bluuebunny/crossref_metadata_2025 huggingface dataset:
Using neutron diffraction to investigate the elastic properties of anisotropic rocks: Results from an olivine + orthopyroxene mylonite
Authors: Covey‐Crump S. J. , Schofield P. F. , Stretton I. C. , Knight K. S. , Ismaïl W. Ben
Published: May 2003

The penetrating power of neutrons means that neutron diffraction may be employed to determine the lattice parameters of mineral phases within samples over 1 cm in size. We have exploited this fact to obtain high‐resolution measurements of the elastic strains experienced by olivine and orthopyroxene during room temperature, uniaxial deformation experiments performed in a neutron beam‐line, on a mylonite from the Oman ophiolite. Specimens were loaded in three orthogonal directions with respect to the macroscopic fabric of the rock. In each case the average olivine and orthopyroxene strains were the same, even though the elastic stiffnesses of the two phases were different. In comparison with the strains expected from the single‐crystal elastic stiffness tensors, the average strains in the different lattice directions for each phase were found to be greater in the stiffer directions, and lesser in the more compliant directions. The overall effect of this tendency toward strain homogenization was for the measured elastic anisotropy of the rock to be significantly lesser than that given by Voigt/Reuss averaging of the single‐crystal elastic properties according to the lattice preferred orientations of each phase. The technique used to derive these conclusions potentially provides an important experimental method for the quantitative examination of nonfracture‐related controls on the elastic anisotropy of geological materials.
An astronomical polarity timescale for the late middle Miocene based on cyclic continental sequences
Authors: Abdul Aziz H. , Krijgsman W. , Hilgen F. J. , Wilson D. S. , Calvo J. P.
Published: May 2003

We present an astronomically tuned polarity timescale for the late middle Miocene based on a cyclic shallow lacustrine/mudflat succession exposed in the Orera Composite Section (OCS; Calatayud basin, NE Spain). Spectral analysis and band‐pass filtering of high‐resolution carbonate and color reflectance records in the depth domain reveal cyclic changes with different cycle lengths, which correspond to lithological alternations observed in the field. An initial age model was constructed by calibrating the OCS magnetostratigraphy to the geomagnetic polarity timescale of Cande and Kent [1995]. Subsequent spectral analysis of the proxy records in the time domain reveals periodicities close to 23, 41, and 400 kyr and, to a lesser extent, 100 kyr, supporting an astronomical origin for the sedimentary cyclicity in the OCS. We established a new age model based on the astronomical calibration of the OCS to the Laskar et al. [1993] (La93) solution by tuning the cycles to the astronomical target curves. Cross‐spectral analysis results of the tuned time series followed by band‐pass filtering reveal a remarkably good and in‐phase relation with precession and with obliquity despite a presumed uncertainty of 20–40 kyr in the tuning in some short intervals. Our tuning provides astronomical ages for sedimentary cycles and subsequently for polarity reversals in the interval between 12.9 and 10.6 Ma. Comparison with Cande and Kent shows that the OCS polarity reversal ages are older by 80 kyr. This age discrepancy decreases with increasing age to 50–60 kyr.
Crustal and upper mantle structure of southernmost South America inferred from regional waveform inversion
Authors: Robertson Maurice Stacey D. , Wiens Douglas A. , Koper Keith D. , Vera Emilio
Published: May 2003

We determine the crustal and upper mantle structure of southern South America by inverting regional seismograms recorded by the Seismic Experiment in Patagonia and Antarctica. We present a waveform inversion method that utilizes a niching genetic algorithm. The niching genetic algorithm differs from the traditional genetic algorithm in that the inversion is performed in multiple subpopulations, thus generating a broader search of the model space and enabling us to examine alternative local error minima. The vertical and transverse waveforms were used, extending from the P arrival to the surface waves, and the inversion was performed at either 0.005–0.06 Hz (larger events) or 0.02–0.06 Hz (smaller events). The inversion included anisotropy by solving for separate SV and SH structures in the upper mantle. Results indicate that crustal thickness varies from 26 to 36 km with thicker values toward the northeast, suggesting that there is little crustal thickening beneath the austral Andes. The average upper mantle velocities are similar to the preliminary reference Earth model (PREM) except that the southernmost region shows velocities of 5% slower than PREM. The upper mantle has up to 5% polarization anisotropy between the Moho and 120 km depth. The strongest anisotropy is localized in a lithospheric lid shallower than 65 km depth, which overlies a pronounced low‐velocity zone. This shallow limit to anisotropy is consistent with the relatively small shear wave splitting values found in this region. These results suggest that the anisotropy is limited to lithospheric depths and may imply the absence of a strong mantle flow pattern in the asthenosphere.
The Gakkel Ridge: Bathymetry, gravity anomalies, and crustal accretion at extremely slow spreading rates
Authors: Cochran James R. , Kurras Gregory J. , Edwards Margo H. , Coakley Bernard J.
Published: May 2003

