I review the recent development of seismographic system and some problems arising with it. Seismology in the present-day is in need of high sensitivity, more broad-band and more wide dynamic range seismic observations. New electrical technology has produced a micro-computer-aided recording system with a large memory and a highly resolvable analog to digital converter as well as a highly sensitive feed-back type seismometer, which have enabled us to make such observations that meet the above requirement.

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Progress in transmitting seismic wave data (i. e. telemetry) is reviewed with special emphasis on those in the past decade. After the pioneering stage in the 1950's and 60's, tele-seismometry became common by the middle of the 1970's, and then came into the new stage of digital transmission in the 1980's, especially with respect to the local and regional microearthquake networks. In both the cases of radio and telephone links, efforts were made to maximize the transmission rate with improved modems, and various methods for data compression were tried to send more data within the limited transmission rates. Efforts were also made to make network timing more accurate. Real-time data exchange between regional networks improved the monitoring capability of micro-seismicity in Japan. The use of communication satellites and the development of dial-up seismic stations are recent topics which seem to be hopeful.

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A review of seafloor observations of seismic waves is presented, highlighting progress in the 80's. Observations made by ocean bottom seismographs have proved superiority in the acquisition of better quality data than conventional surface hydrophone observations and in determining more accurate seismicity and seismic structure in ocean areas. The decade has seen developments of highly reliable instruments which can be deployed in tens of quantity. Efforts are continuing to broaden the observational window in the frequency range and dynamic range, which require to overcome noise problems linked to observational site and instrument coupling parameters. Long-term observation is also another important goal for the next generation of seafloor seismic instruments to construct a truly global seismic network.

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Internal structure of terrestrial planets and moons are best characterized by seismological method. In the present paper, we will briefly describe the seismological instrumentation and results obtained for the moon and Mars. Since the most significant seismological data were obtained by the Apollo seismic network, emphasis is put on the lunar study. The velocity structure of the shallow moon (<120km depth) has been determined by the analysis of man-made impacts. On the other hand, the velocity structure of the mantle (120km-1, 000km) has been determined by using 41 deep moonquakes, which periodically occur at depth from 800km to 1, 150km. The velocity structure at depth below 1, 000km has not yet been established. Several petrological models are made by using the velocity structure of the mantle and other geophysical constraints such as the density distribution, moment of inertia, temperature distribution, and elastic data of the candidate minerals of the moon. These petrological models, however, are not so accurate that they do not constrain the formation process of the moon. The more detailed information on the lunar interior such as the size of core is required for clarifying the formation process of the moon. The forthcoming Japan lunar penetrator (LUNAR-A) mission, which is planned to be launched in 1996, will provide useful data on the lunar interior. A new seismic network of triangle form with about 5, 000km length will be constructed by the penetrator seismometers. We expect the results by the LUNAR-A mission to clarify several unsolved problems of the deep sturucture of the moon.

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Geophysical models are generally estimated from observed data by using some criterion. In most cases observed data are inaccurate, insufficient and inconsistent. To deal with such data we must introduce a new criterion in place of the classical least-squares criterion. The history of the development of inversion theory in geophysics is the history of quest for the new criterion. From such a standpoint we review the development of inversion theory in the last score.
In a series of papers published in 1967, 1968 and 1970, G. Backus and F. Gilbert introduced the concept of model resolution and formulated the geophysical inverse problem as the problem of compromising reciprocal requirements for model resolution and estimation errors in some natural way. This was the starting point of the fruitful development of inversion theory in geophysics. Various criteria to compromise model resolution and estimation errors have been proposed in the early part of the 1970s, but the objectivity of these criteria were not clear. At the end of the 1970s D. D. Jackson introduced the concept of prior information about unknown model parameters and combine two reciprocal criteria for model resolution and estimation erros in a natural way. Jackson's approach for linear inversion, which is based on the minimum variance criterion, has been soon exteded to nonlinear cases by A. Tarantola and B. Valette and also by D. D. Jackson and M. Matsu'ura on the basis of probability theory. The geophysical data inversion is now understood as the process of extracting new information from observed data, combining it with prior information about model parameters, and constructing the more clear image of a physical model.

