An
earthquake (also known as a
tremor or
temblor) is the result
of a sudden release of energy in the
Earth's
crust that creates
seismic waves. Earthquakes are recorded with a
seismometer, also known as a
seismograph. The
moment
magnitude (or the related and mostly obsolete
Richter magnitude) of an earthquake
is conventionally reported, with magnitude 3 or lower earthquakes
being mostly
imperceptible and
magnitude 7 causing serious damage over large areas. Intensity of
shaking is measured on the modified
Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by shaking
and sometimes displacing the ground. When a large earthquake
epicenter is located offshore, the seabed
sometimes suffers sufficient displacement to cause a
tsunami. The shaking in earthquakes can also trigger
landslides and occasionally volcanic activity.
In its most generic sense, the word
earthquake is used to
describe any seismic event — whether a natural
phenomenon or an event caused by humans — that
generates seismic waves. Earthquakes are caused mostly by rupture
of geological
faults, but also by
volcanic activity, landslides, mine blasts, and nuclear
experiments. An earthquake's point of initial rupture is called its
focus or
hypocenter. The term
epicenter refers to the point at ground level
directly above the hypocenter.
Global plate tectonic movement
Naturally occurring earthquakes
Fault types
Tectonic earthquakes will occur anywhere within the earth where
there is sufficient stored elastic strain energy to drive fracture
propagation along a
fault plane. In the
case of
transform or
convergent type plate boundaries, which
form the largest fault surfaces on earth, they will move past each
other smoothly and
aseismically only
if there are no irregularities or
asperities along the boundary that increase the
frictional resistance. Most boundaries do have such asperities and
this leads to a form of
stick-slip
behaviour. Once the boundary 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.
Earthquake fault types
There are three main types of fault that may cause an 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.
Earthquakes away from plate boundaries
Where plate boundaries occur within
continental lithosphere, deformation is
spread out a over a much larger area than the plate boundary
itself.
In
the case of the San Andreas
fault
continental transform, many earthquakes occur away
from the plate boundary and are related to strains developed within
the broader zone of deformation caused by major irregularities in
the fault trace (e.g. the “Big bend” region). The
Northridge earthquake was associated
with movement on a blind thrust within such a zone.
Another example is the
strongly oblique convergent plate boundary between the Arabian and Eurasian
plates where it runs through the northwestern part of the
Zagros
mountains. The deformation associated with
this plate boundary is partitioned into nearly pure thrust sense
movements perpendicular to the boundary over a wide zone to the
southwest and nearly pure strike-slip motion along the Main Recent
Fault close to the actual plate boundary itself. This is
demonstrated by earthquake
focal
mechanisms.
All tectonic plates have internal stress fields caused by their
interactions with neighbouring plates and sedimentary loading or
unloading (e.g. deglaciation). These stresses may be sufficient to
cause failure along existing fault planes, giving rise to
intraplate earthquakes.
Shallow-focus and deep-focus earthquakes
The majority of tectonic earthquakes originate at the ring of fire
in depths not exceeding tens of kilometers. Earthquakes occurring
at a depth of less than 70 km are classified as
'shallow-focus' earthquakes, while those with a focal-depth between
70 and 300 km are commonly termed 'mid-focus' or
'intermediate-depth' earthquakes. In
subduction zones, where older and colder
oceanic crust descends beneath another
tectonic plate,
deep-focus
earthquake may occur at much greater depths (ranging from 300
up to 700 kilometers). These seismically active areas of subduction
are known as
Wadati-Benioff
zones. Deep-focus earthquakes occur at a depth at which the
subducted
lithosphere should no longer
be brittle, due to the high temperature and pressure. A possible
mechanism for the generation of deep-focus earthquakes is faulting
caused by
olivine undergoing a
phase transition into a
spinel structure.
Earthquakes and volcanic activity
Earthquakes often occur in volcanic regions and are caused there,
both by
tectonic faults and the
movement of
magma in
volcanoes.
Such earthquakes can serve as an early
warning of volcanic eruptions, like during the Mount St.
Helens
eruption of 1980
. Earthquake swarms serve as markers for the
location of the flowing magma throughout the volcanoes. In the
United States, these are then recorded by seismometers and
tiltimeters (a device which measures the ground slope) and used as
sensors to predict imminent or upcoming eruptions.
Earthquake clusters
Most earthquakes form part of a sequence, related to each other in
terms of location and time. Most earthquake clusters consist of
small tremors which cause little to no damage, but there is a
theory that earthquakes can recur in a regular pattern.
