chapter 10 geohazards and risk Flashcards
amplitude
the maximum extent of an oscillation, measured from the position of rest.
attenuation
the loss of energy experienced by a wave shown as a reduction in amplitude as it propagates through a material.
moment
the turning effect of a force around a pivot
liquefaction
describes saturated or paritally saturated unconsolidated material losing strength and rigidity in response to an applied stress, usually an earthquake
resonant frequency
the frequency at which the amplitude of the oscillation is greatest.
intensity
a measure of the surface shaking and damage caused by an earthquake. The Mercalli scale measures the intensity of an earthquake. Intensity is based on an earthquake’s local accelerations and how long these persist.
frequency
the number of oscillations per second or the number of waves that pass through a point per second. units are in hertz.
resonance
the tendency to oscillate with greater amplitude at some frequencies than at others.
geological hazard
a geological condition that is dangerous or potentially dangerous to the environment and the people who live within it.
Seismic risk
the possibility of suffering harm or social and environmental loss because of a seismic event that may occur in a specified period of time.
probability
a measure of the likelihood that an event will occur. Probability is quantified as a number between 0 and 1.
return period
the average length of time for an earthquake of a given magnitude to occur again or to be exceeded (sometimes called the recurrence period).
Exceedance
is the probability that an earthquake will generate a level of ground motion that exceeds a specified reference level during a given exposure time.
forecast
a statement of the probable occurrence of an event calculated from data.
prediction
a statement about what you think will happen in the future
determinism
finding outcomes through known relationships ‘cause and effect’.
plasticity
the property a substance has when deformed continuously under a finite force.
tilt
calculated as the change in elevation between 2 points divide by the distance between those points (the first derivative of the subsidence profile). the maximum tilt occurs at the point of inflection in the subsidence trough. Tilt is usually expressed in millimetres per metre.
Inflection point
a point on a curve at which the curve changes from being concave to convex, or vice versa.
Sinkholes
hollows or holes in the ground caused by collapse of a surface layer, usually by dissolution of limestone.t
talus
the debris accumulating at the foot of a slope due to erosion of the rock face above.
translational slide
when material begins to move as an entity, e.g. several beds sliding down slope. It usually breaks up as it gathers speed and the energy increases. Contrast the rotational slide that marks failure of incompetent, clay-rich rocks.
isotropic clay
having equal properties in all directions. this is not the same as being homogeneous, which conveys being made of the same metamorphosed mudstone but is not isotropic in that it has strong cleavage.
tsunamite
a tsunami deposit which is a sedimentary unit deposited onshore during the inundation phase. The offshore deposits as the water retreats are different.
stress
defined as the force per unit area acting on the rock. Compressive= compress/shorten. Shear= acts in a plane parallel to the stressed area. Tensile= stretch/lengthen.
fabric
describes the spatial and geometric configuration of all the ‘components’ of a rock: the mineral crystals and their sizes, shapes and relative orientation.
strain
the response of a system to an applied stress. when a material is loaded with a force, it produces a stress which then causes a material to deform
confining pressure
the combined lithostatic and hydrostatic pressure. At depth all principal stresses are equal.
ductile deformation
occurs when a rock suffers large strains without large-scale fracturing. It bends and flows.
brittle deformation
causes the rock to fracture
lithostatic pressure
the vertical pressure due to the mass of the rock only. It is also referred to as the over burden pressure.
asperity
the term mostly used to describe the roughness of the surface of a discontinuity.
residual strength
the remaining resistance to movement after the rock has failed and been displaced.
joint sets
are fractures across which there is little displacement. They are mostly to dissipate the residual stresses left after folding. As they are the result of regional stresses they tend to form sub-parallel sets.
drift
the superficial deposits such as glacial and fluvioglacial material. The ‘solid’ maps show an overlay of drift. Alluvium refers to the recent deposits by rivers.
seismic refraction surveys
can be made using a simple dropped mass or large hammer as the energy source. Waves are refracted by changes in velocity such as at the base of the weathered layer.