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We review recent progress in the theory on free oscillations of an aspherical Earth. Effects of asphericity (rotation, hydrostatic ellipticity, lateral heterogeneity and anisotropy) are treated in terms of the perturbation theory. For large angular order, free oscillations are related to surface waves.

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It is believed that seismic anisotropy represents dynamics of the Earth's interior, directly reflecting either instantaneous stress, cumulative strain, or deformation of in-situ rocks. A shear wave passing through an anisotropic elastic medium splits into two orthgonally polarized quasi-shear waves with different propagation speeds. This phenomenon is called shear wave splitting, and is useful to understand the Earth's anisotropic fabric, with potential advantages as high lateral resolving power and relatively low sensitivity to seismic wave velocity heterogeneities. During the past decade, a variety of anisotropy-induced shear wave splitting has been observed in many different fields of seismology, indicating that anisotropy is an ubiquitous feature in the Earth's crust and upper mantle. In this review I summarize recent observations of shear wave splitting, with special emphases on their geophysical implications. I also discuss several problems concerned with shear wave splitting analyses, which are expected to be solved in the near future.

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Seismic waves are refracted, diffracted and/or reflected by random inhomogeneities which exixt in the real earth medium. Coherent phases have been well studied especially for the structure study of the lithosphere, where discrete impedance contrasts are supposed. Array obsevations made it possible to measure the precise velocity structure by tomography. Complementary to such a deterministic approach, the random inhomogeneous structure of the earth medium has been statistically studied, where a focus is put on the systematic change in amplitudes of incoherent phases of seismic waves. A latter part of S coda first attracted seismologists to its systematic decay in amplitudes, which offered the way to evaluate quantitatively the short wavelength components of randomness since coda waves are excited due to their large angle scattering. Several models have been proposed to simulate the excitation of coda waves: a single scattering model; a diffusion model; and an energy flux model. The scattering coefficient g which characterizes the coda excitation and coda Q^-1 which characterizes the decay in coda amplitude have been often measured in various areas in the world as tectonic parameters. The energy conservation law suggests that the excitation of coda waves mean the reduction in amplitude of direct waves: the seismic energy of direct waves flows into a coda portion with the increase in propagation distance. Scattering due to random structure has been considered as a dominant mechanism of attenuation. Frequency dependence of attenuation and coda excitation has been studied in relation with the spectral structure of randomness. Recent observations report that Q_s^-1 decreases with frequency for frequency higher than 1Hz. A theoretical model predicts that Q_s^-1 is proportional to the reciprocal of frequency when the randomness is characterized by the exponential autocorrelation function. The separation of intrinsic loss and scattering loss from total attenuation is one of the most important subject. Seismic waves which are pulse-like in time when radiated from the hypocenter collapse with travel distance. Such a characteristic has been studied in relation with the diffraction and multiple forward scattering due to long wavelength components of velocity inhomogeneities. The parabolic approximation well explains the broadening of seismogram envelope and the delay in the maximum amplitude arrival from the onset of direct wave. We have to examine seismograms of not only a short time window around the direct wave but a long time window starting from the direct wave to the end of coda. That is, it is very necessary to make more sophisticated models for full seismogram envelopes adopting the difference in randomness related to seismo-tectonics.

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Recent studies on tsunamis are reviewed. Theoretical studies on tsunami generation and propagation have been done both on coupled and decoupled systems of ocean and solid earth. Both approaches are proved to be equivalent. Hydrodynamic formulation, on the basis of decoupling model, is discussed in detail. Several models have been proposed for the mechanism of tsunami earthquakes that produce abnormally large tsunamis compared to seismic waves. After the 1983 Japan Sea tsunami, some progress has been made on tsunami research. Among them are re-examination of tide-gauge system, studies on free oscillation of the Japan Sea and newly developed inversion technique of tsunami waveforms to estimate complex fault motion. Large-scale realistic computation of tsunamis can be made very rapidly and has potential usage for tsunami warning system. These seismology-related problems are described.

