Earthquake
Earthquakes
can happen quickly and without prior notice. A tectonic plate moving along a
fault line in the earth's crust causes an earthquake, which is characterised by
a severe and rapid shaking of the ground.
Earthquakes can also result in ground tremors, soil liquefaction, landslides, fissures, avalanches, fires, and tsunamis which severely affect the life & nature.
- magnitude
- intensity and duration
- the local geology
- the time of day that it occurs
- building and industrial plant design and materials
- the risk-management measures put in place.
Nearly 750
000 people died worldwide from earthquakes between 1998 and 2017, accounting
for more than half of all deaths from natural catastrophes. Over 125 million
people were impacted by earthquakes during this time period, which means they
were hurt, made homeless, moved, or evacuated during the disaster's emergency
stage.
Causes of Earthquake
A fault slips suddenly, resulting in an earthquake. Even though the tectonic plates are always moving slowly, but friction makes their edges impenetrable. Waves of energy are created when the stress on the edge of tectonic plates are greater than the friction, and these waves move into the earth's crust to produce the shaking we experience.
On Earth,
large earthquakes typically occur in belts along the boundaries of tectonic
plates.
This has been clear for a while from early earthquake catalogues and is considerably more obvious from modern seismicity maps that include instrumentally determined epicentres. The Circum-Pacific Belt, which impacts a number of inhabited coastal areas surrounding the Pacific Ocean, including as those in New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and on the western shores of North and South America, is the most significant earthquake belt. According to estimates, earthquakes with epicentres in this belt account for 80% of the energy now released in earthquakes. There are several branching and variations in the seismic activity throughout the belt, which is by no means homogeneous. The Circum-Pacific Belt is frequently linked to volcanic activity, which is why it is sometimes known as the "Pacific Ring of Fire."
The Mediterranean region is linked to the Circum-Pacific Belt by the Alpide Belt, which runs across Asia and the East Indies. This region is responsible for about 15% of all earthquake energy released worldwide.
Astounding linked seismic activity bands can also be seen, mostly in the rift valleys of East Africa and along oceanic ridges such as those in the Arctic, Atlantic, and western Indian Oceans. The best way to comprehend this worldwide seismicity distribution is in the context of plate tectonics.
Natural forces
The abrupt
release of energy from a specific location of the Earth's crust causes
earthquakes. Elastic strain, gravity, chemical processes, or even the movement
of large bodies can all release the energy. The most significant of them is the
release of elastic strain since it is the only type of energy that can be
stored in the Earth in quantities great enough to create significant
perturbations. Tectonic earthquakes are earthquakes that occur as a result of
this kind of energy release.
Tectonic
After the
San Andreas Fault broke in 1906, causing the great San Francisco earthquake,
American geologist Harry Fielding Reid developed the so-called elastic rebound
theory to explain tectonic earthquakes. The idea states that tectonic
earthquakes happen when strains in rock masses build up to the point when the
resulting stresses surpass the rock's capacity, leading to abrupt rupture. The
fractures spread out quickly through the rock, usually in the same direction,
and can occasionally stretch for miles following a specific zone of weakness.
For instance, the San Andreas Fault shifted along a 430 km (270 miles) long
plane in 1906. The ground was moved horizontally along this line by up to 6
metres (20 feet).
Rock masses
are thrown in opposite directions as a fault rupture moves along or up the
fault, springing back to a location where there is less strain. This movement
could occur in irregular increments rather than all at once at any given site,
and these abrupt slowdowns and restarts are what cause the vibrations that
seismic waves are made of. The modelling of earthquake causes now takes into
account these erratic fault rupture characteristics, both physically and
theoretically. Asperities are irregularities along the fault, and fault
barriers are regions where the rupture slows or ceases. The earthquake focal,
which is typically located close to 5 to 15 kilometres below the surface, is
where the fault rupture begins. The
rupture spreads throughout the fault plane in either one or both directions
until it is stopped or slowed by a barrier. Sometimes the barrier fails to halt
the fault rupture, and it continues on the opposite side; other times, the
barrier is broken by stresses in the rocks, and the rupture continues.
The sort of
fault slip that creates an earthquake will affect the earthquake's
characteristics, as indicated in the picture. A "strike" (i.e., the
direction from north taken by a horizontal line in the fault plane) and a
"dip" (i.e., the angle from the horizontal exhibited by the sharpest
slope in the fault) are two components of the typical fault model.
The
footwall is an inclined fault's lowest wall.
The hanging wall is perched over the footwall. Strike-slip faulting is the movement of rock masses past one another on a line with the strike. Dip-slip faulting is movement that occurs parallel to the dip. Depending on whether the block on the other side of the fault from the observer has moved to the right or left, strike-slip faults are either right lateral or left lateral. In dip-slip faults, "normal" faulting occurs when the hanging-wall block moves downward relative to the footwall block; reverse or thrust faulting occurs when the hanging-wall block moves upward relative to the footwall.
