Earthquakes and landslides are frightening and destructive natural disasters. An earthquake is the sudden, rapid shaking of the earth caused by the breaking and shifting of rock deep underground. If an earthquake occurs in a populated area, it has the potential to cause many deaths and injuries along with extensive property damage. Although earthquakes are sometimes believed to be a West Coast phenomenon, there are actually 45 states and territories throughout the United States (including Pennsylvania) that are at moderate to high risk.

Landslides and debris flows occur in hill and mountain part. In a landslide, masses of rock, earth or debris move down a slope. They can be activated by storms, earthquakes, volcanic eruptions, fires and human modification of land. Landslides and debris flows can move rapidly, striking with little or no warning at avalanche speeds. They also can travel several miles from their source, growing in size as they pick up trees, boulders, cars, and other materials.

Some, but not all, earthquakes are related to volcanoes. For example, most earthquakes are along the edges of tectonic plates. This is where most volcanoes are too. However, most earthquakes are caused by the interaction of the plates not the movement of magma.

Most earthquakes directly beneath a volcano are caused by the movement of magma. The magma exerts pressure on the rocks until it cracks the rock. Then the magma squirts into the crack and starts building pressure again. Every time the rock cracks it makes a small earthquake. These earthquakes are usually too weak to be felt but can be detected and recorded by sensitive instruments. Once the plumbing system of the volcano is open and magma is flowing through it, constant earthquake waves, called harmonic tremor, are recorded (but not felt).

The distribution of earthquakes provides information about magma pathways and the structure of volcanoes. The red dots show earthquakes associated with magma movement. They define the east and southwest rifts of Kilauea. The blue dots show earthquakes associated the sliding of the south flank of Kilauea.

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