Seismic Velocity Discontinuities Research Articles

A global SS precursor method for imaging discontinuities: the Moho and beyond

SUMMARY Imaging seismic velocity discontinuities within the Earth's interior offers important insight into our understanding of the tectonic plate, associated mantle dynamics, and the evolution of the planet. However, imaging velocity discontinuities in locations where station coverage is sparse, is sometimes challenging. Here we demonstrate the effectiveness of a new imaging approach using deconvolved SS precursor phases. We demonstrate its effectiveness by applying it to synthetic seismograms. We also apply it to ∼1.6 M SS precursor waveforms from the global seismic database (1990–2018) for comparison with CRUST1.0. We migrate to depth and stack the data in circular 6° bins. The synthetic tests demonstrate that we can recover Moho depths as shallow as 20 km. Globally, the Moho is resolved at 21–67 km depth beneath continental regions. The Moho increases in depth from 21 km ± 4 km beneath the continental shelf to 45–50 km beneath the continental interiors and is as deep as 67 ± 4 km beneath Tibet. We resolve the Moho in 77 percent of all continental bins, within 10 km of CRUST1.0, with all outliers located in coastal regions. We also demonstrate the feasibility of using this method to image discontinuities associated with the mantle transition zone with both synthetic and real data. Overall, the approach shows broad promise for imaging mantle discontinuities.

2‐D Analytical P‐to‐S and S‐to‐P Scattered Wave Finite Frequency Kernels

AbstractScattered wave imaging provides a powerful tool for understanding Earth's structure. The development of finite frequency kernels for scattered waves has the potential for improving the resolution of both the structure and magnitude of discontinuities in S‐wave velocity. Here we present a 2‐D analytical expression for teleseismic P‐to‐S and S‐to‐P scattered wave finite‐frequency kernels for a homogeneous medium. We verify the accuracy of the kernels by comparing to a spectral element method kernel calculated using SPECFEM2D. Finally, we demonstrate the ability of the kernels to recover seismic velocity discontinuities with a variety of shapes including a flat discontinuity, a discontinuity with a sharp step, a discontinuity with a smooth bump, and an undulating discontinuity. We compare the recovery using the kernel approach to expected recovery assuming the classical common conversion point (CCP) stacking approach. We find that the P‐to‐S kernel increases recovery of all discontinuity structures in comparison to CCP stacking especially for the shallowest discontinuity in the model. The S‐to‐P kernel is less successful but can be useful for recovering the curvature of shallow discontinuity undulations. Finally, although we observe some variability in the amplitude of the kernels along the discontinuities, the kernels show some potential for recovering the magnitude of the velocity contrast across a discontinuity.

Seismic Discontinuities Across the North American Caribbean Plate Boundary From S‐to‐P Receiver Functions

AbstractThe evolution of the Caribbean plate has resulted in the formation of volcanic arcs, the Caribbean Large Igneous Province (CLIP) and micro‐plates across the plate boundary zones. The northern plate boundary with the North American plate has been particularly segmented with the transition from oblique subduction to oblique collision moving from east to west. However, there are few constraints on the seismic structure of the upper mantle across the plate boundary. Here we use S‐to‐P receiver functions to map seismic velocity discontinuities across the plate boundary, placing constraints on crustal and lithospheric thicknesses, as well as the structures associated with subduction and collision. We image a velocity increase with depth, consistently seen at 28–34 ± 4 km along the plate boundary, which corresponds to the Moho. A second strong velocity increase with depth is observed at depths of 64–66 ± 5 km, which is related to the presence of subducting slabs and anisotropic effects. We image a velocity decrease with depth at 95–135 ± 7 km, which reflects a lithosphere‐asthenosphere boundary that varies in depth across the plate boundary. The deepest negative discontinuity spatially maps to the CLIP. We suggest that a deep melting depth at 135 km, associated with an elevated potential mantle temperature of 1585 ± 20°C during CLIP formation, caused a depleted and dehydrated root to the base of melting, thus thickening the lithosphere.

