Earthquake Damage of Site Effect and Soil Liquefaction in Kaohsiung Earthquake
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EARTHQUAKE DAMAGE OF SITE EFFECT AND SOIL LIQUEFACTION IN KAOHSIUNG EARTHQUAKE Zhaoyan LI1,Yu GUAN2 ABSTRACT On February 6, 2016, a magnitude 6.7 earthquake struck Kaohsiung, causing heavy losses in infrastructure and building construction. This paper gives a preliminary analysis and summary of existing achievements. The results show that although the earthquake occurred in the city of Kaohsiung, the earthquake damage was serious in Tainan, mainly due to the site effect and soil liquefaction. The Chianan plain has a soft and thick soil layer, which has a significant effect on the site amplification effect, and increases the degree of seismic intensity. According to this situation, after the earthquake, magnitude effect is significant and the degree of seismicity will be increased. The Chianan Plain is the largest alluvial plain in Taiwan, and the liquefaction area of the earthquake is concentrated in this area. Soil liquefaction not only causes significant collapse and damage to the houses, but also causes long-period seismic wave amplification. The earthquake aggravates the soil liquefaction crisis in Taiwan and the region had to urgently issue zoning maps of liquefaction potentials. This is a good warning to the development and application of liquefaction zoning technology in mainland China. The time of shaking in Tainan region increased significantly from 1.74 seconds epicenter to 8.16 seconds in the other district. It was considered by expert as a result of the joint action of soft soil layer site and soil liquefaction. Its specific phenomena and mechanism needs in-deep study in future. Keywords: Kaohsiung Earthquake, Soil Liquefaction, Site Effect, Seismic Predominant Period 1. INTRODUCTION OF KAOHSIUNG EARTHQUAKE On February 6, 2016, a 6.7 magnitude earthquake with a depth of 15km was recorded at Meinong district of the city in Kaohsiung, Taiwan (22.94 degrees north latitude and 120.54 degrees east longitude). Multiply regions of Taiwan are affected by this earthquake. The distribution of earthquake intensity is shown in Figure 1. From the figure, it can be seen that the earthquakes are greatly affected in Kaohsiung, Tainan, Taitung and Chiayi areas. 1 Institute of Engineering Mechanics, China Earthquake Administration, Harbin, People's Republic ofChina, [email protected] 2 Harbin Guangsha College, Harbin, People's Republic of China, [email protected] Fig 1 Kaohsiung earthquake shake map The epicenter of the earthquake was in the city of Kaohsiung, but the most affected by the earthquake was the city of Tainan, wherethe building destruction, as shown in Figure 2, the roads damages, the widespread phenomenon of soil liquefaction. Especially, the crown building has collapsed from west to east and 115 people dead in this earthquake, which was most serious casualties, as shown in Figure 3. Fig 2 Building slop by the earthquake 2 Fig 3 Building collapse by the earthquake According to experts from Taiwan Earthquake Research Center, there are three main reasons leading to the serious damage in Tainan. Firstly, Tainan may be in the blind fault zone; secondly, the influence of sand liquefaction. Tainan region is located in Chianan plain which is the largest alluvial plain in Taiwan, has the characteristic of shallow groundwater level and thick sand layer. The earthquake caused a large area of sand liquefaction, ground instability and surface damage; lastly, due to the thick alluvial layer in Tainan area, the seismic wave will be amplified in the soft soil, which resultthe amplification of the ground acceleration, the longterm vibration and the further strengthening of the earthquake damage. 2. GEOLOGICAL STRUCTURE OF TAIWAN Taiwan island is located in the southeast of China continental shelf stretches of forming Eocene period by Yayong (2000), is situated in the Eurasia plate and the Philippine plate extrusion by Lin and Rocecker (1993) and Yu et al. (1997), as shown in Figure 4, through the orogeny formed by extrusion sea is described in Weimin et al. (1999).The Philippine plate is pushed to the northwest at a rate of about 8cm each year. The extrusion of the plate leads to a long narrow north-south terrain in Taiwan with frequent faults and earthquakes. During the 100 years from 1914 to 2014, there were 3888 earthquakes that more than 5 magnitudes occurred in China, of which 1396 occurred in Taiwan; there were 126 earthquakes of magnitude 7 and above occurred in China, of which 43 occurred in Taiwan. 3 Fig 4 Sketch map of Taiwan plate tectonic The experts of Taiwan earthquake research point out that the earthquake epicenter located in the Zuozhen fault, Chaozhou fault and Qishan fault between intensive earthquake belt, intensive earthquake belt and densely populated with overlay, intensified human and economic losses. 