- EARTHQUAKES:
Earthquakes are shock waves caused by movements along a fault, or, much less often, by the action of folds.
Focus and Epicenter: The location where the earthquake occurs is called its focus. The surface location directly above the focus is called the epicenter.
Magnitude: The energy or magnitude of earthquakes is measured on the Richter scale, a logarithmic scale in which magnitude 4 is ten times
as powerful as magnitude 3, magnitude 5 is ten times as powerful as magnitude 4 (and 100 times as powerful as magnitude 3), and so on.
Nearly every earthquake is followed by several smaller aftershocks of smaller magnitude.
- Predicting earthquakes:
Along known fault lines, geologists have placed many strain gauges to measure tiny movements on either side of the fault.
When the strain builds up sufficiently, an earthquake occurs and relieves the strain.
Therefore, a large buildup of strain increases the probability that new movement (slippage) along the fault will cause an earthquake.
The size of the earthquake is related to the amount of built-up strain.
- Where earthquakes occur:
Major earthquakes are most frequent along tectonic plate boundaries, especially on the margins of the Pacific Plate, known as the "Ring of Fire".
The Ring of Fire extends along the Andes in South America, the highlands of Nicaragua, the San Andreas Fault of California,
coastal Alaska, Japan,
the Philippines, Papua New Guinea, and the island of New Caledonia.
Another line of frequent major earthquakes extends from Indonesia through northern India, Pakistan, northern Iran,
Turkey, northern Greece, northern Italy, and northern Spain.
The northeast United States, including New England, experiences thousands of tiny and generally unnoticed earthquakes every year.
Scientists think that these tiny earthquakes continually relieve the stress
from building up and causing a major earthquake.
- Earthquake signals:
S-waves (Surface waves or Shear waves) are transverse waves, comparable to the ripples formed on the surface of a pond when a rock falls through the surface.
S-waves spread through the crust only and do not go very deep.
P-waves (Pressure waves) are compression waves, similar to sound waves. They travel faster than S-waves and arrive sooner at locations away from the epicenter.
P-waves travel faster through more dense material, and the waves are bent, or refracted, when they pass from material of one density
into material of a higher or lower density.
By studying the rate at which P-waves arrive at distant locations, scientists can detect changes in density of materials far below the Earth's surface.
This is, in fact, the major evidence for the existence of a core and a mantle, and of regions within each, such as an inner core and outer core.
CLICK HERE for more about S- and P-waves
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