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Seismology – Summary
lectures 1 and 2
Earthquakes -- yearly
Number of events per 12000 km2
Earthquakes -- Why?
Stresses applied to rocks cause brittle failures
Fault types
Normal: Spreading ridges
Mid-Atlantic Ridge
Thrust: Subduction zones
Cascadia
Strike-slip: Transform boundaries
San Andreas
Beachballs
Normal (Mid-Atlantic Ridge)
Beachballs
Reverse thrust (Cascadia)
Beachballs
Reverse oblique (Aleutians)
Beachballs
Strike-slip (San Andreas)
Beachball -- summary
Beachball equivalents for various fault types
Earthquake moment tensor
Components of the moment tensor – 9 elements
Fault Plane
Scale of earthquake:
Mo = fault plane area x slip x breaking strength of rock
First motion wave field
We see two quadrants of compression and two of
dilatations
Earthquake wave types
Body waves (travel through
volume of Earth):
P-waves (sound)
S-waves (shear)
Surface waves (travel across
surface of Earth:
Love waves (horizontal)
Raleigh waves (vertical and
horizontal)
Travel-time curves
P-to-S travel times
P-wave velocity through
rock is usually about
70% faster than S-wave
velocity.
The delay between Pwave and S-wave
arrivals at a
seismograph
determines distance
from source event.
Locating a seismic event
If we can determine
the distance of an
event from 3 or more
seismographs, we
can locate its
epicentre.
...from Press and Seiver
Benioff Zone
Cascadia subduction zone
Earth Structure
Schematic of interior structure of the Earth