বাংলাদেশ অঞ্চলের ভমিকম্প এর ব্যাপারে অধ্যাপক মোহাম্মদ জাফর ইকবালের একটি লেখার কিছু অংশ বাংলাদেশী পাঠকদের জন্যে যুক্ত করা হল এই লেখাটির সঙ্গে ৷ ( সৌজন্য জনকণ্ঠ ও জাফর স্যার )
আট মাত্রার ভূমিকম্প ছয়
মাত্রার ভূমিকম্প থেকে এক হাজার গুণ বেশি শক্তিশালী। তার
অর্থ এই নয় যে, সেই ভূমিকম্পটির তীব্রতা, কম্পন বা ঝাঁকুনি এক হাজার গুণ বেশি! তার
অর্থ ছয় মাত্রার ভূমিকম্প হয় অল্প জায়গা জুড়ে, আট মাত্রার ভূমিকম্প হয় অনেক বেশি জায়গা
জুড়ে। আমাদের পায়ের নিচে শক্ত মাটি দেখে আমরা ধরে নেই ভূমি হচ্ছে
স্থির। আসলে ভূমি স্থির নয়, সেগুলো নানা ভাগে বিভক্ত এবং
সেগুলো এদিক সেদিক নড়ছে। আমরা যে ভূমিখ-ের ওপর আছি তার নাম ইন্ডিয়ান প্লেট। সেটা
বছরে দুই ইঞ্চি করে উত্তর দিকে এগুচ্ছে এবং উত্তরের ইউরেশিয়ান প্লেটকে ধাক্কা
দিচ্ছে। সেই ধাক্কায় মাটি ওপরে উঠতে উঠতে হিমালয় পর্যন্ত
তৈরি হয়ে গেছে!
সব প্লেটেরই একটা পরিসীমা বা বাউন্ডারি থাকে। এই
বাউন্ডারিতে ধাক্কাধাক্কি চলতে থাকে। তাই নিয়মিতভাবে এই
বাউন্ডারিতে ভূমিকম্প হতে থাকে! সেই ভূমিকম্প এতোই নিয়মিত যে বিজ্ঞানীরা আজকাল
মোটামুটি আত্মবিশ্বাস নিয়ে বলেন যে, রিখটার স্কেলে নয় মাত্রার ভূমিকম্প হয়
আনুমানিক দশ বছরে একবার। আট মাত্রার ভূমিকম্প হয় আরও বেশি, আনুমানিক
প্রতি বছরে একবার। হিসেবটি মনে রাখা বেশ সোজা, ভূমিকম্পের
মাত্রা এক কমে গেলে তার সংখ্যা বেড়ে যায় দশগুণ। অর্থাৎ
সাত মাত্রায় ভূমিকম্প বছরে দশটি, ছয় মাত্রার ভূমিকম্প বছরে একশটি, পাঁচ মাত্রার ভূমিকম্প বছরে
প্রায় এক হাজার, চার মাত্রার ভূমিকম্প বছরে দশ হাজার।
এর চেয়ে ছোট ভূমিকম্পের
হিসাব নিয়ে লাভ নেই। সেগুলো ঘটলেও আমরা টের পাই না! কাজেই আসল কথাটা
হচ্ছে বছরে সারা পৃথিবীতে ছোট বড় হাজার হাজার ভূমিকম্প হচ্ছে এবং সেগুলোর প্রায়
বেশিরভাগ পৃথিবী পৃষ্ঠের সঞ্চরণশীল ভূখণ্ড বা টেকটোনিক প্লেটের পরিসীমা বা
বাউন্ডারিতে। সেজন্য নেপাল সিকিম ভুটানে এতো ঘন ঘন ভূমিকম্প হয়। কারণ
আমাদের ভূখণ্ডের পরিসীমা বা ফল্টলাইনটা এই দেশগুলোর ভেতর দিয়ে গিয়েছে।
আমাদের
কপাল অনেক ভালো যে, সেই ফল্টলাইন খুব যতœ করে বাংলাদেশকে বাঁচিয়ে মিয়ানমারের ভেতর দিয়ে নিচে নেমে গেছে। বড়
ফল্টলাইনটা বাংলাদেশের ভেতর দিয়ে না গেলেও উত্তরবঙ্গের পঞ্চগড়, তেঁতুলিয়ার
খুব কাছ দিয়ে গিয়েছে- দূরত্ব পঞ্চাশ কিলোমিটার থেকে কম। তাই
যখন এই ফল্টলাইনে ভূমিকম্প হয় বাংলাদেশের অন্য জায়গা থেকে সেভাবে টের না পেলেও
উত্তরবঙ্গের মানুষ ভালোই টের পায়। বড় ফল্টলাইন থেকে ছোটখাটো
অনেক শাখা-প্রশাখা বের হয় এবং আমাদের দেশে এরকম কিছু ফল্টলাইন থাকতে পারে, সেখান
থেকে ভূমিকম্প হতেও পারে। ভূমিকম্পটি এমন একটি ব্যাপার যে, কোথায়
হবে এবং কোথায় হবে না সেটি
আজকাল তথ্যপ্রযুক্তির
