Basics of Earthquake

1. EARTHQUAKE:
An earthquake is the sudden, rapid shaking of the earth, caused by the breaking and shifting of
subterranean rock as it releases strain (in the form of energy) that has accumulated over a long
time.
For hundreds of millions of years, the forces of plate tectonics have shaped the earth, as the huge
plates that form the earth’s surface slowly move over, under and past each other. Sometimes, the
movement is gradual. At other times, the plates are locked together, unable to release accumulated
energy. When the accumulated energy grows strong enough, the plates break free.
Aftershock - An earthquake of similar or lesser intensity that follows the main earthquake.
Focus: The focus is defined to be that plutonic spot where the energy gets released
Epicenter - The place on the earth’s surface directly above the point on the fault where the earthquake
rupture began. Once fault slippage begins, it expands along the fault during the earthquake and can extend
hundreds of miles before stopping.
Fault - The fracture across which displacement has occurred during an earthquake. The slippage may range
from less than an inch to more than 10 yards in a severe earthquake.
Magnitude - The amount of energy released during an earthquake, which is computed from the amplitude
of the seismic waves. A magnitude of 7.0 on the Richter Scale indicates an extremely strong earthquake.
Each whole number on the scale represents an increase of about 30 times more energy released than the
previous whole number represents. Therefore, an earthquake measuring 6.0 is about 30 times more powerful
than one measuring 5.0.
Seismic Waves - Vibrations that travel outward from the earthquake fault at speeds of several miles per
second. Although fault slippage directly under a structure can cause considerable damage, the vibrations of
seismic waves cause most of the destruction during earthquakes.
Seismic waves analysis helps in outlining:
1. Study of earth’s interior
2. Earthquake as geomorphic agent
Based on propagation, location and behavior, seismic waves are categorized into 2 types:
1. Body waves
a. Compressional
b. Distortional
2. Surface waves
a. Rayleigh
b. Love
BODY WAVES:
The body waves are the waves that propagates via the body (earth’s interior) to be recorded on the
seismograph in different parts of the world.

2. Compressional Body Waves: The compressional waves form those body waves which propagates
via the earth’s interior by compressing the media (longitudinal wave motion). The rocks gets
compressed only to expand beyond their original volume as the energy waves travels via it,
generating alternating sequence of compression and expansion and influencing the volume of the
medium.
These are also called as ‘P’ or ‘longitudinal’ or ‘primary’ waves.
These are the fastest moving seismic waves (7 km/s) and therefore are the first to be recorded on
the seismograph.
Distortional Body Waves: These waves propagates by inducing sheer stress, pulling the rock
particles up and down perpendicular to the direction of wave propagation (transverse wave
motion). In this process, the waves generate distortional effects like influencing the shape of the
rocks.
Also known as S-waves or ‘secondary’ waves, with velocity ~3.5 km/s.
Since a liquid cannot sustain stress perpendicular to it, the S-waves are absent in liquid media.
SURFACE WAVES:
In physics, a surface wave is a mechanical wave that propagates along the interface between
differing media. Surface waves are caused when P waves and S waves released at the focus come
to the surface. These are the slowest moving energy waves with velocity not more than 2.5 km/s.
Love waves (also known as Q waves (Quer: German for lateral)) are surface seismic waves that
cause horizontal shifting of the Earth during an earthquake. Love waves travel with a slower

3. velocity than P- or S- waves, but faster than Rayleigh waves. These waves are observed only when
there is a low velocity layer overlying a high velocity layer/ sub layers.
Rayleigh waves are a type of surface wave that travel near the surface of solids. Rayleigh waves
include both longitudinal and transverse motions that decrease exponentially in amplitude as
distance from the surface increases. There is a phase difference between these component motions.
BEHAVIOR OF BODY WAVES AND EARTH’S INTERIOR:
The P-waves can propagate via all media in the earth’s interior. However, while passing through
different media its velocity and direction (at the interface) gets changed. The inelasticity of the
liquid medium makes the p-waves passing through it reduce its velocity. Also, the disappearance
of secondary waves in the liquid medium is because of the fact that the liquid medium cannot
sustain sheer stress.
In the beginning of the 20th century, analysis of seismographic records made the scholars conclude
the presence of shadow zone, which are defined to be that area of the Earth's surface where
seismographs cannot detect an earthquake after its seismic waves have passed through the Earth.
The P-waves are refracted by the liquid outer core of the Earth and are not detected between 103°
and 143° (between approximately 11,570 and 15,570 km or 7,190 and 9,670 mi) from the
epicenter. The S- waves cannot pass through the liquid outer core and are not detected more than
103° (approximately 11,570 km or 7,190 mi) from the epicenter.
The absence of S-waves in its shadow zone at around 2900 kms from the surface of the earth led
to the conclusion that the outer core of the earth is in liquid form. It is in regards to the analysis of
seismic wave behavior that both plastic media - Asthenosphere and D’ Layer – are referred to be
LOW VELOCITY ZONES. Similarly, Gutenberg and Lehmann Discontinuities are highlighted to
be regulators of seismic wave behavior.

