bluuebunny/crossref_metadata_2025 huggingface dataset:
Galileo radio occultation measurements of Io's ionosphere and plasma wake
Authors: Hinson D. P. , Kliore A. J. , Flasar F. M. , Twicken J. D. , Schinder P. J. , Herrera R. G.
Published: February 2004

Six radio occultation experiments were conducted with the Galileo orbiter in 1997, yielding detailed measurements of the distribution and motion of plasma surrounding Io. This distribution has two components. One is highly asymmetric, consisting of a wake or tail that appears only on the downstream side and extends to distances as large as 10 Io radii. The other resembles a bound ionosphere and is present within a few hundred kilometers of Io's surface throughout the upstream and downstream hemispheres. Motion of plasma within the wake was measured through cross correlation of data acquired simultaneously at two widely separated terrestrial antennas. Plasma near Io's equatorial plane is moving away from Io in the downstream direction. Its speed increases from 30 km s−1 at a distance of 3 Io radii from the center of Io to 57 km s−1 at 7 Io radii. The latter corresponds to corotation with Jupiter's magnetic field, which suggests that bulk plasma motion rather than wave motion is being observed. Results for the bound ionosphere include vertical profiles of electron density at 10 locations near Io's terminator. The ionosphere is substantial, with the peak density exceeding 50,000 cm−3 at 9 out of 10 locations and reaching a maximum of 277,000 cm−3. The peak density varies systematically with Io longitude, with maxima near the center of the hemispheres facing toward (0°W) and away from (180°W) Jupiter and minima near the center of the downstream (90°W) and upstream (270°W) hemispheres. This pattern may be related to the Alfvénic current system induced by Io's motion through magnetospheric plasma. The vertical extent of the bound ionosphere increases from ∼200 km near the center of the upstream hemisphere to ∼400 km near the boundary between the leading and trailing hemispheres. There is a close resemblance between one ionospheric profile and a Chapman layer, and the topside scale height implies a plasma temperature of 202±14 K if Na+ is the principal ion. Two intense volcanic hot spots, Kanehekili and 9606A, may be influencing the atmospheric structure at this location.
Localized modes with zonal neutral wind, diffusion, and shear in equatorial spreadF
Authors: Rappaport H. L.
Published: February 2004

The effects of a zonal neutral wind, diffusion, and shear flow on perturbations in equatorial spreadF(ESF) are examined. A central goal of this work is to consider effects that arise from inhomogeneity of the ionosphere along the magnetic field lines of the Earth. The perturbations discussed here have cross‐field wavelengths which are small compared with the cross‐field inhomogeneity scales of the equilibrium. Inhomogeneity in the zonal wind velocity along a magnetic field line gives rise to a vertical Pedersen current. When the effects of shear are neglected, this current can increase or decrease the growth rate of the perturbations by an amount proportional to the difference between the zonal wind speed and the equilibrium plasma drift speed divided by the ionospheric scale height. Diffusion terms proportional to the product of the ion cross‐field mobility and the electron parallel mobility are found to increase the growth rate of ESF by reducing molecular ion region loading. Effects of shear are considered using the model ofFu et al. [1986], in which the perturbations are driven to smaller scales over time. The “quasi‐local” temporal behavior of the cold kilometer scale instability is described, while retaining, in an approximate way, the effects of the cross‐field scale size dependent mapping efficiency described byRappaport[1996]. In the diffusion regime a variational principle describing the decay of the perturbations is given. This model incorporates the effects of both parallel and cross‐field diffusions, as well as inhomogeneity along the field line.
Magnetic field strength distributions and spectra in the heliosphere and their significance for cosmic ray modulation: Voyager 1, 1980–1994
Authors: Burlaga L. F. , Ness N. F.
Published: February 2004

Annual variations of the hourly averages of the heliospheric magnetic field strengthBobserved by Voyager 1 (V1) have been analyzed for the 15‐year interval 1980–1994 inclusive. During this period, V1 moved from a radial distance of 6.9–58.1 AU and from a heliolatitude of −5.4 to 32.7°. The annual distribution function ofBis nearly lognormal. The width of the distribution, w, does not change systematically with time, distance, latitude or solar activity: 〈w〉 = 0.61 ± 0.02. The standard deviation ofB, SD(B), normalized by the average,Bav, is also invariant: 〈SD(B)/Bav〉 = 0.67 ± 0.03. There is a decadal frequency range for which the power spectral density (PSD) ofB(t) is well described by a power law with an exponent s that ranges from −2.0 near solar maximum (1980, 1990) when GMIRs were present to −1.5 near solar minimum (1987–1988). A Global Merged Interaction Region (GMIR) is a shell‐like region that encircles the Sun, propagates away from the Sun, has a radial thickness of the order of a few AU and contains intense, disturbed magnetic fields. The lower limit of the scaling range corresponds to a period of ≈ 5 days and is invariant in the 15 years studied. The relative amplitude of the PSD at the lower limit of this range is also invariant. However, the relative amplitude of the fluctuations at the solar rotation period of 26 days decreases with increasing distance from the sun asR−(0–9 ± 0.2). These derived observational results in time and frequency domains require theoretical models of the solar wind expansion into the distant heliosphere that include both physical and statistical features. Such models could be based on (1) energy and/or entropy principles or (2) conventional MHD along with temporally complex but statistically simple input functions. Our results suggest an anticorrelation between the annual magnetic field strength and cosmic ray intensity, as is observed during the 15‐year period considered here. Statistical models of the magnetic field might provide a basis for new models of cosmic ray modulation.
The acceleration of pickup ions at shock waves: Test particle‐mesh simulations
Authors: Lipatov A. S. , Zank G. P. , Pauls H. L.
Published: February 2004

