EARTHQUAKES AND NEPAL
AB Thapa
Earthquake, shaking of the Earth’s surface is caused by rapid movement of the Earth’s rocky outer layer. Earthquakes occur when energy stored within the Earth, usually in the form of strain in rocks, suddenly releases. This energy is transmitted to the surface of the Earth by earthquake waves.
Most earthquakes result from the sudden slip along geologic faults. The faults slip because of movement of the Earth’s tectonic plates. The rocky tectonic plates move very slowly, floating on top of a weaker rocky layer. As the plates collide with each other or slide past each other, pressure builds up within the rocky crust. Earthquakes occur when pressure within the crust increases slowly over hundreds of years and finally exceeds the strength of the rocks
The convergence of the Indian plate northward into the Eurasian plate has resulted in the formation of the Himalayas . The Indus-Tsampo suture ( ITS) acted as the primary locale of plate interaction and convergence. There are two main faults running from east to west. Upper one is the Main Central Thrust (MCT) and the lower one the Main Boundary Thrust (MBT). It is regarded that the rocks between the ITS and the MCT, and the rocks between the MCT and the MBT represent successive slices of the Indian plate that have been accreted into the Eurasian plate.
It is considered that since the closing of the Indo-Tsangpo suture, the active plate boundary has shifted progressively southward, first to the Main Central Thrust and more recently to the Main Boundary Thrust. The more northerly MCT represents the older, presently less active boundary, while the MBT represents the present active plate boundary. Thus, periodic great earthquakes ( magnitude 8+ ) occurring in this region are thought to have originated on the MBT.
Earthquake Damages
We all know very well how devastating earthquake can be. In the last 500 years, several million people might have been killed by earthquakes around the world. Earthquakes have also caused severe property and structural damages. The disaster in Tangshan in north- east China, which occurred on 27 July 1976, is reported to have killed no less than 650,000 people and the damage extended to at least 150 km from the epicenter.
The 1934 January 15 Bihar-Nepal earthquake had caused very serious damages in Nepal. Studies of the Bihar-Nepal earthquake have been published by Dunn, Auden and Ghosh in the Memoirs of the Geological Survey of India (1939). The maximum intensity of X on the Modified Mercalli scale was assigned to a belt about 130 km long and 30 km wide, and two spots almost 160 km distant from the main belt, at Monghyr to the south and in the Kathmandu Valley to the north. The devastation at Monghyr was greater than in any other part of Bihar. The entire town was reduced to ruins, scarcely a house or hut escaped destruction or damage. The Kathmandu valley is formed by unconsolidated sediments resting on metamorphic and on partially metamorphosed pre-Tertiary series. The heavy damage was on the unconsolidated ground.
Earthquakes that occur in the area surrounding the Pacific Ocean, at the edges of the Pacific plate, are responsible for an average of 80 percent of the energy released in earthquakes worldwide. Japan is shaken by more than 1,000 tremors greater than 3.5 in magnitude each year. The western coasts of North and South America are also very active earthquake zones, with several thousand small to moderate earthquakes each year.
The destruction an earthquake causes depends on its magnitude and duration, or the amount of shaking that occurs. A structure’s design and the materials used in its construction also affect the amount of damage the structure incurs. Earthquakes vary from small, imperceptible shaking to large shocks felt over thousands of kilometers. Earthquakes can deform the ground, make buildings and other structures collapse, and create tsunamis (large sea waves). Lives may be lost in the resulting destruction.
Reservoir Induced Earthquakes
Fault rupture is not the only cause of earthquakes; human activities can also be the direct or indirect cause of significant earthquakes. Injecting fluid into deep wells for waste disposal, filling reservoirs with water, and underground nuclear test blasts can, in limited circumstances, lead to earthquakes. These activities increase the strain within the rock near the location of the activity so that rock slips and slides along pre-existing faults more easily. While earthquakes caused by human activities may be harmful, they can also provide useful information. Prior to the Nuclear Test Ban treaty, scientists were able to analyze the travel and arrival times of P waves from known earthquakes caused by underground nuclear test blasts. Scientists used this information to study earthquake waves and determine the interior structure of the Earth.