যুুগ, কিছুক্ষণের মাঝেই ভূমিকম্পটির নাড়ি নক্ষত্র ইন্টারনেটে চলে আসবে।
ইউএসজিএসের একটা অসাধারণ
ওয়েবসাইট রয়েছে (earthquake.usgs.gov)। সেখানে
পৃথিবীর যে কোনো প্রান্তে যে কোনো ভূমিকম্প হলেই তার তথ্যটি কয়েক মিনিটেই চলে আসে।
মজার
ব্যাপার হচ্ছে ইউএসজিএসের এই ওয়েবসাইটটি খুলে বসে থাকলে কিছুক্ষণের মাঝেই দেখা
যাবে পৃথিবীর কোথাও না কোথাও একটা ভূমিকম্প হয়েছে। আমরা
যদি নিজের চোখে দেখি সারা পৃথিবীতে হাজার হাজার ছোট বড় ভূমিকম্প হচ্ছে এবং পৃথিবীর
মানুষ এর মাঝেই শান্তিতে দিন কাটাচ্ছে, তাহলে আমার ধারণা আমাদের এই যুক্তিহীন
ভয়টা অনেক কমে আসবে
ভূমিকম্প
হলে তার কেন্দ্র থেকে দুই ধরনের তরঙ্গ বের হয়। একটা
তরঙ্গ শব্দের মতো, মাটির ভেতর দিয়ে সেটা দ্রুত চলে আসে, এটার
নাম প্রাইমারি বা সংক্ষেপে পি. ওয়েভ। দ্বিতীয়টি হচ্ছে সেকেন্ডারি
বা এস. ওয়েভ, এটা হচ্ছে সত্যিকারের কাঁপুনি যেটা আমরা অনুভব করি। এর
গতিবেগ পি ওয়েভ থেকে সেকেন্ডে প্রায় দশ কিলোমিটার কম। কাজেই
দূরে যদি কোথাও ভূমিকম্প হয় তাহলে প্রথমে পি. ওয়েভ এসে একটা ছোট ধাক্কা দেয় এবং
সেকেন্ডে প্রায় দশ কিলোমিটার পিছিয়ে থাকা এস. ওয়েভ একটু পরে এসে ঝাঁকাঝাঁকি
কাঁপাকাঁপি শুরু করে দেয়! কাজেই পি ওয়েভ আসার কত সেকেন্ড পর এস. ওয়েভ এসে আসল
ঝাঁকুনি শুরু করে, সেটা জানলেই আমরা ভূমিকম্পের কেন্দ্রটি কত দূরে সেটা বের করে
ফেলতে পারি! যত সেকেন্ড পার্থক্য তাকে দশ দিয়ে গুণ করলেই দূরত্ব বের হয়ে যায়। (অসমাপ্ত )
ভূমিকম্পের তীব্রতা মাপতে রিখটার স্কেল ব্যবহার করা হয়। ক্যালিফোর্নিয়া ইন্সস্টিটিউট অব টেকনোলজিতে ১৯৩৫ সালে চার্লস ফ্রান্সিস রিখটার এবং বেনো গুটেনবার্গ ভূমিকম্পের তীব্রতা মাপতে এই স্কেলের ব্যবহার শুরু করেন। এই স্কেলে ভূমিকম্প থেকে নির্গত শক্তির পরিমাণ মাপা হয়।
তবে ১৯৭০ সাল থেকে ভূমিকম্পের তীব্রতা মাপতে মোমেন্ট ম্যাগনিচিউড স্কেল (এমএমএস)-এর ব্যবহারও শুরু হয়। এই স্কেলের প্রথম ব্যবহার করে ‘ইউনাইটেড স্টেটস জিওলজিক্যাল সার্ভে’। এখন অধিকাংশ ক্ষেত্রে এমএমএস ব্যবহার করা হলেও সাধারণ মানুষের মধ্যে রিখটার স্কেলের জনপ্রিয়তাই বেশি।
রিখটার স্কেল আসলে লগ স্কেল। এই হিসেবে কোনও ভূমিকম্প থেকে সৃষ্ট তরঙ্গগুলির মধ্যে রেকর্ড করা সবচেয়ে বেশি অ্যামপ্লিচিউড-এর (উচ্চতা), ভূমিকম্পের উৎপত্তিস্থল থেকে সিসমোগ্রাফ (ভূমিকম্প মাপার যন্ত্র) যন্ত্রের গড় দূরত্ব ইত্যাদি ব্যবহার করে ভূমিকম্প থেকে নির্গত শক্তির পরিমাপ করা হয়। এই স্কেলের লগের বেস ১০ ধরা হয়। ফলে রিখটার স্কেলে কোনও ভূমিকম্পের মাপ এক, আর কোনও ভূমিকম্পের মাপ দুই এলে দ্বিতীয়টি প্রথমটির থেকে দশগুণ শক্তিশালী হবে।
British Dictionary definitions for earthquakeExpand
earthquake
The World Fault Line across the globe.
