19.2 Seismic Waves and Earth’s Interior
Seismometer is a machine that detects seismic waves from an earthquake. A seismogram is the actual record of the waves.
Distance from the epicenter: P-waves always arrive first, then S-waves, and surface waves are last.
Seismic waves change speed and direction at the boundaries between different materials. By recording the travel time curves and path of each wave, seismologists can learn about the differences in density and composition within the Earth. S-waves do not travel through liquid, and since they don’t’ travel through
Earths center, seismologists determined that part of the Earths’ core must be at least partially liquid.
Scientists have been able to determine what the inside of the Earth is made of hand the thickness of each layer by how fast the seismic waves travel through the Earth. The speed of seismic waves decreases as temperatures increase. Therefore, waves travel more slowly in hotter areas and faster in cooler areas.
19.3 Measuring and Locating Earthquakes
More than 1 million earthquakes are felt each year, but only the large ones are talked about in the news.
Richter scale-this is a numerical rating system that measures the energy of the largest seismic waves, called the magnitude, that are produced during an earthquake. The numbers in the Richter scale are determined by the height or amplitude of the largest seismic wave. Each successive number represents an increase in the amplitude of a factor of 10. For example, the seismic waves with a magnitude of 8 are
10 times larger than those of a magnitude of 7. The difference in the amount of energy released by earthquakes is even greater than the differences between the amplitudes of the waves. Each increase in magnitude corresponds to about a 32-fold increase in seismic energy. Thus, an earthquake with a magnitude of 8 releases 32 times more energy than a magnitude of 7.
Modified Mercalli scale-measures the amount of damage done during an earthquake. Roman numerals I-
XII are used and the higher the number, the more severe the damage.

Earthquake intensity: The intensity of an earthquake depends on the amplitude of the surface waves generated. Waves decrease in size with increasing distance from the focus of the earthquake. Because of this, intensity also decreases.
Depth of focus: How deep the focus of an earthquake is also determines its intensity. Earthquakes can be classified as shallow, intermediate or deep. Catastrophic earthquakes with high intensity values are almost always shallow-focus events. Deep focus earthquakes generally produce smaller vibrations at the epicenter than those produced by shallow-focus earthquakes. Example: a shallow focus earthquake with magnitude of 6 will have a greater intensity than a deep focus magnitude 8 earthquake.
Seismic Belts: 80% of earthquakes along the Circum-Pacific Belt and about 15% on the Mediterranean
Belt. These belts are subduction zones. Most of the rest occur along narrow the crests of ocean ridges, where tectonic plates are diverging.
Map showing earthquakes from 2003-2011 with magnitude greater than 3. Colors indicate depth of hypocenter, or origin of the earthquake: red is 0-33 km, yellow is 33-100 km, green is 100-400 km, and blue is >400 km depth. Data are from the Advanced National Seismic System.