PacificTWC | How to Make a Tsunami @PacificTWC | Uploaded 11 years ago | Updated 1 minute ago
This cartoon shows schematically how a subduction earthquake generates a tsunami. The green material is the lithosphere (which is brittle) and the purple-red material is the asthenosphere (which, while still solid, is deformable). The lithosphere is divided into the overriding (continental) plate on the right and the downgoing (oceanic) plate on the left. The overriding plate is locked against the downgoing plate, so as the downgoing plate subducts, the overriding plate is progressively deformed. The front edge of the overriding plate is flexed downwards while the coastline is lifted. After a few hundred years, so much strain energy has been stored in the system (think of a spring that is wound up too tight) that something has to give. The result is an earthquake. The earthquake relieves the accumulated strain in just a matter of minutes, during which the overriding plate relaxes back to its original unstrained condition. The toe of the overriding plate kicks up and the shoreline drops.
The distance from the trench axis to the shoreline is about a hundred kilometers, while the distance along strike (perpendicular to the plane of the image) may be as much as a thousand kilometers. So during the earthquake, the shape of the ocean bottom has been changed over a huge area. This change in shape occurs faster than the overlying water can flow out of the way. The result is a deformation of the ocean surface: an excess lump of water over the toe of the overriding plate, a hollow in the ocean along the shoreline.
So now the sea surface has been deformed. Gravity wants the surface to be horizontal, so the lump collapses and the hollow is filled. The flowing water has significant kinetic energy, so it overshoots: the result is a succession of waves (not just one). That succession of waves is the tsunami.
Because the wavelength of the original distortion of the sea surface is so long (if the distance from the trench to the shoreline is 100 km, then that initial wavelength is 60-70 km in this example), then the wavelength of the resulting tsunami is also long. Wavelength, however, continuously varies depending on how fast the tsunami travels (speed depends on the depth of the water: in deep water a stunami travels faster), so it is easier to characterize a tsunami by its period, the time from one wave crest to the next. Regardless of the speed of the tsunami, the period remains constant. Tsunamis typically have periods in the range from 10 to 40 minutes.
Note from the geometry that, on the nearby coast (right), the tsunami first manifests itself as a severe drawdown of water: the sea retreats from the land. Out on the open ocean (left), however, the tsunami first appears as a positive wave.
The cartoon attempts to display physical phenomena on three very different time scales: the time for the initial accumulation of strain is from hundreds to thousands of years, the earthquake itself lasts a few minutes, and the time from one wave to the next of the tsunami is about 20 minutes. The best way to watch the cartoon is as a continuous loop.
This cartoon shows schematically how a subduction earthquake generates a tsunami. The green material is the lithosphere (which is brittle) and the purple-red material is the asthenosphere (which, while still solid, is deformable). The lithosphere is divided into the overriding (continental) plate on the right and the downgoing (oceanic) plate on the left. The overriding plate is locked against the downgoing plate, so as the downgoing plate subducts, the overriding plate is progressively deformed. The front edge of the overriding plate is flexed downwards while the coastline is lifted. After a few hundred years, so much strain energy has been stored in the system (think of a spring that is wound up too tight) that something has to give. The result is an earthquake. The earthquake relieves the accumulated strain in just a matter of minutes, during which the overriding plate relaxes back to its original unstrained condition. The toe of the overriding plate kicks up and the shoreline drops.
The distance from the trench axis to the shoreline is about a hundred kilometers, while the distance along strike (perpendicular to the plane of the image) may be as much as a thousand kilometers. So during the earthquake, the shape of the ocean bottom has been changed over a huge area. This change in shape occurs faster than the overlying water can flow out of the way. The result is a deformation of the ocean surface: an excess lump of water over the toe of the overriding plate, a hollow in the ocean along the shoreline.
So now the sea surface has been deformed. Gravity wants the surface to be horizontal, so the lump collapses and the hollow is filled. The flowing water has significant kinetic energy, so it overshoots: the result is a succession of waves (not just one). That succession of waves is the tsunami.
Because the wavelength of the original distortion of the sea surface is so long (if the distance from the trench to the shoreline is 100 km, then that initial wavelength is 60-70 km in this example), then the wavelength of the resulting tsunami is also long. Wavelength, however, continuously varies depending on how fast the tsunami travels (speed depends on the depth of the water: in deep water a stunami travels faster), so it is easier to characterize a tsunami by its period, the time from one wave crest to the next. Regardless of the speed of the tsunami, the period remains constant. Tsunamis typically have periods in the range from 10 to 40 minutes.
Note from the geometry that, on the nearby coast (right), the tsunami first manifests itself as a severe drawdown of water: the sea retreats from the land. Out on the open ocean (left), however, the tsunami first appears as a positive wave.
The cartoon attempts to display physical phenomena on three very different time scales: the time for the initial accumulation of strain is from hundreds to thousands of years, the earthquake itself lasts a few minutes, and the time from one wave to the next of the tsunami is about 20 minutes. The best way to watch the cartoon is as a continuous loop.
