chapter 10 geohazards and risk

Question 1

amplitude

Answer 1

the maximum extent of an oscillation, measured from the position of rest.

Question 1

attenuation

Answer 1

the loss of energy experienced by a wave shown as a reduction in amplitude as it propagates through a material.

Question 2

moment

Answer 2

the turning effect of a force around a pivot

Question 3

liquefaction

Answer 3

describes saturated or paritally saturated unconsolidated material losing strength and rigidity in response to an applied stress, usually an earthquake

Question 4

resonant frequency

Answer 4

the frequency at which the amplitude of the oscillation is greatest.

Question 5

intensity

Answer 5

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.

Question 6

frequency

Answer 6

the number of oscillations per second or the number of waves that pass through a point per second. units are in hertz.

Question 7

resonance

Answer 7

the tendency to oscillate with greater amplitude at some frequencies than at others.

Question 8

geological hazard

Answer 8

a geological condition that is dangerous or potentially dangerous to the environment and the people who live within it.

Question 9

Seismic risk

Answer 9

the possibility of suffering harm or social and environmental loss because of a seismic event that may occur in a specified period of time.

Question 10

probability

Answer 10

a measure of the likelihood that an event will occur. Probability is quantified as a number between 0 and 1.

Question 11

return period

Answer 11

the average length of time for an earthquake of a given magnitude to occur again or to be exceeded (sometimes called the recurrence period).

Question 12

Exceedance

Answer 12

is the probability that an earthquake will generate a level of ground motion that exceeds a specified reference level during a given exposure time.

Question 13

forecast

Answer 13

a statement of the probable occurrence of an event calculated from data.

Question 14

prediction

Answer 14

a statement about what you think will happen in the future

Question 15

determinism

Answer 15

finding outcomes through known relationships ‘cause and effect’.

Question 16

plasticity

Answer 16

the property a substance has when deformed continuously under a finite force.

Question 17

tilt

Answer 17

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.

Question 18

Inflection point

Answer 18

a point on a curve at which the curve changes from being concave to convex, or vice versa.

Question 19

Sinkholes

Answer 19

hollows or holes in the ground caused by collapse of a surface layer, usually by dissolution of limestone.t

Question 20

talus

Answer 20

the debris accumulating at the foot of a slope due to erosion of the rock face above.

Question 21

translational slide

Answer 21

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.

Question 22

isotropic clay

Answer 22

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.

Question 23

tsunamite

Answer 23

a tsunami deposit which is a sedimentary unit deposited onshore during the inundation phase. The offshore deposits as the water retreats are different.

Question 24

stress

Answer 24

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.

Question 25

fabric

Answer 25

describes the spatial and geometric configuration of all the ‘components’ of a rock: the mineral crystals and their sizes, shapes and relative orientation.

Question 26

strain

Answer 26

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

Question 27

confining pressure

Answer 27

the combined lithostatic and hydrostatic pressure. At depth all principal stresses are equal.

Question 28

ductile deformation

Answer 28

occurs when a rock suffers large strains without large-scale fracturing. It bends and flows.

Question 29

brittle deformation

Answer 29

causes the rock to fracture

Question 30

lithostatic pressure

Answer 30

the vertical pressure due to the mass of the rock only. It is also referred to as the over burden pressure.

Question 31

asperity

Answer 31

the term mostly used to describe the roughness of the surface of a discontinuity.

Question 32

residual strength

Answer 32

the remaining resistance to movement after the rock has failed and been displaced.

Question 33

joint sets

Answer 33

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.

Question 34

drift

Answer 34

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.

Question 35

seismic refraction surveys

Answer 35

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.