb, while that of upwelling margin has primitive or slightly enriched concentrations. These variations can be explained by addition of fluid (flux melting) and subsequent upwelling of enriched material from the deep mantle (e.g., lower‐upper mantle boundary or core‐mantle boundary). The flux melting model also explains the low potential temperature of approximately 1300 °C of the upwelling mantle. Cenozoic volcanism in southwest Japan may have been caused with a plume with abundant volatile and incompatible components rather than a plume of anomalously high temperature (e.g., Hawaii).
Petrogenesis and volatile stratigraphy of the Bishop Tuff: Evidence from melt inclusion analysis
Authors: Dunbar Nelia W. , Hervig Richard L.
Published: February 2008

The Bishop Tuff (BT), erupted from the Long Valley Caldera at 0.74 Ma, is composed of a Plinian tephra and ignimbrite (Hildreth, 1979). Based on ion and electron microprobe analyses of melt inclusions (MI) in quartz and sanidine phenocrysts, a strong H2O gradient was present in the upper portion of the magma chamber, mainly in the first 120 km3 of erupted material. The H2O content of the magma which formed the Plinian tephra drops from 6 wt % to 3.5 wt %. In contrast, the magma which formed the ignimbrite contained a relatively constant amount of H2O, between 2 and 4 wt %. The strong drop in H2O content of the magma which formed the Plinian tephra suggests that only in its extreme upper part, if any, was the PH20 in the magma close to Ptotal. The halogen content of the BT magma was low and relatively constant at ∼700 ppm Cl and ∼ 500 ppm F. The trace and major element composition of MI from the Plinian and first‐erupted ash flow lobes of the BT are similar to that of bulk tephra. The range in the trace element concentrations of inclusions suggests that fractional crystallization may have affected magmatic composition. However, in addition to fractional crystallization, the composition of MI, phenocrysts and bulk pumice lumps suggests that magma mixing may have been an important process in establishing the final trace element zonation within the Bishop magma chamber and may be responsible for some of the most dramatic observed trace element variations. The BT eruption appears to have removed sequential stratigraphic compositional layers from the magma chamber, whereas fhe Lower Bandelier Tuff (LBT) appears to have erupted chaotically, although both magma chambers are characterized by essentially the same volatile and therefore density zonation. However, the H2O content of the Plinian:ignimbrite transition is different for the two (∼3 wt % for the BT, 4–5 wt % for the LBT), suggesting that the LBT Plinian eruption may have ended prematurely, possibly due to caldera collapse. Therefore, the magma withdrawal dynamics of these eruptions may be more strongly controlled by external factors, such as vent configuration, rather that the volatile gradient of the melt.
Continuous gravity observations using Joint Institute for Laboratory Astrophysics Absolute Gravimeters
Authors: Niebauer T. M. , Faller J. E.
Published: February 2008

We discuss two different 1‐month gravity records taken with two different Joint Institute for Laboratory Astrophysics (JILA) gravimeters. These records were corrected for the effects of Earth tides, ocean loading, local and global barometric effects, and polar motion. These data allowed precise determinations of the local air pressure admittance. We also obtained a value for the second‐order gravimetric factor which agreed with the theoretical value to about 0.3% where the predominant uncertainty is the ocean load modeling. The corrected noise spectra were white above 1 cycle/d with a spectral noise density of about
. We note that absolute gravimeters have a sensitivity to time varying gravity that is comparable to that obtained with superconducting relative gravimeters. The sensitivity of these data emphasizes the need for better modeling of ocean loads, air pressure effects, and even man‐made environmental noise.
Interseismic deformation at the Nankai Trough, Japan, subduction zone
Authors: Savage J. C. , Thatcher W.
Published: February 2008