A mechanism for the acceleration of pickup ions by repeated reflection from the electrostatic cross‐shock potential of a quasi‐perpendicular shock was proposed independently by Zank et al. [1996b] and Lee et al. [1996]. The acceleration mechanism, known variously as Multiply Reflected Ion (MRI) acceleration or “shock surfing,” was studied by these authors in the limit of an idealized shock, which possessed neither fine‐scale structure (distinct foot, ramp, or overshoot) nor pickup ion scattering turbulence. Here the acceleration of pickup ions at cometary and interplanetary shocks and at the termination shock is studied in the “test” particle limit on the basis of a particle‐mesh simulation. All simulations assume a shell distribution for the pickup ions (either pickup protons or helium), and the dynamics of pickup ions propagating in a fixed electromagnetic field profile are investigated. The effect of a shock foot, ramp and overshoot on the acceleration of pickup ions at perpendicular and oblique shocks is described. The acceleration of pickup ions in the presence of strong turbulence inside the foreshock region is also addressed. For quasi‐perpendicular shocks with structure, we obtained the following results. First, the accelerated H+ and He+ spectrum is a very hard power law ∝ E−k, k = 0.92–1.2, which is much harder than that predicted by diffusive shock acceleration. Also, as θbn, the angle between the upstream magnetic field and shock normal, decreases from 90°, the accelerated pickup ion spectrum flattens until MRI acceleration ceases. Second, the fine structure of the shock is found to reduce slightly the maximum energy gain for an accelerated pickup ion compared to that gained at an unstructured shock. Third, a flat turbulence spectrum is found to lead to an increase in the maximum energy for transmitted pickup ions, whereas a power law turbulence spectrum leads to a modest reduction in the maximum pickup ion energy gain. However, the basic MRI acceleration mechanism continues to operate in the presence of turbulent magnetic fluctuations and the characteristic hard spectra are preserved. Fourth, MRI acceleration is found to work only for those shocks for which θbn>60°–70°. The results from the test particle simulations described here are also used to interpret particle acceleration in self‐consistent hybrid simulations.
Domain wall structures in single‐crystal magnetite investigated by magnetic force microscopy
Authors: Foss Sheryl , Moskowitz Bruce M. , Proksch Roger , Dahlberg E. Dan
Published: February 2004

Domain walls in bulk single‐crystal magnetite were studied using a variable magnetic field magnetic force microscope (MFM). Classical configurations of 180°, 109°, and 71° walls were observed on (110) surfaces. Magnetostatic effects on these different walls were compared. Profiles of the MFM response above the walls were measured with the MFM tip magnetized in different directions. The contribution to the profiles from the z component of the sample field was distinguished from the in‐plane components. An asymmetry of the z component of the response profiles for all wall types was observed, consistent with the existence of Néel caps which terminate the interior Bloch walls near the surface. The wall profiles of the non‐180° walls were more asymmetric than that of the 180° walls. The 180° walls were observed to be subdivided into alternating polarity segments of average length 15 μm. These walls formed a characteristic zig‐zag structure in which the Bloch lines separating segments were located at the corners of the zig‐zag. Only unusually long 109° walls were observed to contain a single Bloch line, and the 71° walls, although the longest, were never observed to be subdivided. An applied field perpendicular to the sample plane moved the Bloch lines within the walls without translating the walls themselves. Multipolar walls were converted to unipolar in perpendicular applied fields from O to 100 mT. Profiles of opposite polarity segments of a subdivided wall indicated that the Néel cap formation does not alternate sides of the wall from segment to segment. Alignment of opposite polarity segments of parallel subdivided walls provided an example of long range magnetostatic interactions between walls and possibly their Néel caps.
Fine structure of the 410‐km discontinuity
Authors: Melbourne Tim , Helmberger Don
Published: February 2004

The April 14, 1995, earthquake in western Texas (Mw 5.7) produced a strong topside reflection off the 410‐km discontinuity which was recorded on a multitude of seismic arrays throughout the southwestern United States. Data from 394 vertical short‐period and 24 broadband instruments provide dense coverage of this event from distances of 11° to 19° and provide a detailed look at the subcontinental 410‐km structure. The salient features of this data set are (1) the strong dependence on wavelength of the 410‐km triplication range, (2) the uniform amplitude ratio of the direct P and reflected P410 phases on both short‐period and broadband recordings throughout the triplication, and (3) the abrupt termination of the short‐period P410 phase at 13.3°. These features are best modeled by a composite discontinuity in which a sharp velocity jump of 3% is overlain by a linear velocity jump of 3.5% spread over 14 km. The interference of energy turning in the diffuse and sharp portions of this discontinuity structure reproduces both the long‐ and short‐period triplication range and the step‐like behavior of the P410 short‐period amplitude, which cannot be reproduced with either a simple linearly diffuse or a purely sharp discontinuity. This composite structure produces a triplication range which depends on source frequency and has an apparent depth which depends on observation frequency. Additionally, this is the structure expected from mineralogical arguments for the α to β olivine phase transition.
A fluid‐rock interaction cellular automaton of volcano mechanics: Application to the Piton de la Fournaise
Authors: Lahaie Franz , Grasso Jean Robert
Published: February 2004

