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Earthquakes and EQ Prediction

1.1 Fundamentals of Earthquakes

There have been published many books on seismology and earthquakes (EQs), including Richter (1958), Scholtz (1990), Shearer (1999), Uzu (2001), and Rikitake (2001a), so we advise interested readers to consult these books for further details. The information on EQs in Wikipedia (on EQs) was also helpful and useful in writing this chapter.

An EQ is the result of a sudden release of energy in the Earth’s crust that generates seismic waves. At the Earth’s surface, EQs manifest themselves by shaking and sometimes displacement of the ground as in the case of the 1995 Kobe EQ. When the epicenter of a large EQ is located offshore, the seabed may be displaced sufficiently to cause a tsunami. A typical example of this EQ type is the recent 2011 Tohoku EQ. EQs can also trigger landslides and occasionally volcanic activity.

In its most general sense, the word EQ is used to describe any seismic event that gives rise to seismic waves. The point of initial rupture of an EQ is called its focus or hypocenter, and epicenter is defined as the point at ground level directly above the hypocenter. Figure 1.1a and b illustrates the worldwide spatial distributions of EQs with magnitude larger than 5.0 during a period from January 1964 till the end of November 2010. Figure 1.1a refers to shallow EQs (with depth <30 km), while Figure 1.1b refers to deep EQs (with depth >150 km). It is found from these figures that EQs tend to take place in the following major regions: (i) Pacific region; (ii) Southeast Asia, Middle Asia, the Middle East, and South Europe; and (iii) Mid-rim Atlantic ridge and Indian Ocean ridge.

Figure 1.1 Global distribution of EQs (a) shallow (depth ≤30 km) and (b) deep (depth ≥150 km) during the period of 1964–2010.

Reproduced with permission from https://wwweic.eri.u-tokyo.ac.jp/db/isc/index.html, International Seismological Centre

1.1.1 Naturally Occurring EQs

Tectonic EQs can occur anywhere on the Earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. The sides of a fault move past each other smoothly and seismically only if there are no irregularities or asperities along the fault surface that increase frictional resistance. Most fault surfaces do have such asperities, and this leads to a form of stick–slip behavior. Once the fault has locked, continued relative motion between the plates leads to increasing stress and therefore stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault and releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an EQ. This process of gradual buildup of strain and stress punctuated by occasional sudden EQ failure is referred to as the elastic-rebound theory. It is estimated that only 10% or less of the total energy of an EQ is radiated as seismic energy. Most of the EQ energy is used to power the EQ fracture growth or is converted into heat generated by friction. Therefore, EQs lower the Earth’s available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat from the Earth’s deep interior.

1.1.2 EQ Fault Types

There are three main types of fault, all of which may cause an EQ: (i) normal, (ii) reverse (thrust), and (iii) strike slip as shown in Figure 1.2. Normal and reverse faulting are examples of dip slip, where the displacement along the fault is in the direction of dip and movement on them accompanies a vertical component. Normal faults occur mainly in areas where the crust is being extended, such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened, such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other: Transform boundaries are a particular type of strike-slip fault. Many EQs are caused by movement on faults that have components of both dip slip and strike slip; this is known as oblique slip.

Figure 1.2 Three different kinds of fault types (a) strike-slip fault, (b) normal fault, and (c) reverse (thrust) fault

Reverse faults, particularly those along convergent plate boundaries, are associated with the most powerful EQs, including almost all of those with magnitude 8 or more. Strike-slip faults, particularly continental transforms, can produce major EQs up to about magnitude 8. EQs associated with normal faults are generally less than magnitude 7.

This is so because the energy released in an EQ, and thus its magnitude, is proportional to the area of the fault that ruptures and the stress drop. Therefore, the longer the length and the wider the width of the fault area, the larger the resulting magnitude. The topmost, brittle part of the Earth’s crust and the cool slabs of the tectonic plates that are descending into the hot mantle are the only parts of our planet which can store elastic energy and release it in fault ruptures. Rocks hotter than about 300°C flow in response to stress; they do not rupture in EQs. The maximum observed lengths of ruptures and mapped faults are approximately 1000 km. Examples are the EQs in Alaska, 1957; Chile, 1960; Sumatra, 2004; and Japan, 2011, all in subduction zones. The longest EQ ruptures on strike-slip faults, like the San Andreas Fault (1857, 1906), the North Anatolian Fault in Turkey (1939), and the Denali Fault in Alaska (2002), are about half to one-third as long as the lengths along subducting plate margins, and those along normal faults are even shorter.

The most important parameter controlling the maximum EQ magnitude on a fault is, however, not the maximum available length, but the available width, since the latter varies by a factor of 20. Along converging plate margins, the dip angle of the rupture plane is very shallow, typically about 10°. Thus, the width of the plane within the top brittle crust of the Earth can be as much as 50–100 km (Japan, 2011; Alaska, 1964), making the most powerful EQs possible.

Strike-slip faults tend to be oriented vertically, resulting in an approximate width of 10 km within the brittle crust, so EQs with magnitudes much larger than 8 are not possible. Maximum magnitudes along many normal faults are even more limited because many of them are located along spreading centers, where the thickness of the brittle layer is only about 6 km.

In addition, there exists a hierarchy of stress level in the three fault types. Thrust faults are generated by the highest, strike slip by intermediate, and normal faults by the lowest stress levels. This can easily be understood by considering the direction of the greatest principal stress, the direction of the force that pushes the rock mass during the faulting. In the case of normal faults, the rock mass is pushed down in a vertical direction as in Figure 1.2a, where the pushing force (greatest principal stress) equals the weight of the rock mass itself. In the case of thrusting, the rock mass escapes in the direction of the least principal stress, that is, upward, lifting the rock mass up as in Figure 1.2b, and the overburden equals the least principal stress. Strike-slip faulting is intermediate between the other two types described earlier as in Figure 1.2c. This difference in stress regime in the three faulting environments contributes to differences in stress drop during faulting, which contributes to differences in the radiated energy, regardless of fault dimensions.

1.1.3 EQs Away from Plate Boundaries (Interplate EQs)

Where plate boundaries occur within continental lithosphere, deformation is spread over a much larger area than the plate boundary itself. In the case of the San Andreas Fault continental transform, many EQs occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace. The Northridge EQ was associated with movement on a blind thrust within such a zone.

All tectonic plates have internal stress fields caused by their interactions with neighboring plates and sedimentary loading or unloading. These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate EQs.

1.1.4 Shallow-Focus and Deep-Focus EQs

The majority of tectonic EQs originate at the ring of fire in depths not exceeding tens of kilometers. EQs occurring at a depth of less than 70 km are tentatively classified as “shallow-focus” EQs, while those with a focal depth between 70 and 300 km are commonly termed “midfocus” or “intermediate-depth” EQs. In subduction zones, where an older and colder oceanic crust descends beneath another tectonic plate, deep-focus EQs may occur at much greater depths (ranging from 300 up to 700 km). These seismically active areas of subduction are known as Wadati–Benioff zones....