Pict. 6.1. Effusive volcanism, Mauna Kea                         Pict. 6.2. Explosive volcanism

Earthquakes occur all over the world and often occur without significant warning. These geohazards can have far-reaching affects on humans and on the surface of the Earth. Small, localized earthquakes may cause no noticeable damage and may not even be felt by people living in the affected area. In contrast, a large earthquake may cause destruction over a wide area and be felt thousands of miles away. Earthquakes can also occur within plates, although plate-boundary earthquakes are much more common. Less than 10 percent of all earthquakes occur within plate interiors. As plates continue to move and plate boundaries change over geologic time, weakened boundary regions become part of the interiors of the plates. These zones of weakness within the continents can cause earthquakes in response to stresses that originate at the edges of the plate or in the deeper crust. Earthquakes are sudden movements in the earth’s crust that occur along faults (planes of weakness), where one rock mass slides past another one. When movement along faults occurs gradually and relatively smoothly, it is called creep or seismic slip and may be undetectable to the casual observer. When friction prevents rocks from slipping easily, stress builds up until it is finally released with a sudden jerk. The point on a fault at which the first movement occurs during an earthquake is called the epicentre (Fig. 6.3-6.5.).