We developed a cellular automata model of fluid circulation within a rock matrix to test whether considering a volcano as a complex system helps understand the observed eruptive and seismicity patterns of the Piton de la Fournaise (PF) volcano. In this model, fluid and rock cells interact through locally defined rules, including a steady increase of pressure, a threshold dynamics, a fluid redistribution algorithm, and an energy dissipation process. Comparison of the model results and observations for more than 40 simulations provides information on the mechanics of the PF volcano and basaltic volcanoes in general. First, we demonstrate that the multiple lenses magma storage model for the summit reservoir of basaltic volcanoes is consistent with the PF 1920–1992 eruptive pattern, proposing an alternative to the macrochamber model. Second, we show that the great number of interactions between magma storage lenses in a critical state may work as a nonlinear filter, modifying a possible uniform increase of pressure in the input, induced by deep magma supply and gas exsolution during crystallization, into a nonlinear fluid flow emergence in time and size, both unpredictable and organized in nonrandom scaling statistics. The fluid‐rock cellular automaton allows us to explore scales ranging from global Earth mantle to porous rock matrix, thus rationalizing the power law volume distribution of total eruptions on the Earth surface [McClelland et al., 1989] and the hierarchical organization that is reported for many types of fluid‐induced seismicity [Grasso, 1993; Miller et al., 1996], respectively.
The tectonic history of the Tasman Sea: A puzzle with 13 pieces
Authors: Gaina Carmen , Müller Dietmar R. , Royer Jean‐Yves , Stock Joann , Hardebeck Jeanne , Symonds Phil
Published: February 2004

We present a new model for the tectonic evolution of the Tasman Sea based on dense satellite altimetry data and a new shipboard data set. We utilized a combined set of revised magnetic anomaly and fracture zone interpretations to calculate relative motions and their uncertainties between the Australian and the Lord Howe Rise plates from 73.6 Ma to 52 Ma when spreading ceased. From chron 31 (67.7 Ma) to chron 29 (64.0 Ma) the model implies, transpression between the Chesterfield and the Marion plateaus, followed by strike‐slip motion. This transpression may have been responsible for the formation of the Capricorn Basin south of the Marion Plateau. Another major tectonic event took place at chron 27 (61.2 Ma), when a counterclockwise change in spreading direction occurred, contemporaneous with a similar event in the southwest Pacific Ocean. The early opening of the Tasman Sea cannot be modeled by a simple two‐plate system because (1) rifting in this basin propagated from south to north in several stages and (2) several rifts failed. We identified 13 continental blocks which acted as microplates between 90 Ma and 64 Ma. Our model is constrained by tectonic lineaments visible in the gravity anomaly grid and interpreted as strike‐slip faults, by magnetic anomaly, bathymetry and seismic data, and in case of the South Tasman Rise, by the age and affinity of dredged rocks. By combining all this information we derived finite rotations that describe the dispersal of these tectonic elements during the early opening of the Tasman Sea.
High‐resolution, two‐dimensional vertical tomography of the central Pacific mantle using ScS reverberations and frequency‐dependent travel times
Authors: Katzman Rafael , Zhao Li , Jordan Thomas H.
Published: February 2004

A vertical tomogram is constructed for the mantle between Tonga and Hawaii. Using a complete Gaussian‐Bayesian approach, we inverted a data set comprising 304 ScS reverberation travel times with Fréchet kernels computed by the paraxial ray approximation and 1122 frequency‐dependent phase delays of turning and surface waves with Fréchet kernels calculated by a normal‐mode coupling algorithm. The model parameters include shear speed variations, perturbations to shear wave radial anisotropy in the uppermost mantle, and the topographies of the 410‐ and 660‐ km discontinuities. Tests demonstrate that the data set resolves lateral structures in the upper mantle with scale lengths of several hundred kilometers and greater. The most significant feature in our model is a well resolved regular pattern of high and low shear velocities in the upper mantle. These variations have a horizontal wavelength of 1500 km, a vertical dimension of 700 km, and an amplitude of about 3%, and they show a strong positive correlation with seafloor topography and geoid height variations. The geoid highs correspond to a series of northwest trending swells associated with the major hotspots of the Society, Marquesas, and Hawaiian Islands. Where these swells cross the corridor, they are underlain by high shear velocities throughout the uppermost mantle, so it is unlikely that their topographies are supported by thermal buoyancy. Although chemical buoyancy generated by basaltic volcanism and the formation of its low‐density peridotitic residuum can explain the positive correlation between topography and velocity variation at the shallow depth, an additional mechanism is needed to account for the shear velocity pattern at depths greater than 200 km. It is therefore hypothesized that the central Pacific is underlain by a system of Richter‐type convective rolls that are oriented subparallel to the absolute plate velocity, have unit aspect ratio transverse to this orientation, and are confined above the 660‐km discontinuity. This convection pattern appears to be strongly correlated with the locations of the Tahitian, Marquesan, and Hawaiian hotspots, which raises interesting questions for Morgan's hypothesis that these hotspots are the surface manifestations of deep mantle plumes.
Response time of glaciers as a function of size and mass balance: 2. Numerical experiments
Authors: Pfeffer W. Tad , Sassolas Christophe , Bahr David B. , Meier Mark F.
Published: February 2004