Uplift along the coast of southwestern Japan following the 1944 Tonankai (Ms = 8.0) and 1946 Nankaido (Ms = 8.2) earthquakes has been inferred from the 1950–1985 tide gage records. Although uplift rates vary with geographic position, the temporal dependence at each site can be described as the superposition of an exponentially decaying (time constant ∼5 years) transient, significant only in the first decade postseismic, and a steady interseismic rate. The steady interseismic uplift rate is defined only over the 1956–1985 interval, but the linearity in time ofthat uplift record is such that uplift over the remainder of the interseismic cycle (∼80 years) might reasonably be expected to occur at the same rate. Moreover, the spatial pattern and magnitude of the steady uplift rate are reasonably close to those predicted by a simple dislocation model of subduction. However, the currently observed steady uplift rate is not consistent with the uplift rate inferred from geodetic leveling for the final half of the previous earthquake cycle. Thus, either the hypothesis of a steady interseismic uplift rate after the first decade postseismic is wrong or the strain accumulation pattern for one earthquake need not be the same as for the preceding earthquake on the same segment.
Very long period magnetotellurics at Tucson Observatory: Estimation of impedances
Authors: Egbert Gary D. , Booker John R. , Schultz Adam
Published: February 2008

Eleven years (1932–1942) of electric potential and magnetic measurements at the Tucson observatory represent a unique very long period magnetotelluric (MT) data set. We report here on a careful reanalysis of this data using modern processing techniques. We have developed and used novel methods for separating out the quasi‐periodic daily variation fields and for cleaning up outliers and filling in missing data in the time domain. MT impedance tensors, estimated using the cleaned and filled data and using robust frequency domain methods, are well determined and smoothly varying for periods between 4 hours and 10 days. At longer periods the electric field data are swamped by large‐amplitude incoherent noise, particularly after the third year of the experiment. Although we find no evidence for contamination of any field components by oceanic motional induction at tidal periods, the MT impedance estimates do show evidence of small systematic biases due to finite spatial scale geomagnetic sources at harmonies of the daily variation period. These periods are thus removed from the time series and not used in further analysis. We show that the resulting impedance tensor is well modeled by a real, frequency‐independent distortion of a scalar impedance, which is consistent with non‐inductive distortion of the electric fields by local surface geology. To estimate the undetermined static shift of the MT impedance, we compare the long‐period MT results to equivalent MT impedances determined from 46 years of geomagnetic data. Combining the geomagnetic and undistorted MT impedances results in scalar impedance estimates for periods 0.17<T<91 days of unprecedented precision. However, for periods less than one day, the phase and amplitude of this impedance, while individually consistent, are not mutually consistent with any one‐dimensional conductivity distribution. The inconsistency probably results from a combination of subtle multidimensional effects and systematic biases.
Models of quartz overgrowth and vein formation: Deformation and episodic fluid flow in an ancient subduction zone
Authors: Fisher Donald M. , Brantley Susan L.
Published: February 2008

Steady state models of overgrowth and vein formation are developed using kinetic data for quartz dissolution and precipitation and estimates of fluid advection, pore‐fluid and grain‐boundary diffusion. Application of these models to overgrowths and veins in the Kodiak accretionary complex suggests that the Kodiak Formation deformed continuously by a grain‐boundary diffusion‐limited mechanism, accompanied by episodic pore fluid diffusion of quartz from the matrix to vertical fluid‐filled fractures near the base of the accretionary wedge. These processes produced two types of syntectonic crystal textures within the Kodiak Formation: overgrowths containing displacement‐controlled fibers, and throughgoing veins composed of face‐controlled elongate blocky quartz crystals. Based on textural observations, displacement‐controlled quartz growth in overgrowths is rate‐limited by either diffusion along a cohesive interface or the rate of matrix strain. The magnitude of elongation recorded by displacement‐controlled crystal growth varies smoothly (elongation of 1 to 3) from the shallowest to the deepest structural levels of the Kodiak Formation, suggesting that the diffusional component of deformation in the accretionary wedge increases with depth. In contrast, face‐controlled quartz growth is largely restricted to Veins within the deepest level, where the cleavage is subhorizontal and deformation involves a component of simple shear, suggesting proximity to a decollement. The face‐controlled quartz veins represent mode I cracks which seal periodically and contain continuous planar solid inclusion bands, cracks which partially seal periodically and contain discontinuous solid inclusion bands, or cracks that remain open and contain euhedral quartz crystals with no solid inclusions. The initial crack aperture, inferred from spacing of inclusion bands, varies from 8 μm in crack seal features to much larger values in euhedral growth veins. Euhedral growth veins remain open throughout their development (105 to 106 years), and crack seal veins develop as a consequence of many crack‐seal events over a 103–106 year period. In both cases, textural evidence suggests that most transport of silica occurs by local pore‐fluid diffusion from matrix to vein. Wall rock inclusion bands suggest that “crack” events and “seal” events each occurred within periods of 102–104 years. A picture emerges of intermittent fluid flow upward from the decollement into a branching hierarchy of vertical fractures in the accretionary wedge during hydrofracturing events, followed by local transport and precipitation of silica causing sealing of the fractures at depth and propagation of pulses of fracture fluid upward.
Magnetic anisotropy, scanning electron microscopy, and X ray pole figure goniometry study of inclination shallowing in a compacting clay‐rich sediment
Authors: Sun Wei Wei , Kodama K. P.
Published: February 2008

