bluuebunny/crossref_metadata_2025 huggingface dataset:
A two‐dimensional inverse model for extensional sedimentary Basins 2. Application
Authors: Bellingham Paul , White Nicky
Published: November 2002

We apply the two‐dimensional strain rate inversion algorithm described in the companion paper to five sedimentary basins located world wide. This general algorithm makes no assumptions about the number, duration, or intensity of rifting episodes within a basin. Instead, the distribution of strain rate is allowed to vary smoothly through space and time until the misfit between observed and predicted stratigraphy is minimized. Potentially important two‐dimensional effects such as lateral heat flow and elastic thickness have been incorporated. In each case, observed subsidence profiles can be automatically and accurately fitted. The calculated distributions of strain rate are corroborated by independent information about the number and duration of rifting episodes. Peak strain rate estimates are comparable to present‐day measurements in actively extending basins such as the Aegean Sea and the Basin and Range Province. Temporal and spatial strain rate patterns from a large number of sedimentary basins in diverse settings will help to constrain the dynamical evolution of thinning continental lithosphere. In all five sedimentary basins, the smallest acceptable misfit is achieved when the elastic thickness is less than 2–4 km. This surprising result indicates that the lithosphere beneath sedimentary basins remain weak during and after rifting. It also underscores the importance of inverting data sets so that the existence, uniqueness, and resolution of solutions can be analyzed. Our two‐dimensional approach is one small step toward a generalized three‐dimensional inverse model of sedimentary basins.
Microcrack evolution and brittle failure of Inada granite in triaxial compression tests at 140 MPa
Authors: Oda M. , Katsube T. , Takemura T.
Published: November 2002

A tensorial measure, called the crack tensor, is proposed to objectively define the crack geometry in crystalline rock as general as possible. Change of crack populations during brittle failure of rock (Inada granite) is analyzed in terms of the crack tensor and its invariants, which can be evaluated by a conventional petrographic analysis combined with stereology. A crack evolution law is formulated as a function of inelastic strain, so that we can easily depict how much damage accumulates in a sample loaded in a triaxial vessel by measuring the inelastic strain. When rock is loaded up to failure, the crack population satisfies a unique relation in the plot of the crack density versus the anisotropy, which are defined using the invariants of the crack tensors. It is suggested that failure of rock in uniaxial compression may be determined by a local condition such as a stress intensity factor at a crack tip. In the triaxial tests, on the other hand, rock fails when the crack density increases over a threshold value, which is a global condition depending on all cracks existing at failure. The threshold density is about 7 for the rock subjected to differential stress under confining pressures larger than 25 MPa. Under such a high crack density, the rock does not behave like a cracked solid, but rather like an assembly of rock blocks (granular material), and it starts to fail mainly due to the structurally induced instability, not due to the critical stress intensity factor at a crack tip. A throughgoing fault may develop as a result of the relative motion among the disintegrated blocks, sliding and rolling.
Motion and rigidity of the Pacific Plate and implications for plate boundary deformation
Authors: Beavan J. , Tregoning P. , Bevis M. , Kato T. , Meertens C.
Published: November 2002

Using up to 11 years of data from a global network of Global Positioning System (GPS) stations, including 12 stations well distributed across the Pacific Plate, we derive present‐day Euler vectors for the Pacific Plate more precisely than has previously been possible from space geodetic data. After rejecting on statistical grounds the velocity of one station on each of the Pacific and North American plates, we find that the quality of fit of the horizontal velocities of 11 Pacific Plate (PA) stations to the best fitting PA Euler vector is similar to the fit of 11 Australian Plate (AU) velocities to the AU Euler vector and ∼20% better than the fit of nine North American Plate (NA) velocities to the NA Euler vector. The velocities of stations on the Pacific and Australian Plates each fit a rigid plate model with an RMS residual of 0.4 mm/yr, while the North American velocities fit a rigid plate model with an RMS velocity of 0.6 mm/yr. Our best fitting PA/AU relative Euler vector is located ∼170 km southeast of the NUVEL‐1A pole but is not significantly different at the 95% confidence level. It is also close (<70 km in position and <3% in rate) to a pole derived from transform faults identified from satellite altimetry, suggesting that the vector has not changed significantly over the past 3 Myr. Our relative Euler vector is also consistent with all known geological and geodetic evidence concerning the AU/PA plate boundary through New Zealand. The GPS sites offshore of southern California are presently moving 4–5 ± 1 mm/yr relative to predicted Pacific velocity, with their residual velocities in approximately the opposite direction to PA/NA relative motion. Likewise, the easternmost sites in South Island, New Zealand, are moving ∼3 ± 1 mm/yr relative to predicted Pacific velocity, with the residuals in approximately the opposite direction to PA/AU relative motion. These velocity residuals are in the same sense as predicted by elastic strain accumulation on known plate boundary faults but are of a significantly higher magnitude in both southern California and New Zealand, implying that the plate boundary zones in both regions are wider than previously believed.
Three‐dimensional surface motion maps estimated from combined interferometric synthetic aperture radar and GPS data
Authors: Gudmundsson Sverrir , Sigmundsson Freysteinn , Carstensen Jens Michael
Published: November 2002

