5 focal mechanism
TRANSCRIPT
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Earthquake Source Mechanics
Lecture 5
Earthquake Focal Mechanism
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
What is Seismotectonics?
Study of earthquakes as a tectonic component,
divided into three principal areas.
1. Spatial and temporal distribution of seismic
activity
a) Location of large earthquakes and globalearthquake catalogues
b) Temporal distribution of seismic activity
2. Earthquake focal mechanisms3. Physics of the earthquake source through
analysis of seismograms
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Location of large earthquakes and the
global earthquake catalogues
Historically of crucial importance in the developmentof plate tectonics theory
It was the recognition of a continuous belt of seismicity acrossthe North Atlantic (together with profiles measured by marinegeophysicists) that allowed Ewing & Heezen to predict theexistence of a worldwide system of mid-ocean rifts
Goter extended this work in the 60s & 70s tocompile global seismicity maps delineating the plateboundaries
Similar maps at larger scale constructed from regional andlocal seismic networks allow the tectonics to be studied inmuch finer detail
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Global seismicity
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Earthquake focal mechanisms
Using teleseismic earthquake records to determinethe earthquake focal mechanism or fault planesolution and deduce the tectonics of a region
Similar work now done at larger scale for looking at
regional and local tectonics - neotectonics
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
The Seismic Source
Shear faulting
Simple model of the seismic source
1. Fracture criterion
2. Frictional sliding criterion
3. Effect of pore fluid pressure
4. Influence of pressure, i.e. depth, on faulting
Covered more in earthquake source mechanics now start withsimplest model and wont specify whether a fresh fracture orunstable frictional sliding on an existing fault
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
The Seismic Source
Fault plane
Footwall
Hanging wall
Dip
Displacement
+
+
-
-Auxiliary planePerlar to fault plane
Perlar to slip direction
00
Simple normal fault
Look at first motion on seismogram
2 compressional quadrants+
2 dilatational quadrants -
2 nodal planes 0
up on
vertical axis
no motion
no motion
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
First motion
+
-
S1
S2
S3
S4
first motion up
down motion up
S3 & S4 are on
nodal plane
So no motion
or indistinct
first motion in
P wave
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Earthquake Focal Mechanism
Earthquake focal mechanism
Fault plane orientation
Fault plane solution
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Orientation from Seismograms
1. We use a global coverage of seismometers (manystations) to record first motions
In principle we could use any phase (S, pP, PP) but only use Pas later arrivals are more difficult to read
2. Plot onto 2D projection of the Earth
3. Look particularly for nodal planes
where there is no motion as these stations define the faultplane or auxiliary plane
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Orientation from Seismograms
To find a nodal plane we need to know the expected arrival timeaccurately
LP seismogram
Expect here no motion just after arrival, therefore nodal
e.g.
To check arrival time look at high frequency SP record
SP seismogram
Always get some kick on short period
N.B. SP is always more accurate for measurement of times
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Orientation from Seismograms
Examine first motions recorded on long period seismogramsbecause of SP energy from small geological heterogeneities
SP
LP
Theoretical path
Never use SP records for polarity measurements (because of scattering,
multiple reflections, refractions)
e.g. LP period ~20s (seismometer)
for v~8 km/s(mantle), wavelength ~v, T ~ 8x20 = 160km
SP period T~1s (seismometer)
~ v, T ~ 8km
SP records are full of scattered energy
LP records are more reliable (if care taken at nodal planes)
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Orientation from Seismograms
Problem: Fault plane is not uniquely specified by 2 nodalplanes:
Fault breaks (if earthquake has broken surface)Shallow events Ms> 6
2. Aftershocks
occur around fault plane andshow direction of fault plane
3. Isoseismals
elongate along direction of fault plane
(1st discovered after 1906 SF earthquake)
x
x
x x
xx x
x
zones of
damage
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Orientation from Seismograms
4. Source directivity pulse moving along fault
(takes finite time from beginning to end of fault)
analogous to Doppler effect
5. Sub-eventsFracture
stops
Fracture
starts
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Orientation from Seismograms