Aftershocks
An aftershock is an earthquake that occurs after a previous
earthquake, the mainshock. An aftershock is in the same region of
the main shock but always of a smaller magnitude. If an aftershock
is larger than the main shock, the aftershock is redesignated as
the main shock and the original main shock is redesignated as a
foreshock. Aftershocks are formed as the
crust around the displaced
fault plane
adjusts to the effects of the main shock.
Earthquake swarms
February 2008 earthquake swarm near
Mexicali
Earthquake swarms are sequences of earthquakes striking in a
specific area within a short period of time. They are different
from earthquakes followed by a series of
aftershocks by the fact that no single earthquake
in the sequence is obviously the main shock, therefore none have
notable higher magnitudes than the other.
An example of an
earthquake swarm is the 2004 activity at Yellowstone
National Park
.
Earthquake storms
Sometimes a series of earthquakes occur in a sort of
earthquake storm, where the earthquakes
strike a fault in clusters, each triggered by the shaking or stress
redistribution of the previous earthquakes. Similar to
aftershocks but on adjacent segments of fault,
these storms occur over the course of years, and with some of the
later earthquakes as damaging as the early ones. Such a pattern was
observed in the sequence of about a dozen earthquakes that struck
the
North Anatolian Fault in
Turkey in the 20th century and has been inferred for older
anomalous clusters of large earthquakes in the Middle East.
Size and frequency of occurrence
Minor
earthquakes occur nearly constantly around the world in places like
California
and Alaska
in the U.S.,
as well as in Guatemala
. Chile
, Peru
, Indonesia
, Iran
, Pakistan
, the Azores in Portugal
, Turkey
, New Zealand
, Greece
, Italy
, and
Japan
, but earthquakes can occur almost anywhere,
including New York
City
, London
, and
Australia. Larger earthquakes occur less frequently, the
relationship being
exponential; for example, roughly ten
times as many earthquakes larger than magnitude 4 occur in a
particular time period than earthquakes larger than magnitude 5. In
the (low seismicity) United Kingdom, for example, it has been
calculated that the average recurrences are:an earthquake of 3.7 -
4.6 every year, an earthquake of 4.7 - 5.5 every 10 years, and an
earthquake of 5.6 or larger every 100 years. This is an example of
the
Gutenberg-Richter
law.
The number of seismic stations has increased from about 350 in 1931
to many thousands today. As a result, many more earthquakes are
reported than in the past, but this is because of the vast
improvement in instrumentation, rather than an increase in the
number of earthquakes. The
USGS estimates that,
since 1900, there have been an average of 18 major earthquakes
(magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or
greater) per year, and that this average has been relatively
stable. In recent years, the number of major earthquakes per year
has decreased, although this is thought likely to be a
statistical fluctuation rather than
a systematic trend. More detailed statistics on the size and
frequency of earthquakes is available from the USGS.
Most of the world's earthquakes (90%, and 81% of the largest) take
place in the 40,000-km-long, horseshoe-shaped zone called the
circum-Pacific seismic belt,
also known as the
Pacific Ring of
Fire, which for the most part bounds the
Pacific Plate.
Massive earthquakes tend to occur along
other plate boundaries, too, such as along the Himalayan
Mountains
.
With the
rapid growth of mega-cities such as
Mexico
City
, Tokyo
and Tehran
, in areas of
high seismic risk, some seismologists
are warning that a single quake may claim the lives of up to 3
million people.
Induced seismicity
While most earthquakes are caused by movement of the Earth's
tectonic plates, human activity can
also produce earthquakes. Four main activities contribute to this
phenomenon: constructing large
dams and
buildings, drilling and injecting liquid into
well, and by
coal
mining and
oil drilling.
Perhaps the best known
example is the 2008 Sichuan earthquake
in China's Sichuan Province
in May; this tremor resulted in 69,227 fatalities
and is the 19th
deadliest earthquake of all time. The
Zipingpu Dam is believed to have fluctuated the
pressure of the fault away; this pressure probably increased the
power of the earthquake and accelerated the rate of movement for
the fault. The greatest earthquake in Australia's history was also
induced by humanity, through coal mining.
The city of
Newcastle
was built over a large sector of coal mining
areas. The earthquake was spawned from a fault which
reactivated due to the millions of tonnes of rock removed in the
mining process.
Measuring and locating earthquakes
Earthquakes can be recorded by seismometers up to great distances,
because
seismic waves travel through
the whole
Earth's interior. The
absolute magnitude of a quake is conventionally reported by numbers
on the
Moment magnitude scale
(formerly Richter scale, magnitude 7 causing serious damage over
large areas), whereas the felt magnitude is reported using the
modified Mercalli scale (intensity II-XII).
