Earthquakes III
Case studies
Northridge California, 1994
 4:30 AM
 Blind fault: reverse fault, 35 degree dip
 M = 6.7 (moment magnitude)
 Focus: 18 km depth
 Vertical acceleration > gravity
 Houses moved off foundations
Bhuij, India, 2001
 Reverse (thrust) fault
 No ground rupture
 Liquefaction
 Intraplate earthquake (New Madrid ?)
7.9
Denali fault, Alaska
 7.9 magnitude, 2002, aftershock of 6.7 M
 Fault rupture length 320 km
 Highway offset by 2.5 meters
 Alaska pipeline not ruptured (on moveable rails)
 Low population – no dead
Colima Mexico 2003
 7.8 Magnitude
 On coast, 600 miles South USA border
 Subduction zone related
 Only 27 dead
 Compare Mexico city, 1985, 10,000 dead (liquefaction)
Geologic methods
•
•
•
•
•
Paleo-seismicity –
Examine disrupted rocks or sediments for past earthquakes.
Use C14 to date events.
San Andreas: 650 AD – 1857 – disrupted lake deposits
Washington state: Tsunamic deposits (1700)
Some terminology
• Recurrence interval (e.g. Parkfield 22 yrs)
• Aftershocks – on same fault as main shock
• Precursors: warning signs of quake
• Animal behavior, change in water levels, gas emissions, lightning,
• None reliable as predictors.
 Earthquakes are not caused by planet alignments
 Quakes occur on the moon - moonquakes