Dr. Niraj Kumar Singh Assistant Professor,

Department of Geography, S. M. S. G. College, Sherghati,

Gaya, Bihar

EARTHQUAKES: An Overview

Earthquakes: Introduction

• The movement of Earth’s crust resulting from the release of built up potential energy between two tectonic plates.

• An earthquake occurs when Earth’s surface shakes because of the release of seismic energy following the rapid movement of large blocks of the crust along a fault.

• The place of origin of an earthquake is called focus. Earthquake foci occur at a range of depths. The deepest earthquake might have its focus below the ground surface at depth of 700 km.

• The waves generated by earthquake are called Seismic waves. It radiate outward from the focus.

• The Epicentre is the point on the surface directly above the focus.

• It is a point on Earth’s surface that is directly above the focus of an earthquake, where seismic waves are recorded at first time so that shaking is strongest and most damage occurs at epicentre.

• Seismic waves are captured by a recorder known as a seismograph. Earthquakes are measured on Richter Scale

• The Richter magnitude scale is a scale of numbers used to tell the power (or magnitude) of earthquakes.

Focus and Epicentre

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Earthquakes are mainly caused due to disequilibrium or isostatic imbalances in earth’s crust. 1. Vulcanicity 2. Faulting and elastic Rebound Theory (H.F.

Reid, 1906) 3. Hydraulic Pressure and Anthropogenic

Causes 4. Plate Tectonic Theory

A. Constructive Plate Margins B. Destructive Plate Margins C. Conservative Plate Margins

Causes of Earthquakes

Classification on the basis of Focus of Earthquakes Guttenberg has discussed thee types of earthquakes on the basis of its focus. 1. Moderate earthquakes (foci are located at depths between earth surface to 50 km.) 2. Intermediate earthquakes (foci are located at depths between 50km to 250 km.) 3. Moderate earthquakes (foci are located at depths 250 km. To 700km.)

Seismic Waves

During occurrence of earthquakes different amounts of tremors and waves are generated are known as Seismic Waves. Seismic waves represent the energy released from the earthquake focus. Seismic waves can be divided into surface waves that travel on Earth's surface and body waves that travel through Earth. Body waves travel through Earth’s interior. These are further subdivided into P (primary) waves and S (secondary or shear) waves based upon their vibration direction and velocity. P waves are analogous to sound waves. The waves travel at different rates from common source. So that, their arrival at recording station also vary. Based on body waves (P&S) travel speed in different medium, the earth’s interior has been divided in to Crust, Mantle and Core.

P Waves • These waves arrive first. Move with a push pull

motion • P waves vibrate parallel to their travel direction so

it is called Longitudinal or Compressional waves. • P waves travel at speeds of 4 to 6 km per second in

the uppermost part of the crust. • P waves are analogous to sound waves. • These travel with fastest speed in earth interior and

can pass through liquid materials at reduced speed.

S Waves • These waves arrive second. Move with a side-to side motion so it is called transverse

or simply S waves. • These are analogous to water ripples or light waves. • S waves vibrate perpendicular to their travel direction. • S wave velocity is 3 to 4 km per second in the shallow crust. • It can’t pass trough liquid materials. • S waves disappear at the angular distance of 103 to 143 degree is known as Shadow

Zone.

Surface waves • Surface waves travel on Earth’s surface and cause much of the destruction associated with

earthquakes. • Undulations of the land surface during an earthquake are a representation of surface waves. • Rayleigh waves: Surface waves may result in vertical motions are known as Rayleigh waves,

they are like water waves traveling through water, • Love waves: motions in Love waves are entirely horizontal at right angles to the direction of

wave motion. • Surface waves speed is lower than body waves(P&S) but these are very destructive and violent. • Rayleigh waves is named after english physicist Lord Rayleigh. However, love waves is named

after physicist A.E.H. Love.

Fault Classification

A fault is a region on Earth’s surface that is broken into two pieces. Faults are distinguished as dip-slip or strike-slip faults There can be three types of movements. 1.Normal Fault 2.Reverse Fault 3.strike-Slip Fault

1. Normal Fault

• In a normal fault, hanging wall moves down relative to the footwall.

• These are formed due to displacements of both the rocks block in opposite direction.

• The fault plane is found between45 to 90 degree.

