Solid Earth Geophysics

Ali Onceloncel@kfupm.edu.sa

Department of Earth SciencesKFUPM

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

1992 Landers, California EQ, Mw=7.3

Earthquake Mechanisms

Compression and Dilation

ompression: P-wave first motion up

ilatation: P-wave first motion down

P and T axes

irectivity for Compression

irectivity for Tension

Moment Tensors

-D representation of faulting Encompasses the moment (energy)From: modified after http://quake.usgs.gov/recenteqs/beachball.htm

First Motions

the push or pull is recorded on seismogram as either an upward or downward displacement of the arrival of P-wave …look at ‘first motion’ of P-wave on seismogram

Look at first motions from many geographically dispersed seismograms to identify quadrants of compression, dilatation and delineate nodal planes

from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270

..use equal area lower hemisphere projection (stereonet)

Station reports location relative to focus: Azimuth: relative to focus Angle of incidence: between ray vector and vertical

Azimuth and angle of incidence plot on net and first motion noted

From: Kearey and Vine, 1996

From: Rowland and Duebendorfer, 1994

Focal Mechanism

source

receiver

North

center

The angle of incidence, which often called the take-off angle, at the earthquake source is the angle from the vertical at which the ray leaves source, and thus the angle at which the ray intersects the lower hemisphere (Fig.4.2-8 of Stein and Wysession, 2003).

How is distance related to take-off angle?

Source: Stein and Wysession (2003) After Pho and Behe (1972)

Plotting Station LocationsExample 1: Plot seismograph station (azimuth N40°E from the earthquake; angle of first motion ray or take-off angle i= 60°) on an equal-area projection

Fig. 4.2.13 of Stein and Wysession, 2003South

North

Homework: First Motion Polarities Due to May 2

Go to site:http://www.seismo.unr.edu/ftp/pub/louie/class/333/mechanism.html

The data is what you could take from:http://www.seismo.unr.edu/ftp/pub/louie/class/333/weber-seis.pdf

Plot Polarities over the Steronet see Examples on Page 137, Fowler, 2005.

Interpreting Focal Mechanism see example in Fig.4.30 of Fowler’s book

Lon Lat Dist, km176.35° -40.250° 3.403

176.17° -40.292° 14.414

176.37° -40.061° 19.072

176.28° -40.408° 20.154

176.06° -40.106° 25.886

176.47° -40.453° 27.993

176.63° -40.339° 29.118

176.09° -40.429° 29.880

176.27° -40.618° 43.544

176.81° -39.989° 48.965

176.35° -39.699° 59.008

176.88° -39.665° 78.454

176.82° -39.541° 87.502

The Stations

Source: Stein and Wysession, 2003

Stresses Driven Faulting

From: http://earth.leeds.ac.uk/learnstructure/index.htm

Anderson’s theory of faulting

See pp.402, Bolt and Bullen

T

P

P=Maximum Compressive StressT=Minimum Compressive Stress

Stress Orientations

Source: Stein and Wysession, 2003

Let’s come back to discuss earthquake fault mechanism

b d