Introduction to Earth Sciences I
4.1 Some Essentials
4.1.1 Earthquakes Occur On Faults
Three pages at SCEC provide some good definitions.
Earthquakes occur when the Earth breaks. The breakage is called a fault. They are the instantaneous failure of a place on the Earth that is under stress caused by the far field action of the major lithospheric plates that are in constant motion driven by the Earth's heat engine (see Topic 2). This is an important point that the popular press often gets wrong -- earthquakes result from the instantaneous localized release of stresses that are broadly distributed and essentially uniform. They do not result from an instantaneous increase in stresses. In this way they are analogous to the landslide on Bak's sand pile in that the avalanches occur as instantaneous isolated events driven by the steady drip of sand from the sand faucet.
Earthquakes usually occur on faults that were known to exist before the earthquake happened, and have had a prior history of earthquake activity. Many earthquakes occur on the San Andreas Fault, for instance. In a simple way this is a type of prediction. We don't really expect earthquakes to occur in places where there are no faults. Put another way, earthquakes usually occur in places where they have occurred before -- on "active" faults. This is where the sandpile analogy breaks down. To be accurate the sandpile would have to be pre-figured with the scars of previous landslides leading to a tendency for new landslides to occur at the locations of old landslides. The Earth has a memory of where it broke before.
Activity: From SCEDC Section #2 Activity #1 using Seismicity for 1932-1996 chart
Although in detail faults really come in all configurations and accommodate stress relief in many different ways, there is a basic classification of faults into three types -- normal faults, reverse faults and strike-slip faults. Your book has some reasonable diagrams that show the motions associated with each type of fault.
Normal faults relieve tensional stresses and result in two parts of the crust moving apart.
Reverse (or thrust) faults relieve compressional stresses and result in two parts of the Earth moving together. They look much like a normal fault on which the motion has been reversed. Hence the name. SCEDC Section 1, Page 14 and Page 15
Strike-slip faults relieve lateral stresses and result in two parts of the crust sliding along by one another. Strike-slip faults can be right lateral or left lateral.. The difference is easy to recognize -- imagine yourself standing on one side of the fault, the other side moves to the right, the fault is right lateral, left lateral if it moves left. The best known fault in the United States, the San Andreas Fault, is a strike-slip fault. SCEDC Section 1, Page 16
It is rare for one type of fault to exist alone. Strike-slip faults often have compressional stresses associated with the crust on either side of the fault, especially if it isn't perfectly straight (and they never are) so normal and reverse faults are commonly found as secondary features associated with the main strike-slip structure. SCEDC Section 1, Activity 13 and map showing sense of slip density