（鷺谷 威・Angela MENESES-GUTIERREZ）
Geodetic estimates of crustal strain rates in the Japan islands were an order of magnitude larger than geological/geomorphological estimates, which has been an unresolved problem called the strain rate paradox. Ikeda (1996) postulated that geodetic strain mainly reflects elastic strain accumulation due to interactions at plate boundaries. This hypothesis was proven to be correct by the occurrence of the 2011 MW 9.0 Tohoku-oki earthquake. Confusion between elastic strain and inelastic strain was the cause of the paradox. Significant postseismic deformation observed after the Tohoku-oki earthquake made it possible to distinguish the inelastic contribution from the geodetically observed crustal strain through a comparison with the pre-seismic strain rate pattern, which promoted a better understanding of inelastic deformation in the Japan islands. On the other hand, migration of localized deformation and temporal changes of strain rate are identified over a geological time scale, implying that it is essential to carefully review the methods and the uncertainties of geological/geomorphological strain rates. An integrated understanding of crustal deformation in the Japan islands is being advanced through detailed investigations of crustal strain rates on variable temporal and spatial scales.
Thermochronology can reconstruct the thermal history of a rock based on thermally resetting radiometric age, which is useful for estimating a regional exhumation history when applied to rocks exhumed from a great depth. In particular, systems having lower closure temperatures are called “low-temperature thermochronology” and have been used to study tectonics in the shallow crust. In this paper, low-temperature thermochronology and its application to tectonics in the shallow crust are comprehensively reviewed, focusing particularly on the uplift and exhumation histories of mountainous regions. This review paper comprises two parts. In the first part, fundamentals of low-temperature thermochronology are reviewed, including some representative thermochronometers, mathematical descriptions of thermal annealing/diffusion, concepts of closure temperature and partial annealing/retention zone, and inversion method for computing thermal history on the basis of thermochronologic data. In the second part, application to mountain formation is described, including terminology of uplift and exhumation, methodology for estimating cooling and exhumation history based on cooling ages, and some representative case studies around the world.
Flow properties of lower crust and upper mantle are important parameters for better understanding geodynamics. In particular, they are crucial for quantitatively evaluating the process of stress accumulation and relaxation of seismic faults associated with transient crustal deformation, such as post-seismic deformation. Recent activities to illuminate the rheological properties of lower crust and upper mantle are reviewed based on geological and geophysical observations. In particular, large, transient crustal deformation places robust constraints on the rheological properties. Careful observations of exhumed fault rocks limit the range of stress levels in the lower crust and upper mantle. Localized flow in the upper mantle from post-seismic observations can also be constrained. Combining detailed geodetic observations and numerical simulations, taking into account various rheological properties of rocks, can constrain the heterogeneity of viscosities possibly related to the heterogeneities of temperature and water content distribution. Furthermore, recent activities to infer rheological heterogeneity from an inversion analysis are also introduced. Under well constrained conditions, nonlinear flow properties of the upper mantle can be estimated. These activities clearly indicate that various aspects of the rheological (flow) properties of lower crust and upper mantle can be extracted from the analyses.
Because deformation in the lower crust immediately below a seismogenic fault can accumulate stress on the fault, understanding the dominant deformation mechanism and the formation of ductile shear zones in the lower crust is crucial to reveal occurrences of intraplate earthquakes. Plagioclase-rich rocks (e.g., gabbro and gabbronorite) are major constituents of the lower crust. A change in the deformation mechanism from grain-size-insensitive creep to grain-size-sensitive creep would result in significant rheological weakening. The transition potentially increases the strain rate more than an order of magnitude at a constant stress, and the process of grain size reduction in plagioclase-rich rocks is particularly important. In addition to dynamic recrystallization, fracturing is another dominant mechanism of grain-size reduction at high temperatures. The most likely explanation for the occurrence of fracturing in the lower crust is the deeper penetration of large earthquakes, which nucleate in the seismogenic zone and propagate into the underlying ductile region at the stage of coseismic displacement. Zones with very fine grains that result from fracturing would be deformed by grain-size-sensitive creep. Fracturing also facilitates fluid flow due to the increased dilatancy and connectivity of pore spaces. An influx of fluid enhances hydration reactions, which form small, strain-free recrystallized grains. This process promotes grain-size-sensitive creep in narrow zones, and result in the development of ductile shear zones in the lower crust.
