The document provides a comprehensive overview of earthquakes, detailing their causes, mechanisms, and effects, including the 2011 Japan earthquake and tsunami, which resulted in significant loss of life and economic damage. It outlines different fault types, seismic wave categories, and methods for reducing earthquake hazards, emphasizing the need for proper building codes and hazard maps. Furthermore, it introduces the elastic-rebound theory that explains the buildup and release of energy during seismic events.

1. INTRODUCTION TO EARTHQUAKES
 Earthquake, any sudden shaking of the ground caused by the
passage of seismic waves through Earth’s rocks. Seismic waves
are produced when some form of energy stored in Earth’s crust is
suddenly released, usually when masses of rock straining against
one another suddenly fracture and “slip.” Earthquakes occur most
often along geologic faults, narrow zones where rock masses
move in relation to one another. The major fault lines of the world
are located at the fringes of the huge tectonic plates that make up
Earth’s crust.
 At the Earth's surface, earthquakes manifest themselves by
shaking and displacing or disrupting the ground. When the
epicentre of a large earthquake is located offshore, the seabed may
be displaced sufficiently to cause a tsunami. Earthquakes can also
trigger landslides, and occasionally volcanic activity.
In its most general sense, the word earthquake is used to describe any
seismic event—whether natural or caused by humans—that generates
seismic waves. Earthquakes are caused mostly by rupture of geological
faults, but also by other events such as volcanic activity, landslides,
mine blasts, and nuclear tests. An earthquake's point of initial rupture is
called its focus or hypocentre. The epicentre is the point at ground
level directly above the hypocentre.

2. NATURAL OCCURRENCE OF
EARTHQUAKES
 Tectonic earthquakes occur anywhere in the earth where there is
sufficient stored elastic strain energy to drive fracture propagation
along a fault plane. The sides of a fault move past each other smoothly
and seismically only if there are no irregularities or asperities along the
fault surface that increase the frictional resistance. Most fault surfaces
do have such asperities and this leads to a form of stick-slip behaviour.
Once the fault has locked, continued relative motion between the plates
leads to increasing stress and therefore, stored strain energy in the
volume around the fault surface. This continues until the stress has
risen sufficiently to break through the asperity, suddenly allowing
sliding over the locked portion of the fault, releasing the stored energy.
This energy is released as a combination of radiated elastic strain
seismic waves, frictional heating of the fault surface, and cracking of
the rock, thus causing an earthquake. This process of gradual build-up
of strain and stress punctuated by occasional sudden earthquake failure
is referred to as the elastic-rebound theory. It is estimated that only 10
percent or less of an earthquake's total energy is radiated as seismic
energy. Most of the earthquake's energy is used to power the
earthquake fracture growth or is converted into heat generated by
friction. Therefore, earthquakes lower the Earth's available elastic
potential energy and raise its temperature, though these changes are
negligible compared to the conductive and convective flow of heat out
from the Earth's deep interior.

3. 2011 JAPAN EARTHQUAKE AND
TSUNAMI
 A magnitude-9.0 earthquake struck in the Pacific Ocean off
the northeast coast of Japan’s Honshu island on March 11,
2011. The Great East Japan Earthquake — the name given to
the event by the Japanese government — triggered a massive
tsunami that flooded more than 200 square miles of coastal
land. Waves were estimated to be as high as 38 meters, the
height of a 12-story building.
 An estimated 20,000 people were dead or missing and close
to 500,000 people were forced to evacuate. In addition, a
nuclear power plant meltdown triggered a nuclear
emergency. The direct economic loss from the earthquake,
tsunami, and nuclear disaster is estimated at $360 billion.
 Though Japan is a world leader in disaster preparedness, the
2011 quake caused overwhelming damage and humanitarian
needs that required an international response.

4. EARTHQUAKE FAULT TYPES
 There are three main types of fault, all of which
may cause an interpolate earthquake: normal,
reverse (thrust) and strike-slip. Normal and
reverse faulting are examples of dip-slip, where
the displacement along the fault is in the
direction of dip and movement on them
involves a vertical component. Normal faults
occur mainly in areas where the crust is being
extended such as a divergent boundary. Reverse
faults occur in areas where the crust is being
shortened such as at a convergent boundary.
Strike-slip faults are steep structures where the
two sides of the fault slip horizontally past each
other; transform boundaries are a particular
type of strike-slip fault. Many earthquakes are
caused by movement on faults that have
components of both dip-slip and strike-slip; this
is known as oblique slip.

