Earthquake and Tsunami

An earthquake with a magnitude 7-7.9 occurs somewhere in the world about 13 times every year.

An earthquake with a magnitude 8-8.9 occurs somewhere in the world about 1.3 times every year.

An earthquake with a magnitude 9-9.5 occurs somewhere in the world about once every ten years.
See Gutenberg–Richter law

Every large earthquake has a one in twenty chance of being the foreshock of an even larger earthquake.

The magnitude 9.5 1960 Valdivia earthquake was preceded by three foreshocks:

An 8.1 the day before.
A 7.1 that morning.
A 7.8 just 15 minutes before the main earthquake.
The 9.0 2011 Tōhoku earthquake and tsunami was preceded by two foreshocks:

A 7.3 two days before
A 6.4 one day before
See Spatial organization of foreshocks as a tool to forecast large earthquakes

From Richter magnitude scale:

The total energy release of an earthquake closely correlates to its destructive power. A difference in magnitude of 2.0 is equivalent to a factor of 1000 in the energy released. A difference in magnitude of 1.0 is equivalent to a factor of 31.6 in the energy released.

Because of various shortcomings of the original "magnitude scale" developed by Charles F. Richter, and later revised and renamed the **Local magnitude scale**, denoted as "ML" or "ML", most seismological authorities now use other scales, such as the moment magnitude scale (Mw), to report earthquake magnitudes, but much of the news media still refers to these as "Richter" magnitudes.

All scales, except

M
w

{\displaystyle M\_\text{w}}

, saturate for large earthquakes, meaning they are based on the amplitudes of waves which have a wavelength shorter than the rupture length of the earthquakes. These short waves (high frequency waves) are too short a yardstick to measure the extent of the event. The resulting effective upper limit of measurement for

M
L

{\displaystyle M\_L}

 is about 7 and about 8.5 for

M
s

{\displaystyle M\_\text{s}}

.

M
L

{\displaystyle M\_\text{L}}

 is the scale used for the majority of earthquakes reported (tens of thousands) by local and regional seismological observatories. For large earthquakes worldwide, the moment magnitude scale (MMS) is most common, although

M
s

{\displaystyle M\_\text{s}}

 is also reported frequently.

## Tectonic plates

Tectonic plates can do one of three things:

1. Slide past one another.
2. Move away from each other.
3. Move toward each other with one plate sliding below the other.

It is the third type that produces large tsunamis. Areas where this happens are called subduction zones. Subduction zones are colored blue in the image below. Since 1900, all earthquakes greater than magnitude 8.6 (See here and here) have occured at subduction zones. During an earthquake the plates grind past one another creating heat and causing a thin layer of rock along the fault to become molten. See Fault friction

## Tsunami

See International Tsunami Information Center , List of historical tsunamis, and Tsunami earthquake

The figures in the table to the left should only be regarded as a "rule of thumb" for regions very close to the epicenter of the earthquake. Actual values vary considerably. Actual values ranging anywhere from twice the average down to half the average are common. Unexpected tsunamis only a few meters tall have been known to kill hundreds of people. ("Wave height" is twice the "wave amplitude".)

The preliminary computer generated estimate of earthquake magnitude (probably based on ML) is often too small and upon inspection by professional seismologists quickly gets updated to a larger value (probably based on Mw). Increases of 0.5 magnitude are not uncommon. An increase in magnitude of 0.5 doubles the height of the expected tsunami.

If the tsunami wave is funneled into a narrow bay with a progressively decreasing width then the wave gradually becomes narrower but the total energy of the wave remains the same therefore when the bay has become four times narrower then the wave will have become twice as high. See Green's law.

Complicating things even further, even small earthquakes can cause underwater landslides which can produce very large tsunamis.

In general, do not try to escape by car. After a major earthquake roads may be damaged or clogged with those trying to escape. Your best bet is to get to the top of a hill or the roof of a reinforced concrete structure four stories above sea level.

> Then let them flee to the hills.
> Do not let the one who is on the housetop go down to get any thing out of his house.
> Neither let the one who is in the field turn back to get his jacket.
>
>

 Jog
 Sprint
 World record

The 2011 tsunami inundation extended 5 km inland over very flat ground (see the image below). It would take one hour to walk 5 km and half an hour to jog it. The human body can only sprint for about 350 meters. Along the river the inundation extended 10 km inland. (Jogging burns 10 calories a minute)

Close to, and directly in front of, the earthquake the first wave is usually the biggest but the further away the wave travels the less certain that becomes. After the 2011 Japan earthquake it was the fourth wave to hit Tahiti (9500 km from Japan) that was the largest and the all clear had already been broadcast when it arrived. See Sequencing of tsunami waves: why the first wave is not always the largest.

The energy required to lift a section of water 100 km by 15 km by 7 km meters deep a distance of 10 meters is 1018 J. See How Japan's 2011 Earthquake Happened (Infographic)

From Tsunami:

The velocity of a tsunami is the the square root of the depth of the water multiplied by the acceleration due to gravity (approximately 10 m/s2). For example, if the continental shelf is 150 m deep, the velocity of a tsunami would be the square root of (150 × 10) = √1500 = ~40 m/s, which equates to a speed of ~140 km/h or about 90 mph. When the depth decreases by a factor of sixteen then the waves are four times slower and twice as high.