सिन्धुपाल्चोक केन्द्रविन्दू भएर भूकम्प

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काठमाडौं ः सिन्धुपाल्चोकको पाङफुङ केन्द्रविन्दू भएर भूकम्प गएको छ।

राष्ट्रिय भूकम्प मापन तथा अनुसन्धान केन्द्रका अनुसार शनिबार साँझ ७ः५८ बजे पाङफुङ केन्द्रबिन्दु भएर ४.६ रेक्टर स्केलको भूकम्प गएको हो। उक्त भूकम्प गोरखा भूकम्पको परकम्प भएको सो केन्द्रले जनाएको छ।

विसं २०७२ वैशाख १२ गते गएको विनाशकारी भूकम्पपछि हालसम्म नेपालमा ४ रेक्टर स्केलभन्दा माथिका ५६० परकम्प गइसकेको छ।

Earthquakes are the result of sudden movement along faults within the Earth. The movement releases stored-up ‘elastic strain’ energy in the form of seismic waves, which propagate through the Earth and cause the ground surface to shake. Such movement on the faults is generally a response to long-term deformation and the buildup of stress.

The structure of the Earth
Seismic waves from large earthquakes pass throughout the Earth. These waves contain vital information about the internal structure of the Earth. As seismic waves pass through the Earth, they are refracted, or bent, like rays of light bend when they pass through a glass prism. Because the speed of the seismic waves depends on density, we can use the travel-time of seismic waves to map change in density with depth and show that the Earth is composed of several layers.

Hypothetical P-waves
The crust
The mantle
The core
Plate tectonics
The Earth’s outermost layer is fragmented into about 15 major slabs called tectonic plates. These slabs form the lithosphere, which is comprised of the crust (continental and oceanic) and the upper part of the mantle. Tectonic plates move very slowly relative to each other, typically a few centimetres per year, but this still causes a huge amount of deformation at the plate boundaries, which in turn results in earthquakes.

Observations show that most earthquakes are associated with tectonic plate boundaries and the theory of plate tectonics can be used to provide a simplified explanation of the global distribution of earthquakes, while some of the characteristics of earthquakes can be explained by using a simple elastic rebound theory.

Plate tectonic map of the world showing direction of movement
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Plate tectonic map of the world showing direction of movement. BGS ©UKRI.

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What drives the movement of tectonic plates?
Below the tectonic plates lies the Earth’s asthenosphere. The asthenosphere behaves like a fluid over very long time scales. There are a number of competing theories that attempt to explain what drives the movement of tectonic plates. Three of the forces that have been proposed as the main drivers of tectonic plate movement are:

mantle convection currents: warm mantle currents drive and carry plates of lithosphere along a like a conveyor belt
ridge push (buoyant upwelling mantle at mid-ocean ridges): newly formed plates at oceanic ridges are warm, so they have a higher elevation at the oceanic ridge than the colder, more dense plate material further away; gravity causes the higher plate at the ridge to push away the lithosphere that lies further from the ridge
slab pull: older, colder plates sink at subduction zones because, as they cool, they become more dense than the underlying mantle and the cooler, sinking plate pulls the rest of the warmer plate along behind it
Research has shown that the major driving force for most plate movement is slab pull, because the plates with more of their edges being subducted are the faster-moving ones. However, ridge push is also presented in recent research to be a force that drives the movement of plates.

What drives plate tectonics
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Mantle convection currents, ridge push and slab pull are three of the forces that have been proposed as the main drivers of plate movement (based on What drives the plates?). BGS © UKRI. All rights reserved.

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Types of plate boundary
There are three types of plated boundary:

divergent: plates moving apart
convergent: plates coming together
transform: plates moving past each other
Boundaries between tectonic plates are made up of a system of faults. Each type of boundary is associated with one of three basic types of fault, called normal, reverse and strike-slip faults.

Divergent boundary
Convergent boundary
Transform boundary
Elastic rebound theory
Elastic rebound theory was originally proposed after the great San Francisco earthquake in 1906 by the geologist Henry Fielding Reid, to explain the deformation caused by earthquakes.

Before an earthquake, the buildup of stress in the rocks on either side of a fault results in gradual deformation. Eventually, this deformation exceeds the frictional force holding the rocks together and sudden slip occurs along the fault. This releases the accumulated stress and the rocks on either side of the fault return to their original shape (elastic rebound) but are offset on either side of the fault.

Movements in the crust cause stress to build up.
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Over time stresses in the Earth build up (often caused by the slow movements of tectonic plates). At some point the stresses become so great that the Earth breaks. An earthquake rupture occurs and relieves some of the stresses (but generally not all). BGS ©UKRI.

Types of fault
There are three basic types of fault: normal, reverse and strike-slip. Certain types of fault are characteristic of the different plate boundaries, although often more than one type of

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