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Analysis of the Earth's free oscillations using high-quality digital seismograms accumulated over the past 15 years has revealed the large-scale aspherical structure of the Earth. Recently modes of low harmonic degree have been analyzed using a technique of iterative inversion in which spectra of a narrow frequency band which contains a mode of interest are fitted. The modes analyzed fall into two categories: (1) normally split modes which are dominantly sensitive to the structure of the mantle and (2) anomalously split modes which are dominantly sensitive to the structure of the outer and inner cores. Relatively simple mantle models are capable of explaining the structure coefficients of the normally split modes, and these models are quite similar to the models derived from traveltime data. However, all the structure coefficients of the anomalously split modes have not yet been given good fits by a simple aspherical model. While a model including anisotropy of the inner core can give better fits to some of the most anomalous modes, what we can safely say at present is that there is degree-2, axisymmetric asphericity somewhere in the deep Earth. It should be also kept in mind that the core has not yet been given its correct spherically symmetric model.

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The present paper reviews recent studies on the seismological structure in the Earth's core inferred from analyses of seismic body waves. Based on the recent interpretations on the precursors to the PKIKP and the analyses of the PKP waveforms at distance range from 125 to 150 degrees, it is generally accepted that there is no transition layer (F zone) between the inner core and the outer core. It is supported by the rusults of the extremal bounds on the P wave velocity which were also evaluated using the τ-method and the linear programming method. The anelasticity in the outer core is very small, but that in the inner core is very large, especially at the upper part of the inner core, where the process of the solidification is thought to be occurring. The velocity structure in the outer core is homogeneous and isotropic, whereas that in the inner core may be heterogeneous and/or anisotropic. Using the travel time anomalies from ISC bulletines, the hypothesis that the velocity in the inner core may be faster in the N-S direction than that in the E-W direction, has been proposed. The core-mantle boundary topography is obtained by inverting arrival time data of PcP, PKP and PKKP. The CMB topography estimated from PKKP is smoother than that from PcP and PKP.

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This paper reviews recent advances in our knowledge and understanding of the deep structure of the Earth's mantle. Subjects reviewed are mantle discontinuity, aseismic slab, lateral heterogeneity, mantle-core boundary, upper mantle anisotropy, global Q structure.

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Three-dimensional velocity structure beneath island arcs, especially beneath Japan arc, has been investigated in detail by many studies in the last dacade. The low-velocity zone beneath active volcanoes and the high-velocity slab have been delineated by using the block inversion technique. The location of the upper boundary of the descending slab has been estimated precisely by the use of some kinds of converted or reflected waves; the boundary has been located just above the deep seismic zone. In some regions, an oblique thin low-velocity layer has been found on the descending high-velocity slab.

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Deep reflection profilings of the continental crust have been revealing major structures in various types of tectonic settings in the world. Most of these surveys in the 1980's have been carried out by using the Vibroseis technique, which have clarified the detailed characteristics of the crust and the upper mantle. With regard to the Vibroseis method, phase and force control techniques based on the concept of ground force were introduced to produce a stable source signature. Diversity stack which improves signal to noise ratio and migration techniques which map dipping reflectors precisely have helped to improve data quality. Transects across genetic mountain belts produce several types of seismic fabrics which reflect the different geological settings. In particular, the reflective lower crust (RLC) is commonly found in the Phanerozoic crust, while the upper crust and the mantle are relatively transparent. The Moho reflections are found to have relatively continuity and flat orientation beneath complex crustal blocks which have undergone major orogenic deformations. Some of the mid-crustal bright spots are interpreted to be intrusive molten magma bodies. Several scientific drilling programs have been planned to obtain direct information of the continental crust, which are supplemented by deep seismic reflection profilings. Transects across several geological provinces will reveal not only local characteristics but also regional tectonics. Deep seismic reflection surveys in Japan have just commenced, and they are expected to reveal the deep crustal structure which is considered to be more complex than that in other areas.

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This paper reviews recent development of a study of oceanic crust. An old image of crustal structures is replaced by a modern picture with a large amount of new observations. Basically the classical three-layered crustal model is acceptable if we regard it as an averaged structure with depth. The detailed modern model provides a clue to understand crustal dynamics and an origin of the oceanic lithosphere. We are now able to image a magma chamber beneath the mid oceanic ridge, subducting oceanic crust at trench areas, and plutonically solidified mantle melt on passive margins.
A new technology of controlled source seismology includes waveform observation with well-calibrated amplitudes and phases, a two-ship method with multichannel expanding seismic profiling and an ocean bottom seismograph. These methods enable us to use spatially dense seismic data, which make it possible to investigate a detailed structure of the oceanic crust varying vertically and laterally.