Numerical experiments were made using a time‐dependent nonlinear finite element model of glacier flow to seek confirmation of theoretical predictions made in the companion to this paper [Bahr et al., this issue]. The theoretical analysis indicates that under conditions of equal gradients in mass balance along the glacier surface (
), other variables being held constant, response time to mass balance perturbations will decrease as glacier size increases. This behavior is confirmed by the numerical model experiments, which are performed on a suite of 23 radially symmetric ice caps with sizes ranging over ∼4 orders of magnitude. The shape and mass balance profiles of the ice caps are defined so that
is constant over all ice caps, and for uniform perturbations in mass balance, the modeled response times are faster for larger ice caps. The model results also confirm a volume/area scaling relationship established in the theoretical discussion and confirm the equivalence between two measures of response time, one arising from the present theory and the other a conventional expression of response time. The apparent discrepancy between the present results and preexisting theory is discussed, and it is shown that the results are compatible.
Lower Pliocene polarity transitions from the Suva Marl, Fiji
Authors: Clement Bradford M. , Rodda Peter , Sierra Loresley , Smith Edward
Published: February 2004

We present a paleomagnetic record of the lower Cochiti polarity transition and new results from the lower and upper Nunivak transitions obtained from the Lower Pliocene Suva Marl ( Fiji, (18°S, 178°E)). This formation has yielded a well‐defined magnetostratigraphy that, combined with 40Ar/39Ar ages from interbedded tuff beds, provides excellent chronostratigraphic control [Clement et al, 1997]. We previously presented the upper Cochiti and the upper and lower Nunivak polarity transition records from this unit [Clement et al., 1995, 1996]. New results were obtained from oriented block samples spanning these reversals. Each of these reversals was sampled at duplicate sections separated by 4 to 8 km. The agreement between the records obtained from different sections suggests that the paleomagnetic records are not artifacts of local sedimentary disturbances or random degrees of incompletely removed overprints. Instead the similarities in these and other records of these same reversals from different types of paleomagnetic recorders suggest that they provide information about the behavior of the geomagnetic field as it reverses.
Bottom response to a tsunami earthquake: Submersible observations in the epicenter area of the 1993 earthquake off southwestern Hokkaido, Sea of Japan
Authors: Takeuchi Akira
Published: February 2004

We conducted a research campaign of nine dives using the manned submersible vehicle Shinkai 6500 in 1995 beneath the central Okushiri Ridge along the eastern margin of the Sea of Japan. The main objective was to investigate coseismic response and bottom disturbance to the tsunamigenic 1993 Offshore Southwestern Hokkaido earthquake (Ms 7.8; 42°47′N, 139°12′E) in the northern aftershock area including the mainshock epicenter. The whole aftershock area occupies the western half of source area of the related tsunami. Between the Okushiri Ridge and the neighboring Shiribeshi Trough to the east, we found a gentle scarplet of the earthquake fault that bounds the first terrace and a zone of open gashes on the flat terrace. A number of sand blows, open cracks, and other slope failures occupied the slope of the Okushiri Ridge on the uplifted hanging wall side of the tsunami source area. Some fractures and holes whose activity had ceased soon after the earthquake began to be buried by the 2‐year accumulation of recent sediments. In contrast, the Shiribeshi Trough, on the footwall of the fault and in the subsided domain of the tsunami source area, suffered no remarkable disturbance other than gentle folding. Our submersible observations elucidated the seafloor response to tsunamigenic uplift of the hanging wall of a seismogenic thrust fault which displays the present time section of back‐thrusting with an embryonic subduction along the eastern margin of the Sea of Japan.
Sulfur in serpentinized oceanic peridotites: Serpentinization processes and microbial sulfate reduction
Authors: Alt Jeffrey C. , Shanks Wayne C.
Published: February 2004

