9A8-EARTHQUAKES

EARTHQUAKES

Perhaps the Earthquake is the most fearful natural phenomenon in the human life. It is more so, because it is unpredictable and arrives without notice or without announcing it's vigor and strength. Scientists are constantly in search of this unknown. The major events of the earthquakes from the human viewpoint are realized on review of some major catastrophes. Minute observations and records at least have been able to pin point the focus or the epicenter of these earthquakes in the historical past. These studies could reveal two great seismic belts in the entire globe.
So, what is an earthquake?

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 of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, 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.
•Earthquakes are a result of motion within the earth.
•This only occurs where the earth is solid and therefore can only occur within about 100 miles of the surface.
•Earthquakes provide the best evidence regarding the interior structure of the Earth.

Causes of earthquakesEarth's crust is composed of many huge, rocky plates known as tectonic plates. These plates constantly move slowly across the surface of Earth, bumping into each other, overrunning each other, and pulling away from each other. When the strain produced by these movements increases beyond a certain level, the pent-up energy ruptures the crust and creates a fracture known as a fault. The released pressure also causes the ground-shaking vibrations associated with an earthquake.

So, my dear students, tell me, what is an earthquake? You have seen the video clippings. Do you know what is it?
Earthquakes are the shaking, rolling or sudden shock of the earth’s surface. They are the Earth's natural means of releasing stress. More than a million earthquakes rattle the world each year. The West Coast is most at risk of having an earthquake, but earthquakes can happen in the Midwest and along the East Coast. Earthquakes can be felt over large areas although they usually last less than one minute. Earthquakes cannot be predicted - although scientists are working on it!
LET US RECALL THE POINTS:
There are about 20 plates along the surface of the earth that move continuously and slowly past each other. When the plates squeeze or stretch, huge rocks form at their edges and the rocks shift with great force, causing an earthquake. Think of it this way: Imagine holding a pencil horizontally. If you were to apply a force to both ends of the pencil by pushing down on them, you would see the pencil bend. After enough force was applied, the pencil would break in the middle, releasing the stress you have put on it. The Earth's crust acts in the same way. As the plates move they put forces on themselves and each other. When the force is large enough, the crust is forced to break. When the break occurs, the stress is released as energy which moves through the Earth in the form of waves, which we feel and call an earthquake.
Ok, let's go through the lesson again!

•Plates moving in any direction causes earthquakes
•The worst are caused by plates rubbing past each other.
•This happens along the Californian coast

SEA FLOOR SPREADING

•Large continents begin to crack and split apart

•The gaps fill with water

•Small seas become oceans

•The mid ocean ridge continues to produce new crust

WHY SPREAD?

Why is the Atlantic still getting wider?

•The plates are pulled apart by convection currents in the mantle below .

•Caused by heat released from natural radioactive processes

•At the mid Atlantic ridge molten rock from below rises up to fill the gap with new basaltic rock

MORE EVIDENCE

•More evidence has been found for Wegeners theory



•The Earths North and South pole have flipped many times


•These leaves magnetic ‘stripes’ in rock containing iron minerals

SO THE EARTH MUST BE GROWING?

Dense heavy oceanic crust can be subducted below less denser continental crust.

•The friction melts rock .

•This magma rises through the crust to form new volcanoes

•This is happening in South America (The Andes)

CONTINENTS COLLIDE

•Eventually when plates move together the continental crust collides




•The heat and pressure make metamorphic rock




•It also pushes and folds the rocks into high mountains




•The Himalayas rise to 8848m and are still growing today





What is an earthquake and what causes them to happen?
Ans: An earthquake is caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the earth's crust and cause the shaking that we feel during an earthquake. An EQ occurs when plates grind and scrape against each other. In California there are two plates the Pacific Plate and the North American Plate. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coast line. The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. These primary boundary between these two plates is the San Andreas Fault. The San Andreas Fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas Fault Zone. The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year. Parts of the San Andreas Fault system adapt to this movement by constant "creep" resulting in many tiny shocks and a few moderate earth tremors. In other areas where creep is NOT constant, strain can build up for hundreds of years, producing great EQs when it finally releases.

Q: Can we cause earthquakes? Is there any way to prevent earthquakes?
Ans: Earthquakes induced by human activity have been documented in a few locations in the United States, Japan, and Canada. The cause was injection of fluids into deep wells for waste disposal and secondary recovery of oil, and the use of reservoirs for water supplies. Most of these earthquakes were minor. The largest and most widely known resulted from fluid injection at the Rocky Mountain Arsenal near Denver, Colorado. In 1967, an earthquake of magnitude 5.5 followed a series of smaller earthquakes. Injection had been discontinued at the site in the previous year once the link between the fluid injection and the earlier series of earthquakes was established.

