A close coupling between monitoring and modeling is needed for a quantitative prediction of volcanic eruption and earthquake occurrence. The need is demonstrated for the volcano, Piton de la Fournaise, for which a relatively simple model can be constructed because of the isolated tectonic setting and can be tested in a short time because of the high rate of eruptions. Our conceptual model (Fig. 2) explains various observations for each of the 49 eruptions since 1972 in terms of the varying conditions of the model elements, allowing inferences on causal relations between consecutive eruptive periods, recognized as alternating active and quiet ones, as well as predictions on the possible eruption scenarios for the future. Lessons learned from the volcano are applied to earthquake prediction in California, and we found a model of earthquake loading process by plate-driving forces that can be effectively constrained by data from the existing seismic network. The model identified that the periods of several years preceding the 1952 Kern County earthquake and the 1989 Loma Prieta earthquake are in the anomalous phase of the loading process. A companion paper by Jin et al. (2004) in the present issue shows that the 1992 Landers earthquake and the 1999 Hector Mine earthquake were also preceded by the period of anomalous loading process.

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Tokachi-oki earthquake in northern Japan, September 25, 2003, magnitude 8.1, was predicted six months in advance by a short-term earthquake precursor “chain” that reflects an increase of the correlation range among small earthquakes. This prediction is part of the ongoing test of a new short-term prediction method; the test covers territories of Japan, California, and Eastern Mediterranean. Qualitatively, precursory chain is a dense sequence of small earthquakes that had quickly extended over a long distance. A strong earthquake is expected within nine months after such chain is formed, in its formally defined vicinity. Chains are analyzed in conjunction with intermediate-term precursors, emerging with characteristic lead time of years. Methodology of prediction is named “Reverse Tracing of Precursors” (RTP), since precursors are considered in the reverse order of their appearance. That allows detecting short-term precursors not detectable with direct order of analysis. RTP was tested retrospectively for California, Japan, and Eastern Mediterranean, where 22 more strong earthquakes occurred during the time considered. The concept underlying RTP is interaction of lithosphere dynamics on different temporal scales. The described results enhance our fundamental understanding of lithosphere dynamics and, on the practical side, our capability for earthquakes preparedness.

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Two large earthquakes occurred during the last decade on Sakhalin Island, the M_ω7.6 Neftegorskoe earthquake of 27 May 1995 and the M_ω6.8 Uglegorskoe earthquake of 4 August 2000, in the north and south of the island, respectively. Only about five seismograph stations record earthquakes along the 1000 km, mostly strike-slip plate boundary that transects the island from north to south. In spite of that, it was possible to investigate seismicity patterns of the last two to three decades quantitatively. We found that in, and surrounding, their source volumes, both of these main shocks were preceded by periods of pronounced seismic quiescence, which lasted 2.5 ± 0.5 years. The distances to which the production of earthquakes was reduced reached several hundred kilometers. The probability that these periods of anomalously low seismicity occurred by chance is estimated to be about 1% to 2%. These conclusions were reached independently by the application of two methods, which are based on different approaches. The RTL-algorithm measures the level of seismic activity in moving time windows by counting the number of earthquakes, weighted by their size, and inversely weighted by their distance, in time and space from the point of observation. The Z-mapping approach measures the difference of the seismicity rate, within moving time windows, to the background rate by the standard deviate Z. This generates an array of comparisons that cover all of the available time and space, and that can be searched for all anomalous departures from the normal seismicity rate. The RTL-analysis was based on the original catalog with K-classes measuring the earthquake sizes; the Z-mapping was based on the catalog with K transformed into magnitudes. The RTL-analysis started with data from 1980, the Z-mapping technique used the data from 1974 on. In both methods, cylindrical volumes, centered at the respective epicenters, were sampled. The Z-mapping technique additionally investigated the seismicity in about 1000 volumes centered at the nodes of a randomly placed regular grid with node spacing of 20 km. The fact that the two methods yield almost identical results strongly suggests that the observed precursory quiescence anomalies are robust and real. If the seismicity on Sakhalin Island is monitored at a completeness-level an order of magnitude below the present one, then it may be possible to detect future episodes of quiescence in real time.

