Lecture 16 - Tsunami Case Studies and Mitigation

Question 1

What was the deadliest tsunami of all time?

Answer 1

the 2004 Indian Ocean tsunami

Question 2

What was the 2004 Indian Ocean tsunami caused by?

Answer 2

It was caused by a Mw 9.2 earthquake — the third largest ever recorded — on the Sunda megathrust fault, where Indo-Australian oceanic lithosphere subducts beneath Sumatra, Indonesia.

Question 3

What was the rupture duration, length and peak slip of the earthquake that caused the 2004 Indian Ocean tsunami?

Answer 3

The ∼8-minute-long earthquake broke a fault length of ∼1,300 km (the longest rupture duration and length ever observed) with peak slip of ∼20 m.

Question 4

How many people did the 2004 Indian Ocean tsunami and earthquake kill in Indonesia?

Answer 4

The earthquake and tsunami killed ∼160,000 people in Indonesia.

Question 5

How high were run ups in Indonesia during the 2004 Indian Ocean tsunami?

Answer 5

> 30 m

Question 6

Why were so many people killed by the 2004 Indian Ocean tsunami?

Answer 6

There was no warning system in place for the Indian Ocean.

Question 7

How many people were killed by the 2004 Indian Ocean tsunami? (not including Indonesia)

Answer 7

∼60,000 in thirteen other countries across the Indian Ocean, as far away as South Africa.

Question 8

How long did the 2004 Indian Ocean tsunami take to reach Sri Lanka, India, and Thailand?

Answer 8

two hours after the earthquake

Question 9

How high were run ups in some areas of Sri Lanka, India, and Thailand during the 2004 Indian Ocean tsunami?

Answer 9

> 10 m

Question 10

What was the 2011 Tohoku (Japan) tsunami caused by?

Answer 10

It was generated by a Mw 9.0 megathrust earthquake (the fourth largest ever recorded) along the Japan trench, where the Pacific plate subducts westwards between the island of Honshu.

Question 11

How was the 2011 Tohoku (Japan) tsunami different from the 2004 Indian Ocean tsunami?

Answer 11

The Japanese authorities issued a major tsunami alert immediately after the earthquake.

Question 12

Why did the sheer size of the wave in the 2011 Tohoku (Japan) tsunami take people by surprise?

Answer 12

The Japan trench had hosted several Mw 7–8 earthquakes in the previous century, but it was thought that a Mw 9 earthquake was impossible.

Question 13

How did tsunami defences hold up in the 2011 Tohoku (Japan) tsunami?

Answer 13

The wave overtopped tsunami walls that were the tallest in the world and flooded areas that were assumed safe.

Question 14

What was the maximum run up height in the 2011 Tohoku (Japan) tsunami?

Answer 14

Maximum run-up heights were ∼40 m (at Miyako).

Question 15

What was the maximum run up distance in the 2011 Tohoku (Japan) tsunami?

Answer 15

Maximum run-up distances were ∼8 km (at Sendai).

Question 16

How many people died in the 2011 Tohoku (Japan) earthquake and tsunami?

Answer 16

Around 18,500 people died in the earthquake and tsunami, ∼93% by drowning.

Question 17

Waht were the economic costs associated with the 2011 Tohoku (Japan) earthquake and tsunami?

Answer 17

The economic costs exceeded US $3.6 billion, making this the costliest natural disaster ever.

Question 18

Why did only a few fatalities occur outside Japan asa a result of the 2011 Tohoku tsunami?

Answer 18

Due to a successful Pacific- wide alert.

Question 19

What was the 1958 Lituya Bay tsunami caused by?

Answer 19

A Mw 7.8 earthquake triggered a huge rockfall at the head of Lituya Bay, an uninhabited inlet in southeastern Alaska. A ∼30,000,000 m³, intact slab of rock plunged directly into deep water at the head of the bay, causing a giant wave to run up the hillside opposite.

Question 20

Up to what height were trees stripped away by the 1958 Lituya Bay tsunami?