5. Nature of Earthquakes
 Seismologists like to split seismic waves into several categories, but the main types of
seismic waves come in two categories — body waves (which move throughout entire
bodies, such as the Earth), and surface waves )(which travel only on different surfaces, not
through the whole body). The main types of seismic waves are the following:
 Primary waves (P-waves). These are the “first” body waves — the ones that travel the fastest
and through any type of medium (solid, liquid, gas). They propagate longitudinally on the
propagation direction (think of an accordion) and are harmless in terms of earthquake
damage.
 Secondary (S-wave). These are shear waves, which arrive after the P-waves. They’re also
body waves but they only propagate through a solid medium. They also rarely do any
significant damage.
 Surface waves — Rayleigh (R-wave). Surface waves (Rayleigh and Love) do by far the
most damage. As opposed to body waves (S and P waves), they propagate on the surface
and carry the vast majority of the energy felt on the surface — in other words, these are what
you feel when you experience an earthquake. This happens because although they move
slower than body waves, their particle movement is much more pronounced (see below). In
the case of Rayleigh waves, the motion is of a rolling nature, similar to an ocean surface
wave.
 Surface wave — Love (L-wave). Contrary to their name, there’s nothing really lovable
about the Love waves — they were named thusly after Augustus Edward Hough Love, a
Professor for Natural Philosophy at Oxford University who first described the movement of
the waves named after him. Love waves have a transversal (perpendicular) movement and
are the most destructive outside the immediate area of the epicentre. Love waves can be
devastating

6. 2015 NEPAL EARTHQUAKE

7. Methods of Reducing Earthquake Hazards
 Interpreting recorded ground motions
 Most elastic waves recorded close to an extended fault source are
complicated and difficult to interpret uniquely. Understanding such near-
source motion can be viewed as a three-part problem. The first part stems
from the generation of elastic waves by the slipping fault as the moving
rupture sweeps out an area of slip along the fault plane within a given time.
The pattern of waves produced is dependent on several parameters, such as
fault dimension and rupture velocity. Elastic waves of various types radiate
from the vicinity of the moving rupture in all directions. The geometry and
frictional properties of the fault critically affect the pattern of radiation
from it.
 The second part of the problem concerns the passage of the waves through
the intervening rocks to the site and the effect of geologic conditions. The
third part involves the conditions at the recording site itself, such as
topography and highly attenuating soils. All these questions must be
considered when estimating likely earthquake effects at a site of any
proposed structure.
 Experience has shown that the ground strong-motion recordings have a
variable pattern in detail but predictable regular shapes in general (except in
the case of strong multiple earthquakes). An example of actual shaking of
the ground (acceleration, velocity, and displacement) recorded during an
earthquake is given in the . In a strong horizontal shaking of the ground
near the fault source, there is an initial segment of motion made up mainly
of P waves, which frequently manifest themselves strongly in the vertical
motion. This is followed by the onset of S waves, often associated with a
longer-period pulse of ground velocity and displacement related to the near-
site fault slip or fling. This pulse is often enhanced in the direction of the
fault rupture and normal to it. After the S onset there is shaking that
consists of a mixture of S and P waves, but the S motions become dominant
as the duration increases. Later, in the horizontal component, surface waves
dominate, mixed with some S body waves. Depending on the distance of
the site from the fault and the structure of the intervening rocks and soils,
surface waves are spread out into long trains.

8. Methods of Reducing Earthquake
Hazards (2)
 Constructing seismic hazard maps
 In many regions, seismic expectancy maps or hazard maps are
now available for planning purposes. The anticipated intensity of
ground shaking is represented by a number called the peak
acceleration or the peak velocity.
 To avoid weaknesses found in earlier earthquake hazard maps,
the following general principles are usually adopted today:
 The map should take into account not only the size but also the
frequency of earthquakes.
 The broad regionalization pattern should use historical seismicity
as a database, including the following factors: major tectonic
trends, acceleration attenuation curves, and intensity reports.
 Regionalization should be defined by means of contour lines
with design parameters referred to ordered numbers on
neighbouring contour lines (this procedure minimizes sensitivity
concerning the exact location of boundary lines between separate
zones).
 The map should be simple and not attempt to macrozone the
region.
 The mapped contoured surface should not contain
discontinuities, so that the level of hazard progresses gradually
and in order across any profile drawn on the map.
 Developing resistant structures
 Developing engineered structural designs that are able to
resist the forces generated by seismic waves can be achieved
either by following building codes based on hazard maps or
by appropriate methods of analysis. Many countries reserve
theoretical structural analyses for the larger, more costly, or
critical buildings to be constructed in the most seismically
active regions, while simply requiring that ordinary
structures conform to local building codes. Economic
realities usually determine the goal, not of preventing all
damage in all earthquakes but of minimizing damage in
moderate, more common earthquakes and ensuring no major
collapse at the strongest intensities. An essential part of what
goes into engineering decisions on design and into the
development and revision of earthquake-resistant design
codes is therefore seismological, involving measurement of
strong seismic waves, field studies of intensity and damage,
and the probability of earthquake occurrence

9. Made by:
Samvid Sharma
M.A. GEOGRAPHY- 1st YEAR
ROLL NO: 419929