Anisotropy of anhysteretic remanence (AAR), scanning electron microscopy (SEM), and X ray pole figure goniometry studies of clay‐rich sediments were conducted to delineate the interactions between magnetite and clay particles which cause inclination shallowing during compaction. These studies used synthetic sediments composed of kaolinite or illite and two grain sizes of magnetite, 0.45 μm and 2–3 μm. Natural marine sediments which contained 40–50% clay were also used. The sediments were compacted by pressures as high as 0.157 MPa. The sediments' porosity decrease suggests that the compaction experiments model burial depths up to 400–500 m. The main result of this study is that inclination shallowing and void ratio versus pressure curves show two points where behavior changes. A rapid decrease in inclination and void ratio occurs up to a pressure of 0.02 MPa. This is accompanied by a magnetic intensity decrease of 20–30%. Compaction at pressures between 0.02 MPa and 0.05 MPa causes a further decrease in these parameters at moderate rates, while compaction at pressures above 0.05 MPa causes very little change. Percent magnetic anisotropy and X ray pole figure intensity ratio versus pressure curves show only one point where behavior changes at 0.05 MPa with a rapid increase at lower pressures and little change at higher pressures. A model to explain this behavior, based on the SEM observations as well as the magnetic anisotropy, pole figure, and inclination data, suggests that compaction up to 0.02 MPa causes a decrease in pore volume with little reorientation of clay particles. Some magnetic particles are firmly attached to clay particles, whereas other loosely attached particles are either randomized or subvertical ones are preferentially disturbed causing inclination shallowing and an intensity decrease. At pressures between 0.02 MPa and 0.05 MPa, magnetite particles have become firmly attached to clay particles and start to follow the reorientation of clay particles as the clay fabric develops. At 0.05 MPa there is a major change in the clay microstructure with a horizontal fabric becoming evident. Compaction at higher pressures causes little further volume loss and clay particle reorientation and hence little additional inclination shallowing. The randomization/disturbance process suggested by this model causes slightly more than half of the inclination shallowing observed in the samples studied, whereas magnetite‐clay attachment causes the remainder of the inclination decrease.
Sea salt dependent electrical conduction in polar ice
Authors: Moore John , Paren Julian , Oerter Hans
Published: February 2008