The earth is constantly moving because of which there is a continuous movement of the rocks. This movement of the rocks creates fractures or discontinuity which is better known as a fault.
The tectonic forces at work within the rocks create large faults resulting in the release of energy that consequently leads to the eruption of volcanoes and earthquakes.
When a continental/oceanic plate or two continental/oceanic plates or a continental and an oceanic plate move apart, a fault line is created;
while when the plates head-on, a fold is created.
For instance, , when the Nazca Plate and the South American Plate move apart, a fault line is created that leads to the formation of the Andes mountain range.
Similarly, many mountains and other formations are created owing to the movement of other continental and oceanic plates.
The the major fault lines created by the movement of the various continental and oceanic plates.
are:
- North American Plate
- South American Plate
- African Plate
- Arabian Plate
- Eurasian Plate
- Bismarc Plate
- Indo-Australian Plate
- Antarctic Plate
The World Fault Line at the major oceanic plates.
They are:
- Juan De Fuca Plate
- Pacific Plate
- Nazca Plate
- Scotia Plate
- Cocos Plate
- Caribbean Plate
- Philippine Plate
- Fiji Plate
- Carolina Plate
The African Plate is believed to be splitting along the East African Rift Zone.
This causes cracks in the surface that results in the rise of the magma, consequently giving rise to volcanic eruptions.
:
O
earthquake definition
A tremor of the surface of the Earth, sometimes severe and devastating,which results from shock waves generated by the movement of rockmasses deep within the Earth, particularly near boundaries of tectonic plates.
:
Earthquakes &
Tsunamis: Causes & Information
by Nola Taylor
Redd |
What causes
earthquakes?
Earth's crust ranges from 3 to 45 miles deep (5 to 70
kilometers). The crust is a thin, hard shell that floats on the denser, hotter
rock of the mantle. The crust is divided into several
pieces known as tectonic plates that are constantly in motion, grinding past
one another at boundaries known as faults.
As they slide past one another, the tectonic plates snag on
rough patches of rock. They lock together like Velcro. However, even though the
fault boundaries are locked together, the plates still move, pulling at the
entangled sections. This pulling can further crack the Earth's crust, creating
more faults near the plate boundaries.
An earthquake occurs
when the pressure built up along a fault becomes stronger than the pressure
holding the rocks together. Then the rocks on either side of the fault suddenly
rip apart, sometimes at supersonic speeds. The two sides of the fault slide
past one another, releasing the pent-up pressure. Energy from this separation
radiates outward in all directions, including towards the surface, where it is
felt as an earthquake.
Even though the tectonic plates slide at a regular rate over
time, the way that faults release stored energy is different with each earthquake,
said Shimon Wdowinski, a geophysicist at the University of Miami Rosentiel School
A large earthquake is often followed by aftershocks, which are
smaller quakes that result from the crust adjusting to the main shock. These
aftershocks can help scientists target the origin of the main quake, but can
create problems for those suffering its aftermath.
Tsunamis
Measuring earthquakes
An earthquake's size, or magnitude, depends on how large its
parent fault is and how much it has slipped. Because these faults extend from
the surface down to several miles deep, geologists can't simply visit the
source to calculate these numbers. Instead, they rely on a tool known as a
seismograph, which measures seismic waves, or vibrations, from an earthquake.
