Subject: General Knowledge and IQ
Earthquake is the sudden movement of the ground (earth) due to the release of the strain (elastic energy) of the ground. The place where an earthquake originates, below the ground surface, is called hypocenter. The point directly above the hypocenter on the ground is called epicenter. Earthquakes occur all the time, but we do not feel all the vibrations. Occasionally there are quakes that we can feel but they do not damage much. Then there are some which we feel and cause slight damage and others are rare but cause substantial damage. As the intensity of the earthquake increases, the extent of the damage increases.
The intensity of the earthquake is measured in two different scales. One is Richter scale and another is Modified Mercalli Index (MMI). The Ritcher scale measures the amount of energy released in the earthquake and MMI measures the amount of movement that earthquake generates.
Ritcher Scale: It was first developed by Charles F. Richter in 1935. In this scale magnitude of an earthquake is determined by the amplitude of the waves recorded by the seismographs. Each whole number increase in the magnitude of the earthquake has 31 times the higher energy and 10 times higher ground shaking. (USGS, Earthquake Glossary - Richter scale) For example: magnitude 7.0 earthquake has 31 times higher energy than magnitude 6.0 earthquake and 961 times higher energy than magnitude 5.0 earthquake, also the shaking generated by magnitude 7.0 earthquake is 10 times higher than magnitude 6.0 earthquake and 100 times higher than magnitude 5.0 earthquake. This is because it is measured in logarithmic scale. In this scale, there are some variations too. It can be measured in local magnitude, surface-wave magnitude, body-wave magnitude and moment magnitude. First three have limitations so nowadays moment magnitude is mostly used (Bilham, Location, and Magnitude of the 1833 Nepal Earthquake and its Relation to the rupture Zone of Contiguous Great Himalayan Earthquakes).
Modified Mercalli Index (MMI): MMI measures the shaking of the ground as felt by the people. This is not regarded as the scientific method because it uses the observation of people who experienced the earthquake, which might not be accurate also, this is affected by the factors like building designs, distance from epicenter and type of material the city rests on (UPSeis). So, magnitude 7 earthquake in Japan is not as destructive, though large, as magnitude 7 in Nepal, where constructions are not as sturdy as constructions in Japan.
When an earthquake strikes there are two different types of waves that are produced which are body waves and surface waves (Michigan Technological University). There are two different types of body wave, P, and S, out of which S does more damage than P wave (Michigan Technological University). The real destruction is done by the surface wave, which are also of two types: Love and Rayleigh. They are the ones that move ground so violently that it is very hard for structures to remain straight and still (Michigan Technological University). Since the surface wave intensity is greatly reduced in the deep earthquake, they do not produce much damage (Michigan Technological University).
The crust of the earth is divided into different tectonic plates and they move towards or away from each other. They float on the lava and move few centimeters each year. There are about 12 major plates that slide or collide with or, move away from each other (Mooreland School). The reason for the earthquake is the collision of these tectonic plates. The collision of the tectonic plates results in the earthquakes. In the case of Nepal, Indian plate slammed into the Eurasian plate, about 65-45 million years ago and is moving at the rate of 40-50mm/yr (USGS, M7.8 - 34km ESE of Lamjung, Nepal) (Field and Bilham). This collision, over the millions of years, produced the great Himalayas. These both plates are continental plates but Indian plate being less dense is subducted under the Eurasian plate and Eurasian plate resists the movement of the Indian plate. This tug of continents produces a lot of elastic energies that is being stored in the plates. When the enough energy is stored and plate cannot hold the energy they rupture (USGS, M7.8 - 34km ESE of Lamjung, Nepal). This rupture moves along the fault lines. Faults lines are the fractures along which the block of crust on either side have moved relative to one another parallel to the fracture (USGS, Earthquake Glossary - fault). There are three different types of faults, strike-slip, normal fault and thrust fault also called reverse fault (USGS, Earthquake Glossary - fault). The fault line of Himalayan Arc extends about 2200 km and a portion of it runs through Nepal. In Nepal, there are four different fault lines: South Tibetan Detachment Fault System, Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT) which separate the five distinct morph-geotectonic zones of Nepal (Department of Mines and Geology Nepal). These thrusts are the main site of the earthquakes. The active ones are MCT, MBT, and MFT (Yeats, Nakata, and Farah). When an earthquake strikes these thrusts slip and get ruptured; and they travel along the fault line but can create new fault lines too. The magnitude of the earthquake depends on the length of the rupture, which is directly proportional, and slip it produces, which is also directly proportional i.e. higher the length of the rupture and height of the slip greater is the magnitude of the earthquake (Field and Bilham). Slip is the distance, in the fault, that the ground moves relatively away from each other (USGS, Earthquake Glossary - slip). We have detailed mapping of MFT that runs along Terai region of Nepal but for the rest of the fault we know a little (Yeats, Nakata, and Farah). The travel rate of the rupture varies according to the earthquake and the place it ruptured. The energy is carried by the rupture and has highest shaking around the rupture area.
