GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Earthquake Source Mechanics
Lecture 5Earthquake 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
activitya) Location of large earthquakes and global earthquake cataloguesb) Temporal distribution of seismic activity
2. Earthquake focal mechanisms3. Physics of the earthquake source through
analysis of seismograms
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Location of large earthquakes and the global earthquake catalogues
Historically of crucial importance in the development of plate tectonics theoryIt was the recognition of a continuous belt of seismicity acrossthe 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
Goter extended this work in the 60’s & 70’s tocompile global seismicity maps delineating the plate boundariesSimilar maps at larger scale constructed from regional and local seismic networks allow the tectonics to be studied in much finer detail
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Global seismicity
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Earthquake focal mechanisms
Using teleseismic earthquake records to determine the earthquake focal mechanism or fault plane solution and deduce the tectonics of a region
Similar work now done at larger scale for looking at regional and local tectonics - neotectonics
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
The Seismic Source
Shear faultingSimple model of the seismic source
1. Fracture criterion2. Frictional sliding criterion3. Effect of pore fluid pressure4. 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
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
The Seismic Source
Fault plane
Footwall
Hanging wall
Dip
Displacement
++
-
-Auxiliary planePer’lar to fault planePer’lar to slip direction
00
Simple normal faultLook at first motion on seismogram
2 compressional quadrants +2 dilatational quadrants -2 nodal planes 0
↑ up on vertical axis
no motion
no motion
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
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Earthquake Focal Mechanism
Earthquake focal mechanism
Fault plane orientation
Fault plane solution
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Orientation from Seismograms
1. We use a global coverage of seismometers (many stations) to record first motionsIn 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 planeswhere there is no motion as these stations define the fault plane or auxiliary plane
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 time accurately
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
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 λ ~v, T ~ 8x20 = 160km
SP period T~1s (seismometer)λ ~ v, T ~ 8km
SP records are full of scattered energyLP records are more reliable (if care taken at nodal planes)
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 and show direction of fault plane
3. Isoseismalselongate along direction of fault plane(1st discovered after 1906 SF earthquake)
xx
x x
xx x
x
zones of damage
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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Fault Plane Orientation from Seismograms
Problem: Lack of global coverage
Station coverage 2/3 earth is ocean and island stations are noisy so difficult to get good nodal planes
Core shadownear centre of plots (more on this late)
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Fault Plane Orientation from Seismograms
Synthetic seismogramsA large part of modern seismology is devoted to the calculation of seismograms from models of the source and elastic constants -
-
++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
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Faulting
Hanging walls
Footwall
Footwall
Faultstrike
Fault plane
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane OrientationMeasuring strike and dipBy convention the dip is measured to the right of the strike
ϕs ~ 45oN
W E
S ϕs ~ 225o
N
E
S
W
Study the self-taught module on structural geology on the server
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Orientation
Measuring the rake
horizontalstrike direction
normal to fault plane uλ
u is slip directionlies in the fault plane
λ - the rake, measured relative to the strike direction ϕsSo, 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
Normal fault, hanging wall goes down
λ -ve
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Focal Sphere – 3DFocal 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
onto the lower focal hemisphere
Only local seismometers plot onto upper focal sphere
One station → one point on focal sphere
upper
lower
i large close in
i small further out
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Focal Sphere
In principle, azimuth ϕ angle of descent i can be worked out if 1. Location of earthquake2. 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,ϕ)
e.g.+
+--
+C
D-
Strike slip fault
Usually the compressional (+ve polarity) is shaded
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
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Normal Fault
Normal Fault 60o dip 0o strike
+-
6030
N
Fault planeAuxiliary plane
N ϕs ~ 0o
δ = 60oδ = 30o P . T.
Fault planeAuxiliary plane
nodal planes
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Thrust Fault
Thrust Fault 30o dip 0o strike
N ϕs ~ 0o
δ = 60o δ = 30oP . T.
Fault planeAuxiliary plane
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Information from the Fault Plane Solution
Null axisis 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 slipIf it is nearer the edge then predominantly normal or thrust fault
Normal fault – centre is dilatationalThrust fault – centre is compressional
λ
ϕs
RakeSlip direction relative to the azimuth, movement on the fault plane
e.g. angle of slickensides to horizontal
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Fault Plane Solution
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) and maximum compressive (P) stress
ϕs
σmax σintermediate
σmin
Normal faulting
T
P
45o
Deviatoric stress (tectonic) leads to faulting
Fault plane at 45o to P & T axesDefinition 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)
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
+ +-
-