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Lecture

EARTHSC 2GG3 Lecture Notes - Mercalli Intensity Scale, Richter Magnitude Scale, Convergent Boundary


Department
Earth Sciences
Course Code
EARTHSC 2GG3
Professor
Sergei Basik

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Earthquakes
Where do earthquakes occur?
Mostly around convergent plate boundaries
Grenville (S. Ontario) faults in this region are somewhat active and cause minor earthquakes
Most earthquakes (3 plates) coast of B.C.
After affects can be worse than earthquakes itself (ex. Tsunami caused by earthquake)
Waves
Period the time between seismic waves
Wavelength the length from crest to crest of a wave
Frequency the number peaks per second
Seismograph
Tool to measure and record earthquake waves
Old fashioned seismographs also used today
Body waves occur FIRST
After S wave when energy hits earth’s surface
Types of Seismograph:
Short-period seismographs: record local earthquakes (high freq)
Long-period seismographs: record distant earthquake (low freq)
Broadband seismographs: record both local and distant earthquakes; but cannot accurately
measure strong earthquakes in the direct vicinity
Strong-motion Seismographs: record local and very strong earthquakes
Earthquake Wave Types
Body Waves (go through earth’s surface)
P waves (primary or compressional): wave shakes back and forth along the direction of wave
travel
8 km/s upper mantle
5-6 km/s continental crust
S waves (secondary or shear): shakes back and forth perpendicular to the direction of wave
travel (cannot travel through a liquid)
4.5 km/s upper mantle
3.5 km/s crust
Surface Waves (slower)
2 4.5 km/s
Love Waves : move side to side
Rayleigh Waves: move up and down

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Basic Fault Types
Normal Fault
Lower wall will fall
relative to upper wall
Reverse Fault
Lower wall will rise
relative to upper wall
Thrust Fault
Move over each other
Strike-Slip Fault
Back and forth (slipping
Elastic Rebound Theory
The theory applied to
most earthquakes in
which movement on two
sides of a fault leads to
bending of the rocks until
they slip and snap to
release the bending strain
Stress: the force on a
body
Strain: change in size or
shape of a body in
response to an imposed
stress (how much is body
going to deform)
Faults changes, rock goes
back to normal shape
Earthquake Epicentre
Earthquake occurs and energy radiates
Focus is where rupture occurs
Epicentre is origin of Earthquake
Take reading from different seismographs and measure
You don’t have to be right next to epicenter to determine where it is
Determined by a logarithmic scale measure difference between p and s waves (nomograph chart)
and compare to amplitude of waves
Intensity Scale
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