4. EARTHQUAKE AS A GEOMORPHIC AGENT (EFFECTS OF SURFACE WAVES):
When the energy waves released at the focus (i.e. P-waves and S-waves) come to the surface of
the earth there is the development of surface seismic waves which propagates only on the earth
surface. It is the surface waves that cause the tremors and are capable of inducing the secondary
hazards as landslides, avalanches or liquefaction. Primarily they represent the possibilities of
ground shift as the most recognized geomorphic effect. These ground shift can be either horizonta l
or vertical or both depending upon the types of surface waves – Love waves or Rayleigh waves.
The Love waves are responsible for horizontal shifts that involves the wave propagation in a
weeping motion. In comparison, the Rayleigh waves cause vertical ground shift registering sea-swell
motion. These primary out-turns on the continental crust represent restricted
geomorphological influence compared to the oceanic crust.
When the high- intensity submarine quakes result into Rayleigh waves, there is the genesis of
devastating secondary hazards called tsunamis or seiches.
Tsunamis, also known as seismic sea waves (mistakenly called “tidal waves”), are a series of
enormous waves created by an underwater disturbance such as an earthquake, landslide, volcanic
eruption, or meteorite. A tsunami can move hundreds of miles per hour in the open ocean and
smash into land with waves as high as 100 feet or more.
These sea-waves in the open water involves typical characteristics of being long waves
(wavelengths in the range of 150-160 km), short-heighted (wave height not more than 3m) and
fast moving waves with velocities up to 800 km/h. Towards the shoreline, however, these waves
completely transform into slow moving gigantic waves with wave height up to 30m. It is these
gigantic waves that poses as secondary hazards caused by earthquakes. In accordance to its surfing
effect as well as delayed backwash, tsunamis in marginal water bodies are called seiches which
are more specific in creating secondary hazards as they have proximate shoreline.
From the area where the tsunami originates, waves travel outward in all directions. Once the wave
approaches the shore, it builds in height. The topography of the coastline and the ocean floor will
influence the size of the wave. There may be more than one wave and the succeeding one may be
larger than the one before. That is why a small tsunami at one beach can be a giant wave a few
miles away.

5. Earthquake-induced movement of the ocean floor most often generates tsunamis. If a major
earthquake or landslide occurs close to shore, the first wave in a series could reach the beach in a
few minutes, even before a warning is issued. Areas are at greater risk if they are less than 25 feet
above sea level and within a mile of the shoreline. Drowning is the most common cause of death
associated with a tsunami. Tsunami waves and the receding water are very destructive to structures
in the run-up zone. Other hazards include flooding, contamination of drinking water, and fires
from gas lines or ruptured tanks.
Groundwater may help predict earthquakes
Scientists searching for a way to predict earthquakes have uncovered the most promising lead yet,
after uncovering telltale chemical spikes in groundwater up to six months before tremors
struck.
Major earthquakes are the only natural disaster that cannot currently be forecast. Some experts
think a useful prediction of time, place and magnitude may be an impossible dream. Previously,
scientists have examined radon gas leaks, heat maps and unusual animal behaviour as possible
earthquake indicators, without success.
But now geologists taking weekly measurements of groundwater chemistry in northern Iceland
over five years have discovered big shifts four to six months before two separate earthquakes in
2012 and 2013. The quakes were both significant in size — over magnitude five — and 47 miles
from the sampling site.
“This does not mean we can predict earthquakes yet, but at the least we have shown something
happens before earthquakes,” said Prof. Alasdair Skelton, at Stockholm University, Sweden, who
led the research published in Nature Geoscience. “We are highlighting groundwater chemistry as
a promising target for future earthquake prediction studies.”