Physics of the Earth and Planetary Interiors最新文献

Using first-principles density functional theory (DFT), we model the thermoelastic properties of hydrated ringwoodite (γ-(Fe, Mg)₂SiO₄), a potential water reservoir in the Earth's mantle transition zone (MTZ). Our calculations indicate that hydration, in general, leads to a reduction in the sound wave velocity of ringwoodite. However, increased pressure tends to suppress this reduction. For our 1.56 wt% H₂O containing ringwoodite model, we find that the suppression is so large that at pressure and temperature (P-T) conditions relevant to the lower part of the MTZ the sound wave velocities for the hydrous (1.56 wt% H₂O) ringwoodite become very similar to that of the anhydrous ringwoodite. However, when the water concentration is increased to 3.3 wt%, the pressure-induced suppression of the velocity reduction due to hydration is not so significant. We have given a plausible explanation for the same on the basis of the electronic structure of the hydrous ringwoodite models. We conclude that in the lower part of the MTZ, seismic wave data is sufficiently robust to detect regions of very high-water content (∼3.3 wt%). However, if the water concentration is less than 1.56 wt%, sound wave velocities may not be able to precisely detect the state of hydration of the MTZ.

The M 7.8 Kaikoura earthquake occurred in the northern South Island of New Zealand on 3 Nov., 2016, involving the rupture of >20 faults. To understand the complexity of the Kaikoura earthquake, details of the fault geometry, seismic velocity distribution, and stress field are necessary. We have undertaken seismic tomography along the c. 200 km length of the rupture zone. Data from both 51 temporary stations and 22 permanent (GeoNet) stations were collected from March 2011 to December 2018.

The hypocenter of the Kaikoura earthquake and aftershocks near the Kekerengu fault locate along lineaments where seismic velocity changes laterally in the epicentral region. In the uppermost crust, lower velocities occur beneath the Emu Plain and Cape Campbell. A higher velocity region near Kaikoura may have acted as a barrier that prevented eastward rupture from the hypocenter and led to the complex fault distribution in this area. These complexities in the seismic velocity structure may relate to the multi-segment rupture character of the Kaikoura earthquake. Spatial correlations between rupture areas and high Vp/Vs suggest the involvement of overpressured fluid in the nucleation and propagation of rupture segments, which is also supported by the reactivation of unfavourably oriented strike-slip ruptures, many lying at c.70° to the regional maximum compressive stress trajectories.

Active deformation is ongoing in the southeastern margin of the Tibetan Plateau due to the collision of the Indian and Eurasian continents. While large-scale motion of the surface occurs, the nature of deformation at depth remains unresolved. We construct new lithospheric seismic anisotropic (radially and azimuthally) and shear-wave velocity models using fundamental-mode Rayleigh- and Love-wave phase velocity at periods of 20–100 s obtained from the ChinArray experiment to constrain the deformation style of the crust and upper mantle in the southeastern margin of the Tibetan Plateau. The results show that the uppermost mantle (Moho-90 km) underneath the Tibetan Plateau and northwestern part of the western Yangtze block are characterized with NE-SW oriented azimuthal anisotropy, prominent slow velocity and negative radial anisotropy (V_SH < V_SV). We interpret that this seismic pattern reflects the southeastward extrusion of the Tibetan uppermost mantle that may thermally erode the northwestern edge and result in the vertically coherent fabric due to the barriers of the left strong Emeishan large igneous province (i.e., south of the western Yangtze block) dominated by the high shear wave velocities. Low velocity anomaly, N-S trending azimuthal anisotropy, and negative radial anisotropy in the uppermost mantle beneath the eastern Yangtze block are most probably associated with vertical migration of hot mantle material from the lithosphere delamination and/or a branch of the Hainan plume.

We use measurements of Earth's tidal response at the $M_2$ frequency to constrain the rheology of its mantle. The viscoelasticity and anelasticity of the planet are modeled with an Andrade rheology that depends on two parameters: $\alpha$ and $\zeta$. In this paper, we propose an improved algorithm to compute Earth's tidal deformation. Its Love and Shida numbers $k_2$, $h_2$, $k_3$ and $l_2$ as well as the tidal lag $ϵ$ were calculated for two viscosity profiles and for a wide range of values of $\alpha$ and $\zeta$. By comparing our results with geodetic measurements we obtain the range of values of $\alpha$ and $\zeta$ that successfully describes Earth's viscoelastic behavior. Values of $\zeta$ as high as ${10}^5$ can not be excluded. For $\zeta =1$, $\alpha$ should be in the range from 0.19 to 0.33, while for a $\zeta ={10}^5$, $\alpha$ is most likely between 0.11 and 0.17. We believe that a similar rheology should be used in geophysical models of other rocky planets and satellites. The obtained results are mostly representative of the lower mantle.

We have also shown that for several combinations of the two parameters $\alpha$ and $\zeta$ we could obtain nearly identical values of Earth's $\Re \left(k_2\right)$, $\Re \left(h_2\right)$, $\Re \left(l_2\right)$ and $\Re \left(k_3\right)$ with considerably different values of the associated tidal lag. This shows that the approach of always setting $\zeta =1$ might be too simplistic and an Andrade rheology with two free parameters is needed to constrain both the real and imaginary parts of Love and Shida numbers.

