The Relations of Earthquake Disasters with Respect to Surface Fault Rupture, Crustal Movement, and Strong Ground Motion:Using Two Central Taiwan Earthquakes in 1935 and 1999 as Examples

• Main Authors: Tsung-Ming Yu, 余聰明
• Other Authors: Yi-Ben Tsai
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/sn2t63

博士 === 國立中央大學 === 地球物理研究所 === 92 === In this paper we studied two disastrous Guan-Dao-Shan and Chi-Chi earthquakes in Central Taiwan. They were the two most devastating earthquakes in last hundred years in Taiwan. Equally important, the societal and scientific records of the earthquakes were compiled and preserved very well. We started to analyze by townships, then proceeded to analyze by villages. The loss data of townships and villages are complete. Meanwhile, surface ruptures happened in both earthquakes. The fault types were different associated with these two earthquakes. We analyzed the relation of disaster distribution with respect to fault surface, regional surface movement, and strong ground motion. We hope the results will be useful for loss estimation of future earthquakes. Surface fault ruptures and regional surface movements usually occurred in large earthquakes. By comparing the patterns of these phenomena in large earthquakes, we can study their relation with house collapse and human fatality by earthquake. The fault ruptures, crustal movements, and strong ground motions of the Guan-Dao-Shan and Chi-Chi earthquakes were well recorded by observations and surveys. We compared both qualitatively and quantitatively these three causes with the disaster data of the affected townships and villages. There were two segments of fault in the 1935 Guan-Dao-Shan earthquake. In the neighborhood of northern segment (Shihtan fault), the house collapse rate was high (the highest was 99.6% in Dahe village, Sanwan township). But the death rate was relatively low (the highest is 5.16% in Nanfu village, Nanjhuang township). In the neighborhood of southwestern segment (Tun-zu-chiao fault), not only the house collapse rate was high (the highest is 100% in Hsin-Tsuang-Zu village, Shengang township), but also the death rate was high (the highest was 14.7% in Hsin-Tsuang-Zu village, Shengang township). By comparing the distaster data between the fault-zone and non-fault-zone areas, we found symmetric patterns of disaster distribution with respect to fault trace for the Guan-Dao-Shan earthquake. Disasters by the Chi-Chi earthquake were caused by several factors, including fault rupture, strong ground motion, landslide, and house collapse. When the collapsed buildings were located in urban areas, the fatality was concentrated. But because the population is high, thus the average fatality rate is low. For example, there were 84 fatalities in the Donhsin building, in Songshan district of Taipei city. The local fatality rate was 21 % of the building, but was only 1.28 % of the city ward. It was a typical destruction pattern of urban buildings. Besides, the seismic resistance of buildings in urban areas is usually stronger than the buildings in the country areas. The fatality rate is generally low. Many collapsed buildings were made of mud bricks in the country areas. The fatality rate in the Tsaolin village was 4.33 %, because 36 people were buried by a big landslide. The Chelungpu fault was a thrust fault that dips east in the Chi-Chi earthquake. The footwall on the west side did not move much and the seismic intensity was relatively low. Thus the house collapse rate and human fatality rate were low in the west side of the fault, even the population there was high. On the contrary, the damages were severe in the villages in the hanging-wall areas on the east side. In summary, the patterns of both collapsed house and human fatality about the fault zone showed asymmetric distribution with respect to the fault trace in the Chi-Chi earthquake. We also compared the difference of house collapse rate and human fatality rate between these two earthquakes. First, most houses were made by mud bricks and wood in early Taiwan. The structural design against large earthquakes was absent. When the Guan-Dao-Shan earthquake occurred, many houses collapsed and people were killed. In addition, houses made by bricks also collapsed during the Guan-Dao-Shan earthquake. The falling debris hurt many residents when houses collapsed. Second, main types of modern buildings in Taiwan are made by reinforced concrete and steel. Although the destruction of structures were widespread during the Chi-Chi earthquake, owing to the goal of modern architecture design emphasizing some free space for survival of people even when the building collapses. It has effectively reduced the probabilities of fatality. In this study, we have found the modern building types have significantly reduced the damage from earthquake by comparing the disasters of Guan-Dao-Shan and Chi-Chi earthquakes. The average house collapse rate decreased from 32.5 % to 19.2 % and the average fatality rate greatly decreased from 1.12 % to 0.28 % with time. In other word, the modern codes for seismic design of buildings are really effective in reducing the probabilities of house collapse in large earthquakes. Finally, we also analyzed the age distributions of fatality for different towns in both Taichung and Nantou counties during the Chi-Chi earthquake. We obtained an equation as follows: Y% = 0.00022X2 – 0.01X + 0.16 where Y% means the age-specific fatality rate in percentage, and X represents age. According to above equation, the fatalities are significantly different for different ages, especially for age older than 55 years old. There are two significant implications. First, the protection for people during earthquake, should focus especially on old age group. Second, the human fatality rates are almost the same, even though the age dependence of general population and earthquake victims were different for Taichung and Nantou counties. The reasons are worth for further study in the future. We can estimate potential fatalities in future earthquakes based on above age dependence equation of earthquake fatalities by using current population distributions. The results can provide useful information for seismic loss estimation.