The mineralogy, contents, and isotopic compositions of sulfur in oceanic serpentinites reflect variations in temperatures and fluid fluxes. Serpentinization of <1 Ma peridotites at Hess Deep occurred at high temperatures (200°–400°C) and low water/rock ratios. Oxidation of ferrous iron to magnetite maintained low ƒO2 and produced a reduced, low‐sulfur assemblage including NiFe alloy. Small amounts of sulfate reduction by thermophilic microbes occurred as the system cooled, producing low‐δ34S sulfide (1.5‰ to −23.7‰). In contrast, serpentinization of Iberian Margin peridotites occurred at low temperatures(∼20°–200°C) and high water/rock ratios. Complete serpentinization and consumption of ferrous iron allowed evolution to higher ƒO2. Microbial reduction of seawater sulfate resulted in addition of low‐δ34S sulfide (∼15 to ∼43‰) and formation of higher‐sulfur assemblages that include valleriite and pyrite. The high SO4/total S ratio of Hess Deep serpentinites (0.89) results in an increase of total sulfur and high δ34S of total sulfur (mean ∼8‰). In contrast, Iberian Margin serpentinites gained large amounts of 34S‐poor sulfide (mean total S = 3800 ppm), and the high sulfide/total S ratio (0.61) results in a net decrease in δ34S of total sulfur (mean ≈ −5‰). Thus serpentinization is a net sink for seawater sulfur, but the amount fixed and its isotopic composition vary significantly. Serpentinization may result in uptake of 0.4–14 × 1012 g S yr−1 from the oceans, comparable to isotopic exchange in mafic rocks of seafloor hydrothermal systems and approaching global fluxes of riverine sulfate input and sedimentary sulfide output.
Compressional velocity structure and anisotropy in the uppermost mantle beneath Italy and surrounding regions
Authors: Mele Giuliana , Rovelli Antonio , Seber Dogan , Hearn Thomas M. , Barazangi Muawia
Published: February 2004

Travel times of about 39,000 Pn arrivals recorded from regional earthquakes by the Italian Telemetered Seismic Network and by stations of nearby countries are inverted to image lateral variations of seismic velocity and anisotropy at subcrustal depth in Italy and surrounding regions. This method allows simultaneous imaging of variations of Pn velocity and anisotropy, as well as crustal thickness variations. The Po plain, the Adriatic Sea, and the Ionian Sea have normal to high Pn velocities. In contrast, lower velocities (7.9–8.0 km/s) are imaged in Italy beneath the western Alps, the northern Apennines, and eastern Sicily and nearby Calabria, as well as in northern Albania and beneath the Pannonian basin. Low Pn velocities beneath the northern Apennines correlate with present‐day extension and may have resulted from thermal anomalies in the uppermost mantle possibly due to delamination processes. Low velocities are consistent with the high‐attenuation zone inferred in the uppermost mantle beneath the internal Apennine units and the Tyrrhenian margin of the peninsula by Mele et al. [1996, 1997]. On the contrary, low velocities beneath the western Alps may be an apparent effect due to the abrupt thickening of the crustal roots. Pn velocity is anisotropic in the study area with a maximum amplitude of ± 0.2 km/s. The largest anisotropic velocity anomalies are observed along the major arc structures of Italy, i.e., the northern Apennines and the Calabrian Arc, indicating that these features are controlled by uppermost mantle processes. The anisotropy anomaly along the Calabrian Arc extends as far as Albania but ends abruptly north of this area, suggesting that a lithospheric discontinuity is present along the northern Albanian border.
Alteration and mineralization of an oceanic forearc and the ophiolite‐ocean crust analogy
Authors: Alt Jeffrey C. , Teagle Damon A. H. , Brewer Tim , Shanks Wayne C. , Halliday Alex
Published: February 2004

Mineralogical, chemical, and isotopic (O, C, S, and Sr) analyses were performed on minerals and bulk rocks from a forearc basement section to understand alteration processes and compare with mid‐ocean ridges (MOR) and ophiolites. Ocean Drilling Program Hole 786B in the Izu‐Bonin forearc penetrates 103 m of sediment and 725 m into volcanic flows, breccias, and basal dikes. The rocks comprise boninites and andesites to rhyolites. Most of the section was affected by low‐temperature (<100°C) seawater alteration, with temperatures increasing downward. The rocks are partly (5–25%) altered to smectite, Fe‐oxyhydroxide, calcite, and phillipsite, and exhibit gains of K, Rb, and P, loss of Ca, variable changes in Si, Na, Mg, Fe, Sr, and Y, and elevated δ18O and 87Sr/86Sr. Higher temperatures (∼150°C) in the basal dikes below 750 m led to more intense alteration and formation of chlorite‐smectite, corrensite, albite, K‐feldspar, and quartz (±chlorite). A 5 m thick hydrothermally altered and pyritized zone at 815 m in the basal dikes reacted with mixtures of seawater and hydrothermal fluids to Mg‐chlorite, albite, and pyrite, and gained Mg and S and lost Si and Ca. Focused flow of hydrothermal fluids produced serialization halos (Na‐K sericite, quartz, pyrophyllite, K‐feldspar, and pyrite) along quartz veins at temperatures of 200°–250°C. High 87Sr/86Sr ratios of chloritized (∼0.7055) and sericitized (∼0.7065) rocks indicate involvement of seawater via mixing with hydrothermal fluids. Low δ34S of sulfide (−2 to −5.5‰) and sulfate (12.5‰) are consistent with input of magmatic SO2 into hydrothermal fluids and disproportionation to sulfide and sulfate. Alteration processes were generally similar to those at MORs, but the arc section is more intensively altered, in part because of the presence of abundant glassy rocks and mafic phases. The increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near the base of the volcanic section in MOR and the Troodos ophiolite.
Analysis of fault slip inversions: Do they constrain stress or strain rate?
Authors: Twiss Robert J. , Unruh Jeffrey R.
Published: February 2004

