CHAPTER 3: EARTHQUAKE INTENSITY Yang Lihua*
Total Page：16
File Type：pdf, Size：1020Kb
171 CHAPTER 3: EARTHQUAKE INTENSITY DISTRIBUTION AND GROUND FAILURE INTENSITY DISTRIBUTION OF THE TANGSHAN EARTHQUAKE Yang Lihua* Macroscopic intensities in the M7.8 Tangshan earthquake (including the M7.1 Luanxian earthquake) were comprehensively evaluated according to the New Chinese Seismic Intensity Scale combined with the damage characteristics of industrial facilities, foundations and geomorphic conditions in the Tangshan seismic zone. The following four criteria of macroscopic intensities are used: (1) Damage to buildings. This is the main criterion for the evaluation of intensity due to the wide distribution, large amount, and various types of buildings. (2) Damage to smokestacks and brick water tanks, etc. Brick smokestacks and brick water tanks are rather widely distributed in the industrial area of Tangshan, and their damage characteristics may reflect the seriousness of the destruction. (3) Ground failure. Earthquake causative tectonics and ground dislocation in high inten- sity areas are a significant basis for intensity ratings of XI and X areas. Considering that there will be little basis for evaluating intensity if the total or majority of the buildings recol- lapsed the primary ground failure naturally becomes the main criterion for rating. But ground failure induced by soil liquefaction, such as sand boils and waterspouts, local ground subsidence, secondary ground fissures, etc., could not be used as criteria for evaluating high intensities. Therefore, in order to use the damage to railways and bridges as a criterion for high intensity rating we should consider what type of foundation the railway or bridge was built. In the case of unliquefied areas, the local S-shaped bent rails and damage to bridges can be used as a reference criteria for evaluating high intensities; but in liquefied areas, the damage can only be used as an index of degree of liquefaction, but not for evaluating intensity. (4) Human feeling and dropping utensils. The Tangshan seismic area is a densely populated area, so human feeling and dropping of utensils are also an important criteria for intensity ratings. The main phenomena found in different intensity areas are listed in Table 1. Based on intensity ratings at various macroscopic locations investigated one can draw isoseismal lines, which represent the envelopes of certain intensities, which can be drawn from the meizoseismal area to the area of lower intensity. In a small number of anomalous * Seismological Bureau of Hebei Province 172 areas the rated intensity of higher or lower may be included. If the area of anomalous inten- sity is bigger than the area of the meizoseismal area (i.e. area of intensity XI), it is shown on the map of intensity distribution. 1. Intensity Distribution of the M7.8 Tangshan Earthquake and Macroscopic Phenomena of Various Intensity Areas The earthquake intensity distribution of the M7.8 Tangshan earthquake is shown in Fig. 1. The area and length of the major axis and minor axis of various intensity areas are listed in Table 2. Characteristics of various intensity areas are as follows: (1) Area of intensity XI The meizoseismal area of the M7.8 Tangshan earthquake was located on both the north and south sides of the railway in Tangshan, ranging from the Yuehe Commune in Kaiping Town in the east to Zhenggezhuang in the west, and from Nuzhizhai in the south to Xinhua Road in the north. The meizoseismal area exhibits an oval shape, the intensity of which is XI. The major axis extended roughly in the NE direction with a length of 10.5 km, and the minor axis was 3.5 to 5.5 km in width. The area was about 47 km2 (see Fig. 2). In the meizoseismal area most buildings in Tangshan including factories, schools, stores, 1 apartment buildings, and other public buildings, etc. belonged to Class III buildings . The majority of buildings in the countryside belonged to Class II, and a few of the buildings belonged to Class I. After the earthquake all these buildings were extensively collapsed or destroyed. Rails were bent horizontally in an S-shape or vertically in a wave form. For example, at the railway station of Tangshan City the rails were severely bent approximately several tens of centimeters out of line and were out of use due to being unrepairable. Some railway car- riages of goods stopped on inclined rails and some turned over. Highway pavement was seriously damaged due to bulging, tension cracks or foundation failure. For example, a tension crack one meter wide and 1.5 m offset occurred close to the Shengli Bridge. Bridges were extensively destroyed or seriously damaged. For example, Shengli Bridge is a five-span reinforced concrete beam bridge with a pile foundation. It is 66 m in length 1 Buildings are classified as follows: Class I: (1) Adobe houses, (2) Rubble houses, (3) Poor-quality livestock shed. Class II: (1) Dwellings with face bricks and timber frames, (2) Single-story brick buildings, (3) Single-story stone buildings, (4) Few single-story houses with pre- cast slabs. Class III: (1) Buildings with R.C. frames, (2) Industrial plant buildings, (3) Multi-story masonry buildings. The damage level of adobe buildings, dwellings with face brick and timber frames, single-story brick buildings in countrysides and multi-story masonry buildings in city towns, which make up the majority of buildings in town and village is the main basis for evaluating intensity, and others for reference. 