bluuebunny/crossref_metadata_2025 huggingface dataset:
The source of explosive eruptions of Sakurajima volcano, Japan
Authors: Uhira Kohichi , Takeo Minoru
Published: February 2004

To understand the nature of explosive eruptions, in particular the role played by exsolution of magmatic gases, we studied the seismic waveforms produced by two such eruptions of Sakurajima volcano on October 30 and November 14,1986. An explosion begins when a mixture of magmatic gases and fragmented magma flows upward through a narrow vertical channel to the surface. The source region is a reservoir of gas‐rich magma, where the pressure drops as gases flow out. The form of low‐frequency seismic waves can be explained by a source that consists of two components. One component is a single force directed downward that represents the reaction to ejection of gas and magma. The other, more important component is a moment tensor that represents expansion or contraction of rock around a source. The first motion of seismic waves produced by an explosion is directed away from the volcano. Tilt and strain measurements show that the initial motion directed outward is soon followed by subsidence around the volcano. The eigenvectors of the seismic moment tensor are either nearly horizontal or vertical. The ratio of horizontal to vertical eigenvalues was 1.8:1 for the October 30 explosion. This suggests the geometry of the explosive source can be approximated as a buried vertical cylinder. The volume of contraction, 2×l03 m3, can be produced by a pressure change of 0.3 MPa over a cylinder with a radius of 160 m and a height of 300 m. We propose that an explosive eruption occurs after a small shallow source of high pressure forms within a vertical conduit, possibly exsolution of magmatic gases. Eventually, the small source bursts and pushes open a pathway to the surface, providing an upward escape for trapped gases. It is this burst that causes the initial outwardly directed motion at the surface. A deeper reservoir of gas‐rich magma is then subjected to a lower confining pressure and also bursts, causing the explosion.
Synthetic seismogram images of upper mantle structure: No evidence for a 520‐km discontinuity
Authors: Bock Günter
Published: February 2004

Seismological data used by Shearer (1990, 1991) to infer the existence of a seismic discontinuity at 520 km depth are compared with complete long‐period body wave seismograms calculated for the International Association of Seismology and Physics of the Earth's Interior 1991 (Iaspei91) Earth model. The Iaspei91 model does not contain a seismic discontinuity at or near 520 km depth. The observed P and SH multiples caused by topside reflections and SS precursors caused by underside reflections from the 410‐km and 660‐km discontinuities are well reproduced by the synthetic stacks. The synthetics exhibit clear “phase extrema” between these reflections that correlate well with similar features in the observed waveform stacks. Shearer interpreted these “phase extrema” as separate reflections from a seismic discontinuity at 520 km depth. Cross‐correlation analysis of synthetic seismograms gives an apparent discontinuity depth of about 520 km for P and SH multiples as well as SS precursors. Similarly, amplitude analysis of synthetic upper mantle reflections is in reasonable agreement with the observations reported by Shearer (1991). The phase extrema seen in the synthetics are the result of an extended, multicycle wavelet which is composed of depth phases and other structural phases such as reverberations from the crustal layer, convolved with the instrument response of long‐period stations of the Global Seismograph Network. The comparison of observational data, presented by Shearer (1990, 1991), with the synthetic seismogram stacks of this paper shows that the claim of good seismological evidence for a 520‐km seismic discontinuity as a global feature is not compelling.
Evidence for dynamic coupling of surface processes with isostatic compensation in the lower crust during active extension of western Turkey
Authors: Westaway Rob
Published: February 2004

Western Turkey, in the eastern part of the Aegean region, is extending slowly, with extension no faster than ∼0.5 mm yr−1 across any individual active normal fault. Up to 15 km of southward extension has occurred in the most active ∼100 km wide part of this region since ∼15 Ma, indicating a typical strain rate of ∼3×10−16 s−1. Rivers carry a sediment flux of ∼4×106 m3 yr−1 westward to the Aegean Sea coast, with offshore sedimentation at ∼1 mm yr−1. Sediment loading causes coastal areas to subside much faster than would be expected from the extension alone. Most of this sediment is derived from regional erosion of inland areas to the east at a typical rate of ∼0.1 mm yr−1, this erosion being isostatically compensated by uplift. It is suggested that the lower crustal material displaced by sediment loading near the coast flows east to isostatically compensate the erosion. The average eastward velocity of this lower crustal channel flow beneath the Aegean coast is estimated as ∼1 mm yr−1; the maximum velocity is >∼2 mm yr−1. This case study thus indicates the general point that isostatically compensated erosion and lower crustal channel flow can affect the evolution of sedimentary basins that are surrounded by eroding landmasses, factors that are often neglected.
Remagnetization of Jurassic volcanic rocks in the Santa Rita and Patagonia Mountains, Arizona: Implications for North American apparent polar wander
Authors: Hagstrum Jonathan T.
Published: February 2004