2. Reverse Fault

• The hanging wall moves up relative to the footwall in a reverse fault.

• These are formed due to displacements of both the rocks block towards each other.

• The fault plane is found between horizontal(0) to 40 degree.

3. Strike-slip faults Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. Strike Slip Faults are categorised in two types. 1. Right-slip fault 2. Left-slip fault An observer, standing on one side of the fault, sees objects on the other side of the fault move to the right for a right-slip fault or to the left for a left-slip fault.

Distribution of Earthquakes • Earthquakes are most frequent along plate boundaries. • The largest earthquakes are associated with convergent plate boundaries. • Oceanic ridges are characterised by shallow earthquakes. • Deep earthquakes (to depths of 700 km) occur within subduction zones along convergent

plate boundaries. • The most devastating earthquakes are typically shallow earthquakes (0-33 km depth). • There is a strong correlation between earthquake foci and plate boundaries.

Distribution of Earthquakes

• Swarms of earthquakes resulting from collisions of continental plates form a belt across central Asia, through the Middle East and southern Europe.

• Continental interiors that are far removed from plate boundaries have few earthquakes. • A belt of shallow earthquakes can be traced along the global oceanic ridge system from the center

of the Atlantic ocean, through the Indian Ocean, around the southern Pacific Ocean, and into the East Pacific.

• Earthquakes are present under hot spots such as the Hawaiian Islands in the central Pacific Ocean. • The largest earthquakes are associated with convergent plate boundaries.

Distribution of Earthquakes

1. Circum-Pacific Belt• This belt accounts for 65 percent of total earthquakes of the world.• It is also known as Pacific Ring of Fire. • It includes coastal margins of North and South America and Eastern Asia comprising of

eastern and western margins of Pacific ocean respectively.• This belt represents four ideal conditions

1. Continental and Scenic margins2. Young folded mountains.3. Active volcanoes zone.4. Subduction zones of destructive plate margins.

• Recent earthquake in Mexico City in 1985 was related to destructive plate margins.• East Asian earthquakes are caused due to collision of Pacific and Asiatic plates.• It includes Kamchatka, Sakhalin, Japan and Philippines etc. • In Japan, these earthquakes create Tsunamis which flooded towns and drown thousands

people.

Mid-Continental belt •It is known as Mediterranean belt or Alpine- Himalayan belt. •It also comprises with subduction or destructive plate boundaries. •This belt includes epicentre of Alpine Mountains and their off-shoots in Europe, Mediterranean Sea, Northern and Eastern Africa, Himalayan Mountains and Burmese Hills.

•Bhuj earthquake(Jan.26 2001) shook the region for almost a minute. Indian Meteorological Department (IMD) measured the magnitude of quake 6.9 at Richter scale.

•The main cause of Bhuj earthquake was Sea Floor Spreading of Indian Ocean, northward movements of Indian plate and two major faults line located in Kutch region.

•Nepal earthquake (2015) also known as Gorkha Earthquake killed more than 15 thousands people and injured several thousands was recorded magnitude of 7.8 at Richter scale. It was caused due to subduction of (Destructive Plate Margins) Indian plates below the Tibetan Plates.

Mid-Atlantic Ridge It includes epicentre of quakes located near the Mid-Atlantic Ridge. This belt records from moderate to shallow foci earthquakes because it is located near Constructive Plate boundaries. Thus, seafloor spreading and fissure types volcano activities cause moderate and low intensity earthquakes.

Tsunamis The seismic waves caused by earthquakes in oceanic region generate long sea waves that cause huge loss of life in Coastal region. In Pacific region tsunamis are more common with a minimum frequency of 2 tsunamis per year. Indian Ocean quake (2004) with 9.1 magnitudes near Sumatra triggered massive tsunamis which caused 40 mt. sea level rise. The tsunamis generated was so violent and destructive that it rolled over Indian Ocean coast line and severely affected the Sri Lanka, India, Bangladesh, and Southeast Asian countries. Its disastrous impacts were felt at eastern coast of Southern Africa. It claimed more than 2.5 lakh lives and millions homeless.

The effects of earthquakes are : 1. Ground shaking and ground rupture 2. Loss of human lives and property 3. Landslides and slope instability 4. Fires 5. Floods 6. Damage to Town and Cities 7. Tsunamis