Earthquake Size
Earthquake Size and Characteristics
— How big is it?
— Answer in terms of perceived effects:
intensity
— Answer in terms of amount of energy
released: magnitude
Earthquake Intensity
—
Mercalli Intensity Scale developed in 1902
◦ Based on damage and how much people feel shaking
—
Modified Mercalli Intensity Scale maps reflect
subjective observations of people and objective
descriptions of damage
◦ Strongest intensities near epicenter and where ground
conditions favor strong ground shaking
—
Computer-generated ShakeMaps show maximum
ground movement for potential earthquakes
◦ Used to send emergency response to areas with likely
greatest damage
Earthquake Magnitude
—
Richter Magnitude Scale (ML) derived by Charles
Richter in 1935
◦ Based on maximum amplitude of earthquake waves
measured on Wood-Anderson seismograph
(adjusted as though 100 km from earthquake)
◦ Logarithmic scale to compare earthquakes
◦ Amplitude 10 times greater on seismographs
corresponds to magnitude greater by 1.0
Fig. 3-24, p. 48
Estimating
Earthquake
Magnitude
A Nomograph chart
uses the distance
from the earthquake
(P − S time in
seconds) and the Swave amplitude (in
mm) to estimate the
earthquake
magnitude.
Earthquake Magnitude
—
Surface-wave magnitude MS
◦ Similar calculation as for ML
◦ Surface waves usually have largest amplitude on
seismogram
—
Body-wave magnitude MB
◦ Measured from amplitude of body waves
—
Can calculate magnitude from nomograph
◦ Straight line between P – S time and S-wave
amplitude intersects central line at magnitude of
earthquake
Earthquake Magnitude
Larger earthquakes (above 6.5): strongest
shaking with lower frequency, sometimes
below frequency range of seismograph
§ Richter magnitude less accurate above
6.5
§
§
Moment magnitude (Mw) is measure of
total energy expended during earthquake
§ Determined from long-period waves
§ Seismic moment equals (shear strength of
rocks) x (surface area of rupture) x (slip
distance on fault)
Energy of Different Earthquakes*
A Richter magnitude difference of
◦ 0.2 is ~2 times the energy
◦ 0.4 is ~4 times the energy
◦ 0.6 is ~8 times the energy
◦ 1.0 is ~32 times the energy
◦ 2.0 is >1000 times the energy
◦ 4.0 is >1,000,000 times the energy
Earthquake Magnitude
—
Many small events, fewer large ones, rare giant event
—
Gutenberg-Richter frequency-magnitude
relationship: straight-line plot of all earthquakes of
certain size against frequency of occurrence
—
Most energy release from largest earthquakes
◦ 1 magnitude 8 earthquake ~= 1000 magnitude 6
earthquakes
—
The size of an earthquake, as estimated from the
degree of damage at various distances from the
epicenter, is recorded as the Mercalli Intensity.
—
The strength of an earthquake, recorded as the Richter
magnitude, can be determined from its amplitude on a
specific type of seismograph.
—
An increase of one Richter magnitude corresponds to a
10-fold increase in ground motion and about a 32-fold
increase in energy.
Earthquake Size and Characteristics
—
Small, frequent earthquakes are caused by short fault
offsets and rupture lengths
—
Larger earthquakes are less frequent, with longer
offsets and rupture lengths
—
Giant earthquakes are infrequent and caused by
extremely long fault offsets and rupture lengths.
Earthquake Size and Characteristics
Ground Motion and Failure
During Earthquakes
—
How much and how long the ground shakes during an
earthquake depends on
◦ How much fault moved
◦ Where fault moved
—
Relationship between earthquake magnitude and
ground motion
—
Local conditions amplify shaking and increase damage
Ground Acceleration and Shaking Time
—
Acceleration – percentage of acceleration of gravity (g)
—
Duration of shaking depends on size of earthquake
—
Increase above magnitude 6 increases area and total
time of shaking
◦ Does not cause much stronger shaking
—
Amount of shaking decreases with distance from
earthquake
—
Severity of shaking also depends on type of material
waves travel through
◦ Softer material à more intense shaking
—
Activity
PASTA QUAKE
—
Video
MOMENT MAGNITUDE