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Question

Read this website to help you with the following questions:

https://www.pnsn.org/outreach/earthquakesources/CSZ/turbiditeevidencee)

e) What are turbidites?

f) What causes them?

g) How does the fact that the turbidites are simultaneous support the idea that earthquakes cause them?

h) How do cores of ocean sediment allow us to plot the history of catastrophic earthquakes in the Pacific Northwest?

i) What is the recurrence interval for a full Cascadia margin rupture?

j) When did the last one occur?

Explanation / Answer

e) turbidites are the clastic sediment deposition by the turbidity currents in the ocean, this currents spread this sediment deposition through out the deep ocean floor. turbidity is a measure of the amount of sediments, planktons and organic by-products.

f) most of the turbidity currents occur as a result of the earthquake. a large volume of sediment is deposited along the continental shelf and slope, when a shaking experiences in this region they become unstable and collapse, this will moves to the deep ocean like an avalanche, and the speed increases as it moves down. this mass flow will erode the ocean surface and makes some changes over it like under water canyons.

g) the sediment carried by rivers are mainly deposits along the continental shelf which is a more stable area, then it start to move towards the slope regions with the water flow and they becomes close to a unstable position because any kind of movement on the base will causes it to slide over the slope, all other factors other than earthquake is not capable of shake the basement so, the turbidites occur when the it takeplace, and due to the gravity they moves violently in to the dep ocean.

h) core samples in the ocean floor will give the history of deposition, during the turbidies caused by earthquake, a large volume of sediments get deposited in a short period of time, with organic materials, usually the deep see deposits will have fine particle deposition in between these layers we can found the turbidite deposits easly and can leads to a past earth quake that triggered the turbidite.

i) the time interval of recent four earthquakes are, 390,500,980 and 610 years. if we take an average the rupture occurs with a 620 year. it varies for different earthquakes because the rate of accumulation of stress on plate margins are not uniform every time.

j)from the sediment sample its clear that the last turbidity flow occured 300 years ago, so the earthquake caused by the rupture also occured in the same time. so the last rupture occured 300 years ago.

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