High Vp Research Articles

Single-crystal elasticity of the deep-mantle magnesite at high pressure and temperature

Magnesite (MgCO3) is considered to be a major candidate carbon host in the Earth's mantle, and has been found to exist as an accessory mineral in carbonated peridotite and eclogite. Studying the thermal elastic properties of magnesite under relevant pressure–temperature conditions of the upper mantle is thus important for our understanding of the deep-carbon storage in the Earth's interior. Here we have measured the single-crystal elasticity of a natural magnesite using in situ Brillouin spectroscopy and X-ray diffraction in a diamond anvil cell up to 14 GPa at room temperature and up to 750 K at ambient pressure, respectively. Using the third-order Eulerian finite-strain equations to model the elasticity data, we have derived the aggregate adiabatic bulk, KS0, and shear moduli, G0, at ambient conditions: KS0=114.7(±1.3) GPa and G0=69.9(±0.6) GPa. The pressure derivatives of the bulk and shear moduli at 300 K are (∂KS/∂P)T=4.82(±0.10) and (∂G/∂P)T=1.75(±0.10), respectively, while their temperature derivatives at ambient pressure are (∂Ks/∂T)P=−24.0(±0.2) MPa/K and (∂G/∂T)P=−14.8(±0.7) MPa/K. Based on the thermal elastic modeling of the measured elastic constants along an expected normal upper-mantle geotherm and a cold subducting slab, magnesite exhibits compressional wave (VP) anisotropy of approximately 46–49% and shear wave (VS) splitting of 37–41% that are much larger than those of major constituent minerals in the Earth's upper mantle including olivine, pyroxene, and garnet. The modeled aggregate VP and VS velocity in moderately carbonated peridotite and eclogite containing approximately 10 wt.% magnesite (approximately 5 wt.% CO2) show minimal effects of magnesite on the seismic profiles of these rock assemblages at upper mantle conditions, suggesting that the presence of magnesite is likely difficult to be detected seismically. However, due to its unusually high VP and VS anisotropies, magnesite with strong preferred orientations may exhibit sufficient VP and VS anisotropies that can have significant influences on seismic anisotropies of the regionally carbonated upper mantle.

P-wave velocity and density structure beneath Mt. Vesuvius: a magma body in the upper edifice?

<p>A high-resolution image of the compressional wave velocity and density structure in the shallow edifice of Mount Vesuvius has been derived from simultaneous inversion of travel times and hypocentral parameters of local earthquakes and from gravity inversion. The robustness of the tomography solution has been improved by adding to the earthquake data a set of land based shots, used for constraining the travel time residuals. The results give a high resolution image of the P-wave velocity structure with details down to 300-500 m. The relocated local seismicity appears to extend down to 5 km depth below the central crater, distributed into two clusters, and separated by an anomalously high Vp region positioned at around 1 km depth. A zone with high Vp/Vs ratio in the upper layers is interpreted as produced by the presence of intense fluid circulation alternatively to the interpretation in terms of a small magma chamber inferred by petrologic studies. In this shallower zone the seismicity has the minimum energy, whilst most of the high-energy quakes (up to Magnitude 3.6) occur in the cluster located at greater depth. The seismicity appears to be located along almost vertical cracks, delimited by a high velocity body located along past intrusive body, corresponding to remnants of Mt. Somma. In this framework a gravity data inversion has been performed to study the shallower part of the volcano. Gravity data have been inverted using a method suitable for the application to scattered data in presence of relevant topography based on a discretization of the investigated medium performed by establishing an approximation of the topography by a triangular mesh. The tomography results, the retrieved density distribution, and the pattern of relocated seismicity exclude the presence of significant shallow magma reservoirs close to the central conduit. These should be located at depth higher than that of the base of the hypocenter volume, as evidenced by previous studies.</p>

Crustal velocity structure beneath Saurashtra, NW India, through waveform modeling: Implications for magmatic underplating

