Earthquakes

Earthquakes are caused by the release of energy within the Earth’s crust by movement along faults. An earthquake mainshock can be preceded by smaller quakes called foreshocks and followed by other quakes called aftershocks.

Focus: The site within the Earth where movement along a fault releases energy to produce an earthquake. The foci of earthquakes are generally between 5 and 700 kilometers deep.

Epicenter: The geographic location on the surface of the Earth directly above an earthquake’s focus.

Earthquake waves: The energy released by an earthquake is propagated through the Earth in the form of waves. Body waves are the waves that pass though the Earth’s interior. Surface waves travel along the Earth’s surface.

Body waves

P-waves: P-waves or primary waves are pressure waves, resulting in an alternating expansion and contraction of the material they pass through. P- waves can be transmitted through solids, liquids, or gases.

S-waves: S-waves or secondary waves are shear waves, resulting in a side-to-side motion in the material they pass through. S-waves can transmitted through solids only

Surface waves
Rayleigh waves: Rayleigh waves move along the surface producing vertical motions. Love waves: Love waves: move along the surface producing horizontal motions.

Locating earthquakes: P waves travel through the Earth about 1.7 times faster than S-waves. The lag time between the arrival of the first P-wave and the first S-wave from an earthquake can be used to estimate how far away the quake’s focus is from the seismic station receiving the waves. The greater the time lag, the greater the distance. Given the distances from three separate stations, the exact location of the quakes focus can be determined.

The Size of Earthquakes
Magnitude: a measure of the amount of energy released by an earthquake at its focus.

Measures of earthquake magnitude include the Richter scale and the moment magnitude scale. The Richer scale uses the distance to the quake and the amplitude of the largest S-wave on a seismogram to calculate magnitude, but it tends to underestimate the magnitude of extremely large quakes. The moment magnitude scale uses a more complex method, which involves estimating the length of the fault and its amount of slip (plus other factors), and does a better job of characterizing large quakes. Both the Richter and moment magnitude scales are logarithmic scales, which means that at each step of the scale (from <1 to >9) the amplitude of the earthquake waves increases by a factor of 10. The energy released by a quake increases by a factor of 32 with each unit of the scale.

Intensity: a measure of the amount of shaking and damage caused by an earthquake at a given locality. Measures of intensity include the Modified Mercalli Scale, which is based on the observed amount of shaking and destruction caused by a quake. The scale ranges from I (low intensity) to XII (high intensity) and intensity is generally highest near the quake’s epicenter and decreases at distances away from it.

Destructive effects of Earthquakes

Ground shaking is the immediate cause of much earthquake damage. The amplitude of earthquake waves can be amplified by certain substrates, such as soft soils, which results in more intense shaking than on firmer bedrock. The frequency of earthquake waves can also be a factor in the destruction they produce. Tall buildings may resonate with low frequency waves and become damaged, while other buildings remain undamaged. High frequency waves might damage low buildings, but leave taller buildings unaffected.

Liquifaction: vibration during an earthquake can turn soil or sediment layers into a fluid unable to support buildings or roads.

Landslides and land subsidence triggered by earthquakes can also be very destructive.

Tsunami: a destructive ocean wave caused by an earthquake, either due to a submarine landslide or uplift of the sea floor. In the open ocean tsunamis travel extremely rapidly but have very low amplitudes (a meter or less). When they reach shallow water near coastlines they become steeper and higher (up to 30 meters).