House Damage by and Simulation of Ground Motions of the 1997 Magnitude 6.1 Northern Yamaguchi Prefecture Earthquake :

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  • KANAORI,Yuji
    Department of Earth Sciences, Faculty of Science, Yamaguchi University
  • SUGITO,Masata
    Department of Civil Engineering, Faculty of Engineering, Gifu University
  • YAMAMOTO,Tetsuro
    Department of Civil Engineering, Faculty of Engineering, Yamaguchi University

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Abstract

Distribution of aftershocks and data on house damage caused by the magnitude (JMA scale) 6.1 Northern Yamaguchi Prefecture earthquake on 25 June of 1997 are reviewed, and the peak accelerations and seismic intensities of ground motion on freely engineered foundations, defined as having a shear-wave velocity of 500 to 600 m/sec, were simulated using the technique of non-stationary strong motion. Houses damaged by the earthquake are in a circular region with a radius of approximately 10 km that has its center the community of Ikumo-nishibun which sustained the most severe house damage. A zone of aftershocks that accompanied the main shock developed along the NE-SW-trending Sakota-Ikumo fault, which previously had been identified by geological mapping. Taking the aftershock zone into account, a vertical plane with an area of 10 × 10 km and a depth ranging from 5 to 15 km was adopted as the fault model of the Northern Yamaguchi Prefecture earthquake. This fault plane is subdivided into nine (3 × 3) small faults. Earthquake rupture is assumed to have initiated upper from the small fault bottom in the northeast and to have propagated upward to the southwest fault. Seismic ground motions on engineered foundations were simulated by integrating the evolutionary spectra of the constituent small faults as a function of elapsed time. A comparison of the simulation results and observed records of the Northern Yamaguchi Prefecture earthquake show (1) the simulated peak acceleration and seismic intensity of the ground motion on engineered foundations are consistent with the values of the surface observations, (2) the circular pattern of house damage caused by the earthquake is well reproduced by the simulation, and (3) observation sites with high peak accelerations and the seismic intensity from the epicenter are aligned northeastward along the active fault system. The high acceleration and seismic intensity appears to be attributable to the heterog eneity and weakness of the foundation rocks due to the existence of the fault.

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