The Gakkel Ridge in the Arctic Ocean is the slowest spreading portion of the global mid‐ocean ridge system. Total spreading rates range from 12.7 mm/yr near Greenland to 6.0 mm/yr where the ridge disappears beneath the Laptev Shelf. Swath bathymetry and gravity data for an 850 km long section of the Gakkel Ridge from 5°E to 97°E were obtained from the U.S. Navy submarine USSHawkbill. The ridge axis is very deep, generally 4700–5300 m, within a well‐developed rift valley. The topography is primarily tectonic in origin, characterized by linear rift‐parallel ridges and fault‐bounded troughs with up to 2 km of relief. Evidence of extrusive volcanic activity is limited and confined to specific locations. East of 32°E, isolated discrete volcanoes are observed at 25–95 km intervals along the axis. Abundant small‐scale volcanism characteristic of the Mid‐Atlantic Ridge (MAR) is absent. It appears that the amount of melt generated is insufficient to maintain a continuous magmatic spreading axis. Instead, melt is erupted on the seafloor at a set of distinct locations where multiple eruptions have built up central volcanoes and covered adjacent areas with low relief lava flows. Between 5°E and 32°E, almost no volcanic activity is observed except near 19°E. The ridge axis shoals rapidly by 1500 m over a 30 km wide area at 19°E, which coincides with a highstanding axis‐perpendicular bathymetric high. Bathymetry and side scan data show the presence of numerous small volcanic features and flow fronts in the axial valley on the upper portions of the 19°E along‐axis high. Gravity data imply up to 3 km of crustal thickening under the 19°E axis‐perpendicular ridge. The 19°E magmatic center may result from interaction of the ridge with a passively imbedded mantle inhomogeneity. Away from 19°E, the crust appears thin and patchy and may consist of basalt directly over peridotite. The ridge axis is continuous with no transform offsets. However, sections of the ridge have distinctly different linear trends. Changes in ridge trend at 32°E and 63°E are associated with a set of bathymetric features that are very similar to each other and to inside/outside corner complexes observed at the MAR including highstanding “inside corner” ridges, which gravity data show to be of tectonic rather than magmatic origin.
Rheology of calcite–quartz aggregates deformed to large strain in torsion
Authors: Rybacki Erik , Paterson Mervyn S. , Wirth Richard , Dresen Georg
Published: May 2003

High‐temperature, high‐pressure torsion experiments have been conducted on Solnhofen limestone and synthetic calcite–quartz aggregates to investigate the evolution of mechanical strength and microstructure to large strains. Hot isostatic pressing of powders containing 1–30 wt% quartz particles produced synthetic two‐phase aggregates containing trace amounts of wollastonite. The experiments were performed at constant twist rate, 300 MPa confining pressure, and temperatures ranging from 900 to 1300 K in the stability fields of calcite plus quartz and wollastonite plus carbon dioxide. The mechanical data from torsion tests of most samples show a pronounced peak stress at shear strains <1 and subsequent weakening. For most samples, a steady state stress is reached only at shear strains >5. A distinct shape and lattice preferred orientation developed in Solnhofen limestone and in the calcite matrix of the synthetic aggregates. However, at large shear strains, flattened porphyroclasts were almost entirely consumed by recrystallization. Elongation parallel to the specimen axis at temperature below 1100 K is possibly due to rotation of minerals into a preferred crystallographic orientation. Stress exponent and activation energy for creep of the calcite–quartz aggregates increase substantially with increasing quartz content. The strength of the two‐phase aggregates increases with quartz fraction up to 20 wt% by a factor of 2–5 depending on temperature and finite strain. Continuum models underestimate particle strengthening of the calcite–quartz aggregates. An alternative microphysical mechanism for the observed strengthening may be related to the reduced mobility of dislocations through diffusion of silicon into dislocation cores.
Carbon dioxide emissions from soils at Hakkoda, north Japan
Authors: Hernández Perez Pedro , Notsu Kenji , Tsurumi Makoto , Mori Toshiya , Ohno Masao , Shimoike Yoichi , Salazar Jose , Pérez Nemesio
Published: May 2003

A soil gas and CO2 efflux survey was carried out at Hakkoda, north Japan, to investigate the spatial distribution and origin of the gas which caused the accidental death of three Japanese soldiers on 12 July 1997. In the study area, no fumarolic and/or thermal activity is observed. Chemical composition of the gas from areas with high CO2 effluxes (>5,000 g m−2 d−1) and high CO2 contents (>15 vol %) indicate a strong deep sourced contribution for CO2, whereas air and/or air saturated water is suggested for the rest of the discharged gases, which are mainly released from a shallow aquifer. Isotopic analysis of soil gas and hot spring discharges suggests a combination of magmatic degassing and thermal decarbonation, with magmatic degassing being the main contributior to the high CO2 emission. Approximately 74 t d−1 of deep‐seated (magmatic plus thermal decarbonation origin) CO2 were estimated to contribute to the total output of CO2 from this area.
Influence of phyllosilicate mineral assemblages, fabrics, and fluids on the behavior of the Punchbowl fault, southern California
Authors: Solum John G. , van der Pluijm Ben A. , Peacor Donald R. , Warr Laurence N.
Published: May 2003