Paleomagnetic poles for the Jurassic Corral Canyon sequence and Glance Conglomerate in southern Arizona have been used to construct apparent polar wander (APW) paths for the North American plate, but they are controversial and conflict with higher‐latitude poles from New England. Lower Jurassic dacites and ash flow tuffs of the Mount Wrightson Formation in the Santa Rita Mountains were initially sampled to provide an additional paleopole for southern Arizona. These rocks, however, have a predominantly reversed‐polarity characteristic magnetization (in situ, I = −47°, D = 154°, α95 = 9°) which is statistically indistinguishable from that for the nearby latest Cretaceous Elephant Head pluton (I = −48°, D = 165°, α95 = 8°). Although magnetizations of both polarities are observed in the ash flow tuffs, they are mostly carried by hematite, and dual polarity components are observed within some specimens. Moreover, widespread mineralization and a K‐Ar age of ∼67 Ma for altered rocks of the Mount Wrightson Formation imply that these rocks were subjected to a prolonged episode (greater than one polarity interval) of low‐temperature alteration and remagnetization. Middle Jurassic ash flow tuffs of the Corral Canyon sequence in the Patagonia Mountains to the south have a similar geologic setting: They are exposed along strike amid mineralized rocks on the eastern flank of a mountain range cored by Paleocene intrusions. Hematite is also the dominant remanence carrier in most of the Corral Canyon sequence, and its predominantly normal‐polarity direction (in situ, I = 51°, D = 326°, α95 = 9°) is indistinguishable from that for the nearby Patagonia Granodiorite (I = 49°, D = 342°, α95 = 8°). Rocks of the Corral Canyon sequence therefore are likely remagnetized as well. Problems also exist with the Glance Conglomerate pole. These rocks are situated within a caldera structure and have been potassium metasomatized. This potassic alteration could have occurred shortly after emplacement or at a later time, postdeformation. The low‐latitude Jurassic APW path for North America and J‐2 cusp therefore are not well supported and may need revision.
A collision boundary imaged by magnetotellurics, Hidaka Mountains, central Hokkaido, Japan
Authors: Ogawa Yasuo , Nishida Yasunori , Makino Masahiko
Published: February 2004

Magnetotelluric (MT) data were acquired across the Hidaka Mountains, which are thought to have formed by collisional orogenic processes. A regional electrical strike of N18°W, consistent with surficial geology, was derived from impedance tensor decomposition. Two‐dimensional resistivity models were constructed by forward model fitting with preference for reducing the misfit of transverse magnetic mode. The final modeling imaged a collision boundary between Cretaceous forearc basin rocks and continental crust. The major features of the model are (1) a 5‐km‐thick surficial conductive layer corresponding to Cretaceous forearc basin and the underlying resistive basement that shallows and is exposed to the east where Sorachi formations, characterized by oceanic crustal rocks, crop out. Therefore we interpret the underlying resistive layer as oceanic crust. (2) A thin (500 m thick) conductive (3 Ω m) anomaly corresponds to serpentinite exposure in the Kamuikotan metamorphic belt. (3) Exposed deep crustal rocks in the Hidaka belt showed high resistivity (1000–2000 Ω m) at the surface. The subsurface westward extension inferred from the MT data is supported by seismic refraction data. (4) In the Hidaka Mountains, a relatively conductive (500–1000 Ω m) layer implying accretionary prism sequences, was inferred at 2‐to 10‐km depth beneath a more resistive (1000–2000 Ω m) surface layer. (5) A wedge‐shaped resistivity anomaly was found at 5‐to 50‐km depth beneath the Hidaka belt. The resistivity of this wedge decreases eastward from 30,000 Ω m to 2000 Ω m, consistent with the surface decrease of metamorphic grade. The spatial distribution is also consistent with a seismic high‐velocity anomaly. (6) The western side of the resistive wedge is fringed by a dipping conductor. Correlations with a low‐velocity anomaly and high seismicity suggest that this conductor is an enhanced porosity region, and that it may be working as a detachment zone at the collision boundary. The high resistivity contrast between oceanic crust and high‐temperature metamorphosed continental crust permitted us to image the interfingered complex structure beneath the Hidaka Mountains from 300‐m to 50‐km depth.
Viscosity estimates for the crust and upper mantle from patterns of lacustrine shoreline deformation in the Eastern Great Basin
Authors: Bills Bruce G. , Currey Donald R. , Marshall Grant A.
Published: February 2004

