Download MYSTERIES OF PLANET EARTH

Chapter 10 Earthquakes
Definitions
Earthquake-
the vibration of the ground due to the
sudden release of energy accumulated in a deformed
rock
Focus
(Hypocenter)- spot underground where the
rock begins to break
Epicenter-
the point on the land surface directly
above the focus
Aftershock-
tremors that occur as rocks adjust to
their new position
Seismology-
the study of earthquake
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


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


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:
 California Rocks
 Based on Antiquated Wood-Anderson Seismographs
 Measurment Past Magnitude 7.0 ineffective –
Requires Estimates (Scale 8 corresponds to 10m)
Measuring Earthquake-contd.

Moment-Magnitude Scale
– Seismic Moment Factors
 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.
Effects of Earthquakes

Ground Displacement
 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