The Punchbowl fault is an ancient segment of the San Andreas System that was active between 1 and 4 Ma and exhumed from a depth of 2–4 km, thereby providing the opportunity to study processes along the top of the seismogenic zone of today's San Andreas fault. X‐ray diffraction, X‐ray texture goniometry, and scanning and transmission electron microscopy data characterize the detailed microstructural and chemical changes that occurred along the Punchbowl fault and address the importance of mineral transformations, rock fabric, and fluid activity on the hydromechanical behavior of the fault zone. On the basis of bulk and clay fraction X‐ray analyses, the fault rocks have mineral assemblages that are distinct from the protolith, indicating that mineral reactions have occurred. However, microstructural observations show that many of the changes in mineral assemblages occurred after the cessation of faulting. Moreover, measures of phyllosilicate fabrics across the Punchbowl fault from X‐ray goniometry are uniformly weak, and so phyllosilicate‐induced permeability anisotropy in the zone was small. This absence of fabrics limits the effectiveness of fluid focusing along the fault, although some anisotropy may have been imparted by compartmentalization of the fault zone by occasional clay shears. Together these observations indicate that neither clay growth nor fabric‐imposed fluid focusing appeared to have played a significant role during strike‐slip faulting and cataclasite formation, which suggests that fault‐weakening mechanisms that rely on clay growth and elevated fluid pressure cannot be applied to the Punchbowl fault.
Relationship between tremor and volcanic activity during the Southeast Crater eruption on Mount Etna in early 2000
Authors: Alparone Salvatore , Andronico Daniele , Lodato Luigi , Sgroi Tiziana
Published: May 2003

The Southeast Crater of Mount Etna (Italy) was characterized by a violent eruptive activity between 26 January and 24 June 2000. This activity produced 64 lava fountain episodes with repose periods from 3 hours to 10 days. We estimated a volume of about 15–20 × 106 m3 lava and at least 2–3 × 106 m3 of tephra. We compared the paroxysmal volcanic activity to its associated seismic signature: The high number of events highlighted a strict correlation between tremor and volcanic activity. Seismic and volcanic characteristics, such as the frequency of occurrence, the duration of lava fountains and the associated tremor energy, suggested the subdivision of the studied period into two stages separated by the 20 February event. Combining volcanic with seismic data, we observed some useful relationships among lava fountain height, sustained column height and Reduced Displacement; in addition, we found that the entire episode was well correlated with the duration of the amplitude increase. Computing the tremor energy linked to each event, the total energy associated with lava fountains episodes results in 76% of the energy released during the whole period. Finally, the different ratios among the overall spectral amplitude of the seismic signals of the stations located at different altitudes suggested to us the elaboration of a simple qualitative model to explain the dynamic behavior of the tremor source during the whole episode.
The strength and rheology of methane clathrate hydrate
Authors: Durham William B. , Kirby Stephen H. , Stern Laura A. , Zhang Wu
Published: May 2003

Methane clathrate hydrate (structure I) is found to be very strong, based on laboratory triaxial deformation experiments we have carried out on samples of synthetic, high‐purity, polycrystalline material. Samples were deformed in compressional creep tests (i.e., constant applied stress, σ), at conditions of confining pressure P = 50 and 100 MPa, strain rate 4.5 × 10−8 ≤ ≤ 4.3 × 10−4 s−1, temperature 260 ≤ T ≤ 287 K, and internal methane pressure 10 ≤ PCH4 ≤ 15 MPa. At steady state, typically reached in a few percent strain, methane hydrate exhibited strength that was far higher than expected on the basis of published work. In terms of the standard high‐temperature creep law, = Aσne−(E*+PV*)/RT the rheology is described by the constants A = 108.55 MPa−n s−1, n = 2.2, E* = 90,000 J mol−1, and V* = 19 cm3 mol−1. For comparison, at temperatures just below the ice point, methane hydrate at a given strain rate is over 20 times stronger than ice, and the contrast increases at lower temperatures. The possible occurrence of syntectonic dissociation of methane hydrate to methane plus free water in these experiments suggests that the high strength measured here may be only a lower bound. On Earth, high strength in hydrate‐bearing formations implies higher energy release upon decomposition and subsequent failure. In the outer solar system, if Titan has a 100‐km‐thick near‐surface layer of high‐strength, low‐thermal conductivity methane hydrate as has been suggested, its interior is likely to be considerably warmer than previously expected.
Deep structure of the Baikal rift zone revealed by joint inversion of gravity and seismology
Authors: Tiberi C. , Diament M. , Déverchère J. , Petit‐Mariani C. , Mikhailov V. , Tikhotsky S. , Achauer U.
Published: May 2003

The question of plate boundary forces and deep versus shallow asthenospheric uplift has long been debated in intracontinental rift areas, particularly in the Baikal rift zone, Asia, which is colder than other continental rifts. As previous gravity and teleseismic studies support the dominance of opposing mechanisms in the Baikal rift, we reconsidered both data sets and jointly inverted them. This more effective approach brings insight into location of the perturbing bodies related to the extension in this region. Our new joint inversion method allows for inverting the velocity‐density relationship with independent model parametrization. We obtain velocity and density models that consistently show (1) crustal heterogeneities that coincide with the main tectonic features at the surface, (2) a faster and denser cratonic mantle NW of Lake Baikal that we relate to the thermal contrast between old and depleted Archean (Siberian platform) and Paleozoic orogenic belt (Sayan‐Baikal belt), (3) three‐dimensional topographic variations of the crust‐mantle boundary with well‐located upwarpings, and (4) the lithosphere‐asthenosphere boundary uplift up to 70 km depth with a NW dip. Our resulting velocity and density models support the idea of a combined influence of lithospheric extension and inherited lithospheric heterogeneities for the origin of the Baikal rift zone.
Size effect law and fracture mechanics of the triggering of dry snow slab avalanches
Authors: Bažant Zdeněk P. , Zi Goangseup , McClung David
Published: May 2003