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This study attempts to estimate the short-term probabilities of large earthquakes in an offshore area of northeast Japan, where long-term probabilities are assessed based on earthquake intervals. Previous studies report foreshock activity in the area, which could be used for the estimation. From this point of view, a hazard function was constructed based on the concept of “potential foreshocks” of the study area. A total of 14 earthquakes that occurred between 1976 and 2000 are employed in an assessment of models. The probability of a target earthquake occurring at a point in the time-space domain depends on the number of small earthquakes in the vicinity of the point. The parameters for defining potential foreshocks are magnitude, spatial extent, and lead-time to the point, which is optimized by a maximum probability procedure. The most effective hazard function is achieved based on foreshocks of magnitude 4.5 and larger within 1 day and 20 km. A maximum probability gain of more than 20, 000 is obtained if there are two or more foreshocks. We started verification tests applying the model to data from January 1, 2001 to date, but conclusive results were not yet obtained due to the small sample size of target events.

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The principles of self-organized criticality (SOC) provide a framework for understanding the process by which individual earthquakes are generated. The SOC principles of fractality, scaling, hierarchy, and extreme sensitivity to initial conditions, are exhibited by the precursory scale increase (Ψ) phenomenon, which we interpret as evidence of a long-term generation process. We have accordingly included SOC in a three-stage faulting model of seismogenesis. Fractality is represented by the Gutenberg-Richter relation, which is relied on for analysing the precursory scale increase (Ψ) phenomenon. Scaling characterizes the parameters of space, time and magnitude that relate the precursory seismicity to the mainshock and aftershocks. The validity of these relations is supported by application of the EEPAS model. Scaling also underlies the Mogi criteria, which are invoked to explain a selfgenerated transient effect, and hence the long duration of the seismogenic process. Hierarchy clarifies the otherwise complex situations that arise when two or more earthquakes are in process of generation at overlapping places and times. Extreme sensitivity to initial conditions explains why, with rare exceptions, both the seismogenic process and the culminating earthquake are initiated with no recognizable immediate trigger. The only exception so far observed for the seismogenic process is the proposed triggering, on 1992.06.28, of the long-term Hector Mine (California) process by the nearby Landers mainshock.

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We discuss the problem of earthquake forecasting in the context of new models for the dynamics based on statistical physics. Here we focus on new, topologically realistic system-level approaches to the modeling of earthquake faults. We show that the frictional failure physics of earthquakes in these complex, topologically realistic models leads to self-organization of the statistical dynamics, and produces statistical distributions characterizing the activity, notably the Gutenberg-Richter magnitude frequency distribution, that are similar to those observed in nature. In particular, we show that a parameterization of friction that includes a simple representation of a dynamic stress intensity factor is needed to organize the dynamics. We also show that the slip distributions for synthetic events obtained in the model are also similar to those observed in nature

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A thorough discussion is made on what the rational constitutive law for earthquake ruptures ought to be from the standpoint of the physics of rock friction and fracture on the basis of solid facts observed in the laboratory. From this standpoint, it is concluded that the constitutive law should be a slip-dependent law with parameters that may depend on slip rate or time. With the long-term goal of establishing a rational methodology of forecasting large earthquakes, the entire process of one cycle for a typical, large earthquake is modeled, and a comprehensive scenario that unifies individual models for intermediate- and short-term (immediate) forecasts is presented within the framework based on the slip-dependent constitutive law and the earthquake cycle model. The earthquake cycle includes the phase of accumulation of elastic strain energy with tectonic loading (phase II), and the phase of rupture nucleation at the critical stage where an adequate amount of the elastic strain energy has been stored (phase III). Phase II plays a critical role in physical modeling of intermediate-term forecasting, and phase III in physical modeling of short-term (immediate) forecasting. The seismogenic layer and individual faults therein are inhomogeneous, and some of the physical quantities inherent in earthquake ruptures exhibit scale-dependence. It is therefore critically important to incorporate the properties of inhomogeneity and physical scaling, in order to construct realistic, unified scenarios with predictive capability. The scenario presented may be significant and useful as a necessary first step for establishing the methodology for forecasting large earthquakes.