Fault slip data commonly are used to infer the orientations and relative magnitudes of either the principal stresses or the principal strain rates, which are not necessarily parallel or equal. At the local scale of an individual fault, the shear plane and slip direction define the orientations of the local principal strain rate axes but not, in general, the local principal stress axes. At a large scale, the orientations of P and T axes maxima for sets of fault slip data do not provide accurate inversion solutions for either strain rate or stress. The quantitative inversion of such fault slip data, however, provides direct constraints on the orientations and relative magnitudes of the global principal strain rates. To interpret the inversion solution as constraining the global principal stresses requires that (1) the fault slip pattern must have a characteristic symmetry no lower than orthorhombic; (2) the material must be mechanically isotropic; and (3) there must be a linear constitutive relationship between the global stress and the global strain rate. Isotropic linear elastic constitutive equations are appropriate to describe the local deformation surrounding an individual slip discontinuity. Fault slip inversions, however, constrain the characteristics of a large‐scale cataclastic flow, which is described by constitutive equations that are probably, but to an unknown degree, anisotropic and nonlinear. Such material behavior would not strictly satisfy the requirements for the stress interpretation. Thus, at the present state of knowledge, fault slip inversion solutions are most reliably interpreted as constraining the principal strain rates.
Variation of transition zone high‐velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern Kuriles to Izu‐Bonin and Ryukyu
Authors: Tajima Fumiko , Grand Stephen P.
Published: February 2004

The seismic structure of the transition zone beneath the northwestern Pacific ocean is studied by comparing regional distance seismic waveforms with synthetic seismograms. By modeling waveforms we incorporate the travel times of secondary waves that are not included in traveltime data used in most P wave tomography experiments. These secondary waves are highly sensitive to the velocity above the 660 km discontinuity and thus add information on the seismic structure of the transition zone independent of that used in many tomography experiments. We used just two one‐dimensional models in the waveform comparison. The iasp91(slightly modified in the shallower depths as aiasp) is used as a global average structure, and model M3.11 is used as representative of transition zone structure near a deep subducting slab. Model M3.11 is faster than iasp91 from 525 to 660 km depth but also has a 30 km deeper “660” km discontinuity. Our results show strong variations in the transition zone structure beneath the northwestern Pacific. Data that sample beneath the northwestern Japanese subduction zone and the southern Kuriles are well fit by model M3.11. The deep structure beneath the Bohai Sea, to the west of Korea, also appears to show slab‐related velocity anomalies in the transition zone. In contrast, the seismic data sampling beneath the Yellow Sea to the East China Sea are better modeled using the global average model iasp91. Data that sampled beneath the Philippine Sea showed evidence for anomalously high velocity in the transition zone, but there is little apparent change in the depth of the 660 km discontinuity there. The overall results are in agreement with past work indicating fairly broad regions where subducting plate is lying flat or piling up within the transition zone, although we find that the region where this occurs, and thus the total volume of slab within the upper mantle, is considerably less than that seen in past studies. The unusual structure beneath the Philippine plate may be due to the relatively recent descent of slab beneath this region. The slab itself is cold, and thus anomalously fast, but the deeper mantle may not have cooled enough for the 660 km discontinuity to have deepened.
Stress transferred by the 1995Mw= 6.9 Kobe, Japan, shock: Effect on aftershocks and future earthquake probabilities
Authors: Toda Shinji , Stein Ross S. , Reasenberg Paul A. , Dieterich James H. , Yoshida Akio
Published: February 2004

The Kobe earthquake struck at the edge of the densely populated Osaka‐Kyoto corridor in southwest Japan. We investigate how the earthquake transferred stress to nearby faults, altering their proximity to failure and thus changing earthquake probabilities. We find that relative to the pre‐Kobe seismicity, Kobe aftershocks were concentrated in regions of calculated Coulomb stress increase and less common in regions of stress decrease. We quantify this relationship by forming the spatial correlation between the seismicity rate change and the Coulomb stress change. The correlation is significant for stress changes greater than 0.2–1.0 bars (0.02–0.1 MPa), and the nonlinear dependence of seismicity rate change on stress change is compatible with a state‐ and rate‐dependent formulation for earthquake occurrence. We extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of Kobe and calculating the change in probability caused by these stress changes. Transient effects of the stress changes are incorporated by the state‐dependent constitutive relation, which amplifies the permanent stress changes during the aftershock period. Earthquake probability framed in this manner is highly time‐dependent, much more so than is assumed in current practice. Because the probabilities depend on several poorly known parameters of the major faults, we estimate uncertainties of the probabilities by Monte Carlo simulation. This enables us to include uncertainties on the elapsed time since the last earthquake, the repeat time and its variability, and the period of aftershock decay. We estimate that a calculated 3‐bar (0.3‐MPa) stress increase on the eastern section of the Arima‐Takatsuki Tectonic Line (ATTL) near Kyoto causes fivefold increase in the 30‐year probability of a subsequent large earthquake near Kyoto; a 2‐bar (0.2‐MPa) stress decrease on the western section of the ATTL results in a reduction in probability by a factor of 140 to 2000. The probability of aMw= 6.9 earthquake within 50 km of Osaka during 1997–2007 is estimated to have risen from 5–6% before the Kobe earthquake to 7–11% afterward; during 1997–2027, it is estimated to have risen from 14–16% before Kobe to 16–22%.
Suppression of large earthquakes by stress shadows: A comparison of Coulomb and rate‐and‐state failure
Authors: Harris Ruth A. , Simpson Robert W.
Published: February 2004

