Seismology Flashcards
What is an earthquake and what is it caused by?
An event of ground shaking caused by:
- Sudden formation of a new fault (fracture on which sliding occurs)
- Sudden slip of existing fault
- Movement of magma in a volcano/explosion of volcano
- Giant landslides
- Meteorite impact
- Underground nuclear bomb tests
What is the hypocentre and epicentre of an earthquake?
Hypocentre: Actual location of the earthquake at depth
Epicentre: Location on the surface of the Earth above the hypocenter
A fault has two parts, which?
The hanging wall: the top block of a fault
The footwall: the bottom block of the fault
There are three types of faults, which?
Normal fault: From stretching of or extending rock; points on opposite sides of a fault are further apart after an earthquake
Reverse fault: From contracting or squishing rock; points on opposite sides of the fault are closer together after an earthquake
Strike-slip fault: Can form in either areas of stretching or squishing, material slides laterally past each side of the fault.
What is stress and strain?
Stress is defined as force/area while strain is measured change in size or shape. Stress causes strain.
There is normal stress and strain which happens perpendicular to a shape. For example it transforms a square into a rectangle.
There is also shear stress and strain which happens parallel to the shape. It causes deformation
Describe body wave versus surface wave
A body wave is motion through the surface of the Earth.
A surface wave is motion along the surface.
Decribe Compressional (Transverse) waves versus Shear (Longitudinal) waves
Compressional (Transverse) waves: Motion of particles is in the direction of the wave. Causes material to contract and extend
Shear (Longitudinal) waves: Motion is perpendicular to wave direction. Causes material to shear
Which types of seismic waves are there?
- Body waves
P-waves: Compressional body waves. Primary
S-waves: Shear body waves. Secondary - Surface waves – do the most damage in earthquakes
R-waves: Surface shear waves that make the ground move up and down in a retrograde elliptical pattern.
L-waves: Surface shear waves that cause the ground to move horizontally back and forth (like a snake).
How is seismic waves measured?
By seismograph or geophone
The seismograph writes a seismogram which records the arrival times of the seismic waves. P-waves first, S-waves next, possibly aftershocks after that
What is seismic wave fronts?
Seismic sources radiate pulses of energy in all directions (spherically) from a source point and the wave front is then the boundary between the energy pulse and the material that has not yet received the energy.
Think of it like a pebble in water.
What happens with the velocity of seismic rays as they go into the ground?
Generally the velocities increase with depth
What are the shadow zones of the S and the P waves?
The S-wave cannot travel through Earth’s core and therefore the shadow zone will be on the other side of the earth from the origin.
The P-wave gets a different velocity and angle when going into the core so the shadow zones are to the sides of the earth looking from the origin. Waves will travel to the place on the other side of the earth.
What happens with the P-wave velocity in water?
It increases
