EOSC114 - Earthquakes - Part 2
Earthquakes result from the rupture of rocks along a fault
Fault: a fracture in the earth’s crust
Seismic waves: the form in which energy is released when an earthquake occurs
Focus/Hypocentre: the point at which the earthquake originates from (initial rupture),
where seismic waves radiate from
Earthquakes are compared by the amount of energy released, their magnitude, and
by their intensity
Magnitude: can be measured by moment magnitude, the Richter scale, etc.
Intensity: the effects of ground motion on people and structures
Epicentre: the point on the surface of the earth directly above the fault rupture.
Richter Scale: expressed as a single decimal number (ex. 6.8), and quantifies the
magnitude of earthquakes as the logarithm to the base 10
o News reports still use the Richter scale, however, seismologists now use moment
Moment magnitude: determined from an estimate of the area that ruptured along a fault
plane during the quake, the amount of movement or slippage along the fault, and the
rigidity of the rocks near the focus. (ex. An increase from one whole number to the next
represents a 10-fold increase in the amount of shaking and a 30-fold increase in the
amount of energy released
Moment magnitude~=Richter scale, except for with large quakes
Magnitude and Frequency of Earthquakes Worldwide
Descriptor Magnitude Avg. Annual # of Events
Great 8 and higher 1
Major 7-7.9 17
Strong 6-6.9 134
Moderate 5-5.9 1319
Light 4-4.9 13,000 (estimated)
Minor 3-3.9 130,000 (estimated)
Very Minor 2-2.9 1,300,000 (estimated - approx. 150/hour)
Modified Mercalli Intensity Scale: measures the degree to which an earthquake affects
people, property, and the ground.
ShakeMaps show both perceived shaking and potential damage (valuable to
emergency response teams who must locate and rescue people)
Modified Mercalli Intensity Scale (abridged)
I Felt by very few ppl
II Felt by only a few ppl at rest, esp on upper floors. Delicate suspended
objects may swing.
III Felt noticeably indoors, esp on upper floors, no recognized as earthquake.
Vibration feels like truck passing.
IV Day= felt by many indoors, few outdoors. Night=some ppl awakened.
Walls make cracking noise. Stationary cars rock noticeably. Sensation= heavy truck striking a building.
V Felt by nearly everyone, many awakened. Some dishes and windows
broken, unstable objects overturned. Disturbed tall objects sometimes
VI Felt by all; many ppl frightened and run outside. Some heavy furniture
moved. Some fallen plaster/damaged chimneys. Slight damage.
VII Almost everyone runs outdoors. Damage: well built=negligible,
moderate=slight, poor=considerable. Noticed by ppl driving cars.
VIII Damage: well-built=slight, moderate=considerable, poor=great. Tall walls
and columns collapse. Heavy furniture overturned. Changes in well water.
Disturbs ppl driving cars.
IX Damage: considerable in all makes of buildings. Some buildings shifted off
of foundations. Ground cracks conspicuous. Underground pipes are
X Some well-built wooden structures destroyed. Most structures