A 45 m length of ice core from Dolleman Island, Antarctic Peninsula has been dielectrically analysed at 5 cm resolution using the dielectric profiling (DEP) technique. The core has also been chemically analysed for major ionic impurities. A statistical analysis of the measurements shows that the LF (low frequency) conductivity is determined both by neutral salt and acid concentrations. The statistical relationships have been compared with results from laboratory experiments on ice doped with HF (hydrogen fluoride). Salts (probably dispersed throughout the ice fabric) determine the dielectric conductivity. The salt conduction mechanism is probably due to Bjerrum L defects alone, created by the incorporation of chloride ions in the lattice. Samples of ice from beneath the Filchner‐Ronne Ice Shelf were also measured and display a similar conduction mechanism below a solubility limit of about 400 μM of chloride. The temperature dependence of the neutral salt, acid and pure ice contributions to the LF conductivity of natural ice between −70°C and 0°C is discussed. These results allow a comprehensive comparison of dielectric and chemical data from natural ice.
Spatial‐temporal analysis of a seismic series using the principal components method: The Antequera Series, Spain, 1989
Authors: Posadas A. M. , Vidal F. , de Miguel F. , Alguacil G. , Peña J. , Ibañez J. M. , Morales J.
Published: February 2008

A method for the characterization of a seismically active zone from a distribution of hypocenters is presented. This method is based on principal components analysis, a powerful multivariant statistical technique that is used to find the rupture local ellipsoid (RLE). The ellipsoid is a planar structure with which two variations of the method are developed: the spatial principal components analysis and the spatial‐temporal principal components analysis; using these methods, it is possible to find the dominating tendencies in the fracturing of the seismically active volume as well as the temporal evolution of the process. The methodology developed has been applied to a series of earthquakes that occurred near Antequera, Spain, with the result that the main fracture series trends N70°–80°E. Moreover, the temporal evolution of the system from the most relevant RLE has been characterized.
Frequency dependent rheology of vesicular rhyolite
Authors: Bagdassarov N. S. , Dingwell D. B.
Published: February 2008

Frequency dependent rheology of magmas may result from the presence of inclusions (bubbles, crystals) in the melt and/or from viscoelastic behavior of the melt itself. With the addition of deformable inclusions to a melt possessing viscoelastic properties one might expect changes in the relaxation spectrum of the shear stresses of the material (e.g., broadening of the relaxation spectrum) resulting from the viscously deformable geometry of the second phase. We have begun to investigate the effect of bubbles on the frequency dependent rheology of rhyolite melt. The present study deals with the rheology of bubble‐free and vesicular rhyolite melts containing spherical voids of 10 and 30 vol %. We used a sinusoidal torsion deformation device. Vesicular rhyolite melts were generated by the melting (at 1 bar) of an Armenian obsidian (Dry Fountain, Erevan, Armenia) and Little Glass Mountain obsidian (California). The real and imaginary parts of shear viscosity and shear modulus have been determined in a frequency range of 0.005–10 Hz and temperature range of 600°–900°C. The relaxed shear viscosities of samples obtained at low frequencies and high temperatures compare well with data previously obtained by parallel plate viscometry. The relaxed shear viscosity of vesicular rhyolites decreases progressively with increasing bubble content. The relaxation spectrum for rhyolite melt without bubbles has an asymmetric form and fits an extended exponent relaxation. The presence of deformable bubbles results in an imaginary component of the shear modulus that becomes more symmetrical and extends into the low‐frequency/high‐temperature range. The internal friction Q−1 is unaffected in the high‐frequency/low‐temperature range by the presence of bubbles and depends on the bubble content in the high‐temperature/low‐frequency range. The present work, in combination with the previous study of Stein and Spera (1992), illustrates that magma viscosity can either increase or decrease with bubble content, depending upon the rate of style of strain during magmatic flow.
Crustal structure of Endeavour Ridge Segment, Juan de Fuca Ridge, from a detailed seismic refraction survey
Authors: Cudrak Connie F. , Clowes Ron M.
Published: February 2008

