GLOSSARIES - Geotechnical Earthquake Engineering - page 520

Isolator Unit A horizontally flexible and vertically stiff structural element that allows for large lat-
eral deformation under the seismic load.
Isoseismal Line A line connecting points on the earth's surface at which earthquake intensity is
the same. It is usually a closed curve around the epicenter.
Leaking Mode A surface seismic wave which is imperfectly trapped so that its energy leaks or
escapes across a layer boundary, causing some attenuation or loss of energy.
Liquefaction The sudden and large decrease of shear strength of a submerged cohesionless soil
caused by contraction of the soil structure, produced by shock or earthquake-induced shear strains,
associated with a sudden but temporary increase of pore water pressures. Liquefaction occurs when the
increase in pore water pressures causes the effective stress to become equal to zero and the soil behaves
as a liquid.
Lithosphere The outermost layer of the earth. It commonly includes the crust and the more rigid
part of the upper mantle.
Love Wave Surface waves that are analogous to S waves in that they are transverse shear waves
that travel close to the ground surface. It is named after A. E. H. Love, the English mathematician, who
predicted it.
Low-Velocity Zone

Any layer in the earth in which seismic wave velocities are lower than in the

layers above and below.
Magnitude (of the Earthquake) A measure of the size of the earthquake at its source. Many dif-
ferent methods are used to determine the magnitude of an earthquake, such as the local magnitude
scale, surface wave magnitude scale, the body wave magnitude scales, and the moment magnitude
scale.
Major Earthquake An earthquake having a magnitude of 7.0 or larger on the Richter scale.
Mantle The layer of material that lies between the crust and the outer core of the earth. It is approx-
imately 2900 km thick and is the largest of the earth's major layers.
Maximum Capable Earthquake According to the Uniform Building Code (1997), in seismic
zones 3 and 4, the level of earthquake ground motion that has a 10 percent probability of being
exceeded in a 100-year period.

Maximum Credible Earthquake (MCE) Often considered to be the largest earthquake that can
reasonably be expected to occur based on known geologic and seismologic data. In essence, the max-
imum credible earthquake is the maximum earthquake that an active fault can produce, considering the
geologic evidence of past movement and recorded seismic history of the area. According to Kramer
(1996), other terms that have been used to describe similar worst-case levels of shaking include safe
shutdown earthquake (used in the design of nuclear power plants), maximum capable earthquake,
maximum design earthquake, contingency level earthquake, safe level earthquake, credible design
earthquake, and contingency design earthquake. In general, these terms are used to describe the upper-
most level of earthquake forces in the design of essential facilities.
The maximum credible earthquake is determined for particular earthquakes or levels of ground
shaking. As such, the analysis used to determine the maximum credible earthquake is typically referred
to as a deterministic method.

Maximum Probable Earthquake Commonly the largest earthquake that a fault is predicted
capable of generating within a specified time period of concern, say, 50 or 100 years. There are many
different definitions of the maximum probable earthquake. The maximum probable earthquake is
based on a study of nearby active faults. By using attenuation relationships, the maximum probable
earthquake magnitude and maximum probable peak ground acceleration can be determined. Maximum
probable earthquakes are most likely to occur within the time span of most developments and, there-
fore, are commonly used in assessing seismic risk.
Another commonly used definition of a maximum probable earthquake is an earthquake that will
produce a peak ground acceleration a max with a 50 percent probability of exceedance in 50 years.