Transcript Earthquakes: fault classification, terminology, stress

Japan Earthquake and Tsunami
What happened?
• Large earthquake
• Earthquake hazards:
– Tsunami
– Ground shaking
– Liquefaction
– Landslides
• People and structures in the way of the
hazards = catastrohe
Note:
• Japan is a developed country: per capita
income is $32,433
• Well prepared for earthquakes in terms of
monitoring, education, and preparedness
• 8,000 fatalities and 12,000 missing
Japan was prepared. The earthquake and tsunami caused so much
destruction that emergency services and education of population were
not enough to save many lives.
Today’s lecture
• What is an earthquake?
– Causes of earthquakes
– Foreshocks and aftershocks
– Earthquake terminology
– Fault classification
What is an earthquake?
Release of stored energy
• Elastic rebound theory explanation to how
earthquakes occur
• Plate movement concentrates energy in crust
• When the stored energy exceeds the strength
of the crust, the crust ruptures
• The rupture generally occurs along faults
because this is the weakest point
• Japan’s earthquake was produced on a plate
boundary
Japan moved eastward 8 feet
How Faulting Generates Earthquakes
• Movement on the fault causes a release in
energy
• As the energy passes through an area, the
vibration is felt
• The energy is transferred through the earth
and man-made structures
• The bigger the amount of slip the more energy
released and therefore, the more vibrations
are produced
Causes of Earthquakes
• Tectonic stress (most
common)
• Water added under
pressure
• Geothermal gradient
(variation due to
boundary)
• Rock type
• Fast/cold versus
slow/warm rate and
temperature
Causes of Earthquakes
• Stress accumulation and energy release at
plate boundaries.
Types of Stress (think of plate boundaries)
• Compressional stress- crust shortens
• Tensional stress- crust thins
• Shear stress- one piece of crust slides past
another piece of crust
Stress: causes rock to change volume
or shape
Permanently
deformed
Maximum
strength
before
rupture or
failure
If stress is
released the
material will
return to the
original shape
• Strain measures the amount of deformation
Response to stress
Brittle : rock breaks
Ductile: rock folds
Geothermal gradient
• Quartz makes a transition from brittle to ductile at
about 350 degrees centigrade
• This is to a depth of about 12 miles in California
Fast/cold versus slow/warm
Fast/cold versus slow/warm
1906 San Francisco Earthquake: fast
movement causes offset
Offset: amount of
displacement
Marin County, 16 feet of offset
Calaveras Fault: creep causes deformation
Elastic Rebound Theory: Reid, 1906
Elastic Rebound Theory
earthquake
• Stress is added to the crust
• Strain is accumulated and deformation occurs
• Stress exceeds the frictional strength of the
fault plane then rupture occurs
The San Andreas fault: right lateral strike-slip
Strike-slip Fault Example
(which way is the window relative to the manure
pile?)
Fault
scarp
1906 San Francisco Earthquake
Japan is part of the Ring of Fire
Ring of Fire: trenches and associated
subduction zones that surround the Pacific
Ocean
Tectonic Plates
Tectonic plates may be composed of oceanic crust, continental crust or
both types of crust. Describe the extent of the North American plate and
the Pacific plate.
Tectonic Setting: complex
Magnitude 9.0 NEAR THE EAST COAST OF HONSHU, JAPAN
Friday, March 11, 2011 at 05:46:23 UTC
This earthquake was the
result of thrust faulting along
or near the convergent plate
boundary where the Pacific
Plate subducts beneath
Japan.
This map also shows the rate
and direction of motion of the
Pacific Plate with respect to
the Eurasian Plate near the
Japan Trench. The rate of
convergence at this plate
boundary is about 83 mm/yr
(8 cm/year). This is a fairly
high convergence rate and
this subduction zone is very
seismically active.
Japan Trench
Magnitude 9.0 NEAR THE EAST COAST OF HONSHU, JAPAN
Friday, March 11, 2011 at 05:46:23 UTC
What does the pink line represent?
This earthquake occurred 130 km (80
miles) east of Sendai, Honshu, Japan and
373 km (231 miles) northeast of Tokyo,
Japan.
Images courtesy of the US Geological Survey
Magnitude 9.0 NEAR THE EAST COAST OF HONSHU, JAPAN
Friday, March 11, 2011 at 05:46:23 UTC
The map on the right shows historic
earthquake activity near the epicenter
(star) from 1990 to present.
As shown on the cross section,
earthquakes are shallow (orange dots) at
the Japan Trench and increase to 300 km
depth (blue dots) towards the west as
the Pacific Plate dives deeper beneath
Japan.
Seismicity Cross Section across the subduction zone
showing the relationship between color and
earthquake depth.
Images courtesy of the US Geological Survey
Magnitude 9.0 NEAR THE EAST COAST OF HONSHU, JAPAN
Friday, March 11, 2011 at 05:46:23 UTC
Globally, this is the 4th largest earthquake since 1900.
Great (M > 8) Earthquakes Since 1900
9.6
Chile 1960
9.4
Alaska 1964
9.2
Magnitude
Sumatra 2004
Russia 1952
9
8.8
Japan 2011
Ecuador 1906
Chile 2010
Alaska 1965
8.6
8.4
8.2
8
7.8
1900
1920
1940
1960
Year
1980
2000
2020
Foreshocks and aftershocks
• Relative measurement
• Foreshocks: smaller
earthquakes before the
main event;
– a portion of the fault
moves
• Aftershocks: larger
earthquakes after the
main event;
– Adjustment of the fault
plane
Worldwide Seismicity
Classification of Faults
Fault scarp: a portion of
the fault plane exposed
after an earthquake
Fault plane: described by an
area; length x width; where
movement occurs
• Based on relative movement
along the fault plane
Focus or hypocenter: point of
movement initiation
Epicenter
• Point on the Earth’s surface directly above the
hypocenter or focus
• The earthquake is generally named after the
epicenter
USGS
Phil Stoffer, USGS geologist
Cold, brittle crust breaks and moves
•
•
•
•
Compressional stress causes reverse faults
Extensional stress causes normal faults
Shear stress causes strike-slip faults
Oblique movement on strike-slip faults occurs
when there is also vertical slip along the fault
plane
Thrust Fault
Waterton Lakes National Park, Alberta,
Canada
Alps
A close-up of the thrust plane at this location. The rocks underlying the fault plane
are intensely deformed
Older rocks on top of younger rocks
Strike-slip faults
North Anatolian Fault, Turkey
Identification of faulting
• Rocks along fault may be ground up or
polished
• Ground up rocks are easier to erode so
often depressions on the Earth’s surface are
indicative of active faults.
Identification of faulting
• Rocks along fault may
be ground up
• ground up rocks are
easier to erode, so
linear gullies or valleys
form
Fault trace of the San Andreas Fault
LIDAR image, similar location
Prince William Sound, 1964 Alaska earthquake, marine terrace
exposed
Flat surface formed by wave action below sea level. Uplifted above
sea level during the earthquake.
Uplifted marine
terraces, California
coast north of Santa
Cruz
Michael Rymer, USGS
Identification of faulting
• Offset is the distance
of displacement along
the fault plane
• Offset features such as
offset streams, roads,
fences are indicative
of movement
• Changes in
topography
Identification of faulting
• Fault Scarp produced
by fault movement
• When fault plain rises
higher than the Earth’s
surface
• Hector Mine
Earthquake, 1999
• Mojave Desert
Understand the relation between
tectonic setting, stress, and fault type
Tectonic setting Stress
Fault