We provide a technique to efficiently produce high‐resolution three‐dimensional surface motion maps by combining information about the motion of the Earth's surface from interferometric observations of synthetic aperture radar images and repeated Global Positioning System (GPS) geodetic measurements. Unwrapped interferograms, showing pixel‐wise change in range from ground to satellite, and sparse values of three‐dimensional movements are required as input. The problem of finding the full three‐dimensional motion field is separated into two two‐dimensional problems. Initially, the vertical component of the deformation field and its horizontal component in the look direction of the satellite are found. Later, the look direction component is resolved into north and east components. Initial values for the motion fields are assigned to each pixel of interferograms from interpolation of available GPS observations. These values are then updated and optimized by comparison with the interferograms and the GPS observations. An additional constraint is an assumption of a smoothly varying motion field. Markov random field‐based regularization and simulated annealing algorithm are used for the optimization. The technique is applied to create surface motion maps for the Reykjanes Peninsula, SW Iceland.
Migration of seismicity and earthquake interactions monitored by GPS in SE Asia triple junction: Sulawesi, Indonesia
Authors: Vigny Christophe , Perfettini Hugo , Walpersdorf Andrea , Lemoine Anne , Simons Wim , van Loon Danny , Ambrosius Boudewijn , Stevens Colleen , McCaffrey Rob , Morgan Peter , Bock Yehuda , Subarya Cecep , Manurung Parluhutan , Kahar Joenil , Abidin Hasanuddin Z. , Abu Samad H.
Published: November 2002

Global Positioning System (GPS) measurements made in Sulawesi, Indonesia, from 1992 to 1999 detected coseismic and transient postseismic deformation related to the 1 January 1996, Mw = 7.9 earthquake on the North Sulawesi (Minahassa) trench. These motions are superimposed on the long‐term secular motion (40 mm/yr) of the left‐lateral Palu fault in central Sulawesi and continued for about 1.5–2 years. Following the earthquake, a string of earthquakes (of magnitude >6) migrated along the Minahassa trench, from west to east. Subsequently, two earthquakes of magnitude >6 occurred on or near the Palu fault migrating toward the south. Modeling the increase in Coulomb stress generated by the successive earthquakes agrees with the hypothesis of interacting events. An unclamping effect, possibly due to fluid migration in the Palu area, is also suggested by the stress computations and the detected (GPS) displacements.
Morphology, displacement, and slip rates along the North Anatolian Fault, Turkey
Authors: Hubert‐Ferrari Aurélia , Armijo Rolando , King Geoffrey , Meyer Bertrand , Barka Aykut
Published: November 2002

Geological and geomorphological offsets at different scales are used to constrain the localization of deformation, total displacement, and slip rates over various timescales along the central and eastern North Anatolian Fault (NAF) in Turkey. The NAF total displacement is reevaluated using large rivers valleys (80 ± 15 km) and structural markers (Pontide Suture, 85 ± 25 km; Tosya‐Vezirköprü basins, 80 ± 10 km). These suggest a Neogene slip rate of 6.5 mm/yr over 13 Myr. The river network morphology shows offsets at a range of scales (20 m to 14 km) across the main fault trace and is also used to estimate the degree to which deformation is localized. At a smaller scale the morphology associated with small rivers is offset by 200 m along the NAF. The age of these features can be correlated with the Holocene deglaciation and a slip rate of 18 ± 3.5 mm/yr is determined. This is consistent with a rate of 18 ± 5 mm/yr deduced independently from the 14C dating of stream terrace offsets. Over the short term, GPS data gives a similar rate of 22 ± 3 mm/yr. All our results tend to show that most of the deformation between the Anatolian and Eurasian lithospheric plates has been accommodated along, or very close to, the active trace of the NAF. The difference between the Neogene and the Holocene slip rate may be due to the recent establishment of the current plate geometry after the creation of the NAF.
Dilatancy, brittle strength, and anisotropy of foliated rocks: Experimental deformation and micromechanical modeling
Authors: Rawling Geoffrey C. , Baud Patrick , Wong Teng‐fong
Published: November 2002

Triaxial compression experiments were performed on the Four‐mile gneiss. The biotite foliation in the Four‐mile gneiss results in dilatancy and strength anisotropies, which become more pronounced with increasing confining pressure. Microstructural observations indicate that when there is high resolved shear stress on the macroscopic foliation, dilatancy arises from extensile microcracks nucleated by frictional slip on biotite grains. Evolution of crack geometry and coalescence are also influenced by the biotite foliation. Motivated by these observations, a damage mechanics model based on sliding wing cracks was adopted to analyze the anisotropic development of dilatancy and brittle fracture. Frictional coefficients for the sliding cracks are inferred to be comparable to those of cleavage surfaces of biotite. The strength anisotropy data of the Four‐mile gneiss can be explained by the variation of the initial damage with the foliation angle. The damage derives from a set of preexisting microcracks with random orientation, and a set of cleavage cracks in mica grains preferentially oriented along the foliation angle. Hence, the initial damage is higher for the intermediate angles, and, consequently, the strength is somewhat lower. The observation that the mechanical strengths of a variety of foliated rocks decrease with increasing mica content can be explained by the same model, with the implication that the initial damage and mica content are linearly related. The mechanical and microstructural data show that dilatancy anisotropy may significantly influence the progressive development of borehole breakout and strain localization.
Fluid flow near reservoir lakes inferred from the spatial and temporal analysis of the electric potential
Authors: Trique Michaël , Perrier Frédéric , Froidefond Thierry , Avouac Jean‐Philippe , Hautot Sophie
Published: November 2002