The Saurashtra peninsula in the northwestern segment of the Deccan volcanic province of India, is characterized by several high gravity and magnetic anomalies, which correspond to mafic crustal intrusions. This study attempts to quantify the alterations to the crust caused by the Deccan volcanism, by estimating the crustal Poisson’s ratio, shear wave velocity (Vs) structure and the shear velocity contrast across the Moho, through teleseismic waveform modeling. The P receiver functions (RFs) for six broadband seismic stations were constructed using about 575 high quality (S/N≥2.5) teleseismic waveforms of earthquakes (M≥5.5) recorded during the period 2004–2010. The moveout corrected RF summation stacks were inverted using the Neighborhood algorithm (NA) to estimate the shear velocity structure beneath each station. The crustal thickness is estimated to range from 38km in western Saurashtra to 33km close to the southern extension of the Cambay rift. A low velocity zone possibly corresponding to sub-basaltic sediments is detected beneath all the stations. The average crustal Vs and Poisson’s ratio are estimated to be 3.68km/s and 0.276 respectively. The crustal Poisson’s ratio indicates an intermediate to mafic composition for the crust. Modeling reveals a relatively high velocity lower crust with an average Vs≈3.88±0.1km/s which is consistent with the high Vp ≈ 7.1km/s reported for this region through seismic refraction and wide angle reflection studies, overlying a relatively low velocity (Vs ≈ 4.4±0.1km/s) upper mantle. The shear velocity contrasts across the Moho, derived from the amplitudes of the P-to-s (Pms) conversions from the Moho range from 0.08 to 0.17 which are much smaller than those (>0.20) observed across the Indian shield, implying a gradational Moho due to underplating. This study reveals that the crust beneath Saurashtra is distinctly different from that of the Archean Indian shield largely due to significant alterations by the Deccan volcanism.

Constraints on the composition of the Aleutian arc lower crust from VP/VS

AbstractDetermining the bulk composition of island arc lower crust is essential for distinguishing between competing models for arc magmatism and assessing the stability of arc lower crust. We present new constraints on the composition of high P‐wave velocity (VP = 7.3–7.6 km/s) lower crust of the Aleutian arc from best‐fitting average lower crustal VP/VS ratio using sparse converted S‐waves from an along‐arc refraction profile. We find a low VP/VS of ~1.7–1.75. Using petrologic modeling, we show that no single composition is likely to explain the combination of high VP and low VP/VS. Our preferred explanation is a combination of clinopyroxenite (~50–70%) and alpha‐quartz bearing gabbros (~30–50%). This is consistent with Aleutian xenoliths and lower crustal rocks in obducted arcs, and implies that ~30–40% of the full Aleutian crust comprises ultramafic cumulates. These results also suggest that small amounts of quartz can exert a strong influence on VP/VS in arc crust.

Crustal structure of the Siberian craton and the West Siberian basin: An appraisal of existing seismic data

We present a digital model SibCrust of the crustal structure of the Siberian craton (SC) and the West Siberian basin (WSB), based on all seismic profiles published since 1960 and sampled with a nominal interval of 50 km. Data quality is assessed and quantitatively assigned to each profile based on acquisition and interpretation method and completeness of crustal model. The database represents major improvement in coverage and resolution and includes depth to Moho, thickness and average P-wave velocity of five crustal layers (sediments, and upper, middle, lower, and lowermost crust) and Pn velocity. Maps and cross sections demonstrate strong crustal heterogeneity, which correlates weakly with tectono-thermal age and strongly with tectonic setting. Sedimentary thickness varies from 0–3 km in stable craton to 10–20 km in extended regions. Typical Moho depths are 44–48 km in Archean crust and up-to 54 km around the Anabar shield, 40–42 km in Proterozoic orogens, 35–38 km in extended cratonic crust, and 38–42 km in the West Siberian basin. Average crustal Vp velocity is similar for the SC and the WSB and shows a bimodal distribution with peaks at ca. 5.4 km/s in deep sedimentary basins and ~ 6.2–6.6 km/s in parts of the WSB and SC. Exceptionally high basement Vp velocities (6.8–7.0 km/s) at the northern border between the SC and the WSB indicate the presence of magmatic intrusions and are proposed to mark the source zone of the Siberian LIP. The cratonic crust generally consists of three layers and high-velocity lowermost crust (Vp ~ 7.4 km/s) is observed only locally. Pn velocities are generally ~ 8.2 km/s in the SC and WSB and abnormally high (8.6–8.9 km/s) around kimberlite fields. We discuss the origin of crustal heterogeneity and link it to regional crustal evolution.

