TOPIC 1 TECTONIC PROCESSES AND HAZARDS Lesson 10

TOPIC 1: TECTONIC PROCESSES AND HAZARDS • (Lesson 10 b)

ENQUIRY QUESTIONS: • 1. Why are some locations more at risk from tectonic hazards? • 2. Why do some tectonic hazards develop into disasters?

Previous lesson Objectives • To describe and explain the global distribution of tsunamis • To understand the causes of tsunamis by submarine earthquakes at subduction zones as a result of sea bed and water column displacement • To be able to analyse a tsunami time travel map to aid prediction • To have knowledge of strategies to modify future tsunami events: including tsunami warning systems • Ref: Hodder p 11 -12 Oxford p 20 -23

Today’s lesson objectives • To understand the social economic and environmental impacts of a tsunami • Case study examples Indian Ocean 2004 and Tohoku, Japan 2011

Recap: What is a tsunami? (Japanese: tsu – harbour; nami – wave) Definition A series of waves caused by the sudden displacement of a large volume of water in an ocean or large lake, as a result of: • earthquakes (most common; shallow focus <30 km) • undersea volcanoes • undersea landslides • landslides or glaciers breaking off into lakes •

As they approach shallower water they slow down but the height of tsunami waves increase- ‘shoaling effect’. .

Are tsunamis rare? It depends on the scale of event (ocean-wide or local) and whether we are talking about time or space Ocean-wide, 2004 NOAA

Source: NOAA Ocean-wide, 2011

Do tsunamis have to be devastating? The highest tsunami wave ever recorded was in Lituya Bay, Alaska, in 1958. Caused by a rock fall (30 million m 3) from a height of 1000 m. It created a tsunami wave 520 m high. Impact? 5 deaths, 1 million trees lost

Do tsunamis have to be devastating? Indian Ocean Banda Aceh 2004 – low-lying 31, 000 deaths Simeulue - island folklore recounted an earthquake and tsunami in 1907, and islanders fled to inland hills before the tsunami struck. 7 deaths (from a population of 78, 000) www. nirmalya. net

Do tsunamis have to be devastating? Since the start of the international warning system for the Pacific in 1965, situated in Hawaii, less than 1% of tsunami deaths have occurred at distances greater than 3+ hrs tsunami travelling time, compared to ~17% of tsunami deaths prior to 1965. NOAA Take home message – warnings save lives! In 2004, there was no equivalent organisation in place for the Indian Ocean. The Sumatra earthquake and its tsunami risk were detected by NOAA and others, but there was not a proper communications network with countries at risk.

The Physical and Human factors which affect the IMPACT of a tsunami… A tsunami hazard is a flood hazard… hazard Garry Welch. U. S. Marine Corps Sunichi Koshimura

WHITEBOARD-PAIRED TASK • What physical factors do you think could influence the degree of devastation by a tsunami?

Physical factors that can influence the degree of devastation • Wave energy – Shoaling – Depth of water – Shape of seabed – Interference waves ACT News • Shape of coastline • Topography( shape) of the land • Lack of natural defences – coral, mangroves

Physical factors that can influence the degree of devastation Waikato Regional Council Wave energy • The rate at which a wave loses its energy is inversely related to its wavelength. • Tsunamis can have very large wavelengths, up to 100 km and therefore large volumes of water. • In very deep water, a tsunami will travel at high speeds with little loss of energy. Can travel across the Pacific Ocean in less than one day.

Physical factors that can influence the degree of devastation Waikato Regional Council Shoaling • Shoaling is the major destructive process. As water becomes more shallow, the water is pushed upwards. • Even when tsunamis have shoaled, they still have a relatively long wavelength, which is why they seem to keep flowing inland over many minutes. The first wave is not always the worst. • Underwater ridges are shallower areas and can intensify a tsunami.

Physical factors that can influence the degree of devastation www. nbcnews. com Constructive interference patterns When two waves travelling in different directions meet, they combine their energies and form interference patterns, which can increase the amplitude of the waves.

Physical factors that cause an increase in devastation Nature of the coastline • Wave heights increase in bays that narrow from the entrance to the head, funnelling energy on to the coastline. • Loss of coral reefs and mangroves means that coastlines are no longer protected. • Low-lying coasts are vulnerable.