SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Fig 4.19 • lithosphere is broken up into 12 major plates • move about (a few cm per year) • divergent • driven by mantle convection • convergent • transform Mendocino Triple Junction SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries Image: P. Abbott “Natural Disasters”
Fig. 5.3 Normal Events Reverse and Thrust Events watch Short Video 5a typically along convergent typically along divergent plate boundaries plate boundaries typically along transform plate boundaries Strike-slip events SAF is right-lateral strike-slip fault SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries Images: S. Marshak “Earth, Portrait of a Planet”
Figs. 4.19/4.21 • most EQs along Courtesy: David Sandwell, SIO all 3 types of plate boundary • these EQs are shallow • some intra-plate EQs! shallow (< 100km) intermediate (100-300 km) deep (>300 km) SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Fig. 4.21 deep Eqs only • ocean-ocean convergent • ocean-continent convergent = subduction zones shallow (< 100km) continent-continent convergent intermediate (100-300 km) has no subduction! deep (>300 km) SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Fig. 4.21 shallow (< 100km) • shallow EQs mainly along trenches intermediate (100-300 km) • deep earthquakes only behind trenches deep (>300 km) • distance from trench -> steepness of slab SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
• steep • normal • shallow SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
• earthquakes trace the subduction slab • shallow EQs behind trench • Wadati-Benioff zone in OVERRIDING plate • most EQ near top of slab Kiyoo Wadati (1902-1995) slab: part of the subducting plate that is already in the mantle Hugo Benioff (1899-1968) SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Fig. 4.21 • deep earthquakes occur only to 670 km • below pressure too high for cracks in crust • deepest EQ due to phase transformations subducting plate heats up � no longer brittle but ductile Image: S. Marshak “Earth, Portrait of a Planet” SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
the largest EQs in the last 100 year occurred in subduction zones SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
16ºN rupture propagation of 2004 Sumatra-Andaman EQ 0ºN • the largest EQs occurred along long faults • large EQs along short faults are unlikely SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Fig. 6.26 updated PGA 2% in 50 yrs source: USGS SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Mag 7.9/8.0, 1811/1812; largest U.S. EQ outside of Alaska Earthquakes recorded (together with 1857 Fort Tejon, CA) between 1974 and 1995 Mercalli Intensity Short Video 5b Fig. 6.35 Fig. 6.35 Failed Ancient Rift (~550 my ago)! SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Fig. 6.36 SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Source: Wikipedia.org SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Fig. 5.9 Lisbon 1755 Mercalli Intensity ~ 100,000 fatalities watch Short Video 5b I: not felt II: felt by only few people at rest suspended objects may swing • oldest scale (1902) V: felt indoors by nearly everyone some broken dishes, • twelve points minor cracks in plaster • reported damage • not a measure for EQ size XII: damage nearly total objects thrown up in air SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Waste water injection from typically < M4 but: Shale Oil Production - exponential increase in last 10 years 9/3/16: M 5.8 strike-slip (most likely natural) 2011: M 5.6; $1M tied with 2011 Virginia; strongest in 70 years Image: wikipedia/USGS problem: no EQ building code SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Oil Production Reservoirs/Dams • fracking • waste water disposal Image: wikipedia/USGS SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
2010: OGS scientists suspected induced seismicity 2013: OGS and USGS – “activities such as wastewater disposal may contribute” 2015: OGS “natural cause very unlikely” 2016: after 9/3 EQ, Oklahoma Corporation Commission ordered emergency shut down of 37 waste water disposal wells � Traffic light systems to warn which well need to shut down Earthquake insurance: only 18% of claims were filled $4.5M $1.2M for 1 mansion Image: wikipedia/USGS SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Image: wikipedia/USGS SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Fig. 5.1 hanging wall foot wall • fault plane: contact zone along which an EQ occurs • fault: surface expression of fault plane • hanging wall: wall above fault plane • foot wall: wall below fault plane • epicenter: surface projection of hypocenter SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Fig. 5.1 h 0 : source depth earthquake size depends on • rupture area within the fault plane • slip (displacement), rupture length • source duration i.e. large EQs are unlikely along short faults SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Fig. 5.2 • strike (direction of surface trace) • dip (dip of fault plane) • rake (direction of motion of hanging wall) • slip vector (motion of block) hanging wall foot wall SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
• > 3.5 Mio per year • small events often • large events rare SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
Fig. 5.13 Get recurrence time from # of EQ per year E.g. 3 Mio EQ/year -> 1 EQ every 10.5s 0.3 EQ/year -> 1 EQ every 3.3 years SIO15-SS1 2020: Topic 5 Earthquakes and Plate Boundaries
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