GLOSSARIES - Geotechnical Earthquake Engineering - page 522

Risk See Seismic Risk.
Rupture Zone The area of the earth through which faulting occurred during an earthquake. For
great earthquakes, the rupture zone may extend several hundred kilometers in length and tens of kilo-
meters in width.
Sand Boil Also known as sand blow, sand volcano, or silt volcano. The ejection of sand at ground
surface, usually forming a cone shape, is caused by liquefaction of underlying soil during an earth-
quake. Sand boils can also be caused by piping (see Glossary 4).
Seiche Identical to a tsunami, except that it occurs in an inland body of water, such as a lake. It can
be caused by lake-bottom earthquake movements or by volcanic eruptions and landslides within the
lake. A seiche has been described as being similar to the sloshing of water in a bathtub.
Seismic or Seismicity Dealing with earthquake activity.
Seismic Belt An elongated earthquake zone. Examples include the circum-Pacific, Mediterranean,
and Rocky Mountain seismic belts.
Seismic Risk The probability of human life and property loss due to an earthquake.
Seismogram A written record of an earthquake that is produced by a seismograph.
Seismograph An instrument that records the ground surface movement as a function of time
caused by the seismic energy of an earthquake.
Seismology The study of earthquakes.
Shear Wall Sometimes referred to as a vertical diaphragm or structural wall, a shear wall is designed
to resist lateral forces parallel to the plane of the wall. Shear walls are used to resist the lateral forces
induced by the earthquake.
Spreading Center An elongated region where two plates are being pulled away from each other.
New crust is formed as molten rock is forced upward into the gap. An example is seafloor spreading,
which has created the mid-Atlantic ridge. Another example is a rift valley, such as the East African rift.
Strike and Dip (of a Fault Plane) A description of the orientation of the fault plane in space.
Strike is the azimuth of a horizontal line drawn on the fault plane. The dip is measured in a direction
perpendicular to the strike and is the angle between the inclined fault plane and a horizontal plane.
Strike-Slip Fault See Fault.
Subduction Zone An elongated region along which a plate descends relative to another plate. An
example is the descent of the Nazca plate beneath the South American plate along the Peru-Chile
trench.
S Wave A body wave that is also known as the secondary wave, shear wave, or transverse wave.
The S wave causes shearing deformations of the materials through which it travels. Because liquids
have no shear resistance, S waves can only travel through solids. The shear resistance of soil and rock
is usually less than the compression-dilation resistance, and thus an S wave travels more slowly
through the ground than a P wave. Soil is weak in terms of its shear resistance, and S waves typically
have the greatest impact on ground surface movements.
Syncline Layers of rock that have been folded in a generally concave upward direction. The core
of a syncline contains the younger rocks.
Travel Time The time required for a seismic wave train to travel from its source to a point of obser-

vation.
Tsunami A Japanese term that means harbor wave. It is a long-period ocean wave that can be cre-
ated by seafloor earthquake movements or by submarine volcanic eruptions and landslides.