Seismic Tomography Studies Research Articles

The structure of the lithosphere of the Black Sea by the results of 3D gravity analysis and seismic tomography

3D gravity analysis and local seismic tomography have been performed to study the lithosphere structure of the Black Sea. Due to gravity analysis gravity effects of different layers and levels of the model (sea water, four layers in sediments, consolidated crust) have been distinguished and residual gravity anomalies of mantle origin have been obtained. Long-wave component of these anomalies forms the vast area of positive values with small amplitude that indicates good isostatic balance of deep structures of the Black Sea. Velocity structure of lithospheric mantle of the Black Sea (down to the depth of 85 km) was studied by local seismic tomography which used travel times of P-waves from the earthquakes occurred inside the region and recorded by a net of seismic stations around the Black Sea. Results of seismic tomography study indicate heterogeneous structure of the upper mantle of the Black Sea with dominant areas of increased velocities in the western and eastern parts of the sea. Joining of two methods - gravity analysis and seismic tomography has been carried out by calculation of gravity effect of the upper mantle heterogeneities obtained by seismic tomography. Correspondence of the sign and order of magnitude of anomalous mantle gravity field obtained by two mentioned independent methods gives us an opportunity to conclude on the presence of sufficiently rigid continental lithosphere under the Black Sea.

Geodetic evidence of viscoelastic relaxation after the 2008 Iwate-Miyagi Nairiku earthquake

Continuous GPS observations, for over two years, detected long-term postseismic deformation after the 2008 Iwate-Miyagi Nairiku earthquake (Mj 7.2). The displacement field exhibits ESE-WNW shortening and subsidence near the focal area. These features are attributed to a viscoelastic relaxation caused by the mainshock. A simple two-layered structural model, which consists of an elastic layer having a thickness of 19.0–23.5 km and an underlying Maxwell viscoelastic layer having a viscosity of 2.4–4.8 × 1018 Pa s, explains the far-field deformation pattern, which probably reflects the viscoelastic response exclusively. These estimated parameters are consistent with the deeper limit of the seismogenic layer in the upper crust and the previous rheological model in northeastern Japan. However, near-field deformation requires additional sources in order to reproduce the observed postseismic deformation, such as long-term afterslip and/or a complicated response due to the highly heterogeneous structure suggested by seismic tomography studies.

沈み込み帯の地震の発生機構

The mechanisms that generate the three main types of earthquake in subduction zones are discussed addressing their relations to geofluids. Studies on the spatial distribution of earthquakes and seismic velocity structure within the subducted slab provide evidence that strongly supports the dehydration embrittlement hypothesis for the generation of intermediate-depth intraslab earthquakes. Detailed imaging of the seismic velocity structure in and around plate boundary zones suggests that interplate coupling is mainly controlled by local fluid over-pressure. Seismic tomography studies show the existence of inclined sheet-like seismic low-velocity zones in the mantle wedge, not only in Tohoku but also in other areas in Japan, which perhaps correspond to the upwelling flow of the subduction-induced convection system. These upwelling flows reach the Moho directly beneath the volcanic areas, suggesting that those volcanic areas are formed by the upwelling flows. Aqueous fluids derived from the slab are probably transported up through the upwelling flows to the arc crust, where they might weaken the surrounding crustal rocks and finally cause shallow inland earthquakes. All of these observations suggest that geofluids expelled from the subducting slab play an important role in the generation of earthquakes in subduction zones.

