![]() Primary waves can travel through both fluids and solids. ![]() Particles of rock move forward and back during the passage of the P waves. They move through the rock via compression, very much like sound waves move through the air. Primary waves are the fastest seismic waves. Body waves include primary waves (P waves) and secondary waves (S waves). When seismic energy is released, the first waves to propagate out are body waves that pass through the planet’s interior. Seismic waves are an expression of the elastic energy released after an earthquake that travel either along the Earth’s surface ( surface waves) or throughout the Earth’s interior ( body waves). ( 1) Figure 5.3.1 Focus and Epicenter by the Utah Geologic Survey, Public Domain. The focus is the point along the fault plane from which the seismic waves spread outward. The epicenter is also the location that most news reports give because it is the center of the area where people are affected. The epicenter is the location on the Earth’s surface vertically above the point of rupture (focus). ![]() More shaking is usually the result of more seismic energy released. The larger the displacement and the further it propagates, the more significant the seismic waves and ground shaking. Notice that the location of the fault scarp may be a distance from the epicenter. The displacement produces shock waves, creating seismic waves. From the focus, the displacement propagates up, down, and laterally along the fault plane. The focus is always at some depth below the ground surface in the crust, and not at the surface, as shows in Figure 5.3.1. Retrieved 3 September 2010.The point of initial breaking or rupturing, where the displacement of rocks occurs along the rupture surface, is called the focus. Archived from the original on 24 January 2013. ↑ "Magnitude 7.0 - South Island of New Zealand: Summary".↑ "State of emergency declared in Canterbury".↑ "Strong earthquake rocks New Zealand's South Island"."Marlborough, Kaikoura escape worst of quake". ↑ Van Der Heide, Maike (4 September 2010). ![]() Archived from the original on 6 September 2010.
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