1.3 Seismic Hazard
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1 Introduction Natural disasters inflicted by earthquakes, landslides, flood, drought, cyclone, forest fire, volcanic eruptions, epidemics etc. keep happening in some parts or the other around the globe leading to loss of life, damage to properties and causing widespread socio-economic disruptions. EM- DAT, a global disaster database maintained by the Centre for Research on the Epidemiology of Disasters (CRED) in Brussels, records more than 600 disasters globally every year (http://www. cred.be). Earthquakes are the major menace to the mankind killing thousands of people every year in different parts of the globe. An estimated average of 17,000 persons per year has been killed in the 20th century itself. Statistics taken for the period 1973-1997 (http://www.cred.be), organized in 5-year bins, exhibit that earthquakes are amongst the disasters with larger death impact as depicted in Figure 1.1 even though the occurrences of flood events are twice per year. According to the International Disaster database (i.e. CRED) the total human fatality occurred in Asia for the period between 1900 to 2015 is estimated to be 18,23,324 persons while in case of only the Indian subcontinent the casualty is estimated to be around 78,209 with total economy loss of 5222.7 million (US$). Thus earthquakes are considered to be one of the worst among all the natural disasters. Figure 1.1 Comparison amongst different types of natural catastrophes (after Ansal, 2004). A comparative analysis performed by CRED in terms of total damage in billions of US$ reportedly caused by natural disasters as shown in Figure 1.2 illustrates that Asia is more prone to earthquake disaster than any other continental regions in the world. From the global earthquake perspective, the 21st century began quite ominously due to the occurrence of some great earthquakes causing damage both in terms of fatalities and 1 2 Introduction Figure 1.2 Total damages (US $billion) reportedly caused by natural disasters (source: EMDAT, http://www.emdat.be). socio-economic context. In 2001, the Bhuj earthquake of Mw 7.7, killed around 20,085 people with 1,66,836 injuries, approximately 3,39,000 buildings destroyed and 7,83,000 damaged in the Bhuj- Ahmadabad-Rajkot area and other parts of the region. In 2003, the Mw 6.6 Bam earthquake occurred in southeastern Iran which killed 31,000 people, rendered 30,000 injured, 75,600 homeless and 85 percent of buildings damaged or destroyed in the Bam region. The greatest earthquake of the century occurred in 2004, the Mw 9.1 Sumatra-Andaman earthquake which is the third largest earthquake in the world since 1900 and the largest since 1964. In total 2,27,898 people were killed or missing, presumably dead and about 1.7 million people were displaced by the earthquake and the subsequent tsunami that propagated across 14 countries in South Asia and East Africa. In 2005, the Mw 7.6 Kashmir earthquake killed 86,000 people. The Wenchuan, China earthquake of Mw 7.9 occurred in 2008 which killed around 87,587 people and rendered around 3,74,177 injured. The 2010 Haiti earthquake of Mw 7.0 killed 3,16,000 people, rendered 3,00,000 injured, 1.3 million displaced, 97,294 houses destroyed and 1,88,383 damaged in the Port-au-Prince area and in most of southern Haiti. The 2011 Japan earthquake of Mw 9.0 killed 20,896 people and caused wide spread damage and destruction. This earthquake serves as a warning that even developed and well- prepared countries are not immune to terrifying disasters. Though large loss of life in earthquakes has been observed previously also, the occurrence of so many deadly earthquakes within a single decade is unprecedented. These deadly earthquakes have incited assumption that the planet has experienced the consequences of a large population growth in the 20th century (Holzer and Savage, 2013). The global growth of population implies that earthquakes have become increasingly deadly because the expected number of earthquake disaster is proportional to the exposed population and their vulnerability components. 