Receiver function mapping of mantle transition zone discontinuities beneath Alaska using scaled 3-D velocity corrections

SUMMARYThe mantle transition zone is the region between the globally observed major seismic velocity discontinuities around depths of 410 and 660 km and is important for determining the style of convection and mixing between the upper and the lower mantle. In this study, P-to-S converted waves, or receiver functions, are used to study these discontinuities beneath the Alaskan subduction zone, where the Pacific Plate subducts underneath the North American Plate. Previous tomographic models do not agree on the depth extent of the subducting slab, therefore improved imaging of the Earth structure underneath Alaska is required. We use 27 800 high quality radial receiver functions to make common conversion point stacks. Upper mantle velocity anomalies are accounted for by two recently published regional tomographic S-wave velocity models. Using these two tomographic models, we show that the discontinuity depths within our CCP stacks are highly dependent on the choice of velocity model, between which velocity anomaly magnitudes vary greatly. We design a quantitative test to show whether the anomalies in the velocity models are too strong or too weak, leading to over- or undercorrected discontinuity depths. We also show how this test can be used to rescale the 3-D velocity corrections in order to improve the discontinuity topography maps.After applying the appropriate corrections, we find a localized thicker mantle transition zone and an uplifted 410 discontinuity, which show that the slab has clearly penetrated into the mantle transition zone. Little topography is seen on the 660 discontinuity, indicating that the slab has probably not reached the lower mantle. In the southwest, P410s arrivals have very small amplitudes or no significant arrival at all. This could be caused by water or basalt in the subducting slab, reducing the strength at the 410, or by topography on the 410 discontinuity, preventing coherent stacking. In the southeast of Alaska, a thinner mantle transition zone is observed. This area corresponds to the location of a slab window, and thinning of the mantle transition zone may be caused by hot mantle upwellings.

Characterization of Crustal and Uppermost‐Mantle Seismic Discontinuities in the Ontong Java Plateau

AbstractThe Early Cretaceous Ontong Java Plateau (OJP) in the southwestern Pacific Ocean is the largest oceanic plateau by volume on Earth, and a broad range of observations has been conducted to reveal its formation and evolution. However, because seafloor seismic observations of the OJP and surrounding areas have been insufficient so far, such experiments are capable of generating additional information regarding the crustal and mantle structure of the OJP. To image seismic velocity discontinuities from the crust to the uppermost mantle, we applied receiver function analysis to seismic records acquired by 17 broadband ocean bottom seismometers deployed across the region in and around the OJP and 3 broadband stations located on ocean islands in Micronesia (one: permanent, two: temporary). The results revealed midcrustal discontinuities and the Moho at depths of 10–20 and 30–40 km (from the top of the basement), respectively, in the central OJP. Moreover, a mantle discontinuity was also imaged at the depth of 55–60 km (from the top of the basement) in the central OJP. These boundaries were not imaged outside the OJP, implying that they are characteristic features of the OJP. In addition, receiver function images showed Moho signals at the depth of 20 km in the eastern OJP, where few previous seismic exploration surveys have been conducted. This depth is comparable with that found in the Manihiki and Hikurangi Plateaus that were potentially separated from the OJP.

Thin lithosphere beneath the central Appalachian Mountains: A combined seismic and magnetotelluric study

A joint analysis of magnetotelluric and Sp receiver function data, collected along a profile across the central Appalachians, highlights variations in regional lithospheric structure. While the interpretation of each data set by itself is non-unique, we identify three distinct features that are consistent with both the resistivity model and the receiver function image: 1) thin lithosphere beneath the Appalachian Mountains, 2) somewhat thicker lithosphere to the east of the mountains beneath the Coastal Plain, and 3) a lithosphere-asthenosphere boundary that deepens to the west of the mountains. In some regions, the correspondence between seismic velocity discontinuities and resistivity mark the base of the lithosphere, while in other locations we see seismic discontinuities that are contained within the lithosphere. At the western end of our profile a transition from highly resistive lithosphere to more conductive mantle represents the transition across the Grenville front. The thickness of lithosphere beneath the Grenville terrain is ∼140 km. Lithosphere at the eastern end of the profile has a thickness that is not well constrained by our coverage, but is at least 110 km thick. This lithosphere can be associated with a broader region of high resistivity material seen to extend further south. Directly beneath the Appalachian Mountains, lithospheric thickness is inferred to be as thin as ∼80 km, based on observations of elevated mantle conductivities and a westward-dipping seismic converter. Electrical conductivities in the uppermost asthenospheric mantle are sufficiently high (>0.1 S/m) to require the presence of a small volume of partial melt. The location of these elevated conductivities is close (offset ∼50 km to the west) to Eocene volcanic outcrops in and around Harrisonburg, VA. Our observations speak to mechanisms of intraplate volcanism where there is no divergent or convergent plate motion to trigger mantle upwelling or obvious fluid release, either of which can facilitate melting. Instead, we suggest that small scale mantle convection related either to pre-existing lithospheric thickness variations, or to lithospheric loss through delamination, coupled with relative plate motion with respect to the underlying asthenosphere, can trigger small amounts of melting. This melt migrates upslope, along the base of the lithosphere, potentially thermally eroding the lithosphere resulting in further thinning.