Though Nepal lies in the seismically hazard zone, there has not been a proper study of earthquakes. There have been substantial studies in Pakistan area and Bhutan area of the same Himalayan Thrust that Nepal is part of but not in Nepal. There are many tales of huge earthquakes in Nepal and some of them do corroborate with the geological findings. There have been various earthquakes and the great ones in the entire fault system but the major ones that strike Kathmandu are rare though we see the smaller quakes frequently. The several earthquakes that are recorded in or near Nepal are in Kumaon 1505, Nepal 1833, Nepal-Bihar 1934, Chamoli 1999 etc. (Bilham, Earthquakes in India and the Himalaya: tectonics, geodesy, and history). There is a brief reference of a massive earthquake in 1255 by Wright but the proper documentation, even in other text, is not found; so the credibility of this claim is also disputed (Bilham, Earthquakes in India and the Himalaya: tectonics, geodesy, and history). The earthquake in 1505 which was about Mw > 8.2 based on its probable rupture area (Bilham, Earthquakes in India and the Himalaya: tectonics, geodesy, and history). The records of an earthquake during the 18th century are poorly documented like in the case of 1720 quake near Delhi which caused liquefaction, but little is known about the event.
Another historical earthquake happened in Kangra in 1905 which was of magnitude 7.8 in Richter scale. This earthquake had its oddities as the shaking of intensity VIII, in MMI, was felt 250 km far from the epicenter, so the magnitude was inflated to 8 (Ambry says and Bilham) but the geologic study shows no facts of magnitude 8 earthquake.
The earthquake that rattled Nepal came in the year of 1934 as Nepal-Bihar earthquake. An earthquake of magnitude 8.1 struck near the border of Nepal and India on January 15, 1934, at 2:28 pm (USGS, Historic Earthquake: Bihar, India - Nepal). The epicenter of this earthquake was reported at 26.60 N, 86.80E, which lies in India (Singh and Gupta), but later was reported as 26.770N, 86.760E (Engdahl and Villasenor). The uncertainty of the epicenter is by +- 50km (Bilham, Bodin, and Jackson, Entertaining a Great Earthquake in Western Nepal: Historic Inactivity and Geodetic Tests for the Present State of Strain) and it occurred near the border of the two countries, the location of the epicenter is still disputed. The rupture then moved east towards Kathmandu and the destruction done by the earthquake consistent with the movement of the rupture (Bilham, Bodin, and Jackson, Entertaining a Great Earthquake in Western Nepal: Historic Inactivity and Geodetic Tests for the Present State of Strain). There was an extensive damage done by this earthquake in Bihar and Nepal. The especially hit areas of Nepal were the districts of the eastern terai and Kathmandu city. Some part of the valley felt MMI of X and rest felt IX, which completely destroyed 19% of the homes and 38% were badly injured (Pandey and Molnar). The destruction subsided as we move west from the Kathmandu valley as the MMI felt was decreasing in order though there was damage reported till Nuwakot, which felt MMI of VI (Pandey and Molnar).
The earthquake of 25 May 2015 was the recent earthquake that hit Nepal. The earthquake struck at 11:56:26 was 7.8 in Ritcher moment magnitude scale with MMI of IX in the epicentral area, Gorkha, and Kathmandu valley (USGS, M7.8 - 34km ESE of Lamjung, Nepal). This earthquake ruptured 120 km of the fault length and rupture moved eastward towards Kathmandu and 80 km of fault breadth towards the south, 9600 sq. km of the area (USGS, M7.8 - 34km ESE of Lamjung, Nepal). Though long predicted (Bilham, Earthquakes in India and the Himalaya: tectonics, geodesy, and history) (Bilham, Bodin and Jackson, Entertaining a Great Earthquake in Western Nepal: Historic Inactivity and Geodetic Tests for the Present State of Strain). Nepal was not as prepared as it should be, so the loss of life and property was extensive. But good thing that happened was it did not damage as it was expected by the seismologists. The reason for this is still under research. This earthquake moved Kathmandu 3m south and 1.6m high. The death toll in Nepal is 8601, as of 6/6/15, out of which 3440 is in Sindupalchok district (Emergency Management Center). Ten districts near the rupture area are hardly hit, but total 39 districts reported at least a death by the earthquake and about forty districts reported damaged buildings due to this earthquake.
We do know that Nepal lies on one of the most active seismic faults and sees major earthquake every 80 or so years. There is no way to predict the earthquake, a seismologist can say how much of strain the fault has and what can be the outcome of that strain, but the exact time is unknown to everybody. So we have to be vigilant and prepare ourselves for the future earthquakes. Know the basis of the earthquake and do not follow the irrelevant news or rumors. We should know what to do when the earthquake strikes and after that. Now Nepal is going through the rebuilding phase. We have to create structures that are resilient to earthquakes and that can minimize the casualties.
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