The deformed shorelines of Lake Bonneville constitute a classic source of information on lithospheric elastic thickness and upper mantle viscosity. We describe and apply a new model to a recently augmented data set. New data better constrain both the complex spatio‐temporal pattern of the lake load and the crustal deformation response to that load. The history of lake level fluctuations has been significantly refined and somewhat modified. This is due to both more radiocarbon dates from within the Bonneville basin and to an improved calibration of the radiocarbon timescale itself. The data which constrain the crustal deformation pattern consist of ages and shoreline elevations from several hundred points which sample three major levels of Lake Bonneville and corresponding elevations from the high stands of three smaller lakes situated to the west of Lake Bonneville. The data from the smaller lakes help elucidate the pattern of deflection which occurred beyond the edge of the big lake. The geometry of the Earth model incorporates an arbitrary number of layers overlying a half‐space, and the rheology of each level can accommodate an arbitrary number of Maxwell viscoelastic elements in parallel. The inverse modeling comprises three complementary approaches: for the simplest configurations, we performed a direct search of the parameter space and delineated the irregular boundary of the subspace of acceptable models. For more complex configurations, we constrained the elastic parameters to their seismically determined values and then solved for viscosity versus depth profiles by either expressing the log(viscosity) versus log(depth) profile as a series of specially constructed orthogonal polynomials, or by allowing each of 8–10 layers (plus the half‐space) to have an independently determined viscosity. We found that the data do not strongly support (nor can they conclusively exclude) a more complex rheology than simple Maxwell viscoelasticity. The orthogonal polynomial solution exhibits an essentially monotonic decrease in viscosity with depth. The most rapid change occurs at shallow depths, decreasing from 1023 Pa s at 3 km to 1020 Pa s at 30 km. The decrease is much more gradual below, with only another factor of 5 decrease between 30 and 300 km depth. The unconstrained solution exhibits a rapid decrease in viscosity with depth from 2×1024 Pa s in the top 10 km to 4×1017 Pa s at a depth of 40 km. A nearly isoviscous asthenospheric region extends from 40 to 150 km and is underlain by a mantle lithospheric region with increased viscosity (2×1020 Pa s) extending from 150 to 300 km depth and by a uniform viscosity (1019 Pa s) half‐space below.
Tectonic structure across the accretionary and erosional parts of the Japan Trench margin
Authors: von Huene R. , Klaeschen D. , Cropp B. , Miller J.
Published: February 2004

Structure sections across the Japan Trench from prestack depth migrated seismic data define the accretionary prism, a wedge‐shaped seaward end of the continental framework, and a tectonized middle slope between them. The 10‐km‐wide accretionary prism is underthrust by sediment remaining with the subducted oceanic crust. Structure beneath the steep middle slope more nearly resembles the adjacent continental margin than the accreted mass. This deformed middle slope domain forms a buffer between the compliant accretionary wedge and a more rigid older continental framework. Along the plate boundary, subducted strata thicken beneath the middle and upper slopes and, where last imaged, are four times the input thickness beneath the accretionary prism. This interplate stratified layer appears to control interplate friction because the layer is imaged 45 km landward from the trench axis to 12‐km depth and in that expanse, few upper plate earthquakes are recorded. Erosion of the base of the upper plate beneath the middle continental slope is indicated by margin subsidence and material flux. This erosion is concurrent with accretion at the front of the margin.
Chaos in geomagnetic reversal records: A comparison between Earth's magnetic field data and model disk dynamo data
Authors: Cortini Massimo , Barton Christopher C.
Published: February 2004

The Earth's geomagnetic field reverses its polarity at irregular time intervals. However, it is not clear whether a reversal is a deterministic (low‐dimensional) or a random (high‐dimensional) process; the duration‐frequency distribution of the polarity time intervals resembles those generated by random processes, but many models suggest that a geomagnetic field reversal can be the outcome of a deterministic dynamics, that of the convection in the Earth's outer core. The latter, in turn, is only a part of an extremely complex system, made up of both terrestrial and extraterrestrial subsystems nonlinearly interacting with each other over a wide range of time scales. We studied the geomagnetic field reversal patterns by means of several techniques of nonlinear dynamics and compared the results obtained on actual geomagnetic reversal data with synthetic reversal sequences generated by the Rikitake and Chillingworth‐Holmes models of the Earth's magnetic field. We analyzed both the geomagnetic and the synthetic reversal scales by nonlinear forecasting and found that we cannot predict the geomagnetic reversal sequence with nonlinear forecasting. Predictability of the synthetic data varies widely depending on the model parameters. Phase portraits of data obtained from the magnetic field models show fractal structures similar to those associated with the Lorenz attractor. We measured the correlation dimension DC of the synthetic and geomagnetic data by means of the Grassberger‐Procaccia method and found that DC always has a value of about one for the synthetic data. The correlation integrals for the geomagnetic reversal sequence behave very differently from those of randomized reversal sequences and suggest that the Earth's geomagnetic field reversal dynamics is not random. However, the limited size of the magnetic reversal data set (282 points) and the poor convergence of the correlation integrals make a quantitative assessment of low‐dimensional chaos impossible. Our analysis sets a lower limit to the correlation dimension of the geomagnetic reversal dynamics: DC > 3.
Three‐dimensional crustal structure of the Mendocino Triple Junction region from local earthquake travel times
Authors: Verdonck David , Zandt George
Published: February 2004