Seismic Imaging of the North American Midcontinent Rift Using S-to-P Receiver Functions.

North America's ~1.1‐Ga failed Midcontinent Rift (MCR) is a striking feature of gravity and magnetic anomaly maps across the continent. However, how the rift affected the underlying lithosphere is not well understood. With data from the Superior Province Rifting Earthscope Experiment and the USArray Transportable Array, we constrain three‐dimensional seismic velocity discontinuity structure in the lithosphere beneath the southwestward arm of the MCR using S‐to‐P receiver functions. We image a velocity increase with depth associated with the Moho at depths of 33–40 ± 4 km, generally deepening toward the east. The Moho amplitude decreases beneath the rift axis in Minnesota and Wisconsin, where the velocity gradient is more gradual, possibly due to crustal underplating. We see hints of a deeper velocity increase at 61 ± 4‐km depth that may be the base of underplating. Beneath the rift axis further south in Iowa, we image two distinct positive phases at 34–39 ± 4 and 62–65 ± 4 km likely related to an altered Moho and an underplated layer. We image velocity decreases with depth at depths of 90–190 ± 7 km in some locations that do not geographically correlate with the rift. These include a discontinuity at depths of 90–120 ± 7 km with a northerly dip in the south that abruptly deepens to 150–190 ± 7 km across the Spirit Lake Tectonic Zone provincial suture. The negative phases may represent a patchy, frozen‐in midlithosphere discontinuity feature that likely predates the MCR and/or be related to lithospheric thickness.

HyMaTZ: A Python Program for Modeling Seismic Velocities in Hydrous Regions of the Mantle Transition Zone

AbstractMapping the spatial distribution of water in the mantle transition zone (MTZ, 410‐ to 660‐km depth) may be approached by combining thermodynamic and experimental mineral physics data with regional studies of seismic velocity and seismic discontinuity structure. HyMaTZ (Hydrous Mantle Transition Zone) is a Python program with graphical user interface, which calculates and displays seismic velocities for different scenarios of hydration in the MTZ for comparison to global or regional seismic‐velocity models. The influence of water is applied through a regression to experimental data on how H2O influences the thermoelastic properties of (Mg,Fe)2SiO4 polymorphs: olivine, wadsleyite, and ringwoodite. Adiabatic temperature profiles are internally consistent with dry phase proportion models; however, modeling hydration in HyMaTZ affects only velocities and not phase proportions or discontinuity structure. For wadsleyite, adding 1.65 wt% H2O or increasing the iron content by 7 mol% leads to roughly equivalent reductions in VS as raising the temperature by 160 K with a pyrolite model in the upper part of the MTZ. The eastern U.S. low‐velocity anomaly, which has been interpreted as the result of dehydration of the Farallon slab in the top of the lower mantle, is consistent with hydration of wadsleyite to about 20% of its water storage capacity in the upper MTZ. Velocity gradients with depth in absolute shear velocity models are steeper in all seismic models than all mineralogical models, suggesting that the seismic velocity gradients should be lowered or varied with depth and/or an alternative compositional model is required.