Electric self‐potential (SP) variations have been monitored continuously from 1995 to 1998 at 14 points on a ridge separating the Roselend and La Gittaz reservoir lakes in the French Alps. The lakes have level variations of at least 50 m over yearly cycles. Seasonal variations of SP associated with lake‐level variations are observed on five points of the array. For three points located on the banks of the lakes, a positive correlation to the lake‐level variations is observed with a maximal amplitude of about 180 mV, corresponding to an average response of 2.4 mV per meter of water. For two points located on the bottom of each lake, the correlation is negative, with a maximal magnitude of about −50 mV, corresponding to an average response of −1.1 mV per meter of water. Two independent temporary electrical arrays located on the banks of each lake confirm these measurements and allow a better spatial characterization of the sources associated with the observed SP variations. In particular, near the Roselend lake, the electrical response to lake‐level variations is increasing for decreasing altitude. The measured SP variations are proposed to result from the electrokinetic coupling associated with a vertical groundwater flow connecting a constant pore pressure source to the bottom of the lakes. Numerical modeling indicates that the spatial variation of the response and the nonlinear response observed at one point can be explained by leakage currents in the conductive lake water. The values of the streaming potential coefficient (SPC), measured in the laboratory with crushed rock samples from the site, range from 14 to 50 mV/0.1 MPa for an electrolyte resistivity of 40 Ω m and are compatible, to first order, with the magnitude of the observed seasonal SP variations. A detailed quantitative electrokinetic modeling is currently limited mainly by the poor knowledge on the contribution of electrical leakage currents and the local variability of the SPC. This experiment indicates that spatial and temporal variations of the electric potential are promising tools to characterize and monitor shallow groundwater flow and provide practical data for the investigation of groundwater flow associated with volcanic or tectonic activity.
Volcanic mound fields on the East Pacific Rise, 16°–19°S: Low effusion rate eruptions at overlapping spreading centers for the past 1 Myr
Authors: White Scott M. , Macdonald Ken C. , Sinton John M.
Published: November 2002

Volcanic mound fields identified on SeaMARC II and HMR1 12 kHz side‐scan data from the southern East Pacific Rise (SEPR) occur near overlapping spreading centers (OSCs) and migration traces of OSCs. The volcanic mound fields appear as a distinctive hummocky seafloor fabric due to side‐scan backscatter reflections from clusters of mound‐shaped reflectors. The lack of growth of the mound fields away from the ridge axis, and their occurrence in association with OSC traces, suggests that mound fields form along the ridge crest near OSCs. Volcanic mound fields are found where 120 kHz side‐scan and visual observations find fields of pillow mounds. Since pillow mounds are constructed by low effusion rate eruptions, the volcanic mound fields found near the OSCs and in their migration traces indicate that volcanic effusion rates tend to be lower near ridge discontinuities than midsegment regions. This tendency for low effusion rate eruptions at OSCs is documented for the past ∼1 Myr. Three independent measurements of ridge segmentation, (1) volcanic segment boundaries marked by the low effusion rate eruptions, (2) tectonic segments defined by OSCs, and (3) magmatic segment boundaries based on continuity of parental magma composition, all coincide in the study area. High backscatter off‐axis lava fields not associated with seamounts are found on seafloor younger than ∼0.2 Ma. The ∼0.2 Ma corridor corroborates previous results from the distribution of small isolated volcanoes that indicates randomly distributed off‐axis eruptions mainly occur on crust younger than ∼0.2 Ma.
East to west retardation in the onset of the recent warming across Canada inferred from inversions of temperature logs
Authors: Majorowicz Jacek , Safanda Jan , Skinner Walter
Published: November 2002

Canadian well temperature logs for 141 sites are analyzed and show evidence of extensive ground surface temperature (GST) warming beginning in the eighteenth and nineteenth centuries following a lengthy period of cooling (the Little Ice Age) over most of the past millennium. The method of functional space inversion (FSI) is applied to the complete set of 141 well precise temperature logs from wells located in low‐conductivity clastic sediments of the western Canadian Sedimentary basin and higher thermal conductivity crystalline rocks of the Canadian Shield in central and eastern Canada. Marked regional differences in the onset of recent warming are evident (nineteenth century compared to the seventeenth to eighteenth centuries, respectively), following a prolonged cold period through earlier centuries. Analysis of spatial patterns of GST history changes across Canada based on the individual inversions of temperature logs shows a systematic east‐to‐west retardation in the onset of the recent warming yet a higher overall warming magnitude in western Canada in the twentieth century.
Processes leading to eclogitization (densification) of subducted and tectonically buried crust
Authors: Bjørnerud M. G. , Austrheim H. , Lund M. G.
Published: November 2002