The large‐scale, three‐dimensional geometry of the Mendocino Triple Junction at Cape Mendocino, California, was investigated by inverting nearly 19,000 P wave arrival times from over 1400 local earthquakes to estimate the three‐dimensional velocity structure and hypocentral parameters. A velocity grid 175 km (N‐S) by 125 km (E‐W) centered near Garberville, California, was constructed with 25 km horizontal and 5 fan vertical node spacing. The model was well resolved near Cape Mendocino, where the earthquakes and stations are concentrated. At about 40.6° N latitude a high‐velocity gradient between 6.5 and 7.5 km/s dips gently to the south and east from about 15 km depth near the coast. Relocated hypocenters concentrate below this high gradient which we interpret as the oceanic crust of the subducted Gorda Plate. Therefore the depth to the top of the Gorda Plate near Cape Mendocino is interpreted to be ∼15 km. The Gorda Plate appears intact and dipping ∼8° eastward due to subduction and flexing downward 6°–12° to the south. Both hypocenters and velocity structure suggest that the southern edge of the plate intersects the coastline at 40.3° N latitude and maintains a linear trend 15° south of east to at least 123° W longitude. The top of a large low‐velocity region at 20–30 km depth extends about 50 km N‐S and 75 km E‐W (roughly between Garberville and Covelo) and is located above and south of the southern edge of the Gorda Plate. We interpret this low velocity area to be locally thickened crust (8–10 km) due to either local compressional forces associated with north‐south compression caused by the northward impingement of the rigid Pacific Plate or by underthrusting of the base of the accretionary subduction complex at the southern terminous of the Cascadia Subduction Zone. South of Cape Mendocino and southwest of the Garberville fault, high velocities indicative of oceanic crust are detected at 15 km depth. We interpret this evidence as tectonic underplating of the Pacific oceanic crust beneath the coastal Franciscan Complex, similar to the situation seismically imaged in central coastal California (Page and Brocher, 1994). This geometry indicates that the westernmost portion of the slab window has been filled by tectonic underplating and crustal thickening, while deep‐seated mantle upwelling is confined to an area centered about 75 km north of Clear Lake (Benz et al, 1992; Zandt et al., 1992).
Fluid dynamic and geochemical aspects of entrainment in mantle plumes
Authors: Hauri Erik H. , Whitehead John A. , Hart Stanley R.
Published: February 2004

A similarity solution has been developed for vertical, steady‐state mantle plume conduits by considering the boundary layer flow emanating from a point source of heat in an axisymmetric geometry. This model includes the effects of temperature and shear stress on viscosity, and incorporates depth‐dependent viscosity and thermal expansivity. Plumes with variable viscosity have upward velocities of 0.30–100 m/yr and radii of 30–250 km, depending on temperature, rheology and buoyancy flux. These results demonstrate the small lateral scale of plume features relative to the resolution of most large‐scale convection models and seismological studies. All of the plumes studied showed significant entrainment of ambient mantie surrounding the plume conduit, driven by the radial conduction of heat from the plume. This heat raises the buoyancy and lowers the viscosity of the ambient mantel, thereby entraining it into the conduit flow. For buoyancy fluxes of 0.1–10 Mg/s, similar to the range estimated for plumes in the Earth's mantle, we calculate a range of entrainment of >90% to <5% ambient mantle, correlating negatively with buoyancy flux. Examination of the streamlines of mantle material which is entrained into thermal plumes indicates that most of the entrained fraction originates from approximately the lower half of the layer traversed by the plume, and shows minor entrainment of upper level material. This is especially true for non‐Newtonian and depth dependent rheologies, and for depth‐dependent thermal expansivity. Upwelling of depleted upper mantle, viscously coupled to the plume flow, is proposed as a mechanism for generating post‐shield stage alkalic basalts erupted on oceanic island chains and their associated flexural arches. The existing Sr‐Nd‐Pb isotope data for oceanic basalts indicate the presence of a component which is common to hotspot basalts worldwide, and which is distinct from the upper mantle source of mid‐ocean ridge basalts. This component (termed “FOZO” by Hart et al. (1992)) has moderately depleted Sr and Nd signatures, radiogenic Pb isotopes, and elevated 3He/4He ratios. The high He isotope ratios of FOZO suggest an origin for this component in the lower mantle, and would appear to provide independent evidence to support the fluid dynamic observations for significant entrainment of lower mantle in plumes and exclusion of upper mantle. If the composition of FOZO is representative of the isotopic composition of the lower mantle, then it would appear that this reservoir has been differentiated relative to estimates for the bulk silicate earth (BSE). This may be due either to melting and differentiation at higher levels in the mantle, or to fractionation of high pressure phases from a terrestrial magma ocean.
The Krafla spreading segment, Iceland: 1. Three‐dimensional crustal structure and the spatial and temporal distribution of local earthquakes
Authors: Arnott S. K. , Foulger G. R.
Published: February 2004

The geothermal seismicity of the Krafla Volcanic System, NE Iceland, was monitored for 3 months in 1985 using a dense, local seismometer network. The seismicity was continuous, and the spatial and temporal distributions were roughly known prior to monitoring. The instruments could thereby be deployed in a well‐positioned array. A total of 489 locatable events were recorded within the network, and 1771 arrival times were inverted to calculate the three‐dimensional P‐wave velocity structure and hypocentral locations. Low‐velocity volumes were imaged beneath the Krafla and Námafjall geothermal areas and indicate zones of hydrothermal alteration. High‐velocity bodies beneath the Krafla caldera rim are interpreted as gabbroic intrusions. Using a three‐dimensional velocity structure instead of a refraction‐based one‐dimensional model to locate the hypocenters significantly improves their location quality and illustrates the shortcomings of using refraction‐based models to locate earthquakes in local, anomalous areas. Seismic activity was concentrated within the Krafla and Námafjall geothermal areas and in a narrow zone where dike injections had occurred 8 and 5 years earlier. The activity occurred in the depth range 0–3 km. The seismic rate for the whole area was one magnitude 3.2 event per year and the b value was 0.77±0.10. Most of the seismicity appears to result from geothermal processes in the manner proposed for other Icelandic areas (Foulger and Long, 1984; Foulger, 1988b). Seismicity directly beneath the Bjarnarflag well field within the Námafjall area is probably induced partly by geothermal exploitation. Considerable seismicity also occurred immediately below Leirhnjúkur, a site of intense geothermal activity in the center of the Krafla caldera that overlies a roof pendant in the magma chamber below. This volume of high seismicity is probably highly fractured and may provide a conduit for magma escaping from the magma chamber.
Joint inversions of seismic and geodynamic data for models of three—dimensional mantle heterogeneity
Authors: Forte Alessandro M. , Woodward Robert L. , Dziewonski Adam M.
Published: February 2004

