Topic 5: Earthquakes

Earthquake

The vibration of Earth produced by the rapid release of energy when a rock mass breaks or slips

Fracture

When slippage occurs along a break in a rock

Focus

Where the energy radiates in all directions from the earthquake's source

Epicentre

The point at surface directly above the foc

Elastic Rebound Hypothesis

Rough rock masses may become locked together with tectonic forces still acting upon them, deforming them, and causing them to store more elastic energy

Stress

Force per unit area

Strain

Deformation or change in shape as a result of that stress

Ductile Deformation

Occurs in soft, warm rocks which may deform plastically and form folds

Brittle Deformation

Occurs when hard, cold rocks accumulate stress until they fail along a fracture, forming faults

Elastic Deformation

When the deformed rock "springs" back to its original shape

Compressive Force

Is associated with a convergent boundaries, subduction zones, the ductile regime folds while the brittle regime forms a reverse fault

Tensional Force

Is associated with divergent boundaries, forces move apart, and the ductile regime stretches and thins, while the brittle regime forms a normal fault

Strike-Slip Fault

It occurs in subvertical faults (with no hanging wall or footwall) or along with normal/reverse faulting

Hanging Wall

The block of rock above an inclined geological fault

Footwall

The block of rock beneath an inclined fault plane

Left Lateral Strike-Slip Fault

When the stationary feature appears to offset to the left when standing across from it

Right Lateral Strike-Slip Fault

When the stationary feature appears to offset to the right when standing across from it

Oceanic Transform Faults

The direction of movement along transform faults (at MORS) provides evidence for seafloor spread

Normal Fault

The hanging wall moves upwards relative to the footwall

Reverse Fault

The hanging wall moves downward relative to the footwall

Strike

The direction of a line created by the intersection of a fault plane and a horizontal surface (0° to 360°)

Dip

The angle between the fault and a horizontal plane (0° to 90°)

Seismic Waves

When an earthquake occurs, elastically stored energy is carried outward from the focus of vibrations

Body Waves

These waves travel outward in all directions from the focus through Earth’s interior

Primary Waves

They are the first to arrive, moves through compression and expansion, and it passes through solids, liquids, and gases

Secondary Waves

This wave has a shearing motion (perpendicular to wave movement) and passes through solids only since they are shear waves

Surface Waves

These waves travel around Earth, analogous to ocean waves, but along all of the Earth’s solid surface

Love Waves

This wave moves the ground from side to side

Raleigh Waves

This wave has a motion which follows the shape of an ellipse (like rolling ocean waves) and are generally slower

Seismograph

Can help us record seismic waves and map out when the waves arrive; they help us visualize the Earthquake with P waves arriving first, followed by an S-P interval before the arrival of the S wave, and the surface waves arriving last

Core-Mantle Boundary

Casts S-wave and P-wave shadows, which uses triangulation from many stations to calculate the epicentre

Mantle-Crust Boundary

Also called the Mohorovicic (Moho) discontinuity which determines the P-wave velocity since the continental crust, oceanic crust, and the mantle have different compositions

Richter Scale

A logarithmic scale with each unit corresponding to 10x increase in wave amplitude measuring an Earthquake's magnitude

Modified Mercalli Scale

A more general scale based on an Earthquake's intensity and is useful for hazard planning

Magnitude

Estimates the amount of energy released by the earthquake

Intensity

A measure of earthquake shaking at a given location based on amount of damage