Influence of iron on the elastic properties of wadsleyite and ringwoodite

[1] We investigate by first‐principles the influence of iron on the elastic properties of the b–phase (wadsleyite) and g–phase (ringwoodite), polymorphs of olivine, the most abundant minerals of the upper and lower parts of the transition zone, respectively. Our study aims to complement experiments to understand details of the 410 km and 520 km discontinuities. The full elastic‐tensor Cij, bulk (K), and shear (G) moduli are determined under static conditions for b–g–(Mg1–xFex)2SiO4 with x = 0.125. Wave propagation anisotropy in single crystals and polarization anisotropy in aggregates with preferred orientation are investigated and compared with those of iron‐free wadsleyite and ringwoodite for a thorough understanding of the effect of iron. We examine the effect of iron on velocity contrasts due to phase changes and conclude that iron enhances DVP and DVS across the a → b transition but suppresses them across the b → g transition. The latter might contribute to suppress locally the 520 km discontinuity if this has a significant contribution from the b → g transition. We show that lateral variation of iron, dx, produces lateral velocity heterogeneity ratios similar to those produced by lateral variations of temperature, dT, both producing ratios comparable to values extracted from seismic tomography studies. However, in contrast with dT, dx produces negative values for density to longitudinal and shear wave velocity ratios. This might be considered the fingerprint of lateral variations of iron concentration. These negative ratios appear similar to results inferred from geodynamical models compiled by Karato and Karki (2001) for the upper mantle and transition zone.

Lithospheric thermal structure and rheology of the eastern China

Temperature distributions of the lithosphere beneath the eastern China were estimated by local isostasy equilibrium constrained geothermal calculation in this study. Maps of the lateral temperature variation at depths of 50 and 100km as well as Moho boundary are presented for the eastern China continent, and the 600°C and 1100°C isotherm depth maps are also given. Meanwhile, the thermal thickness of lithosphere is calculated as the depth of 1350°C isotherm. The rheological strength of lithosphere is calculated based on a four-layer model, which consists of the upper, lower, lowermost crust layers and a wet lithospheric mantle layer. The results show that, the lithosphere of 160–180km thick exists under Sichuan basin in the western part of Yangtze Craton, but keel is absent beneath the entire North China Craton. It is notable that the lithospheric thickness is only 130–140km beneath Ordos basin in the western part of North China Craton; accordingly, there is no lithospheric keel beneath Ordos basin. These geothermal modeling results are in good agreement with those of seismic tomography and other geothermal studies. The temperatures in the lower crust of the North China Craton estimated are in range of 500–600°C. Consequently, the composition of lower crust of the North China Craton should be more mafic than that of previous estimation based on much cooler geotherms. The spatial variation of lithospheric rheology in the eastern China is influenced by local geotherms and crustal compositions. The “crème brûlée” model, which is represent by a strong crust portion but a weak lithospheric mantle portion in vertical strength profile, approximates the lithospheric rheological layering for the regions of the western half of North China Craton and the northern part of NE China, which has thicker crust or moderate-to-high geotherm. On the other hand, the “jelly sandwich” model demonstrates the rheological layering of the western part of the Yangtze Craton and the eastern half of the North China Craton, which has low-to-moderate geotherm and/or thinner crust. In general, thermal age of the tectonic province controls the present-day thermal structure and rheology of the lithosphere of the eastern China.

Effects of seismic anisotropy on P-velocity tomography of the Baltic Shield

The upper mantle structure of Swedish part of Baltic Shield with its isotropic and anisotropic seismic velocity characteristics is investigated using telesesismic body waves (i.e. P waves and shear waves) recorded by the Swedish National Seismological Network (SNSN). Nonlinear high-resolution P and SV and SH wave isotropic tomographic inversions reveal velocity perturbations of ± 3 % down to at least 470 km below the network. Separate SV and SV models indicate several consistent major features, many of which are also consistent with P-wave results. A direct cell by cell comparison of SH and SV models reveals velocity differences of up to 4%. Numerical tests show that differences in the two S-wave models can only be partially caused by noise and limited resolution, and some features are attributed to the effect of large scale anisotropy. Shear-wave splitting and P-travel time residual analyses also detect anisotropic mantle structure. Distinct back-azimuth dependence of SKS splitting excludes single-layer anisotropy models with horizontal symmetry axes for the whole region. Joint inversion using both the P and S data reveals 3D self-consistent anisotropic models with well-defined mantle lithospheric domains. These domains of differently oriented anisotropy most probably retain fossil fabric since the domains' origin, supporting the idea of the existence of an early form of plate tectonics during formation of continental cratons already in the Archean. The possible disturbing effects of anisotropy on seismic tomography studies are investigated, and found to be potentially significant. P-wave arrival times were adjusted based on the estimates of mantle anisotropy, and re-inverted. The general pattern of the velocity-perturbation images was similar but changed significantly in some places, including the disappearance of a slab-like structure identified in the inversion with the original data. Thus the analysis demonstrates that anisotropy of quite plausible magnitude can have a significant effect on the tomographic images, and should not be ignored. If, as we believe, our estimates of anisotropy are reasonably correct, then the model based on the adjusted data should give a more robust and correct image of the mantle structure.

