GEOL 3400 Geologic Hazards Tsunami Mechanics http resource

GEOL 3400: Geologic Hazards Tsunami Mechanics http: //resource. isvr. soton. ac. uk/spcg/tutorial/Tutorial_files/Web-basics-frequency. htm 31 -Aug-21 1

Tsunami: Mechanics GEOL 3400: Geologic Hazards Introduction • a tsunami are giant, long-period sea waves caused by oceanic disturbances – produced by sudden displacement of a large volume of water • tsunami are, in fact, wave systems that spread out from a point of energy release – the number of waves produced vary • commonly the 3 rd through 8 th wave are the largest – have periods of 10 -20 minutes or longer – when they arrive at a distance shore, these waves may be separated by tens of minutes to hours • a seiche is a slow, rhythmic oscillation of water in enclosed or nearly enclosed bodies of water – results from seismic action or storm conditions – water sloshes back and forth with waves usually no more than 5 feet 31 -Aug-21 2

Tsunami: Mechanics GEOL 3400: Geologic Hazards Wave Characteristics • Tsunamis behave very differently from typical wind-produced ocean waves. Because of this, the mechanics of tsunamis are different and detecting them is difficult. 31 -Aug-21 3

Tsunami: Mechanics Wave Characteristics GEOL 3400: Geologic Hazards • a wave is basically an energy carrier 31 -Aug-21 – thus, tsunami is carrying energy away from where it has been suddenly released by some geologic process • understanding tsunami hazards requires knowing about the characteristic of waves • primary wave features that are independent of time are: 4

Tsunami: Mechanics Wave Characteristics GEOL 3400: Geologic Hazards • terminology used to describe tsunamis are same as that for any wave – seismic, water, sound, etc. • the physical form of a wave is characterized by the following parameters: – wavelength: distance from one point on the waveform to the adjacent identical point, e. g. trough to trough, or crest to crest – amplitude: height from still water line to wave crest or from same line to wave trough – wave height: distance from trough to crest, equal to twice the amplitude – trough: lowest point of wave – crest: highest point of wave • the terminology used to describe a tsunami is the same as that used for all other waves, e. g. sound, light, ocean wave, etc. 31 -Aug-21 5

Tsunami: Mechanics Wave Characteristics GEOL 3400: Geologic Hazards • is a longitudinal wave of compressional nature 31 -Aug-21 6

Tsunami: Mechanics Wave Characteristics: Wavelength GEOL 3400: Geologic Hazards • wavelength (L) of a tsunami can be greater than 200 km – nearly 200 times normal ocean wave with a wavelength of about 100 m – the depth to which a wave affects the water column is related to its wavelength, i. e. L/2 • with wavelengths of hundreds of kilometers, a tsunami affects the entire water column • thus, tsunami “feel” the ocean bottom even in the deepest part of the ocean basin – consequently, tsunami are ‘shallow-water’ waves • because energy loss is inversely proportional to wavelength, not much energy is dissipated as the tsunami crosses even a large ocean basin 31 -Aug-21 7

Tsunami: Mechanics GEOL 3400: Geologic Hazards Wave Characteristics: Wavelength • shallow vs deepwater waves • tsunami/diagrams/wave. gif 31 -Aug-21 8

Tsunami: Mechanics GEOL 3400: Geologic Hazards Wave Characteristics: Wavelength 31 -Aug-21 9

Tsunami: Mechanics GEOL 3400: Geologic Hazards Wave Characteristics: Wavelength Robke & Vott (2017) 31 -Aug-21 10

Tsunami: Mechanics GEOL 3400: Geologic Hazards Wave Characteristics: Wavebase • One important way in which tsunamis differ from wind-generated waves is their wavelength • The wavelength of a tsunami is hundreds of kilometers compared to wavelengths measured in meters for wind-produced waves • Since wavebase, the depth at which water is disturbed by the passage of the wave, is related to wavelength, tsunami and wind waves behave very differently • Wavelength (L) and wavebase are related by relation: 31 -Aug-21 11

Tsunami: Mechanics Wave Characteristics: Amplitude GEOL 3400: Geologic Hazards • at sea, the amplitude of a tsunami is on the order of a meter or less – noticeable in other ocean waves – thus, tsunami are not hazards to ships at sea • as the wave approaches shallow water, its height builds and its energy is concentrated into a smaller region – because the depth decreases, the wave slows and its height increases – wavelength decreases thereby piling waves on top of each other – bottom of the wave is slowed by the seafloor, but the crest is not • top gets ahead of the base and finally crashes forward or breaks 31 -Aug-21 12

Tsunami: Mechanics GEOL 3400: Geologic Hazards Wave Characteristics: Amplitude Robke & Vott (2017) 31 -Aug-21 13

