Chapter 10 Flashcards
Earthquakes
Earthquake
The vibration/shaking of the earth caused by a sudden release of energy.
Elastic rebound
Rock is strained around the fault and stores elastic energy for thousands of years. When the stress is greater than the frictional fault strength, kinetic energy is released as an earthquake and the rock springs back to the original shape.
Causes of earthquakes (3)
Fault movements related to tectonic activity, movement of magma below volcanoes, human activity.
Hypocenter:
Location where the fault initially breaks or explosion occurs. Energy spreads out from the focus like ripples on a pond.
Epicenter
Point on the earth’s surface directly above the hypocenter
Seismology
Study of earthquakes
Seismograph
Instrument that measures ground movement as earthquake occurs
Seismogram
Record of an earthquake made on a seismograph
Seismologist
Person who studies seismology
Seismic Waves
Generated by an earthquake and travel away from the hypocentre. Separated into body waves and surface waves.
Body waves
Waves that travel through the Earth (parallel and shear)
P-waves
Compressional and longitudinal body waves that are faster than S-waves and travel parallel to the direction of travel. Divided into reflection and refraction
S-waves
Shear and transverse body waves that travel normal to the direction of travel and can’t go through liquids.
Surface waves
Waves that travel along or just below the surface with an amplitude that decreases with depth. (Rayleigh and Love waves)
Rayleigh waves
Waves that travel in a retrograde elliptical particle motion
Love waves
Waves that travel with horizontal (back and forth) motion
P-wave velocity through sandstone
2.0-2.5 km/s
P-wave velocity through quartzite
5.5-6.0 km/s
P-wave velocity through limestone
5.0-5.5 km/s
P-wave velocity through granite
5.5-6.0 km/s
P-wave velocity through gabbro
6.5-7.0 km/s
P-wave velocity through ultramafic rocks
7.5-8.5 km/s
P-wave velocity through air
0.3 km/s
P-wave velocity through water
1.4-1.5 km/s
P-wave velocity through petroleum
1.3-1.4 km/s
How to locate the source of an earthquake (5 steps)
- Measure the arrival times of P-waves and S-waves waves and find the difference in time between them.
- Compare this to the calculated travel times and look for the distance between lines where the difference in arrival times is the same.
- Calculate The Distance To The Earthquake
- Repeat steps 1-3 at at least three seismic stations.
- Draw a circle at each station. The intersection of these circles is the epicenter.
Results of locating the source of an earthquake (3)
- Direction of the waves
- Time at which the earthquake occurred
- Depth of the hypocenter
Earthquake distribution
80% of all earthquakes occur in the circum-Pacific belt. 15% within the Mediterranean Asiatic belt. 5% occur along oceanic spreading ridges or within plate interiors.
Annual earthquake numbers
900,000+ earthquakes occur per year with 31,000+ of them strong enough to be felt.
Wadati-Benioff zones.
Dipping seismic zones in a subduction zone that defines plate geometry.
Magnitude
Quantitative measure of the energy released by an earthquake
Richter scale scale
Increase of 1 in magnitude is a 10-fold increase in amplitude and a 30-fold increase in energy.
How to use the Richter scale
- Measure A = maximum amplitude of oscillation with period of 0.8 s in mm
- Determine the distance to epicenter (Δ) in kilometers from time between P-wave and S-wave arrivals.
- Use M = log(A) + 2.76log(Δ) - 2.48
Seismic-moment magnitude scale
Uses the area of the fault that slips and offset (D) to compute the magnitude.
Seismic wave velocity ranking
- P waves (fastest)
- S-waves
- Surface waves.
Seismic wave propagation
As pressure increases, P-wave velocity increases and causes P-wave paths to bend.
Where the composition changes, we have a discontinuity
Repeat time of Canadian earthquakes
450 years
Direct earthquake destruction
Multi-story concrete buildings are prone to collapse with horizontal ground motion (S-waves).
Steel Frame Buildings Do Much better as they flex.
Factors determining earthquake intensity (7)
- Earthquake magnitude
- Distance from the epicenter
- Focal depth of the earthquake
- Population density
- Building construction
- Duration of ground shaking
- Near surface geology (rock type, amplitude of wave increases in softer material)
Indirect earthquake destruction (4)
Liquefaction (soil loses its cohesive strength during ground shaking and acts like dry sand), fires, tsunamis, landslides
Earthquake induction
Earthquakes can be induced in the earth by the injection of water, and by filling reservoirs. However, it might be possible to release small amounts of the energy stored in rock sand thus avoiding a large earthquake.
One possible means of earthquake control is fluid injections along locked segments of an active fault.
Crust
Felsic rocks
Mantle
Mafic rocks
Mohorovicic discontinuity
30-40 km thick separation between the crust and mantle.
P-Wave velocity rule
Gets faster the deeper in the earth, the more magic the rock
