214 Proc. Japan Acad., 71, Ser. B (1995) [Vol. 71(B), Extremely High Damage Potential Comparison of the Hyogo-ken of Near Field Earthquake Ground Nanbu and the Northridge Earthquakes Motion By Hirokazu IEMURA Department of Civil Engineering, Kyoto University, (Communicated by Shunzo OKAMOTO, M. J. A., Sept. Kyoto 606-01 12, 1995) Abstract : The Hyogo-ken Nanbu Earthquake of January 17, 1995 caused severe damage to buildings, highway bridges, railways, lifeline systems, port facilities, and so on. This event is the first instance in which engineering structures which were designed for the highest seismic forces in the world have been subjected to such destructive ground motions. This paper shows several calculated response spectra for the Kobe record together with the Sylmar record obtained during the Northridge Earthquake, which occurred on exactly the same day of 1994. Comparing these response spectra with those for previously obtained historical earthquake records, extremely high damage potential is revealed for near field earthquake ground motion. Urgent retrofit of old structures and re-examination of present seismic design codes are essential. Key words : Earthquake ground motion; spectrum; inelastic displacement response. near field; damage potential; evolutionary power Outline of the Hyogo-ken Nanbu Earthquake. The Hyogo-ken Nanbu earthquake (the name officially given by the Japanese Meteorological Agency (JMA), indicating the location of the epicenter and the fault zone) was of magnitude 7.2 and occurred at 5:46 a.m, on January 17, 1995 in the Hanshin area, one of the most densely populated areas in Japan. However, as the extent of the damage caused by this earthquake was revealed by the intense media coverage and the investigations following this earthquake, the "Great Hanshin (-Awaji) Earthquake Disaster" has become widely referred to as the worst natural disaster in Japan since the 1923 Great Kanto earthquake. This Magnitude 7.2 earthquake was felt throughout the western part of Japan, as well as in the Kinki area. The epicenter was located in the Awaji-shima island near the Akashi strait. The identified location of the epicenter by JMA is Latitude=34.6 N, Longitude= 135.0 E, with a focal depth of 20 km. The fault rupture penetrated to the ground surface in Awaji-shima. The visible ground rupture indicates the fault's right-slip movement, as confirmed by the analysis of the measured seismic records. The movement of a segment of the Nojima Fault, followed by that of the Rokko fault and others on the Honshu side, is inferred to cause this earthquake; this conjecture is also supported by the distribution of aftershock hypocenters. Although earthquakes of this magnitude were not unexpected in this area by researchers, it is the largest to strike Kobe at such a short distance from the fault zones. The shaking intensity on the JMA scale at several regions in the Awaji-shima island, and areas extending from Suma-ku of Kobe to Nishinomiya, was announced to be 7; that roughly corresponds to a shaking intensity of X or more on the Modified Mercalli Intensity (MMI) scale. The distribution of damage to traditional Japanese house is concentrated in these areas, which as shown in Fig. 1 encompasses a region with a length of about 20 km and a width of 1 km. The extent of the fault zone, as well as the specific soil conditions and topography of the area (narrowed by mountains in the north and by coastline in the south), played a very strong role in the regional damage distribution for this earthquake. Peak horizontal accelerations of the ground exceeded 0.8 g in Kobe and were as high as 0.23 g at a distance of 50 km in Osaka. One of the measured ground accelerations, recorded at Kobe Marine Meteorological Observatory in Chuo-ku, is shown in Fig. 2. This record also shows a peak acceleration over 0.8 g, and strong shaking lasted only 10 seconds.
No. 7] Extremely High Damage Potential of Near Field Earthquake Ground Motion 215 Fig. 1. Area of JMA seismic intensity 7 in the prefecture and Awaji island (after Ref. 2). Hyogo I IMt ~S@C) Fig. 3. Comparison of the recorded accelerograms. Fig. 2. Acceleration records obtained meteorological observatory close to at Kobe marine Sannomiya. Comparison of the recorded accelerograms. In Fig. 3, the N-S components of recorded earthquake ground motion are compared for the Kobe record of the Hyogo-ken Nanbu Earthquake, the Sylmar record of the Northridge Earthquake, the El Centro record of the Imperial Valley Earthquake in 1940, and the Hachinohe record of the Tokachi-oki Earthquake in 1968. The latter two records have been used for earthquake resistant design of critical structures in Japan. It is clear that the Kobe and Sylmar records obtained very close to the faults have high intensity but short duration, which are quite unique features of near field earthquake ground motion. Comparison of acceleration, velocity and displacement response spectra. As can be seen in Fig. 4(a), the absolute acceleration response spectra for the Kobe and Sylmar records show higher acceleration response than the El Centro and Hachinohe records in all period ranges. The response of Kobe shows more than 1 g in the period range from 0.15 to 1.2 seconds. Especially in the 0.3 to 0.5 range, the response even