Dynamics of post-slab breakoff in convergent plate margins: a "jellyfish" Model

The Earth9s subduction zones are characterized by a complex architecture of the subducting slabs, including necking, tearing, detachment from the surface plate, breakoff or ultimate slab loss, in contrast to the idealized models as popularly envisaged. Here we address the fate of the subducted slab after its breakoff by employing a simple two dimensional numerical modeling. Our results agree well with the tectonic scenario and geometric evolution of slab breakoff and reveal that during the sinking of broken slab fragments, the shape of the low viscosity slab (1-1000 times more viscous than the surrounding mantle) evolves into an inverted plume, similar to the “<i>jellyfish</i>” model proposed in some recent studies on deep subduction slabs. Our results confirm that the penetration criteria of the jellyfish into the lower mantle are primarily governed by the viscosity contrast between the lower to upper mantle. An evaluation of the various parameters suggest that the broken slab fragments may achieve a diverse type of morphology, correlating well with a similar diversity observed from high resolution seismic tomographic studies.

Tracking repeated subduction of the Hikurangi Plateau beneath New Zealand

The Hikurangi Plateau has been subducted beneath New Zealand twice — firstly at ca. 100 Ma during N–S convergence with Gondwana, and currently during E–W convergence between the Pacific and Australian plates. Here we use a nationwide 3-D seismic velocity model derived from seismic tomography studies, and seismicity relocated with this model, to determine the location of the subducted western and southern edges of the plateau. In the Hikurangi subduction zone in the North Island and northern South Island we identify the western edge in the 37–140 km depth range, based on the distribution of seismic velocities in the top of the subducted Pacific plate, changes in the thickness of the dipping seismic zone, and steepening of the subducted plate downdip of the buoyant plateau. In the central South Island, where the Hikurangi Plateau is subducting for a second time, the plateau dips less steeply. Its southern edge is well-defined by the collision of the delaminated lower crust of the plateau with the northern Fiordland subduction zone, where it bends the younger subducted Australian plate to vertical. The Hikurangi Plateau was originally rifted from the Ontong Java Plateau, and the subducted portions of the Hikurangi Plateau identified here fill a gap in reconstructions of this large igneous province.

Effects of the Fe 3 + spin transition on the properties of aluminous perovskite—New insights for lower-mantle seismic heterogeneities

We have measured the effects of the coupled substitution of Fe 3 + and Al on the density and compressibility of mantle silicate perovskite (Pv) up to 95 GPa. X-ray emission spectroscopy and synchrotron Mössbauer spectroscopy reveal a rapid increase in the population of low-spin Fe 3 + in Fe 3 + , Al-bearing Pv over a narrow pressure range near 70 GPa, which is in sharp contrast with Al-free Fe 3 + -bearing Pv, where Fe 3 + undergoes a gradual spin transition, and with Al-free Fe 2 + -bearing Pv, where Fe 2 + does not become low spin. At low pressure, Fe 3 + and Al expand the perovskite lattice. However, near the pressure range of the abrupt increase in the low-spin population, the unit-cell volume of Fe 3 + , Al-bearing Pv becomes similar to that of Mg-endmember Pv, while those of Al-free Fe 3 + -bearing Pv and Al-free Fe 2 + -bearing Pv remain larger throughout the lower mantle. Consequently, Pv in Al-rich systems should have lower density in the shallow lower mantle but similar or greater density than Pv in pyrolite in the deep lower mantle, affecting the buoyancy and mechanical stability of heterogeneities. Although the Fe 3 + spin transition in Pv is unlikely to cause a seismic discontinuity at mantle temperatures, it may result in a large change in bulk sound speed at 1200–1800 km depth, such that a vertically extending structure with an elevated amount of Fe 3 + would generate slower and faster anomalies above and below the depth of the spin transition, respectively, relative to the surrounding mantle. This may have important implications for bulk sound speed anomalies observed at similar depths in seismic tomography studies.

