The strength of a seismic event can be characterized by its magnitude
and its intensity, defined thus:
magnitude: a measure of the energy released; it therefore has a
single value for a specific seismic event. It is categorized
on the Richter scale, ranging upwards from M1.0 to
M9.0.
intensity: a measure of the violence of seismic shaking attaching
to an event, and hence of its destructiveness, at a spe-
cific location. Intensity thus varies with position and dis-
tance from the epicentre, and is commonly expressed
on the modified Mercalli scale of MMI to MMXII.
The terminology associated with seismic safety evaluation includes a range
of definitions, some of which are especially significant in the context of
dams, thus:
Maximum Credible Earthquake (MCE): the event predicted to
produce the most severe level of ground motion possible for the geo-
logical circumstances of a specific site.
Safety Evaluation Earthquake (SEE):the event predicted to produce
the most severe level of ground motion against which the safety of a
dam from catastrophic failure must be assured.
SEE may be defined as a proportion of the MCE or equal to it; an altern-
ative is to specify SEE on the basis of a notional return period (cf. flood
categorization in spillway design, Section 4.2). In recommended UK prac-
tice (Charles et al., 1991) SEE takes the place of MCE as employed in US
practice, e.g. USBR (1987).
Other terms employed in seismic design include Controlling
Maximum Earthquake (CME), Maximum Design Earthquake (DBE) and
Operating Basis Earthquake (OBE). These are not referred to further in
this text.
Ground motions associated with earthquakes can be characterized in
terms of acceleration, velocity or displacement. Only peak ground acceler-
ation, PGA, generally expressed as a proportion of gravitational accelera-
tion,g, is considered here. PGA can be rather imprecisely correlated with
intensity, as indicated by Table 2.7. It has been suggested that in general
terms seismic events with a high PGA of short duration are less destruc-
tive than events of lower PGA and greater duration.
Most energy released as ground motion is transmitted in the sub-
10 Hz frequency band. An elastic structure having a natural frequency, fn,
within that range is therefore potentially at risk. Embankment dams will
92 EMBANKMENT DAM ENGINEERING
