Download Earthquake

Geology
Plate tectonics – the dynamic Earth
The rock cycle – from a tectonic view
Earth’s structure and geophysics
Earthquakes
Plate tectonics – the dynamic Earth
Plate tectonics is one of the most important scientific revolution of the
20th century.
It provides an explanation for practically all observations on the
surface of the Earth, and processes originating below the surface.
The Old Theories
1. The “Shrinking Apple”
After being hot planet Earth was cooled and shrank
2. The concept of Geosyncline:
In the 19th century: sedimentary rocks accumulate in basins, magmatic
rocks uplifted by internal processes created mountain ranges.
1. “The moving continents” - Alfred Wegener proposed “Continental Drift” in
1915 he noticed that continents fitted together like pieces of a puzzle.
The theory was not accepted at the time - scientists couldn’t imagine horizontal
movement of the crust and no one knew what would drive such motion.
2. The continents don’t only fit together –
There is an overlap of fossil assemblages, structures, and rock types
up to the time in geologic history when the plates split apart.
3. Glacial deposits from the same period in the past can be
found in different latitudes in the Southern Hemisphere.
4. The mountain belts, volcanic activity and earthquakes are localized along
narrow belts such as “The Ring of Fire”
5. Sea floor maps reveal very interesting and strange data:
A. The depth of most of the sea floor is 4 km
B. There is more then 70,000 km of mountain belts (ridges) in the
oceans. Most of them are built from volcanic rocks.
C. There are very deep trenches along the margins of the pacific ocean,
near Asia and South America.
6. There are magnetic stripes along the Mid-Atlantic Ridge.
Symmetry can be found by measuring the magnetic data.
Actual Data (South of Iceland)
Magnetic ``Stripes'' on Seafloor
Tectonics plate boundaries
Divergent Plate Boundaries
1. Mid-Ocean Ridges (spreading centers)
2. Continental rift
1.Rifting: Our Rift Valley
2. Embryonic ocean: The Red Sea
3. Young Ocean: The Atlantic Ocean
Mid-Atlantic Ridge
(color indicates seafloor age)
Generation of sea floor magnetic anomalies
Divergent Plate Boundaries
= Plate Tectonic Forces
Plate #1
Plate #2
faulting
• Extension, spreading, rifting, tension, thinning of the
lithosphere, normal faulting
Convergent Plate Boundaries
Subduction Zones: Oceanic-Continental
Subduction Zones: Oceanic vs. Oceanic
Pachapaqui Mining Area, Andes, Peru
Machu Picchu, Andes, Peru
Mt. Fuji, Japan
Convergent Plate Boundaries
Continental Collision Zones: Continent-Continent
Himalayas and Tibet
Because both are buoyant,
neither continental plate subducts
* broad areas of high elevation (large
mountain ranges and high plateaus)
result.
* No volcanism.
Convergent Plate Boundaries
= Plate Tectonic Forces
Plate #1
•
•
Plate #2
Convergence, compression, mountain building, thickening of the
lithosphere, thrust faulting
Types:
– Ocean-ocean (subduction; island arc; volcanoes)
– Ocean-continent (subduction; long mountain chains; volcanoes;
strongest earthquakes)
– Continent-continent (continental collision zone; wide, high
mountainous region; no volcanism)
‫סדרו לפי הסדר ותארו את התהליך‪ .‬אילו סוגי גבול לוח מתוארים?‬
Tectonics plate boundaries
Transform Plate Boundaries
Mid-Atlantic Ridge
(ocean floor age)
Divergent
Transform
Divergent
http://www.ngdc.noaa.gov/mgg/image/images.html#crustage
Transform Plate Boundaries
= Plate Tectonic Forces
Plate #2
Plate #1
•
•
•
Shear, no change in lithospheric thickness, strike-slip faulting
No volcanism, no significant change in topography
Shallow faulting
Tectonics - plate configuration
Tectonics – what drives the motion:
What causes the plates to move?
Mantle convection produces lateral forces at the boundary
between the mantle and the crust
Ridge push
Trench rollback
Slab pull
Hawaii Islands – Hot spot
Oceanic
plate
move
above hot
spot
Mantle
Plume
As the plate moves, each island is
dragged away from the heat source
and a new volcano forms.