A size effect law for fracture triggering in dry snow slabs of high enough length‐to‐thickness ratio is formulated, based on simplified one‐dimensional analysis by equivalent linear elastic fracture mechanics. Viscoelastic effects during fracture are neglected. The derived law, which is analogous to Bažant's energetic size effect law developed for concrete and later for sea ice, fiber composites, rocks, and ceramics, is shown to agree with two‐dimensional finite element analysis of mode II cohesive crack model with a finite residual shear stress. Fitting the proposed size effect law to fracture data for various slab thicknesses permits identifying the material fracture parameters. The value of preexisting shear stress in a thin weak zone of finite length is shown to have significant effect. There exists a certain critical snow depth, depending on the preexisting stress value, below which the size effect disappears. Practical applications require considering that the material properties (particularly the mode II fracture toughness or fracture energy) at the snow slab base are not constant but depend strongly on the slab thickness. This means that one must distinguish the material size effect from the structural size effect, and the combined size effect law must be obtained by introducing into the structural size effect law dependence of its parameters on snow thickness. The thickness dependence of these parameters can be obtained by matching the combined law to avalanche observations. Matching Perla's field data on 116 avalanches suggests that the mode II fracture toughness is approximately proportional to 1.8 power of snow thickness.
Scaling of stress drop with recurrence interval and loading velocity for laboratory‐derived fault strength relations
Authors: He Changrong , Wong Teng‐fong , Beeler Nick M.
Published: May 2003

The dynamics of a spring‐slider system with a single degree of freedom was investigated, focusing on two different rate‐ and state‐dependent friction laws. While the inertia‐controlled behavior and stick‐slip cycles for a system that obeys the slip law have been extensively simulated, this study presents a comparative study of a system that obeys the slowness law. A key conclusion is that for both friction laws the overall stress drops are linearly related to the logarithm of the loading velocity (and the recurrence time) through the velocity‐weakening parameter b − a and normal stress. Relatively higher peak stresses, larger quasi‐static stress drop, and larger effective fracture energy are associated with a system that obeys the slowness law. Consequently, the partitioning of stress drop between quasi‐static and dynamic slips, as well as dynamic overshoot and strength recovery, varies according to whether the slowness or slip law has been adopted. Analytic approximations were derived that elucidate the interplay of dynamics, energetics, and frictional constitutive behavior in controlling the scaling of stress drops with loading velocity and recurrence time. Seismological implications of the scaling behavior are also discussed.
Constraints on the long‐term colluvial erosion law by analyzing slope‐area relationships at various tectonic uplift rates in the Siwaliks Hills (Nepal)
Authors: Lague Dimitri , Davy Philippe
Published: May 2003

The large database of topographic form and uplift rates that exists for the Siwaliks Hills (central Nepal) makes possible a thorough analysis of the long‐term erosion model. The study especially focuses on drainage areas larger than 5.10−3 km2, fixed by the database resolution, and smaller than 1 km2 above which a fluvial signature is recorded. This area range corresponds to colluvial valleys in which the dominant erosion process is likely debris flow. We evaluate a phenomenological model wherein erosion is considered to depend on drainage area and slope. We test this model by assuming that the uplift rate is in approximate equilibrium with erosion. The stream power law model, formulated by analogy to river incision and transport problems, is found to be consistent with data since an inverse power law relationship between slope and drainage area is systematically observed between 7.10−3 and ∼1 km2, with little variability on the exponent ∼−0.24. Thanks to the range of uplift rates, we obtain constraints on the slope dependency of erosion law, which appears linear and which predicts a significant erosion threshold. The linear dependence on slope in the debris‐flow zone is consistent with findings by Kirby and Whipple [2001] in the fluvial downstream zone and with the linear relationship between local relief and uplift rate documented by Hurtrez et al. [1999]. The transition between this colluvial‐channel regime and the fluvial regime appears quite sharp in contrast with recent studies, but the latter regime is not sufficiently documented to derive definite conclusions.
Can in situ measurements of mantle electrical conductivity be used to infer properties of partial melts?
Authors: Park Stephen K. , Ducea Mihai N.
Published: May 2003

