5 focal mechanism

8/3/2019 5 Focal Mechanism

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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Earthquake Source Mechanics

Lecture 5

Earthquake Focal Mechanism


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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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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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Global seismicity


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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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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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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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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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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 (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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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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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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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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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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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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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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Faulting

Hanging walls

Footwall

Footwall

Faultstrike

Fault plane


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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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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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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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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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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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Normal Fault

Normal Fault 60o dip 0o strike

+-

60

30

N

Fault planeAuxiliary

plane

N s ~ 0o

= 60o

= 30o P . T.


plane

nodal planes


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Thrust Fault

Thrust Fault 30o dip 0o strike

N s ~ 0o

= 60o = 30o

P . T.


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 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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Fault Plane Solution


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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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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

++

-

-

5 focal mechanism

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