Lecture 16 Earthquakes

• What are earthquakes?

• Elastic rebound theory

• Waves generated by earthquakes:

P waves, S waves, Surface waves

• Locating earthquakes

• Earthquake magnitude

• Earthquake distributions

Denali Earthquake 2002/11/03, 5km, M7.9

• Northridge earthquake, Jan 1994. The parking lot at Cal State Univ. Northridge campus collapsed during the earthquake.

What are earthquakes?

Earthquakes are vibrations of earth, produced by the rupture and sudden movement of rocks, which are caused by stresses beyond the elastic limits of the rocks.

• The point inside the Earth where the rupture generates earthquake energy is called focus or hypocenter. The point at the Earth's surface directly above the focus is the epicenter.

Elastic rebound theory

• Reid in early 20th century suggested that earthquakes are caused by "spring back" of deformed rocks (termed elastic rebound).

• Example: The displacement in the 1906 San Francisco earthquake was as much as 15 feet.

• Elastic rebound. As rock is deformed, it bends, storing elastic energy. Once strained beyond its breaking point, the rock cracks, releasing the stored-up energy, which generates earthquake waves.

• This fence was offset 2.5 m in the 1906 San Francisco earthquake (G.K. Gilbert)

• Fault scarp produced by vertical displacement in the 1964 Alaska earthquake. (USGS)

Waves generated by earthquakes

Three types of waves are generated by an earthquake:

P waves (primary waves),S waves (secondary waves)Surface waves (Rayleigh waves, and Love

waves)

• P waves are compressional waves, which push (compress) and pull (expand) rock particles in the direction the waves are traveling.

• S waves are shear waves. The particles in S waves moves at right angles to the direction the waves are traveling.

Surface waves

• The motions of surface waves are more complex. There are two types of surface waves that propagate along Earth's surface: Rayleigh waves and Love waves.

• Surface waves are generally strongest waves. Their amplitudes decay strongly with depth (i.e. they are generally confined near the surface).

• The particle motions of Love waves (top) are perpendicular to the propagation direction and parallel to the surface. The particle motions of Rayleigh waves (bottom) are along the propagatio

n direction and the vertical plane.

Wave speeds

• P waves are faster than S waves.

• S waves are faster than surface waves.

• Thus, in a typical seismogram, P wave arrives first, then S wave, and then surface wave.

• Typical seismogram, showing P wave, S wave, and surface wave.

• Seismograph. The inertia of the suspended mass tends to keep it motionless, while the recording instrument vibrates with the earth.

Locating earthquakes• Locating an earthquake requires four basic parameters: origin ti

me, and latitude, longitude, and depth of the hypocenter.

• A basic method for locating an earthquake is using travel times of P waves and S waves recorded at seismic stations.

• For example, epicenter can be quickly estimated from the time interval between the P wave and the S wave recorded at 3 or more stations:

(1) Using one station, the distance between the epicenter and the station can be estimated from the S-P time at the station.

(2) The epicenter can then located using distance estimates from 3 or more stations.

• The distance to the epicenter can be determined using S-P time. Here, the S-P time is 5 minutes, the epicenter is about 3800 km (2350 miles) away.

• Earthquake epicenter can be located using the distances obtained from travel times of three or more seismic stations.

Earthquake magnitude

• Earthquake magnitude measures the amount of energy released by an earthquake.

• The best-known magnitude scale is Richter scale, developed by Charles Ricther of Caltech.

• Richter magnitude is determined by (1) the largest amplitude of the seismogram recorded at the Wood-Anderson instrument and (2) the distance from the focus.

• Richter scale can be determined graphically using a seismogram recorded at a Wood-Anderson instrument.

• Richter scale is logarithmic. Each unit increase corresponds to tenfold increase in amplitude, and 32-fold increase in energy.

• Thus, the largest amplitude of a magnitude 6 earthquake is 10 times as great as that of magnitude 5. The energy released by a magnitude 6 earthquake is 32 times that of a magnitude 5 earthquake.

Earthquake distributions

• A great majority of earthquakes originates in a few narrow zone of the globe along plate boundaries.

Earthquake depths

• Earthquakes occur at depths from near the surface to nearly 700 km, although a great majority of earthquakes are shallow.

Shallow earthquakes: 0-70 km intermediate earthquakes: 70 - 300 km deep earthquakes: 300-700 km

• Wadati-Benioff zone at Tonga subduction zone. Almost all deep earthquakes are associated with subduction zones. The focal depths increase with the distances from the trench. These subduction seismic zones are called Wadati-Benioff zones.