GLOSSARIES - Geotechnical Earthquake Engineering - page 521

According to Kramer (1996), other terms that have been used to describe earthquakes of similar size
are operating basis earthquake, operating level earthquake, probable design earthquake, and strength
level earthquake.
Microearthquake An earthquake having a magnitude of 2 or less on the Richter scale.
Modified Mercalli Intensity Scale See Intensity (of an Earthquake).
Mohorovicic Discontinuity (or Moho Discontinuity) The boundary surface or sharp seismic-
velocity discontinuity that separates the earth's crust from the underlying mantle. Named for Andrija
Mohorovicic, the Croatian seismologist who first suggested its existence.
Normal Fault See Fault.
Paleomagnetism The natural magnetic traces that reveal the intensity and direction of the earth's
magnetic field in the geologic past. Also defined as the study of these magnetic traces.
Paleoseismology The study of ancient (i.e., prehistoric) earthquakes.
Peak Ground Acceleration (PGA) Also known as the maximum horizontal ground acceleration.
The peak ground acceleration can be based on an analysis of historical earthquakes or based on prob-
ability (see Sec. 5.6). An attenuation relationship is used to relate the peak ground acceleration to the
earthquake magnitude and closest distance between the site and the focus of the earthquake.
Period The time interval between successive crests in a wave train. The period is the inverse of the
frequency.
Plate Boundary The location where two or more plates in the earth's crust meet.
Plate Tectonics According to the plate tectonic theory, the earth's surface contains tectonic plates,
also known as lithosphere plates, with each plate consisting of the crust and the more rigid part of the
upper mantle. Depending on the direction of movement of the plates, there are three types of plate
boundaries: divergent boundary, convergent boundary, and transform boundary (see Sec. 2.1).
Pseudostatic Analysis A method that ignores the cyclic nature of the earthquake and treats it as
if it applied an additional static force upon the slope or retaining wall.
P Wave A body wave that is also known as the primary wave, compressional wave, or longitudinal
wave. It is a seismic wave that causes a series of compressions and dilations of the materials through
which it travels. The P wave is the fastest wave and is the first to arrive at a site. Being a compression-
dilation type of wave, P waves can travel through both solids and liquids. Because soil and rock are
relatively resistant to compression-dilation effects, the P wave usually has the least impact on ground
surface movements.
Rayleigh Wave Surface wave similar to the surface ripple produced by a rock thrown into a pond.
These seismic waves produce both vertical and horizontal displacement of the ground as the surface
waves propagate outward. They are usually felt as a rolling or rocking motion and, in the case of major
earthquakes, can be seen as they approach. They are named after Lord Rayleigh, the English physicist
who predicted their existence.
Recurrence Interval The approximate length of time between earthquakes in a specific seismi-
cally active area.
Resonance A condition where the frequency of the structure is equal to the natural frequency of the
vibrating ground. At resonance, the structure will experience the maximum horizontal displacement.
Response Spectrum See Design Response Spectrum.
Richter Magnitude Scale Also known as the local magnitude scale; a system used to measure the
strength of an earthquake. Professor Charles Richter developed this earthquake magnitude scale in
1935 as a means of categorizing local earthquakes.
Rift Valley A long and linear valley formed by tectonic depression accompanied by extension. A
divergent boundary between tectonic plates can create a rift valley. Earthquakes at a rift valley are
often due to movement on normal faults. Examples of rift valleys are the East African rift and the Rhine
Graben.