Download Lecture 2

Learning Objectives
Saturday, August 22, 2009
5:05 PM
LO2-1
Describe the physical characteristics of the earth's layers.
1. crust
2. lithosphere
3. asthenosphere
4. mantle
5. outer core
6. inner core
LO2-2
Describe scientific evidences that support plate tectonics.
LO2-3
Describe the different types of plate boundaries and the types of earthquakes
they generate. Give an example or location of each of these boundaries.
1. spreading or rift zone (divergent plate boundary)
2. convection zone (convergent plate boundary)
3. Transform fault boundaries
LO2-4
Describe the characteristic of fault types in terms of relative movement of the
fault plane.
1. dip slip
2. strike slip
3. oblique slip
LO2-5
Describe the various earthquake magnitude scales and how to calculate each.
1. Local magnitude
2. Surface wave magnitude
3. Body wave magnitude
4. Moment magnitude
Lecture 2 - Science of Earthquakes Page 1
Learning Objectives
Saturday, August 22, 2009
5:05 PM
LO2-6
Know how to calculate the distance to the earthquake focus from p and s-wave
arrival information.
LO2-7
Know how to calculate earthquake magnitude, maximum and average fault
displacement from geological data.
LO2-8
Define the following:
 focus
 epicenter
 epicentral distance
 hypocentral distance
 right lateral fault
 fault directivity
 fault fling
 elastic rebound theory
 seismic moment
 modified Mercalli intensity scale
 maximum credible earthquake
 characteristic earthquake
 asthenosphere
 continental shelf
 mid Atlantic ridge
 fault dip
 fault strike
 return period
 active fault
Lecture 2 - Science of Earthquakes Page 2
Important Concepts
Tuesday, August 18, 2009
3:07 PM
1. The earth is a dynamic planet. The earth’s crust and landforms have changed
throughout geologic time. The planet materials have segregated by density
into distinct layers: (1) core, (3) mantle, (3) lithosphere, (4) surface fluids
(i.e., water and atmosphere).
2. The theory of continental drift (plate tectonics) was proposed in the early
1900s and was supported by a variety of geologic evidence. However,
without a knowledge of the nature of the oceanic crust, a complete theory
of earth dynamics could not be developed.
3. A major breakthrough in the development of plate tectonic theory occurred
in the early 1960s when the topography of the ocean floors was mapped and
magnetic and seismic characteristics were determined.
4. The earth’s tectonic system involves the movement of material from the
earth’s interior which results in seafloor spreading, creation of new crust,
continental drift, volcanism, earthquakes and mountain-building.
5. The lithosphere can be divided into a series of plates bounded by the
oceanic ridge, trenches, mountain ranges, and transform faults.
6. The plates move apart where the convecting mantle rises and spread
laterally beneath the oceanic ridge at rift zones. The plates descend
(subduct) into the mantle beneath ocean trenches and are consumed in the
mantle by heat in subduction zones.
7. The energy for plate tectonics is internal heat, probably generated by
radioactivity in the asthenosphere.
8. There are three types of plate boundaries: (1) rift or spreading zones, (2)
subduction zones, (3) transform faults. Subduction zone boundaries can
produce the largest earthquakes, followed by transform faults and then rift
zones.
9. Faults can have relative movements which consist of: (1) dip slip, (2) strike
slip, and (3) oblique.
Lecture 2 - Science of Earthquakes Page 3
Important Concepts
Sunday, August 23, 2009
7:45 PM
10. The energy-releasing function of earthquakes suggests that a period of
time for strain energy accumulation should be expected between large
earthquakes at the same location. It also suggests that earthquakes should
be most likely to occur along portions of a fault for which little seismic
activity has been observed-unless the plate movement has occurred
aseismically.
11. Earthquake intensity is a qualitative measure of the effects of an
earthquake at a particular location. It is related to the size of the
earthquake but is also influenced by other factors. Isoseismal maps can be
used to describe the spatial variation of intensity for a given earthquake.
Because no instrumental measurements are required, historical accounts
can be used to estimate intensity values for pre-instrumental earthquakes.