An earthquake's magnitude is ranked on the moment magnitude
scale, not the Richter scale. The moment magnitude scale provides a better idea
of the shaking and possible damage from earthquakes of all kinds around the
world. [Related: Whatever Happened to the Richter Scale?]
Earthquakes with magnitudes less than 3 occur every day, and are generally not felt by
people. A magnitude of 3 to 5 is considered minor, while a quake with a
magnitude of 5 to 7 is moderate to strong. At the higher end, these quakes can
be destructive to cities. Earthquakes from 7 to 8 are major; about 15 of these
occur annually. Every year, at least one earthquake with a magnitude over 8 — a
"great" quake — wreaks havoc. An earthquake with a magnitude of 10
has never been measured, but it would create widespread devastation.
[Scary Scenario: Devastating Earthquake Visualized]
[Scary Scenario: Devastating Earthquake Visualized]
.
Preparing for disaster
Scientists have not yet come up with a way to forecast
earthquakes. Although animals are reputed to have a sixth sense when it comes
to these vibrations, no research has confirmed it, much less determined how
such predictions might occur. In many cases, animals are simply sensing the
arrival of earthquake waves that go unnoticed by people.
.
Earthquake
From
Wikipedia, the free encyclopedia
An earthquake (also known as a quake, tremor or temblor)
is the result of a sudden release of energy in the Earth's crustthat 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 magnitude
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 March 2014), 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.
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 March 2014), 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 theepicenter 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 itsfocus or hypocenter. The epicenter is
the point at ground level directly above the hypocenter.
Naturally occurring
earthquakes
Fault types
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.
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 interplate 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.
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, megathrust
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.
For every unit increase in magnitude, there is a roughly thirtyfold increase in the energy released. For instance, an earthquake of magnitude 6.0 releases approximately 30 times more energy than a 5.0 magnitude earthquake and a 7.0 magnitude earthquake releases 900 times (30 × 30) more energy than a 5.0 magnitude of earthquake. An 8.6 magnitude earthquake releases the same amount of energy as 10,000 atomic bombs that were used in World War II.
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.
For every unit increase in magnitude, there is a roughly thirtyfold increase in the energy released. For instance, an earthquake of magnitude 6.0 releases approximately 30 times more energy than a 5.0 magnitude earthquake and a 7.0 magnitude earthquake releases 900 times (30 × 30) more energy than a 5.0 magnitude of earthquake. An 8.6 magnitude earthquake releases the same amount of energy as 10,000 atomic bombs that were used in World War II.
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 a single rupture) 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.
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 a single rupture) 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.
Earthquakes away from plate boundaries
Where plate boundaries occur within the continental lithosphere, deformation is spread
out 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.
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 earthquakes 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 where 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.
In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes 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 where 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, as during the Mount St. Helens eruption of 1980. Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltmeters (a device that measures ground slope) and used as sensors to predict imminent or upcoming eruptions.
Such earthquakes can serve as an early warning of volcanic eruptions, as during the Mount St. Helens eruption of 1980. Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltmeters (a device that measures ground slope) and used as sensors to predict imminent or upcoming eruptions.
Rupture dynamics
A tectonic earthquake begins by an initial
rupture at a point on the fault surface, a process known as nucleation.
The scale of the nucleation zone is uncertain, with some evidence, such as the rupture dimensions of the smallest earthquakes, suggesting that it is smaller than 100 m while other evidence, such as a slow component revealed by low-frequency spectra of some earthquakes, suggest that it is larger.
The possibility that the nucleation involves some sort of preparation process is supported by the observation that about 40% of earthquakes are preceded by foreshocks. Once the rupture has initiated it begins to propagate along the fault surface. The mechanics of this process are poorly understood, partly because it is difficult to recreate the high sliding velocities in a laboratory. Also the effects of strong ground motion make it very difficult to record information close to a nucleation zone.
The scale of the nucleation zone is uncertain, with some evidence, such as the rupture dimensions of the smallest earthquakes, suggesting that it is smaller than 100 m while other evidence, such as a slow component revealed by low-frequency spectra of some earthquakes, suggest that it is larger.
The possibility that the nucleation involves some sort of preparation process is supported by the observation that about 40% of earthquakes are preceded by foreshocks. Once the rupture has initiated it begins to propagate along the fault surface. The mechanics of this process are poorly understood, partly because it is difficult to recreate the high sliding velocities in a laboratory. Also the effects of strong ground motion make it very difficult to record information close to a nucleation zone.