A detailed seismic refraction survey was carried out over the Endeavour Segment of the Juan de Fuca Ridge, a medium‐rate spreading center which lies off western North America, to investigate the creation and evolution of oceanic crust. A bathymetric high and the presence of hydrothermal vents suggested that the study area was the most recent locus of spreading. Travel time and amplitude data from 15 in‐line air gun/ocean bottom seismometer profiles were forward modeled using asymptotic ray theory to obtain two‐dimensional velocity models consisting of four primary layers which correlate well with classic models of oceanic crust. Significant lateral variations in thicknesses and velocities on the scale of a few to 10 km are superimposed on this basic velocity structure, but they appear to be random rather than distributed symmetrically about the ridge. We attribute them to fracturing which causes porosity changes, hydrothermal circulation which fills voids and fractures with alteration products, and variations in magmatic and/or deformational processes at the spreading center. Layer 2A is found to have low (2.6–2.8 km/s) velocities, to average 0.4 km in thickness with variations up to 0.2 km, and to be bounded at its base by a sharp velocity increase to 4.8 km/s. Along the axial ridge, velocities 0.4–0.6 km/s higher than average are interpreted for layers 2B and 2C, but these values are confined to a 2‐km‐wide zone centered below the ridge. Velocities along ridge‐parallel lines offset 10 km are normal, indicating that maturation to off‐ridge structure has occurred within at most 0.3 Ma. Layer 3 velocities decrease by 0.1–0.2 km/s for arrivals traveling along and under the axial ridge, perhaps caused by higher temperatures. However, we find no anomalously low velocities beneath the ridge, indicating that no large crustal magma chamber exists. On the basis of this study, we conclude that magmatic accretion is a fully three‐dimensional process within ridge segments such as Endeavour Ridge.
Tomographic inversion of Rg wave group velocities for regional near‐surface velocity structure
Authors: Kocaoğlu Argun H. , Long Leland T.
Published: February 2008

The tomographic inversion of group velocities of Rg waves is a viable technique for mapping lateral variations in the lithology of sedimentary basins. In order to determine a velocity structure, we first use the moving‐window maximum entropy spectral analysis to measure the average Rg wave group travel times. Second, the group velocities within blocks (subregions) covering the area sampled by the Rg wave propagation paths are determined by tomographic inversion. Third, the vertical shear wave velocity profile within each block is determined from the inversion of group velocities for that block. Finally, a structural cross section is obtained by connecting blocks located along a profile. Rg waves recorded from mining explosions in Alabama were used in a test of the technique by modeling the Paleozoic sedimentary rocks in the Black Warrior Basin of northern Alabama. The high resolution of moving‐window maximum entropy spectral analysis allows estimation of the Rg wave group velocities with a precision of better than 3 %. The usable frequency range of 0.5 to 1.5 Hz allows determination of structures to depths of about 2.4 km with layers of 0.2 km thickness. Blocks of widths of 20 km or less could be resolved with appropriate data. Shear wave velocities in the Black Warrior Basin range from 1.6 to 4.0 km/s. The layering in the velocity structure corresponds to major geologic units. Within units, an increase in velocity to the northeast corresponds to observed facies changes.
A paleomagnetic study of Tertiary formations from the Kyrgyz Tien‐Shan and its tectonic implications
Authors: Thomas J.‐C. , Perroud H. , Cobbold P. R. , Bazhenov M. L. , Burtman V. S. , Chauvin A. , Sadybakasov E.
Published: February 2008

As part of a general investigation of Cenozoic deformation in Central Asia, we studied the paleomagnetism of Tertiary red beds and lava flows from intermontane basins in the Tien‐Shan region of Kyrgyzstan. We collected 532 cores and hand samples from 78 sites at 12 localities and progressively demagnetized them, thermally or with alternating fields. For most sites, there are magnetic components with unblocking temperatures higher than 600°C. We infer that the magnetic carriers are mainly hematite and magnetite. For most localities, the high‐temperature component appears to predate tectonic folding. For all localities, mean inclinations are shallower than expected from apparent polar wander paths. Inclination anomalies range from 16°±5° for the Issyk‐Kul basin, to 26°±7° for the Fergana basin. If due to changes in latitude, these anomalies imply at least 2000 km of northward displacement of the Tien‐Shan during the Tertiary, for which there is no tectonic evidence. We thus consider that the paleomagnetic reference directions cannot be directly compared with our Tertiary data. We explored other possible reasons for this anomaly, which has also been reported from other parts of the Alpine belt, but we could not find a satisfactory explanation. Absolute rotations cannot be accurately determined, because of problems with the reference direction. Nevertheless, the mean declination for the Fergana basin lies counterclockwise by 20°±11° from the mean declination of the Issyk‐Kul basin. This result is consistent with the counterclockwise rotation inferred for the Fergana basin from the pattern of Cenozoic faults and folds. It suggests a Cenozoic right‐lateral displacement of 110±60 km on the Talas‐Fergana fault.
Linear and nonlinear propagation of water wave groups
Authors: Pierson W. J. , Donelan M. A. , Hui W. H.
Published: February 2008