Seismotomographic detection of major structural discontinuity in northern Sicily

We present the results of tomographic inversion computed with the use of the LOTOS code for Sicily and surroundings, a region of great geodynamic interest located on the Nubia-Europe margin where previous analyses have progressively improved the knowledge of seismic velocity structure without, however, permitting fine detection of tectonic units and structural discontinuities. We used LOTOS's devices for inversion, grid rotation and adaptation to ray density for application to a dataset of 7105 local earthquakes of the period 1990-2012. Our tomographic model highlights a previously undocumented major discontinuity which is located approximately along the northern coast of Sicily and is characterized by a sudden transition from low velocity imbricate thrust sheets and accretionary wedge in mainland Sicily (to the south) to relatively high velocity Tyrrhenian continental crust (to the north). Combining this finding with available geological and geodynamic information, we conclude that this northern Sicily seismic velocity discontinuity, which approximately corresponds to a regional fault system known as Kumeta-Alcantara, may have played a major role in the Miocene to Middle Pliocene, when lithosphere tearing occurred between the Tyrrhenian sea and Sicily in response to trench retreat. The more recent geodynamic settings of northern Sicily and the southern Tyrrhenian can be unravelled from Quaternary geological observations, seismicity and GPS data, which indicate that (i) the northern Sicily discontinuity has ceased to be active in more recent times; and (ii) the reorganized slow convergence of Nubia with respect to Europe is currently accommodated ~100 km north of Sicily, along the east-trending seismogenic belt enclosing Ustica and the Aeolian Islands.

Constraints on the anisotropic contributions to velocity discontinuities at ∼60 km depth beneath the Pacific.

Strong, sharp, negative seismic discontinuities, velocity decreases with depth, are observed beneath the Pacific seafloor at ∼60 km depth. It has been suggested that these are caused by an increase in radial anisotropy with depth, which occurs in global surface wave models. Here we test this hypothesis in two ways. We evaluate whether an increase in surface wave radial anisotropy with depth is robust with synthetic resolution tests. We do this by fitting an example surface wave data set near the East Pacific Rise. We also estimate the apparent isotropic seismic velocity discontinuities that could be caused by changes in radial anisotropy in S‐to‐P and P‐to‐S receiver functions and SS precursors using synthetic seismograms. We test one model where radial anisotropy is caused by olivine alignment and one model where it is caused by compositional layering. The result of our surface wave inversion suggests strong shallow azimuthal anisotropy beneath 0–10 Ma seafloor, which would also have a radial anisotropy signature. An increase in radial anisotropy with depth at 60 km depth is not well‐resolved in surface wave models, and could be artificially observed. Shallow isotropy underlain by strong radial anisotropy could explain moderate apparent velocity drops (<6%) in SS precursor imaging, but not receiver functions. The effect is diminished if strong anisotropy also exists at 0–60 km depth as suggested by surface waves. Overall, an increase in radial anisotropy with depth may not exist at 60 km beneath the oceans and does not explain the scattered wave observations.

Structure of the mantle transition zone under the Yunnan region and its geodynamic implications

