Information about earthquakes

•	The Earth’s Structure
	The Earth’s interior is composed of three major layers, similar to an egg. The Earth’s shell is called the crust, which is made up of more than a dozen major tectonic plates. The whites are called the mantle, which is composed of solid rock. The yolk is called the core, and is composed of liquid iron and other materials.
•	Inter-plate earthquakes
	Although the mantle is solid rock, it slowly flows in a convection current (1) because of heat dissipating from the core. This causes the plates on the mantle to move several centimeters a year. When plates move, they collide with each other or grind past each other at plate boundaries, creating stress (strain energy). When sufficient strain energy is stored, the plates try to return to their original position, causing an earthquake. This is called an interplate earthquake. Interplate earthquakes can reach magnitudes of over 8, and cause tsunamis and other extensive damage over wide areas. The Pacific Plate is subducting under the Okhotsk Plate at the bottom of the sea east of Sendai. The Miyagi-oki Earthquake occurs when the Okhotsk Plate, which is dragged down by the Pacific Plate, rebounds to its original position.
	(1) Convection is a phenomenon that occurs when part of a fluid is warmed. The warmed fluid expands, becomes less dense, and rises upwards. Cold and denser parts of the fluid sink and fill the space previously occupied by the warmed fluid.
	Figure 1. Inter-plate Earthquakes
	1. Plate A subducts under Plate B, dragging Plate B with itself. 2. Plate B tries to return to its original position, causing an earthquake.
•	Intra-plate Earthquakes
	Strain energy caused by plate movement is also stored within the plates. When weak parts of the bedrock cannot resist the energy, they slip, causing intra-plate earthquakes. The discontinuities caused by intra-plate earthquakes are called faults. Faults that have caused earthquakes within the last 1.8 million years are called active faults. Active faults are like injuries that haven’t completely healed yet. They’re weaker than other parts of the ground and frequently cause earthquakes. Intra-plate earthquakes do not cause tsunamis, and their magnitudes rarely exceed 7. However, they occur at relatively shallow depths, within 20km from the ground. Therefore, intra-plate earthquakes can also cause large damage. There are 6 known active faults in the Sendai area, the Nagamachi-Rifu Fault Zone, the Dainenjiyama Fault, the Shishiochizaka Fault, the Tsubonuma Fault, the Ayashi Fault, and the Sakunami-Yashikidaira Fault.
•	Magnitudes and Seismic Intensity
	The magnitude of an earthquake is the measurement of an earthquake’s energy. When the magnitude of an earthquake increases by one step, its energy increases by a factor of 32. If the magnitude increases by two steps, the energy increase is a factor of 32 times 32 – approximately 1,000. Seismic intensity refers to the strength of shaking experienced in a particular location. The seismic intensity of an earthquake can vary according to location and geological conditions. The Japan Meteorological Agency Seismic Intensity Scale is used in Japan. The scale runs from 0 to 7 and has 10 different measurements because intensities of 5 and 6 are divided in to “weak” and “strong” intensities. Seismic intensity used to be measured by human perception and damage to buildings, but it is now quickly measured by mechanical seismic scales. Readmore:http://en.wikipedia.org/wiki/Earthquake