Imaging the Hikurangi Plate interface region, with improved local-earthquake tomography

The properties of the plate interface region influence plate coupling and rupture behaviour during large earthquakes. Plate coupling varies greatly along the Hikurangi subduction zone in the southern half of the North Island, New Zealand, and heterogeneous material properties can be examined with the well-recorded seismicity. For this study, we have used a modified velocity inversion to incorporate earthquake differential times from selected groups of distributed earthquakes, which improves the spatial resolution of features in the lower crust, and sharpens velocity gradients. Data are selected from temporary deployments and the permanent GeoNet network, which has expanded in the last decade. The resulting 3-D velocity model shows that the overlying plate exhibits patterns related to geologic terranes. The Rakaia terrane has low Vp/Vs and high Vp, and is spatially related to the zone of strong plate coupling. Seismicity occurs throughout the overlying plate, and extends to the plate interface without a lower crustal aseismic zone. The Wairarapa–Waewaepa fault zone may form the updip limit of strong coupling in future plate interface earthquakes. The crust of the subducting plate is characterized by abundant seismicity and is bounded by strong velocity gradients. Low-velocity zones above the plate interface are indicated from 30 to 50km depth. Seismic velocities near the plate interface show an excellent correlation with the distribution of plate coupling, and provide insight into what controls such coupling. The plate interface has the highest Vp/Vs (>1.85) and sharpest Vp/Vs gradient in the region of strongest coupling, consistent with the suggestion that strong coupling is related to the inability of fluid to cross the plate interface. In regions of recurrent slow slip, Vp/Vs is still high, but the gradient of Vp/Vs near the plate interface is much broader, suggesting movement of fluid across the plate interface. The area of deep slow slip corresponds to the most extensive high Vp/Vs mantle region above the slab.

A 3D Vs model of the upper mantle beneath Italy: Insight on the geodynamics of central Mediterranean

We compute a high resolution Vs model of the upper mantle underneath Italy by inverting relative residuals of S-wave arrivals from teleseisms recorded at the Italian broad-band national seismic network. We also present a model of Vp/Vs perturbations obtained from similarly resolved P- and S-wave velocity models. The joint analyses of Vs and Vp/Vs models suggests that compositional variations concur with temperature variations to generate the observed velocity patterns, giving deeper insight into the upper mantle structure. The investigation of coherent high Vs and high Vp anomalies permits a comprehensive picture of the main oceanic domains involved in the puzzling subduction system of the central Mediterranean. Slab windows and tears developed in the Ionian slab are highlighted by regions of low Vp, low Vs and high Vp/Vs anomalies. The Adria continental lithosphere is characterized by high Vp, high Vs and low Vp/Vs anomalies that trace the sinking of eclogitized material underneath the Apennines and the Eastern Alps chains. The overall tomographic results yield the development of a new conceptual model for the Alps and Apennines formation, in which three basic processes occurred in different time and different locations all around the Alpine–Apennines system: continental collision, slab break-off and delamination of the upper-plate lithosphere.

Multiscale seismic signature of a small fault zone in a carbonate reservoir: Relationships between VP imaging, fault zone architecture and cohesion

The seismic (P-waves velocity, VP) signature of a small fault zone intersecting carbonate reservoir layers with contrasted properties of the southeastern French sedimentary basin was studied from the micro-scale to the fault zone pluri-meter scale architecture. VP measurements were done both at the meter scale, at 250-m depth in a gallery within the LSBB-URL, and at the centimeter scale (laboratory scale), on samples collected through boreholes. Results were compared to a VP tomography at the reservoir scale and to the fault rock mechanical properties. Our investigations indicated that P-wave velocity variations across the fault zone are strongly correlated to some key parameters, such as the uniaxial compressive strength (σc) of the fault core, the intact rock porosity (ϕ) and the fracturation intensity (RQD) of the damage zone. The seismic visibility of the fault zone depends on the contrasts between the porosity and the fracturation density of the sedimentary layers. In porous layers the fault induced deformations are mainly accommodated at the micro-scale (grain scale) with few macroscopic fractures, and the damage zone is thin leading to a high VP contrast with the fault core. In the low-porosity layers where deformations are mainly accommodated through brittle fractures, the seismic visibility of the fault is moderate, characterized by a decrease in the VP value which remains within the magnitude of the VP variations within the layers outside the fault zone. Interestingly, the fault seismic signature in the highly fractured layers appears clearly in the frequency domain at 3 dominant frequencies (2000, 9000 and 28,000Hz), each of which exhibiting different spectral amplitudes for each components of the fault zone. Finally, the seismic signature of a relatively small fault zone included in a layered sedimentary series appears discontinuous, characterized by more or less thick high velocity patches more or less extended within the stratigraphic layers. We showed that the VP variations across the fault zone are related to its architecture, and this can help to estimate the fault zone cohesion evolution. Logarithmic correlations were found between VP and the fault core's cohesion, and the cohesive component of the damage zone. In the fault core, the P-waves propagation depends mainly of the surfaces conditions, like the cohesion, between numerous blocks, whereas in the damage zone the cohesion is mainly influenced by the fracturing degree of the rock mass.