Other human activities, even nuclear detonations, have not been linked to earthquake activity. Energy from nuclear blasts dissipates quickly along the Earth's surface. Earthquakes are part of a global tectonic process that generally occurs well beyond the influence or control of humans. The focus (point of origin) of earthquakes is typically tens to hundreds of miles underground. The scale and force necessary to produce earthquakes are well beyond our daily lives. We cannot prevent earthquakes; however, we can significantly mitigate their effects by identifying hazards, building safer structures, and providing education on earthquake safety.

Q: What are plate tectonics?
Ans: Plate tectonics is the continual slow movement of the tectonic plates, the outermost part of the earth. This motion is what causes earthquakes and volcanoes and has created most of the spectacular scenery around the world.

Q: What is a fault and what are the different types?
Ans: A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.
Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse, depending on their motion. Faults that move horizontally are known as strike-slip faults and are classified as either right-lateral or left-lateral. Faults, which show both dip-slip and strike-slip motion are known as oblique-slip faults.
The following definitions are adapted from The Earth by Press and Siever.
Normal fault- a dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.
Thrust fault- a dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block. This type of faulting is common in areas of compression, such as regions where one plate is being sub ducted under another as in Japan. When the dip angle is shallow, a reverse fault is often described as a thrust fault.
Strike-slip fault - a fault on which the two blocks slide past one another. The San Andreas Fault is an example of a right lateral fault.
A left-lateral strike-slip fault is one on which the displacement of the far block is to the left when viewed from either side.
A right-lateral strike-slip fault is one on which the displacement of the far block is to the right when viewed from either side.

Q: At what depth do earthquakes occur?
Ans: Earthquakes occur in the crust or upper mantle, which ranges from the earth's surface to about 800 kilometers deep (about 500 miles).
Q: What is "surface rupture" in an earthquake?
Ans: Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL earthquakes result in surface rupture.

Q: What is the relationship between faults and earthquakes? What happens to a fault when an earthquake occurs?
Ans: Earthquakes occur on faults - strike-slip earthquakes occur on strike-slip faults, normal earthquakes occur on normal faults, and thrust earthquakes occur on thrust or reverse faults. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth. The slip direction can also be at any angle.

Q: How do we know a fault exists?
Ans: if the earthquake left surface evidence, such as surface ruptures or fault scarps (cliffs made by EQs).
if a large EQ has broken the fault since we began instrumental recordings in 1932.
if the faults produces small earthquakes that we can record with the denser seismographic network established in the 1970s.


Q: Where can I go to see the/a fault?
Ans: The closest fault depends on where you live. Some earthquakes produce spectacular fault scarps, and others are completely buried beneath the surface. Sometimes you may not even know that you are looking at a fault scarp.

Q: What does an earthquake feel like?
Ans: Generally, during an earthquake you first will feel a swaying or small jerking motion, then a slight pause, followed by a more intense rolling or jerking motion. The duration of the shaking you feel depends on the earthquake's magnitude, your distance from the epicenter, and the geology of the ground under your feet. Shaking at a site with soft sediments, for example, can last 3 times as long as shaking at a stable bedrock site such as one composed of granite. If the site is in a building, then the height of the building and type of material it is constructed from are also factors. For minor earthquakes, ground shaking usually lasts only a few seconds. Strong shaking from a major earthquake usually lasts less than one minute.

Q: Foreshocks, aftershocks - what is the difference?
Ans: "Foreshock" and "aftershock" are relative terms. Foreshocks are earthquakes, which precede larger earthquakes in the same location. Aftershocks are smaller earthquakes, which occur in the same general area during the days to years following a larger event or "mainshock", defined as within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the main shock. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30km) are much less likely to be followed by aftershocks than shallow earthquakes.

Q: Two earthquakes occurred on the same day. Are they related?
Ans: Often, people wonder if an earthquake in Alaska may have triggered an earthquake in California; or if an earthquake in Chile is related to an earthquake that occurred a week later in Mexico. Over these distances, the answer is no. Even the Earth's rocky crust is not rigid enough to transfer stress fields efficiently over thousands of miles.

Words to Know
Epicenter: The location where the seismic waves of an earthquake first appear on the surface, usually almost directly above the focus.
Fault: A crack running through rock that is the result of tectonic forces.
Focus: The underground location of the seismic event that causes an earthquake.
Modified Mercalli scale: A scale used to compare earthquakes based on the effects they cause.
Richter scale: A scale used to compare earthquakes based on the energy released by the earthquake.
Seismic waves: Classified as body waves or surface waves, vibrations in rock and soil that transfer the force of the earthquake from the focus into the surrounding area.
Seismic waves traveling along Earth's surface are called surface waves or L waves (long). The two main types, Rayleigh waves and Love waves, are named after two prominent seismologists (scientists who study earthquakes). Although surface waves move slower than body waves—less than 2 miles (3.2 kilometers) per second—they cause greater damage. Rayleigh waves cause the ground surface in their path to ripple with little waves. Love waves move in a zigzag along the ground. Both Rayleigh and Love waves set off avalanches, landslides, and other earthquake damage.