Rupture propagation is generally modeled using
a fracture mechanics approach, likening the rupture to a
propagating mixed mode shear crack.
The rupture velocity is a function of the fracture energy in the volume around the crack tip, increasing with decreasing fracture energy. The velocity of rupture propagation is orders of magnitude faster than the displacement velocity across the fault.
Earthquake ruptures typically propagate at velocities that are in the range 70–90% of the S-wave velocity and this is independent of earthquake size. A small subset of earthquake ruptures appear to have propagated at speeds greater than the S-wave velocity. These supershear earthquakes have all been observed during large strike-slip events. The unusually wide zone of coseismic damage caused by the 2001 Kunlun earthquake has been attributed to the effects of the sonic boom developed in such earthquakes. Some earthquake ruptures travel at unusually low velocities and are referred to as slow earthquakes. A particularly dangerous form of slow earthquake is the tsunami earthquake, observed where the relatively low felt intensities, caused by the slow propagation speed of some great earthquakes, fail to alert the population of the neighbouring coast, as in the 1896 Meiji-Sanriku earthquake.
The rupture velocity is a function of the fracture energy in the volume around the crack tip, increasing with decreasing fracture energy. The velocity of rupture propagation is orders of magnitude faster than the displacement velocity across the fault.
Earthquake ruptures typically propagate at velocities that are in the range 70–90% of the S-wave velocity and this is independent of earthquake size. A small subset of earthquake ruptures appear to have propagated at speeds greater than the S-wave velocity. These supershear earthquakes have all been observed during large strike-slip events. The unusually wide zone of coseismic damage caused by the 2001 Kunlun earthquake has been attributed to the effects of the sonic boom developed in such earthquakes. Some earthquake ruptures travel at unusually low velocities and are referred to as slow earthquakes. A particularly dangerous form of slow earthquake is the tsunami earthquake, observed where the relatively low felt intensities, caused by the slow propagation speed of some great earthquakes, fail to alert the population of the neighbouring coast, as in the 1896 Meiji-Sanriku earthquake.
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 that
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.
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
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 aftershocksby 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. In August 2012, a swarm of earthquakes shookSouthern California 's Imperial Valley , showing the most
recorded activity in the area since the 1970s.
An example of an earthquake swarm is the 2004 activity at Yellowstone National Park. In August 2012, a swarm of earthquakes shook
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 inTurkey Middle East .
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
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:
storing large amounts of water behind a dam (and possibly building an extremely
heavy building), drilling and injecting liquid into wells, 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 1,650 feet (503 m) away; this pressure probably increased the power of the earthquake and accelerated the rate of movement for the fault.[
The greatest earthquake inAustralia
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 1,650 feet (503 m) away; this pressure probably increased the power of the earthquake and accelerated the rate of movement for the fault.[
The greatest earthquake in
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 intensity scale (intensity II–XII).
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 intensity scale (intensity II–XII).
Every tremor produces different types of
seismic waves, which travel through rock with different velocities:
·
Longitudinal P-waves (shock-
or pressure waves)
·
Surface waves — (Rayleigh and Love waves)
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 Earth's interior
the shock- or P waves travel much 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.
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 Earth's crust up to 7 km/s in the deep
mantle. As a consequence, the first waves of a distant earthquake arrive at an
observatory via the Earth's mantle.
On average, the kilometer distance to the
earthquake is the number of seconds between the P and S wave times 8. Slight deviations are caused by inhomogeneities of
subsurface structure. By such analyses of seismograms the Earth's core was
located in 1913 by Beno Gutenberg.
Earthquakes are not only categorized by their
magnitude but also by the place where they occur. The world is divided into 754 Flinn–Engdahl regions (F-E regions), which are based on
political and geographical boundaries as well as seismic activity. More active
zones are divided into smaller F-E regions whereas less active zones belong to
larger F-E regions.
Standard reporting of earthquakes includes its magnitude,
date and time of occurrence, geographic
coordinates of its epicenter, depth of the epicenter,
geographical region, distances to population centers, location uncertainty, a
number of parameters that are included in USGS earthquake reports (number of
stations reporting, number of observations, etc.), and a unique event ID.
Where Do Earthquakes Happen?
FIGURE 1 - AN IMAGE OF THE WORLD'S PLATES AND THEIR BOUNDARIES. NOTICE THAT MANY PLATE BOUNDARIES DO NOT COINCIDE WITH COASTLINES.