173 and 10 m in width. After the earthquake the bridge piers in the west fractured, beams in two spans fell down and the other three piers inclined to the west with an inclination angle equal to 18°. Tall structures such as brick smokestacks, brick water tanks, etc., totally collapsed or were fractured near the bottom. The fractured zone of the causative fault passed through the high intensity areas (intensity XI and X), and basically controlled the trend of isoseismals. The fractured zone was arranged en echelon, the general strike of which was roughly N30°E, and the strike of each en echelon crack was more than 45°NE. The fractured zone and en echelon cracks exhibited right lateral rotation. The east side of the fault moved in a SW direction and the west side moved in a NE direction. The underground pipeline was broken and the water supply was out of service. During the earthquake vertical bumping appeared along the fracture zone of the causative fault in its vicinity. In the range of approximately 200 m from both sides of the causative fault people felt as if they were being thrown upward. In the area with thin overburden or in the hilly region with outcrop of bedrock, such as Fenghuangshan Hill, Dachengshan Hill, and Jiajiashan Hill, etc., buildings were slightly damaged and an anomalous area of lower intensity was formed causing nonhomogeneity in the intensity distribution (2) Area of intensity X The area of intensity X ranged from Guye in the east to Langao Commune in the west and from the town of Daodi in Fengnan County in the south to Fujiazhuang and Wangnianzhuang Communes in the north. The major axis of isoseismal lines was in a N45°E direction with a length of 36 km. It was narrow in the north-east and wide in south-west like a gourd in shape and the widest distance was 15 km with an area of 370 km2 (including the area of higher intensity). Most buildings in the cities, towns and communes in the area were Class III buildings. In villages located in this area the majority were Class III buildings and few were Class II buildings. A rather high percentage of collapse of the Class III buildings was caused by poor earthquake resistance of multi-story buildings which form the major part of the buildings in that category. Only a small number of Class III buildings constructed with cement mortar were slightly damaged. In the Gaozhuangzi Commune of Fengnan County the buildings were mainly one-story Class II buildings and more than 90% of the buildings collapsed. Only individual one-story new brick buildings were damaged or slightly damaged. The majority of factory buildings were severely damaged. In a portion of the area rails bent horizontally in an S-shape or vertically in a wave form such as the rails at the Fengnan Railway Station which were deformed up to several tens of centimeters. Transverse bulging and longitudinal tension cracks occurred extensively on the highway pavement. Various bridges were seriously destroyed in general. Along the Tangshan-Guye 174 Highway near Wali a longitudinal tension crack 1 m wide and more than 1 m of vertical dis- location occurred thus, the traffic was blocked. The majority of tall structures such as brick smokestacks and brick water tanks, etc., col- lapsed at the bottom or middle portion and a small number were seriously damaged. In Daodi the abutments of a reinforced concrete highway bridge settled due to ground subsidence, the bridge cracked at the beam joints, and the piers tilted. Sand boils and waterspouts due to liquefaction accompanied by ground subsidence occurred but these phenomena were not widespread in the area. This only occurred in parts of the south of Gaozhuangzi and Daodi. (3) Area of intensity IX The area of intensity IX was located in a region from Leizhuang in Luanxian County in the east to Yuelongzhuang in Ninghe County of Tianjin City in the west and from Xiaoji, Huituo, Xigezhuang in Fengnan County in the south to Xinzhuangzi and Lizhuangzi in Fengren County in the north. It is like an irregular ellipse in a north-east direction with a major axis of 78 km in length and a minor axis of 42 km in length and a total area of 1800 km2.