The transformation of subducted and tectonically buried crustal rocks to denser eclogite plays a fundamental role in the dynamics of mountain building and crustal recycling. However, a complex of partially eclogitized granulites on the island of Holsnøy in the western part of the Norwegian Caledonides reveals that such densification depends in part on the antecedent history of rock masses and that large bodies of untransformed rock can persist metastably even after long residence times in the lower crust. The rocks on Holsnøy suggest that conversion to eclogite was delayed until lower or subcrustal seismic events allowed aqueous fluids to enter the previously dry and unusually strong granulite complex. Metamorphism then occurred in a spatially heterogeneous manner, with several distinct physical processes involving feedbacks between deformation, fluid infiltration, and chemical reactions operating simultaneously in different parts of the rock mass. Field relations show that the introduction of fluids and conversion to eclogite significantly weakened the rocks. A reservoir‐flux systems model is used to represent the various processes contributing to eclogitization of the granulites, which operated at vastly different rates over disparate intervals of time. The model suggests that the metamorphic process may have been brief (≪1 Myr), and ultimately self‐limiting, arrested by the change in rheology associated with the conversion of brittle granulite to ductile eclogite.
Geochemistry of dikes and lavas from the north wall of the Hess Deep Rift: Insights into the four‐dimensional character of crustal construction at fast spreading mid‐ocean ridges
Authors: Stewart Michael A. , Klein Emily M. , Karson Jeffrey A.
Published: November 2002

An investigation of ∼1‐m.y.‐old dikes and lavas from the north wall of the Hess Deep Rift (2°15′N, 101°30′W) collected during Alvin expeditions provides a detailed view of the evolution of fast spreading oceanic crust. The study area encompasses 25 km of an east‐west flow line, representing ∼370,000 years of crustal accretion at the East Pacific Rise. Samples analyzed exhibit depleted incompatible trace element abundances and ratios [(La/Sm)N < 1]. Indices of fractionation (MgO), and incompatible element ratios (La/Sm, Nb/Ti) show no systematic trends along flow line. Rather, over short (<4 m) and long (∼25 km) distances, significant variations are observed in major and trace element concentrations and ratios. Modeling of these variations attests to the juxtaposition of dikes of distinct parental magma compositions. These findings, combined with studies of segmentation of the subaxial magma chamber and lateral magma transport in dikes along rift‐dominated systems, suggest a more realistic model of the magmatic system underlying the East Pacific Rise relative to the commonly assumed two‐dimensional model. In this model, melts from a heterogeneous mantle feed distinct portions of a segmented axial magma reservoir. Dikes emanating from these distinct reservoirs transport magma along axis, resulting in interleaved dikes and host lavas with different evolutionary histories. This model suggests the use of axial or flow line lava compositions to infer the evolution of axial magma chambers should be approached with caution because dikes may never erupt lava or may transport magma significant distances along axis and erupt lavas far from their axial magma chamber of origin.
Rapid postseismic transients in subduction zones from continuous GPS
Authors: Melbourne Timothy I. , Webb Frank H. , Stock Joann M. , Reigber Christoph
Published: November 2002

Continuous GPS time series from three of four recently measured, large subduction earthquakes document triggered rapid postseismic fault creep, representing an additional moment release upward of 25% over the weeks following their main shocks. Data from two Mw = 8.0 and Mw = 8.4 events constrain the postseismic centroids to lie down dip from the lower limit of coseismic faulting, and show that afterslip along the primary coseismic asperities is significantly less important than triggered deep creep. Time series for another Mw = 7.7 event show 30% postseismic energy release, but here we cannot differentiate between afterslip and triggered deeper creep. A fourth Mw = 8.1 event, which occurred in the broad Chilean seismogenic zone, shows no postseismic deformation, despite coseismic offsets in excess of 1 m. For the three events which are followed by postseismic deformation, stress transferred to the inferred centroids (at 34, 60, and 36 km depths) by their respective main shock asperities increased reverse shear stress by 0.5, 0.8, and 0.2 bar with a comparatively small decrease in normal stress (0.01 bar), constraining the Coulomb stress increase required to force slip along the metastable plate interface. Deep triggered slip of this nature is invisible without continuous geodesy but on the basis of these earthquakes would appear to constitute an important mode of strain release from beneath the seismogenic zones of convergent margins. These events, captured by some of the first permanent GPS networks, show that deep moment release is often modulated by seismogenic rupture updip and underscore the need for continuous geodesy to fully quantify the spectrum of moment release in great earthquakes.
Locating sources of volcanic tremor and emergent events by seismic triangulation: Application to Arenal volcano, Costa Rica
Authors: Métaxian Jean‐Philippe , Lesage Philippe , Valette Bernard
Published: November 2002

We address the issue of locating the sources of volcanic tremor and emergent events with a method requiring a limited amount of equipment. A network of several triangular seismic antennas made of vertical sensors is used. The slowness vectors are estimated at each array on a sliding window by inverting the time delays between the sensors calculated with the cross‐spectral method. A probabilistic approach is adopted whereby each measure and its error are represented by a probability density function (PDF). A weighted summation of the PDFs is carried out in which the stable directions of propagation are enhanced. The effects of the structural heterogeneities are taken into account by introducing an additional error associated to a robust hyperbolic secant (sech)‐type PDF. The resulting PDFs of the back‐azimuth are combined to calculate a PDF of the source location. The maximum likelihood of this PDF is taken as an estimate of the source position and its spread is characterized by a covariance analysis. Data from an experiment carried out at Arenal volcano (Costa Rica) with four arrays are analyzed. The precision and robustness of the method are tested by exploring the influence of the array configuration and other parameters. The mean standard deviation on the position of the sources is 600 m for the tremor and 400 m for the explosions and long‐period (LP) events. Several tremors, explosions and LP events are analyzed and their sources located. The seismogenic zone is located in a 600 m radius area centered on the active crater.
Local shear wave observations in João Câmara, northeast Brazil
Authors: do Nascimento Aderson F. , Pearce Robert G. , Takeya Mario K.
Published: November 2002