The mantle transition zone (MTZ) bounded by physical discontinuities at 410 and 660 km depth is believed to play a key role in controlling mantle flow. Mineral physics experiments reveal that phase changes from olivine to β phase and γ -spinel to perovskite+magnesiowustite, at pressures equivalent to the existing at those mantle discontinuities, largely explain the variations in seismic velocity observed by the seismologists. The globally observed seismic velocity discontinuity at 660 km depth marks the bottom of the MTZ, which is the natural boundary between the upper and lower mantle. This discontinuity is considered an important mantle boundary, since it is especially associated with the mantle dynamics, the source of mantle plumes and the sinking of the subducted plates. On the other hand, temperature variations are the cause of thickening and thinning of the MTZ in correspondence with the positive and negative slopes of the Clapeyron curve at the 410 and 660 km mantle discontinuities, respectively. Receiver function analysis is a straightforward and commonly accepted method of studying the crust and upper-mantle structure through the use of teleseismic waveforms recorded at three-component seismic stations. This method requires the rotation of the ZNE displacement components to the radial and transversal components in the horizontal plane; and later the transformation of the radial and vertical components, which lie in the same vertical plane, into the L and Q components of the ground motion. By deconvolving the L component from the Q -component, both the source, far field path and instrument effects are removed. The resulting receiver function waveform is the ground’s impulse response. The analysis of each receiver function recorded at an array station provides, after moveout correction, a detailed estimation of the depth of the interface where the P-to-S seismic phase conversion is produced. However, the converted Ps phases at the discontinuities of 410 and 660 km depth are so weak that stacking of a large amount of waveforms is needed to enhance the signal-to-noise ratio in the receiver functions. In this study, we obtained 8600 teleseismic P-wave receiver functions recorded at 48 permanent broadband stations deployed in Yunnan and surroundings, which later were properly stacked in a single trace in 1°×1°-sized grid cells. Before stacking, the receiver functions were migrated from time domain to depth domain with the help of a reference earth model. Finally, we obtained 6 stacking images of receiver functions at latitude of 28°, 27°, 26°, 25°, 24° and 23°N, being the stacking depth in the 0–800 km range. The results indicate: (1) further north of 26°N, the average depths of the 410 and 660 km mantle discontinuities are in a range of 407–408 and 663–670 km, respectively, while the average thickness of the MTZ is within the range of 255–269 km, which are values very close to the global average thickness of 250 km. (2) South of 26°N, the average depths of the 410 and 660 km discontinuities are in a range of 412–426 and 675–703 km, respectively, and the average thickness of the MTZ is within the range of 262–279 km, which is obviously larger than the global average value of 250 km. The deepening of the 410 and 660 km discontinuities in the Yunnan region is obviously associated with the eastward subduction of the Indian plate below the Burma Arc; then, based on the greater thickness of the MTZ beneath the Yunnan region, we suggest that the eastward subduction of the Indian plate occurs mainly south of 26°N in the Yunnan region.

Modelo petrofísico del borde oriental de las sierras de Valle Fértil- La Huerta, Argentina a partir de datos sísmicos y petrológicos

This paper is a contribution to the knowledge of the crustal structure of the eastern flank of the Valle Fértil - La Huerta ranges (Western Sierras Pampeanas, San Juan, Argentina) at 31°S, in the Andean foreland region, where the Nazca plate is subducting horizontally at about 100 km depth. A 1D velocity model was constrained, combining petrographic and seismological observations from analysis of 19 igneous and metaigneous plutonic rocks belonging to the Famatinian (Ordovician) magmatic arc, which make up most of the crystalline basement of these ranges. Granitoid lithologies predominate in the northern region whereas mafic lithologies are more common to the south. The seismological analysis consisted of modeling teleseismic receiver functions near three seismological stations: LUNA, MAJA and CHUC, in places where those rocks are dominant. Thus, P and S seismic-wave velocities (Vp and Vs) and Poisson´s coefficient (ν), among other elastic parameters, were obtained. The seismic velocity model indicates an overthickened crust with an average thickness between 55 and 60 km, which matches with global average values (~41km); this agrees well with the hypothesis of partial eclogitization in the lower crust. The presence of two seismic velocity discontinuities at mid-crustal levels (12 and 28 km depths), likely associated to décollements, might be related to the accretion of the Cuyania terrane to the Pampia terrane. We obtained low P seismic-wave velocities (Vp ~5.8 km/s), Vp/Vs ratio (~1.70) in upper crust levels consistent with granitoid lithologies, as well as high P seismic-wave velocities (Vp ~6.76 km/s), Vp/Vs ratio (~1.78) in lower crust levels; these figures match with mafic lithologies of a more dense (~3.00 g/cm3), lower crust with respect to other back-arc Andean regions. Also, these values are consistent with the existence of mafic rocks composed of olivine, ortho- and clinopyroxene, which constitute the root of the Famatinian magmatic arc. These results indicate high-grade metamorphic conditions and depths corresponding to geophysical properties of middle to lower crust and correlate with the hypothesis of a dehydrated, cool and magnesium-enriched mantle located in the region between the subducted Nazca slab and the bottom of the Cuyania terrain crust.

