Magnitude:

Measure of the amount of energy released during an earthquake. It is usually expressed using Richter scale.

Professor Charles Richter noticed that at a constant distance, seismograms records of earthquake ground vibration of larger earthquake have bigger wave amplitude than those of smaller earthquakes, and for given earthquake, seismograms at farther distances have smaller wave amplitude than those at close distances. Now commonly used magnitude scale, the Richter scale.

There are other magnitude scales viz,

  1. Body wave magnitude
  2. Surface wave magnitude and
  3. Wave energy magnitude.

It is defined as logarithm to the base 10 of the maximum trace amplitude, expressed in microns, which the standard short-period torsion seismometer would register due to the earthquake at an epicenter distance of 100 km.

Earth quake are often classified into different groups based on their magnitude. Table is given by:

Earthquake groups based on their magnitude

Intensity:

Intensity is a qualitative measure of the actual shaking at a location during an earthquake, and is assigned in Roman Capital Numerical.

It refers to the effects of earthquakes. Modified Mercalli scale is the standard measurement.

There are many intensity scales.

Two commonly used, ones are the Modified Mercalli Intensity (MMI) scale and MSK scale. Both are quite similar and range from I to XII . These intensity scale are based on the features of shaking- perception by people and animals, performance of buildings, and changes to natural surroundings.

Table 2 for Ressi-Forrel scale of earthquake Intensity:


Class

Name

Effects

1.

Imperceptible

Recorded by sensitive seismograph

2.

Feeble

Recorded by seismograph difference may be felt by number of person at rest.

3.

Very slight

Felt by several persons at rest, is strong enough for the duration and direction to be recorded.

4.

Slight

Disturbs persons in motion movable objected distributed, creaking of doors and windows.

5.

Weak

Disturbances of furniture and ringing of bells.

6.

Moderate

General awakening of those asleep, stopping of clocks, visible disturbances of trees.

7.

Strong

Overthrow of movable objects, fall of plaster, general panic with out serious damage to building.

8.

Very strong

Fall of chimneys, cracks in the walls.

9.

Severe

Partial or total destruction of some buildings.

10.

Destructive

All structures distributed.

Shaking intensity as per MSK scale:

Intensity VIII- Destruction of buildings

  1. Fright and panic. Also, persons driving motorcars are disturbed. Here and there branches of trees break off. Even heavy furniture moves and partly overturns. Hanging lamps are damaged in part.
  2. Most buildings of type C suffer damage of Grade 2, and few of grade 3. Most buildings of Type B suffer damage of Grade 3, and most buildings of Type A suffer damage of Grade 4. Occasional breaking of pipe seams occurs. Memorials and monuments move and twist. Stone walls collapse.
  3. Small landslips occur in hollows and on banked roads on steep slopes, cracks develop in ground up to widths of several centimeters. Water in lakes becomes turbid. New reservoirs come into existences. Dry wells refill and existing wells become dry. In many cases, changes in flow and level of water are observed.

Note:

Type A – structures- rural constructions.

Type B – Ordinary masonry constructions.

Type C – Well-built structures

Single Few- about 5%

Many – about 50%

Most – about 75%

Grade 1 Damage – Slight damage

Grade 2 moderate damage

Grade 3 Heavy damage

Grade 4 Destruction

Grade 5 Total damage.

Epicenter:

The geographical point on the surface of earth vertically above the focus of the earthquake.

Focus:

The originating source of the elastic waves inside the earth which cause shakings of ground due to earthquake. The point of maximum shock/stress release during an earthquake. Deeper focus earthquakes are often less damaging because the rocks absorb more energy before the waves hit the surface.

Shallow Focus Earthquake:

Earthquake of focus less than 70 km deep from ground surface are called shallow focus earthquakes.

Teleseism:

A teleseism is an earthquake recorded by a seismograph at a distance. By international convention the distance is over 1000 Kilometers from the epicenter. Earthquake originating near the recording station are termed as near earthquake or local earthquake.

Microseism:

These are more or less continuous disturbances in the ground recorded by seismographs.

Micro earthquake:

Very small earthquake having magnitude less than measurable than three on Richter scale are called Micro-earthquake. Highly sensitive seismographs are employed to monitor these for seismological and engineering applications.

Accelerogram:

The ground acceleration record produced by Accelerograph is called Accelerogram.

Accelerograph:

This is an earthquake-recording device designed to measure the ground motion in terms of acceleration in the epicentral region of strong shaking. It writes the time wise history of ground acceleration at a particular site.

Focal distance:

The straight-line distance between the places of recording/observation to the hypocenter is called the focal distance.

Intermediate Focus Earthquake:

The focus is between 70 to 300 km deep.

Epicentral Distance:

Distance between epicenter and recording station in km.

Fore Shocks:

Smaller earthquake that precede the main earthquake.

After shocks:

Smaller earthquake that follow the main earthquake

Asthenosphere:

The layer of the upper mantle which is close to melting point and behaves in a semi-plastic way. Convection currents in this layer are believed to influence the movement of tectonic plates.

Asthenosphere:

Benioff zone:

A region of earthquake activity inclined at an angle underneath a destructive boundary. Deeper earthquakes occur further from the boundary.

Constructive boundary:

A part of the earth’s crust where tectonic plates are moving away from each other, constructing new crustal material where they part. Associated with basic volcanism and frequent, shallow earthquakes.

Debris avalanche:

A sudden, large scale avalanche of rock. May be set off by heavy rain or by earthquake activity.

Destructive boundary:

A part of the earth’s crust where tectonic plates move towards one another, resulting in the seduction of one below the other.

Direct hazard:

A threat to life or property arising from the direct action of a hazard (eg shaking in an earthquake or blast in a volcano).

EARTHQUAKE TERMS

Fault:

A fracture in the rocks along which strain is occasionally released as an earthquake. By definition, only active faults are associated with earthquakes.

Lithosphere:

The rigid outer shell of the earth which normally comprises crust (oceanic or continental) and part of the upper mantle above the asthenosphere.

Liquefaction:

The process by which sediments and soil collapse, behaving like a thick liquid when shaken by earthquake waves.

Magnitude/frequency relationship:

The observed relationship (with most hazards) that bigger scale events occur less frequently while smaller scale events are relatively common.

Richter scale:

A measure of earthquake magnitude allowing an estimate of energy levels involved.

Rossi-Forrel scale:

An observational scale for measuring earthquake intensity. This was improved and expanded by Mercalli to produce the "Modified Mercalli Scale".

Seismograph:

A printout from a seismometer. Studies of seismograph traces can be used to pinpoint both the epicenter of an earthquake and the nature of the fault movement.

Accelerograms

Seismometer:

An instrument for detecting and recording earthquake waves.

Seismometer

Seduction zone:

A narrow region along a destructive plate boundary where one plate is consumed underneath another.

Transform boundary:

A plate boundary where the relative movement is sideways. The classic example of a transform boundary is in California where the San Andreas fault is a part of a transform plate boundary.

Tsunami:

An earthquake generated sea wave. Can travel thousands of miles and reach many metres in height when approaching shallow water.

techtonic plates

techtonic plates and past earthquakes in India