Earthquakes
An
earthquake (also known as a
quake,
tremor or
temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The
seismicity,
seismism or
seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.
Earthquakes are measured using observations from
seismometers. The
moment magnitude
is the most common scale on which earthquakes larger than approximately
5 are reported for the entire globe. The more numerous earthquakes
smaller than magnitude 5 reported by national seismological
observatories are measured mostly on the local magnitude scale, also
referred to as the
Richter
scale. These two scales are numerically similar over their range of
validity. Magnitude 3 or lower earthquakes are mostly almost
imperceptible or weak and magnitude 7 and over potentially cause serious
damage over larger areas, depending on their depth. The largest
earthquakes in historic times have been of magnitude slightly over 9,
although there is no limit to the possible magnitude. The most recent
large earthquake of magnitude 9.0 or larger was a
9.0 magnitude earthquake in Japan in 2011
(as of October 2012), and it was the largest Japanese earthquake since
records began. Intensity of shaking is measured on the modified
Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal.
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the
epicenter 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
hypocenter. The
epicenter is the point at ground level directly above the hypocenter.
Naturally occurring 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
aseismically 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.
Earthquake fault types
There are three main types of fault, all of which 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.
Reverse faults, particularly those along
convergent plate boundaries
are associated with the most powerful earthquakes, including almost all
of those of magnitude 8 or more. Strike-slip faults, particularly
continental
transforms
can produce major earthquakes up to about magnitude 8. Earthquakes
associated with normal faults are generally less than magnitude 7.
This is so because the energy released in an earthquake, and thus its
magnitude, is proportional to the area of the fault that ruptures
and the stress drop. Therefore, the longer the length and the wider the
width of the faulted area, the larger the resulting magnitude. The
topmost, brittle part of the Earth's crust, and the cool slabs of the
tectonic plates that are descending down into the hot mantle, are the
only parts of our planet which can store elastic energy and release it
in fault ruptures. Rocks hotter than about 300 degrees Celsius flow in
response to stress; they do not rupture in earthquakes.
The maximum observed lengths of ruptures and mapped faults, which may
break in one go are approximately 1000 km. Examples are the earthquakes
in
Chile, 1960;
Alaska, 1957;
Sumatra, 2004, all in subduction zones. The longest earthquake ruptures on strike-slip faults, like the
San Andreas Fault (1857, 1906), the
North Anatolian Fault in Turkey (1939) and the
Denali Fault
in Alaska (2002), are about half to one third as long as the lengths
along subducting plate margins, and those along normal faults are even
shorter.
Aerial photo of the San Andreas Fault in the
Carrizo Plain, northwest of Los Angeles
The most important parameter controlling the maximum earthquake
magnitude on a fault is however not the maximum available length, but
the available width because the latter varies by a factor of 20. Along
converging plate margins, the dip angle of the rupture plane is very
shallow, typically about 10 degrees.
[6] Thus the width of the plane within the top brittle crust of the Earth can become 50 to 100 km (
Tohoku, 2011;
Alaska, 1964), making the most powerful earthquakes possible.
Strike-slip faults tend to be oriented near vertically, resulting in an approximate width of 10 km within the brittle crust,
thus earthquakes with magnitudes much larger than 8 are not possible.
Maximum magnitudes along many normal faults are even more limited
because many of them are located along spreading centers, as in Iceland,
where the thickness of the brittle layer is only about 6 km.
In addition, there exists a hierarchy of stress level in the three
fault types. Thrust faults are generated by the highest, strike slip by
intermediate, and normal faults by the lowest stress levels.
This can easily be understood by considering the direction of the
greatest principal stress, the direction of the force that 'pushes' the
rock mass during the faulting. In the case of normal faults, the rock
mass is pushed down in a vertical direction, thus the pushing force (
greatest
principal stress) equals the weight of the rock mass itself. In the
case of thrusting, the rock mass 'escapes' in the direction of the least
principal stress, namely upward, lifting the rock mass up, thus the
overburden equals the
least principal stress. Strike-slip
faulting is intermediate between the other two types described above.
This difference in stress regime in the three faulting environments can
contribute to differences in stress drop during faulting, which
contributes to differences in the radiated energy, regardless of fault
dimensions.
/ˈælbərt ˈaɪnstaɪn/; German: [ˈalbɐt ˈaɪnʃtaɪn] (
listen); 14 March 1879 – 18 April 1955) was a German-born theoretical physicist who developed the general theory of relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and the most influential physicist of the 20th century. While best known for his mass–energy equivalence formula E = mc2 (which has been dubbed "the world's most famous equation"),he received the 1921 Nobel Prize in Physics "for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect".The latter was pivotal in establishing quantum theory.