We present shear wave splitting data from a 10 km aperture, eight‐station digital three‐component seismograph network, which operated from December 1992 to August 1994 near João Câmara (5°33′S, 35°51′W), to record shear waves from continuing microearthquake activity persisting 6 years after a main shock in November 1986. Previous aftershock recording had shown remarkably impulsive simple seismograms and hypocentral errors of typically 200 m in a structure of near‐uniform wave speed, but only one station was three component. The pattern of polarization directions shown is consistent with the trend of Precambrian foliation observed in the field. It is therefore concluded that these shear wave observations are controlled by the seismic anisotropy associated with the Precambrian foliation and hence bear the signature of the “paleo‐stress field,” i.e., the Precambrian deformation regime. This conclusion has important implications for the interpretation of shear wave splitting observations made in crustal crystalline rock, because other authors have asserted that shear wave splitting normally results from the effect of stress‐aligned fluid‐filled cracks (extensive dilatancy anisotropy or EDA). EDA is, by contrast, a signature of the present‐day stress field. In this region, there is firm evidence that the present‐day maximum compressive stress is E‐W. This evidence comes from earthquake focal mechanisms and borehole breakout data and is summarized in the World Stress Map. We therefore conclude that our present data are in maximum conflict with an interpretation in terms of EDA. Our interpretation is supported by the observation that delay times are consistent with those expected from modeling of elastic waves in the crust.
Three‐dimensional crustal structure in the Southern Alps region of New Zealand from inversion of local earthquake and active source data
Authors: Eberhart‐Phillips Donna , Bannister Stephen
Published: November 2002

P and S‐P arrival time data from 311 earthquakes and several thousand offshore and onshore shots have been used in simultaneous inversion for hypocenters, three‐dimensional (3‐D) Vp and Vp/Vs models in the Southern Alps region, New Zealand. The combined data result in a highly nonuniform ray path distribution, and linked nodes are used in sparsely sampled areas. Gravity data are used to improve the model below 20‐km depth, where it is poorly sampled by local earthquakes. The crustal Vp from 5 to 25 km depth is fairly uniform, generally ranging from 5.5 to 6.5 km/s, typical of graywacke and schist. Active fault zones tend to be correlated with low‐velocity zones. Where the Alpine fault is primarily strike slip, it is characterized by a vertical low‐velocity zone, to at least 15‐km depth. Where the fault is dipping and has a large dip‐slip component, it is characterized by a large region of low velocity above and southeast of the fault, to at least 14‐km depth, consistent with fluids and fracture density from active deformation. A large high‐velocity, high‐resistivity feature in the eastern Southern Alps may represent Mesozoic schist of higher metamorphic grade than its surroundings, which is relatively rigid and serves to both reduce deformation in the overlying basin and concentrate deformation in the adjoining low‐velocity region. The imaged crustal root is deepest 80‐km south of Mt. Cook and is asymmetric with a sharper gradient on the northwestern side. The approximate Moho shows regional variation, with 5–10 km thicker crust in Otago than Canterbury.
Fragmentation of a porous viscoelastic material: Implications to magma fragmentation
Authors: Ichihara Mie , Rittel Daniel , Sturtevant Bradford
Published: November 2002

Fragmentation of vesicular magma by rapid decompression is one of the most likely triggers for explosive eruptions. In this phenomenon the decompression rate and the viscoelastic nature of magma are considered to be key factors. In order to obtain a clear idea on the effects of these two factors, controlled fragmentation experiments have been conducted. These experiments have three advantages. First, the specimen is made of a viscoelastic material with controlled porosity and geometry. Second, the fragmentation process is directly monitored. Finally, both the magnitude and rate of decompression are controlled. Brittle fragmentation and ductile expansion were both observed in the same porous material at different timescales. The various mechanical responses of the specimen (elastic, flow, and fragmentation) were correlated with the pressure profile measured at the base of the specimen. Fragmentation was noted to occur when the decompression exceeded a critical value within a critical time. Two relevant timescales are discussed in terms of physical mechanisms of relaxation. The first is the measured glass transition time. The second is the estimated timescale for the onset of viscous bubble expansion. The observed phenomena bear several similarities with natural magma fragmentation. It is thus considered that the present results are a useful step toward constructing a model for magma fragmentation.
Crustal and upper mantle structure in the Amazon region (Brazil) determined with broadband mobile stations
Authors: Krüger F. , Scherbaum F. , Rosa J. W. C. , Kind R. , Zetsche F. , Höhne J.
Published: November 2002