Constraints on bulk conductivity from magnetotelluric measurements and petrological analyses of late Quaternary peridotite xenoliths from the southern Sierra Nevada allow evaluation of models commonly used to relate electrical conductivity to the physical and chemical state of the upper mantle. In these models, two conductive melts (basalt and sulfide) are embedded in a resistive matrix. Bounds on the amount of sulfide (0.06–0.4%) and the bulk conductivity (0.03–0.1 S/m) place constraints on the degree of interconnection between the melts. Because the sulfide melt is very conductive, even a small fraction of well‐connected melt results in a bulk conductivity larger than 0.1 S/m. Similarly, completely disconnected melts result in bulk conductivities much less than 0.03 S/m. The only models which matched both the bulk conductivity and sulfide bounds consisted of a small fraction (<1%) interconnected basalt melt with a discontinuous sulfide phase. Such a texture is observed in laboratory experiments with much larger sulfide melt fractions, but has not been reported for small melt fractions. A variant of the Hashin‐Shtrikman model and a hybrid model consisting of cascaded Hashin‐Shtrikman calculations were successful in matching the magnetotelluric and petrologic constraints. With a model that appropriately simulates the melt interconnectivity, we suggest that electrical conductivity may be used to infer in situ melt properties in the mantle.
Ascent and emplacement of buoyant magma bodies in brittle‐ductile upper crust
Authors: Burov Evgene , Jaupart Claude , Guillou‐Frottier Laurent
Published: May 2003

The emplacement of silicic magma bodies in the upper crust may be controlled by density (such that there is no buoyancy to drive further ascent) or temperature (such that surrounding rocks are too cold to deform significantly over geological timescales). Evidence for the latter control is provided by negative gravity anomalies over many granitic plutons. Conditions of diapir ascent and emplacement in this case are studied with a numerical model for deformation and heat transport allowing for ductile, elastic and brittle behavior. A large‐strain formulation is used to solve for temperature, stress, strain, and strain rate fields as a function of time for a range of diapir sizes, density contrasts, and background geotherms. The method allows for large viscosity contrasts of more than 6 orders of magnitude and determines the dominant deformation mechanism depending on the local values of temperature, strain, and strain rate. Emplacement depth and final deformation characteristics depend on diapir size and buoyancy. Small diapirs (less than about 5 km in diameter) cannot reach shallow crustal levels and do not involve brittle deformation. In the ductile regime the diapir flattens significantly upon emplacement due to stiff roof rocks and to the free surface above. Late stage deformation proceeds by horizontal spreading, with little upward displacement of roof rocks and is likely to be interpreted as “ballooning.” Large diapirs (more than about 5 km in diameter) rapidly rise to shallow depths (1–5 km) and induce brittle faulting in the overlying rocks. In this regime, buoyancy forces may lead to faulting in roof rocks. In this case, late stage ascent proceeds by vertical intrusion of a plug of smaller horizontal dimensions than the main body. Buoyant diapirs keep on rising after solidification, long after the relatively short‐lived high‐temperature magmatic stage. This may account for some phases of late caldera resurgence in extinct volcanic systems.
Melting processes and fluid and sediment transport rates along the Alaska‐Aleutian arc from an integrated U‐Th‐Ra‐Be isotope study
Authors: George Rhiannon , Turner Simon , Hawkesworth Chris , Morris Julie , Nye Chris , Ryan Jeff , Zheng Shu‐Hui
Published: May 2003

A comprehensive data set for young lavas erupted along the Alaska‐Aleutian arc is used to examine how fluid and sediment transport rates and melting processes vary in response to systematic changes in subduction rate and dip along the arc and across the ocean‐continent boundary. Positive correlations between convergence rate, volcano volume, and238U excesses suggest that magmatic output is closely linked to the size of the fluid flux which occurred <10 kyr prior to eruption. Sediment‐sensitive tracers like Th/Nb, Ce/Ce*, and10Be/9Be also increase with convergence rate. However, the inferred10Be/9Be ratio of this component is low relative to that in the incoming sediments, and this could reflect either sediment removal by accretion or storage in the mantle wedge for ∼0.5–1 Myr. Th/Nb and Th/Nd ratios exceed those expected from bulk sediment addition in the center of the arc suggesting that the enhanced sediment signal reflects transfer in a partial melt phase, whereas partial melts from the subducted oceanic crust appear to be restricted to the western tear in the Pacific plate. Therefore the thermal structure at the slab‐wedge interface in the center of the arc must lie close to, and possibly between, the sediment and basalt solidii and is thus constrained to be in the region of 670–740°C at 3 GPa. Elevated (230Th/232Th) ratios, the occurrence of230Th excesses and only modest226Ra excesses in many of the lavas are modeled as the products of dynamic melting effects superimposed upon a fluid‐fluxed mantle wedge. The transition from oceanic to continental lithosphere seems to play an important role in determining the relative effects of fluid addition and partial melting upon U‐Th disequilibria. We infer that the change from fast and steep plate subduction in the Aleutians to shallow and slow subduction, coupled with increasing lithospheric lid thickness, in Alaska together control slab and wedge temperatures and the rate of matrix flow through the melting region.
Double seismic zone and dehydration embrittlement of the subducting slab
Authors: Yamasaki Tadashi , Seno Tetsuzo
Published: May 2003

Dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate‐depth earthquakes. We explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast Japan, southwest Japan, northeast Taiwan, northern Chile, Cape Mendocino, and eastern Aleutians. We calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions. We then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams. The depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age. The dehydration loci of serpentine produce a double‐layered structure. Because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab, we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust, and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab. We find that the lower plane seismicity is located at the lower dehydration loci of serpentine, which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement.
Source mechanisms of explosions at Stromboli Volcano, Italy, determined from moment‐tensor inversions of very‐long‐period data
Authors: Chouet Bernard , Dawson Phillip , Ohminato Takao , Martini Marcello , Saccorotti Gilberto , Giudicepietro Flora , De Luca Gaetano , Milana Giuliano , Scarpa Roberto
Published: May 2003

