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EAS/BIOEE 154 Lecture 12 Introduction to Fundamental Principles Ideally, waves represent a propagation of energy, not matter.  (but waves are not always ideal). Three kinds:  Longitudinal (e.g., sound )  Transverse (e.g., seismic “S” wave) - only in solids  Surface, or orbital wave  Occur at the interfaces of two different densities.  These are the common wind-generated waves. Some Definitions  Wave Period: it Takes a Wave Crest to Travel one (units of time)  Wave : Number of Crests Passing A Fixed Location per Unit Time (units of 1/time)  Frequency = 1/Period  Wave Speed: Distance a Wave Crest Travels per Unit Time (units of distance/time)  Wave Speed = Wave Length / Wave Period for deep water waves only  Wave : /2  Wave Steepness: Wave Height/Wavelength Wave Interference  Crossing waves can “interfere” to create either a bigger wave (constructive interference) or smaller wave (destructive interference). Generation of Waves  Most surface waves generated by wind; therefore called wind waves Waves are also generated by  ,  Atmospheric pressure changes ()  of the and Height of Wind-Generated Waves depends on:  Wind Speed  Duration of Wind Event  Fetch - the distance over which wind can blow without obstruction Waves and  Once generated, waves can propagate as swell without wind  : Large waves travel faster than small ones  Because waves are not ideal, energy is dissipated and waves die out  Small ones die out, or damp out, faster. Wave Speed and Water Depth Deep-Water waves  travel in water that is deeper than 1/2 the wave’s wavelength; Depth > L/2  Speed is function of wavelength only – long move faster  Waves have nearly ideal shape and thus propagate energy but very little mass EAS/BIOEE 154 Lecture 2

Shallow-Water waves  travel in water that is shallower than 1/20 of the wave’s wavelength; Depth

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depends on depth).  In water of average depth (4000 m), a will travel at 700 km/hr. The December, 26 2004 Sumatran and Tsunami  Generated by a magnitude 9.3 earthquake as the Indian Plate thrust under the Sunda Plate  Calculated vertical displacements were as much as 5 meters  The ruptured along more than 1200 km  One of the largest earthquakes in past 100 years.  May have generated submarine landslides  Extensive damage and loss of life throughout the Eastern Can we predict tsunami’s and save lives? Generating a Tsunami Warning  Earthquake occurs  Seismic waves travel through the (at about 8 km/sec) to seismometers  Earthquake detected by seismometers  Determine location - did it occur in the ?  Determine size - is it big enough to generate a tsunami?  Pacific Tsunami Warning Center issues bulletin  About 75% are false alarms - most earthquakes don’t generate damaging tsunamis  Governments must then take action to warn & evacuate  Detect tsunamis using detectors - NOAA’s DART system: only 6 detectors deployed so far – most in the northeast Pacific; 32 planned for the Pacific by 2007 (at a cost of over $1 million a piece). What areas are vulnerable to tsunamis?  is most vulnerable  is particularly vulnerable  , , S. America   The plate boundary system in the Pacific NW is behaves very similarly to the one, with very large, very infrequent earthquakes. Geologic evidence and Japanese historical records indicate a very large tsunami generated there in 1700.  Indian Ocean  Atlantic and Caribbean  Many of the Caribbean islands are zone volcanoes - in addition to earthquakes, volcanic eruptions and volcanic landslides could generate tsunamis  Eastern Mediterranean  Both volcanically and seismically active  Salt beds beneath Mediterranean are particularly subject to landslides. Other Waves – resonant oscillations Internal Waves – waves that propagate along density boundaries within

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the ocean Kelvin and Rossby waves:  Low amplitude, long wavelength waves related to wind changes – notably El Niño

Some Study Questions How deep must the water be for a wave with a 100 m wavelength to behave as a strictly deep water wave? How shallow must the water be for that same wave to behave as a strictly shallow water wave? What is the restoring force for capillary waves? If a wave has a period of 10 , what is the frequency of that wave? Describe what happens as waves move out from a storm? How does the distribution of short and long wavelength waves change? Why would wave refraction cause headlands to erode, and sediment to be deposited in bays? Explain how waves can transport sediment along the coast in “”. Why are tsunamis considered “shallow water waves”? About how long did it take the Dec. 26 tsunami to reach ? What role do NOAA’s DART buoys play in the Pacific ? How do the DART instruments detect a tsunami? How does water motion differ between tsunamis and wind-driven waves?

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