Intrusive mechanism of the 2008–2009 Mt. Etna eruption: Constraints by tomographic images and stress tensor analysis

Since January 2008, several geophysical parameters have evidenced a recharging phase at Mt. Etna volcano culminating with an effusive eruption that began on May 13, 2008. Seismic activity recorded at Mt. Etna from January 2007 to May 2008 was analyzed in order to provide seismological constraints to the volcano dynamics leading to the eruption. A total of 336 selected earthquakes, with ML≥1.5, were used as data source for this study. Specifically, we calculated 3D velocity and attenuation tomography, including a 3D relocation of the events, and we computed 53 selected fault plane solutions (FPSs) that were used for stress tensor inversion. The most important result obtained from the joint analysis of VP, VP/VS and P-wave attenuation is an anomalous zone with normal to high VP (values between 3.5 and 4.5km/s) and low VP/VS (values≤1.64), which partially overlaps with a low QP (values≤50) volume located along a NS trending channel beneath the central crater. This can be interpreted as a shallow volume characterized by high temperature where the magma is located with the presence of supercritical fluids. The analysis of seismic stress tensor evidenced an extensional regime in the depth range 3–13km with a vertically oriented σ1. This finding may suggest an extensional stress regime, probably related to the kinematic response of the volcanic edifice to both a deep magmatic intrusion and a condition of decreased regional compressive stress facilitated by sliding processes of the eastern flank of the volcano.

Ranolazine prevents heart failure with preserved ejection fraction, diastolic dysfunction, and left ventricular hypertrophy in a hypertensive murine model

Background: Recent in vitro studies on failing human heart muscle has suggested that ranolazine (RAN), a novel antianginal drug known to inhibit late sodium current, may improve diastolic dysfunction (DD) caused by hypertension (HTN)-induced oxidative stress. By improving DD, RAN can improve heart failure with preserved ejection fraction (HFPEF). Methods: Amouse model of DDwas developed by inducing mild HTN by unilateral nephrectomy, deoxycorticosterone acetate (DOCA) pellet implantation, and substituting drinking water with 1% NaCl. Control animals underwent sham operation and drank regular tap water. RAN 50 (mg/kg) was given orally twice a day. Weekly blood pressures (BPs) and heart rates (HRs) were measured using tail-cuff plethysmography. Cardiac function and left ventricular posterior wall thickness (LVPWd) were measured by echocardiogram under light anesthesia using isoflurane. Treatment durationwas for 3weeks and 7weeks. Pulmonary congestion was assessed by lung to body weight (L/BW) ratio. Results: Compared to controls, DOCA mice were hypertensive (BP 112 ±4 vs. 89±2 mmHg) and had DD evidenced by a low LV inflow propagation velocity Vp (28.1±0.5 vs. 47.2±1 cm/s⁎), a low mitral annulus early longitudinal velocity E' (2.70 vs. 4.40 cm/s⁎), and a high E/E' ratio (29.3±1.5 vs. 18.0±0⁎). DOCA mice had LVH as evidenced by high heart/BW ratio (6.22±0.15 vs. 4.90±0.22 mg/g⁎) and increased LVPWd (0.82±0.22 vs. 0.74±0.01 mm⁎). There was no systolic dysfunction in either group. RAN pretreatment effectively prevented DD and LVH in DOCA mice as evidenced by high E' (3.9 Vs 2.70 cm/s⁎), low E/E' (18.8±1.0⁎), high Vp (41±2Vs 28.1±0.5 cm/s⁎), lowheart/BW ratio (4.79±0.11 mg/g⁎), and low LVPWd thickness (0.76±0.01 vs. 0.82±0.02 mm⁎). RAN had no effect on HR [625±18 vs. 630±20 bpm, P=not significant (NS)]. RAN pretreatment effectively prevented development of heart failure in DOCA mice at 7 weeks as evidenced by significantly lower L/BW ratios when compared with untreated DOCA mice (0.41±0.002 vs. 0.52±0.02 mg/g⁎). Conclusions: RANhas prevented DD, LVH, and HFPEF in a hypertensive murine model of DD. Prevention of heart failure in our model warrants additional animal andhuman studies to further evaluate the therapeutic potential of RAN in HFPEF (Mean±S.E.M., ⁎Pb.05).

