Divergent plate boundaries
- Divergent motion and high temps = rock failure easy in tension
- Small earthquakes generally
Transform plate boundaries
Plates slide horizontally; locked at irregularities in plate boundaries
Convergent plate boundaries
- Huge energy required
- Largest earthquakes
The energy required to move plates is released as
large earthquakes
Spreading Center Earthquakes: Iceland
- Divergent boundary earthquakes are uncommon
Iceland: - Volcanic island hot spot on the mid-Atlantic ridge
- Swarms of small earthquakes
Red Sea and Gulf of Aden
- Young spreading center, new ocean basin
- Hot upper mantle under Africa melts and uplifts forming a rift
- Flooding by ocean = new sea
- At south end of red sea = 3 rifts meet at triple junction
- Arabian plate diverges from African plate
- Somali plate may diverge from African plate
Subduction-zone earthquakes
- Great earthquakes
- At current rates, oceanic plates completely submerged in 180mil years
- Shallow earthquakes:
~ compressive movements of overriding plate and subducting plate - Lithospheric bending eventually snaps
- Largest, most damaging earthquakes
Deep earthquakes:
Subducting plate generates earthquakes to 700km in depth
- Earthquake energy dissipates before reaching the surface
Seismic-Gap Method
- Predict sites of future earthquakes
- Faults fail in segments:
~ Recent failure segments = low future prob
~ Non-failed segments = high future prob
Subduction-Zone Earthquakes: Indonesia, 2004
- Indian-Australian plate collides Sunda plate = frequent, high subduction-caused earthquakes
- Dec 26, 2004: 1,500km fault rupture in 11min
- March 28, 2005: 400km rupture south of earlier rupture
- Additional quakes in years after: 9 7.4+ mag events over 8 years
- More seismic gaps to fill
Subduction-Zone Earthquakes: Mexico City, 1985
Resonance:
- 5,700 buildings damaged, 15% collapsed
- Small towns near epicenter had much less damage
- Seismic resonance amplified ground motion due to soft, unconsolidated lake-sediment
- T < 10s waves
- Weak building code enforcement
- 6-16 story buildings resonate to destruction
Pacific Northwest: The Upcoming Earthquake
- Cascadia subduction zone: small plates subducting North America
- Why no major earthquakes in last 200yrs in Pacific NW? Volcanism is still occurring so subduction is too
- Are plates subducting so smoothly earthquakes don’t occur? Prob not, since subducting lithosphere is so young
Pacific Northwest: Earthquake in 1700, The Trees Tell the Story
- Drowned coastal forests at end of grow season 1699
- Tsunami hits Japan indicating a large Washington-Oregon costal EQ ~21:)), Jan 26, 1700
- Likely to occur again
- Major cities in the region
Transform-Fault Earthquakes: Haiti, 2010
- 7mag EQ, 230,000 killed, 300,000 injured, displaced 1.1mil
- ~250,000 houses and 30,000 commercial buildings collapsed
Transform-Fault Earthquakes: Turkey, 1999
- Transform fault b/w Arabian and Eurasian plates
11 EQs > 6.7M - Unique, semi-regular pattern
- Interval of 3months to 32 years
- Next event? Prob near Istanbul, home to 13mil ppl
Transform Fault EQs: San Andreas
- Opening of Atlantic Ocean basin pushes North and South America westward
San Andreas Fault: different sections behave differently:
- South of San Fran: frequent small to moderate EQs and creep
- North of LA: locked zone with recent deficit of earthquake activity, most recent large rupture in 1857 Fort Tejon EQ of mag 7.9
- Southernmost section from San Bernardino to Salton Sea: complex zone with locked sections but little large EQ activity in recorded history
San Andreas Fault: Bay Area EQ Probabilities
- San Fran Bay region: 62% prob of 6.7mag EQ on fault before 2032
- Hayward fault: 27% prob of causing 6.7mag or greater EQ before 2032
- Next movement of Hayward fault will likely cost bils of dollars and thousands of deaths
