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Earthquake and earthquake ground motion
Abstract: This pages and next page summarizes the fundamental concepts of earthquake engineering. Causes of earthquakes, earthquake measuring scales, seismicity and characteristics of strong ground motion are discussed. The intensity and magnitude of the earthquake are differentiated. An introduction to response of structures to earthquake ground motion is presented in next pages.
Key words: magnitude, intensity, peak ground acceleration, faults, tsunami, rebound theory.
Vibrations in structural systems may result from a wide variety of sources. Some of the most common and significant dynamic waves imposed on the structure are those carried by environmental activities such as wind, earthquakes and waterways. In these environmental sources, earthquakes rank as the most important in terms of their enormous potential for damage to structure and loss of human life. An earthquake has long been feared as one of the most terrifying natural phenomena. Early in human history, the sudden shaking of the earth and the death and destruction that resulted were seen as mysterious and uncontrollable. We now understand the origin of earthquakes and know that they must be accepted as a natural environmental process. An earthquake is one of the periodic adjustments that the Earth makes in its evolution. Arriving without warning, the earthquake in few seconds can create a level of death and destruction that can be equalled only by the most extreme weapons of war. This uncertainty combined with terrifying sensation of earth movement creates our fundamental fear of earthquake.
The Tangshan, China earthquake of 1976 is officially reported to have caused 255 000 deaths. The city of Tangshan was essentially levelled as if struck by an atomic bomb. Tangshan was built mostly from non-engineered, unreinforced masonry buildings, and this level of destruction is not expected in a city built to recent seismic codes.
Every year, approximately 10 000 people on average die from earthquakes worldwide. In addition to this staggering loss of life, earthquakes are responsible for hundreds of millions of dollars in property damage annually. The costs of renovating structures after the Loma prieta earthquake of 1989 and the North ridge earthquake of 1994 are estimated at $6.8 billion and $25 billion respectively. For the 1995 Kobe earthquake, the repair costs exceed $100 billion.
India is a land of approximately 3 280 483 km2. More than 60% of the area is in an earthquake-prone zone. During the last 100 years, India has lost about 100 000 lives to earthquakes. On average this is 1000 lives/year but 20 000 lives have perished in a matter of seconds. In 1905 the Kangra earthquake of Richter scale M = 8.0 in Himachal Pradesh resulted in 13 800 lives lost. In Gujarat in the Kachchh earthquake of 2001, hundreds of thousands of houses collapsed or were severely damaged. In each event one can imagine the misery of the survivors and the impact on the economy of the affected region.
The tragedy of Al Asnam in Algeria should indeed be an eye-opener to planners, engineers and builders. In October 1980 an earthquake of 7.5 M rocked the lives and reduced most of its sandstone modern buildings to rubble. The estimate of lives lost was 2500, with 200 000 injured. The worst part of the story is, however, the fact that barely 26 years earlier, the same town had been destroyed by an earthquake affecting 1600 people at that time, and the present town was mostly built anew after the earthquake.
In India itself, after the tragic occurrence of the Bhutanâ€'Nepal earthquake of 1934 (M = 8.4) in which more than 13 000 people died, no improvements have been made in construction practice, as a result of which even during the moderate earthquake of 21 August, 1988, with M = 6.6, the results were catastrophic with a loss of lives of about 900 and property millions have been destroyed. The Bhujj earthquake resulted in calamity in five districts of Gujarat, and also showed that lessons had not been learnt even after the tragedy of 1956 earthquake.
Table 16.1 gives some of the largest and deadliest earthquakes that have occurred worldwide during 1900â€'2007. Table 16.2 shows the number of earthquakes worldwide between 2000 and 2007 and mortality figures.
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