UrEDAS, Urgent Earthquake Detection and Alarm System, is an integrated real-time earthquake warning system using a single station with three-component seismometer. Detecting the initial P-wave of an earthquake, UrEDAS judges the destructive potential of the impending earthquake on the basis of the relation between magnitude and epicentral distance of earthquake, and issues a necessary warning within four seconds after the P-wave arrival for a certain area which may sustain damage. UrEDAS is the fastest earthquake alarm system in the world, practically in use since 1988. At the arrival of S wave, it improves the estimates and updates the warning. When several UrEDAS units are used, the data from each unit are synthesized to produce more definitive information.
HERAS, Hazards Estimation and Restoration Aid System, estimates the degree of damage caused by an earthquake based on the synthesized information from UrEDAS units within a few minutes using the relation between earthquake strong motion and damage for the prompt correct countermeasures just after the earthquake. The prototype system of HERAS completed in 1992.
As a method to estimate the degree of damage, usefulness of vulnerability indexes K values are proposed in 1992-1995. K values can be derived accurately from microtremors measured easily and quickly by using PIC, Portable Intelligent Collector, developed in our laboratory in 1986.

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A theoretical relation between the characteristic periods and peak values of strong ground motions is derived based on random-vibration theory. The relation, summarized as a simple form of theorem, is analogous to the one between period and amplitude of a harmonic motion. The theorem leads to the bounded prediction of central periods whose upper and lower limit values are dependent only on peak acceleration, peak velocity and peak displacement. Following the confirmation of its validity in comparison with observed strong-motion records, the theorem is further expanded to theoretically scale acceleration, velocity and displacement spectra in seismic source area.

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The orientation error of accelrographs in borehole array stations was discussed using the records obtained during the South Hyogo Earthquake of Jan. 17, 1995. The four stations, three of them looted on reclaimed land and the other on stiff ground, were selected The displacement orbits on the horizontal plane were carefully examined, and significant orientation errors were detected The significance of the correction of orientation error was discussed The effective stress-based 2D and/or 3D liquefaction analyses were applied for one of the array stations. It was demonstrated that the method could reproduce the records well in the case that the orientation-corrected base motion was used as the input motion.

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This paper presents a safety check method of steel frame check dam by using an elastic-plastic reliability analysis method. The proposed method makes use of the holonomic elastic-plastic analysis to combine with the first order reliability analysis method. In order to combine the holonomic elastic plastic analyisis with reliability analysis into one formulation, the conventional holonomic elastic plastic analysis formulation is modified in cojunction with the elastic-plastic mode generation method. Then, the first order reliability method based on elastic-plastic displacement limit state is formulated as a mathematical programming problem. Furthermore, it is pointed out that some collapse modes are defined as an excessive displacement limit, and the safety check method for this limit state is also proposed by using the modified reliability analysis method. Finally, The numerical examples of the three bar truss and the steel frame check dam subject to uncertain loads show the validity and the applicability of the proposed method.

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The elasto-plastic behavior of steel structures under cyclic loading is one of the significant subjects to estimate design criteria and design procedures based on the ultimate state. Analyzing the cyclic response of steel structures, it is necessary to estimate a mechanical model which can represent well a cyclic stress-strain relationship of steel and numerical techniques for the solution of non-linear structure problem. In this paper, we propose a solution method for an elasto-plastic large deformation analysis under uniaxial cyclic loading, which introduces a new iteration technique of minimizing the residual displacements for correcting residual force and a modified two-surface model as a constitutive equation. A few numerical simulations are presented to show the accuracy and efficiency of the proposed method.

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S-wave records for 17 earthquakes observed by a dense accelerograph array (Chiba array) are analyzed to investigate the source, path and site effects on polarization of strong ground motion. The polarization characteristics averaged over the array do not agree with those expected from the focal mechanism because of the wave scattering effect along the path. The direction of the average polarization varies with frequency, and the degree of the average polarization decreases with increasing in the earthquake magnitude and the hypocentral distance. Each of the observation points shows its own local site effect on polarization of strong ground motion. It is suggested that the local site effect is caused by some anisotropic property of the surface layers at the Chiba array site.

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