Tectonics ... and earthquakes and tsunamis

After the second big quake off the coast of Indonesia I looked up the location of the various tectonic plates and ...

The surface of the earth is made up of lots of plates that move and slide and crash into each other ... in slow motion.
To see how this causes continents to move, over hundreds of millions of years, click here.

Two plates may crunch together, build up pressure, then slide abruptly, one plate moving up over the other.
If this happens under land, it might look like this (Convergent: West coast of S. America) or this (Divergent: mid-Atlantic).
If this happens under water, it might look like this.

>That last one looks like a tsunami, right?
Yes, it could be.
Apparently those convergent (or compressional) types can cause quakes exceeding 9 on the Richter scale ... as off the coast of Indonesia.

It's interesting to look at the location of the tectonic boundaries and the (historical) location of quakes.
The quakes tend to occur along a tectonic plate boundary.
Those quakes that occur beneath the sea may cause huge waves.
However, the two plate need not rise, one over the other. They may slide ... like this.
(Transform-fault: see the coast of California?)

The recent quakes near Indonesia (Dec, 2004 and Mar, 2005) are indicated by the black oval.
Notice the quake density in that area !!.

>You mentioned the Richter scale. How does that ...?
When the plates move abruptly, they release lots of energy and that creates a seismic wave that travels through the ground.
That makes the needle in a seismometer swing violently.
Let A be the amplitude of the needle swing, in millimetres.
Then the Richter value (or "Local Magnitude" value ML) is:

      ML = log10(A) + (correcton for distance).

The correction for distance is necessary because the amplitude of the needle swing depends upon how far away the seismometer is located.
If the seismometer were located at the epicentre and had an amplitide B, then at a distance D, the amplitude A would be less.

      A = k B/Dn ... where k is some contant and n is some constant dependant upon the rate at which the amplitude decays with distance.


      log10(A) = log10(k) + log10(B) - n log10(D)   or   log10(B) = log10(A) + n log10(D) - log10(k)
In 1935, the seismologist Charles Richter studied various earthquakes in California and generated values for k and n which lead to the nomogram shown in the Figure

Note that it's "normalized" so that A = 1 mm and D = 100 km gives Richter = 3.
In 1989 there was an earthquake south of San Francisco,
called the Loma Prieta earthquake.

In Eureka (D = 480 km away),
a seismometer registered A = 255 mm.

That earthquake measured ...

>7.0 on the Richter scale, right?
Actually, it was a 7.1 earthquake.

Note:   There's lots of interesting stuff on Tectonic Plates