seismology measuring the interior-4

Solid Earth Geophysics

Ali [email protected].

saDepartment of Earth SciencesKFUPM

Today’s class: Seismology Measuring the InteriorReading: Fowler Chapter 4

Earthquake Faults

PP. 215-17 of Stein and Wysession, 2003

Earthquake Fault Parameters

PP. 218 of Stein and Wysession, 2003

Directivity of Slip Motion


Earthquake Focal Mechanisms


Black = Compression

White = Dilatation

Earthquake Mechanism

Focal mechanism solution for a fault is commonly a lower focal sphere projection.

For a dip-slip fault, this projection is equivalent to the compression/dilatation pattern viewed by a bird flying over the earthquake focus.

Cross sections of Dip-Slip Faults

The opposite pattern is observed for a normal fault (b)

Reverse FaultReverse Fault Normal FaultNormal Fault

The focal mechanism solution for a reverse fault (a) has a compression (black) in the inside portion of the circle, surrounded by regions of dilatation (white).

Source Parameter

sTry to write about the parameters:

Strike =?

Dip of Fault Plane=?

Dip of Auxiliary Plane?

Inside Portion=?

5 Minutes

A=?

B1=? B2=?

C=?

what type of earthquake along what fault orientation is this?

Ambiguity: • which is fault plane? • which is auxiliary plane?

can be either: • right-lateral on EW fault • left-lateral on NS faultFrom: Kearey and Vine, 1996

2 Minutes

Thrust faults

(b): focal mechanism same for both

use geological setting to determine most reasonable

From: Kearey and Vine, 1996From: Kearey and Vine, 1996

shaded: compressional

(a): W dipping fault(c): E dipping fault

also Anderson’s theory: thrusts dip < 45°

http://www.seismo.unr.edu/ftp/pub/louie/class/plate/seismology.html

Normal faults

shaded: compressional

(a): W dipping fault(c): E dipping fault

(b): focal mechanism same for both

use geological setting to determine most reasonable

From: Kearey and Vine, 1996

Beach Ball Representation of Typical Faults

Fig. 4.2.14 of Stein and Wysession, 2003

Bolt, 1978

What type of faulting?

A normal faulting mechanism.

Which plane do you think is the fault plane ? A reverse faulting mechanism. Which plane do you think is the fault plane ?

Strike-slip faulting. Which strike?

Fournier and Petit, 2007, JSG

www.lgs.jussieu.fr/tectonique/tectonique2/vie_du_labo/pdf/oblique_rifting_JSG_2007.pdf

World Stress Map

Different colors correspond to different methods of measurementmaximum horizontal stress

Plotting Planes-1: Strike & Dip


To plot a plane striking N45E and dipping 60E, rotate the stereonet (or tracing paper above it) so that the strike is at the top and the dip can be measured along the equator. After plotting the appropriate meridian, rotate the net back to the geographic orientation with north at the top.

Plotting Planes : Strike & DipStrike = N45°E, Dip = 60°, Rake = 70°


WAVEFORM MODELINGS&W 4.3-11

-Mechanism has significant dip-slip components but one of other plane has not resolved well.-Additional information is obtained by comparing the observed body and surface waves to theoretical, or synthetic waveforms computed for various source parameters, and finding a model that best fits the data, either by forward modeling or inversion.

Why Waveform-Modeling?

?

? ?

In brief, waveform analysis also gives information about earthquake depths and rupture processes that can’t be extracted from first motions.

It provides about the information about structural parameters as source of faulting and moment magnitude, focal depth. It works also for cased in case FIRST MOTIONS DON’T CONSTRAIN FOCAL MECHANISM due to lack of deployed instruments (say few stations).

Regard ground motion recorded on seismogram as a combination of factors:

- earthquake source- earth structure through which the waves propagated-Seismometer-Create synthetic seismogram as Fourier domain convolution of these effects

Synthetic Seismogram: CONVOLUTION

seismology measuring the interior-4

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