Seismic imaging of the 2001 Bhuj Mw7.7 earthquake source zone: b-value, fractal dimension and seismic velocity tomography studies

We use precisely located aftershocks of the 2001 Mw7.7 Bhuj earthquake (2001–2009) to explore the structure of the Kachchh seismic zone by mapping the 3-D distributions of b-value, fractal dimension (D) and seismic velocities. From frequency–magnitude analysis, we find that the catalog is complete above Mw = 3.0. Thus, we analyze 2159 aftershocks with Mw ≥ 3.0 to estimate the 3-D distribution of b-value and fractal dimensions using maximum-likelihood and spatial correlation dimension approaches, respectively. Our results show an area of high b-, D- and Vp/Vs ratio values at 15–35 km depth in the main rupture zone (MRZ), while relatively low b- and D values characterize the surrounding rigid regions and Gedi fault (GF) zone. We propose that higher material heterogeneities in the vicinity of the MRZ and/or circulation of deep aqueous fluid/volatile CO 2 is the main cause of the increased b-, D- and Vp/Vs ratio values at 15–35 km depth. Seismic velocity images also show some low velocity zones continuing in to the deep lower crust, supporting the existence of circulation of deep aqueous fluid / volatile CO 2 in the region (probably released from the eclogitasation of olivine rich lower crustal rocks). The presence of number of high and low velocity patches further reveals the heterogeneous and fractured nature of the MRZ. Interestingly, we observe that Aki (1981)'s relation (D = 2b) is not valid for the spatial b-D correlation of the events in the GF (D 2 = 1.2b) zone. However, the events in the MRZ (D 2 = 1.7b) show a fair agreement with the D = 2b relationship while the earthquakes associated with the remaining parts of the aftershock zone (D 2 = 1.95b) show a strong correlation with the Aki (1981)'s relationship. Thus, we infer that the remaining parts of the aftershock zone are probably behaving like locked un-ruptured zones, where larger stresses accumulate. We also propose that deep fluid involvement may play a key role in generating seismic activity in the MRZ, therefore, a similar episode of deep fluid inflow could play a crucial role in causing the next strong earthquakes in the surrounding blocks of NWF.