PLATINUM Japan SV: platinum chromium everolimus-eluting stent in small vessels

Background: Recent in vitro studies on failing human heart muscle has suggested that ranolazine (RAN), a novel antianginal drug known to inhibit late sodium current, may improve diastolic dysfunction (DD) caused by hypertension (HTN)-induced oxidative stress. By improving DD, RAN can improve heart failure with preserved ejection fraction (HFPEF). Methods: Amouse model of DDwas developed by inducing mild HTN by unilateral nephrectomy, deoxycorticosterone acetate (DOCA) pellet implantation, and substituting drinking water with 1% NaCl. Control animals underwent sham operation and drank regular tap water. RAN 50 (mg/kg) was given orally twice a day. Weekly blood pressures (BPs) and heart rates (HRs) were measured using tail-cuff plethysmography. Cardiac function and left ventricular posterior wall thickness (LVPWd) were measured by echocardiogram under light anesthesia using isoflurane. Treatment durationwas for 3weeks and 7weeks. Pulmonary congestion was assessed by lung to body weight (L/BW) ratio. Results: Compared to controls, DOCA mice were hypertensive (BP 112 ±4 vs. 89±2 mmHg) and had DD evidenced by a low LV inflow propagation velocity Vp (28.1±0.5 vs. 47.2±1 cm/s⁎), a low mitral annulus early longitudinal velocity E' (2.70 vs. 4.40 cm/s⁎), and a high E/E' ratio (29.3±1.5 vs. 18.0±0⁎). DOCA mice had LVH as evidenced by high heart/BW ratio (6.22±0.15 vs. 4.90±0.22 mg/g⁎) and increased LVPWd (0.82±0.22 vs. 0.74±0.01 mm⁎). There was no systolic dysfunction in either group. RAN pretreatment effectively prevented DD and LVH in DOCA mice as evidenced by high E' (3.9 Vs 2.70 cm/s⁎), low E/E' (18.8±1.0⁎), high Vp (41±2Vs 28.1±0.5 cm/s⁎), lowheart/BW ratio (4.79±0.11 mg/g⁎), and low LVPWd thickness (0.76±0.01 vs. 0.82±0.02 mm⁎). RAN had no effect on HR [625±18 vs. 630±20 bpm, P=not significant (NS)]. RAN pretreatment effectively prevented development of heart failure in DOCA mice at 7 weeks as evidenced by significantly lower L/BW ratios when compared with untreated DOCA mice (0.41±0.002 vs. 0.52±0.02 mg/g⁎). Conclusions: RANhas prevented DD, LVH, and HFPEF in a hypertensive murine model of DD. Prevention of heart failure in our model warrants additional animal andhuman studies to further evaluate the therapeutic potential of RAN in HFPEF (Mean±S.E.M., ⁎Pb.05).

Seismicity and velocity structures along the south-Alpine thrust front of the Venetian Alps (NE-Italy)

In this paper we show the seismicity and velocity structure of a segment of the Alpine retro-belt front along the continental collision margin of the Venetian Alps (NE Italy). Our goal is to gain insight on the buried structures and deep fault geometry in a “silent” area, i.e., an area with poor instrumental seismicity but high potential for future earthquakes, as indicated by historical earthquakes (1695 Me = 6.7 Asolo and 1936 Ms = 5.8 Bosco del Cansiglio). Local earthquakes recorded by a dense temporary seismic network are used to compute 3-D Vp and Vp/Vs tomographic images, yielding well resolved images of the upper crust underneath the south-Alpine front. We show the presence of two main distinct high Vp S-verging thrust units, the innermost coincides with the piedmont hill and the outermost is buried under a thick pile of sediments in the Po plain. Background seismicity and Vp/Vs anomalies, interpreted as cracked fluid-filled volumes, suggest that the NE portion of the outermost blind thrust and its oblique/lateral ramps may be a zone of high fluid pressure prone to future earthquakes. Three-dimensional focal mechanisms show compressive and transpressive solutions, in agreement with the tectonic setting, stress field maps and geodetic observations. The bulk of the microseismicity is clustered in two different areas, both in correspondence of inherited lateral ramps of the thrust system. Tomographic images highlight the influence of the paleogeographic setting in the tectonic style and seismic activity of the region.