Every tremor produces different types of seismic waves which travel
through rock with different velocities: the longitudinal
P-waves (shock- or pressure waves), the transverse
S-waves (both body waves) and several
surface waves (
Rayleigh and
Love
waves). The
propagation
velocity of the seismic waves ranges from approx. 3 km/s
up to 13 km/s, depending on the
density
and
elasticity of the medium. In the
Earths interior the shock- or P waves travel much more faster than
the S waves (approx. relation 1.7 : 1). The differences in
travel time from the
epicentre to the observatory are a measure of the
distance and can be used to image both sources of quakes and
structures within the Earth. Also the depth of the
hypocenter can be computed roughly.
In solid rock P-waves travel at about 6 to 7 km per second;
the velocity increases within the deep mantle to ~13 km/s. The
velocity of S-waves ranges from 2–3 km/s in light sediments
and 4–5 km/s in the Earths crust up to 7 km/s in the deep
mantle. As a consequence, the first waves of a distant earth quake
arrive at an observatory via the Earths mantle.
Rule of thumb: On the average, the kilometer
distance to the earthquake is the number of seconds between the P
and S wave
times 8 [1210]. Slight deviations are caused by inhomogenities
of subsurface structure. By such analyses of seismograms the
Earth's core was located in 1913 by
Beno
Gutenberg.
Effects/impacts of earthquakes
The effects of earthquakes include, but are not limited to, the
following:
Shaking and ground rupture
Shaking and ground rupture are the main effects created by
earthquakes, principally resulting in more or less severe damage to
buildings and other rigid structures. The severity of the local
effects depends on the complex combination of the earthquake
magnitude, the distance from
the
epicenter, and the local geological
and geomorphological conditions, which may amplify or reduce
wave propagation. The
ground-shaking is measured by ground
acceleration.
Specific local geological, geomorphological, and geostructural
features can induce high levels of shaking on the ground surface
even from low-intensity earthquakes. This effect is called site or
local amplification. It is principally due to the transfer of the
seismic motion from hard deep soils to soft
superficial soils and to effects of seismic energy focalization
owing to typical geometrical setting of the deposits.
Ground rupture is a visible breaking and displacement of the
Earth's surface along the trace of the fault, which may be of the
order of several metres in the case of major earthquakes. Ground
rupture is a major risk for large engineering structures such as
dams, bridges and
nuclear power stations and requires
careful mapping of existing faults to identify any likely to break
the ground surface within the life of the structure.
Landslides and avalanches
Earthquakes, along with severe storms, volcanic activity, coastal
wave attack, and wildfires, can produce slope instability leading
to landslides, a major geological hazard. Landslide danger may
persist while emergency personnel are attempting rescue.
Fires
Earthquakes can cause
fires by damaging
electrical power or gas lines. In the
event of water mains rupturing and a loss of pressure, it may also
become difficult to stop the spread of a fire once it has started.
For example, more deaths in the
1906 San Francisco earthquake
were caused by fire than by the earthquake itself.
Soil liquefaction
Soil liquefaction occurs when, because of the shaking,
water-saturated
granular material (such as
sand) temporarily loses its strength and transforms from a
solid to a
liquid. Soil
liquefaction may cause rigid structures, like buildings and
bridges, to tilt or sink into the liquefied deposits. This can be a
devastating effect of earthquakes. For example, in the
1964 Alaska earthquake, soil
liquefaction caused many buildings to sink into the ground,
eventually collapsing upon themselves.
Tsunami
Tsunamis are long-wavelength, long-period sea waves produced by the
sudden or abrupt movement of large volumes of water. In the open
ocean the distance between wave crests can surpass 100 kilometers,
and the wave periods can vary from five minutes to one hour. Such
tsunamis travel 600-800 kilometers per hour, depending on water
depth. Large waves produced by an earthquake or a submarine
landslide can overrun nearby coastal areas in a matter of minutes.
Tsunamis can also travel thousands of kilometers across open ocean
and wreak destruction on far shores hours after the earthquake that
generated them.
Ordinarily, subduction earthquakes under magnitude 7.5 on the
Richter scale do not cause tsunamis, although some instances of
this have been recorded. Most destructive tsunamis are caused by
earthquakes of magnitude 7.5 or more.
Floods
A flood is an overflow of any amount of water that reaches land.
Floods occur usually when the volume of water within a body of
water, such as a river or lake, exceeds the total capacity of the
formation, and as a result some of the water flows or sits outside
of the normal perimeter of the body. However, floods may be
secondary effects of earthquakes, if dams are damaged. Earthquakes
may cause landslips to dam rivers, which then collapse and cause
floods.