Flat-Slab Thermal Structure and Evolution Beneath Central Mexico

Recent seismic and magnetotelluric experiments, aimed at better characterizing the shape and state of the subducting slab and continental crust beneath Central Mexico, exposed significant differences with conclusions of previous studies. A new slab geometry is revealed in which the subducting Cocos slab is perfectly flat between 120 to 290 km from the trench, after which it plunges into the asthenosphere at a dip angle of ~65°, in sharp contrast with the previously proposed ~20° dip angle. Seismic tomography studies show negative P-wave velocity anomalies (−2 to −4%) in the mantle wedge beneath the Mexican Volcanic Belt, and positive anomalies (+2 to +3%) for the subducted Cocos slab. Magnetotelluric experiments exposed a very low-resistivity area (1–10 Ωm) located within the continental crust just below the Mexican Volcanic Arc. Finally, several spots of non-volcanic tremors (NVTs) have been recorded inside the continental crust above the flat-slab segment. While all these experiments provide a better picture of the subduction system beneath Central Mexico, several key processes need further investigation. In this study, we take advantage of these new observations to better constrain the thermal structure beneath Central Mexico. Two different thermal models are computed for a mantle potential temperature (T p) of 1,350 and 1,450°C, respectively. The new thermal structures are then converted into P-wave velocity anomalies and compared with the observed V p anomalies. We found that a T p of 1,450°C produced larger V p anomalies that do not fit the observations. However, using a T p of only 1,350°C, our predicted V p anomalies are positive (+2 to +3%) for the cold slab and negative (−2 to −4%) in the mantle wedge. These V p estimates are consistent with the observed seismic tomography from P-wave arrivals, and therefore we conclude that a T p of 1,350°C is a better estimate for the mantle potential temperature beneath Central Mexico. The new thermal model, in conjunction with phase diagrams for sediments, hydrated basalt and lithospheric mantle, have been used to estimate the amount and location of fluids released from the subducting Cocos slab. Several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat-slab. We found a good correlation between the pattern of these dehydration pulses and the location of NVTs, suggesting that slab dehydration is responsible for triggering the tremors. We suggest that NVT bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening, a process that has been going on since 15 Ma ago when the Cocos slab entered into a flat-slab regime. Such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15 Ma, allowing the slab to easily roll back. The continuous low-resistivity region recorded beneath the volcanic front in Central Mexico might represent the evidence of slab dehydration and crust weakening over time.

Lithospheric layering in the North American craton

How cratons-extremely stable continental areas of the Earth's crust-formed and remained largely unchanged for more than 2,500 million years is much debated. Recent studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at depths too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. Here we show that changes in the direction of azimuthal anisotropy with depth reveal the presence of two distinct lithospheric layers throughout the stable part of the North American continent. The top layer is thick ( approximately 150 km) under the Archaean core and tapers out on the surrounding Palaeozoic borders. Its thickness variations follow those of a highly depleted layer inferred from thermo-barometric analysis of xenoliths. The lithosphere-asthenosphere boundary is relatively flat (ranging from 180 to 240 km in depth), in agreement with the presence of a thermal conductive root that subsequently formed around the depleted chemical layer. Our findings tie together seismological, geochemical and geodynamical studies of the cratonic lithosphere in North America. They also suggest that the horizon detected in receiver function studies probably corresponds to the sharp mid-lithospheric boundary rather than to the more gradual lithosphere-asthenosphere boundary.

A model for the evolution of the Earth's mantle structure since the Early Paleozoic

Seismic tomography studies indicate that the Earth's mantle structure is characterized by African and Pacific seismically slow velocity anomalies (i.e., superplumes) and circum‐Pacific seismically fast anomalies (i.e., a globally spherical harmonic degree 2 structure). However, the cause for and time evolution of the African and Pacific superplumes and the degree 2 mantle structure remain poorly understood with two competing proposals. First, the African and Pacific superplumes have remained largely unchanged for at least the last 300 Myr and possibly much longer. Second, the African superplume is formed sometime after the formation of Pangea (i.e., at 330 Ma) and the mantle in the African hemisphere is predominated by cold downwelling structures before and during the assembly of Pangea, while the Pacific superplume has been stable for the Pangea supercontinent cycle (i.e., globally a degree 1 structure before the Pangea formation). Here, we construct a proxy model of plate motions for the African hemisphere for the last 450 Myr since the Early Paleozoic using the paleogeographic reconstruction of continents constrained by paleomagnetic and geological observations. Coupled with assumed oceanic plate motions for the Pacific hemisphere, this proxy model for the plate motion history is used as time‐dependent surface boundary condition in three‐dimensional spherical models of thermochemical mantle convection to study the evolution of mantle structure, particularly the African mantle structure, since the Early Paleozoic. Our model calculations reproduce well the present‐day mantle structure including the African and Pacific superplumes and generally support the second proposal with a dynamic cause for the superplume structure. Our results suggest that while the mantle in the African hemisphere before the assembly of Pangea is predominated by the cold downwelling structure resulting from plate convergence between Gondwana and Laurussia, it is unlikely that the bulk of the African superplume structure can be formed before ∼230 Ma (i.e., ∼100 Myr after the assembly of Pangea). Particularly, the last 120 Myr plate motion plays an important role in generating the African superplume. Our models have implications for understanding the global‐scale magmatism, tectonics, mantle dynamics, and thermal evolution history for the Earth since the Early Paleozoic.

