Earthquake and volcanoes - class-VIII

Thermal springs, geysers, and fumaroles are classic examples of post-volcanic or hydrothermal activities that occur in areas where volcanic activity has subsided or is waning.

The epicenter is the point on the Earth's surface directly above the focus (or hypocenter) where the earthquake originates.

A tsunami is a series of large ocean waves caused by underwater earthquakes, volcanic eruptions, or landslides.

The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs.

• Volcanic activities refer to pouring out of hot magma from weak layers of the earth's crust while seismic occurrences are the shaking of the earth's crust due to the release of energy at plate boundaries. Both the phenomenon occur at the plate boundaries of the earth crust, and they draw energy from the same sources. Thus both are interrelated.

An earthquake is a moment of tremor of the earth's crust. It originates naturally and below the surface. It sometimes causes a permanent change of level at the surface of the earth.

According to the theory of plate tectonics, the world has been divided into six major plates and several smaller movable plates. Each of the plates is composed of crust up to a depth of 100 km from the surface of the earth. Due to the forces at the earth's interior, these plates move.

The epicenter of an earthquake  is the point on the earth's surface vertically above the focus. Maximum damage is caused at the epicenter. 

Following activities of humans can trigger an earthquake:

P- Waves or the primary waves are also known as longitudinal or compressional waves. It can pass through solid, liquid and gas. P- waves travel at the fastest speed through solid materials. Though these also pass through liquid and gaseous materials their speed is slowed down. 

S-Waves or secondary waves are also called transverse or distortional waves. These are analogous to water ripples or light waves wherein the particles move at right angles to the rays. S- Wave cannot pass through liquid materials. It can only pass through solids.  

Krakatoa is a volcanic island situated in the Sunda Strait between the islands of Java and Sumatra in the Indonesian province of Lampung. The name is also used for the surrounding island group comprising the remnants of a much larger island of three volcanic peaks which was obliterated in a cataclysmic 1883 eruption, unleashing huge tsunamis (killing more than 36,000 people) and destroying over two-thirds of the island. The explosion is considered to be the loudest sound ever heard in modern history, with reports of it being heard up to 4,800 km from its point of origin. The shock waves from the explosion were recorded on barographs worldwide for days afterward.

The Focus or Seismic Focus is also called the hypocenter of an earthquake. The vibrating waves travel away from the focus of the earthquake in all directions. The waves can be so powerful they will reach all parts of the Earth and cause it to vibrate like a tuning fork. The focus is the exact point inside the crust of the earth where the quake begins.

The San Andreas Fault is a transform plate boundary where the movement of tectonic plates causes folding and faulting of the Earth's crust, leading to earthquakes.

Duration is the vibration of earthquakes which can be felt by human beings that last from a few seconds to several minutes. Generally, the greater the intensity of the shocks, the longer they last. The average duration of shocks of sufficient intensity to produce much damage is perhaps from one to two minutes.

There are annually 8,000 - 10,000 earthquakes in the world, which comes to about an earthquake every hour. And there are many more that go undetected because there are no stations to record them over the oceans covering a very large surface of our earth. This is known as the Earthquake Frequency

The epicenter is the point on the earth's surface vertically above the focus of the earthquake. Maximum damage is caused at the epicenter.

Isoseismic Line is the line connecting all points on the surface of the earth where the intensity is the same. These lines show great irregularity in their form. 

According to the Bureau of Indian Standards, India is divided into four seismic zones (II, III, IV, and V) based on the intensity of earthquake risk.

The Gorkha earthquake, which occurred on April 25, 2015, had a magnitude of 7.8 and caused significant destruction in Nepal.

Tectonics is a geological process involving the Earth's crust, whereas centrifugal force, solar energy, and precipitation are atmospheric or physical forces related to weather and climate.