Three broadband stations operated from March 1997 to September 1998 in the Amazon region north of Manaus, Brazil, which, including the IRIS station PTGA (Pitinga), covered an area of roughly 60 × 200 km. Applying the receiver function technique to determine the crustal structure, there is evidence for an increase of the Moho depth from about 38 km below the Amazon Basin to approximately 48 km north of the basin. In addition, we analyzed the polarization of SKS‐waves to determine anisotropy. The observed splitting parameters in a majority of the data set can be accounted for by a layer of axial symmetric anisotropy with the fast direction oriented roughly N110E. This direction cannot be associated with the present‐day plate motion of the South American plate. However, the anisotropy may be related to a lithospheric fabric that was generated during the early tectonic history of the Amazonian Craton.
Buoyant decompression melting: A possible mechanism for intraplate volcanism
Authors: Raddick M. Jordan , Parmentier E. M. , Scheirer Daniel S.
Published: November 2002

Volcanic activity away from plate boundaries occurs in a variety of settings. Linear, age‐progressive volcanic chains have been explained as the manifestation of fixed hot spots, possibly generated by plumes originating deep in the mantle. However, important examples of oceanic intraplate volcanism cannot be explained in this way, including short‐lived chains and chains violating the predicted age‐distance behavior. Furthermore, a significant fraction of volcanism does not occur in linear chains, and observations suggest control by lithosphere structure in many cases. In this study, melting in mantle upwellings that result from the buoyancy associated with the melting itself is considered as a mechanism of intraplate volcanism. Numerical models of this “buoyant decompression melting” in a layer that is initially at its melting temperature over some depth range are used to determine the duration and amount of melting in upwellings which organize from an initially small, random perturbation for a range of model parameters. A predicted inverse correlation between the amount of melt produced and the duration of melting may be diagnostic of the buoyant melting process. Buoyant melting could occur in a number of geologic settings. Since the oceanic upper mantle may have previously melted beneath a spreading center and subsequently cooled, spontaneous buoyant decompression melting may be possible only in regions where a large‐scale mantle upwelling can counteract conductive cooling, keeping the mantle at its solidus temperature over some depth range. Where the mantle is at, or sufficiently near, its solidus, buoyant decompression may spontaneously generate “convective storms” of melting. This may explain the abundance of volcanism associated with the South Pacific Superswell. Where mantle is slightly cooler than its melting temperature, buoyant melting may not occur spontaneously but may be triggered by some initial upwelling. This may provide a physical explanation for volcanism that is controlled by lithosphere structure.
Magma system recharge of Mount St. Helens from precise relative hypocenter location of microearthquakes
Authors: Musumeci Carla , Gresta Stefano , Malone Stephen D.
Published: November 2002

Four hundred forty‐seven well‐recorded earthquakes at Mount St. Helens during the late 1990s are relocated with high precision using a combination of a cross‐correlation technique and then a procedure to obtain a one‐dimensional velocity structure solution together with station corrections. The resulting high‐resolution pattern of the spatial‐temporal evolution of the earthquake activity, along with focal mechanisms of events provide additional evidence for the presence of a magma reservoir in the depth range 5.5–10 km, connected to the surface by a thin vertical conduit. Moreover, the distribution of the events below 5.5 km yielded a much clearer view of the structures that generate the seismicity at that depth. Some of the deeper events occur along a sequence of faults under the influence of a radial stress field; however, a large number of the events occur on a NNE‐SSW striking steeply dipping fault with slip consistent with magma being periodically injected into a truncated dike on the northwest side of this fault.
True triaxial strength of the KTB amphibolite under borehole wall conditions and its use to estimate the maximum horizontal in situ stress
Authors: Haimson Bezalel C. , Chang Chandong
Published: November 2002

Estimation of the maximum horizontal in situ stress from logged borehole‐breakout dimensions in brittle elastic rocks requires knowledge of the true triaxial compressive strength of the medium. We employed our true triaxial loading apparatus to determine the strength criterion of the nearly impermeable amphibolite penetrated by the German Continental Deep Drilling Program (KTB) superdeep scientific hole at depths of 3200–7800 m. To better simulate borehole wall conditions, we left one pair of the prismatic specimens' faces unjacketed and in direct contact with the confining fluid. This testing variation brings about a fundamentally different failure mechanism from that in fully jacketed (dry) specimens. Brittle fracture occurs at or soon after dilatancy onset and results from the development of densely spaced extensile fractures subparallel and adjacent to one of the unjacketed faces. We infer that the confining fluid intrudes newly opened microcracks, which are predominantly subparallel to the unjacketed faces, and promotes their elongation into throughgoing fractures. For any given least principal stress, the compressive strength typically increases with the rise in the intermediate principal stress. The true triaxial strength criterion of the unjacketed amphibolite can be expressed as a linear relationship between the octahedral shear stress and the octahedral normal stress at failure. Employing this criterion together with all the other known data from the KTB hole, we recomputed the magnitude of the maximum horizontal in situ stress there. Our results show that it increases steadily with depth, with a relatively narrow band of uncertainty, confirming previous assessments of a strike‐slip stress regime.
Temporal evolution of a hydrothermal system in Kusatsu‐Shirane Volcano, Japan, inferred from the complex frequencies of long‐period events
Authors: Kumagai Hiroyuki , Chouet Bernard A. , Nakano Masaru
Published: November 2002