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This paper discusses various slip modes on a plate boundary on the basis of a two-degree-of-freedom block-spring model and large-scale biaxial experiments, including a new experimental result on afterslip. We conducted slip experiments using large granite blocks with a pre-existing fault surface of 100 cm in length. Velocity-strengthening friction was given over a half of the fault length by inserting a thin Teflon sheet, while the other half retained velocity-weakening friction of the bare rock surface. Under a loading at a constant velocity, dynamic stick-slip repeated on the velocity-weakening region, causing afterslip on the velocity-strengthening region. The velocity-strengthening region experienced small coseismic slip as well, with the magnitude decreasing with the distance from the velocity-weakening region. The behaviors observed in the laboratory experiments were quantitatively simulated by a two-degree-of-freedom block-spring model, in which two blocks (Block 1 and Block 2) are connected by a liner spring and driven by a slowly moving driver. The friction on each block was assumed to obey rate and state dependent friction law. When a-b was assumed to be negative for Block 1, and positive for Block 2, afterslip occurred at Block 2. This model can also reproduce wide spectrum of slip modes by adjusting frictional parameters.

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We have investigated spatio-temporal variation in small repeating earthquake activity in the 1989 earthquake swarm and used them to infer quasi-static slip distribution on the plate boundary off Sanriku, northern Honshu, Japan. Seismicity and inferred quasi-static slip accelerations propagated to the west and to the south during the swarm activity to trigger the occurrence of the largest earthquake (M7.1). To explain the migration of the seismicity and inferred quasi-static slip acceleration, we propose a conceptual model named the ‘chain reaction model’ in which large earthquakes generate large afterslips and then the afterslips accelerate the ruptures of the nearby asperities to generate the next earthquakes which are also followed by large afterslips, and so on. The model is applicable to a plate boundary where asperities are located sparsely but the afterslip associated with the rupture of an asperity can reach nearby asperities. Aftershock area expansion, which is conspicuous off Sanriku, is also explained by the model. If we can evaluate the slip deficit of a large asperity correctly, we will be able to issue a warning of large earthquake occurrence in some areas when we detect the acceleration of quasi-static slip near the asperity although the prediction is inevitably probabilistic.

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In the past decade, nine silent earthquakes were documented along the Nankai and the Sagami Troughs in Japan, which form the northwestern margin of the Philippine Sea plate. They occurred in the stable-unstable transition zone at depths of around 30 km on the subduction interface and were segregated from major asperities of the 1923 Kanto, the 1944 Tonankai and the 1946 Nankai earthquakes. Their equivalent magnitudes were less than 7 and overall slips were less than 0.2 m, one-order smaller than those of the major asperities of ordinary great earthquakes. Moment release rates of the silent earthquakes were less than 10¹⁴ Nm/s, five-orders smaller than 10¹⁹ Nm/s of the great earthquakes. Two methodologies are attempted to obtain order of magnitude estimates of the roughness and friction parameter of source areas of some of the silent earthquakes. One method compares observed waveforms to synthetics with an empirical source time function based on laboratory experiment. The other relates sizes of silent earthquakes to the friction parameter a-b.

