Earthquake Source Mechanics
Lecture 5
Earthquake Focal Mechanism
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 global
earthquake catalogues
b) Temporal distribution of seismic activity
2. Earthquake focal mechanisms
3. Physics of the earthquake source through
analysis of seismograms
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Location of large earthquakes and the
global earthquake catalogues
ß
Historically of crucial importance in the development
of plate tectonics theory
a
It was the recognition of a continuous belt of seismicity across
the North Atlantic (together with profiles measured by marine
geophysicists) that allowed Ewing & Heezen to predict the
existence of a worldwide system of mid-ocean rifts
ß
Goter extended this work in the 60’s & 70’s to
compile global seismicity maps delineating the plate
boundaries
a
Similar maps at larger scale constructed from regional and
local seismic networks allow the tectonics to be studied in
much finer detail
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Global seismicity
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Earthquake focal mechanisms
ß
Using teleseismic earthquake records to determine
the earthquake focal mechanism or fault plane
solution and deduce the tectonics of a region
ß
Similar work now done at larger scale for looking at
regional and local tectonics - neotectonics
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The Seismic Source
ß Shear faulting
a
Simple model of the seismic source
1.
2.
3.
4.
Fracture criterion
Frictional sliding criterion
Effect of pore fluid pressure
Influence of pressure, i.e. depth, on faulting
Covered more in earthquake source mechanics – now start with
simplest model and won’t specify whether a fresh fracture or
unstable frictional sliding on an existing fault
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The Seismic Source
Simple normal fault
Look at first motion on seismogram
2 compressional quadrants +
2 dilatational quadrants 2 nodal planes
0
Dip
Displacement
Footwall
-
+
no motion
0
Auxiliary plane
Per’lar to fault plane
Per’lar to slip direction
-
Hanging wall
↑ up on
vertical axis
+
0
Fault plane
no motion
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First motion
S3 & S4 are on
nodal plane
So no motion
or indistinct
first motion in
P wave
+
-
S4
S1
S3
S2
↑ first motion up
↓ down motion up
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Earthquake Focal Mechanism
Earthquake focal mechanism
Fault plane orientation
Fault plane solution
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Fault Plane Orientation from Seismograms
1.
We use a global coverage of seismometers (many
stations) to record first motions
In principle we could use any phase (S, pP, PP) but only use P
as 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 fault
plane or auxiliary plane
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Fault Plane Orientation from Seismograms
To find a nodal plane we need to know the expected arrival time
accurately
LP seismogram
e.g.
Expect here – no motion just after arrival, therefore nodal
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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Fault Plane Orientation from Seismograms
Examine first motions recorded on long period seismograms
because of SP energy from small geological heterogeneities
Theoretical path
SP
LP
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 λ ~v, T ~ 8x20 = 160km
SP
period T~1s (seismometer)
λ ~ 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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Fault Plane Orientation from Seismograms
Problem: Fault plane is not uniquely specified by 2 nodal
planes:
ß
Fault breaks (if earthquake has broken surface)
Shallow events Ms> 6
x
3.
Aftershocks
occur around fault plane and
show direction of fault plane
x
x
2.
x
Isoseismals
elongate along direction of fault plane
(1st discovered after 1906 SF earthquake)
x
x
x
x
zones of
damage
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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
Fracture
starts
5.
Sub-events
Fracture
stops
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Fault Plane Orientation from Seismograms
Problem: Lack of global coverage
ß
Station coverage
2/3 earth is ocean and island stations are noisy so
difficult to get good nodal planes
ß
Core shadow
near centre of plots (more on this late)
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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 elastic
constants
-
+
By 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
+
45o
-
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Faulting
Hanging walls
Footwall
Fault
strike
Fault plane
Footwall
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Fault Plane Orientation
ß
Measuring strike and dip
a
By convention the dip is measured to the right of the strike
N
W
ϕs ~ 45o
E
S
N
E
W
ϕs ~ 225o
S
Study the self-taught module on structural geology on the server
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Fault Plane Orientation
ß
u is slip direction
lies in the fault plane
Measuring the rake
normal to fault plane
u
λ
strike direction
horizontal
λ - the rake, measured relative to the strike direction ϕs
So,
λ = 0o
strike slip (pure) [e.g. San Anreas]
λ = -90o
normal (pure)
λ = +90o
reverse/thrust (pure)
Slip direction refers to the
relative movement of the
hanging wall
Hanging
wall
Foot
wall
λ -ve
Normal fault, hanging wall goes down
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Focal Sphere – 3D
Focal sphere for a seismic point source is a sphere centred on the
source and having arbitrarily small radius. It is a convenient
device for displaying radiation patterns, since information
recorded by seismometers (distributed over the Earth’s 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
i large close in
onto the lower focal
hemisphere
i small
upper
further out Only local seismometers
plot onto upper focal sphere
lower
One station → one point on focal sphere
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Focal Sphere
In principle, azimuth ϕ angle of descent i can be worked out if
1.
Location of earthquake
2.
Location of station
3.
Velocity profile i(∆)
Use computers to do this, and so one may specify a point on the
focal sphere by angular coordinates (i,ϕ)
e.g.
-
+
Strike slip fault
-
+
-
+
D
Usually the compressional
(+ve polarity) is shaded
C
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Equal Area Projection (2D) of the
Focal Sphere – Strike Slip Fault
Schmidt net
T.
preserves area
We map a plan view of the
horizontal plane, i.e. an equal
area projection of the lower
focal hemisphere
Strike slip fault
P.
.P
D
C – compression
D – dilatational
C
T.
→ auxiliary plane
→fault plane
T – tension axis
Use equal area projection, so that all data
collected over area have same weight
P – pressure axis
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Normal Fault
60o dip
Normal Fault
0o strike
N
N ϕ ~ 0o
s
30
60
Auxiliary
plane
δ = 30o
P.
T.
δ = 60o
+
Fault plane
Auxiliary
plane
Fault plane
nodal planes
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Thrust Fault
Thrust Fault 30o dip
0o strike
N ϕ ~ 0o
s
δ=
60o
P.
Auxiliary
plane
T.
δ = 30o
Fault plane
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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 is
predominantly strike slip
If it is nearer the edge then predominantly normal or thrust fault
Normal fault – centre is dilatational
Thrust fault – centre is compressional
Rake
Slip direction relative to the azimuth,
movement on the fault plane
ϕs
λ
e.g. angle of slickensides to horizontal
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Fault Plane Solution
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Information from the Fault Plane Solution
P & T axes correspond roughly to the directions of minimum (T) and
maximum compressive (P) stress
Normal
faulting
P σmax
σintermediate
ϕs
45o
T
σmin
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 σmax is at 30o to fault plane, i.e. dip of 60o in rocks)
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Information from the Fault Plane Solution
P & T axes
P
Section
P axis – dilatational quadrant
-
T axis – compressional quadrant
+
+
-
T
P-axis direction of tectonic
movement ±15o
Good for plate tectonics as gives
direction, c.f. neotectonics
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