We present a detailed description of temporal variations in the complex frequencies of long‐period (LP) events observed at Kusatsu‐Shirane Volcano. Using the Sompi method, we analyze 35 LP events that occurred during the period from August 1992 through January 1993. The observed temporal variations in the complex frequencies can be divided into three periods. During the first period the dominant frequency rapidly decreases from 5 to 1 Hz, and Q of the dominant spectral peak remains roughly constant with an average value near 100. During the second period the dominant frequency gradually increases up to 3 Hz, and Q gradually decreases from 160 to 30. During the third period the dominant frequency increases more rapidly from 3 to 5 Hz, and Q shows an abrupt increase at the beginning of this period and then remains roughly constant with an average value near 100. Such temporal variations can be consistently explained by the dynamic response of a hydrothermal crack to a magmatic heat pulse. During the first period, crack growth occurs in response to the overall pressure increase in the hydrothermal system caused by the heat pulse. Once crack formation is complete, heat gradually changes the fluid in the crack from a wet misty gas to a dry gas during the second period. As heating of the hydrothermal system gradually subsides, the overall pressure in this system starts to decrease, causing the collapse of the crack during the third period.
Paleomagnetism of Paleozoic and Mesozoic sediments from the southern margin of Mongol‐Okhotsk ocean, far eastern Russia
Authors: Kravchinsky Vadim A. , Sorokin Andrey A. , Courtillot Vincent
Published: November 2002

This paper presents new data from paleomagnetic investigations of Middle Paleozoic (Early‐Middle‐Late Devonian, Early Carboniferous) and Middle Jurassic geological units from the Amuria block south of the Mongol‐Okhotsk suture zone. With fold tests for all localities, and two polarities for the Devonian, our new results constrain the evolution of the Mongol‐Okhotsk ocean from the Devonian to the Middle Jurassic. The corresponding paleopoles lie at 21.6°N, 6.3°E (dp/dm = 5.3°/10.6°) for Early Devonian, 26.3°N, 345.3°E (dp/dm = 6.4°/12.8°) for Early‐Middle Devonian, 24.6°N, 12.9°E (dp/dm = 8.7°/16.9°) for Middle‐Late Devonian, 40.5°N, 352.4°E (dp/dm = 9°/16.7°) for Late Devonian, 39.8°N, 31.6°E (dp/dm = 10°/15.5°) for Early Carboniferous, and 46°N, 37.9°E (dp/dm = 9.4°/13.4°) for Middle Jurassic. The poles confirm that the large Paleozoic Mongol‐Okhotsk ocean closed during the Jurassic, ending up in the late Jurassic or early Cretaceous at the eastern end of the suture zone, as originally proposed on geological grounds. The new paleomagnetic results exhibit large tectonic rotations about local vertical axes, which we interpret as probably arising from both the collision process and left‐lateral shear movement along the suture zone, related to incipient extrusion of Amuria due to indentation of India into Asia.
Evidence for ionic conductivity in lower mantle (Mg,Fe)(Si,Al)O3 perovskite
Authors: Xu Yousheng , McCammon Catherine
Published: November 2002

Electrical conductivity of (Mg,Fe)(Si,Al)O3 perovskite was measured in the multianvil press, using impedance spectroscopy at 25 GPa in the temperature range 100–1700°C, in conjunction with Mössbauer spectroscopic measurements of quenched phases. Conductivity results confirm previous observations that activation enthalpies are larger at high temperatures than at low temperatures. Mobilities of charge carriers in (Mg,Fe)(Si,Al)O3 perovskite were calculated from the conductivity and Mössbauer data, and are consistent with Fe2+‐Fe3+ electron hopping (small polaron) as the dominant conduction mechanism at low temperatures and ionic conduction at high temperatures. A laboratory‐based conductivity model is constructed for the lower mantle based on the new results.
A method for separating Antarctic postglacial rebound and ice mass balance using future ICESat Geoscience Laser Altimeter System, Gravity Recovery and Climate Experiment, and GPS satellite data
Authors: Velicogna Isabella , Wahr John
Published: November 2002

Measurements of ice elevation from the Geoscience Laser Altimeter System (GLAS) aboard the Ice, Cloud, and Land Elevation Satellite can be combined with time‐variable geoid measurements from the Gravity Recovery and Climate Experiment (GRACE) satellite mission to learn about ongoing changes in polar ice mass and viscoelastic rebound of the lithosphere under the ice sheet. We estimate the accuracy in recovering the spatially varying ice mass trend and postglacial rebound signals for Antarctica, from combining 5 years of simulated GRACE and GLAS data. We obtain root‐mean square accuracies of 5.3 and 19.9 mm yr−1 for postglacial rebound and ice mass trend, respectively, when smoothed over 250 km scales. The largest source of error in the combined signals is the effect of the unknown time‐variable accumulation on the density of the ice column. To estimate this contribution and so obtain better estimates of ice mass trend and postglacial rebound, we add Global Positioning System (GPS) measurements of vertical velocities as additional constraints. Using an empirical relation between the errors in postglacial rebound and ice mass trend that result from the unknown density variation within the ice column, we are able to solve for all three unknowns in the problem: ice mass trend, postglacial rebound, and the snow compaction trend. The addition of a plausible distribution of GPS measurements reduces the errors in estimates of postglacial rebound and ice mass trend to 3.4 and 15.9 mm yr−1, respectively.
Creep along the Imperial Fault, southern California, from GPS measurements
Authors: Lyons Suzanne N. , Bock Yehuda , Sandwell David T.
Published: November 2002