Small scale heterogeneity in the mid-lower mantle beneath the circum-Pacific area

Data from a western United States short-period seismic network are investigated in order to detect anomalous later P-wave arrivals within a time window from 50s to 170s after direct P waves for deep and intermediate-depth earthquakes at circum-Pacific subduction zones. We interpret the anomalous arrivals as S-to-P scattered waves from heterogeneities in the mid-lower mantle. The searched regions (depths from ∱/4800km to 2200km) cover approximately 2% of the entire lower mantle. The heterogeneous scattering objects are located by array processing of the seismic waves using semblance, with careful investigations of arrival time, slowness and azimuth of the later phases relative to direct P waves. We usually observe two or three later phases which are separated in arrival time by 5•20s and have similar arrival azimuth and slowness. This observation suggests that a few scatterers often form a cluster with a typical spacing of 100•200km vertically. More than ten clusters or groups of S-to-P scattering objects are detected in the mid-lower mantle beneath the circum-Pacific regions; northern Kuril, Izu-Bonin, Mariana, northern Fiji, and west coast of South America. The S-to-P scattering objects are found in a depth range from 1100 to 1800km, and the most prominent objects of each region tend to be located around the middle of the depth range. The amplitudes of the scattered waves range from 1% to 10% of the direct P waves, with exceptionally large values as large as 40%. The associated magnitudes of the changes in elasticity is not tightly constrained but probably are nearly 1% and may occasionally exceed 4% of shear velocity. Considering their high efficiency at converting short-period waves (wavelengths of 5•10km), the changes in elastic properties at the heterogeneous objects appear to occur within several kilometers. Seismic tomography studies have not detected prominent seismic wave speed anomalies except for those beneath South America which appear to be located in the former Farallon plate and those at north of Fiji which appear to be located around the Pacific superplume. The mid-lower mantle heterogeneity should represent sharp chemical variations in major element composition of the lower mantle rocks. The most plausible tectonic explanation of the variations is remnant of subducted and folded former oceanic crust which preceded the current subduction of the Pacific plate. The extreme estimates of low shear velocity (>4%) may indicate the prominent drops in the shear modulus due to ferro-elastic phase transition in stishovite (SiO2). Our observations imply that heterogeneity of kilometer-scale persists for billions of years despite of stirring due to convection and therefore is ubiquitous in the lower mantle. It is qualitatively consistent with some recent convection models.

Detection of Systematic Errors in Travel-Time Data Using a Minimum 1D Model: Application to Costa Rica Seismic Tomography

Many seismological studies depend on the accuracy of timing of seis- mological data. In seismic tomography, travel-time residuals defined as differences between the observed and calculated arrival times of seismic phases are minimized to constrain 3D velocity structure. Inconsistencies and large errors in data sets that result from incorrect station coordinates, errors in the timing acquisition system, errors in the merging procedure, inconsistency in the picking and phase misidentification can also generate travel-time residuals, and because of their systematic nature, these errors cannot be treated as random noise even by exploiting a large number of travel times. While the inverse problem is perfectly set up to deal with random noise, systematic errors lead to significant artifacts in the solution, but may not be detected by a poste- rior error assessment. For this reason, detecting and removing systematic travel-time errors from data sets before inversion is crucial for seismic tomography studies. We present a methodology based on the use of a minimum 1D model to detect and remove systematic errors in travel-time data by detailed analysis of station delays and observation residuals and apply it to a local earthquake data set from Costa Rica. The determination of the exact nature of detected inconsistencies needs further investiga- tions in each individual case. If the cause of detected systematic errors cannot be determined beyond any doubt and the afflicted data may not be corrected, they must be deleted from the data set. To assess the extent of influence of systematic errors on hypocenter locations and their uncertainties, we present two examples showing the effects of station mislocation.