Differential patterns of SH and P wave velocity structures in the transition zone beneath northwestern Tibet

Triplicate waveform modeling is used to resolve SH (Vs) and P (Vp) wave velocity structures in the upper mantle transition zone (TZ) beneath northwestern (NW) Tibet. Focal depth move out stacking is proposed to enhance the identification of triplicate phases, and can be used to test consistency of our data. Our results show that the Vs and Vp structures are decorrelated, and that a large Vs jump occurred across the 660-km discontinuity, with a small Vs gradient above it. Conversely, the Vp model is characterized by a relatively small contrast across the discontinuity, accompanied by a high Vp gradient in the TZ. There seem no significant depth anomalies of the 660-km discontinuity in both models. The seismic structures in TZ beneath NW Tibet are similar to recent studies beneath the central Qiangtang and western Lhasa terrains. Taking the lower TZ structures under India as references, Vs is normal but Vp appears slightly high, and thus a high ratio of Vp/Vs was indicated beneath NW Tibet. Combined results with experiment information from mineral studies, we suggest that the differential anomalies of Vp and Vs can be attributed to a chemical heterogeneity, such as increased Al content in the lower TZ. Considering the tectonic evolution of Tibet, the chemical heterogeneity may be associated with subduction or detachment of the Tethys oceanic slab.

Magmatic processes that generate chemically distinct silicic magmas in NW Costa Rica and the evolution of juvenile continental crust in oceanic arcs

Northwestern Costa Rica is built upon an oceanic plateau that has developed chemical and geophysical characteristics of the upper continental crust. A major factor in converting the oceanic plateau to continental crust is the production, evolution, and emplacement of silicic magmas. In Costa Rica, the Caribbean Large Igneous Province (CLIP) forms the overriding plate in the subduction of the Cocos Plate—a process that has occurred for at least the last 25 my. Igneous rocks in Costa Rica older than about 8 Ma have chemical compositions typical of ocean island basalts and intra-oceanic arcs. In contrast, younger igneous deposits contain abundant silicic rocks, which are significantly enriched in SiO2, alkalis, and light rare-earth elements and are geochemically similar to the average upper continental crust. Geophysical evidence (high Vp seismic velocities) also indicates a relatively thick (~40 km), addition of evolved igneous rocks to the CLIP. The silicic deposits of NW Costa Rica occur in two major compositional groups: a high-Ti and a low-Ti group with no overlap between the two. The major and trace element characteristics of these groups are consistent with these magmas being derived from liquids that were extracted from crystal mushes—either produced by crystallization or by partial melting of plutons near their solidi. In relative terms, the high-Ti silicic liquids were extracted from a hot, dry crystal mush with low oxygen fugacity, where plagioclase and pyroxene were the dominant phases crystallizing, along with lesser amounts of hornblende. In contrast, the low-Ti silicic liquids were extracted from a cool, wet crystal mush with high oxygen fugacity, where plagioclase and amphibole were the dominant phases crystallizing. The hot-dry-reducing magmas dominate the older sequence, but the youngest sequence contains only magmas from the cold-wet-oxidized group. Silicic volcanic deposits from other oceanic arcs (e.g., Izu-Bonin, Marianas) have chemical characteristics distinctly different from continental crust, whereas the NW Costa Rican silicic deposits have chemical characteristics nearly identical to the upper continental crust. The transition in NW Costa Rica from mafic oceanic arc and intra-oceanic magma to felsic, upper continental crust-type magma is governed by a combination of several important factors that may be absent in other arc settings: (1) thermal maturation of the thick Caribbean plateau, (2) regional or local crustal extension, and (3) establishment of an upper crustal reservoir.

