Solid Earth Geophysics

Ali Onceloncel@kfupm.edu.sa

Department of Earth SciencesKFUPM

Today’s class: Earthquake MechanismReading: Fowler Chapter 4

Solid

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UPM

Fault Model: Summing

Subevents

Ms 7.5 on Motagua fault, transform segment of Caribbean-North American plate boundary caused enormous damage and 22,000 deaths.

1976 Guatemala Earthquake

Source: Stein, 2003

Land shift

Motagua, Guatemala February 4, 1976

Magnitude 7.5

Photo Credit: U.S. Geological Survey

Building Digital Solid Earth

atlas.geo.cornell.edu Interactive Mapping

Focal Mechanism: the Interactive Database

Select your area by cursorThen, click on button “submit”Then, click on “Show Data Set”From Geophysics, select “CMT Focal Mechanism”Then, Click on “Submit Data Set”

Focal Mechanism Map

Download the map under different options. JPG is easy to download but *.PS is good if making up for picture is needed. Then, just click the option “JPG”, and see the regional map of focal mechanisms.

World-wide Data Search

http://pubs.usgs.gov/ds/2007/241/

3D Focal Mechanisms

Search Data for area of your term project from the below site and try to make a 3D map of Focal Mechanism Using the Software “ArcScene” above by USGS.

http://neic.usgs.gov/neis/sopar/

http://pubs.usgs.gov/ds/2007/241/

Data Search: Example for Marmara Region (38-42N, 26-31 E)

Homework due May 23

Through the Interactive Data Module which is explained so far http://neic.usgs.gov/neis/sopar/

Prepare the 3D focal mechanism map of the Marmara Area using the software:

http://pubs.usgs.gov/ds/2007/241/

Also, try to provide a discussion about one paragraph:Mechanism of Faulting• Direction of Regional Stress

Nuclear Explosions

An underground nuclear test can cause earth shaking like an M5 earthquake.

A lot of effort has gone into trying to being able to determine whether ground shaking is caused by a natural earthquake or a nuclear explosion.

Establishment of the World Wide Seismic Network in the 1960s was actually funded based on the need to monitor bombs, not study earthquakes.

Nuclear Bomb: Compressional Source. P wave first motion in all directions.

Slip on a fault: P wave

first motion compressional and extensional.

From: Vince Cronin, 2004

Radiation patterns for Bomb and Earthquake are totally different due to differences in their sources as: Compressional and Shear.

How can we tell?

Earthquake or Bomb?

Bomb

Earthquake

Data recorded at Nilore, Pakistan. Nuclear tests are shallower than most earthquakes.

Nuclear tests generally have weaker surface waves and stronger P wave arrivals.

Source: https://www.llnl.gov/str/Zucca.html

Nuclear explosions vs. Earthquakes

Physics are different Explosions are compressional sources

Generates strong P-waves, little shear energy (S-waves, Surface waves) Earthquakes are shear sources

Generate all wave types, but dependent on radiation pattern

Empirical methods are preferred for monitoring Easy to implement Quick (no heavy computations)

Must be able to record and understand “regional” recordings Waves that travel through crust are much more complex than

those traveling through body of the earth (mantle)

Source: Aaron A. Velasco, SACNAS, 2005.

Raw Data Filtered Data

Forensic Seismology

Magnitude-Energy

Mag.-Energy Plot

Source

Mag.-Energy Relation

Source: Murphy, 1996

Forensic Seismology ResultsEvent locates to North KoreaEvent has strong Rg waves, implies shallow sourceEvents has high ratio of P/S, implies explosionFirst motions up, implies explosionAssuming an explosion, the magnitude (4.2 mb)

indicates a yield of about 1 ~ kt

Russian Earthquake Explosion

Source: http://www.atomicforum.org/russia/russiantesting.html

Detonated on August 29, 1949, it had a yield of 22 kilotons.

Detonated on August 12, 1953, it had a yield of 400 kilotons.

Detonated on September 14, 1954, it had a yield of 40 kilotons.

Magnitude as a discriminator

http://www.geoscienceworld.org/

Asperity Hazard Model

●

● within the northern Marmara Sea region.

TL(M) = dT/10 (a-bM)

Oncel and Wyss, 2000

Log N = a – b M

MAGNITUDE DISTRIBUTION

log

N

0

1

2

3

4

0 2 4 6 8

MAGNITUDE

23-31°E

b-value

aterial heterogeneity Applied shear stress level Thermal gradient Fault complexity

b-value :