Seismicity Flashcards
Focus
The point within the crust where the pressure release occurs
Types of focus
Shallow focus- 0-70km, greatest damage caused
Intermediate focus- 70-300km deep
Deep focus- 300-700km deep
Epicentre
The place on the earth’s surface immediately above the surface
How are earthquakes caused?
Earthquakes originate along faults.
Parts of the crust are being forced to move in opposite directions
These huge masses of rock get stuck but the forces on them continue, building up stresses in the rock.
Eventually the strain overcomes the friction, releasing large amounts of energy
Primary waves
Body waves Travel the fastest Can move through solids and liquids Compressions Least amount of damage
Secondary waves
Body waves Only travel through solids Half the speed of primary waves Pushes the ground surface up and down More damage than primary waves
Surface waves
Travel the slowest and near the ground surface
Most destructive
Destroy buildings
Two types- love and Rayleigh
Love waves
Shake the ground up and down and then in a longitudinal movement
Rayleigh waves
Cause a complex heaving or rolling motion
How can scientists tell how far away the earthquake is?
By looking at the time between the primary and secondary waves on a seismograph
If you are close to the earthquake, the primary and secondary waves will come one right after the other but if you are far away, there will be more time between them
How are earthquakes recorded?
Recorded by instruments called seismograms
The seismograms has a base that sera firmly in the ground and a heavy weight than hangs free
When an earthquake causes the ground to shake, the base of the seismograms shakes too but the hanging weight doesn’t.
Instead, the Spring that is hanging from absorbs all the movement
Earthquakes at destructive margins
Most powerful earthquakes
Create frequent shallow to deep focus earthquakes
Eg around the Circum Pacific belt
Earthquakes at conservative margins
Where plates slip past each other, the boundary is marked by a fault or a series of faults
Eg San Andreas fault
Earthquakes at constructive margins
Often submarine and distant from people so there is a relatively little hazard.
They occur where the upwelling magma forces the places apart and faulting occurs on the edge of the central Rift Valley
E.g mid Atlantic ridge areas
Earthquakes away from plate boundaries
Some earthquakes occur along old fault lines that have become reactivated
The earthquakes in China and Central Asia occur along the lines of weakness connected to the collision of India with Eurasian plate over 50 million years ago
