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Learning objectives• Understand the relationship of earthquakes to faulting• Familiarization with earthquake & wave (energy)
terminology• Understand the concept of earthquake magnitude (and
its calculation)• How seismic risk is estimated• Familiarization with the major effects of earthquakes• The prediction of earthquakes • Mitigation of earthquake damage
Earthquake Processes
Earthquakes:• Occur predominately at plate boundaries• Mainly in conjunction with faults• Due to movement along fault planes
• Relationship to plate tectonics evidenced by:– Geographic distribution & correlation with plate boundaries– Relationship to plate boundaries
• Shallow earthquakes (at both divergent and convergent boundaries)• Deep earthquakes (at convergent boundaries; subduction zones)
Earthquake Terminology• Seismic (seismicity): (earth) movement/shaking relating to
earthquake (also artificial) vibration/activity
• Focus: Actual (centered) area of movement on the fault plane (where the rupture starts)
• Epicenter: Point on the earth’s surface directly above the focus (apparent focal point)
• (more later)
Map of world-wide seismicity
Major Earthquake Belts
Types of Plate Boundaries & Seismicity• Divergent-Margin Earthquakes
• Convergent-Margin Earthquakes
• Transform-Margin Earthquakes
• Intraplate Earthquakes– Basin and Range; Mid-Continent
Dip-Slip• normal• reverse (thrust)
Strike-Slip• Left Lateral• Right Lateral
Mid-Continent Faulting/Seismicity: Basin-Range Type
Fault-Related Landforms
+ Wine glass valleys
Slip Rate & Recurrence Interval
• Slip Rate: Rate of displacement per year (units of e.g., mm/yr; 1m/1000 yrs = 1 mm/yr)
• Recurrence Interval: based on– Seismicity: Historical time interval between events
– Paleoseismic data: Time interval between events from the geologic record
– Recurrence Interval; displacement/event slip rate• e.g., 1m 2mm/yr = 500 yr interval
Seismic Waves and Ground Shaking
• Types of seismic waves– Body waves: waves travel within the earth
• P- waves: Primary compression waves
• S- waves: Shear waves
– Surface waves: waves that travel only along/within surface layers (shallow)
• L-(Love) waves: horizontal ground movement
• Rayleigh waves: rolling motion
• Focus: Point/area where rupture starts• Epicenter: point on earth’s surface directly
above the focus
Waves=Forms of energy release• Motion/propagation types• Frequency: Number of waves passing
a reference point/sec (in Hz)
• Period: Number of seconds between successive peaks
• Amplitude: Measure of ground motion
• Attenuation/amplification
Seismic Waves
Nature & Propagation of Seismic Waves
• Waves travel at different rates, & depend on:– intrinsic velocities
• P-waves: 5.5 km/sec
• S-waves: 3 km/sec (cannot propogate through liquids)
– rock material type (density); denser faster; less amplification
– refraction– reflection
Comparing/Measuring Earthquakes• Magnitude
– Measure of energy released (log scale)
– measurement scale = Richter scale (0-10)
• Intensity:– Relative scale: based on perceived damage
– Modified Mercalli Scale (1-12)
• Ground acceleration during earthquakes– Rate of change of horizontal or vertical velocity of the ground
– Normalized/compared to earth’s gravity; 9.8 m/sec2= 1g
– e.g., M =6.0-6.9 quake 0.3-0.9 g
Measurement Methods: Seismometers• Horizontal Motion• Vertical Motion
Seismograms
0
200
400
600
800
1000
1200
0 50 100 150 200 250
Arrival Time (sec)
Dis
tan
ce
Tra
ve
led
(k
m)
P-Waves: 5.5 km/sec
S-Waves: 3 km/sec
S-P Interval
0
500
1000
1500
2000
2500
0 50 100 150 200 250 300 350
S-P Arrival Interval (sec)
Dis
tan
ce
(k
m)
X
Calculation of Magnitude & Location of Epicenter
•P-waves: 5.5 km/sec
•S-waves: 3 km/sec