Tsunami: Mechanics Wave Characteristics: Amplitude GEOL 3400: Geologic Hazards • normally most damage is done from tsunami generated locally – of 104 damaging tsunami in the past century, only 9 have produced damage outside their local areas • the maximum amplitude of a tsunami is related to magnitude of earthquake and its focus depth as well as the water depth above the epicenter • with distance from source, amplitude decreases (but slowly), but the number of waves increases 31 -Aug-21 14

Tsunami: Mechanics GEOL 3400: Geologic Hazards Tsunami Characteristics: Drawdown • drawdown is the sudden lowering of water level before a tsunami strikes – because wavelength so great, the path followed by water particles is long • draws water out toward wave – has proven deadly to onlookers – first indication of a tsunami need not be a drawdown • often arrives as a series of crest – largest need not be the first – it can take successive crests up to an hour to reach shore – because frequency (time between crests) varies with velocity and wavelength 31 -Aug-21 15

Tsunami: Mechanics GEOL 3400: Geologic Hazards Wave Characteristics – Time Dependent • a wave’s is not fixed in time, that is the wave propagates in the direction of wave motion • terms that describe the change in a wave with time: – wave period (p) is the length of time required for a wavelength to pass a fixed point • that is the time between adjacent crests, troughs or any equivalent point on the wave form • the faster a wave is moving, the shorter the period • measured in seconds (f): number of waves that pass a point in one second – frequency (f) • measured in Hertz (Hz), i. e. inverse seconds – relation between period and frequency is: 31 -Aug-21 16

Tsunami: Mechanics GEOL 3400: Geologic Hazards Wave Characteristics - Time 31 -Aug-21 17

Tsunami: Mechanics Tsunami Characteristics: Run-up GEOL 3400: Geologic Hazards • factors influencing run-up include: • water depth • sea floor profile and topography • coastline shape, e. g. bay configuration • along steep coasts, the waves do not slow and break but grow rapidly as approaches shore • allows a very large run-up 31 -Aug-21 18

Tsunami: Mechanics Period GEOL 3400: Geologic Hazards • the period of a tsunami is related to earthquake magnitude by: where T = period, and M = Richter magnitude – scales for defining the magnitude of a tsunami are based on logarithm of maximum wave amplitude observed locally • similar to Richter scale • Imamura-Iida scale is calculated as a function of maximum wave height along coast – different magnitudes for same tsunami at different places along coast 31 -Aug-21 19

Tsunami: Mechanics GEOL 3400: Geologic Hazards Wave Characteristics - Frequency 31 -Aug-21 20

Tsunami: Mechanics GEOL 3400: Geologic Hazards Wave Characteristics - Frequency 31 -Aug-21 21

Tsunami: Mechanics Tsunami Characteristics: Run-up GEOL 3400: Geologic Hazards • run-up is the elevation a tsunami reaches above sea level – measured in meters (or feet) 31 -Aug-21 22

Tsunami: Mechanics GEOL 3400: Geologic Hazards Wave Characteristics - Time 31 -Aug-21 23

Tsunami: Mechanics Velocity GEOL 3400: Geologic Hazards • the velocity of a tsunami is related to ocean depth by: where g is the acceleration due to gravity (9. 81 m/s 2 [32. 19 ft/s 2]) and D is water depth, i. e. depth of the ocean – for depth of 4, 000 m, the tsunami velocity would be 198 m/s (649 ft/s) or 713 km/hr (443 mi/hr) • for the average Pacific Ocean depth of 5, 500 m (18, 040 ft), tsunami velocity is 232 m/s (761 ft/s) or 834 km/hr (518 mi/hr) – because tsunami always feels the bottom, they are effectively slowed to 675 -722 km/hr (420 -480 mi/hr) • takes less than 24 hours to cross Pacific Ocean – as tsunamis approach shore, they are slowed down by the shallowing seafloor • can still be moving 50 -80 km/hr (30 -50 mi/hr) • much too fast for human to outrun 31 -Aug-21 24

Tsunami: Mechanics GEOL 3400: Geologic Hazards Velocity • For any type of water wave, the wave's wavelength (L), velocity (c) and period (T) are related by the following relations: 31 -Aug-21 25

Tsunami: Mechanics GEOL 3400: Geologic Hazards Energy • the energy transported by tsunami is distributed from the ocean floor to the ocean surface • tsunami contain enough energy to destroy most man-made structures – in the Krakatau tsunami (1883), the steamer Berouw was carried 2. 5 km inland deposited 24 m above sea level 31 -Aug-21 26

Tsunami: Mechanics GEOL 3400: Geologic Hazards Summary 31 -Aug-21 27