Learning objectives Understand the relationship of earthquakes to faulting Familiarization with earthquake & wave (energy) terminology Understand the.

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