Subduction zone megathrusts

Question 1

What are the controls of megathrust rupture area?

Answer 1

downdip
updip
lateral extent

Question 2

What does seafloor uplift near a trench create?

Answer 2

tsunami (harbor wave)

Question 3

How is slip at subduction zone megathrusts calculated?

Answer 3

Slip roughly ≤ plate convergence rate x time since last megathrust

Question 4

What is the seismogenic zone?

Answer 4

the part of the Earth’s crust or lithosphere where most earthquakes occur

Question 5

What is the limit of the updip from subduction zone megathrusts and what crontrols it?

Answer 5

~4 km bsf, controlled by clay dewatering

Question 6

What is the limit of the downdip from subduction zone megathrusts and what crontrols it?

Answer 6

20-30 km depth, controlled by Temp and presence of hydrated mantle rocks

Question 7

what controls the lateral extent of subduction zone megathrusts?

Answer 7

margin structure

Question 8

What is the limitation of subduction zone megathrust extent and what does it suggest?

Answer 8

extent reaches but dos not overlap the limits of other megathrusts showing some long lived segemntation of the plate boundary

Question 9

Why are megathrusts such a major hazard?

Answer 9

Not big EQ hazard as most shaking occurs at sea but tsunami generated big threat

Question 10

What occurs in the interseismic period to megathrusts?

Answer 10

bending of overriding plate (bend before the snap) leads to uplift

Question 11

What areas subsided or uplifted as a result of Alaska 64?

Answer 11

Areas close to fault lifted up exposing the old water line whereas more inland places subsided leading to flooding of coastal areas

Question 12

What causes uplift and subsidence at megathrusts?

Answer 12

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

Question 13

What issue with tsunami peak and triugh is there?

Answer 13

If peak arrives before trough there will be no receeding of water just wave

Question 14

What are most tsunamis caused by?

Answer 14

megathrusts

Question 15

WHat is the speed of tsunamis and how does this affect the wave height?

Answer 15

900km/hr in water and 40 when reaches land this causes the back of the wave to catch up

Question 16

WHat makes tsunamis more devestating then wind waves?

Answer 16

sealevel rise that rushes inland for ~15minutes (repeating several times) over a very large area

Question 17

What are some past examples of tsunmanogenic earthquakes off sumatra?

Answer 17

1883: 8.7
1797: 8.2
1861: 8.5

Question 18

How much displacement was there at sumatra 04?

Answer 18

5m uplift 4m subsidece

and 5m slip

Question 19

THe rupture from sumatra 04 lasted 9 minutes what would the outcome be if it stopped sooner?

Answer 19

low magnitude eq
Thailand sheltered from worst of tsunami

Question 20

What was one of the reasons Tohoku 2011 was so much larger than expected and bigger than sumatra?

Answer 20

sumatra extent double and 60% longer duration but due to the vast amount of slip (60m) in japan tsunami was larger

Question 21

Where was though to be the highest risk tsunami area in japan?

Answer 21

Nankai trough in the south not the Japan trench

Question 22

What is an orphan tsunami?

Answer 22

a tsunami with no local EQ

Question 23

What is an examle of a orhpan EQ?

Answer 23

Japan 1700

Question 24

What is thought to be the origin of the 1700 japan orphan tsunami?

Answer 24

Spreading ridge in NA creating Juan de fuca plate which subducts beneath north west America but this area is of very little seismicity

Question 25

What factors in North America hint at one large event for the orphan tsunami?

Answer 25

Tsunami sands with fire pits which can be dated to around 1700 and gost forests which when using dendrochnology died after summer 1699

Question 26

What are turbidites?

Answer 26

Sediments deposited on the edge of the continental shelf which fail from EQ or storm

Question 27

What happens when continental shelf deposist are reactvate?

Answer 27

Fail as landlside entraining water move rapidly downslope as tubidity current carving canyons as current slows it deposits coarsest to finests (grading up)

Question 28

What is a tell that turbidite currets happened at same time?

Answer 28

If multiple channels with turbidites converge and is not the sum of both channels (1+1=2) then happened simultaneously

Question 29

What is dated to age turbidites?

Answer 29

hemipelagic just beneath