Seismic data recorded in the 2–30 s band at Stromboli Volcano, Italy, are analyzed to quantify the source mechanisms of Strombolian explosions during September 1997. To determine the source‐centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous elastic medium that takes topography into account. Two source centroids are identified, each representative of the distinct event types associated with explosive eruptions from two different vents. The observed waveforms are well reproduced by our inversion, and the two source centroids that best fit the data are offset 220 and 260 m beneath and ∼160 m northwest of the active vents. The source mechanisms include both moment‐tensor and single‐force components. The principal axes of the moment tensor have amplitude ratios 1:1:2, which can be interpreted as representative of a crack, if one assumes the rock matrix at the source to have a Poisson ratio ν = 1/3, a value appropriate for hot rock. Both imaged cracks dip ∼60° to the northwest and strike northeast–southwest along a direction parallel to the elongation of the volcanic edifice and a prominent zone of structural weakness, as expressed by lineaments, dikes, and brittle structures. For our data set, the volume changes estimated from the moments are ∼200 m3 for the largest explosion from each vent. Together with the volumetric source is a dominantly vertical force with a magnitude of 108 N, consistent with the inferred movement of the magma column perched above the source centroid in response to the piston‐like rise of a slug of gas in the conduit.
Repeating earthquakes and interplate aseismic slip in the northeastern Japan subduction zone
Authors: Igarashi Toshihiro , Matsuzawa Toru , Hasegawa Akira
Published: May 2003

On the basis of a waveform similarity analysis, we detected 321 earthquake clusters with very similar (cross‐correlation coefficient >0.95) waveforms on the plate boundary in the northeastern Japan subduction zone. Most of them were not found within the subducting Pacific plate with a few exceptions. Moreover, even on the plate boundary, they were not located in the large moment release areas of large interplate earthquakes that occurred recently or in the areas where the plates are inferred to be strongly coupled from GPS data analyses. These observations suggest that these similar earthquakes are caused by repeating slips of small asperities with a dimension of around 0.1 to 1 km surrounded by stable sliding areas on the plate boundary. If the aseismic slip portion in these small asperities is negligible, we can estimate the cumulative amount of aseismic slip in the area surrounding each asperity. In other words, repeating earthquake data potentially can be used to estimate the spatiotemporal aseismic slip distribution on the plate boundary. We estimated the spatial distribution of slip rate on the plate boundary from repeating earthquake data. The scaling relation between seismic moment and seismic slip by Nadeau and Johnson [1998] is used for the estimation of the slip amount by each repeating earthquake cluster. Obtained spatial distribution is consistent with that estimated from GPS data on land.
Spacing of faults at the scale of the lithosphere and localization instability: 2. Application to the Central Indian Basin
Authors: Montési Laurent G. J. , Zuber Maria T.
Published: May 2003

Tectonic deformation in the Central Indian Basin (CIB) is organized at two spatial scales: long‐wavelength (∼200 km) undulations of the basement and regularly spaced faults. The fault spacing of order 7–11 km is too short to be explained by lithospheric buckling. We show that the localization instability derived byMontési and Zuber[2003]provides an explanation for the fault spacing in the CIB. Localization describes how deformation focuses on narrow zones analogous to faults. The localization instability predicts that localized shear zones form a regular pattern with a characteristic spacing as they develop. The theoretical fault spacing is proportional to the depth to which localization occurs. It also depends on the strength profile and on the effective stress exponent,ne, which is a measure of localization efficiency in the brittle crust and upper mantle. The fault spacing in the CIB can be matched byne∼ −300 if the faults reach the depth of the brittle–ductile transition (BDT) around 40 km orne∼ −100 if the faults do not penetrate below 10 km. These values ofneare compatible with laboratory data on frictional velocity weakening. Many faults in the CIB were formed during seafloor spreading. The preexisting faults near target locations separated by the wavelength of the localization instability were preferentially reactivated during the current episode of compressive tectonics. The long‐wavelength undulations may result from the interaction between buckling and localization.
Volcanic and tectonic deformation on Unimak Island in the Aleutian Arc, Alaska
Authors: Mann Dörte , Freymueller Jeffrey
Published: May 2003