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We extended the analyses of temporal variation of coda Q^-1 and seismicity by Jin and Aki (1989, 1993) for central and southern California to year 2003. We use the relative frequency N(M_c) of earthquakes in a certain magnitude range around M_c characteristic to a seismic region to quantify the seismicity. The cross-correlation function between coda Q^-1 and N(M_c) is calculated using a 10-year moving time window. The correlation coefficient for the entire period of about 60 years is peaked at the zero-time shift with the value close to 0.8 for both regions. We found, however, the simultaneous correlation is disturbed before major earthquakes. The disturbance is, consistently, a delay in the change of coda Q^-1 relative to that of N(M_c) before the occurrence of a major earthquake. We attribute the temporal change in coda Q^-1 to fractures in the ductile part of the lithosphere and that in N(M_c) to the response of the brittle part to the ductile fracture. We believe that M_c characteristic to a seismic region is originated from a characteristic size of fractures in the ductile zone of the lithosphere. The observed delay of coda Q^-1 change relative to N(M_c) before a major earthquake can be explained simply by the strain energy stored in the brittle part of lithosphere reaching a saturation limit and starting to flow back to the ductile part.

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The nature and origin of the concentrated deformation zone along the Japan Sea coast (NKTZ: Niigata-Kobe Tectonic Zone) were clarified by analyzing various observations. We made a qualitative modeling for the stress state and deformation style in and around the NKTZ. In this model a weak zone with low viscosity exists in the lower crust beneath the NKTZ. In the surrounding region, however, the viscosity in the lower crust is very high and can be regarded as elastic for the periods of a recurrence interval of intraplate earthquakes. The concentrated deformation is basically attributed to the low viscosity in the weak zone. In more details, the concentrated deformation is thought to be generated by a postseismic deformation of the weak zone to the previous large intraplate earthquake in the interseismic period (the brittle-ductile interaction model) and/or anelastic deformation in both the upper and lower crusts in the NKTZ, under a finite constant force (the anelastic deformation model).

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Odawara City in central Japan, in the northernmost margin of the Philippine Sea (PHS) plate, suffered from severe earthquake disasters five times during the last 400 years with a mean repeat time of 73 years; in 1633, 1703, 1782, 1853 and 1923. In this region, non-volcanic Izu outer arc (IOA), the easternmost part of the PHS plate, has been subducted beneath Honshu (Japanese main island), and volcanic Izu inner arc (IIA) on the west of IOA has made multiple collision against Honshu. I hypothesize ‘West-Sagami-Bay Fracture’(WSBF) beneath Odawara, a north-south striking tear fault within the PHS plate that has separated the descending IOA crust from the buoyant IIA crust, through examinations of multiple collision process and the PHS plate configuration. WSBF is considered a blind causative fault of the 1633, 1782 and 1853 M 7 Odawara earthquakes, and is inferred to have ruptured also during the 1703 and 1923 great Kanto earthquakes simultaneously with the interplate main fault. A presumable asperity on WSBF just beneath Odawara seems to control the temporal regularity of earthquake occurrence. Though WSBF has not yet been detected directly, it is considered an essential tectonic element in this region, which might be a fracture zone with a few or several kilometer thickness actually. The WSBF hypothesis is the only conceptual model to explain the earthquake recurrence beneath Odawara.

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Takahiro Hagiwara, Professor Emeritus of the University of Tokyo, was born in 1908, and passed away in 1999. His name is inseparably tied with earthquake prediction, especially as the founder of the earthquake prediction program of Japan, and as a distinguished leader of earthquake prediction research in the world. This short article describes the career of Prof. Hagiwara focusing on his contribution to earthquake prediction research. I also sketch his activities in the development of instruments, and the multi-disciplinary observation of the Matsushiro earthquake swarm to show the starting point of his scientific strategy: good observation.