Although tectonic
movements along faults are frequently gradual and the majority of geologically
old faults are now aseismic (that is, they no longer create earthquakes), it is
considered that every known fault has once been the epicentre of one or more
earthquakes. It is frequently unclear if an earthquake's total energy
originates from a single fault plane because the actual faulting that causes it
might be complex.
In contrast to the
abrupt slide offsets that cause seismic waves, observed geologic faults
occasionally exhibit relative displacements on the order of hundreds of
kilometres over the course of geologic time. For instance, the causative fault
east of Beijing saw a surface strike-slip of around one metre during the
Tangshan earthquake in 1976, while the Chelung-pu fault experienced up to eight
metres of vertical slide during the Taiwan earthquake in 1999.
Volcanism
Volcanic
earthquakes are a distinct subtype of earthquake that are connected to volcanic
activity. However, even in these circumstances, it is likely that the
disturbance is caused by the abrupt slippage of rock masses close to the
volcano and the subsequent release of elastic strain energy. The heat generated
by magma moving in r, however, may partially account for the stored energy's
hydrodynamic origin.Major earthquakes and volcanoes are clearly correlated with
one another's locations, especially in the Circum-Pacific Belt and along
oceanic ridges. However, most big shallow earthquakes have epicentres hundreds
of kilometres away from volcanic vents, and many earthquake sources are located
far from active volcanoes. There is probably no direct causal link between the
two events, even when an earthquake's focal point is located immediately
beneath structures identified by volcanic vents; most likely, both events are
the product of the same tectonic processes.
Artificial induction
Human activities
such as the dumping of liquids into deep wells, the detonation of massive
underground nuclear explosions, the mining industry, and the filling of
enormous reservoirs can occasionally create earthquakes. In the case of deep
mining, the strain around the tunnels changes as a result of the removal of
rock. It is possible for nearby, existing faults to slip or for rock to break
apart and fall into the new voids. As in the case of tectonic earthquakes, it
is believed that in the case of fluid injection, the slip is caused by the
premature release of elastic strain after the liquid has lubricated the fault
surfaces. On existing strained faults close to the test devices, large
underground nuclear explosions have been known to cause slip.
Reservoir induction
The filling of big
reservoirs is among the most significant of the several earthquake-causing
actions mentioned above. There have been more than 20 notable incidents when
local seismicity has increased as a result of the storage of water behind
towering dams. Because there is sometimes no data to compare earthquake
occurrence before and after the reservoir was filled, causality is frequently
impossible to prove. The effects of reservoir induction are most noticeable in reservoirs
that are deeper than 100 metres (330 feet) and larger than 1 cubic kilometre
(0.24 cubic miles). The Hoover Dam in the United States, the Aswan High Dam in
Egypt, and the Kariba Dam on the border between Zimbabwe and Zambia are three
locations where such connections have very likely taken place.
In these
situations, the most widely accepted theory for why earthquakes happen is that
the rocks close to the reservoir have already been pushed by regional tectonic
forces to the point that surrounding faults are almost ready to slip. The
pressure perturbation caused by the reservoir's water causes the fault rupture.
The fact that the rocks along the fault have less strength due to higher
water-pore pressure may have an even greater impact on the pressure effect.
Despite these concerns, the majority of large reservoir fillings have not
resulted in earthquakes that are dangerous in size.
In a few instances,
the precise seismic source processes linked to reservoir induction have been
identified. The evidence supports strike-slip faulting motion for the primary
shock at the Koyna Dam and Reservoir in India (1967). The producing technique
used at the Kremasta Dam in Greece (1965) and the Kariba Dam in Zimbabwe-Zambia
(1961) was dip-slip on typical faults. On the other hand, the thrust mechanisms
for the origins of earthquakes at the lake behind Tajikistan's Nurek Dam have
been identified. In the first nine years following the water's impoundment in
this 317-meter-deep reservoir in 1972, there were more than 1,800 earthquakes,
which is four times the typical quake frequency occurs in the area before the
reservoir was filled.
Seismology and nuclear explosions
Representatives
from a number of nations, including the US and the USSR, met in 1958 to explore
the technical foundation for a treaty banning nuclear tests. One of the topics
discussed was the viability of creating efficient tools for spotting
subterranean nuclear explosions and telling them apart seismically from
earthquakes. Following that meeting, seismology received a lot of specialized
research, which greatly improved the ability to detect and analyse seismic
signals.