The seismic models of three‐dimensional (3‐D) mantle heterogeneity may be interpreted in terms of the density perturbations which drive mantle flow and thus provide important predictions of flow‐related observables, such as the nonhydrostatic geoid. The current models of global‐scale shear velocity heterogeneity provide reasonably good fits to the long‐wavelength nonhydrostatic geoid data (60–70% variance reductions) but rather poor fits to the corresponding free‐air gravity anomalies (30–40% variance reductions). This major difference is due to two factors: (1) the very different amplitude spectrum of the gravity anomalies, which is nearly flat for degrees ℓ < 8, and (2) the relatively poor match between the individual harmonic components of the predicted and observed gravity anomalies for degrees ℓ > 3. The largest mismatch between the pattern of predicted and observed gravity anomalies is in the southern hemisphere. This observation suggests that one reason for the poor overall match between the two fields may be that the global seismic data are not resolving sufficiently well the heterogeneity in the southern hemisphere portion of the deep mantle. In contrast, the gravity data provide accurate and geographically uniform constraints on the vertically integrated heterogeneity in the mantle. We therefore perform a series of experiments in which we simultaneously invert a large set of seismic data (which includes long‐period waveforms, SS ‐ S and ScS ‐ S differential travel times, and normal‐mode structure coefficients) and the long‐wavelength gravity anomaly data. We thus determine whether it is possible to derive new 3‐D heterogeneity models which satisfy both data sets. The gravity anomaly data are interpreted in the context of spherically‐symmetric viscous flow models of the mantle. In these inversion experiments we test several radial viscosity and δlnρ/δlnν profiles and thereby assess their plausibility. The joint seismic‐geodynamic inversions reveal that it is indeed possible to greatly improve the fit to the free‐air gravity anomalies (with variance reductions of 80–90% readily accessible) while preserving the fit to the seismic data. This improvement is achieved with some adjustment to the heterogeneity in the depth range 1500–2500 km, where the seismic data constraints appear to be weakest. The joint inversions also reveal new structures in the southern hemisphere portion of the lower mantle which apparently are not resolved by the seismic data alone.
Ionic surface electrical conductivity in sandstone
Authors: Glover Paul W. J. , Meredith Philip G. , Sammonds Peter R. , Murrell Stanley A. F.
Published: February 2004

Recent analyses of complex conductivity measurements have indicated that high‐frequency dispersions encountered in rocks saturated with low‐salinity fluids are due to ionic surface conduction and that the form of these dispersions may be dependent upon the nature of the pore and crack surfaces within the rock (Ruffet et al., 1991). Unfortunately, the mechanisms of surface conduction are not well understood, and no model based on rigorous physical principles exists. This paper is split into two parts: an experimental section followed by the development of a theoretical description of adsorption of ions onto mineral surfaces. We have made complex conductivity measurements upon samples of sandstone saturated with a range of different types and concentrations of aqueous solution with a frequency range of 20 Hz to 1 MHz. The frequency dependence of complex conductivity was analyzed using the empirical model of Cole and Cole (1941). The “fractal” surface models of Le Méhauté and Crépy (1983), Po Zen Wong (1987), and Ruffet et al. (1991) were used to calculate apparent fractal pore surface dimensions for samples saturated with different solution types and concentrations. These showed a pronounced decrease of apparent fractal surface dimension with decreasing electrolyte concentration and a decrease of apparent fractal dimension with increasing relative ionic radius of the dominant cation in solution. A model for ionic surface concentration (ISCOM I) has been developed as the first step in producing a rigorous physicochemical model of surface conduction in quartz‐dominated rocks. The results from ISCOM I show that quartz surfaces are overwhelmingly dominated by adsorbed Na+ when saturated with NaCl solutions of salinities and pH found in actual geological situations. ISCOM I also shows that the concentration threshold for dominance of surface conduction over bulk conduction is aided by depletion of ions from the bulk fluid as a result of their adsorption onto the mineral surfaces as well as by changes in the ionic mobility in the surface conduction double‐layer as the wetting solution becomes more dilute.
Excitation of atmospheric oscillations by volcanic eruptions
Authors: Kanamori Hiroo , Mori Jim , Harkrider David G.
Published: February 2004

