Last Friday’s tsunami on the eastern coast of Japan, with waves that reached heights of more than 10 feet, was generated due to a seismic event whose epicenter lay approximately 60 miles to the east of the country.
Earthquakes are a relatively common occurrence in the country, as it lies at the junction of four of Earth’s tectonic plates: the Eurasian plate, Philippine plate, Okhotsk plate and Pacific plate.
The northern half of Japan lies on a portion of the Okhotsk plate that is directly west of the Pacific plate. The area at which two tectonic plates meet is called the fault plane.
In last Friday’s quake, the earthquake’s magnitude was a recorded 9.0 on the moment magnitude scale. Because this scale is reported logarithmically, an earthquake with a magnitude of 8 has a shaking amplitude that is ten times greater than an earthquake with a magnitude of 7.
For reference, the earthquake that caused the tsunami in Indonesia in 2004 was a magnitude 9.1 quake, and the one that happened in Haiti last year was magnitude 7.0.
However, scientists measured the surface energy of the Japan quake as nearly double that of the 2004 earthquake.
The Earth’s plates are constantly in motion. The rigid plates that make up the Earth’s crust float on the relatively more plastic upper mantle, known as the aesthenosphere.
The edges of the plates are constantly rubbing against and sliding over or underneath each other. The Pacific plate, which was one of the plates involved in Friday’s quake, typically travels westward at a rate of four inches per year, and is pushed beneath the Okhotsk plate at the boundary of the two in a process known as subduction.
The Pacific is the largest tectonic plate, encompassing the majority of the Pacific Ocean, while the Okhotsk is much smaller.
At the boundary between these two plates, the front edge of the Pacific plate is drawn downward into the earth’s second layer, the mantle, where it is transformed into molten rock by the higher temperatures and pressures found in the mantle.
However, this process is sometimes interrupted by the cracking and breaking of the rigid plates. In this case, the westward movement of the Pacific plate beneath the Okhotsk plate caused the Pacific plate to break, sending the seafloor upward by several meters and initiating the earthquake.
Because this occurred at a relatively shallow depth of 17 miles below sea level, most of the energy contained in the break was released on the seafloor.
As the force of the seafloor thrusting upward is transmitted to the water, the waves that are created have long wavelengths but low amplitudes. These waves are capable of traveling at speeds greater than 500 miles per hour. As the waves reach shallower waters, however, the speed of the waves decreases.
Because the energy of the tsunami stays the same, an increase in wave height compensates for the slowing of the tsunami front. As a result, tsunamis are usually imperceptible out at sea, travelling at a height of a meter or less.
However, the effects are significantly more dramatic once the wave approaches land and begins to “feel bottom.” Friction between the bottom of the wave and the seafloor causes the wave to slow.
The wavelength of the tsunami wave decreases closer to shore, so that a great deal of water and energy begins to accumulate behind the front of the tsunami.
This is how the tsunami is able to push inland with such great force and over large distances. Eventually, the energy is dissipated because of friction, water turbulence and physical obstructions such as buildings and trees.
Because waves are sent in all directions from the epicenter of the earthquake, a tsunami warning was issued across the Pacific Ocean, as far as the West Coast of the U.S.
The waves were able to reach California as early as 10 hours after the initial earthquake and South America approximately 20 hours later.