Earthquakes and Related Phenomena. Earthquake Web Resources

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 0.3 8.0 - 8.4 1.1 7.5 - 7.9 3.1 7.0 - 7.4 15 6.5 - 6.9 56 6.0 - 6.4 210

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 (right-or

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