Stress shadows generated by California's two most recent great earthquakes (1857 Fort Tejon and 1906 San Francisco) substantially modified 19th and 20th century earthquake history in the Los Angeles basin and in the San Francisco Bay area. Simple Coulomb failure calculations, which assume that earthquakes can be modeled as static dislocations in an elastic half‐space, have done quite well at approximating how long the stress shadows, or relaxing effects, should last and at predicting where subsequent large earthquakes will not occur. There has, however, been at least one apparent exception to the predictions of such simple models. The 1911M>6.0 earthquake near Morgan Hill, California, occurred at a relaxed site on the Calaveras fault. We examine how the more complex rate‐and‐state friction formalism based on laboratory experiments might have allowed the 1911 earthquake. Rate‐and‐state time‐to‐failure calculations are consistent with the occurrence of the 1911 event just 5 years after 1906 if the Calaveras fault was already close to failure before the effects of 1906. We also examine the likelihood that the entire 78 years of relative quiet (only fourM≥6 earthquakes) in the bay area after 1906 is consistent with rate‐and‐state assumptions, given that the previous 7 decades produced 18M≥6 earthquakes. Combinations of rate‐and‐state variables can be found that are consistent with this pattern of large bay area earthquakes, assuming that the rate of earthquakes in the 7 decades before 1906 would have continued had 1906 not occurred. These results demonstrate that rate‐and‐state offers a consistent explanation for the 78‐year quiescence and the 1911 anomaly, although they do not rule out several alternate explanations.
Lower crustal anisotropy or dipping boundaries? Effects on receiver functions and a case study in New Zealand
Authors: Savage Martha Kane
Published: February 2004

We examine the effects on receiver functions of transverse anisotropy and of dipping isotropic boundaries. Splitting of the Moho Ps phase predicts the anisotropy from a postulated 10‐km‐thick layer of highly anisotropic crustal material only when other phases can be well isolated and when allowance is made for the rotation of the incident energy out of the plane of energy propagation expected for isotropic models. We examine azimuthal variations of the synthetic radial and transverse receiver functions. For both transversely anisotropic layers and dipping isotropic boundaries, radial receiver functions are symmetric and transverse receiver functions are antisymmetric about a given back azimuth (the horizontal projection of the axis of symmetry or the dip direction). Differences include the following: (1) Transverse energy from dipping boundaries arrives at the time of the initial P phase no matter at what depth the dipping boundaries occur, while transverse energy arrives at the time of the initial P phase for anisotropic material only when the anisotropy is at the surface. (2) Transversely anisotropic systems with horizontal symmetry axes have waveforms with 180° periodicity as a function of back azimuth, while dipping symmetry axes or dipping boundaries just have 360° periodicity. A consequence of the symmetry is that stacking the initial P and Ps phases from transverse receiver functions for back azimuths that are 180° apart tends to decrease the arrivals from shallowly dipping isotropic boundaries and doubles the arrivals from anisotropic layers with horizontal symmetry axes. We examine receiver functions for station SNZO in New Zealand, above a subducting plate dipping at 15° beneath the station. Opposite polarities of transverse receiver functions separated by 180° in back azimuth suggest that either dipping isotropic layers or dipping axes of symmetry of anisotropic layers are present. The 15° dip of the subduction zone is insufficient to explain the energy on the transverse receiver functions by the isotropic structures previously determined from refraction and earthquake travel time inversion. However, modifying the isotropic models for the region by anisotropy determined via shear wave splitting measurements and refraction studies, allows many of the features of both the radial and transverse receiver functions to be explained. A relatively highly anisotropic layer about 7 km thick at the base of the crust with an axis of symmetry dipping at 20°–40° may be caused by metamorphosed oceanic crust.
Eurasian fundamental mode surface wave phase velocities and their relationship with tectonic structures
Authors: Curtis Andrew , Trampert Jeannot , Snieder Roel , Dost Bernard
Published: February 2004

We automatically analyzed 32,000 fundamental mode Love and Rayleigh wave signals with earthquake‐station paths traversing Eurasia and Indonesia and obtained robust average phase velocity measurements between 20 s and 170 s periods along 4389 Love and 4020 Rayleigh paths. These were inverted to give phase velocity maps at 14 fixed periods. Resolution tests suggest that features with diameter >750 km and >500 km are resolved over most of Eurasia and central/SE Asia respectively. Low‐period Love waves image areas with thick sedimentary cover as low‐velocity zones, and almost all periods image mountainous regions since these have thick crust and hence low average lithospheric shear velocity. At long periods, both Love and Rayleigh waves define high phase velocity zones across shield and cratonic areas reflecting their deep lithospheric roots. We observe significant along‐strike heterogeneity in the Zagros fold belt and Tien Shan‐Altai system. Taking sections across Eurasian phase velocity space allows us to make approximate interpretations in terms of shear velocity structure directly. For example, the Red River and East Vietnam Boundary faults are traced on their eastern side by low velocities which extend at depth into Indonesia. We relate this to mantle upwelling associated with early Eocene rotation of Indochina and reversal of the sense of shear across the Red River fault post‐20 Ma. We observe dipping subduction of the Mediterranean beneath the Aegean, of the Philippine Sea beneath Indonesia, and of the Indian shield beneath Tibet. We also image a fossil subducted plate beneath NE Borneo which we associate with subduction of the proto‐South China Sea between 50 Ma and 15 Ma.
Dike propagation through an elastic plate
Authors: Mériaux Catherine , Jaupart Claude
Published: February 2004