Upper crustal structure, seismicity and pore pressure variations in an extensional seismic belt through 3-D and 4-D VPand VP/VSmodels: The example of the Val d'Agri area (southern Italy)

[1] We use local earthquake tomography and background seismicity to investigate static and transient features of the crustal velocity structure in the Val d'Agri (southern Apennines, Italy), one of the regions in central Mediterranean with the highest seismogenic potential. The upper crust is dominated by two broad high-velocity anticlines of the buried Apulia Carbonate Platform ramping on two parallel high-angle thrusts interpreted as preexisting inverted normal faults. The deep core of the anticlines consists of very high VP (up to 6.9 km/s) and low VP/VS rocks, suggesting the involvement of the Apulian crystalline basement in the Apennine belt. These results provide valuable constraints on the Apennine belt tectonic evolution, supporting a thick-skinned interpretation for the Pliocene terminal phase of the compressional tectonics. The geometry of the Val d'Agri Quaternary basin is controlled by these inherited compressive features, whereas the presently active extensional tectonics barely reworked the structure. We find inconsistency between the structure of the Apulia Carbonate Platform and the location and geometry of the Quaternary normal faults mapped at the surface. This suggests either the immaturity of the normal faults or their secondary role in accommodating the extension. We observe spatiotemporal (4-D) changes of VP and VP/VS models defining transient variations of pore fluid pressure in the upper crust. A strong change in the VP/VS ratio heralds a raise in the seismicity rate that can be related to large water level changes in a nearby artificial lake. This evidence is consistent with a mechanism of reservoir-induced seismicity by fluid pressure increase and pore pressure diffusion. The 4-D velocity variations are confined in the shallow portion of the upper crust (3–6 km depth) where fluids are stored in a highly fractured medium. Pore pressure fluctuations can affect the strength of fault segments, favoring seismicity rate changes along the active faults and possibly promoting large future earthquakes.

Fore-arc mantle wedge seismicity under northeast New Zealand

Relocated upper mantle seismicity, using a 3D velocity model, delineates three tightly clustered (about 12 km diameter) zones of seismicity that occur in the mantle wedge under the fore-arc region of the northern Hikurangi margin, North Island, New Zealand. These clusters extend from the subducting plate at about 55 km depth up to about 35 km, and coincide with strong seismic reflectivity recorded active source seismic reflection measurements, suggesting a fluids association. They have maximum magnitudes of about Mw 4.5. The clusters correspond closely with a change from low to high Vp and Qp, and with average Vp/Vs that is interpreted to be caused by the transition from serpentinised to normal peridotite. The location of this transition appears to correspond to the locus of dehydration of peridotite, suggesting that the seismicity is caused by dehydration embrittlement. The release of these fluids where the serpentinised mantle wedge reaches the locus of dehydration may provide a mechanism for transferring additional fluids or fluid fluxed partial melt into the adjacent back-arc basin and facilitate the generation of voluminous magmas that are erupted there. The occurrence of serpentinite in the fore-arc mantle under northeastern North Island may influence the shallow extent of interplate coupling under the region. The location and spacing of the clusters is consistent with thermally driven diapiric or fluid upwellings, but they also show a spatial relationship with proposed lithospheric fractures and associated volcanic seamounts on the subducting Hikurangi plateau.

Fault zone properties affecting the rupture evolution of the 2009 (Mw6.1) L'Aquila earthquake (central Italy): Insights from seismic tomography

[1] We have inverted P- and S-wave travel times from seismograms recorded by a dense local network to infer the velocity structure in the crustal volume where the April 6th 2009 main shock nucleated. The goal is to image local variations of P-wave velocity and Poisson ratio along the main shock fault zone for interpreting the complexity of the rupture history. The initial stages of the mainshock rupture are characterized by an emergent phase (EP) followed by an impulsive phase (IP) 0.87 s later. The EP phase is located in a very high VP and relatively low Poisson ratio (ν) region. The IP phase marks the beginning of the large moment release and is located outside the low ν volume. The comparison between the spatial variations of VP and Poisson ratio within the main shock nucleation volume inferred in this study with the rupture history imaged by inverting geophysical data allows us to interpret the delayed along-strike propagation in terms of heterogeneity of lithology and material properties.

