Earthquake

General Definition: The momentary shocks experienced by earth at different locations & different time.

Technical Definition : Vibration induced in earth's crust due to internal  and external causes that virtually shake up a part of crust , structures & living & non-living thins existing on it.

Terminology :

1.       Seismology : The science dealing with the study of earthquake in all their aspect is called seismology.

2.       Seismic : Used to qualify anything related to earthquake e.g.: seismic wave seismic intensity, seismic zone, seismic region.

3.       Focus : Place or point of origin of an earthquake below surface of earth is focus (hypocenter). It may lie from a few hundred meters to hundred of km below earth's surface.

4.       Epicenter : Point or place on the surface vertically above focus is epicenter. Its placed on surface of earth where vibration of particular earth quake  reach first of all .

5.       Seismic wave : During each earthquake elastic wave are generated at the focus. These wave are called seismic wave & they travel in all directions. with their characteristics and   velocity.

Types of seismic wave :

1.       P Waves :

a)       Primary waves

b)       Push & pull / long compression wave

c)       Fastest wave

d)       Velocity (Vp =

e)       P wave should travel fast in rigid bodies.

2.       S- waves :

a)       Secondary waves

b)       Shear waves, transverse wave, directional wave

c)       Vs=

d)       S – wave wouldn't be propagated through a liquid medium

P waves + S waves = Body waves.

3.       L-waves

a)       Long wave or surface wave

b)       Sluggish wave, only recorded after the arrival of P and S –wave

c)       Type of L wave

-          Rayleigh wave

-          Love wave.

Rayleigh wave	Love wave
1.       The complex nature partly in the direction of propagation & party in rt. angle. 2.       Tend to distort horz. surface into a wavy shape.	1.            Horizontal Propagation in direction of propagation. 2.            To create shearing , breaking & rupture

 Magnitude: Rating of an earthquake on the basis of amplitude of seismic wave recorded in seismograph. The scale used is Richter scale (8.9 highest ever)            

Intensity	Type	Magnitude	Effects
I	Instrumental		Recorded by  sismograph only
II	Very feeble	3.5-4.2	Felt by some person
III	Slight		Felt by every one at rest
IV	Moderate	4.3 - 4.8	Felt by people in motion
V	Rather strong		Felt outdoors and indoors
VI	Strong	4.9-5.4	General Panic
VII	Very strong	5.5-6.1	Breaking of furniture
VIII	Destructive	6.2 - 6.9	Fall of tanks projection C-D class building collapsing
IX	Ruinous		Cracks develop in the ground
X	Disastrous	7-7.3	General destruction of all classes of building, bridges dams.
XI	Very disastrous	7.4-8.1	Fissuring of ground
XII	Catastrophic	>8.1(8.9)	Total destruction in the region in terms of life.

Intensity :

Rating in a qualitative manner on the basis of its effect on living & non-living things. Scale used is Mercalli scale

Classification of Earthquake :

a)   Based on depth of focus.

1.   Shallow – up 60 km

2.   Intermediate 60-300

3.   Deep seated 300-700

b)   Based on magnitude

                 Class                                                        Magnitude

                    A                                                        7.8 and above

                    B                                                              7.0-7.7

                    C                                                              6.0-7.7

                    D                                                              5.3-6.0

                    E                                                          less than 5.3

c)   Based on cause of origin :

1)   Tectonic earthquake : due to faulting or relative displacement of plates along rupture surface

2)   Non tectonic earthquake : Volcanic eruption, landslide, atomic explosion., subsidence.

Engg. Consideration

a)      To know seismic history of any area

b)      Assessment of seismic risk

c)      A seismic designing

Geo Physics

i)        Is a tool to detect the physical properties of the subsurface materials

ii)      Main methods used are

a)      Resistively method

b)      Seismic method.

c)      Reflection method

d)     Magnetic

iii)                The sensing devices used are called geophones (microphone) converts seismic vibration or motions of the earth to electrical impulses.

Used to Detect

1.      Depth of bedrock

2.      Shape of bedrock surface

3.      Depth of water table

4.      Fault location

5.      Rip ability assessment

6.      Blasting assessment

7.      Sand and gravel assessment

8.      Cavity detection

9.      Determination of dynamic elastic constant.

General Rules

1.      Velocity is roughly proportional to the degree of consolidation of rock or soil.

2.      In consolidated materials, velocity increases with water content.

3.      Weathering of rock will greatly reduce it's velocity.

4.      A particular rock type will include a range of velocities and these ranges may overlap for different rock types.

5.      Correlation of velocity with the type of earth materials, to a great extent will depon upon the overall geological characteristics of the area under study.

Electric resistivity method – Give a physical property that characterizes a material almost as definitely as its density.

-          Resistivity depends upon the quality and the quantity of the water filling the open spaces in the rock.

-          The flow of current through soil and rock is by ion conduction which is dependent on a combination of the conductively of the fluid present, porosity and percentage of saturation.

-          Dissolved salt – provide ion conductance of the electrical current

-          Rock forming mineral-high resistive to current flow.

-          The resistivity method is a valuable one for identifying layered subsurface materials that have different electrical properties.

Volcanic processes

-          1984-<500 volcanoes have been active

-          Around the margin of the pacific ocean

-          Location where molten rock or magma, issues from deep within the earth.

-          Explosiveness a viscosity and water content

-          volcanic Hazards Lava flows, Hot avalanches, mudflows and flood, volcanic ash (Tephra) and gases.

-          Mitigation

a)      Prevent land use regulation in high hazard area.

b)      Explosives and barriers to divert lave flow.