5 focal mechanism - ucl focal mechanism.pdf · relative movement of the hanging wall hanging ... if...

GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD

Earthquake Source Mechanics

Lecture 5Earthquake Focal Mechanism


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


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


Global seismicity


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


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


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


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


Earthquake Focal Mechanism

Earthquake focal mechanism

Fault plane orientation

Fault plane solution


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



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



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)



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



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



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)



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


Faulting

Hanging walls

Footwall

Footwall

Faultstrike

Fault plane


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


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


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


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


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


Normal Fault

Normal Fault 60o dip 0o strike

+-

6030

N

Fault planeAuxiliary plane

N ϕs ~ 0o

δ = 60oδ = 30o P . T.


nodal planes


Thrust Fault

Thrust Fault 30o dip 0o strike

N ϕs ~ 0o

δ = 60o δ = 30oP . T.



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


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)



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 - ucl focal mechanism.pdf · relative movement of the hanging wall hanging ... if...

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