Answer 20

Up to a height of ∼500 m above sea-level, a peak run-up that ranks this the largest amplitude wave in modern history.

Question 21

Why is the 1958 Lituya Bay tsunami considered a local tsunami?

Answer 21

The effects of the tsunami were hardly visible outside of the narrow inlet, making this an example of a local tsunami.

Question 22

How many people were killed by the 1958 Lituya Bay tsunami?

Answer 22

Three fishing boats had anchored near the entrance to the fjord; two were sunk, drowning two people (two others survived), while the third boat, remarkably, rode out the ∼25 m high wave and was left undamaged.

Question 23

What is another example of a tsunami caused by a rock fall?

Answer 23

The June 2017 Nuugaatsiaq tsunami (which killed four people in a small community in western Greenland) was also generated by a large rockfall on steep hillslopes above a fjord, though in this case there does not appear to have been an earthquake trigger.

Question 24

What was the trigger for the 1929 Grand Banks tsunami? (Newfoundland/Nova Scotia, Canada)

Answer 24

A Mw 7.2 intraplate strike-slip earthquake which then triggered collapse of the steep continental slope in a huge submarine landslide.

Question 25

What was the estimated volume of the submarine landslide associated with the 1929 Grand Banks tsunami?

Answer 25

∼200 km³

Question 26

How was the submarine landslide associated with the 1929 Grand Banks tsunami discovered?

Answer 26

It generated a turbidity current which broke several transatlantic telegraph cables.

Question 27

How did the submarine landslide cause with the 1929 Grand Banks tsunami?

Answer 27

In the landslide source area, an abrupt increase in sea-floor depth caused the sea surface to warp downwards, generating a tsunami.

Question 28

How long did it take the 1929 Grand Banks tsunami waves to reach the Burin Peninsula?

Answer 28

Just over two hours.

Question 29

What was the size of the tsunami waves from the 1929 Grand Banks tsunami and what was the size of the run up in coastal inlets?

Answer 29

4–7 m waves became 15–30 m run-ups.

Question 30

How many people were killed in the 1929 Grand Banks tsunami?

Answer 30

28 people were killed, making this the deadliest earthquake or tsunami in modern Canadian history.

Question 31

Why is the 1929 Grand Banks tsunami considered a regional tsunami?

Answer 31

Although waves were recorded as far south as South Carolina and across the Atlantic Ocean in Portugal, major damage was restricted to Newfoundland.

Question 32

Why was a ~2 m-high tsunami inJanuary 1700, recorded in eastern Japan, known there as the “orphan tsunami”?

Answer 32

They felt no earthquake. This orphan tsunami was believed to have been caused by a mega thrust earthquake on the Cascadia subduction zone.

Question 33

What do tsunami early warning systems aim to do?

Answer 33

They aim to alert authorities to incoming waves and prompt them to evacuate coastal regions.

Question 34

What did the first tsunami early warning system rely on?

Answer 34

The Pacific Tsunami Warning Center, relied upon rapid estimates of earthquake locations, magnitudes and depths. However, not all large, shallow, marine earthquakes generate tsunamis, and so this system produced many false alarms.

Question 35

What do modern tsunami early warning systems rely on?

Answer 35

More sophisticated warning systems rely upon direct measurements of the tsunami wave, and include ocean-bottom pressure sensors (e.g. Deep-ocean Assessment and Reporting of Tsunamis, DART) and coastal radar systems (e.g. the WERA wave radar system in Tofino, BC).

Question 36

What can tsunami hazard maps be used to indicate?

Answer 36

areas that are safe from run ups

Question 37

What can be built in cities (e.g. Padang, Sumatra) where it is difficult to evacuate quickly to higher ground?

Answer 37

raised earth parks

Question 38

What are examples of structural countermeasures to tsunamis?

Answer 38

tsunami walls to block waves

offshore breakwaters and berms to dissipate their energy

Question 39

What is an example case study of a tsunami where the first wave was not the largest?

Answer 39

In the 1960 Chilean earthquake and tsunami at Hilo, Hawaii the third wave was the highest. 61 people were killed.