Tectonics – the continent cycle and the rock cycle:
Tectonics – the continent cycle:
.1
.2
.5
:‫דוגמאות‬
:‫מחזור ווילסון‬
Earth’s structure:
The Earth Interior’s
mantle convection
Heat transfer from the core to the
mantle produces slow convection
of the mantle material
(in the order of centimetres per year)
1.
Volcanoes are found at which type of plate boundary?
a)
b)
c)
d)
e)
2.
Convergent (continent-continent)
Transform
Subduction zone
Divergent
Both c and d
Seafloor magnetic anomalies
a)
b)
c)
d)
e)
documents the history of magnetic polarity reversals on Earth
are associated with island arc formation
form at ocean-ocean convergence zones
are caused by sediment accumulating atop the seafloor
trend perpendicular to mid-ocean ridges
‫עוצמת השדה המגנטי גדולה יותר ליד הקטבים‪,‬‬
‫כוח המשיכה קטן יותר במשווה כי כדה"א פחוס‬
Earthquakes
Basic Terminology
•
•
•
•
Fault: discontinuity in the
Earth’s crust; the two sides
of the fault may move
relative to one another
(analogous to fracture or
crack)
Earthquake: abrupt motion
across a fault
Focus (a.k.a. hypocenter):
point
within the Earth where
the fault rupture starts
Epicenter: point of the
Earth’s surface directly
above the focus
‫רעידות אדמה וסיכונים סיסמיים‬
Earthquakes: Types of faults
• Three types: Normal, thrust and strike-slip
How do plate motions
cause earthquakes?
Earthquakes: Seismology
• Seismic waves:
Vp= ~6 km/sec
Vs= ~3 km/sec
Fast P-wave
Surface waves
Tp
Ts
S-wave
Earthquakes: Seismology
• Measurement of seismic waves from earthquake
Fast P-wave
Surface waves
Time between arrivals
determines the distance from
the earthquake hypocenter.
Tp
DT
Ts
S-wave
Why is this so important?
Earthquakes: Seismology
• Measurement of seismic waves from earthquake
Fast P-wave
Surface waves
Time between arrivals
determines the distance from
the earthquake hypocenter.
Tp
DT
Ts
Why is this so important?
S-wave
X=VT
DT=Ts-Tp
;
DT=X/Vs–X/Vp = …= X(Vp-Vs)/VpVs
x
Vp
Vs
t
;
X=DTVpVs/(Vp-Vs)
Earthquakes: Seismology
• Measurement of seismic waves from earthquake
Fast P-wave
Surface waves
Time between arrivals
determines the distance from
the earthquake hypocenter.
Vp= ~6 km/sec
Vs= ~3 km/sec
DT
S-wave
An easy way to calculate:
X=VT ; X= Xp=Xs
X = Vp *Tp = Vs * (Tp+DT) = Vs * Tp + Vs * DT
Tp= Vs * DT / (Vp-Vs)
Here: Tp = 3km/sec * 300sec / 3km/sec = 300sec
Where Earthquakes Occur
Intraplate
Earthquake
Shallow earthquakes (most damaging)
– Tension at mid-ocean ridges; Lateral slip on transform faults
Deep earthquakes
- Thrusting; Down-dip compression of subducting plates
The very active Mediterranean
X=VT ; X= Xp=Xs
Vp *Tp = Vs * (Tp+DT) …..
Tp=? …..
X=?
.‫א‬
Timeearthquae= TimeP_arrival – Tp
.‫ב‬
T = Timeearthquae+4000/Vs
.‫ג‬
Waves – seismic frequency and wavelength :
Wavelength –
Velocity Frequency –
Period -
‫אורך גל‬
‫מהירות‬
‫תדירות‬
‫זמן מחזור‬
t
l
V
f
T
X=VT
f=1/T
T=1/f
[Hz]=[1/sec]
l=VT = V/f
T
T
t
T
Waves – seismic frequency and wavelength :
l Wavelength –
‫אורך גל‬
V Velocity ‫מהירות‬
f Frequency –
‫תדירות‬
f=1/T
T Period ‫זמן מחזור‬
T=1/f
X=VT
[Hz]=[1/sec]
l=VT = V/f
Waves – Tsunami:
X=VT
‫בהצלחה‬