Along‐arc variation in water distribution in the uppermost mantle beneath Kyushu, Japan, as derived from receiver function analyses

AbstractWe investigated the seismic velocity discontinuities in the uppermost mantle of Kyushu, a subduction zone in Japan, using receiver function analyses developed especially for discontinuities with high dipping angles. We elucidated the regional variation of discontinuities, as being the continental Moho, oceanic Moho, and upper boundary of the Philippine Sea slab. From the geometry of these discontinuities and contrast in S wave velocities, we discovered that the oceanic crust of the Philippine Sea slab has a low velocity and is possibly hydrated down to 70 km beneath south Kyushu, 80–90 km beneath central Kyushu, and less than 50 km beneath north Kyushu. The fore‐arc mantle wedge beneath central Kyushu has a low‐velocity zone and possibly contains hydrated materials and free fluid but such a low‐velocity zone does not exist in the fore‐arc mantle wedge beneath north Kyushu and south Kyushu. Beneath south Kyushu, water dehydrated from the slab could move to the back‐arc side and cause arc volcanism. Beneath central Kyushu, water dehydrated from the slab could move to the fore‐arc side and cause a gap in volcanism and hydration of fore‐arc mantle materials. Beneath north Kyushu, the oceanic crust does not appear to convey water abundantly in the mantle wedge. The low‐velocity hydrated fore‐arc mantle extends landward beneath the northern part of central Kyushu and could produce volcanic rocks partially contaminated by slab‐derived fluid.

Insights into the evolution of the Italian lithospheric structure from S receiver function analysis

To advance our understanding of the complex and episodic tectonic history of the central Mediterranean, we image the characteristics and structure of the convergent African and Eurasian lithospheric plates using S receiver functions. Specifically we investigate the lithospheric structure in terms of seismic velocity discontinuities, including the crust–mantle boundary (Moho) and the lithosphere–asthenosphere boundary (LAB) beneath Italy. The geometry and continuity of these structures are explored using teleseismic S-to-P converted phases, applied extensively throughout this region for the first time. The continental lithospheric thickness varies between ∼60 and ∼170km with an average of ∼90km, and the pulse characteristics of the converted phases can be characterized by tectonic region and history. Beneath the Northern Apennines, where subduction is still active, we find evidence for a complex lithospheric structure, where two different negative S-velocity jumps are present at approximately 90 and 180km depth. The shallowest negative phases are interpreted as the base of the lithosphere, but the second negative phases are much deeper than expected for the overriding plate lithosphere. When compared to hypocenter locations and P-wave tomography these deep converted phases correlate with the base of fast velocities perturbations in the upper mantle and regions with subcrustal earthquakes. In contrast beneath the Southern Apennines, the lithospheric structure appears less complex, with only one significant negative arrival in the receiver functions and where no subcrustal earthquakes are present. These pulses are generally deeper (∼110km) than those beneath the Northern Apennines and are broader in shape indicating a less sharp velocity contrast. Farther south along the Italian peninsula, the structure becomes complex again, with two sharp negative amplitude phases at depth. These deep converted phases that originate in the uppermost mantle beneath the Calabrian arc and Sicily are also co-located with deep (>35km) earthquakes. These observations suggest that these deepest negative phases correspond to the base of the actively subducting lithosphere beneath the Northern Apennines and Calabria, and where these phases are absent in the receiver functions there is no subducting slab. Seismic imaging of the lithosphere and uppermost mantle using S-to-P conversions provides another geophysical dataset to aid in interpretation of the complex, segmented slab structure beneath Italy.

Evidences of buried loads in the base of the crust of Borborema Plateau (NE Brazil) from Bouguer admittance estimates