We investigated the mechanism of atmospheric oscillations with periods of about 300 s which were observed for the 1991 Pinatubo and the 1982 El Chichón eruptions. Two distinct spectral peaks, at T = 270 and 230 s for the Pinatubo eruption and at T = 195 and 266 s for the El Chichón eruptions, have been reported. We found similar oscillations for the 1980 Mount St. Helens and the 1883 Krakatoa eruptions. To explain these observations, we investigated excitation problems for two types of idealized sources, “mass injection” and “energy injection” sources, placed in an isothermal atmosphere. In general, two modes of oscillations, “acoustic” and “gravity” modes, can be excited. For realistic atmospheric parameters, the acoustic and gravity modes have a period of 275 and 304 s, respectively. For a realistic time history of eruption, atmospheric oscillations with an amplitude of 50 to 100 Pa (0.5 to 1 mbar) can be excited by an energy injection source with a total energy of 1017 J. This result is consistent with the observations and provides a physical basis for interpretation of atmospheric oscillations excited by volcanic eruptions.
Long‐lived mid‐ocean ridge segmentation of the Pacific‐Antarctic Ridge and the Southeast Indian Ridge
Authors: Hayes Dennis E. , Kane Kimberlee A.
Published: February 2004

The Pacific‐Antarctic and the Southeast Indian ridges are examined for evidence of long‐lived (tens of millions of years) tectonic segmentation of the mid‐ocean ridge by examining a variety of geophysical parameters similar to those previously documented in the South Atlantic (Kane and Hayes, 1992). These parameters include subsidence rate of the oceanic crust (individual flanks and asymmetry), zero‐age depth, and geoid height decrease with age (geoid rate). The variability in these parameters along‐strike of the mid‐ocean ridge is systematic and serves to define a large scale ridge segmentation of the order of hundreds of kilometers. The Southeast Indian ridge exhibits long‐lived segmentation in all parameters examined and is subdivided into five flow line corridors with boundaries between corridors occurring at or near major fracture zones. While the Pacific‐Antarctic ridge exhibits conspicuous asymmetric crustal subsidence, evidence of segmentation exists primarily in the along‐strike geoid rate. The observations in these two areas are found to be internally inconsistent with simple thermal conduction models for oceanic crust, strongly suggesting that factors other than cooling and isostasy are influencing the creation and subsequent modification of the oceanic crust and lithosphere.
A new relationship between subsidence rate and zero‐age depth
Authors: Kane Kimberlee A. , Hayes Dennis E.
Published: February 2004

The relationship between subsidence rates and axial depths for zero‐age crust has been documented along the mid‐ocean ridge systems of three separate oceans. A systematic, linear relationship exists between these two parameters. Shallow zero‐age depths are associated with high subsidence rates, and deep zero‐age depths are associated with low subsidence rates. The linear subsidence rate versus zero‐age depth trends have similar slopes for all three oceans, but their zero‐age depth intercepts are offset by a few hundred meters. The scatter of individual subsidence rate and zero‐age depth values about each linear trend varies from ocean to ocean and is related to the presence and extent of long‐lived, large‐scale segmentation of each mid‐ocean ridge system. One‐dimensional thermal conduction models typically used to quantify subsidence rates cannot account for the observed relationships. A qualitative model that was used previously to explain long‐lived tectonic segmentation can also explain the subsidence rate versus zero‐age depth relationships and associated variance by assuming the existence of average regional and local mande temperature variations of ∼20°C to ∼170°C, respectively.
An in situ estimation of anisotropic elastic moduli for a submarine shale
Authors: Miller Douglas E. , Leaney Scott , Borland William H.
Published: February 2004

Direct arrival times and slownesses from wide‐aperture walkaway vertical seismic profile data acquired in a layered anisotropic medium can be processed to give a direct estimate of the phase slowness surface associated with the medium at the depth of the receivers. This slowness surface can, in turn, be fit by an estimated transversely isotropic medium with a vertical symmetry axis (a “TIV” medium). While the method requires that the medium between the receivers and the surface be horizontally stratified, no further measurement or knowledge of that medium is required. When applied to data acquired in a compacting shale sequence (here termed the “Petronas shale”) encountered by a well in the South China Sea, the method yields an estimated TIV medium that fits the data extremely well over 180° of propagation angles sampled by 201 source positions. The medium is strongly anisotropic. The anisotropy is significantly anelliptic and implies that the quasi‐shear mode should be triplicated for off‐axis propagation. Estimated density‐normalized moduli (in units of km2/s2) for the Petronas shale are A11 = 6.99 ± 0.21, A33 = 5.53 ± 0.17, A55 = 0.91 ± 0.05, and A13 = 2.64 ± 0.26. Densities in the logged zone just below the survey lie in the range between 2200 and 2400 kg/m3 with an average value close to 2300 kg/m3.
Mantle discontinuity structure beneath China
Authors: Revenaugh Justin , Sipkin Stuart A.
Published: February 2004

