An earthquake, which may be termed as a quake, tremor or temblor is the result of a sudden release of energy in the earth’s crust that creates seismic waves that cause a lot of destruction in the social and economic environment and the natural environment itself. The seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. In its most general sense, the word earthquake is used to describe any seismic event, whether natural or caused by humans that generate seismic waves.
The earthquake magnitude is a number that characterizes the relative size of an earthquake. , (Guinness and Nagle 2002). The seismic hazard maps, which provide the basis of risk estimates, lead to underestimates of the casualties by more than two orders of magnitude. Hence, relying on these maps, communities were unprepared in Kashmir, India and Wenchuan of China where killer earthquakes wiped out schools, hospitals and residences, killing and injuring hundreds of thousands of people.
How serious the earthquake problem is for megacities like Lima in Peru can be understood when one considers that the population is about 12 million, and, depending on the building quality, the soil conditions, and the distance from the earthquake, the number of fatalities in different districts may range from 0. 5 to 4 percent— a potential death toll of 60,000 to 480,000. The looming problem is most dramatically demonstrated by our calculations of the likely fate of the children: In a worst-case scenario of a magnitude-8. earthquake close to the city, one must expect that about 20,000 children, about 2 percent of the 1 million school-aged populations, will probably die, and a multiple of this number will be maimed for life, (Max 2012). Earthquake magnitude only gives the size of the quake and nothing else about where the earthquake has happened, the population of economic activities near the epicentre and how prepared were the people to the earthquake.
So as a guide to impact assessment, the magnitude is proved to be poor because there are other factors which can be used on measuring the seismic impact assessment such as the distribution of economic activities, density of buildings in an area and even location of the affected area. Although seismologists and engineers have generated a world map of seismic hazard, which shows the level of ground shaking likely not to be exceeded, all of these disasters are always surprising: The ground motions and death tolls far exceeded expectations, causing consternation among the scientific community.
The standard method to estimate seismic hazard has been brought into question and its assumptions and calculation methods have come under scrutiny, to mean even magnitude can be nothing, except a figure of no use in the seismic impact assessment. The amount of damage and loss of life associated with earthquakes depends largely on the population density, the nature of the buildings, the nature of the bedrock, building density, and the accessibility or isolation of the region. The relative importance of these factors varies in a great deal. For example, the Kobe earthquake of January 1995 had a magnitude of 7. and caused over 5000 deaths. By contrast, the Northridge earthquake, which affected part of Los Angeles in January 1994, was 6. 6 on the Richter scale but caused only 57 deaths. Yet an earthquake of force 6. 6 at Mahashtra in India in September 1993 killed over 22000 people. So the magnitude in this respect can be a poor guide to impact assessment. Kobe and Los Angeles are known earthquake zones, buildings are built to withstand earthquakes, and the local people are ever prepared for the earthquakes, to mean the impact assessment in such a case cannot be taken as the earthquake magnitude, (Guinness and Nagle 2002).
The Richter magnitude scale is any of a number of ways to assign a single number to quantify the energy contained in an earthquake. In all cases, the magnitude is a base-10 logarithmic scale obtained by calculating the logarithm of the amplitude of waves measured by a seismograph. An earthquake that measures 5. 0 on the Richter scale has a shaking amplitude 10 times larger and corresponds to an energy release of v1000 ? 31. 6 times greater than one that measures 4. 0. The impact depends on the distance of economic and social environment from the epicentre because even if an earthquake records more than 9. on the Richter scale if the location of the earthquake is in the desert, the impact of such an earthquake is totally different from one with a magnitude of 3. 5 in a small town of India that is heavily populated by unprepared people whose building structures are not built to withstand the earthquake,(Plait 2009).. At the Earth’s surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicentre of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami.
Earthquakes can also trigger landslides, and occasionally volcanic activity. With this brief note of the occurrence, to use earthquake magnitude in seismic impact assessment become difficult as this cannot tell the real impact expected from the approaching earthquake as far as the distance from the epicentre is concerned. If the quake occurs in the dessert, its impacts are limited because there are very few developed infrastructure except for the natural environment, (the fauna and flora),(Simkim et al 2006).
The most important parameter controlling the maximum earthquake magnitude on a fault is however not the maximum available length, but the available width because the latter varies by a factor of 20. Along converging plate margins, the dip angle of the rupture plane is very shallow, typically about 10 degrees. Thus the width of the plane within the top brittle crust of the Earth can become 50 to 100 km (Tohoku, 2011; Alaska, 1964), making the most powerful earthquakes possible, thus the width can be used in the place of the magnitude in forecasting the possible impact to be brought by an earthquake, (Max 2012).