GPS measurements on Unimak Island in the eastern Aleutian arc between 1998 and 2001 show deformation of Westdahl volcano and Fisher caldera. Westdahl is inflating, with the best fit point source located at 7.2−1.2+ 2.3 km depth and a volume change rate of 6.7−1.8+ 3.3 × 106 m3 yr−1. The GPS data indicate that inflation may have slowed down slightly compared with interferometric synthetic aperture radar (InSAR) observations between 1993 and 1998. The accumulated subsurface volume increase during the GPS and InSAR observation period (1993–2001), ∼70 × 106 m3, already accounts for at least 15% more than the erupted volume from the last eruption in 1991–1992. Fisher caldera shows subsidence and contraction across the caldera center. The data are fit best with a rectangular dislocation source at a shallow depth. It is 14 km long and 0.5 km wide, dips 80° to the NW, and strikes N35°E, with rather large uncertainties for most of these parameters. Its volume decrease is 2.0 × 106 m3yr−1. The main mechanisms to explain the subsidence and contraction are degassing and contractional cooling of a shallow magma body and depressurization of Fisher's hydrothermal system, possibly triggered by an earthquake in 1999. At the 95% confidence level, no significant strain accumulation due to subduction is observed across the entire island. The low coupling across the rupture zone of the 1946 earthquake is a strong argument for the idea that the earthquake and tsunami did not result from a purely double‐couple (earthquake) source.
Response of subglacial sediments to basal freeze‐on 2. Application in numerical modeling of the recent stoppage of Ice Stream C, West Antarctica
Authors: Bougamont Marion , Tulaczyk Slawek , Joughin Ian
Published: May 2003

Ross ice streams supply over 90% of the ice volume flowing out of the Ross sector of the West Antarctic ice sheet (WAIS). Stoppage of Ice Stream C (ISC) ca. 150 years ago appears to have pushed this sector of WAIS from negative into positive mass balance [Joughin and Tulaczyk, 2002]. We propose an explanation for the unsteady behavior of ISC using a new numerical ice‐stream model, which includes an explicit treatment of a subglacial till layer. When constrained by initial conditions emulating prestoppage geometry, dynamics, and mass balance of ISC, the model yields a rapid (∼100 years) stoppage of the main ice‐stream trunk. The stoppage is triggered by basal freeze‐on, which consolidates and strengthens the subglacial till. Our numerical simulations produce results consistent with a number of existing observations, for example, continuing activity of the two tributaries of ISC. The model always yields rapid stoppage unless we specify ice‐stream width that is smaller than its prestoppage values (maximum of ∼80 km). We conjecture that if ISC was active for at least a few thousand years before slowdown, its width was significantly smaller than today to sustain the long active phase. Ice‐stream width is a key control that helps determine whether ice‐stream flow is sustainable over a long term. Our work indicates that the recent stoppage of Ice Stream C could have been part of inherent ice‐stream cyclicity, and it leaves open the possibility that other active ice streams may evolve in the future toward rapid shutdowns.
Source area and rupture parameters of the 31 December 1881 Mw = 7.9 Car Nicobar earthquake estimated from tsunamis recorded in the Bay of Bengal
Authors: Ortiz Modesto , Bilham Roger
Published: May 2003

On the morning of 31 December 1881 a submarine earthquake beneath the Andaman Islands generated a tsunami with a maximum crest height of 0.8 m that was recorded by eight tide gauges surrounding the Bay of Bengal. Since the earthquake occurred 8 years before the construction of the world's first teleseismic recording seismometer, little has been known about its rupture parameters or location. Waveform and amplitude modeling of the tsunami indicate that it was generated by a Mw = 7.9 ± 0.1 rupture on the India/Andaman plate boundary resulting in 10–60 cm of uplift of the island of Car Nicobar. The rupture consisted of two segments: the northern 40‐km‐long segment is separated from the southern 150‐km‐long segment by a 100‐km region corresponding to the westward projection of the West Andaman spreading center. The main rupture occurred between 8.5°N and 10°N with a total area of 150 km × 60 km dipping 20°E with a mean slip of 2.7 m. The recurrence time for 1881‐type events is estimated to be 114–200 years on the basis of inferred GPS convergence rates and inferred plate closure vectors, although slip partitioning in the region may extend this estimate by as much as 30%.
From subduction to collision: Control of deep processes on the evolution of convergent plate boundary
Authors: Regard Vincent , Faccenna Claudio , Martinod Joseph , Bellier Olivier , Thomas Jean‐Charles
Published: May 2003

Using laboratory experiments, we investigate the dynamics of the collisional process that follows the closure of an oceanic basin. The evolution of these experiments systematically shows four successive episodes of deformation, which correspond to (1) the initiation of oceanic subduction, (2) a mature period of oceanic subduction, (3) an episode of continental subduction, during which the trench absorbs all the convergence and the superficial tectonic regime does not change significantly within the continental plates, and (4) continental collision that starts when the trench locks and convergence is absorbed by a system of thrust faults and folds. We observe that the amount of continental material that subducts before the onset of collision depends on the slab pull exerted by the subducted oceanic lithosphere. The slab‐pull force, in turn, depends on the amount of subducted oceanic material, on the thickness of the convective mantle, and also on the rheology of the slab. Our experiments, indeed, suggest that parts of the oceanic slab may separate from the superficial slab to sink rapidly in the mantle, decreasing the slab‐pull level and triggering the rapid onset of collision. We observe two possible modes of slab deformation: slab break‐off and development of viscous instabilities. We define two dimensionless rheological numbers to characterize the possible occurrence of these modes of deformation. In all cases, oceanic closure is followed by episode of continental subduction during which the continental lithosphere may reach depths varying between 50 and 450 km, prior to the onset of continental collision.
Triggering of seismicity at short timescales following Californian earthquakes
Authors: Marsan D.
Published: May 2003

