Effects of the hanging wall and footwall on peak acceleration during the Jiji (Chi-Chi), Taiwan Province, earthquake

The M=7.6 Jiji (Chi-Chi) earthquake, Taiwan Province, on September 21, 1999 (local time) is a thrust fault style earthquake. The empirical attenuation relations of the horizontal and vertical peak ground accelerations (PGA) for the Jiji (Chi-Chi) earthquake are developed by regression method. By examining the residuals from the Jiji (Chi-Chi) earthquake-specific peak acceleration attenuation relations, it is found that there are systematic differences between PGA on the hanging-wall and footwall. The recorded peak accelerations are higher on the hanging-wall and lower on the footwall. The clear asymmetry of PGA distribution to the surface rupture trace can also be seen from the PGA contour map. These evidences indicate that the PGA attenuates faster on the hanging-wall than on the footwall. In the study of near-source strong motion, seismic hazard assessment, scenario earthquake and seismic disaster prediction, the style-of-faulting must be considered in order that the attenuation model can reflect the characteristic of ground motion in various seismic environmental regions.

Characteristics of frequency content of near-fault ground motions during the Chi-Chi earthquake

The frequency content of earthquake ground motion is very important because it affects the dynamic response of structural systems. In the paper, we use five scalar parameters (the response spectral predominant period Tp, smoothed spectral predominant period To, Fourier amplitude spectral mean period Tm, equivalent pulse period Tv, and the pseudo-velocity spectral predominant period Tpv) reflecting the characteristics of frequency content of strong ground motion to examine the near-fault three-component motions during the 1999 Chi-Chi earthquake. The result indicates that the frequency content of near-fault motions at the Hanging wall is less than that at the foot wall; Tp shows a smaller value than that of To and Tm and it emerges a reverse relation of three-component motions as compared with that of To and Tm; Tv and Tpv of the near-fault motions at the north end of the rupture display a similar trend to that generated by the rupture directivity effect of strike-slip faulting. We therefore conclude that these observations are useful in the formulation of near-fault design spectra for seismic codes and in zoning studies in seismic risk.

Slope safety control during mining below a landslide

The East Open Pits of Daye Iron Mine were closed in year 2000 and the ore beyond the pit limit was planned to be mined by underground ways. In order to facilitate the smooth transition to underground mining, hanging-wall ores in the old pits were scheduled to be mined. Xiangbishan pit is one of the four mining spots selected. In 1979 when the pit was mined to around 36 m a.s.l., a landside of around 70000 m3 occurred in its North Slope, leading to the closure of the pit. The pit was then backfilled to 60 m a.s.l. to maintain the slope stability. The hanging-wall ore to be mined is just located below the old slide, so whether the slide will be reactivated and how to control its deformation and stability are the key issues concerned. Based on geological investigation and numerical analysis, it was predicted that the slide would be reactivated and a control measure using the intact ore body as a retaining wall was thus suggested. During mining, displacements were monitored to assure safety. By July, 2003, 383000 tons of ore were successfully mined. Though the observed displacement reached 8795 mm, no major failure happened. The final slope is 110 m high with an angle of 70°.