12. Earthquake magnitude is a quantitative measure of the size of an
earthquake. Most magnitude scales are based on measured ground
motion characteristics. The local magnitude is based on the trace
amplitude of a particular seismometer, the surface wave magnitude on
the amplitude of Rayleigh waves, and the body wave magnitude on the
amplitude of p-waves. Because these amplitudes tend to reach limiting
values, these magnitude scales may not accurately reflect the size of very
large earthquakes. The moment magnitude, which is not obtained from
ground motion characteristics, is able to describe the size of any
earthquake.
13. Earthquake magnitude scales are logarithmic, hence a unit change in
magnitude corresponds to a 10-fold change in the magnitude parameter
(ground motion characteristic or seismic moment). The energy released by
an earthquake is related to magnitude in such a way that a unit change in
magnitude corresponds to a 32-fold change in energy.
Lecture 2 - Science of Earthquakes Page 4
Internal Structure of Earth
Sunday, August 23, 2009
8:26 PM
The internal structure
of the earth as
deduced from
variations in
velocities of seismic
waves at depth, The
velocity of both P and
S waves increases to
a depth of
approximately 100
km; then it decreases
abruptly to a depth of
200 km. This lowvelocity layer is called
the asthenosphere.
There is then a
general increase in
velocities of P and S
waves to a depth of
about 3000 km,
where both change
abruptly. The S wave
does not travel
through the central
part of the earth, and
the velocity of the P
wave decreases
drastically. This is the
most striking
discontinuity in the
earth and is
considered to be the
boundary between
the core and the
mantle. Another
discontinuity in P
waves occurs at a
depth of 5000 km,
indicating an inner
core.
Lecture 2 - Science of Earthquakes Page 5
Physiography of Atlantic Ocean
Sunday, August 23, 2009
8:40 PM
Lecture 2 - Science of Earthquakes Page 6
Physiography of Pacific Ocean
Sunday, August 23, 2009
8:45 PM
1
2
3
4
6
5
1 = Kuril Trench
2 = Japan Trench
3 = Mariana Trench (Challenger Deep 36198 feet)
4 = Philippine Trench
5 = Tongan Trench
6 = Pacific Rift Zone
Lecture 2 - Science of Earthquakes Page 7
Plate Boundaries
Sunday, August 23, 2009
9:11 PM
(a) Earthquake Epicenters (dots) and Subduction Zones (sawtooth lines)
(b) Major Plate Tectonic Features and Direction of Movement of Plates
Lecture 2 - Science of Earthquakes Page 8
Plate Boundaries
Sunday, August 23, 2009
9:01 PM
Lecture 2 - Science of Earthquakes Page 9
Subduction Zone
Sunday, August 23, 2009
9:03 PM
Subduction zone at converging plate margins
Cascadian subduction zone
Lecture 2 - Science of Earthquakes Page 10
Fault Geometry
Sunday, August 23, 2009
9:17 PM
Lecture 2 - Science of Earthquakes Page 11
Fault Movement
Sunday, August 23, 2009
9:20 PM
Lecture 2 - Science of Earthquakes Page 12
Fault Movement
Sunday, August 23, 2009
9:25 PM
Lecture 2 - Science of Earthquakes Page 13
Fault Solutions from P-waves
Sunday, August 23, 2009
9:31 PM
positive polarity =
compression
negative polarity
= extension
Lecture 2 - Science of Earthquakes Page 14
Polarity
from
direction
of first
wave
arrival
Elastic Rebound Theory
Monday, August 24, 2009
7:41 AM
Elastic Rebound Theory (Video)
Lecture 2 - Science of Earthquakes Page 15
Elastic Rebound Theory
Monday, August 24, 2009
7:44 AM
see Multiple Epicentral Locations - 1964 Alaska Earthquake
see Fault Directivity
see Fault Fling
Lecture 2 - Science of Earthquakes Page 16
Multiple Epicentral Locations - 1964 Alaska Earthquake
Thursday, January 17, 2013
11:43 AM
© Steven F. Bartlett, 2013
Lecture 2 - Science of Earthquakes Page 17
Fault Directivity
Thursday, January 17, 2013
11:45 AM
Earthquake directivity is the focusing of wave energy along the fault in the
direction of rupture. This means that, exclusive of local site conditions such
as soft soil, the stronger ground motions (and damage, if the earthquake is
large enough) will be distributed in an elongated pattern centered along the
axis of the fault. In other words, distance to the fault is not the only
consideration for ground motion amplitude: direction is also important.