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This paper reviews recent experimental studies on phase transitions at high pressure, determination of elastic and thermodynamic properties of mantle and core materials conducted in this decade. These data enabled us to assess the chemical and mineralogical structure of the mantle and core. The observed density and seismic wave velocity profiles of the upper mantle and transition zone can be explained by phase transitions in peridotite. On the other hand, the chemical composition of the lower mantle is not well constrained by the present experimental data, due to a large uncertinty in the pressure and temperature derivatives of the physical properties of the high pressure phases and a temperature distribution in the lower mantle. Both a homogeneous peridotite mantle and a chemical stratified mantle with a silica-rich lower mantle can be consistent with the observed seismic data. Experiments to determine the melting relations of the mantle materials and element partitioning at ultrahigh pressure are also reviewed. The MORB source upper mantle derived from the MORB chemistry has chondritic abandances of the refractory lithphile elements and it shows no clear evidence of ultrahigh pressure fractionation. Lack of evidence of the ultrahigh pressure fractionation, however, does not necessarily conflict with the model of global melting in the accretional stage of the earth. Solid state convection might have homogenized the stratification formed by the melting and fractionation during the accretional stage of the earth. Some komatiites observed in the early Archean (3.5-3.7 Ga old) show chemical evidence derived from non-chondritic source mantle, which was depleted in majorite. Various high pressure experiments have been conducted in this decade to discuss the chemical and physical constitution of the core. High pressure studies made by a multianvil press in the systems of iron and light elements such as hydrogen, oxygen, sulphur, and carbon imply that these elements can be dissolved into the molten iron, and thus they are the strong candidates for the light elements in the core. On the other hand, extension of the pressure range generated by the diamond anvil cell made it possible to conduct melting and iron-silicate reaction experiments under the core conditions, although the results are still preliminary and qualitative at present. These results provides important constraints on the temperature profile and the chemical structures in the lower part of the mantle and the core.

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Understanding of the structure of electrical conductivity of the Earth has progressed much in recent 20 years. This is due to the advancements in 1. data acquisition in geomagnetic/geoelectric exploration, 2. data reduction technique aided by large computers and 3. mineralogical research including high pressure/high temperature experiments. Geomagnetic/geoelectric explorations has clarified that the Earth's crust and mantle are heterogeneous in conductivity structure. The origins of these heterogeneities are reviewed and several representative cases of the heterogeneities are described.

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Isotopic compositions of MORBs and OIBs indicate presence of large scale chemical heterogeneities in the earth's mantle. More than at least four independent source materials must be present in order to account for Rb-Sr, Sm-Nd and Pb-Pb isotopic variations in MORBs and OIBs. Distribution of different source materials in the mantle, however, is highly debatable. Because current geochemical discussions on mantle heterogeneities are based on isotopes and trace elements, direct comparison with the 3-D seismic tomography is unwarranted. In order to make such comparison, a theory must be developed which will connect the mantle heterogeneity in isotopes with the compositional heterogeneity in major elements and mantle mineralogy. Seismic tomography shows that Archean cratons have deep roots up to 200km which are characterized by anomalously high P and S wave velocities yet have densities lower than ambient mantle. Petrologic studies on mantle xenoliths in south African kimberlites show that the sub-continental lithosphere beneath the Archean craton consists of very depeleted garnet peridotites. These depleted peridotites may have been formed as residues after extensive extraction of komatiitic magma in the Archean mantle, and they may have been separated from convective mantle materials for a long geologic time due to their low intrinsic densities.