Abstract In the Borborema Province (BP) – northeastern Brazil – two important Cenozoic events occurred at the surface: the Macau magmatism and the Borborema Plateau epeirogenesis. To obtain appropriated-scale geophysical data to explain the deep origins of these two events, different gravimetric/elevation databases were integrated with new surveys. Bouguer admittance estimates reveal that isostatic condition of the BP, especially in the Borborema Plateau, can be explained using elastic models to the lithosphere only if surface and buried loadings are combined. If the buried load is applied in the base of the crust, the ratio between buried and surface weights is circa 15 for a lithosphere with effective elastic thickness around 15 km and crust thickness around 33 km. From an interpretative viewpoint of the buried load, it is assumed that the lower crust under the Borborema Plateau might have an anomalous high value of density. Magmatic underplating might explain this fact as well as the observed surface magmatism and epeirogenesis. Crustal thickening of about 4 km under the Borborema Plateau and intracrustal seismic velocity discontinuity with high Vp/Vs ratio are geophysical facts consistent with magmatic underplating. However, the surface magmatism presents low volume and mainly alkaline composition – facts that are not entirely consistent with the hypothesis of magmatic underplating. Regardless the validity of this hypothesis, Cenozoic-to-present events in BP might be somewhat associated with imbalances in lithosphere-asthenospheric mantle and/or crust-lithospheric mantle systems. The existence of free-air anomalies showing no null integral over area and of an expressive positive geoid anomaly are geophysical evidences of these imbalances. Possibly, the Borborema Plateau is still suffering epeirogenesis. Post-depositional deformation found in Barreiras Formation strata, Late Quaternay fault reactivations, and AFT thermochronology analysis suggesting the existence of a cooling stage between 20 and 0 Ma might be geologic evidences of the continued action of epeirogenesis until the present. In addition, the relatively high level of the present intraplate seismicity recorded in several regions of the BP is another unequivocal geophysical evidence that the crust of the province is still submitted to accommodation processes.

Water transportation through the Philippine Sea slab subducting beneath the central Kyushu region, Japan, as derived from receiver function analyses

[1] Receiver function analyses are performed to detect seismic velocity discontinuities in the uppermost mantle beneath the Kyushu subduction zone, Japan. The Philippine Sea slab subducting beneath Kyushu is young (26–50 Ma) and steeply dipping (at greater than 30°). We detect a seismic velocity contrast larger than 10% corresponding to the oceanic Moho down to 90 km in depth, implying that the subducting oceanic crust contains more than 3.0 wt.% water down to this depth. We also detect a discontinuity with downward decreasing seismic velocity at depths of 50–80 km, which is parallel to the oceanic Moho and 10 km shallower than it. This fact indicates that there is a sharp discontinuity between the mantle wedge and the hydrous oceanic crust. The existence of such a sharp discontinuity would require a large temperature gradient around the boundary or a permeability barrier at the upper boundary of the slab. We delineate the continental Moho with downward decreasing seismic velocity and the upper boundary of the slab with upward decreasing seismic velocity beneath the forearc region, which implies the existence of serpentinite and/or free fluid which causes high pore pressure in the forearc mantle.

Common-conversion-point stacking of receiver functions for estimating the geometry of dipping interfaces

SUMMARY We develop a new method to estimate geometries of dipping seismic velocity discontinuities from common conversion point stacking of receiver functions. The multistage fast-marching method is applied to the ray tracing with refraction at dipping interfaces. Although geometry of interfaces dipping at 30°–70° cannot be correctly estimated with a 1-D velocity model, it can be correctly estimated when refraction at dipping interfaces is taken into account. We apply this method to synthetic and observed data and confirm that geometries of dipping interfaces are successfully estimated. This method will contribute to revealing the structures beneath subduction zones where dipping interfaces exist.

Traveltime seismic tomography with adaptive wavelet parameterization

An algorithm for solving the inverse kinematic problem of traveltime seismic tomography is developed and tested. The algorithm is intended for imaging the three-dimensional (3D) velocity model composed of a layer underlain by a half-space. This algorithm considers the bottom boundary of the layer as a first-order seismic velocity discontinuity with unknown position that has to be determined in the inversion together with the velocity variations inside the overlying layer and the sub-interface boundary velocities. The inversion can be applied to the travel times of refracted, head and reflected waves. The main idea behind the algorithm is the adaptive parameterization of the medium by the sparse Haar wavelet series expansion. In order to throw off the poorly resolved coefficients of expansion, we suggest using two empirical local resolution measures: the number of seismic rays crossing the support of the corresponding wavelet support area and their angular coverage, i.e., the spread in the azimuths of these rays. The adequacy of these measures is tested by their comparison with the estimation of the diagonal elements of the resolution matrix on the synthetic examples. This comparison proved that the proposed measures can be successfully applied for statistical estimation of the resolution and for constructing the adaptive parameterization. It was shown also that the best results are achieved while using the number of rays normalized to the size of the wavelet support together with their angular coverage. An automated procedure for throwing off poorly resolved unknowns is developed. The parameters of this procedure can be tuned to provide the desired level of detail of the model to be reconstructed. The synthetic checkerboard testing proved the efficiency of the algorithm. The proposed algorithm can be applied to solve different types of problems, including regional seismic studies, as well as exploration and engineering seismology. The use of this algorithm is especially convenient when the medium is essentially three-dimensional and when the conventional seismic methods implying regular network measurements directly above the studied structure (such as the common depth point method) are inapplicable, e.g., in the seismic studies of the foundations of buildings and in rugged terrains.