Mantle reflectivity profiles obtained from long‐period recordings of multiple‐ScS reverberations from intermediate and deep‐focus earthquakes surrounding China reveal remarkably sharp images of China deep structure. The Tibetan Plateau and fold belts to the north are underlain by a midlithospheric, low‐velocity zone (LVZ) evidenced by coincident G (lid‐LVZ boundary) and L' (base of the LVZ) reflectors and a thickened lithosphere consistent with uniform thickening as the dominant mode of north‐south deformation. The mantle beneath northeastern China, Mongolia and southeastern Russia has a typically cratonic reflectivity signature with H (Hales) and deep L (Lehmann) discontinuities. The Sino‐Korean Paraplatform of eastern China is a region of active extensional tectonics, as expressed by an “oceanic” reflectivity pattern at shallow depths (a G discontinuity at ∼80 km), but a more continental structure below. The southern Sea of Japan, Korean Peninsula, Yellow Sea and bordering regions of the China mainland are underlain by a ∼5.4% impedance decrease near 330 km depth that we interpret as the upper limit of dense silicate melt atop the 410‐km discontinuity produced at the subduction zones to the east. Regional inversion of the reflection coefficient of the 410‐km discontinuity, R(410), reveals a pronounced high beneath Mongolia and north to northwestern China which is best explained by increased upper mantle olivine content, signifying extensive basalt depletion in a tectosphere that extends below the 410‐km discontinuity. A low value of R(660) associated with topographic depression of the 660‐km discontinuity beneath eastern Asia is consistent with a horizontally inclined slab atop the phase transition marking the seismic discontinuity. Reflection defocusing is an alternative but requires ≥38 km of intermediate‐scale topography within the depressed region, exceeding the amplitude of the depression by a factor of 1.5–2. Either mechanism requires strong heterogeneity far west of seismically active slabs. Two lower mantle reflectors (one beneath southern Honshu, Kyushu, and the Sea of Japan; the other below Shantung province (China) and the Yellow Sea) correlate with horizontally elongated velocity anomalies in western Pacific seismic tomography models. We speculate that they may represent slabs stagnating below the 660‐km discontinuity.
Seismic heterogeneity in the upper mantle
Authors: Nolet G. , Grand S. P. , Kennett B. L. N.
Published: February 2004

This paper gives an overview of recent seismological evidence on heterogeneity in the upper mantle. While there is a growing consensus among seismologists on the low‐order spherical harmonic expansion of upper mantle structure, short‐ and intermediate‐wavelength features remain the subject of intense debate. Regional studies of Earth structure yield evidence that the upper mantle is heterogeous at all resolvable scales and that global models underestimate the amplitudes of velocity anomalies. Some important problems still outstanding concern: (1) the fate of subducting slabs, (2) the depth extent of low‐velocity zones under spreading centers and (3) the depth at which hotspots originate. The gross radial layering of the upper mantle has become much more evident to us in recent years, mainly as a result of the analysis of waveforms observed by the modern generation of digital, broadband stations and by densely spaced, narrow band arrays of seismometers. Nevertheless, important questions on the average radial structure still persist. The low shear velocity zone has been mapped in detail beneath the continents, and Jordan's tectosphere hypothesis is largely confirmed by regional one‐dimensional (1‐D) and global tomographic studies. However, it is less clear to what extent these low shear velocities are matched by low compressional velocities. Anisotropy, for which compelling evidence exists in the observations of split shear waves, may play a role in shaping our images of low velocity zones. Below the asthenosphere, the existence of the Lehmann discontinuity at about 220 km depth as a worldwide feature is unlikely, but early indications for a phase transition at 520 km have found some confirmation in stacked wave sections. Anomalously low velocities at greater depth have been observed near current subduction zones and even beneath more ancient suture zones. Recent progress in analysis of seismic waves has followed major advances in seismic instrumentation of two kinds: (1) the installation of very broadband stations around the world, and (2) the increased density of stations in seismic networks. Future progress will most likely depend on the expansion of dense networks, including temporary and portable stations; on our ability to increase the station coverage of oceanic regions; and on advancing the forward problem of computing waveforms for heterogeneous Earth models to a level of efficiency that allows for nonlinear optimization.
Transition from continental to oceanic crust on the Wilkes‐Adelie margin of Antarctica
Authors: Eittreim Stephen L.
Published: February 2004

The Wilkes‐Adelie margin of East Antarctica, a passive margin rifted in the Early Cretaceous, has an unusually reflective Moho which can be traced seismically across the continent‐ocean transition. Velocity models and depth sections were constructed from a combined set of U.S. and French multichannel seismic reflection lines to investigate the transition from continental to oceanic crust. These data show that the boundary between oldest oceanic crust and transitional continental crust is marked by a minimum in subsediment crustal thickness and, in places, by a shoaling of Moho. The Moho reflection is continuous across the edge of oceanic crust, and gradually deepens landward under the continental edge. A marginal rift basin, some tens of kilometers in width, lies in the transition between continental and oceanic crust, contains an average of about 4 km of synrift sediment that is prograded in places, and has characteristics of a former rift valley, now subsided to about 10 km. Three types of reflections in the seismic data are interpreted as volcanic deposits: (1) high‐amplitude reflections that floor the marginal rift basin, (2) irregularly seaward dipping sequences that comprise an anomalously thick edge of oceanic crust, and (3) highly irregular and diffractive reflections from oceanic crustal basalts that cap a normal‐thickness ocean crust. The present depth to the prerift surface of continental crust is compatible with passive margin subsidence since 95 Ma, corrected for its load of synrift and postrift sediment and mechanically stretched by factors of β = 1.8 or higher. Comparison of seismic crustal thickness measurements with inferred crustal thinning from subsidence analysis shows agreement for areas where β < 4. In areas where β > 4, measured thickness is greater than that inferred from subsidence analysis, a result that could be explained by underplating the crust beneath the marginal rift basin.
Equation of state and wave propagation in hysteretic nonlinear elastic materials
Authors: McCall K. R. , Guyer R. A.
Published: February 2004

