Chapter 10 Earthquakes
Worldwide distribution of Earthquakes
Release of pent-up energy
Causes of Earthquakes
 Sudden release of accumulated strain energy – at shallow depths, stressed rocks
accumulate strain energy
 Creation of new faults by rupturing rocks
 Shifting of rocks at preexisting faults
 Sporadic recurrence of Earthquakes: Accumulation of Energy ---- Sudden release -----Rocks lock back in place
Seismic Waves
(Earthquake’s energy is transmitted through the earth as seismic waves)
 Two types of seismic waves
 Body waves- transmit energy through earth’s interior
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Primary (P) wave- rocks vibrate parallel to direction of wave
 Compression and expansion (slinky example)
Secondary (S) wave- rocks move perpendicular to wave direction
 Rock shearing (rope-like or ‘wave’ in a stadium)
 Surface waves- transmit energy along earth’s surface
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Rock moves from side to side like snake
Rolling pattern like ocean wave
Primary Waves
Secondary Waves
Two most common types of surface waves
Functioning of Seismograph
Measuring of Earthquakes
 Seismograph- device that measures the magnitude of earthquake
 Seismogram is visual record of arrival time and magnitude of shaking associated with
seismic wave
 Mercalli Intensity scale
 Measured by the amount of damage caused in human terms- I (low) to XII (high); drawback:
inefficient in uninhabited area
 Richter Scale- (logarithmic scale)
 Magnitude- based on amplitude of the waves
 Earthquake total energy- uses moment magnitude scale
Measuring Earthquake-contd.
 Richter Scale
– Amplitude scale is logarithmic (10-fold increase for every whole number increase)
– Scale 1 ---- 0.001 mm; 2---- 0.01 mm; 5---- 10mm; 7---- 1 meter
– Earthquake Energy: Each whole number represents a 33-fold increase in Energy; Energy difference
between 3 & 6 means ~1000 times
– Drawbacks:
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California Rocks
Based on Antiquated Wood-Anderson Seismographs
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Measurment Past Magnitude 7.0 ineffective – Requires Estimates (Scale 8 corresponds to 10m)
Measuring Earthquake-contd.
 Moment-Magnitude Scale
– Seismic Moment Factors
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Length of Fault Rupture
Depth of Fault Rupture
Amount of Slip along Rupture
– Moment = (Total Length of Fault Rupture) X (Depth of Fault Rupture x Total amount of
Slip along Rupture x Strength of Rock)
– Measurement Analysis requires Time
Locating Epicenter and Focus Depth (EQ Classfication)
 Use Arrival time at a recording station (time lag between P & S waves) to locate the epicenter
of an earth quake
 Need three stations to determine the epicenter
 Depth of Focus
 Shallow focus EQ < 70 km (45 mi) most earthquakes
 Intermediate focus EQ- 70-300 km (45- 180 mi)
 Deep focus EQ- > 300 km (> 180 mi)
Seismograph Waves
Epicenter
Liquefaction
Frequency vs Depths
 90% of Earthquakes occur within depths less than 100 km
 Majority of Catastrophic Earthquakes occur within Depths less than 60-km Deep
– 1964 Alaska EQ ---- 33 km from surface
– 1995 Kobe, Japan--- 20 km from surface
Magnitude vs Depth:
a) Shallow: Up to 9.5 RM (Moment-Magnitude Scale)
b) Intermediate: Up to 7.5 RM
c) Deep: Up to 6.9 RM
Earthquake Depth
Map of Tennessee Earthquake
Graphs & Maps of Denver Earthquake
Graphs & Maps of Denver Earthquake-contd.
 Ground Displacement
Effects of Earthquakes
 Lateral and vertical (In 30-Myr, Rocks & Landforms on the West Side have shifted ~560-km toward Northwest); Vertical
Displacement occur during movement along Dip-slip Faults
 -Landslides
 Liquefaction
 Conversion of formally stable fine grain materials to a fluid mass
 Seiches
 The back and forth movement in a semi-closed/closed body of water- could cause flooding- Alaskan EQ in 1964 – 6000 km
away felt in TX Swimming pools
 Tsunamis More from submarine landslide (Large fast-moving sea waves); Sea-floor displacement during faulting and submarine slides
 Fire (Tokyo 1923; San Francisco, 1906)
Principal Earthquake zones
 Earthquake zones at Plate Boundaries
– Shallow Earthquakes occur at
– Oceanic Divergent zones, continental rift and collision zones, and transform boundaries
– Subduction-zone earthquake regions are called Benioff-Wadati zones
– In subduction zones: Depth of earthquakes correspond to depth of portions of descending slab; up to
300-km deep: strong earthquakes; 300-700 km: weaker earthquakes; >700 km: earthquakes are rare
– Magnitude of EQ ~80% of EQ Energy released in Pacific Rim Region; ~20% of world’s EQ energy
released in collision Zone from Turkey to Burma
Specific Areas
 Japan: Subduction of Pacific Plate beneath Eurasian Plate; Quakes occur in Tokyo
every 69 yrs; 15% of world seismic energy released
 Alaska: Pacific Plate subducts beneath North American and Eurasian Plates
 Mid-plate Eqs are shallow, weak
Stratigraphy of a fault zone
Hidden faults
Maps of seismic gaps around the Pacific Ocean
A Close-up Map of S. California
Dilatancy of stressed rocks
Dilatancy of stressed rocks-contd.
Coping with Earthquakes
 Earthquake zone-identification
 Plate boundaries
 Assessing local seismic history and future risks
 Land use planning-situating critical facilities
 Quake reinforcement of building/structures: wood, steel, reinforced concrete are
preferable (heavy masonry, unreinforced concrete, etc are not good)
 Short term and Long term forecast
 Contingency plan
USGS-Survey Plan
USGS-Survey Plan-contd.
Earthquake Prediction
 Successful Prediction in 1975 in China, but no prediction in 1976
 Reduction of Energy build up along segments of individual faults – Plans to lubricate
San Andreas Fault
 Increase in Radon concentration as a precursor (and other noble gases, solubilites of
these gases)
 Animal Behavior??
Earthquakes in Moon
CHAPTER SUMMARY
CAUSES OF EARTHQUAKES
P-WAVES, S-WAVES, VELOCITIES
SEISMIC WAVES, SEISMOGRAPH, SEISMOGRAM, SEISMOMETER
RICHTER SCALE-AMPLITUDE AND ENERGY
SEISMIC MOVEMENT, STRENGTH OF ROCKS
SEICHE, TSUNAMI
CAUSES FOR THE EARTHQUAKE IN JAPAN, CALIFORNIA
SAFEST LOCATION FOR EARTHQUAKE – LOG CABIN VS BRICK HOUSE
RADON AS A PRECURSOR