FIGURE 2 - A NORMAL FAULT. THE 'FOOTWALL' IS ON THE 'UPTHROWN' SIDE OF THE FAULT, MOVING UPWARDS. THE 'HANGING WALL' IS ON THE 'DOWNTHROWN' SIDE OF THE FAULT, MOVING DOWNWARDS.
FIGURE 3 - A REVERSE FAULT. THIS TIME, THE 'FOOTWALL' IS ON THE 'DOWNTHROWN' SIDE OF THE FAULT, MOVING DOWNWARDS, AND THE 'HANGING WALL' IS ON THE 'UPTHROWN' SIDE OF THE FAULT, MOVING UPWARDS. WHEN THE HANGING WALL IS ON THE UPTHROWN SIDE, IT 'HANGS' OVER THE FOOTWALL.
FIGURE 4 - TWO STRIKE-SLIP FAULTS. (LEFT), A LEFT-LATERAL STRIKE-SLIP FAULT. NO MATTER WHICH SIDE OF THE FAULT YOU ARE ON, THE OTHER SIDE IS MOVING TO THE LEFT. (RIGHT), A RIGHT-LATERAL STRIKE-SLIP FAULT. NO MATTER WHICH SIDE OF THE FAULT YOU ARE ON, THE OTHER SIDE IS MOVING TO THE RIGHT.
Where Do Earthquakes Happen?
Earthquakes occur all the time all over the world, both along plate edges and along faults.
Along Plate Edges
Most earthquakes occur along the edge of the oceanic and continental plates. The earth's crust (the outer layer of the planet) is made up of several pieces, called plates. The plates under the oceans are called oceanic plates and the rest are continental plates. The plates are moved around by the motion of a deeper part of the earth (the mantle) that lies underneath the crust. These plates are always bumping into each other, pulling away from each other, or past each other. The plates usually move at about the same speed that your fingernails grow. Earthquakes usually occur where two plates are running into each other or sliding past each other.
FIGURE 1 - AN IMAGE OF THE WORLD'S PLATES AND THEIR BOUNDARIES. NOTICE THAT MANY PLATE BOUNDARIES DO NOT COINCIDE WITH COASTLINES.
Along Faults
Earthquakes can also occur far from the edges of plates, along faults. Faults are cracks in the earth where sections of a plate (or two plates) are moving in different directions. Faults are caused by all that bumping and sliding the plates do. They are more common near the edges of the plates.
Types of Faults
Normal faults are the cracks where one block of rock is sliding downward and away from another block of rock. These faults usually occur in areas where a plate is very slowly splitting apart or where two plates are pulling away from each other. A normal fault is defined by the hanging wall moving down relative to the footwall, which is moving up.
FIGURE 2 - A NORMAL FAULT. THE 'FOOTWALL' IS ON THE 'UPTHROWN' SIDE OF THE FAULT, MOVING UPWARDS. THE 'HANGING WALL' IS ON THE 'DOWNTHROWN' SIDE OF THE FAULT, MOVING DOWNWARDS.
Reverse faults are cracks formed where one plate is pushing into another plate. They also occur where a plate is folding up because it's being compressed by another plate pushing against it. At these faults, one block of rock is sliding underneath another block or one block is being pushed up over the other. A reverse fault is defined by the hanging wall moving up relative to the footwall, which is moving down.
FIGURE 3 - A REVERSE FAULT. THIS TIME, THE 'FOOTWALL' IS ON THE 'DOWNTHROWN' SIDE OF THE FAULT, MOVING DOWNWARDS, AND THE 'HANGING WALL' IS ON THE 'UPTHROWN' SIDE OF THE FAULT, MOVING UPWARDS. WHEN THE HANGING WALL IS ON THE UPTHROWN SIDE, IT 'HANGS' OVER THE FOOTWALL.
Strike-slip faults are the cracks between two plates that are sliding past each other. You can find these kinds of faults in California. The San Andreas fault is a strike-slip fault. It's the most famous California fault and has caused a lot of powerful earthquakes.
FIGURE 4 - TWO STRIKE-SLIP FAULTS. (LEFT), A LEFT-LATERAL STRIKE-SLIP FAULT. NO MATTER WHICH SIDE OF THE FAULT YOU ARE ON, THE OTHER SIDE IS MOVING TO THE LEFT. (RIGHT), A RIGHT-LATERAL STRIKE-SLIP FAULT. NO MATTER WHICH SIDE OF THE FAULT YOU ARE ON, THE OTHER SIDE IS MOVING TO THE RIGHT.
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