Recent
seismological work on treaty verification has comprised deploying seismic
arrays, estimating explosion yield, researching wave attenuation in the Earth,
determining wave amplitude and frequency spectra discriminants, and using
high-resolution seismographs in a global network. The results of this research
have demonstrated that, in contrast to natural earthquakes, underground nuclear
explosions typically produce seismic waves that travel through the body of the
Earth that are substantially bigger in amplitude than the surface waves. A
global monitoring network of 270 seismographic stations should be able to
identify and locate all earthquakes of magnitude 4 and above (equivalent to an
explosive output of around 100 tonnes of TNT), according to this telling
difference and other types of seismic evidence.
There are many
different repercussions of earthquakes, including modifications to the earth's
surface, harm to man-made structures, and an impact on both human and animal
life. The majority of these consequences take place on solid ground, but as the
majority of earthquake foci are actually found beneath the ocean floor, severe
effects are frequently seen at ocean borders.
Surface phenomena
Ground
movements—vertical or horizontal—along geologic fault traces, rising, falling,
and tilting of the ground surface, changes in the flow of groundwater,
liquefaction of sandy ground, landslides, and mudflows are among the dramatic
geomorphological changes that earthquakes frequently bring about. Geodetic
measurements, which are routinely taken in a number of nations severely
affected by earthquakes, are helpful in examining topography changes.
Buildings, bridges, pipelines, railways, embankments, and other structures are all susceptible to major damage from earthquakes. The kind and level of damage sustained depend on the force of the ground vibrations and how the foundation soils behave. The repercussions of a major earthquake are typically complex and dependent on the topography and the makeup of the surface materials in the area that has been damaged the most, known as the meizoseismal area. On soft alluvium and unconsolidated sediments as opposed to hard rock, they are frequently more severe. Seismic waves travelling along the surface are the primary cause of damage at distances greater than 100 km (60 miles) from the source. In mines, there is typically little damage beyond a few hundred metre depths, despite significant damage to the land surface immediately above.
There are often accounts of unusual sounds and lights during earthquakes. They have been compared to the low-pitched noise of an underground train passing through a station because of their characteristic low pitch. Such sounds are compatible with the ground being penetrated by high-frequency seismic waves. Occasionally, after earthquakes, dazzling flashes, streamers, and bright balls have been observed in the night sky. Electric induction in the atmosphere near the earthquake source is what is thought to be responsible for these lights.
Tsunamis
Very
long-wavelength water waves in oceans or seas can reach ashore after some
earthquakes. Although they are more appropriately referred to as tsunamis or
seismic sea waves (tsunami is the Japanese term for "harbour wave"),
tidal waves are not formed by the Moon and Sun's gravitational pull. They may
be very damaging and occasionally come ashore at tremendous heights—tens of
metres above ordinary tide level.
How Do Scientists Predict Tsunamis After Earthquakes?
A quick movement in
the seabed that results in the abrupt lifting or lowering of a huge body of
water is typically the immediate source of a tsunami. This deformation might be
a submarine landslip that results from an earthquake, or it might be the fault
that causes the earthquake. There have also been major tsunamis caused by large
volcanic eruptions near coastlines, such as those at Thera (about 1580 BCE) and
Krakatoa (1883 CE). On December 26, 2004, an earthquake off the Indonesian
coast of Sumatra moved the seabed, causing the most catastrophic tsunami yet
seen. A succession of waves that inundated coasts from Indonesia to Sri Lanka
and even came ashore on the Horn of Africa killed more than 200,000 people.
Water waves
propagated in all directions once the sea surface was initially disturbed. The
formula (Square root of gh), where g is the acceleration of gravity and h is
the sea depth, determines their speed of motion in deep water. When h is 1,000
metres (3,300 feet), this speed—100 metres per second, or 225 miles per
hour—might be significant. Although the principal wavelength may be on the
order of hundreds of kilometres, the amplitude (or height of disturbance) at
the water's surface does not exceed a few metres in deep water. As a result,
the principal wave period—the amount of time between the arrival of successive
crests—may be on the order of tens of minutes. Because of these, tsunami waves
are not noticed by ships far out at sea.
However, the wave
amplitude grows as tsunamis approach shallow water. In U- and V-shaped harbours
and inlets, the waves can occasionally reach heights of 20 to 30 metres above
mean sea level. Around such inlets, they frequently cause significant damage to
low-lying ground. The wave front at the entrance is frequently almost vertical,
similar to a tidal bore, and the onrush speed can reach 10 metres per second.
There may be multiple large waves in some situations, spaced by periods of
several minutes or longer. A remarkable retreat of water from the shore may
start several minutes or even a half hour before the first of these waves,
which is common.
There are
organisations that have been established to provide tsunami warnings,
particularly in Japan, Siberia, Alaska, and Hawaii. The Seismic Sea Wave
Warning System, a system with worldwide assistance intended to lessen
fatalities in the Pacific Ocean, is a significant development. It is situated
in Honolulu and issues alerts based on reports of earthquakes from
seismographic stations around the Pacific.