3 Introduction Though large earthquakes cause immense damage & destruction, it also provides an opportunity for the seismologists to get more insight into the internal structure of the earth to gain a better understanding of the mechanism of earthquakes. The studies carried out after the occurrence of strong earthquakes have provided basic knowledge and information on the phenomenon to provide necessary input for the assessment of seismic hazard and its possible mitigation. These post- earthquake surveys gave insight into the destructive pattern of earthquakes caused due to three complex processes. The first process is the solid earth system that is made of (a) seismic source, (b) propagation of the seismic wave through a medium, and (c) the local geology. The second process is the anthropogenic system that consists of the man-made structures like buildings, dams, bridges, tunnels etc. and the quality of construction and the last factor is the socio-economic development of the settlement before it is struck by an earthquake (Panza et al., 2001). The amount of loss of life and damages to human properties not only depend on the magnitude of the earthquake but also on the aforesaid three processes. Due to the heterogeneous nature of the earth’s crust, the seismic waves undergo multiple reflections, refractions and transformations along their path from the source to the site of observation. The changes are more prominent near the surface underlain by soil, where the geological and geotechnical properties of the soil layers play an important role in the amplification of the seismic energy. Prediction of an earthquake has been a subject of controversy with divided opinions. Earthquake prediction gained momentum with the prediction of the Blue Mountain Lake earthquake in 1971 and the success claimed at Haicheng which occurred in 1975 but proved to be short lived. Sykes et al. (1999) gives an account on the possibility and limitation of earthquake prediction. Researchers like Geller (1997) and Main (1995) argue that short-term prediction with certainty is inherently difficult and that very high resolution is required for mitigation measures. Usually a time scale is involved that corresponds to long-term prediction considering an earthquake with a return period of 50, 100 or 500 years. Prediction of individual earthquake may not be possible but the long-term rates of earthquakes can be forecasted with considerable accuracy especially in the regions of high seismic activities, like the plate margins, such as Japan, Italy, Turkey, Mexico, and California. Time and again, catastrophic earthquakes across the globe annihilate vast population and cause severe socio-economic breakdowns incurring substantial setbacks in the developmental efforts. They continue to pose significant threat to the sustainable development and growth of civilization. Scientific understanding of the phenomena is of vital importance for mitigation measures to withstand the impacts of the hazard. The seismic hazard paradigm embodies ‘where’, ‘when’ and ‘how’ the earthquakes occur, associated ground motions, and their effects on the structures (McGuire, 2004). In several cases, considerable damages are also caused by secondary effects such as tsunami, landslide, soil liquefaction, rockfall, and ground subsidence. However, structural engineering is based mostly on ground shaking intensity levels that act as primary hazard proxy. Seismologists, therefore, predict the ground shaking intensity from potential earthquakes to support building codes for seismic-resistant constructions, landuse planning, and earthquake insurance purposes. Earthquake occurrences do not have spatial uniformity entailing differential exposures to earthquake effects across large areas. Furthermore, intensities of ground motions also vary widely depending on regional tectonics and local geological conditions. By delineating zones of different hazard levels using state-of-the-art technology, we deliver knowledge products about the expected ground motion quantities. The vulnerability of modern society towards earthquake hazard is increasing with time. Although the occurrence of earthquakes is inevitable, the reduction of the social and economic setback during earthquakes can be achieved through a comprehensive assessment of Seismic 4 Introduction Hazard Microzonation and Risk. This can be accomplished by creating public awareness and by upgrading or retrofitting the existing buildings & engineering structures and also taking appropriate measures in case of upcoming urban structures. 1.1 Earthquakes in India India, with its unique geological setting and socio-economic conditions is highly vulnerable to disasters. Seismic vulnerability in India is well evidenced by numerous past earthquake-related calamities. According to the vulnerability atlas of India prepared by Building Materials and Technology Promotion Council (BMTPC), more than 59 percent of the total landcover in the country is susceptible to seismic hazard (BMTPC, 1997). Incidentally, India is the second most populous country in the world. On the other hand, unplanned urbanization is expanding rapidly across the country to accommodate the burgeoning population. Information regarding earthquake magnitude and frequency are essential for proper assessment of seismicity of a region. There has been a consistent sequence of earthquakes in the Indian subcontinent since ancient times and it is one of the most earthquake prone regions of the world and is