Specific waveforms with known analytical group shapes were generated, and their evolution observed, in a wave tank in the form of both transient wave groups and the cnoidal (cn) and dnoidal (dn) wave trains as derived from the nonlinear Schrödinger equation. Low‐amplitude transients behaved as predicted by linear theory. The cn and dn wave trains of moderate steepness behaved almost as predicted by the nonlinear Schrödinger equation. There is no adequate theory for the higher nonlinear transient wave groups. The functions of time at successive wave staffs were analyzed in terms of calculations of Fourier integral spectra to interpret the nonlinear behavior of these groups. Dispersed wave groups that were less nonlinear at the wave maker and that became highly nonlinear as they traveled along and coalesced provided an unusual data set. The effects of sum and difference frequencies increased. The apparent phase and group velocities increased for the higher frequencies. The Fourier integral spectra changed shape from one wave staff to the next over the entire range of frequencies that could be analyzed as the waves coalesced. In general, the spectra broadened, shifting energy to both lower and higher frequencies. This experimental exploration of the properties and evolution of transients is motivated by the possibility that the chance occurrence of steep transient wave groups on the ocean may be an important aspect of the evolution of a wind‐driven sea.
Low‐frequency vibrational motion of Arctic pack ice
Authors: Dugan J. P. , DiMarco R. L. , Martin W. W.
Published: February 2008

The mechanisms responsible for vibrational motions of Arctic ice at frequencies below about 100 Hz are know to be a combination of wave types, namely, gravity waves, flexural waves, and a variety of acoustic and/or elastic waves. Surface motion measurements using geophones and accelerometers on multiyear pack ice are used to establish the energy level of these motions and to differentiate between the different modes of propagation. Gravity waves are dominant below about 0.05 Hz, with their direction and dispersion relation being confirmed with vector measurements of acceleration from two stations. Motions driven by surface pressure fluctuations in the local wind appear at somewhat higher frequencies up to about 0.3 Hz and are particularly evident on thinner first‐year ice during periods of high wind. The band from 0.1 to 10 Hz comprises mainly flexural waves, as determined by their dispersion relation, and these are prominent during local ridge building events. At frequencies above 10 Hz, there is a combination of acoustic and flexural modes of propagation, with one or the other being dominant during specific environmental events. The probability density function of power in octave wide bands is approximately lognormal as a result of the high level of intermittence of local events. The variance in power level is significantly smaller for waterborne noise from more distant sources. Specific examples are used to illustrate the nature of the events.
Tropical Pacific Ocean response to observed winds in a layered general circulation model
Authors: Miller Arthur J. , Oberhuber Josef M. , Graham Nicholas E. , Barnett Tim P.
Published: February 2008