3. SAND LIQUEFACTION PHENOMENON Soil liquefaction is a very practical and important research topic in soil dynamics. Soil liquefaction is typical earthquake damage after earthquake and has a significant disruptive effect on all kinds of structures and engineering. It has always been noticed by engineering and academia at home and abroad. Researchers have conducted in-depth studies on liquefaction phenomena and occurrence conditions to the mechanism of earthquake damage by Zhaoyan et al. (2014) and Xiaoming et al. (2009). Many regions have suffered severe losses caused by earthquake liquefaction, and countries all over the world may face the risk of liquefaction after the earthquake in the future is described by Chisheng et al. (2004) and Idriss et al. (2006). After the earthquake in Kaohsiung, a large scale of sand liquefaction occurred in the southern part of Tainan area, as shown in Figure 5a, which resulted in the further increase of the intensity of the Tainan area.During the earthquake in Taiwan's Kaohsiung, the liquefaction of the Annan area in the south of Taiwan was 30cm-100cm, which caused a lot of damage to buildings and infrastructure.In the zenith of the Annan district in Tainan, the liquefaction of sandy soil has resulted in the collapse of the house strata of the entire community, with more than half of the most serious floors falling into the ground, as shown in Figure 5b.The study also found that the most serious of the Weiguan buildings was once a fish pond, which experts believe was caused by the construction quality of the building.On the other hand may also be the soil liquefaction effect after the earthquakes, namely the soil liquefaction caused by earthquake in the earthquake components are increased for a long period of time, duration of vibration increase, increase the quality of coarse high-rise building damage. 4 (a) Building crack by soil liquefaction (b)Building subsidence by soil liquefaction Fig 5 Soil liquefaction in Kaohsiung earthquake The Kaohsiung earthquake occurred in the southern part of Chianan plain in the southwest of Taiwan, which is the largest alluvial plain in Taiwan, which has been accumulated by many rivers.The underground water level of Chianan plain is shallow, the sand soil layer is thick, sothe soil liquefaction of sandy soil is a great threat after the earthquake. The Kaohsiung earthquake also shook up the crisis of sand liquefaction in Taiwan. Professor Li Hongyuan from the Department of Civil Engineering, National Taiwan University believes that if a magnitude 6 earthquake occurs in Taipei, because of soil liquefaction, the magnitude will expand to 7.3 and a large number of houses will collapse in Kaohsiung. The Taiwan Geological Survey is divided into three liquefaction potential areas of red (high-risk area), yellow (medium-risk area) and green (low-risk area) according to the liquefaction hazard rating of sand in different areas of Taiwan. Taking the results of liquefaction potential areas published by Taipei City and New Taipei City as an example, most of the areas were found to be in a highly threatened area under the seismic soil liquefaction as shown in Figure 6. At the same time, it is also found that there are a wide range of liquefaction potential areas in Taiwan. 151 schools are located in the danger zone of sand liquefaction. The threat of sand liquefaction to the island of Taiwan cannot be ignored. 5 Fig 6 Soil liquefaction potential map in the city of Taipei and Xinbei 4. SITE EFFECT The influence of the earthquake on the soft ground is greater than that of the bedrock by Kun (2010). The softer and thicker of the soil layer will be longer seismic predominant period Kexu et al. (1980),which is called Site effects. The site effect is caused by the change of ground motion due to the difference of hard and soft soil. Many domestic and foreign experts have carried out a large number of quantitative studies on the amplification effect of seismic sites by Holzer (2005) and Xiaojun (2001), and found that the difference between the values of ground motion parameters on the bedrock and those considering the field amplification effect can reach 250% by Allen (2007). Seismic source is located in Meinong region of Taiwan Kaohsiung area, which is located in the mountains, geological hard, degree is only 5 levels, shaking time of 1.74 seconds, the Qishan stations of Kaohsiung measured acceleration response spectrum, velocity response spectrum is shown in Figure 7. Tainan city is located in Chianan plains, soft soil, seismic wave to Tainan, the amplification effect because of the influence of the field effect of an earthquake parameters, degree of 7 magnitude, earthquake damage is serious, the surface peak acceleration, predominant period longer, vibration amplitude, the earthquake shaking time longer, up to 8.16 seconds, theXinhua stations of Tainan measured acceleration response spectrum, velocity response spectrum is shown in Figure 8. 6 Fig 7 Acceleration and velocity response spectrum in Qishan station of Kaohsiung Fig 8 Acceleration and velocity response spectrum in Xinhua station of Tainan 5.