Combined three-dimensional electric and seismic tomography study on the Åknes rockslide in western Norway

We present a combined 3-D geoelectric and seismic tomography study conducted on the large Åknes rockslide in western Norway. Movements on the slope are strongly influenced by water infiltration, such that the hydrogeological regime is considered as a critical factor affecting the slope stability. The aim of our combined geophysical study was to identify and visualize the main shallow tension fractures and to determine their effect on hydraulic processes by comparing the geophysical results with information from borehole logging and tracer tests. To resolve the complex subsurface conditions of the highly fractured rock mass, a three-dimensional set-up was chosen for our seismic survey. To map the water distribution within the rock mass, a pattern of nine intersecting 2-D geoelectric profiles covered the complete unstable slope. Six of them that crossed the seismic survey area were considered as a single data set in a 3-D inversion. For both methods, smoothing-constraint inversion algorithms were used, and the forward calculations and parameterizations were based on unstructured triangular meshes. A pair of parallel shallow low-velocity anomalies (< 1400 m/s) observed in the final seismic tomogram was immediately underlain by two anomalies with resistivities <13 kΩm in the resistivity tomogram. In combination with borehole logging results, the low-velocity and resistivity anomalies could be associated with the drained and water-filled part of the tension fractures, respectively. There were indications from impeller flowmeter measurements and tracer tests that such tension fractures intersected several other water-filled fractures and were responsible for distinct changes of the main groundwater flow paths.

日本列島下のスラブの三次元構造と地震活動

A dense nationwide seismic network recently constructed in Japan has been yielding large volumes of high-quality data that have made it possible to investigate the seismic structure in the Japanese subduction zone with unprecedented resolution. We introduce a precise configuration of the Philippine Sea and Pacific plates subducting beneath the Japanese Islands, which was recently obtained by seismic tomographic imaging, precise earthquake hypocenter determinations, and focal mechanism studies. Seismic tomographic studies show that the Philippine Sea plate subducting beneath southwest Japan is continuous throughout the entire region, from Kanto to Kyushu, without disruption or splitting even beneath the area north of the Izu Peninsula. The estimated geometry of the subducted Pacific and Philippine Sea slabs shows a broad contact zone between the two slabs located directly beneath the Kanto plain. It further shows the wavy configuration of the Philippine Sea slab subducting beneath the entire region of southwestern Japan. Contact between the Philippine Sea plate and the Pacific plate causes anomalously deep interplate and intraslab earthquake activity in Kanto. Moreover, the interplate coupling coefficient estimated from repeating earthquake data shows a distinct change across the northeastern edge of this slab contact zone, suggesting that the overlying plate controls large-scale interplate coupling. High-resolution studies of spatial variations of intraslab seismicity and the seismic velocity structure of the slab crust strongly support the dehydration embrittlement hypothesis for the generation of intraslab earthquakes.

An analysis of the first-arrival times picked on the DSS and wide-angle seismic section recorded in Italy since 1968

We performed an analysis of refraction data recorded in Italy since 1968 in the frame of the numerous deep seismic sounding and wide-angle reflection/refraction projects. The aims of this study are to construct a parametric database including the recording geometric information relative to each profile, the phase pickings and the results of some kinematic analyses performed on the data, and to define a reference 1D velocity model for the Italian territory from all the available refraction data. As concerns the first goal, for each seismic section we picked the P-wave first-arrival-times, evaluated the uncertainties of the arrival-times pickings and determined from each travel time-offset curve the 1D velocity model. The study was performed on 419 seismic sections. Picking was carried out manually by an algorithm which includes the computation of three picking functions and the picking- error estimation. For each of the travel time-offset curves a 1D velocity model has been calculated. Actually, the 1D velocity-depth functions were estimated in three different ways which assume: a constant velocitygradient model, a varying velocity-gradient model and a layered model. As regards the second objective of this work, a mean 1D velocity model for the Italian crust was defined and compared with those used for earthquake hypocentre locations and seismic tomographic studies by different institutions operating in the Italian area, to assess the significance of the model obtained. This model can be used in future works as input for a next joint tomographic inversion of active and passive seismic data.