A total of one to ten ocean masses of water (including H2O, OH, H in gas, liquid, and solid phases) is probably stored in the Earth's crust-mantle system as a variety of forms and components. Of these, the liquid phase involves aqueous solution, hydrous melt, and supercritical fluid, which may change composition continuously from a diluted aqueous fluid to a silicate melt above the second critical endpoint. These fluids are termed “geofluids” in this review paper, which describes physicochemical conditions required for the presence of geofluids, their compositions, and related tectonic setting and geodynamic phenomena. On the basis of the maximum water content that can be stored in crust-mantle rocks, subduction zone processes are discussed in which slab-derived fluids that are enriched in solid components play important roles: viscosity-flow-thermal field and melting in the mantle wedge, and origin of deep-seated fluid such as Arima-type brine. The fluid flux in the arc crust may be captured by geophysical and geochemical observations, such as seismicity, seismic velocity, electrical conductivity, and helium isotopic ratios. Several specific phenomena such as migrating seismic events and localized crustal deformation may be attributed to such fluid fluxes from the depths, which have been induced by an elevated fluid pressure and a reduction of rock strength and viscosity.
Magmatic and seismic activities are typically monitored using geophysical techniques, although geochemical analyses of groundwater can also be employed. Given their inert nature and high diffusivity, helium isotopes in volcanic gases, hydrothermal fluids, and groundwaters have been used to investigate magmatic and seismic activities. Helium isotopic ratios (3He/4He) are known to change before and after volcanic eruptions and large earthquakes. The migration and flux of crustal fluids are also important in this regard, because fluids are considered to play a major role in triggering earthquakes by changing fault strength through variations in pore pressure. In this contribution, the current state of helium-based monitoring of magmatic and seismic activities is reviewed; how to estimate the flux of helium and other volatile species is described; and, a method for estimating water flux from groundwater isotopic data is provided.
In order to understand crustal dynamics, including the occurrence of earthquakes and the development of mountain ranges, it is important to estimate the stress state in the Earth's crust from observed data. This paper reviews stress tensor inversion techniques using seismological data. The techniques were originally applied to a dataset of slip orientations taken from focal mechanisms. Subsequently, other techniques, which use P wave first-motion polarities or centroid moment tensor (CMT) solutions, were developed. This paper clarifies the principles and basic hypotheses, on which each technique is built. In the techniques using focal mechanisms and P wave first-motion data, the Wallace–Bott hypothesis that a fault slips in the direction of maximum resolved shear stress plays the principal role; basically, we search for a stress state that satisfies observed data on the basis of the Wallace–Bott hypothesis. On the other hand, the stress inversion technique using CMT data is not based on the Wallace–Bott hypothesis; instead, it is assumed that stress released by earthquakes is proportional to the stress tensor in the region surrounding the hypocenter. The characteristics and advantages of these techniques are also compared from physical and pragmatic viewpoints. It would be valuable to further improve these techniques, as well as to compare their performance using synthetic and actual data to clarify the differences and advantages of their characteristics in more detail.
Modeling deformation and stress states in the island arc's crust and mantle is reviewed and discussed, and examples of numerical modeling for northeastern Japan are presented. When modeling deformation and stress states of an island arc, rheological properties of the island arc—friction and flow laws for brittle and ductile regions, respectively—need to be assumed. Consideration of gravity as a body force is important because friction and flow laws depend on absolute stress. The interactions between a subducting oceanic plate and an island arc lithosphere also need to be considered. Based on recent observations, a detailed rheological structure of the island-arc crust and mantle can be developed. Through 3D modeling of deformation, by considering geothermal structure and nonlinear viscoelasticity and plasticity, long-term mountain-building and stress state in northeastern Japan can be reproduced. The stress state in the region from the trench to the island arc in northeastern Japan is modelled considering the interaction between the subducting Pacific Plate and the island arc lithosphere. Although the frictional strength of the subducting plate interface is quite low, the region where friction acts is quite wide. Therefore, a large compressional force acts on the island arc crust. In future work, the interactions of plates with realistic 3D geometries of plate interfaces need to be considered, as well as the rheological structure of the island arc inferred from various observational information, such as seismic velocity and electrical resistivity structures.