I. What is an earthquake?
Earthquake defined
A.
•
•
An earthquake is a series of vibrations (shaking) at the
earth’s surface produced by the sudden release of
energy by movement along a fault.
Other mechanisms of earthquake generation…
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B.
C.
Volcanic activity
Landslides
Meteorite impacts
Major explosions
Fractures and faults
Some terminology
I. What is an earthquake?
Earthquake defined
Fractures and faults
A.
B.
•
•
•
Faults are fractures within the earth (rocks and soils)
along (across) which movement has occurred
Faults are 2 dimensional planes within the 3D earth
Faults appear as 1 dimensional lines at the earth’s 2D
surface, on maps of the earth’s surface and on vertical
cross-sections
[the fault is a plane, the surface is a plane where two plane intersect you get a line]
•
Fractures with no movement are called joints.
Strike-Slip
Thrust
Normal
Slide source: Lisa Wald – USGS Pasadena
Faults: Normal faults offsetting layers in volcanic ash deposits
2D cross-section view of fault in road cut
(plane of the faults intersect the plane of the vertical road cut, looks like a line)
Faults cutting
layered
volcanic rocks
Near Kingman AZ; image from R.J. Varga
San Andreas Fault
(right-lateral strike-slip
fault)
Wallace Creek,
Carrizo Plain, CA
2D map view of fault
from above
(plane of vertical fault
intersects plane of the
surface of the earth = line
SERC and USGS
Faults: Strike-slip fault near Valley of Fire (Clark Co, NV)
East of Las Vegas: image from M.B. Miller
I. What is an earthquake?
Earthquake defined
Fractures and faults
Some terminology
A.
B.
C.
•
•
Seismology – field of study of earthquakes
Seismic waves – vibration energy transmitted through earth
materials (rocks and soils)
•
Earthquake focus (or hypocenter) – site within the earth
•
where fault fails, source of seismic waves
Earthquake epicenter – position on the earth’s surface above
the focus
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Magnitude – calculated energy released at the earthquake
focus
•
Intensity – experienced effects of seismic waves on people
and materials (varies with location and materials, type of
building, rocks vs. loose soils)
M5
M6
M7
Magnitude scale is logarithmic
M7 = 32*M6 = 1024*M5
Slide source: Lisa Wald – USGS Pasadena
Earthquake Intensity – Shake Maps