In May of 1999 and 2000, we surveyed with Global Positioning System (GPS) 46 geodetic monuments established by Imperial College, London, in a dense grid (half‐mile spacing) along the Imperial Fault, with three additional National Geodetic Survey sites serving as base stations. These stations were previously surveyed in 1991 and 1993. The Imperial College sites were surveyed in rapid‐static mode (15–20 min occupations), while the NGS sites continuously received data for 10 h d−1. Site locations were calculated using the method of instantaneous positioning, and velocities were determined relative to one of the NGS base stations. Combining our results with far‐field velocities from the Southern California Earthquake Center (SCEC), we fit the data to a simple elastic dislocation model with 35 mm yr−1 of right‐lateral slip below 10 km and 9 mm yr−1 of creep from the surface down to 3 km. The velocity field is asymmetrical across the fault and could indicate a dipping fault plane to the northeast or a viscosity contrast across the fault.
Style of density stratification in the mantle and true polar wander induced by ice loading
Authors: Sabadini R. , Marotta A. M. , De Franco R. , Vermeersen L. L. A.
Published: November 2002

The present‐day velocity of true polar wander (TPW) and the displacement of the axis of rotation of the Earth in response to ice ages, resulting from stratified, viscoelastic Earth models, are sensitive to the nonadiabatic density gradient in the mantle. Previous studies, based on a fully nonadiabatic, or chemically stratified mantle, overestimated the present‐day TPW for lower mantle viscosities in the range 1021–1022 Pa s. For a density profile in agreement with the reference seismological model, where nonadiabaticity is confined to the transition zone between 420 and 670 km, the predicted present‐day TPW for viscosities on the order of 1021 Pa s is 0.65–0.9 deg/Myr, substantially lower than the 3.0 deg/Myr obtained for the chemical mantle. This decrease is due to the lack of an isostatic restoring force in the adiabatic case or to a global reduction of the buoyancy, which favors the attainment of rotational equilibrium. The correctness of this physical interpretation is demonstrated by the behavior of a fully adiabatic phase change that can be satisfactorily modeled by deleting the buoyancy restoring modes due to chemical density contrasts. This finding provides quantitative support to the procedure used in previous studies to simulate the rotational behavior of an adiabatic transition zone by simply deleting the buoyancy modes triggered by the (chemical) density contrasts at 670 km, as done by Mitrovica and Milne [1998] for present‐day TPW and by Ricard and Sabadini [1990] for long‐term TPW driven by mantle density anomalies. The reduction of present‐day TPW induced by the Pleistocenic deglaciation, for an adiabatic mantle with nonadiabatic density gradients due to phase changes localized in the transition zone, impacts the inversion of the lower mantle viscosity. Predictions based on such a model are characterized by two best fit values in proximity of 1021 and 1022 Pa s, resembling the behavior of the time derivative of the second‐degree component of the gravity field. The reduction of predicted present‐day TPW due to Pleistocene deglaciation suggests that other mechanisms, such as present‐day ice mass instability in Antarctica and Greenland, are presently contributing to the drift of 0.9 deg/Myr of the axis of rotation toward Newfoundland. The secular drift of the adiabatic mantle model during the continuous occurrence of ice ages is increased by 50% with respect to the chemically stratified one, showing a longer decay time after termination of the ice cycles. This mantle model confirms that ice cycles cannot be the major source of TPW for timescales of 106–107 years.
Partial anhysteretic remanent magnetization in magnetite 1. Additivity
Authors: Yu Yongjae , Dunlop David J. , Özdemir Özden
Published: November 2002

We have tested the additivity of partial anhysteretic remanent magnetization (pARM) for suites of eight synthetic magnetites with mean grain sizes from 65 nm to 18 μm and 18 natural samples including lake sediments, oceanic and continental volcanic rocks, gabbros, and granites. In both synthetic and natural sample suites, domain states inferred from hysteresis and other magnetic properties vary from single‐domain (SD) through pseudosingle‐domain (PSD) to multidomain (MD). For each sample, total ARM intensity was compared with sums of partial ARMs of three different types: four conjugate pairs of parallel pARMs; four pairs of perpendicular pARMs; and one set of five neighboring parallel pARMs. In each case, the intervals of alternating field (AF) over which a steady field H was applied to produce the partial ARMs are nonoverlapping and cover the entire AF range (0–100 mT) used to produce the total ARM. Additivity of partial ARMs was verified to better than ±3% for all the samples, whatever the domain state (SD, PSD, and MD) or composition (ranging from pure magnetite to x = 0.6 titanomagnetite). The universality of pARM additivity is unexpected because its analog, partial thermoremanent magnetization (pTRM), deviates from ideal behavior as the grain size increases and the domain structure becomes MD. The different behaviors probably result from the fact that pARM is produced at ordinary temperatures over short times, whereas the most intense pTRM is produced at temperatures approaching the Curie point with significant dwell times, promoting such processes as isothermal remanence acquisition, domain nucleation, and domain wall reequilibration. Verification of the law of additivity of pARMs is an encouraging first step toward validating pseudo‐Thellier and other methods of paleointensity determination that use ARM in place of, or in addition to, TRM.