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quantity for classifying the dynamic characteristics of the ground (see Section 4.3.3). Since vs
is lower than the velocity Vp of P-waves, the latter are always the first to arrive at a station
recording the seismic motion, followed by S-waves, which are associated with large
amplitudes of motion.
When body waves reach the earth surface they are reflected back into the crust, and a
vibration of the surface is initiated, which propagates through surface waves. Depending on
the way these waves propagate along the earth surface, they are classified as Rayleigh waves
or Love waves. Surface waves, along with S-waves, account for the strongest part of the
seismic motion (i.e. these are the ones that may cause losses). P-waves are generally small
amplitude and of interest to the seismologists only; they use the difference in arrival times
between P and S-waves for determining the epicentre of an earthquake.

4.2.2 Measures of earthquakes
Designing against earthquakes presupposes that the phenomenon can be adequately quantified.
There are two main ways for measuring the size (or strength) of earthquakes: One based on
instrumental data, and one based on observation of the effects of earthquake motions on
humans and structures; both are indispensable for hazard assessment and seismic design.
There are two types of instruments that can be used for recording earthquake motions:

●The seismographs, which record the displacement of the ground with time. These
instruments are designed to magnify weak motions, so they can record motions caused by
very distant earthquakes. Their recordings are of interest mainly for the seismologists, since
they are used for locating earthquakes and characterizing their sources.
●The accelerographs, which record the acceleration of the ground with time. Until recently
these instruments were recording (on film) whenever they were triggered by a minimum
level of acceleration (e.g. 0.01 g), but more advanced instruments are currently available,
which record in a digital form on reusable medium, hence they can operate continuously
and save only records of interest; this has the extra advantage that the initial part of the
motion is not lost. Accelerograms are the main type of earthquake record used for deriving
design seismic actions.

Magnitude
The magnitude of an earthquake is a measure of the earthquake size or the source strength;
usually, though not necessarily, the magnitude measures the amount of energy released by an
earthquake. The Richter magnitude or local magnitude M is defined as the (base 10)
L
logarithm of the maximum amplitude A (in µm) of the earthquake, corrected to a distance of
100 km; the correction is done by subtracting