Problem: Lack of global coverage
Station coverage
2/3 earth is ocean and island stations are noisy sodifficult to get good nodal planes
Core shadow
near centre of plots (more on this late)
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Orientation from Seismograms
Synthetic seismograms
A large part of modern seismology is devoted to the calculation
of seismograms from models of the source and elasticconstants
-
-
+
+
45oBy building up these
seismograms from a model of
an earthquake source, varying
a wide range of physical
parameters, until the synthetic
seismograms matches the real
observed seismograms
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Faulting
Hanging walls
Footwall
Footwall
Faultstrike
Fault plane
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Orientation
Measuring strike and dip
By convention the dip is measured to the right of the strike
s~ 45o
N
W E
S
s~ 225o
N
E
S
W
Study the self-taught module on structural geology on the server
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Orientation
Measuring the rake
horizontalstrike direction
normal to fault plane u
u is slip direction
lies in the fault plane
- the rake, measured relative to the strike direction sSo, = 0o strike slip (pure) [e.g. San Anreas]
= -90o normal (pure)
= +90o reverse/thrust (pure)
Slip direction refers to therelative movement of the
hanging wall
Hanging
wall
Foot
wall
Normal fault, hanging wall goes down
-ve
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Focal Sphere 3D
Focal sphere for a seismic point source is a sphere centred on thesource and having arbitrarily small radius. It is a convenientdevice for displaying radiation patterns, since informationrecorded by seismometers (distributed over the Earths surface)
may be transferred back to the focal sphere.
Remember p = r sin i / v = constant for a spherical Earth
If velocity at station = velocity near source, then isource = istation
(applies best to shallow earthquakes, correction can be applied for deeper
earthquakes) All teleseismic stations plot
onto the lower focal
hemisphereOnly local seismometers
plot onto upper focal sphere
One station one point on focal sphere
upper
lower
i large close in
i smallfurther out
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Focal Sphere
In principle, azimuth angle of descent i can be worked out if
1. Location of earthquake
2. Location of station3. Velocity profile i()
Use computers to do this, and so one may specify a point on the
focal sphere by angular coordinates (i,)
e.g.
+
+
--
+
CD-
Strike slip fault
Usually the compressional
(+ve polarity) is shaded
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Equal Area Projection (2D) of the
Focal Sphere Strike Slip Fault
T.
T.
. PP .
Schmidt netpreserves area
C
D
We map a plan view of the
horizontal plane, i.e. an equal
area projection of the lower
focal hemisphere
Use equal area projection, so that all data
collected over area have same weight
Strike slip fault
C compression
D dilatational
auxiliary plane
fault plane
T tension axis
P pressure axis
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Normal Fault
Normal Fault 60o dip 0o strike
+-
60
30
N
Fault planeAuxiliary
plane
N s ~ 0o
= 60o
= 30o P . T.
Fault planeAuxiliary
plane
nodal planes
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Thrust Fault
Thrust Fault 30o dip 0o strike
N s ~ 0o
= 60o = 30o
P . T.
Fault planeAuxiliary
plane
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Information from the Fault Plane Solution
Null axis
is the interception of 2 nodal planes (direction of movement)
If the null axis is nearer the centre of the projection, the mechanism ispredominantly strike slip
If it is nearer the edge then predominantly normal or thrust fault
Normal fault centre is dilatational
Thrust fault centre is compressional
s
Rake
Slip direction relative to the azimuth,
movement on the fault plane
e.g. angle of slickensides to horizontal
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Solution
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Information from the Fault Plane Solution
P & T axes correspond roughly to the directions of minimum (T) andmaximum compressive (P) stress
s
maxintermediate
min
Normal
faulting
T
P
45o
Deviatoric stress (tectonic) leads
to faulting
Fault plane at 45o to P & T
axes
Definition of P & T
90o to intermediate axis (strike)
45o to auxiliary plane
45o
to fault plane
(Usually max is at 30o to fault plane, i.e. dip of 60o in rocks)
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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Information from the Fault Plane Solution
P & T axes
Section
T
P
P axis dilatational quadrant
T axis compressional quadrant
P-axis direction of tectonic
movement 15o
Good for plate tectonics as gives
direction, c.f. neotectonics
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