Seiches
Seiches, which can occasionally be caused by earthquakes and tsunamis, are rhythmic water movements in almost landlocked bays or lakes. Such oscillations may endure for several hours or even several days.
The big Lisbon earthquake of 1755 generated noticeable oscillations in the waters of lakes and canals as far away as Scotland and Sweden. Seismic surface waves from the 1964 Alaska earthquake began to cause surges in lakes in Texas, in the southwest of the United States, 30 to 40 minutes after the event.
When seismic waves
from an earthquake travel through saltwater after being refracted by the
seafloor, they have a similar effect. Approximately 1.5 kilometres (0.9 miles)
per second, or the speed of sound in water, is the speed of these waves. Such
waves can give the impression that a ship has impacted a submerged item if they
strike it with enough force. This occurrence is referred to as a seaquake.
What is the significance of the depth?
Earthquakes take place in the crust or upper
mantle, which covers the planet's surface and extends down to a depth of around
800 km (500 miles).
The power of shaking at the surface from an
earthquake that happens at 500 km depth is significantly less than if the same
earthquake had occurred at 20 km depth because the intensity of shaking from an
earthquake reduces with increasing distance from the earthquake's source.
Additionally, the depths of earthquakes reveal
vital details about the tectonic environment and the structure of the Earth.
This is most visible at subduction zones, when plates are colliding and one
plate is being subducted beneath another. By carefully tracking the location
and depth of earthquakes linked to the zone, we may identify structural
features of a subduction zone, such as how steeply it dips and whether the
down-going plate is flat or curved. These details, which are vital, help us to
understand the mechanics and characteristics of the deformation in the
subduction zone.
The most powerful earthquakes occur deep within
subducting slabs, oceanic plates that drop into the Earth's mantle along
convergent plate boundaries when a thick oceanic plate collides with a less
dense continental plate and falls beneath it.
A plate boundary contact between two of these plates, which is only active down to a depth of around 60 km, which is fairly shallow, is what produced both the Sumatra 2004 M9.1 earthquake and the Japan 2011 M9.0 earthquake. In contrast, these faults remain brittle and can cause earthquakes to occur as deep as 700 km in deeper subduction zone settings (for example, the Pacific Plate beneath Japan, Kamchatka, and Tonga), when oceanic slabs are comparatively cool in relation to the surrounding mantle.
As the slab sinks further into the mantle, rheology changes (variations in viscosity qualities) take place, which lead to these earthquakes. Subduction zone locations known as "Wadati-Benioff Zones" are where these events seem to occur more frequently.
Faults are exclusively active within continents and along continental plate boundary transform faults like the San Andreas, possibly down to depths of about 20 km.
Unless there is a seismic station nearby and
above the epicentre, accurately identifying the depth of an earthquake is often
more difficult than determining its position. Therefore, errors on depth
calculations tend to be a little bigger than errors on location calculations.
Impact
Health risks resulting from earthquakes might
vary depending on the earthquake's severity, the built environment (such as
substandard housing or urban slums), and any subsequent consequences, such as
tsunamis or landslides. Human & Animal health can be affected by
earthquakes in both the short and long term.
The following have immediate health effects:
- trauma-related deaths and injuries from building collapse;
- secondary effects of the earthquake that cause trauma-related deaths and injuries, such as burns from fires or drowning from tsunamis.
Medium-term health impacts include:
- Secondary infections from untreated wounds;
- increased morbidity and risk of complications during pregnancy and childbirth as a result of interrupted obstetric and neonatal services;
- potential risk of communicable diseases, particularly in areas affected by overcrowding; increased morbidity and risk of complications as a result of discontinuing treatment for chronic diseases;
- increased psychosocial needs;
- and potential environmental contamination by chemical/radiological agents.
In addition to harming transport and healthcare infrastructure, earthquakes can also cause service delivery and care access to be hampered. Medical supplies can be misplaced and health personnel might not be able to access hospitals that are still operating.
How to Remain Safe
As the leader of the health cluster for
international emergencies, WHO collaborates with partners to prepare for,
mitigate, and respond to earthquakes everywhere.
- Limiting the danger of exposure to earthquakes by enhancing the built environment, with better land-use control, including controlling building construction, and strengthening health emergency risk management systems
- Ensuring that health facilities are robust to hazards, that they can continue to operate, that they can respond to increased and altered health demands following earthquakes, and that their staff is properly trained in doing so
- Mobilising medical response teams, setting up field hospitals and temporary health facilities as well as emergency medical supplies.
- Making investments in community readiness, as locals frequently serve as first responders.
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