レシーバ関数解析から推定された日本列島の地殻構造

Recently, several researchers have elucidated crustal structures over a large area of the Japanese Islands from travel time inversion analyses. However, very few studies have paid attention to velocity discontinuities due to the limitations of spatial resolution. In this study, we apply a receiver function analysis to estimate seismic velocity structure and seismic velocity discontinuities of the crust in the Japanese Islands. We search for the best-correlated velocity structure model between an observed receiver function at each station and synthetic ones using a grid search method. Synthetic receiver functions are calculated from many assumed simple velocity structures that consist of a sediment layer to compensate for the effects of the low-velocity sediment layer and two velocity discontinuities. As a result, we clarified the spatial distributions of the crustal S-wave velocities and the tops of mantle depths. Several plain and basin areas are covered with thick low-velocity sediment layers. There are low-velocity layers corresponding to volcanoes in the upper crust (5-15 km deep). In the lower crust (15-25 km deep), our results show low-velocity structures in the eastern part of the Niigata-Kobe Tectonic Zone (NKTZ) and the Median Tectonic Line. Around the crust-mantle boundary, we see clear low-velocity zones beneath volcanoes, the western part of the NKTZ, and in the occurrence regions of the non-volcanic low-frequency tremor. High velocities near the southern coastline of the Japanese Islands correspond to the crust-mantle discontinuity of the subducting Philippine Sea plate. The crustal structure beneath the Itoigawa-Shizuoka Tectonic Line (ISTL) shows relatively low velocities in the shallower part and high velocities in the deeper part compared to neighborhood areas. The ISTL is also the boundary of the velocity structure of the upper crust in the Japanese Islands. The northeastern Japan region has heterogeneities of velocity perturbations, whereas the southwestern Japan region has the relatively stable high-velocity zones. The tops of mantle depths tend to increase in mountain regions with some undulations. The discontinuity of the subducting Philippine Sea plate at a depth of more than 40 km is extracted in several areas from the Kanto district to the Kyushu district. This suggests that the velocity discontinuity of the subducting Philippine Sea plate is larger than that of the overriding plate.

Crustal structure beneath Aso Caldera, Southwest Japan, as derived from receiver function analysis

Aso Volcano experienced a huge pyroclastic eruption 90 thousand years ago, and formed a large caldera (18 km × 25 km). In order to test the hypothesis of a magma body in the mid and lower crust that has been suggested geophysically and geochemically, we investigated seismic velocity discontinuities and velocity structure beneath Aso Caldera using receiver functions and a genetic algorithm inversion. We confirm the existence of the Moho at depths between 30 km and 35 km and a large velocity anomaly should exist in the deep portion of the crust beneath Aso Caldera, from imaging of receiver functions observed only at stations outside the caldera. As a result of a more detailed examination with GA inversion, a low velocity layer is detected at depths between 10 km and 24 km beneath the western part of the caldera. S-wave velocity of the layer is estimated to be 2.0–2.4 km/s. We estimate that the low velocity layer contains at most 15% melt or 30% aqueous fluid. The layer exists near the Conrad and at the same depths as the swarm of the low frequency earthquakes and a compressional and dilatational deformation source which are expected to be caused by fluid movement beneath the middle-eastern part of the caldera. Fluid contained in the layer might be related with huge pyroclastic eruptions of Aso Volcano.