We study the vertical propagation of a magma‐filled crack which rises due to buoyancy through an elastic plate of finite thickness. The plate rests on a magma source of large volume. Full time‐dependent solutions of the coupled problem with elastic deformation and viscous flow in a thin fracture are obtained numerically. A range of initial conditions corresponding to different regimes of fracture filling, from passive conditions at negligible magma velocity to forceful injection, are investigated. Three phases are distinguished during ascent. In a first phase, as the dike grows out of the initial magma‐filled fracture, the elastic pressure distribution adjusts to a fully developed distribution, with values close to buoyancy in a nose region and smaller values in a conduit region below the nose. Subsequently, the dike develops increasingly wider conduit and nose regions. Close to the plate upper boundary, stress and displacement fields become sensitive to the width of the plate. The initial conditions determine quantitative characteristics of subsequent dike propagation and, in particular, the flux of magma into the dike. As a dike propagates away from source, magma encounters increasingly colder rock but flows through an increasingly wider channel. This provides a self‐shielding mechanism against freezing. At the upper boundary, eruption initially occurs out of the inflated nose region and is driven by the relaxation of elastic stresses.
Dynamics and energetics of bubble growth in magmas: Analytical formulation and numerical modeling
Authors: Proussevitch A. A. , Sahagian D. L.
Published: February 2004

We have developed a model of diffusive and decompressive growth of a bubble in a finite region of melt which accounts for the energetics of volatile degassing and melt deformation as well as the interactions between magmatic system parameters such as viscosity, volatile concentration, and diffusivity. On the basis of our formulation we constructed a numerical model of bubble growth in volcanic systems. We conducted a parametric study in which a saturated magma is instantaneously decompressed to one bar and the sensitivity of the system to variations in various parameters is examined. Variations of each of seven parameters over practical ranges of magmatic conditions can change bubble growth rates by 2–4 orders of magnitude. Our numerical formulation allows determination of the relative importance of each parameter controlling bubble growth for a given or evolving set of magmatic conditions. An analysis of the modeling results reveals that the commonly invoked parabolic law for bubble growth dynamics R∼t1/2 is not applicable to magma degassing at low pressures or high water oversaturation but that a logarithmic relationship R∼log(t) is more appropriate during active bubble growth under certain conditions. A second aspect of our study involved a constant decompression bubble growth model in which an initially saturated magma was subjected to a constant rate of decompression. Model results for degassing of initially water‐saturated rhyolitic magma with a constant decompression rate show that oversaturation at the vent depends on the initial depth of magma ascent. On the basis of decompression history, explosive eruptions of silicic magmas are expected for magmas rising from chambers deeper than 2 km for ascent rates >1–5 m s−1.
Theory for the effects of free gas in subsea formations on tidal pore pressure variations and seafloor displacements
Authors: Wang Kelin , Davis Earl E. , van der Kamp Garth
Published: February 2004

Loading of the seafloor by regional‐scale pressure variations, such as those imposed by ocean tides, is supported by both the rock matrix and interstitial fluid. The nature of the partitioning of the support between the two depends primarily on the compressibility of the fluid and the compressibility and fluid‐transport properties of the rock matrix. In this paper, we examine theoretically the influence of free gas on pore fluid compressibility, on the nature of time‐dependent load partitioning, and on the consequent vertical rock deformation and seafloor displacement. An example is the gas trapped below deep‐sea gas hydrate. We have derived an expression for the steady state compressibility of pore fluid considering the influence of gas solubility in water. The effect of gas solubility is seen to be important at low, such as tidal, loading frequencies and thus must be included when observations of tidally induced pore fluid pressure variations or seafloor displacements are used to constrain gas content. For very low gas concentrations ng (much less than 0.1%), the steady state fluid compressibility can be enhanced by gas solution/dissolution over the loading cycle by several factors at high ambient pressure and more than an order of magnitude at low ambient pressures (< 5 MPa). At ng > 2%, the fluid compressibility increases sensitively with ng and greatly affects the tidal response of the pore fluid pressure regardless of the solubility. Thus, with careful experimental design, tidally induced pore pressure variations may be used to detect very small amounts of free gas and constrain the quantity if ng > 2%. This method is complementary to using acoustic velocity to constrain the quantity of free gas, which works well in the ng = 0.2–2% range. We have also given an expression for the vertical deformation of subsea formations and hence of the seafloor displacement under tidal loading. The presence of free gas enhances tidally induced seafloor displacement, but the maximum effect is limited by the compressibility of the matrix frame. Given relatively low frame compressibility, tidally induced seafloor displacement is small, of the order of 1 mm, which is presently difficult to detect at tidal frequencies.