Seismic structure and composition of the crust beneath the southern Scandes, Norway

New results on P and S-wave seismic velocity structure in southern Norway indicate that the crust has an average Poisson's ratio of 0.25, is predominantly of felsic-intermediate composition and lacks a significant mafic lower crust. A crustal scale refraction seismic study (Magnus-Rex — Mantle investigations of Norwegian uplift structure, refraction experiment) acquired data along three 300 to 400 km long active source seismic profiles across the Southwest Scandinavian Domain in southern Norway, the youngest section of the Fennoscandian shield. Moho depths in the Domain are 36–40 km, thinning towards the continental shelf and Oslo Graben. The high Vp lower crust beneath the Southwest Scandinavian Domain (Vp > 7 km/s) is around 4 km thick. Crustal structure in the adjacent Svecofennian Domain differs significantly; Moho depths reach ~ 50 km and an up to 24 km thick high Vp lower crust is present. Strong P and S-wave arrivals are recorded from Magnus-Rex, from which Poisson's ratio for the crust is calculated. The unusually strong S-wave arrivals allow rare insight into crustal Poisson's ratio structure that is not commonly available from active source data. The thinness of the mafic lower crust in southern Norway is in significant contrast to other parts of the Fennoscandian shield and delamination or convective removal of the lower crust is proposed. Tectonic events that may have triggered detachment of the lower crust include extensional collapse after the Caledonian orogeny and Permian extension.

Linking the Alps and Apennines subduction systems: New constraints revealed by high-resolution teleseismic tomography

We report a new model of the upper mantle structure beneath Italy obtained by means of P-wave teleseismic tomography. Besides the recent and remarkable development of the Italian Seismic Network, a high model resolution has been achieved improving the inversion method upon the ACH method used in previous investigations and picking high quality arrival times with the Multi-Channel Cross-Correlation technique. The finer details of our Vp model yield new insights into the heterogeneous structure of the Adria continental lithosphere involved in the collision between the Africa and Europe plates. A wide low Vp anomaly located in the northern Adria mantle, facing the Alpine high Vp slab, supports the idea that the Adria lithosphere has been hydrated and thinned during the Alpine subduction. We argue that this mantle softening may have played a key role in favoring the subsequent delamination of the Adria lithosphere in the northern Apennines. We hypothesize that delamination of continental lithosphere previously thinned in a back-arc setting may be considered a key process to favor subduction polarity reversal and recycling of continental material into the mantle circulation. Conversely, in the central-southern Apennines, the velocity structure is consistent with the existence of a deeper oceanic slab that flattens at the base of the upper mantle, in agreement with the widely accepted geodynamic evolution of the central Mediterranean by slab retreat and back-arc spreading. The oceanic slab is discontinuously detached from the surface plate, suggesting a different structure of the Adria lithosphere, which resists subduction instead of favoring delamination.

P-wave velocity and anisotropy of lawsonite and epidote blueschists: Constraints on water transportation along subducting oceanic crust

P-wave velocity (Vp) and the anisotropy of lawsonite and epidote blueschists were measured up to 1.0GPa and 400°C using the ultrasonic pulse transmission technique. The slowest Vp in the direction normal to foliation is similar between lawsonite and epidote blueschists (7.0–7.2km/s at 1.0GPa and room temperature), while the fastest Vp in the direction parallel to lineation markedly differs between lawsonite blueschists (7.4–7.6km/s at 1.0GPa and room temperature) and epidote blueschist (7.9km/s at 1.0GPa and room temperature). Crystallographic orientation measurements for main constituent minerals revealed that both epidote [010] axes (fastest Vp direction in epidote single crystal) and amphibole [001] axes (fastest Vp direction in amphibole single crystal) are preferentially oriented parallel to lineation to enhance Vp anisotropy of the epidote blueschist. In contrast, lawsonite [001] axes (fastest Vp direction in lawsonite single crystal) are oriented subnormal to foliation, whereas amphibole [001] axes are oriented subparallel to lineation, so that relatively weak Vp anisotropy was observed in the lawsonite blueschist. Our experimental results, in conjunction with recent seismological observations, suggest that the Vp of the subducting oceanic crust at <50km beneath NE and SW Japan is similar to those of blueschists (9–12% lower Vp than peridotite). In contrast, the Vp in the subducting oceanic crust markedly increases at deeper than ∼50km depth beneath NE Japan, and such a slight low-velocity layer (5–8% slower Vp) at >∼50km has been observed in several subducting slabs. However, the high Vp values at >∼50km depth are difficult to be explained by blueschists. This indicates that the blueschist would be at least partially transformed to hydrous mineral-bearing eclogite at ∼50km depth in subducting oceanic crusts.