5.3.1 Earthquakes

5.3.1.1 General notes

For any nuclear installation, the risk of earthquakes at the location concerned must be assessed in principle and it must be designed to deal with seismic effects. Details here can be found in the relevant IAEA Safety Standards (cf. Section 3.4.1) and corre­sponding national rules and regulations, such as the German KTA 2201.1 [37], which many other countries also use.

Earthquakes can be defined as shocks to solid rock emanating from an underground source (hypocentre) attributable to natural causes. Earthquakes can be divided into a number of types, depending on what causes them:

— Collapse earthquakes

When underground cavities suddenly collapse

— Volcanic earthquakes

Incandescent molten rock rises to the surface from inside the Earth under high pressure

— Tectonic earthquakes

Sudden violent shifts of rock strata along geological fault lines or faults; with faults, there are three basic kinds of movement: gravity faults, upthrusts and horizontal faults.

In what follows, we will concentrate on tectonic earthquakes, as they account for more than 90% of all earthquakes (cf. [38]). The effects of such tectonic earthquakes, which induce seismic effects, manifest themselves in considerable amounts of energy being released, due to the rock strata shifting. From the earthquake hypocentre, shock waves spread out at different speeds and amplitudes, referred to as compression or primary waves (P waves) and shear or secondary waves (S waves). These shock waves can also be recognised in recorded acceleration time displacements (Figure 5.2). The earth­quakes themselves which trigger these waves can be defined and/or quantified either by their magnitude or their intensity. Magnitude, which is normally used as local or close earthquake magnitude (ML), measures the energy released at the hypocentre of the earthquake underground. This scale was introduced by C. F. Richter in 1935, and is therefore often referred to as the Richter magnitude, or magnitude on the Richter scale. This magnitude is obtained as the logarithm of the maximum deflection of recorded seismograms, allowing for the distance to the hypocentre (Figure 5.3). That means each additional unit of magnitude increases the energy released by around approximately 30 times. One of the greatest earthquakes recorded to date occurred in Alaska in 1964, and reached a magnitude of around 8.8.

Intensity can be defined as the impact of an earthquake at a given location on the surface of the Earth (normally a land surface) as a function of its magnitude at a given hypocentre depth. Intensity is a measure of the impact of seismic waves and dislocations at the surface of the Earth on people, objects and building structures. The strength of these effects is classified in qualitative terms based on the effects observed in a limited area. Intensity is divided into 12 degrees, which are defined as macro-seismic scales, such as the MSK scale (Medvedev-Sponheuer-Karnik; cf. Table 5.2) or the EMS scale 1998 (European Macroseismic Scale). Comparing two earthquakes of the same magnitude but whose hypocentres are at different depths (shallow and deep hypocentres) shows that earthquakes are more intensive the closer their hypocentre is to the surface.

The level of earthquake governing earthquake design, or design basis earthquake, is given generally by the intensity to be expected for the site. In line with this site-specific intensity, with its associated ground movements (accelerations, velocities, displace­ments), a ground response spectrum must be defined as the basis for the further design of building structures or components. Such a response spectrum, in the form of an acceleration spectrum, represents the maximum acceleration amplitudes of the

vibration of single mass oscillators with different eigenfrequencies and damping in response to a non-stationary excitation (Figure 5.4).