Subduction zone megathrusts Flashcards
What are the controls of megathrust rupture area?
downdip
updip
lateral extent
What does seafloor uplift near a trench create?
tsunami (harbor wave)
How is slip at subduction zone megathrusts calculated?
Slip roughly ≤ plate convergence rate x time since last megathrust
What is the seismogenic zone?
the part of the Earth’s crust or lithosphere where most earthquakes occur
What is the limit of the updip from subduction zone megathrusts and what crontrols it?
~4 km bsf, controlled by clay dewatering
What is the limit of the downdip from subduction zone megathrusts and what crontrols it?
20-30 km depth, controlled by Temp and presence of hydrated mantle rocks
what controls the lateral extent of subduction zone megathrusts?
margin structure
What is the limitation of subduction zone megathrust extent and what does it suggest?
extent reaches but dos not overlap the limits of other megathrusts showing some long lived segemntation of the plate boundary
Why are megathrusts such a major hazard?
Not big EQ hazard as most shaking occurs at sea but tsunami generated big threat
What occurs in the interseismic period to megathrusts?
bending of overriding plate (bend before the snap) leads to uplift
What areas subsided or uplifted as a result of Alaska 64?
Areas close to fault lifted up exposing the old water line whereas more inland places subsided leading to flooding of coastal areas
What causes uplift and subsidence at megathrusts?
when plate is locked convergence causes bending and uplift in coastal areas when slipping overiding plate springs forward producing tsunami oceanward overiding plate shoots up but in land sinks causing flooding
What issue with tsunami peak and triugh is there?
If peak arrives before trough there will be no receeding of water just wave
What are most tsunamis caused by?
megathrusts
WHat is the speed of tsunamis and how does this affect the wave height?
900km/hr in water and 40 when reaches land this causes the back of the wave to catch up
WHat makes tsunamis more devestating then wind waves?
sealevel rise that rushes inland for ~15minutes (repeating several times) over a very large area
What are some past examples of tsunmanogenic earthquakes off sumatra?
1883: 8.7
1797: 8.2
1861: 8.5
How much displacement was there at sumatra 04?
5m uplift 4m subsidece
and 5m slip
THe rupture from sumatra 04 lasted 9 minutes what would the outcome be if it stopped sooner?
low magnitude eq
Thailand sheltered from worst of tsunami
What was one of the reasons Tohoku 2011 was so much larger than expected and bigger than sumatra?
sumatra extent double and 60% longer duration but due to the vast amount of slip (60m) in japan tsunami was larger
Where was though to be the highest risk tsunami area in japan?
Nankai trough in the south not the Japan trench
What is an orphan tsunami?
a tsunami with no local EQ
What is an examle of a orhpan EQ?
Japan 1700
What is thought to be the origin of the 1700 japan orphan tsunami?
Spreading ridge in NA creating Juan de fuca plate which subducts beneath north west America but this area is of very little seismicity
What factors in North America hint at one large event for the orphan tsunami?
Tsunami sands with fire pits which can be dated to around 1700 and gost forests which when using dendrochnology died after summer 1699
What are turbidites?
Sediments deposited on the edge of the continental shelf which fail from EQ or storm
What happens when continental shelf deposist are reactvate?
Fail as landlside entraining water move rapidly downslope as tubidity current carving canyons as current slows it deposits coarsest to finests (grading up)
What is a tell that turbidite currets happened at same time?
If multiple channels with turbidites converge and is not the sum of both channels (1+1=2) then happened simultaneously
What is dated to age turbidites?
hemipelagic just beneath