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The possibility of a great shallow earthquake (M 8) in the Tokai region, central Honshu, in the near future was pointed out by Mogi in 1969 and by the Coordinating Committee for Earthquake Prediction (CCEP), Japan (1970). In 1978, the government enacted the Large-Scale Earthquake Countermeasures Law and began to set up intensified observations in this region for short-term prediction of the expected Tokai earthquake. In this paper, two serious issues are pointed out, which may contribute to catastrophic effects in connection with the Tokai earthquake: 1. The danger of black-and-white predictions: According to the scenario based on the Large-Scale Earthquake Countermeasures Law, if abnormal crustal changes are observed, the Earthquake Assessment Committee (EAC) will determine whether or not there is an imminent danger. The findings are reported to the Prime Minister who decides whether to issue an official warning statement. Administrative policy clearly stipulates the measures to be taken in response to such a warning, and because the law presupposes the ability to predict a large earthquake accurately, there are drastic measures appropriate to the situation. The Tokai region is a densely populated region with high social and economic activity, and it is traversed by several vital transportation arteries. When a warning statement is issued, all transportation is to be halted. The Tokyo capital region would be cut off from the Nagoya and Osaka regions, and there would be a great impact on all of Japan. I (the former chairman of EAC) maintained that in view of the variety and complexity of precursory phenomena, it was inadvisable to attempt a black-and-white judgment as the basis for a “warning statement”. I urged that the government adopt a “soft warning” system that acknowledges the uncertainty factor and that countermeasures be designed with that uncertainty in mind. 2. The danger of nuclear power plants in the focal region: Although the possibility of the occurrence of a great shallow earthquake in the Tokai region was pointed out by CCEP in 1970, soon after, plans for construction of a nuclear power plant started in this region. Since 1976, Hamaoka nuclear power plants (Units 1-4) have been operating near the center of the expected focal region of the great Tokai earthquake, and Unit 5 is under construction. This is quite a dangerous situation.

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The Japanese national earthquake prediction program started in 1962 with a blue print for the scope and direction of research to follow. Substantial time and efforts were subsequently devoted to the construction of new observation networks and the study on the earthquake generation mechanisms. An important result has been the recognition of the great difficulty in identifying creditable precursors due to a diversity of earthquake generation process. In recent years, a new age of near real time observations of Earth's crustal processes by dense arrays of seismic and the GPS (Global Positioning System) stations has arrived. The results of the real time monitoring may lead to a new approach in the earthquake prediction research, i.e., the quantitative forecasting of the crustal activities. The new national program, which inherits its essential observational network from all the previous programs, emphasizes the importance of modeling as well as monitoring for a sound scientific development of earthquake prediction research.

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After the disastrous 1995 Kobe earthquake, a new national project has started to drastically improve seismic observation system in Japan. A large number of strong-motion, high-sensitivity, and broadband seismographs were installed to construct dense and uniform networks covering the whole of Japan. The new high-sensitivity seismograph network consisting of 696 stations is called Hi-net, while the broadband seismograph network consisting of 71 stations is called F-net. At most of Hi-net stations strong-motion seismographs are also equipped both at depth and the ground surface. The network of these 659 stations with an uphole/downhole pair of strong-motion seismographs is called KiK-net, while another network consisting of 1034 strong-motion seismographs installed at the ground surface is called K-NET. Here, all the station numbers are as of April 2003. High-sensitivity data from Hi-net and pre-existing seismic networks operated by various institutions have been transmitted to and processed by the Japan Meteorological Agency since October 1997 to monitor the seismic activity in and around Japan. The same data are shared to university group in real time using satellite communication for their research work. The data are also archived at the National Research Institute for Earth Science and Disaster Prevention and stored in their database system for public use under a fully open policy.

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The dense continuous GPS network of Japan, now called GEONET, has been operated since 1994 by the Geographical Survey Institute. GEONET provides precise daily coordinates of all the stations, with which displacement rates and strain rates are calculated nationwide. Various characteristics of tectonic deformation in the Japanese Islands have been revealed. GEONET is also quite useful in earthquake studies, precisely detecting co-seismic, post-seismic, and inter-seismic deformation signals. These observations are utilized to infer physical processes at earthquake sources. Slow slip events on plate boundaries have been found from GPS data. Such slow events provide an important constraint on the mechanism of faulting. On the other hand, there has been no success in detecting pre-seismic deformation. Lack of a precursory signal before the 2003 Tokachi-Oki (M8.0) earthquake has posed a serious question to short-term earthquake prediction. GEONET enables a good linkage between monitoring and modeling studies, opening a possibility of practical data assimilation. For further contribution to earthquake studies, it is necessary to continue GEONET with high traceability on the details in observation and analysis.

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