We examine the climatological and anomalous response of an isopycnic coordinate general circulation model, with embedded bulk surface mixed layer model, when forced by observed monthly mean wind stress variability. Our results show that model fields of sea surface temperature (SST) and currents are fairly similar to those arising in other models as well as published observations. The model generates a climatology of SST which is usually within 0.5°–1.5°C of the Comprehensive Ocean‐Atmosphere Data Set observations. Anomalous SST compares best with observations when averaged over rather large spatial regions. The mean horizontal currents tend to resemble the observed, but the amplitudes are smaller than observed (primarily because the model predicts vertical mean currents in the mixed layer rather than the surface currents) and the springtime reversal of the South Equatorial Current is absent in the model. Anomalous zonal currents are similarly reduced in amplitude though the variability is comparable to the available observations. We also describe the model's basin‐scale mean mixed‐layer structure and the mean vertical velocity field (which is fundamentally different from that of the Geophysical Fluid Dynamics Laboratory model). We discuss various aspects of the 1982–1983 El Niño in particular detail, including the heat budget of the variable‐depth tropical Pacific surface mixed layer, and the relationships among anomalies of mixed‐layer depth, thermocline depth, and SST. By these analyses, we identify the strengths and weaknesses of the model's capability to simulate observations, particularly with respect to that of other models. Throughout the paper, we attempt to identify the primary areas which require improvement in the model and suggest possible remedies.
The Sverdrup circulation for the Atlantic along 24°N
Authors: Schmitz William J. , Thompson J. Dana , Luyten James R.
Published: February 2008

The 30 Sv transported by the Florida Current through the Straits of Florida off Miami could consist of a wind‐driven contribution of 17 Sv along with a thermohaline component of 13 Sv. The latter might flow from the South Atlantic as upper layer compensation for a net lower layer cross‐equatorial flow southward. The Sverdrup transport along 24°N in the interior North Atlantic east of 55°–60°W is about 17 Sv, potentially matching the wind‐driven component of the Florida Current. The circulation in the vicinity of 24°N and west of 55°–60°W up to the Bahama Banks contains energetic, shorter spatial scale flows, where there is a similarity between the regional pattern of the Sverdrup transport contours and the scales and structure of the observed C‐shaped pattern of dynamic height.
Double structure of the coastal front in the Kii Channel, Japan, during winter
Authors: Toda Takashi
Published: February 2008

Analysis of satellite thermal imagery showed that the coastal front in the Kii Channel, Japan, has a double structure that consists of the well‐known S‐shaped front and a newly found mouth front. The S‐shaped front is in a state of equilibrium as has been shown by previous numerical experiments, whereas the mouth front is produced as a direct response to the intermittent arrival of warm water from the Kuroshio. As was shown by a previous study, the warm water comes along the eastern or western coast, and the western path (the Geitou Branch) occurs less frequently than the eastern (the Kinan Branch). We found, however, that each appearance of the western path has a greater effect on the heat budget of the channel and the shape of the front than the eastern path does. The western path tends to produce a large temperature difference and to form a sharp mouth front which is often accompanied by tongue and outflowing structures. Both structures are produced by water intruding across the mouth front; the tongue structure by warm water intruding toward the coast, and the outflowing structure by cold water intruding toward the offshore region. The tongue structure is a transient process toward equilibrium, the S‐shaped front. A hypothetical model that shows how the tongue and outflowing structures are formed also is presented.
Influence of precipitating convection on the surface energy budget observed during a tropical ocean global atmosphere pilot cruise in the tropical western Pacific Ocean
Authors: Young G. S. , Ledvina D. V. , Fairall C. W.
Published: February 2008

A short‐term observational study of the surface energy budget of the equatorial western Pacific Ocean was conducted in preparation for the Tropical Ocean and Global Atmosphere program Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). Data were obtained between February 17 and March 10, 1990, during the TOGA pilot cruise of the R/V Wecoma near the equator and 147°E longitude. Analysis of the 443 hours of turbulent and radiative fluxes collected was focused on the influence of deep precipitating convection on the day‐to‐day variability of the surface energy budget. Partitioning the data series according to the degree of local convective activity allowed comparison of the surface energy budgets for various convective regimes. As was observed in the tropical Atlantic during the GARP Atlantic Tropical Experiment (GATE), the surface fluxes of sensible and latent heat were larger during periods influenced by deep precipitating convection. Moreover, the mechanisms by which convection affected these fluxes were similar to those described in the tropical Atlantic. Variations in the shortwave radiation caused by convective clouds and their by‐products were found to contribute even more than the surface fluxes to the day‐to‐day variability of the surface energy budget. It is concluded that the existence of convectively disturbed days had a significant impact on the surface energy budget for the entire cruise period as well as on the day‐to‐day variability within that period.