Topic 3 - Earthquakes, Processes, Hazards, and their impacts Flashcards
what are earthquakes
shaking/trembling caused by sudden release of energy usually associated with faulting/breaking of rocks
where are earthquakes frequent
- 80% occur in Pacific Ring of Fire and most result from convergent margin activity
- 15% in Mediterranean-Asian belt
- remaining 5% occur in interiors of plates and on spreading ridge centers
- more than 150,000 strong enough to be felt are recorded
what is the elastic rebound theory
explains how energy is stored in rocks
- rocks bend until strength is exceeded
- rupture occurs and rocks quickly rebound to undeformed shape
- energy released in waves that radiate outward from fault
what is the focus and epicentre of an earthquake
FOCUS = point where pressure release occurs in crust where faulting begins
EPICENTRE - point directly above the focus on the surface, closest point to the focus
explain the depths of the focus
SHALLOW focus = 0-70km deep, common along crests of mid-oceanic ridges
INTERMEDIATE focus = 70-300km deep, nearly all occur in Benioff zone
DEEP focus = 300-700km deep, nearly all occur in Benioff zone
what are the 3 types of waves
- P-waves (primary waves)
- S-waves (secondary waves)
- Surface waves (L-waves (long waves) and Raleigh waves)
what is a P wave
- travel fastest through Earth
- are compressional (longitudinal) waves in which rock vibrates back and forth in direction in which waves are travelling
- can move through solids and liquids and arrive at any point at the earth’s surface before next fastest waves
what is an S wave
- travel half the speed of P waves
- shear the rock by vibrating the earth at right angles (transverse) to direction of travel
- cannot move through liquids but do more damage than P waves
what are both P and S waves
- can be called body waves
- travel out from the focus in all directions through the earth’s interior
what do surface waves do
- travel much nearer surface and slower than P/S waves but are more destructive
- L-waves (long waves) shake ground at right angles to direction of wave movement
- Raleigh waves have rolling motion (like water surface wave) that produces vertical ground movement
- they’re especially damaging to buildings and travel along earth’s surface away from epicentre
what is a seismograph and seismometer and what does it record
- P/S waves detected at earth’s surface with seismometer
- seismograph measures the waves, recording time of arrival of wave, and amplitude (depends on magnitude which is amount of energy released)
how is an earthquakes epicentre located
seismic wave behaviour:
- P waves arrive first, then S waves and then L and R
- average speed for waves is known
- after earthquake, difference in arrival times at a seismograph station can be used to calculate distance from seismograph to the epicentre
- time-distance graph shows average P/S wave travel times and farther the seismograph from focus = longer the interval between arrivals of P/S waves
how can a graph help locate the epicentre
- three stations are needed
- circle where the radius equals the distance to epicentre is drawn
- the intersection of the earthquakes is where epicentre is
how is size/strength measured
INTENSITY is subjective measure of damage, isoseismal lines identify areas of equal intensity
MAGNITUDE represented by richter scale measuring total amount of energy released by earthquake, amplitude of largest wave produced is corrected for distance
what are the geomorphological effects of earthquakes
geomorphological effects due to ground shaking causes:
- liquefaction
- landslides/avalanches
- tsunamis
- permanent displacement of land surface
primary vs secondary effects
PRIMARY occur immediately, all due to shaking of the ground eg destruction and collaping
SECONDARY happens afterwards but can be more devastating eg tidal waves, disease, landslides
