GEOL 106 Lecture Notes - Lecture 8: Richter Magnitude Scale, Mercalli Intensity Scale, Strong Ground Motion
15 May 2018
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3.3 Earthquake Shaking
- 3 factors determining type of shaking: earthquake magnitude, location in relation to epicenter and
direction of rupture, local soil and rock conditions
- strong shaking may be expected from moderate magnitude earthquake or larger
EARTHQUAKE MAGNITUDE
- Richter scale by Charles Richter: determined by measuring maximum amount of ground shaking due
to S wave
o seismograph/ seismometer records ground motion in either vertical or horizontal direction
o log scale; scale is specific to only one location (amount of ground shaking is affected by
numerous variables)
- moment magnitude scale: for absolute size of earthquake; measurement of actual energy released
during earthquake
o determined from estimate of area that ruptured along fault plane during quake, amount of
movement or slippage along fault, rigidity of rocks near focus of quake
o estimates can take days to months to calculate (unlike Richter)
- except for large earthquakes, Richter magnitude = moment magnitude
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- more than 3500 very minor earthquakes with magnitude of less than 3 occur each day
- amount of ground motion: related to its magnitude, depth, geologic environment in which it occurs
- as earthquake magnitudes increase, amount of ground motion changes less drastically than amount
of energy released
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EARTHQUAKE INTENSITY
- qualitative; modified Mercalli Intensity scale: 12 categories with Roman numerals
o description of how people perceived shaking from quake and extent of damage to buildings
and other human-made structures
o Mercalli intensity can vary from I to XII depending on distance form epicenter and on local
geologic conditions
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- can also vary from country to country due to inconsistent building codes → generally greater where
human made structures are less well constructed
- based on questionnaires sent to residents near epicenter, newspaper articles, reports from damage
assessment teams
- instrumental intensity: used to immediately produce shake map that shows both intensity of shaking
and potential changes by networks that transmit direct measurements of ground motion as soon as
shaking stops
o shake maps can be used to locate areas where gas lines and other utilities are likely to be
damaged
DEPTH OF FOCUS
- deeper the focus → less shaking that will occur on surface
o seismic waves lose much of their energy during travelling to surface = attenuation = loss of
energy
DIRECTION OF RUPTURE
- although rupture may proceed in many directions from focus, the path of greatest rupture can focus
earthquake energy
- directivity = this behaviour that contributes to amplification of seismic waves and thus to increased
shaking
DISTANCE TO THE EPICENTER
- on seismogram, P and S waves appear as oscillating line that looks similar to electrocardiograph
o P waves travel faster → always appear first on seismogram
o seismologists use the difference between the time that first P and S waves arrive from a quake
to determine how far away the epicenter is
o seismograph that is furthest away from epicenter will observe greatest difference between P
and S wave arrival times
- use difference between P and S arrival times at seismographs in different locations to pinpoint
epicentre of earthquake
o distance to epicenter calculated for each of three seismographs and the respective values are
used for radius of circle drawn around each seismic station
o circles will intersect in one location that is the epicentre
find more resources at oneclass.com
find more resources at oneclass.com
