Solid Earth Geophysics-KFUPM
Solid Earth Geophysics
Today’s class: Earthquake Mechanism
Reading: Fowler Chapter 4
Ali Oncel
oncel@kfupm.edu.sa
Department of Earth Sciences
KFUPM
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 cursor
 Then, 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
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/ne
is/sopar/
http://pubs.usgs.gov/d
s/2007/241/
3D Focal Mechanisms
Data Search: Example for Marmara
http://pubs.usgs.gov/ds/2007/241/
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
a) 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.
Earthquake or Bomb?
How can we tell?
Nuclear Bomb: Compressional
Source. P wave first motion in all
directions.
Slip on a fault: P wave
first motion compressional and
extensional.
Radiation patterns for Bomb and Earthquake are totally different due to
differences in their sources as: Compressional and Shear.
From: Vince Cronin
Bomb
Data recorded at Nilore, Pakistan. Nuclear tests are
shallower than most earthquakes.
Earthquake
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.
Forensic Seismology
Raw Data
Filtered Data
Magnitude-Energy
Mag.-Energy Plot
 Source
Source: Murphy, 1996
Mag.-Energy Relation
Forensic Seismology Results
 Event locates to North Korea
 Event has strong Rg waves, implies shallow source
 Events has high ratio of P/S, implies explosion
 First motions up, implies explosion
 Assuming an explosion, the magnitude (4.2 mb)
indicates a yield of about 1 ~ kt
Russian Earthquake Explosion
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.
Source: http://www.atomicforum.org/russia/russiantesting.html
Magnitude as a discriminator
http://www.geoscienceworld.org/
Asperity Hazard
Model
MAGNITUDE DISTRIBUTION
"
"
within the northern Marmara Sea
region.
4
log N
3
b-value
2
1
0
23-31°E
0
2
4
6
8
MAGNITUDE
Log N = a – b M
b-value :
Material heterogeneity
Applied shear stress level
Thermal gradient
Fault complexity
Oncel and Wyss, 2000
TL(M) = dT/10
(a-bM)