Modified Mercalli Intensity Scale Mana loa (1931 Abridged and rewritten) Intensi

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Modified Mercalli Intensity Scale Mana loa (1931 Abridged and rewritten) Intensity value (in bold) followed by description = L. Not felt Marginal and long-period effects of large earthquakes. leanses II. Felt by persons at rest, on upper floors, or favorably placed III. Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration estimated. May not be recognized as an earthquake Hazards IV. Hanging objects swing. Vibration like passing of heavy trucks; or sensation of a jolt like a heavy ball striking the walls Standing cars rock Windows, disbes, door rattle. Glasses clink. Crockery frame creak clashes. In the upper range of IV, wooden walls and V. Felt outdoors; direction estimated. Sleepers awakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. Shutters, pictures move. Pendulum clocks stop, start, change rate. VI. Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Knickknacks, books, etc, off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster and masonry D cracked. Small bells ring (church, school). Trees, bushes shaken visibly, or heard to rustle. VII. Difficult to stand. Noticed by drivers. Hanging objects quiver. Furniture broken. Damage to masonry D, including cracks. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, conices, also unbraced parapets and architectural ornaments. Some cracks in masonry C. Waves on ponds, water turbid with mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged. VIII. Steering of cars affected. Damage to masonry C: partial collapse. Some damage to masonry B; none to masonry A Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Decayed piling broken off. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes. IX. General panic. Masonry D destroyed, masonry C heavily damage, sometimes with complete collapse; masonry B seriously damaged. General damage to foundations. Frame structures, if not bolted, shifted off foundations. Frames racked. Serious damage to reservoirs. Underground pipes broken. Conspicuous cracks in ground. In alluviated areas, sand and mud ejected, earthquake fountains, sand craters

Explanation / Answer

2) When a certain strength of shaking is given, it is expected that the buildings of equivalent strength will not respond to the shaking in a uniform way but there shall be a modal level of damage observed. Like, some buildings will suffer less damage while others will suffer more. This is because the intensity/impact of an earthquake in a particular area depends mainly on the geological structures in that particular area as well as the types of buildings or structures present there. Those houses built on rocks will suffer less damage than those built on the loose sediments, both located at the same distance from the earthquake’s epicenter. Also, poorly built houses will obviously be damaged to a greater extent than those that have been reinforced or built specially to withstand earthquakes. Thus,the net effect will approximate to a normal distribution. Hence, for any particular level of shaking, it is expected that different percentages of the building of a given strength will suffer different degrees of damage (and this is true of the lower degrees as well as the damaging ones). In assessing Mercalli scale(intensity) we are usually dealing with a sample/estimate of the percentages that were observed, and attempt to match these to the expected ranges for one of the intensity degrees. This is how the scale is prepared and used. The Mercalli scale measures the intensity of an earthquake which is mainly affected by two factors: The depth of the earthquake - A very deep earthquake does much less damage than a shallow earthquake, so it is less intense.
The other factor is its distance from the epicenter - An earthquake that occurs very close to a city will do more damage. Thus, the information or phrases that were the most helpful when trying to access the Mercalli scale are mainly the degree of ground shaking, damage caused to structures and buildings, effect on the ground surface, experience of the quake on the population, it takes information from or the readings from the amount of death, damages caused to roads, bridges, accounts of witnesses who were there etc where a scale corresponding to 1 being almost undetectable and scale corresponding to 12 being of almost complete destruction.

3) The modified Mercalli scale or the MMI scale as is used as a reference to the intensity of damage inflicted by an earthquake in a particular area. So it is also known as an Intensity Scale. It is a local scale for a given area. The scale is particularly useful for measuring earthquakes in inhabited areas only because it is mainly based on observation. It is not considered scientific, as the experiences of the witnesses may vary and the damage caused may or may not accurately reflect an earthquake’s strength. However, it is used to compare the damage caused by an earthquake in different areas.

4) The Mercalli scale does not tell us about the earthquake source or its magnitude. It only describes the intensity of a quake and is mainly based on an earthquake's effect on people, buildings, structures, etc. It only measures the damage on society or how intense it was for a given area which means it is not concerned about the earthquake source and is thus a subjective measure. The limitations of the scale can be well explained by the followig example such as a very strong earthquake in the middle of an uninhabited land would register low on the Mercalli scale because there were no people to witness or structures to be affected or damaged. On the contrary, if a weak earthquake occurred right in an area with many people living there and having unstable houses, then this would cause a lot of damage or death and therefore register high on the Mercalli. So it is not an accurate scale. Therefore the main limitations can be stated as the fact that the eyewitness accounts are not always accurate because different people feel & see different things during a traumatic event especially during an earthquake and the intensity of shaking largely depends on the near-surface and regional geologic conditions.

5) Yes, there is a strong relation between what the ground material is made of and the intensity value. This is because the structural damage may not always be the same for all buildings. The structural stability & how well the sediments are consolidated beneath those foundations all play a major role in determining if a building will withstand an earthquake or will break down. Structures constructed on loose sediments are more prone to damages because the shaking caused during a quake can easily displace those sediments thus making those structures to fall off. The houses built on strong and hard rocks will suffer less damage and corespond to a lower value on the Mercalli scale showing low intensity than those built on the sediments, both located at the same distance from an earthquake’s epicenter.

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