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
Strike-Slip
• normal
• reverse (thrust)
• Left Lateral
• Right Lateral
Mid-Continent Faulting/Seismicity: Basin-Range Type
+ Wine glass valleys
Fault-Related Landforms
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
• Focus: Point/area where rupture starts
• Epicenter: point on earth’s surface directly
above the focus
• 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
Seismic Waves
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
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
1200
P-Waves: 5.5 km/sec
Distance Traveled (km)
1000
800
S-Waves: 3 km/sec
600
400
S-P Interval
200
0
0
50
100
150
Arrival Time (sec)
200
250
2500
Distance (km)
2000
1500
1000
500
X
0
0
50
100
150
200
S-P Arrival Interval (sec)
250
300
350
Calculation of Magnitude & Location of Epicenter
•P-waves: 5.5 km/sec
•S-waves: 3 km/sec