216 H. IEMURA [Vol. 71(B), Fig. 4. ment Comparison response spectra. of acceleration, velocity and displace- exceeds 2 g. It is interesting to see that the Kobe and the Sylmar records show similar acceleration responses in almost all period ranges. In Figs. 4(b) and (c), velocity and displacement response spectra are shown. In the short period range (less than 0.25 seconds), not much difference is found among the four records. However, the Kobe and Sylmar records show much higher velocity and displacement response in the period range from 0.3 to 3.0 seconds. In field investigations of the JMA Intensity 7 region just after the Hyogo-ken Nanbu earthquake, severe damage was observed for relatively rigid buildings (lower than 10 stories) and short bridge piers, but not much damage was found in structures for which the period is longer than 1.0 second. This field observation of damage and nondamage does not coincide with the response spectra shown in Figs. 4(b) and (c). The reason for the discrepancy may be due to the linearity assumption used in calculation of the response spectra. With extremely high level and shock-type earthquake ground motion, the structural response must have gone deeply into the nonlinear and inelastic range. Hence, actual structural damage may not be adequately explained from only the linear response spectra. Evolutionary power spectra. To examine the nonstationary power imported to structures with different natural periods, evolutionary power spectra for the four earthquake ground motion records are calculated and compared in Figs. 5(a)-(d). The evolutionary power spectra is the power of a simple structure with natural period To=2,r/coo and damping ratio h0=0.05, as given by following equation; 3 2 G(t, W )= 2h0 w x2(t)+ (t) 1 0 where, x(t) and x(t) are displacement and velocity time history responses. The Kobe record in Fig. 5(a) shows a very high and sharp peak in power around 1.0 second after the beginning of the earthquake motion. This peak was felt as an extremely strong shock by local residents. Two strong peaks of the power are observed in the first several seconds, which could have contributed to the serious damage of structures. The Sylmar record in Fig. 5(b) also shows sharp peaks, but the level of the power is not as high as the Kobe record. Instead, in the longer period range (larger than 1.5 seconds), moderate but relatively high peaks of the power are seen, which could have contributed to the observed damage of flexible steel frame buildings. Compared to the Kobe and the Sylmar records which were measured very near the faults, the Hachinohe and El Centro records in Figs. 5(c) and (d) show a very low level of the power, even though the duration of strong motion is about 30 seconds long.
No. 7] Extremely High Damage Potential of Near Field Earthquake Ground Motion 217 Fig. 5. Comparison of the evolutionary power spectra. Fig. 7. Inelastic displacement response spectra. Fig. 6. Calculation of inelastic displacement spectra. Inelastic displacement response spectra. The structural damage we observe at the local site just after the earthquake may be highly correlated with the experienced maximum displacement response relative to the yielding displacement. Hence, in this section, inelastic displacement response spectra of a single degree of freedom structure with a perfect elastoplastic bilinear hysteretic restoring force shown in Fig. 6(a) is calculated. The yielding level, set as shown in Fig. 6(b), was used for most of the damaged old structures. The calculated inelastic displacement response spectra are shown in Fig. 7. Comparing with the corresponding linear displacement response spectra shown in Fig. 4(c), the Kobe and Sylmar records give much larger displacements in the short period range (0.1-0.7 seconds). This effect is due to the large plastic deformation caused by the high intensity, shock-type loading. The yielding displacement oy corresponding to a yielding acceleration and natural period of the structures (by ff wo) is plotted by the dashed line. Also plotted are 2, 5, and 10 times by. In the short
218 H. IEMURA [Vol. 71(B), period range, the inelastic displacement response exceeds the allowable ductility level, thus the yielding level of adequately designed structures needs to be increased. On the contrary, in the medium period range (0.7-3.0 seconds), the inelastic displacement response shows lower values than those of linear structures. The reason is that hysteretic energy dissipation suppresses the displacement response. For long period structures (over 3 seconds), the inelastic displacement becomes smaller than 2 by, and not much difference is found between the linear and nonlinear response spectra. Conclusions. The recently obtained Sylmar records from the Northridge Earthquake (Jan. 17, 1994) and Kobe records from the Hyogo-ken Nanbu Earthquake (Jan. 17, 1995) have been analyzed based on different types of response spectra. Compared with the El Centro records obtained during the Imperial Valley Earthquake (May 18, 1940) and the Hachinohe record obtained during the Tokachi-oki Earthquake (May 16, 1968). Extremely high damage potential of these two recent earthquake ground motions has been revealed. Especially for short period structures in which the natural period is less than 0.7 second, the present elastic design level of 0.2 g has to be raised up by at least a factor of two, otherwise extremely large inelastic deformation can be expected due to shocktype loading. Urgent retrofit of old structures and re-examination of present seismic design codes are essential. Acknowledgments. The author would like to thank Dr. Shunzo Okamoto, M. J. A., for his recommendation and encouragement to submit this paper to the proceedings of the Japan Academy. Special thanks are also given to Prof. B. F. Spencer of University of Notre Dame for reviewing this paper, and Dr. A. Igarashi and Mr. Y. Takahashi of Kyoto University for preparing the manuscript. References 1) Japan Society of Civil Engineers (1995) The First and the Second Reconnaissance Report of the Great Hanshin Earthquake Disaster (in Japanese). 2) Oka, F. Sugito, M. Yashima, A., and Bardet, J. P. (1995) Preliminary Investigation Report of the Great Hanshin Earthquake Disaster. 3) CSMIP Strong-Motion Records from the Northridge, California Earthquake of January 17, 1994 (1994). 4) Kameda H. (1975) Proc. of the Japan Society of Civil Engineers, no. 235, pp. 55-62. 5) Japan Road Association (1990) Part V Seismic Design, Specifications of Highway Bridges.