Mechanical waves

Table of Contents

1. Overview
2. Mechanical waves
   1. Types of mechanical waves
   2. Characteristics of mechanical waves
3. Wave behaviour
   1. Reflection
   2. Reflection in 2-D
   3. Refraction
   4. Diffraction
4. Oscillations
   1. Free oscillations
   2. Forced oscillations
   3. Resonance
5. Interference
   1. Principle of superposition
   2. Standing waves
   3. Interference in 2-D
   4. Applications of interference
6. Task: 2004 Sumatran Tsunami

1. Additional resources:
   1. Glossary
   2. Print content

Task: 2004 Sumatran Tsunami

Read the information below and answer the questions that follow.

Plate tectonics theory tells us that large slabs of the earth's surface extending for thousands of kilometres are being driven in different directions by deep-seated convection currents. For millions of years the northward-moving Indian plate has been experiencing a slow motion collision with the Eurasia plate (creating the Himalayas) and the Sunda plate, upon which China and South-East Asia ride.

On its journey north, the eastward-dipping Indian plate drags down a portion of the Burma microplate along a boundary in a process known as subduction. For a period of about 300 years a section of the boundary became locked together; storing an enormous amount of strain energy.

On Sunday 26 December 2004, 7:59 am local time, the two plates slipped non-uniformly, unzipping for 420 seconds along a 1200 km north-south zone, north-west of Sumatra, Indonesia. The Burma plate sprung upwards on the zone's western side and subsided on the zone's eastern side like a gigantic wrist action. This created an average 4-5 m vertical displacement of seabed and the second largest earthquake in recorded history, vibrating the whole planet up to 1 cm.

Interference between long period (~0.3 mHz) travelling seismic waves generated standing waves that oscillated through the earth's interior. Later analysis of the amplitudes of these standing waves revealed the earthquake's magnitude to be an incredible 9.3 on the moment magnitude scale (a modern earthquake scale).

Measurement of differences in arrival times of seismic P-waves that radiated through the earth's interior placed the initial rupture at a depth of about 35 km and a surface distance of 250 km south-south east of Banda Aceh, the capital city of Aceh province in northern Sumatra.

Tsunamis were generated across the entire rupture zone and moved with astonishing speed. In two hours they reached Sri Lanka, the Maldives in four hours and Perth in six hours – as fast as a commercial passenger aircraft.

Tsunamis have small amplitudes and very long wavelengths in the open ocean, but as they approach shallow waters their amplitudes increase significantly. The maximum height of the Sumatran Tsunami was 35 m; determined by the devastation wreaked on forested hills near beaches at Leupung, a small town in Aceh province. Along the western coastline facing the rupture zone the tsunamis ranged from 15 m to 30 m in height.

Apart from Indonesia the countries with the highest death tolls included Sri Lanka, Thailand and India. It was one of the deadliest earthquakes known with official figures stating that about 184 000 lives were confirmed lost and about 45 000 people missing. To avoid such tragedies, early warning systems consisting of sensitive seabed sensors and floating buoys linked to orbiting satellites are strategically placed in the Pacific Ocean and, to a lesser extent, the Indian Ocean. Nevertheless, for many poor coastal nations, educating inhabitants about tsunami danger signs is their best defence for saving lives.