A method for measuring seismicity rate changes caused by the occurrence of major earthquakes in the surrounding crust is proposed. It is based on a nonstationary Poisson modeling of earthquake activity. An estimate of the seismicity rate change and a probability measure of seismicity triggering following a large shock are derived, and the removal of the influence of independent aftershock sequences of other nearby, previous earthquakes is attempted, without using more traditional declustering techniques. The seismicity rate change estimator is corrected for its natural bias in favor of positive changes, at short timescales, that has so far strongly hindered measures of seismicity quiescences. Three Californian earthquakes are examined using this method: the 1989 M7.1 Loma Prieta, the 1992 M7.3 Landers, and the 1994 M6.7 Northridge earthquakes. Positive triggering is very commonly observed, as expected, but quiescence is, however, more seldomly obtained, for timescales up to 100 days after the main shock. This relative absence of quiescence, as compared to typical predictions of Coulomb modeling, for example, is also independently observed through direct correlation analyses of Californian seismicity. The possibility that the stresses caused by the main shock are spatially highly variable may explain these observations.
Source characteristics of large deep earthquakes: Constraint on the faulting mechanism at great depths
Authors: Tibi Rigobert , Bock Guenter , Wiens Douglas A.
Published: May 2003

Source parameters for eight large deep earthquakes are inferred from broadband data of the Global Seismograph Network. We completed an unprecedented data set composed of 14 largest deep earthquakes (Mw ranging from 7.0 to 8.3) from six different subduction zones studied with identical source parameter inversion methods. The study confirms earlier findings that deep earthquake source properties vary with the temperature of the subducting plate. Events in cold slabs show high aftershock activity, high rupture velocity, and high seismic efficiency. In contrast, earthquakes in warm slabs generally have low aftershock productivity, low rupture velocity, and low seismic efficiency. However, the 23 June 1991 Argentina event has source properties that are rather characteristic for earthquakes in cold slabs, and that differ from those of other large South American events. We suggest that two types of deep earthquakes occur in South America, and we interpret these results in the context of the shear instability model for the mechanism of deep earthquakes. In this model, shear failure occurs through a thermal runaway mechanism within a shear zone at great depth, and low seismic efficiency might indicate significant energy dissipation due to heating and possible melt production. The 1991 Argentina event, located in a pocket of deep seismicity within the slab core, may represent a shear instability with little melting as would be the case for large deep earthquakes in cold subduction zones, whereas the other known South American large deep events such as 1963 Peru, 1970 Colombia and 1994 Bolivia, are located in relatively aseismic regions and may involve significant melting within warmer regions of the slab.
Linearized inverse scattering of teleseismic waves for anisotropic crust and mantle structure: 1. Theory
Authors: Bostock M. G.
Published: May 2003

In this study we examine the recovery of vertically varying anisotropic crust and upper mantle structure using converted teleseismic waves recorded on three‐component seismograms from individual broadband stations. Our analysis is cast in terms of inverse scattering theory for a one‐dimensional medium and begins with a derivation of the plane wave Green's function for a homogeneous medium exhibiting arbitrary elastic anisotropy. This Green's function is employed within the single‐scattering approximation to derive formulae that relate the amplitude of the scattered wave in time to perturbations in material properties at corresponding depths. Inclusion of seismograms from multiple events representing a range of azimuths and incident angles leads to the construction of a linear system of equations that is readily solved using singular value decomposition. A useful by‐product of this “amplitude‐versus‐slowness” approach is the identification of simple and compact expressions for linearized reflection and conversion coefficients in anisotropic media that are accurate for small contrasts at near‐vertical angles of incidence. We demonstrate the accuracy of the linearized P‐to‐S transmission coefficient for an idealized upper mantle model. In a companion paper we will examine the application of this approach to field data recorded at stations of the Canadian National Seismograph Network.
Linearized inverse scattering of teleseismic waves for anisotropic crust and mantle structure: 2. Numerical examples and application to data from Canadian stations
Authors: Bank Carl‐Georg , Bostock Michael G.
Published: May 2003

The objective of this study is to characterize elastic properties, including anisotropy, at the base of the crust and uppermost mantle using the Moho PMs phase recorded in the teleseismic P coda at Canadian stations. We use linearized inverse scattering and singular value decomposition to identify those parameter combinations to which idealized teleseismic data sets are most sensitive. Five to seven independent parameter combinations are likely to be resolvable, one of which is sensitive to isotropy, whereas the remainder quantify different harmonic orders (1θ, 2θ, 3θ) of back azimuthal response. Aside from the isotropic component which is resolved only by P‐SV interactions, P‐SV and P‐SH conversions exhibit a redundant sensitivity to model parameters for uniform back‐azimuthal sampling. We use parameter combinations from the idealized, uniform back‐azimuthal data distribution to compare Moho anisotropy at 25 broadband stations on the Canadian landmass. The isotropic component dominates at all stations and corresponds to shear velocity contrasts ranging between 10 and 35%. Perturbations to anisotropic material property parameters are more modest, generally between 3 and 7% when consistent between SV and SH responses and in many cases suggest an anisotropic lower crust. Inconsistent responses may manifest contamination by lateral heterogeneity, upper crustal reverberations, or pervasive crustal anisotropy leading to shear wave splitting.