Physics of Earthquake, Tsunami Part 2

This is part 2 of a blog I started on the physics of the earthquake and tsunami in Japan.  A HREF=”https://www.alabamawx.com/?p=41853″>You can read part 1 here.

The above map (from USGS) shows the ground shaking intensity at the peak of the main earthquake.  In the dark orange/brown zone near the coast, peak ground velocities were around 1 mph, and peak accelerations were about 0.4g, 40% of the gravitational pull of the earth, or about 4 meters per second per second.  The ground shook beneath people’s feet at 1 mph.

This earthquake has been rated a 9.0.  It ties with for 4th most powerful earthquakes on record with an earthquake in Kamchatka, Russia in 1952.   The rating used to be based on the Richter scale, and is now based on moment magnitude scale, that provides similar numbers for most earthquakes.  These scales are logarithmic, meaning that a magnitude 5 earthquake is 32 times stronger than a magnitude 4, a magnitude 6 is about 1000 times stronger than a magnitude 4. 

The map shows that the earthquake was centered underneath the ocean.  This is why a tsunami was generated.  Basically, the movement of the ocean bottom produces a wave (or many waves).  In the open ocean away from land, tsunamis are fairly small in amplitiude (only about 3 feet high), but they have very long wavelengths (a typical wind-driven wave coming into the beach has a wavelength of about 6 feet, while a tsunami has a wavelength about 100 miles).

Much of the open Pacific Ocean is 20,000 feet deep. The speed of water waves in shallow water is proportional to the depth of the water.  Most waves we see on the ocean or on lakes are considered deep water waves (the speed is proportional to wavelength), since the shallow vs. deep distinction depends on how large the wavelength is in comparison to the depth of the water (does the wave reach the bottom, see below).  However, in the case of tsunami, even in 20,000 foot water, a wavelength of 100 miles (500,000 feet) acts like a shallow water wave.  So, its speed is determined by the depth of the water.

In the open ocean,  a long wavelength, shallow-water wave like a tsunami moves very fast, over 500 mph.  However, as the water gets shallower (around land), it slows down.  By the time the depth of the water is 9 feet, the speed is down to 12 mph.  Since the waves keep coming in and converging on each other, energy must be conserved, so the waves grow in amplitude, a process called shoaling, shown below.  This shoaling is what causes the tsunami to be so large and come so far inland. 

(Shoaling waves, Lachaume)

For a NOAA model simulation of the tsumani, go to https://www.youtube.com/watch?v=P-T00d-0MGQ.

The waves slow down as they go over shallower water, especially near islands (Hawaii, the Aleutians, etc.). Some also reflect off islands and the west coast of America.

Category: Headlines, Met 101/Weather History