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Following a brief summary of the progress in the study of brittle deformation (frictional sliding), recent developments in the laboratory studies of ductile rock deformation are summarized with the emphasis on the following topics: (1) the effects of phase transformations, (2) the effects of fluids, (3) deformation microstructures including the microstructural paleopiezometers and the preferred orientation, (4) shear instability and the origin of deep earthquakes. An important observation is a correlation between plastic properties and crystal structure and/or rock type, which suggests rheological stratifications in the Earth. A well established example is the rheological stratification of continental lithosphere in which a soft lower crust is sandwiched between stronger upper crust and upper mantle. Another example is a strong garnet-rich layer, which might occur at the top of subducting slabs and/or at the deeper part of the transition zone. The development in the study of the role of fluids has been significant, major progress being the better characterization of fundamental thermodynamic properties associated with the presence of fluids (solubility and diffusivity of hydrogen, wetting angle between fluids and minerals). The study of deformation microstructures has provided with some useful tools to infer the stress magnitude or strain pattern in the Earth. Finally, current progress in the understanding of the origin of deep earthquakes is critically reviewed.

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Our understanding of three dimensional mantle structure has been greatly advanced in the last decade. We review recent seismological studies on the Earth's interior with special emphasis on implication for mantle convection. Studies reviewed in the present paper involve large-scale seismic anisotropy, large-scale mantle heterogeneity, fine structure and topography of the 650km discontinuity and the core-mantle boundary, and the slab penetration into the lower mantle.

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The recent developments in the theories and applications of moment tensor inversions are reviewed. A special attention is paid to the observability and interpretation of non-double couple components of moment tensors. Some seismic events which cannot be modelled by moment tensors are also discussed.

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This paper reviews studies on the scaling of rupture size from earthquake to microfracture. The studies were published mainly in the 1980s. These investigated whether the cube-law (M₀∝f₀^-3) holds for smaller events (M₀: seismic moment, f₀: corner frequency). It is found that the cube-law appears to be incorrect for earthquakes and microfractures of which M₀ are less than 10²¹ dyne·cm. Croner frequencies of these events are lower than those expected from the M₀-f₀ relation which holds for large earthquakes. Almost all the studies attributed this fact to small stress drops. They calculated the fault dimension from f₀, assuming that the rupture velocity is constant independently of earthquake size. However, the rupture velocity of small earthquakes has been never determined. The rupture velocity of a microfracture in the mine was estimated as 0.4km/s; this value is much smaller than that of large earthquakes. The lower corner frequency is possibly generated by lower rupture velocity.

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Rock mechanics experiments in a laboratory have provided useful information on the physical process of earthquakes. Our knowledge of rock fracture has increased rapidly because of recent developments in experimental equipment and techniques. Acoustic emission activity, spatial variation of AE hypocenter distribution, and focal mechanism solution of AE have the same properties as natural earthquakes. These suggest the processes earthquakes and microfracturing are related. However, the large time-scale and space gaps between laboratory experiments and real earth phenomena must be bridged in order to apply the results of laboratory experiments to natural earthquakes. Time-dependent fracture properties of rocks and frictional behavior have been described by a strain-rate or slip-rate dependent law. Size-scale dependence of physical parameters is controlled by the inhomogeneous structure of rocks and/or surface roughness of fault.

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Recent studies on source complexity such as multiple shock nature of large earthquakes are reviewed. Brief notes on the historical background as well as inversion techniques for complex body waves are also included. Through the body wave inversions, complex rupture patterns have so far been obtained for individual earthquake sources. The source complexity is characterized by the following three properties: (1) discreteness of rupture propagation; (2) heterogeneity of stress distribution; (3) hierarchy of rupture elements. These properties suggest the existence of some rate-determining processes which may be associated with ‘ductile’ barriers surrounding ‘brittle’ asperities.

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Use of near-field seismograms makes it possible to represent the rupture process of a target earthquake indetail. Nevertheless, few sets of strong motion data have been obtained in fault regions yet. This paper describes the observations of strong ground motions of some earthquakes occurring in the recent 20 years and reviews the results of the analyses of these data in the context of strong motion seismology. Some new aspects of observing and analyzing strong motions are discussed for seismology and engineering seismology in the next decade.

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Recently, telemetered microearthquake observation networks have been developed, and data acquisition and processing procedures have been automated in many networks, including automatic hypocenter determinations by using computers. As a result the detection capability and the accuracy of hypocenter determination of microearthquakes have been tremendously improved, which made a great advance in detailed microearthquake studies. We present a brief review on recent studies of seismicity based on microearthquake observations, focusing on spatial and temporal distributions of microearthquakes and their related problems.

