EPP

地球与行星物理

ISSN  2096-3955

CN  10-1502/P

2018 Vol.2(3)

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REVIEW
SOLID EARTH
Evidence supporting New Geophysics
Stuart Crampin, Yuan Gao
2018, 2(3): 173-188. doi: 10.26464/epp2018018
Abstract:
In the last decade a New Geophysics has been proposed, whereby the crust and uppermost ~400 km of the mantle of the Earth are so pervaded by closely-spaced stress-aligned microcracks (intergranular films of hydrated melt in the mantle) that in situ rocks verge on failure by fracturing, and hence are critical-systems that impose a range of fundamentally-new properties on conventional sub-critical geophysics. Enough of these new properties have been observed to confirm that New Geophysics is a new understanding of fluid/rock deformation with important implications and applications. Evidence supporting New Geophysics has been published in a wide variety of publications. Here, for clarification, we summarise in one document the evidence supporting New Geophysics.
RESEARCH ARTICLE
SOLID EARTH: GEODYNAMICS
Reproducing past subduction and mantle flow using high-resolution global convection models
JiaShun Hu, LiJun Liu, Quan Zhou
2018, 2(3): 189-207. doi: 10.26464/epp2018019
Abstract:
Plate subduction drives both the internal convection and the surface geology of the solid Earth. Despite the rapid increase of computational power, it remains challenging for geodynamic models to reproduce the history of Earth-like subduction and associated mantle flow. Here, based on an adaptive approach of sequential data assimilation, we present a high-resolution global model since the mid-Mesozoic. This model incorporates the thermal structure and surface kinematics of tectonic plates based on a recent plate reconstruction to reproduce the observed subduction configuration and Earth-like convection. Introduction of temperature- and composition-dependent rheology allows for incorporation of many natural complexities, such as initiation of subduction zones, reversal of subduction polarity, and detailed plate-boundary dynamics. The resultant present-day slab geometry well matches Benioff zones and seismic tomography at depths < 1500 km, making it possible to hindcast past subduction dynamics and mantle flow. For example, the model produces a flat Farallon slab beneath North America during the Late Cretaceous to Early Cenozoic, a feature that has been geodynamically challenging to reproduce. This high-resolution model can also capture details of the 4-D evolution of slabs and the ambient mantle, such as temporally and spatially varying mantle flow associated with evolving slab geometry and buoyancy flux, as well as the formation of shallow slab tears due to subduction of young seafloors and the resulting complex mantle deformation. Such a geodynamic framework serves to further constrain uncertain plate reconstruction in the geological past, and to better understand the origin of enigmatic mantle seismic features.
SOLID EARTH: SEISMOLOGY
Seismic detection of the X-discontinuity beneath the Ryukyu subduction zone from the SdP conversion phase
QingHui Cui, WenLan Li, GuoHui Li, MaiNing Ma, XiaoYu Guan, YuanZe Zhou
2018, 2(3): 208-219. doi: 10.26464/epp2018020
Abstract:
The X-discontinuity, which appears at the depth of approximately 300 km, is an important seismic interface with positive velocity contrasts in the upper mantle. Detecting its presence and topography can be useful to understand phase transformations of relevant mantle minerals under the high-temperature and high-pressure circumstance of the Earth's interior. In this study, we detect the X-discontinuity beneath the Ryukyu subduction zone using five intermediate-depth events recorded by the dense Alaska Regional Network (AK). The X-discontinuity is successfully revealed from the robust slant stacking of the secondary down-going and converting SdP phases. From the depth distribution of conversion points, we find that the X-discontinuity's depth ranges between 269 km and 313 km, with an average depth of 295 km. All the conversion points are located beneath the down-dipping side of the Philippine Sea slab. From energy comparisons in vespagrams for observed and synthetic seismograms, the strong converted energy is more likely from a thin high-velocity layer, and the S-wave velocity jumps across the X-discontinuity are up to 5% to 8% with an average of 6.0%. According to previous petrological and seismological studies, the X-discontinuity we detected can be interpreted as the phase transformation of coesite to stishovite in eclogitic materials within the oceanic crust.
SOLID EARTH: SEISMOLOGY
Joint tomographic inversion of first-arrival and reflection traveltimes for recovering 2-D seismic velocity structure with an irregular free surface
XinYan Zhang, ZhiMing Bai, Tao Xu, Rui Gao, QiuSheng Li, Jue Hou, José Badal
2018, 2(3): 220-230. doi: 10.26464/epp2018021
Abstract:
Irregular surface flattening, which is based on a boundary conforming grid and the transformation between curvilinear and Cartesian coordinate systems, is a mathematical method that can elegantly handle irregular surfaces, but has been limited to obtaining first arrivals only. By combining a multistage scheme with the fast-sweeping method (FSM, the method to obtain first-arrival traveltime in curvilinear coordinates), the reflected waves from a crustal interface can be traced in a topographic model, in which the reflected wave-front is obtained by reinitializing traveltimes in the interface for upwind branches. A local triangulation is applied to make a connection between velocity and interface nodes. Then a joint inversion of first-arrival and reflection traveltimes for imaging seismic velocity structures in complex terrains is presented. Numerical examples all perform well with different seismic velocity models. The increasing topographic complexity and even use of a high curvature reflector in these models demonstrate the reliability, accuracy and robustness of the new working scheme; checkerboard testing illustrates the method’s high resolution. Noise tolerance testing indicates the method’s ability to yield practical traveltime tomography. Further development of the multistage scheme will allow other later arrivals to be traced and used in the traveltime inversion.
SPACE PHYSICS: MAGNETOSPHERIC PHYSICS
In situ detection of the electron diffusion region of collisionless magnetic reconnection at the high-latitude magnetopause
Qiu-Gang Zong, Hui Zhang
2018, 2(3): 231-237. doi: 10.26464/epp2018022
Abstract:
Magnetic reconnection is the most fundamental energy-transfer mechanism in the universe that converts magnetic energy into heat and kinetic energy of charged particles. For reconnection to occur, the frozen-in condition must break down in a localized region, commonly called the ‘diffusion region’. In Earth’s magnetosphere, ion diffusion regions have already been observed, while electron diffusion regions have not been detected due to their small scales (of the order of a few km) (Paschmann, 2008). In this paper we report, for the first time, in situ observations of an active electron diffusion region by the four Cluster spacecraft at the Earth’s high-latitude magnetopause. The electron diffusion region is characterized by nongyrotropic electron distribution, strong field-aligned currents carried by electrons and bi-directional super-Alfvénic electron jets. Also observed were multiple micro-scale flux ropes, with a scale size of about 5 c/ωpe (12 km, with c/ωpe the electron inertial length), that are crucial for electron acceleration in the guide-field reconnection process (Drake et al., 2006a). The data demonstrate the existence of the electron diffusion region in collisionless guide-field reconnection at the magnetopause.
PLANETARY SCIENCE
Mapping of the lunar surface by average atomic number based on positron annihilation radiation from Chang’e-1
LiangQuan Ge, JianKun Zhao, QingXian Zhang, YaoYao Luo, Yi Gu
2018, 2(3): 238-246. doi: 10.26464/epp2018023
Abstract:
A map of the average atomic number of lunar rock and soil can be used to differentiate lithology and soil type on the lunar surface. This paper establishes a linear relationship between the average atomic number of lunar rock or soil and the flux of position annihilation radiation (0.512-MeV gamma-ray) from the lunar surface. The relationship is confirmed by Monte Carlo simulation with data from lunar rock or soil samples collected by Luna (Russia) and Apollo (USA) missions. A map of the average atomic number of the lunar rock and soil on the lunar surface has been derived from the Gamma-Ray Spectrometer data collected by Chang’e-1, an unmanned Chinese lunar-orbiting spacecraft. In the map, the higher average atomic numbers (ZA > 12.5), which are related to different types of basalt, are in the maria region; the highest ZA (13.2) readings are associated with Sinus Aestuum. The middle ZA (~12.1) regions, in the shape of irregular oval rings, are in West Oceanus Procellarum and Mare Frigoris, which seems to be consistent with the distribution of potassium, rare earth elements, and phosphorus as a unique feature on the lunar surface. The lower average atomic numbers (ZA < 11.5) are found to be correlated with the anorthosite on the far side of the Moon.
PLANETARY SCIENCE
Simultaneous characterization of the atmospheres, surfaces, and exomoons of nearby rocky exoplanets
WenLiang Cui, JinSu Zhang, Frederic Schmidt, Duo Cui, XiaoMeng Huang, Tong Li, Feng Tian
2018, 2(3): 247-256. doi: 10.26464/epp2018024
Abstract:
Atmospheric composition is an important indicator of habitability and life. The presence or absence of a large exomoon around an Earth-size exoplanet could have important consequences for planet climate stability. Thus the detection of exomoons and retrieval of information regarding atmospheric composition of Earth-size exoplanets are important goals of future exoplanet observations. Here a data analysis method is developed to achieve both goals simultaneously, based on reflection spectra of exoplanet-exomoon systems. We show that the existence of exomoons, the size of exomoons, and the concentrations of some atomic and molecular species in the atmospheres of their hosting Earth-like exoplanets can be retrieved with high levels of reliability. In addition, the method can provide well-constrained fractions of basic surface types on the targets because of the characteristic spectral features of atmospheric species and surface types in the analyzed spectral range.