Scientists have determined that as water level in a reservoir increases, water pressure in pores inside the rocks along local faults also increases. The increased pressure may cause the rocks to slip, generating earthquakes. Beginning in 1935, the first detailed evidence of reservoir-induced earthquakes came from the filling of Lake Mead behind Hoover Dam on the Nevada-Arizona state border. Earthquakes were rare in the area prior to construction of the dam, but seismographs registered at least 600 shallow-focus earthquakes between 1936 and 1946. Most reservoirs, however, do not cause earthquakes.
Earthquake Scales
Earthquake size can be measured by seismic intensity as well, a measure of the effects of an earthquake. Before the advent of seismographs, people could only judge the size of an earthquake by its effects on humans or on geological or human-made structures. Such observations are the basis of earthquake intensity scales first set up in 1873 by Italian seismologist M. S. Rossi and Swiss scientist F. A. Forel. These scales were later superseded by the Mercalli scale, created in 1902 by Italian seismologist Giuseppe Mercalli. In 1931 American seismologists H. O. Wood and Frank Neumann adapted the standards set up by Giuseppe Mercalli to California conditions and created the Modified Mercalli scale. Many seismologists around the world still use the Modified Mercalli scale to measure the size of an earthquake based on its effects. The Modified Mercalli scale rates the ground shaking by a general description of human reactions to the shaking and of structural damage that occur during a tremor. This information is gathered from local reports, damage to specific structures, landslides, and peoples’ descriptions of the damage.
Modified Mercalli and Richter Scales
The Modified Mercalli and Richter scales are used to rate and compare the intensity of earthquakes. The Modified Mercalli scale is somewhat subjective, because the apparent intensity of an earthquake depends on how far away from its center the observer is located. Rating intensities from I to XII, it describes and rates earthquakes in terms of human reactions and observations. The Richter scale measures the motion of the ground 100 km (60 mi) from the earthquake’s epicenter, or the location on the earth’s surface directly above where the earthquake occurred. The rating scale is logarithmic; each increase of 1 on the scale represents a tenfold increase in the motion of the ground
All magnitude scales give relative numbers that have no physical units. The first widely used seismic magnitude scale was developed by the American seismologist Charles Richter in 1935. The Richter scale measures the amplitude, or height, of seismic surface waves. The scale is logarithmic, so that each successive unit of magnitude measure represents a tenfold increase in amplitude of the seismogram patterns. This is because ground displacement of earthquake waves can range from less than a millimeter to many meters. Richter adjusted for this huge range in measurements by taking the logarithm of the recorded wave heights. So, a magnitude 5 Richter measurement is ten times greater than a magnitude 4; while it is 10 x 10, or 100 times greater than a magnitude 3 measurement.
Today, seismologists prefer to use a different kind of magnitude scale, called the moment magnitude scale, to measure earthquakes. Seismologists calculate moment magnitude by measuring the seismic moment of an earthquake, or the earthquake’s strength based on a calculation of the area and the amount of displacement in the slip. The moment magnitude is obtained by multiplying these two measurements. It is more reliable for earthquakes that measure above magnitude 7 on other scales that refer only to part of the seismic waves, whereas the moment magnitude scale measures the total size. The moment magnitude of the 1906 San Francisco earthquake was 7.6; the Alaskan earthquake of 1964, about 9.0; and the 1995 Kōbe, Japan, earthquake was a 7.0 moment magnitude; in comparison, the Richter magnitudes were 8.3, 9.2, and 6.8, respectively for these tremors.
Minimizing Earthquake Damages
Earthquakes cannot be prevented, but the damage they cause can be greatly reduced. Adequate precautions, such as education, emergency planning, and constructing stronger, more flexible, safely designed structures, can limit the loss of life and decrease the damage caused by earthquakes. In response to the tragic loss of life and great cost of rebuilding after past earthquakes, many countries have established earthquake safety and regulatory agencies. These agencies require codes for engineers to use in order to regulate development and construction. Buildings built according to these codes survive earthquakes better and ensure that earthquake risk is reduced.
Geologists and engineers use risk assessment maps, such as geologic hazard and seismic hazard zoning maps, to understand where faults are located and how to build near them safely. Engineers use geologic hazard maps to predict the average ground motions in a particular area and apply these predicted motions during engineering design phases of major construction projects. Engineers also use risk assessment maps to avoid building on major faults or to make sure that proper earthquake bracing is added to buildings constructed in zones that are prone to strong tremors. They can also use risk assessment maps to aid in the retrofit, or reinforcement, of older structures.