Earth's magnetic field changes in both space and time: the temporal changes are called geomagnetic and paleomagnetic secular variations. Westward drift has been noted as a feature of secular variation for several centuries, but eastward drift has received less attention. We use three global geomagnetic field models covering the past 100 kyr to extend temporal coverage for tracking the zonal (azimuthal) motion of the radial magnetic field. The models we use are GGF100k (100–0 ka), GGFSS70 (70–15 ka), LSMOD.2 (50–30 ka); the extent of the models enables the inclusion of the extreme secular variations found during excursions, particularly the Laschamp excursion (42–40 ka). GGFSS70 and LSMOD.2 have higher temporal resolution than GGF100k, but their underlying data have poorer spatial coverage. Spatial structure is greatly diminished in all models for spherical harmonic degrees $l>4$.

We use two types of time-longitude plots, one of the full radial field to expose reverse and intense flux patches at the core-mantle boundary. The second time-longitude plot is processed to enhance zonal motion signatures and allows us to use Radon drift analyses to uncover characteristic time scales of both westward and eastward drift at mid to high latitudes in both the northern and southern hemispheres. Our results differ across the three models, which we attribute to varying degrees of resolution, accuracy, and data distribution. Nevertheless, recurrent episodes of both eastward and westward drift ranging from $\pm {0.05}^o/$yr to $\pm {0.18}^o/$yr occur in both the northern and southern hemispheres. Westward drift dominates. We also observe 8–20 kyr intervals between occurrences of high-latitude reverse flux patches correlated with strong drift signals. Focusing on the period 50–30 ka, we observe dominant eastward drift preceding the Laschamp excursion and westward drift subsequently. In a period not associated with an excursion, 90–80 ka, we see strong mid to high latitude drift signatures in the northern hemisphere.

The Lockne impact structure in Sweden formed due to a meteorite impact into the Proterozoic basement rocks in the Ordovician. The dykes of Asby dolerite are part of the target basement and are located outside the crater rim. We investigated the variation in thermomagnetic properties and X-ray diffraction (XRD) spectra of Asby dolerite with distance from the crater centre to find evidence of a potential low shock overprint. Ti-poor, (Ti)-magnetite with dominantly pseudo-single domain behaviour is the main magnetic carrier in the studied samples. Towards the centre of the crater, the Verwey transition temperature (T_v) and Curie temperature (T_c) decrease, whereas alteration indices increase systematically. Higher alteration indices and lower T_v near the crater centre may be owed to more intense fracturing of the silicate minerals which facilitated low-temperature oxidation of (Ti)-magnetite and alteration of mafic dykes by improving fluid circulation in a network of fractures in the silicate minerals. Higher deformation near the crater centre is supported by our XRD results, as the peak position (2θ value) of plagioclase (040), pyroxene $(\overline{3}11$) and magnetite (311) peaks decrease, while the width of the plagioclase (040) and pyroxene $(\overline{3}11$) peaks increase towards the centre. This study shows that changes in thermomagnetic properties and lattice parameters of magnetite are a powerful tool to characterise low shock pressure deformation in impacted target rocks.

We have investigated the microstructure of naturally deformed olivine (chemically equilibrated at 1000 °C) by conventional transmission electron microscopy and electron tomography. The peridotite specimen, from Oman ophiolite, has a mylonitic microstructure with remnant, strongly deformed, millimetric porphyroclasts co-existing with small newly formed olivine grains generated by dynamic recrystallization. Imaging by transmission electron microscopy reveals that both newly formed grains and porphyroclasts display [100] and [001] dislocations activity. Subgrain boundaries are composed of either [100] or [001] dislocations. The characterization of this natural sample also permits to identify sporadic [100] dislocation loops, rare [010] dislocation, infrequent melt, and intragranular bubbles or along subgrain boundaries. Electron tomography permits to identify several glide planes, which are similar to previous observations acquired on experimentally deformed polycrystalline olivine, more importantly electron tomography also permits to evidence combination of glide, climb and mixed climb (dislocation moving in an intermediate plane between the plane of glide plane and the plane of pure climb). Our study further infers the diversity of linear defects responsible for plastic deformation of olivine at lithospheric conditions.

Seismic tomography allows us to image the interior of the Earth. In general, to determine the nature of seismic anomalies, constraints on more than one seismic parameter are required. For example, the ratio $R$ between perturbations in $v_s$ and $v_p$ ($\mathrm{dln}{v}_s$ and $\mathrm{dln}{v}_p$, respectively) is studied extensively in the lowermost mantle and interpreted in terms of thermal and/or chemical anomalies. However, to jointly interpret tomographic models of variations in $v_s$ and $v_p$ or their ratio $R$, it is essential for them to share the same local resolution. Most existing models do not provide resolution information, and thus cannot guarantee to honour this condition. In addition, uncertainties are typically not provided, making it difficult to robustly interpret the ratio $R=\mathrm{dln}{v}_s/\mathrm{dln}{v}_p$. To overcome these issues, we utilise the recently developed SOLA tomographic method, a variant of the linear Backus–Gilbert inversion scheme. SOLA retrieves local-average model estimates, together with information on their uncertainties, whilst it also provides direct control on model resolution through target kernels. In this contribution, we apply SOLA to normal-mode data with sensitivity to both $v_s$ and $v_p$, as well as density throughout the mantle. Specifically, we aim to develop models of both $v_s$ and $v_p$ with the same local resolution. We test our methodology and approach using synthetic tests for various noise cases (random noise, data noise or also additional 3D Earth noise due to variations in other physical parameters than the one of interest). We find that the addition of the 3D noise increases the uncertainties in our model estimates significantly, only allowing us to find model estimates in six or four layers for $v_s$ and $v_p$, respectively. While the synthetic tests indicate that no satisfactory density models can be obtained, we easily manage to construct models of $\mathrm{dln}{v}_s$ and $\mathrm{dln}{v}_p$ with almost identical resolution, from which the ratio