When using the earthquake magnitude as a tool for seismic impact assessment, there is a problem in timing. The window of time from the announcement of an Earthquake Early Warning until the arrival of the main tremors is very short, that is it’s only a matter of seconds or between several seconds and a few tens of seconds). In areas that are close to the focus of the earthquake, the warning may not be transmitted before strong tremors hit.
In this case magnitude will be of no importance as the timing on detecting the quake will make all the expectations or forecasted figures idle due to the time of occurrence in relation to the forecasted. In this regard the preparedness of the people will be of greater importance. For example, in countries like Japan, they are ever prepared as compared to other areas like California in the USA. Thus impact assessment will be done looking at the time scale and how prepared were the people to evacuate the area before the earthquake.
The soil type here will also help in determining the speed of the quake, if loose it means the impact is extreme. False alarms are also another problem when using the magnitude as an impact assessment tool. When using data from only one seismograph, false Earthquake Early Warnings may occur because of noise from accidents, lightning or device failure. This is usually because of faulty machines, which are not regularly serviced or the underwater movements. There is also a problem of magnitude estimation. There are limits to the accuracy of estimating magnitude, especially for large earthquakes.
It is difficult to separate earthquakes and provide accurate warnings when multiple earthquakes occur almost simultaneously or in close proximity to each other. Taking for example, the earthquake of 2011 in Japan, it recorded 9. 0, which was not even expected. Thus, the impact of such quakes is hard to understand using magnitude as a guide to their impact assessment since magnitude has been estimated well after the occurrence of the previous earthquake, (Reitherman 2012). Seismic intensity estimation can be used instead of the magnitude for impact assessment.
There are limits to the accuracy of estimating seismic intensity by statistical attenuation formula, as well as limits to the prediction of land surface amplification, but this measure can be used instead of the magnitude. The scale of the nucleation zone is uncertain, with some evidence, such as the rupture dimensions of the smallest earthquakes, suggesting that it is smaller than 100 m while other evidence, such as a slow component revealed by low-frequency spectra of the some earthquakes suggests that it is larger.
The possibility that the nucleation involves some sort of preparation process is supported by the observation that about 40% of earthquakes are preceded by foreshocks. Once the rupture has initiated it begins to propagate along the fault surface. The mechanics of this process are poorly understood, partly because it is difficult to recreate the high sliding velocities in a laboratory. Also the effects of strong ground motion make it very difficult to record information close to a nucleation zone.
Thus using earthquake magnitude as a guide to seismic impact assessment will be inappropriate due to the limits of accuracy as provided by the seismologists, (Simkim et al 2006). Propagation velocity of the seismic waves ranges from approx. 3 km/s up to 13 km/s, depending on the density and elasticity of the medium. In the Earth’s interior the shock waves travel much faster than the waves (the approximate relation is 1. 7 : 1). The differences in travel time from the epicentre to the observatory are a measure of the distance and can be used to image both sources of quakes and structures within the Earth.
In solid rock waves travel at about 6 to 7 km per second; the velocity increases within the deep mantle to ~13 km/s. The velocity of waves ranges from 2–3 km/s in light sediments and 4–5 km/s in the Earth’s crust up to 7 km/s in the deep mantle. Consequently, the first waves of a distant earthquake arrive at an observatory via the Earth’s mantle. Thus these are the issues earthquake magnitude cannot fully present, hence a poor tool in the seismic impact assessment,(Plait 2009).
Conclusively, earthquake magnitude can be a poor guide to seismic impact assessment because it only gives the data about the size of the earthquake and nothing else about the nature of buildings, the nature of the bed rock, building density, accessibility or isolation of the affected region, the population of the area affected and how prepared were the area before the hit.
Paul Guinness and Garrett Nagle, (2002), Advanced Geography: Concepts and Cases, Hodder Education, U. K. Max Wyss, (2012), Earthquake Hazards Seismology, American Geosciences Institute, retrieved from earthmagazine. rg. on the 7th of October 2012. Reitherman, Robert (2012). Earthquakes and Engineers: An International History. Reston, VA: ASCE Press. pp. 208-209. ISBN 9780784410714. Phil Plait (2009). “Anniversary of a cosmic blast”. discovermagazine. com. Retrieved on the 7th of October 2012. Simkin, Tom et al (2006). “This dynamic planet. World map of volcanoes, earthquakes, impact craters, and plate tectonics. Inset VI. Impacting extraterrestrials scar planetary surfaces”. U. S. Geological Survey. Retrieved from www. mineralsciences. si. edu/tdpmap/pdfs/impact. pdf on the 8th of October 2012.
Cite this Development Studies
Development Studies. (2017, Jan 23). Retrieved from https://graduateway.com/development-studies-2/