During an earthquake, surface waves are the seismic waves that cause the most damage. Unlike other seismic waves that move deep inside the Earth, surface waves move along just under the surface of the Earth like waves in water. Surface waves are the slowest seismic waves and are the final waves to hit an area after an earthquake. First, P-waves move through compression through the deep layers of the Earth. They result in minimal damage. The second group of waves is S-waves. They move up and down starting deeper in the Earth. Stronger S-waves do some damage when they reach the surface. Surface waves move in last. Their slow roll just under the surface provides the greatest risk for damage to man-made structures and changes to natural landforms

Earthquake is a movement of tremor of the earth's crust. It originated naturally and below the surface. An earthquake produces infrasound before the mainshock wave begins. Infrasound is low-frequency sound less than 20hz. It can travel hundreds of km through the earth's surface. So by detecting this sound, we can predict an earthquake.

• Earth's crust is divided into various plates which float on hot magma layer called asthenosphere. When these plates move, they interact with each other which releases energy, that is felt on the surface of earth as tremors or shaking movement, which is termed as earthquake.
  
• Cyclones, thunderstorms and lightenings are the atmospheric phenomena and are not related with earth's surface, hence these 3 options are incorrect.

An earthquake  is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves.

An earthquake's point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter

The processes for building new land and coastal plains are called constructive forces. Three of the main constructive forces are crustal deformation, volcanic eruptions, and deposition of sediment.

Crustal deformation occurs when the shape of land (or crust) is changed or deformed. One of the main causes is movement of the Earth's plates. When the plates collide or push toward each other, pressure builds. This can cause two things to happen. The rock can either fold or fault.

Both A and B

Coastal plains are formed when ocean levels fall on continental shelves that are at sea level, or when rivers carry sediment to areas of the ocean. These plains can take hundreds of years to form, and a few are found hundreds of miles inland.

1. Earthquake may sometimes create new coastal plains, by the emergence of a portion of ocean floor.

2. Submergence of coastal land may result in the formation of bays and gulfs which are favorable sites for the development of ports harbors.

3.Earthquakes result in the formation of hot springs and geyser beneficial for medicinal purposes

• Vertical displacement of overlying water causes a stark difference between the elevation of the ocean floor due to which water from higher elevation quickly move towards lower elevation seafloor. 
• This generates very high waves which travel towards the coastal regions and are known as tsunami or harbour waves. Hence option C is correct.
• Note- Horizontal displacement does not cause displacement of a very significant amount of water hence cannot generate tsunamis.

July 9, 1958

On 9 July 1958, a 7.8Mw strike-slip earthquake in southeast Alaska caused 90 million tonnes of rock and ice to drop into the deep water at the head of Lituya Bay. The block fell almost vertically and hit the water with sufficient force to create a wave that surged up the opposite side of the head of the bay to a height of 524 m, and was still many tens of metres high further down the bay, when it carried eyewitnesses Howard Ulrich and his son Howard Jr. over the trees in their fishing boat. They were washed back into the bay and both survived

• The primary effect of the earthquake- shaking of earth's surface.
• Secondary effects- effects caused by primary effect i.e. effects caused by the shaking of the earth's surface viz tsunami, landslides, and fires.
• Hence all options are correct.

The primary destructive effect of Earthquakes include the damage to man-made structures, such as roads, bridges, dams, and buildings from the ground motion depends on the type of construction:

1. concrete and masonry structures are brittle and thus more susceptible to damage and collapse

2. damage to wood and steel structures is far less because of its flexibility.

Underwater earthquakes push water upward to create the initial movement, then gravity pulls the water downward, creating the horizontal force that forms the tsunami. The waves travel through the ocean in a similar fashion to ripples created by throwing a stone into a body of water.

Earthquakes occur often on the earth's crust that forms the sea floor; however, not all result in a tsunami. For a tsunami to form, the magnitude of the earthquake must measure a minimum of 7.0 on the Richter scale. Below this magnitude, the earthquake does not displace enough water to create the big wave. The movement must also lift or lower the sea bed. Sideways movement does not create the initial wave to start the process. The epicenter of the earthquake must be near the surface of the sea floor to cause the correct movement.

• An Earthquake at the bottom of ocean can cause large scale displacement of water leading into very high harbour waves called Tsunami. Tsunamis can in turn cause large scale flooding of the coastal areas. 
• Also Earthquakes the shaking of the mountaneous regions caused by earthquakes can cause landslides. Hence earthquakes can cause tsunami,flooding and landslides. 
• Lightening is an atmospheric phenomenon, hence is unrelated to earthquake.