Geologic Hazards

Asthenosphere

Under the lithosphere, it is the weak upper mantle where isostatic adjustments are made, magma generated, and seismic waves are strongly attenuated

Attenuation

A reduction in the amplitude of seismic waves produced by divergance, scattering, reflection, and absorption

Avalanche

A large mass of snow, ice, rock, of mixtures of the above, falling, sliding, or flowing very rapidly due to gravity

Body-Wave Magnitude (Mb)

Magnitude of an earthquake as estimated from the amplitude of body waves

Crown

The material that is still in place and adjacent to the highest parts of the main scarp of a landslide

Differential Settlement

The uneven lowering of different parts of an engineered structure, often resulting in damage to the structure

Dilatancy

The increase in volume of rocks due to elastic and nonelastic changes during deformation

Elastic Rebound Theory

A theory proposed by Reid to explain the cause of Earthquakes. As rocks on opposite sides of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded

Epicenter

Point on Earth's surface directly above the focus

Factor of Safety

A measure of the stability of a slope in which the resisiting forces are compared to the driving forces. If the resisting force is equal to or less than the driving force, the slope will fail

Fall

A very rapid downward movement of rock or earth that travels most of the distance through the air, either by free fall, leaps and bounds, or rolling

Fault Creep

Slow ground displacement usually occurring without accompanying earthquakes. It may be of tectonic origin or result from oil or groundwater withdrawal

Fault Scarp

A steep slope formed directly by movement along a fault and representing the exposed surface of the fault before erosion and weathering modify it

Fault-Line Scarp

A steep slope formed by differential erosion along a fault line

Flow

Movement within the displaced mass of rock or soil that continually deforms it. Slip surface are not generally visible

Focus (Hypocenter)

The place where earthquake rupture originates

Ground Lurching

The movement of an unsupported cliff or stream bank towards the free face during an earthquake. A series of more or less parallel cracks or formed, separating the ground into blocks

Ground Shattering

Bedrock or soil on a ridge top that has been thoroughly pulverized due to high accelerations over many cycles of earthquake shaking

Growth Fault

A fault that moves contemporaneously with deposition causing the throw (amount of vertical displacement) to increase with depth and the strata to be thicker on the downthrown side as compared to the upthrown side

Head Scarp (Main Scarp)

A steep surface on the undisturbed ground at the top of the slide, which is the upper slip surface

Hydrocompaction

The process whereby soils collapse when they are wetted (aka hydroconsolidation)

Intensity

A measure of the effects of an earthquake at a particular place. Observed effects are the damage to human structures, ground disturbances, and animal reactions

Intensity Scale

A descriptive way to assess earthquake intensity. The scale used is the Modified Mercalli Intensity Scale

Lateral Spread

Movement of a fractured mass laterally, often along a basal shear surface or zone of plastic flow

Liquefaction

The sudden large decrease of shearing resistance of a cohesionless soil caused by a collapse of thestructure by a shock (such as an earthquake), and associated with an increase in pore pressure

Lithosphere

Composed of the Earth's crust and part of the upper mantle, it is approximately 100 km thick and is relatively strong compared to the underlying asethenosphere

Local Magnitude (ML)

Local or Richter magnitude is the log10 of the maximum seismic-wave amplitude (in thousandths of a millimeter) recorded on a standard seismograph at a distance of 100 km from the earthquake epicenter

Magnitude

A measure of the strength of an earthquake or the strain energy released by it. Four common types are Richter (aka local), body wave, surface, and moment magnitude

Microseism

A more or less continuous motion in the Earth unrelated to earthquakes, with a period of 1-9 seconds

Moment Magnitude (M)

The magnitude of an earthquake estimated by using the seismic moment

Period

The interval of time required for the completion of a cyclic motion or recurring event

Recurrence Interval

The average time interval between earthquake occurances of equal magnitude on the same fault

Rigidity

The ratio of the shearing stress to the amount of angular rotation it produces in a rock sample

Rotational Slide or Slump

Landslide movement due to forces that cause a turning moment about a point above the center of gravity of the unit. The slip surface is concave upwards

Sand Boil

A cone-shaped deposit of sand formed during an earthquake when subsurface sand layers liquefy and then are blown to the surface through cracks

Seiche

A wave oscillation of the surface of water in an enclosed basin (such as a lake or bay) initiated by an earthquake or changes in atmospheric pressure

Seismic Moment (Mo)

A measure of earthquake size that depends on the rock rigidity, amount of slip, and area of rupture

Settlement

The gradual downward movement of structure due to the compression of the soil below the foundation

Soil Creep

The gradual downward movement of soil and loose rock material

Subsidence

A localized mass movement that involves the gradual downward settling or sinking of the Earth's surface

Surface-Wave Magnitude (Ms)

Magnitude of an earthquake estimated from measurements of the amplitude of surface waves

Toe

The outermost margin of displaced landslide material, farthest away from the head scarp

Topple

Slope movement due to forces that cause an overturning moment about a pivot point below the center of gravity of the unit

Translational Slide

Landslide movement that occurs predominantly along planar or gently undulating surfaces

Triggered Creep

Creep that occurs on a fault which has been triggered by a strong earthquake on some other fault

Tsunami

Gravitational sea wave produced by large-scale, short-duration earthquake on ocean floor, submarine earth movement, subsidence or volcanic eruption