Transcript L13_Earthquakes1

Earthquakes and Related Phenomena
Earthquake Web Resources
http://earthquake.usgs.gov/
Deadliest Earthquakes on Record
Introduction to Earthquakes
• ~1 million earthquakes a year
• Change of 1 Mw = ~30 X energy
• Great earthquakes; Ms >8
• Major earthquakes; Ms=7-7.9
(widespread damage)
• Moderate earthquakes: Ms=6
(considerable damage)
• Population factors: density, land use,
and building construction
Ms Earthquakes/year
8.5 - 8.9
8.0 - 8.4
7.5 - 7.9
7.0 - 7.4
6.5 - 6.9
6.0 - 6.4
0.3
1.1
3.1
15
56
210
Where do earthquakes occur?
Plate boundaries, but not only – intraplate earthquakes are also common
Earthquake Occurrence
• Most occur along plate
boundaries.
– Loma Prieta (1989)
– North Ridge (1994)
• Intraplate earthquakes
– Can be large and quite
damaging.
• New Madrid zone is a
foundered rift zone
• Salt Lake City and south
into New Mexico
• Charleston, S.C.
Largest US Earthquakes
Relationship of Earthquakes to Faults
Earthquake Processes
• Faults, Folds & Concealed Faults, and Movement
– Fractures
• Faults are fractures or systems along which rocks have
been displaced
– Model: brick -bungee cord on a winch; sand paper
• Joints are fractures and fractures systems along with
rocks have NOT been displaced
• Slip rate aver. long-term rate of movement ( mm/yr or
m/1000 yrs); risky measurement
• Seismic waves produced by rupture
• Faults-seismic sources; used to evaluate the earthquake
risk in a given area
Earthquake Processes
• Fault types
• Strike-slip faults only have horizontal displacement (rightor left-lateral)
• Normal faults occur where the hanging wall moves down
with respect to the foot wall; results in crustal extension
• Reverse faults occur when the hanging wall moves up with
respect to the foot wall; results in crustal shortening; low fault
plane angle ( less than 45°)= thrust fault
• Oblique faults have both horizontal and vertical
components
• Buried faults may cause earthquakes and have to surface
expression
Faults – Understanding Strike and Dip