A boundary layer model for submarine hydrothermal heat flows at on‐axis and near‐axis regions

Hydrothermal discharge from oceanic crust makes up a significant portion of Earth's overall heat flux. In this paper we investigate two aspects of oceanic hydrothermal heat flux that are not well understood: (1) the relative partitioning of heat flux between high‐temperature and low‐temperature flows at oceanic spreading ridges and (2) hydrothermal behavior of the near‐axis region, where seismic data suggest that a zone of partial melt extends quasi‐vertically into the lower crust at the East Pacific Rise. We apply steady state boundary layer theory to each system by assuming circulation occurs near a hot isothermal wall that laterally transfers heat to and induces convection within an adjacent fluid‐filled medium. We vary the permeability of the medium over a range of 10−14 to 10−10 m2. For model 1 we assume a wall temperature of 150°C, corresponding to a relatively impermeable anhydrite wall within layer 2A extrusives, whereas for model 2 we assume a wall temperature of 800°C that extends to the base of the crust. The boundary layer results for model 1 suggest that diffuse flow makes up 50% or less of the heat output at the ridge axis, which is at the low end of some observations. Should more accurate and complete heat flux data be obtained and indicate that a high fraction of ridge axis heat loss results from diffuse flow, another mechanism in addition to the boundary layer model proposed here may be required. Boundary layer results for near‐axis circulation, model 2, indicate that temperature distributions are in reasonable agreement with those inferred from seismic tomographic studies of the oceanic crust and with other numerical models. For the near‐axis model, results also show that heat transfer in the hydrothermal boundary layer is greater than the input from steady state generation of the oceanic crust by seafloor spreading. This difference in heat input suggests that the width of the partial melt zone may be decreasing with time. Thus, the application of the boundary layer theory to complex hydrothermal processes near oceanic spreading centers provides important insights into key system processes.

Effect of iron content on electrical conductivity of ringwoodite, with implications for electrical structure in the transition zone

Electrical conductivity of ringwoodite with various iron contents [Fe/(Fe + Mg) = 0.09, 0.2 and 0.3] was measured at pressure (20 GPa) and temperature (up to 1900 K) conditions of the lower part of the mantle transition zone in a Kawai-type multi-anvil apparatus. The conductivity increased with increasing total iron content. All electrical conductivity data were fitted to the formula of electrical conductivity σ = σ 0 X Fe exp(− H/ kT), where σ 0 is the pre-exponential term, X Fe is the mole fraction of iron content in the Mg site, H is the activation enthalpy, k is the Boltzmann constant and T is absolute temperature. The activation enthalpy becomes higher at a certain temperature. At high temperatures, the activation enthalpy decreased from 1.44 to 0.92 eV with increasing total Fe content. At low temperatures less than 1000 K, the activation enthalpy also decreases from 1.15 to 0.74 eV with total Fe content. Dependence of the activation enthalpy on Fe content suggests that the dominant mechanism of charge transport is Fe 2+–Fe 3+ hopping (small polaron). Recent electrical conductivity-depth profiles of the transition zone beneath the Pacific Ocean obtained from the electromagnetic induction study shows that the conductivity values between 520 and 660 km depths may be explained by ringwoodite with Fe/(Fe + Mg) = 0.10. On the other hand, assuming a normal geotherm, conductivity values beneath the continent or stable craton are considerably lower than those of ringwoodite with Fe/(Fe + Mg) = 0.10. Taking into consideration results from the global seismic tomographic studies, relatively low conductivity in these regions can be explained by the existence of a cooler region compared with the surrounding mantle, rather than the presence of iron-poor ringwoodite, or a combination of both.