The
terrain below the Sarez
Lake
in Tajikistan
is in danger of catastrophic flood if the landslide dam formed by the earthquake, known
as the Usoi Dam, were to fail during a
future earthquake. Impact projections suggest the flood
could affect roughly 5 million people.
Human impacts
Earthquakes may lead to
disease, lack of
basic necessities, loss of life, higher insurance premiums, general
property damage, road and bridge
damage, and collapse or destabilization (potentially leading to
future collapse) of buildings. Earthquakes can also precede
volcanic eruptions, which cause further problems; for example,
substantial crop damage, as in the "
Year Without a Summer" (1816).
Preparation
In order to determine the likelihood of future seismic activity,
geologists and other scientists examine
the rock of an area to determine if the rock appears to be
"strained". Studying the
fault of an
area to study the buildup time it takes for the fault to build up
stress sufficient for an earthquake also serves as an effective
predicition technique. Measurements of the amount of pressure which
collocates on the fault line each year, time passed since the last
major temblor, and the energy and power of the last earthquake are
made. Together the facts allow scientists to determine how much
pressure it takes for the fault to generate an earthquake.
Though
this method is useful, it has only been implemented on California's
San Andreas
Fault
.
Today, there are ways to protect and prepare possible sites of
earthquakes from severe damage, through the following processes:
earthquake engineering,
earthquake preparedness,
household seismic safety,
seismic retrofit (including special
fasteners, materials, and techniques),
seismic hazard,
mitigation of seismic motion,
and
earthquake
prediction.
History
An image from a 1557 book
Pre-Middle Ages
From the lifetime of Greek
Anaxagoras to
the 14th century, earthquakes were attributed to "air (vapors) in
the cavities of the Earth". Tales of Milet, who lived from 625-547
(BCE) was the only documented person who believed that earthquakes
were caused by tension between the earth and water. Other theories
existed, including Greek philosopher Anaxamines of Milet's (585-526
BCE) beliefs that short incline episodes of dryness and wetness
caused seismic activity. Greek philosopher Democritus (460-371BCE)
blamed water in general for earthquakes.
Pliny the Elder called earthquakes
"underground thunderstorms".
Earthquakes in culture
Mythology and religion
In
Norse mythology, earthquakes were
explained as the violent struggling of the god
Loki. When Loki,
god of mischief
and strife, murdered
Baldr, god of beauty and
light, he was punished by being bound in a cave with a poisonous
serpent placed above his head dripping venom. Loki's wife
Sigyn stood by him with a bowl to catch the poison,
but whenever she had to empty the bowl the poison would drip on
Loki's face, forcing him to jerk his head away and thrash against
his bonds, causing the earth to tremble.
In
Greek mythology,
Poseidon was the cause and god of earthquakes. When
he was in a bad mood, he would strike the ground with a
trident, causing this and other calamities. He also
used earthquakes to punish and inflict fear upon people as
revenge.
In
Japanese mythology,
Namazu
(鯰) is a giant
catfish who causes
earthquakes. Namazu lives in the mud beneath the earth, and is
guarded by the god
Kashima who restrains the
fish with a stone. When Kashima lets his guard fall, Namazu
thrashes about, causing violent
earthquakes.
Popular culture
In modern
popular culture, the portrayal of
earthquakes is shaped by the memory of great cities laid waste,
such as Kobe in
1995
or San
Francisco in 1906. Fictional earthquakes tend to strike
suddenly and without warning. For this reason, stories about
earthquakes generally begin with the disaster and focus on its
immediate aftermath, as in
Short Walk to Daylight (1972),
The Ragged Edge
(1968) or
Aftershock: Earthquake in New
York (1998). A notable example is Heinrich von Kleist's
classic novella,
The
Earthquake in Chile, which describes the destruction of
Santiago in 1647.
Haruki Murakami's
short fiction collection,
After the Quake, depicts the
consequences of the Kobe earthquake of 1995.
The most
popular single earthquake in fiction is the hypothetical "Big One"
expected of California's San Andreas Fault someday, as depicted in the novels Richter 10 (1996) and Goodbye California (1977)
among other works. Jacob M. Appel's widely-anthologized
short story,
A Comparative Seismology, features a con
artist who convinces an elderly woman that an apocalyptic
earthquake is imminent. In
Pleasure Boating in Lituya Bay,
one of the stories in
Jim Shepard's
Like You'd Understand, Anyway, the "Big One" leads to an
even more devastating tsunami.
See also
References
External links
Educational
Seismological data centers
Europe
Japan
New Zealand
United States
Seismic scales
Scientific information
Miscellaneous
Wikis
Quiz
Facts
Map
Misc
Search