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Hydro-geochemical anomalies associated with large earthquakes are reviewed and discussed from the viewpoints of (1) the response of confined groundwater to minute deformations of crustal rocks, (2) ground-water anomalies related to earthquake source mechanisms, (3) origins of groundwaters and dissolved gases in them, and (4) factors to cause groundwater gas anomalies.
Theoretical models for deformations of water-saturated porous or cracked rocks under the undrained condition indicate that compressive and extensive stress changes induce increasing and decreasing changes in the pore water pressure, respectively. The pore pressure change is to be the same order of the external stress change, but the maximum change of pore pressure cannot exceed the stress change. These are consistent with observed responses of confined groundwaters to the earth tides.
Coseismic groundwater anomalies accompanied by the 1946 Nankaido earthquake and the 1923 Great Kwanto earthquake are directly related to their residual fields of megathrusting faults. Preseismic groundwater anomalies of these great earthquakes commonly suggest that megathrusting fault motions began to start preceding main seismic events.
Chemical compositions of groundwater gases are controlled by admixing of the dissolved atmospheric air with the subsurface gases enriched in CH₄, He and others. Groundwater gas anomalies can be described by the change in mixing ratio of the two different gases and/or by the compositional change of the admixing subsurface gas itself. Such an admixing process could be affected by anomalous pore pressure distributions induced by the deformation of crustal rocks related to earthquakes. H₂ gas anomalies are distinctive among subsurface gas anomalies reported so far, but the origins are not fully understood. The characteristics of H₂ anomalies are discussed.

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Recent researches on crustal deformation are briefly reviewed in relation to earthquake studies and other geophysical researches. Main topics discussed in this article are as follows;
(1) Based on the elasticity theory of dislocation, simple analytical expressions and computational tools were developed to estimate static source parameters from the surface deformation data. Starting from this simple source model, further developments were made in chiefly three courses of studies: (a) introduction of inversion technique to directly derive source parameters from geodetic data, (b) introduction of quasi-static viscoelastic process to interpret post-seismic relaxation process, (c) examinations on stress redistribution due to an earthquake and the interaction of active faults.
(2) Seismic cycles are discussed with special focus on the coastal deformation in the southwest Japan. From time intervals of a cycle and seismic uplift data, the time-predictable model was proposed. The mechanism of accumulation of remnant deformation in the seismic cycle is still open to question in spite of various proposed mechanisms.
(3) Developments of space techniques were briefly reviewed; they are Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Global Positioning System (GPS). Recent results from these new techniques may have capabilities to provide sufficient data to discuss not only overall plate motions but also detailed intraplate deformations. These techniques are opening a new era for the crustal deformation researches.

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The modern seismology took over the classical seismology in 1950s. The former is based on the analysis of individual earthquakes, while the latter took the statistical approach as a whole in understanding earthquakes. The earthquake phenomenon is a good example of fractal, in which the part is as complex as the whole. For understainding this type of complex phenomena, a new view is required other than the reductionist's (modern) view or the wholist's (classical) one. The whole is not a mere sum of the part, due to the nonlinear interaction among the parts. Recent advances in the nonlinear dynamical system theory are clarifying the various effects of nonlinear interactions. Earthquakes are re-examined on this line. In order to explain power-law relations known in seismology, earthquakes are viewed as a self-organized critical phenomenon (SOC). Earthquakes occur as an energy dissipation process in the earth's crust to which the energy is continuously input due to plate tectonics. The crust self-organizes into the critical state and the temporal and spatial fractal structure emerges naturally. Power-law relations are the expression of the critical state of the crust. The SOC model for earthquakes explains the Gutenberg-Richter relation, the Omori's formula of aftershocks and the fractal distribution of hypocenters.

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In Greece, astonishing success has reportedly been achieved in predicting earthquakes by monitoring the geoelectric potential changes. The method, called the VAN-method taking the initials of Varotsos, Alexopoulos and Nomikos, claims that earthquakes with magnitude greater than ca. 5 occurring in Greece can be predicted within the errors of 100km in epicenter and 0.7 in magnitude. The lead time is between several hours and ca. 20 days. The actual success rate and alarm rate for the recent one year are both estimated to be about 60%. Some technical aspects and the outline of suggested physical mechanism of the method are reviewed. A brief introduction of our attempt to apply the method in Japan is also given.

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