Heterogeneous materials, such as rock, have extreme nonlinear elastic behavior (the coefficient characterizing cubic anharmonicity is orders of magnitude greater than that of homogeneous materials) and striking hysteretic behavior (the stress‐strain equation of state has discrete memory). A model of these materials, taking their macroscopic elastic properties to result from many mesoscopic hysteretic elastic units, is developed. The Preisach‐Mayergoyz description of hysteretic systems and effective medium theory are combined to find the quasistatic stress‐strain equation of state, the quasistatic modulus‐stress relationship, and the dynamic modulus‐stress relationship. Hysteresis with discrete memory is inherent in all three relationships. The dynamic modulus‐stress relationship is characterized and used as input to the equation of motion for nonlinear elastic wave propagation. This equation of motion is examined for one‐dimensional propagation using a Green function method. The out‐of‐phase component of the dynamic modulus due to hysteresis is found to be responsible for the generation of odd harmonics and to determine the amplitude of the nonlinear attenuation.
Skewness of marine magnetic anomalies created between 85 and 40 Ma in the Indian Ocean
Authors: Dyment J. , Cande S. C. , Arkani‐Hamed J.
Published: February 2004

We have performed a detailed analysis of the skewness of marine magnetic anomalies in Indian Ocean basins created between 85 and 40 Ma as a result of the northward motion of India. Visual and semiautomated methods of skewness determination were applied to the data. Both provide consistent results, but the visual method is prefered for its ability to deal with noisy data. Plots of apparent effective remanent inclination (or skewness corrected for present geomagnetic field inclination) versus time for conjugate basins display the combination of three effects: a gradual increase with time, related to the northward motion of the ridges attached to India in the geomagnetic reference frame; a gap between conjugate curves, which represents anomalous skewness senso stricto; and short‐period fluctuations, which represent the sequence effect, i.e., the effect of neighboring magnetic sources on the skewness of a given anomaly. The anomalous skewness decreases with faster spreading rate and completely disappears above 50 km/m.y., an observation which negates geomagnetic field behavior as a possible cause of the observed anomalous skewness.
Active hydrothermal systems during the recent uplift of Nanga Parbat, Pakistan Himalaya
Authors: Chamberlain C. Page , Zeitler Peter K. , Barnett Daniel E. , Winslow David , Poulson Simon R. , Leahy Tim , Hammer Julia E.
Published: February 2004

During the last 10 m.y., the Nanga Parbat Haramosh Massif in the northwestern Himalaya has been intruded by granitic magmas, has undergone high‐grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m.y. We suggest that the rapid uplift of the massif created a dual hydrothermal system, consisting of a near‐surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif along the transpressional faults causing δ18O and δD depletions in the gneisses and marked oxygen isotopic disequilibrium between mineral pairs from the fault zones. The discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif. At deeper levels within the massif, infiltration of low δ18O magmatic fluids caused δ18O depletions in the gneisses within the migmatite zone. These low δ18O fluids were derived from the young (<4 Ma), relatively low δ18O granites (∼8‰c) that are found within the core of the massif. Geochronological evidence in the form of fission track and 40Ar/39Ar cooling ages and U/Pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tato traverse was coeval with metamorphism, granite emplacement, and rapid denudation, in the interval 0.8–3.3 Ma. Finally, we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the Nanga Parbat‐Haramosh Massif.
Estimation of nonisotropic scattering in western Japan using coda wave envelopes: Application of a multiple nonisotropic scattering model
Authors: Hoshiba Mitsuyuki
Published: February 2004

Scattering by heterogeneities is an important factor controling shapes of seismograms. Analyzing horizontal component seismograms in the band 1.0–8 Hz of local earthquakes in western Japan, we found that coda energy concentration just after the S wave arrival is more than expected for any multiple isotropic scattering model in uniform infinite medium. This behavior was seen even for deep earthquakes and became stronger with increase of frequency. To try to interpret this observation, nonisotropy is introduced into the multiple scattering model. The observed envelopes are compared with the model in order to estimate the amount of nonisotropy in scattering. Coda wave envelopes are synthesized using the Monte Carlo method for the nonisotropic scattering media. Two types of angular dependent scattering are considered as models of the nonisotropy with relation to the perturbation of seismic wave velocity in the lithosphere: perturbed media with exponential and Gaussian autocorrelation functions. It is found that forward scattering is dominant and the strength of sideways scattering is at least 10 times larger than that of backward scattering. The amount of the nonisotropic scattering means that correlation distance is estimated to be larger than several hundred meters in the case of Gaussian random media.