EXPLORATION: Earthquakes and Volcanoes
1. Earthquakes
Within the earth, about a million tremors are produced each year. Of
these, about 1000 are major earthquakes. Most of these do not make
headlines because they occur in unpopulated areas or under oceans.
Charles F. Richter devised a scale in 1935 that measures the amount of
energy released by a shock or earth movement. The shock is graded on a
scale of 1 to 10.
Earthquakes are the result of a sudden
release of stress across a fault zone . The stress takes many years to
build up and then is suddenly released. Strain builds up until the
blocks along the fault are moved a few centimetres or many meters. A
slip at one weak point may trigger slips all along the fault. The sudden
movement causes seismic waves that travel out from the centre of the
slip. Example: 1906 San Francisco earthquake caused by a build up of
pressure along the San Andreas Fault. The pressure had been building up
on the Pacific side for 100 years.
- Shallow Earthquakes : cause the greatest destruction since their source of energy is very close to the earth's surface. Most quakes are of this variety. Shock lasts a fraction of a second to 10 seconds. Depth of shallow quakes varies from about 1.6 to 96 km.
- Deep Focus Earthquakes : Most of these occur near the deep oceanic trenches and are usually 6 to 7 in magnitude. Depth between 96 km and 960 km.
Epicentre: spot on earth's surface directly above the focus.
They can cause vertical and lateral displacement of parts of the crust. In Sagami Bay (Japan) in 1923, parts of the bay were uplifted by 215 metres. This can then trigger a tsunami In Alaska in 1899 some coastal rocks were uplifted by 16 metres.
- Near coastlines, harbors and rivers, they can make the wet, sandy soil jiggle, turning it temporarily from a solid to a liquid state. Heavy sand and rock sinks, while water and lighter sand bubble to the surface. The slurry spreads, often toward the water, and the surface shifts. This is known as liquefaction.
They can cause landslides.
They can cause the devastation of cities . (i.e. collapse of buildings, fires, death, disruption of utilities, spread of disease).
Earthquakes 101 - National Geographic
Watch
this video, noting your response in your learning guide as you do. Pay
attention to how the video shows you how earthquakes occur. Listen to
what it says about our ability to anticipate when and where an
earthquake will strike.
Causes of Earthquakes
There are two main belts: Pacific Rim and Portugal to South Asia
- In the Pacific, earthquakes are related to the giant convection currents moving the ocean floor.
- The belt of earthquakes passing through the continental mountain ranges of the Alps, Andes, Himalayas could be the result of mountains growing and decaying - this is called orogenesis . Erosion wears down the mountains making them lighter. To reach a state of equilibrium other areas rise causing earthquakes.
- The movement of continents and ocean floors exerts pressure at the trenches, ridges, and throughout the faults in the earth's crust.
- The earth wobbling on its axis may cause earthquakes. The earth wobbles a few centimetres each day totaling approximately 20 to 23 metres each year. These wobbles tend to maximize about every seven years.
- Human interference in the balance of nature.
- eg water injection: pumping fluids at high pressures into oil- bearing rock formations underground - forcing more oil into production wells. Quakes as large as 4.3 on the Richter scale have been produced at the Eagle oilfield north of Fort St. John, BC, underground nuclear bomb testing
Magnitude
Magnitude is the total amount of energy released by an earthquake, and consequently, the amount of ground motion it causes. Magnitude is measured using the Richter Scale. The scale grows by a power of 10 for each 1.0 increase in magnitude. Hence, a 5.0 earthquake is 1000 times (10 x 10 x 10) greater than a 2.0 earthquake.
Intensity
The intensity of an earthquake is a value that reflects the effect produced. Intensity is measured using the Modified Mercalli scale.