When a fault ruptures unilaterally (with the epicenter at or near one end of
the fault break), the radiated waves are stronger in one direction along the
fault. Pasted from <http://earthquake.usgs.gov/regional/nca/rupture/directivity/>
Peak accelerations and epicenter of the San Juan Bautista
earthquake of August 12, 1998. The epicenter is near the
southeast end of the fault break. Notice the asymmetrical
distribution, with higher values in the northwest, along the
strike of the fault. Pasted from <http://earthquake.usgs.gov/regional/nca/rupture/directivity/>
© Steven F. Bartlett, 2011
Lecture 2 - Science of Earthquakes Page 18
Fault Fling
Thursday, January 17, 2013
11:45 AM
see more from: Effects of Fling Step and Forward Directivity on Seismic Response of Buildings
Erol Kalkan,a… S.M.EERI, and Sashi K. Kunnath,a… M.EERI
© Steven F. Bartlett, 2011
Lecture 2 - Science of Earthquakes Page 19
Elastic Wave Propagation
Thursday, September 10, 2009
10:52 AM
Elastic Properties and Wave Propagation
The elasticity of a homogeneous, isotropic solid can be defined by two elastic constants, K and G.
• K = bulk modulus
= 27 x 1010 dynes per cm2 (for granite)
= 2.0 x 1010 dynes per cm2 (water)
(Note that 1 dyne = 1 x 10 -5 N)
• G = shear modulus (sometime also called the modulus of rigidity, )
= 1.6 x 1011 dynes per cm2 (for granite)
= 0 (for water) (water cannot sustain a shear stress)
Speed of wave propagation in an elastic body
• Velocity of P wave = ((K + 4/3*G1/2
where = mass density
• Velocity of S wave = (G/)1/2
Location of Earthquakes
The difference in arrival time between the first P wave and the first S wave can be used to calculate
the distance to the earthquake source.
t = D(Vp - Vs )/(Vp Vs ) = D [1/Vs - 1/Vp ]
where t = difference in arrival time between the P and S wave
D = distance to the earthquake focus
VP = P wave velocity
Vs = S wave velocity
D2
D1
D3
Lecture 2 - Science of Earthquakes Page 20
Seismic Moment and Moment Magnitude
Monday, August 24, 2009
7:45 AM
Lecture 2 - Science of Earthquakes Page 21
Other Measures of Magnitude
Monday, August 24, 2009
7:56 AM
Lecture 2 - Science of Earthquakes Page 22
Earthquake Intensity
Monday, August 24, 2009
8:01 AM
Lecture 2 - Science of Earthquakes Page 23
Earthquake Intensity - Masonary Classification
Monday, August 24, 2009
8:02 AM
Lecture 2 - Science of Earthquakes Page 24
Earthquake Intensity
Monday, August 24, 2009
8:00 AM
Isoseismal map for the 1811 New Madrid Earthquake
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Earthquake Intensity
Monday, August 24, 2009
8:00 AM
Isoseismal map for the 1886 Charleston, South Carolina Earthquake
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Magnitude from Geological Data
Monday, August 24, 2009
8:04 AM
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Magnitude from Geological Data
Monday, August 24, 2009
8:05 AM
Should use coefficients for all slip type (All) in this table.
Lecture 2 - Science of Earthquakes Page 28
Magnitude from Geological Data
Monday, August 24, 2009
8:10 AM
Lecture 2 - Science of Earthquakes Page 29
Blank
Wednesday, August 17, 2011
12:45 PM
© Steven F. Bartlett, 2011
Lecture 2 - Science of Earthquakes Page 30