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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