10. Mechanism of an Earthquake
Tectonic earthquakes occur anywhere in 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 move past each other smoothly and aseismically only if there are no irregularities 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 irregularity, 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 . Earthquakes in volcanic regions are caused there, both by tectonic faults and the movement of magma in volcanoes.
9. What We Feel Is Just 10% of What’s Going Inside
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.
8. 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 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 503 meters 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 is also claimed to be induced by humanity, through coal mining. The city of Newcastle was built over a large sector of coal mining areas. The earthquake has been reported to be spawned from a fault that reactivated due to the millions of tonnes of rock removed in the mining process.
7. Fault Lines
In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement. Large faults within the Earth’s crust result from the action of tectonic forces. Energy release associated with rapid movement on active faults is the cause of most earthquakes. A fault line is the surface trace of a fault, the line of intersection between the fault plane and the Earth’s surface. Geologists can categorize faults into three groups based on the sense of slip: (the picture above explains more than enough)
- a fault where the relative movement (or slip) on the fault plane is approximately vertical is known as a dip-slip fault;
- where the slip is approximately horizontal, the fault is known as a transcurrent or strike-slip fault;
- an oblique-slip fault has non-zero components of both strike and dip slip.
6. Common Wrong Perceptions about Quakes
There is a common myth (particularly in New Zealand where earthquakes are common) that if you have a lot of small earthquakes, it helps to alleviate the pressures building up that can cause a big one. But this is not true. Seismologists have observed that for every magnitude 6 earthquake there are 10 of magnitude 5, 100 of magnitude 4, 1,000 of magnitude 3, and so forth as the events get smaller and smaller. This sounds like a lot of small earthquakes, but there are never enough small ones to eliminate the occasional large event. It would take 32 magnitude 5′s, 1000 magnitude 4′s, 32,000 magnitude 3′s to equal the energy of one magnitude 6 event. So, even though we always record many more small events than large ones, there are never enough to eliminate the need for the occasional large earthquake.
There is also a perception that “lubricating” faults with water or some other substance will reduce the quakes or the intensity, well injecting high pressure fluids deep into the ground is known to be able to trigger earthquakes to occur sooner than would have been the case without the injection. However this would be a dangerous pursuit in any populated area, as one might trigger a damaging earthquake. And by the way, there is no such thing as earthquake weather. They seem to occur the same number of times in all different types of weather. It is impossible for the weather to affect the forces beneath the earth’s surface.
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. Most aftershocks are located over the full area of fault rupture and either occur along the fault plane itself or along other faults within the volume affected by the strain associated with the main shock. Typically, aftershocks are found up to a distance equal to the rupture length away from the fault plane. The pattern of aftershocks helps confirm the size of area that slipped during the main shock. Aftershocks are dangerous because they are usually unpredictable, can be of a large magnitude, and can collapse buildings that are damaged from the main shock. Bigger earthquakes have more and larger aftershocks and the sequences can last for years or even longer especially when a large event occurs in a seismically quiet area.
4. Earthquake Swarms
Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are differentiated from earthquakes succeeded by a series of aftershocks by the observation that no single earthquake in the sequence is obviously the main shock. Earthquake swarms are one of the events typically preceding eruptions of volcanoes. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park.
3. Earthquake Storms
An earthquake storm is a recently proposed theory about earthquakes, where one triggers a series of other large earthquakes—along the same plate boundary—as the stress transfers along the fault system. This is similar to the idea of aftershocks, with the exception that they take place years apart. These series of earthquakes can devastate entire countries or geographical regions. Possible events may have occurred during the end of the Bronze Age, and the latter part of the Roman Empire. It has been suggested that this is what may be occurring in modern day Turkey.
2. Earthquake in 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 dripped on Loki’s face, forcing him to jerk his head away and thrash against his bonds, which caused the earth to tremble. In Greek mythology, Poseidon was the cause and god of earthquakes. When he was in a bad mood, he struck the ground with a trident, causing earthquakes and other calamities. He also used earthquakes to punish and inflict fear upon people as revenge. In Japanese mythology , Namazu 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.
Thales of Miletus, who lived from 625-547 (BCE) was the only documented person who believed that earthquakes were caused by tension between the earth and water.
1. Animals Can Predict Quakes!
In April 2009, British researchers were studying the common toad at a breeding site in central Italy when they “observed a mass exodus of toads”, Jill Lawless reports for the Associated Press. Just five days later, a 6.3-magnitude earthquake hit, killing some 150 people and causing extensive damage to the town of L’Aquila. Rachel Grant, a researcher at Open University and lead author of one of the first studies to document animal behavior surrounding earthquakes, believes “that toads are able to detect pre-seismic cues such as the release of gases and charged particles, and use these as a form of earthquake early warning system.” According to the study, “Predicting the unpredictable; evidence of pre-seismic anticipatory behaviour in the common toad,” the toad population at the breeding site dropped to zero three days prior to the quake. “A day after the earthquake, they all started coming back,” Grant told the AP. “The numbers were still lower than normal and remained low until after the last aftershock.” The belief that animals can predict earthquakes has been around for centuries,” Maryann Mott wrote for National Geographic News in 2003. In 373 B.C., historians wrote that rats, weasels and snakes made a mass exodus from the Greek city of Helice days before an earthquake destroyed the city. Other examples exist from throughout the centuries. Reports include bees leaving their hive, catfish moving violently and chickens refusing to lay eggs. Pet owners also have examples of their cats and dogs behaving strangely before a quake. To date, seismologists can’t predict when or where the next earthquake will hit, and scientists don’t know what, if anything, animals sense before a quake. Some, like Grant, think they can detect changes in the Earth’s gases. Others wonder if animals’ more sensitive hearing and other senses allow them to feel vibrations that humans can’t, or detect electrical changes. Whatsoever the exact mechanism but the fact is that they feel it and flee.