1. Figure 18.3: An earthquake is a shaking or vibration of the ground usually resulting when there is a sudden break or slip along a fault.

2. Figure 18.1: The elastic rebound theory serves as a theory for why earthquakes occur.

3. Figure 18.1: The distance of displacement between the two blocks is called the slip.

4. The focus of an earthquake is the point at which the slip is initiated.

5. The epicenter is the point on the Earth's surface directly above the focus.

6. Figure 18.3: When the two blocks on either side of a fault plane suddenly slip, the energy travels outward from the focus as intense vibrations called seismic waves. These waves cause the ground near the epicenter to shake violently.

1. Figure 18.5: The seismic waves generated by earthquakes are recorded using seismographs, instruments that measure the vibrations in the ground.

2. A typical observatory measures three components of ground motion: up-down, horizontal east-west, and horizontal north-south.

1. Figure 18.6: The waves that travel from the earthquake and through the Earth will arrive at the seimograph in three distinct groups.

2. P-waves are primary waves that travel through solid rock at ~5 km/sec. P-waves are also called compressional waves because they travel through solid, liquid or gaseous materials in a manner that pushes or pulls particles of matter in the direction of their path of travel.

3. Figure 18.7a: P-waves therefore appear as a succession of contractions and relaxations in the direction of wave travel.

4. S-waves are secondary waves that travel through solid rock at about half the speed of P-waves.

5. Figure 18.7b: S-waves are also called shear waves because they push material at right angles to their path of travel. Unlike P-waves, shear waves cannot travel through liquids or gases.

6. Figure 18.8: Surface waves are confined to the Earth's surface and outer layers and are similar to waves in the ocean. The speed of surface waves is slightly less than that of S waves. One type of surface wave produces a rolling motion of the ground while another type shakes the ground sideways. Surface waves do most of the destruction.

1. The further seismic waves travel, the wider becomes the interval between the arrival of the different waves.

2. Figure 18.9: Seismologists can translate the time interval between first arrival of a P- and S-wave to determine the distance from the epicenter. The relationship between the time interval and actual distance is established by recording seismic waves from earthquakes or underground nuclear explosions at known distances from that particular seismograph.

3. Knowing the distance from the epicenter of three different stations, seismologists can pinpoint the location of the epicenter using a map and some simple geometry.

4. In reality, the process described actually involves a large number of seismographic stations and the calculations are carried out with repeated iterations on computers until a large number of stations agree on where the epicenter is located.

1. The modified Mercalli Intensity Scale was devised in 1931 to assign a measure of the destructiveness of an earthquake.

2. The scale ranges from I (very weak, not felt by people) to XII (total damage).

3. Unfortunately, the destructiveness of an earthquake depends on many factors such as the strength of buildings, distance from the epicenter, nature of the soil and bedrock (buildings resting on unconsolidated soil experience more damage than buildings constructed on solid bedrock), and other local conditions.

1. Figure 18.10: In 1935, Charles Richter, a California seismologist, devised a procedure to measure the size of an earthquake on the basis of ground motion rather than amount of destruction. According to this method, the size of the ground movement caused by seismic waves is determined by measuring the maximum amplitude on the seismic curve.

2. Figure 18.11: The Richter magnitude scale is used to indicate the size of an earthquake. The numbers are based on a logarithmic scale so that a magnitude one difference in the Richter scale indicates a factor of 10 difference in the amplitude of ground motion.

3. Figure 18.11: The energy released by seismic waves increases even faster by a factor of 33 for each unit on the Richter scale.

4. Seismic waves weaken as they spread out from the focus so that stations successively further from the epicenter record weaker ground motions relative to stations closer to the earthquake. The amplitude measured at each seismographic station must therefore be adjusted to account for this weakening of ground motion with distance so that seismographs all over the world come up with nearly the same value for magnitude.

1. Although the Richter magnitude is the most popular way of measuring the strength of an earthquake, seismologists prefer a scale that measures the actual energy released at the earthquake source rather than measure how much the ground shakes at some distance from the epicenter.

2. The moment magnitude depends on:

(a) the amount of slip on the fault plane

(b) the area of the fault break

(c) rigidity or strength of the rock

3. The total energy of an earthquake can be related to its Richter magnitude using the following equation:

log E = A + BM

E = total energy in ergs

A and B are constants which depend on local geology

M = Richter magnitude

1. Figure 18.12: In addition to determining the earthquake epicenter and magnitude, seismologists can also determine the type of fault movement that produced the earthquake.

2. The first motion of P waves arriving at seismographic stations is used to determine the orientation of the fault plane and the directions of slip.

3. Figure 18.13: In some directions from an earthquake, the first seismic movement recorded is a push (upward motion of the seismogram trace) away from the focus. On seismograms located in other directions, the initial movement may be a pull (downward motion on seismogram trace) towards the focus. These differences indicate that the fault slip looks like a push when viewed from some stations, but appears like a pull when viewed from other stations.

4. By plotting the positions of various stations, pushes and pulls can be divided into different sections.

1. Figure 18.19: Tsunamis are often called tidal waves.

2. Tsunamis can result from vertical displacement of the ocean floor during an earthquake.

3. They can travel at speeds between 500 and 950 km/hr.

4. These waves often undetected in the open ocean, but can dramatically increase in height upon entering shallower coastal waters.

5. The ultimate surge is capable of extending hundreds of meters inland.

6. Tsunamis can be very destructive.

1. Animals may behave strangely just before earthquake.

2. Rapid tilting of the ground is sometimes observed prior to an earthquake.

3. Other types of ground deformation are sometimes detected in the area of future fault slippage.

4. Sometimes there are changes in the physical properties of rocks such as the way the rocks conduct electric current.

5. Sometimes there are changes in the P-wave velocity of smaller earthquakes prior to a major one.

6. Changes in the level of water in wells have sometimes been detected prior to an earthquake.

7. There is also evidence of an increase in the frequency of smaller earthquakes prior to the main shock.

1. Seismic gaps along the fault.

2. Paleoseismology

3. Maps of earthquake probability

4. Figure 18.21: Seismic hazard maps.