Earthquakes are normally assumed to originate from a single point known as the focus or hypocenter (Fig. 2.1), which is invariably within about 700 km of the surface. In reality, however, most earthquakes are generated by movement along a fault plane, so the focal region may extend for several kilometers. The point on the Earth's surface vertically above the focus is the epicenter. The angle subtended at the center of the Earth by the epicenter and the point at which the seismic waves are detected is known as the epicentral angle A. The magnitude of an earthquake is a measure of its energy release on a logarithmic scale; a change in magnitude of one on the Richter scale implies a 30-fold increase in energy release (Stein & Wysession, 2003).
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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.