A problem with Intensities

More off the wall disageeableness.
Well what do you want from me? Sensible?

You'll be lucky. …

The Modified Mercalli Scale of Earthquake Intensity

In seismology a scale of seismic intensity is a way of measuring or rating the effects of an earthquake at different sites. The Modified Mercalli Intensity Scale is commonly used in the United States by seismologists seeking information on the severity of earthquake effects. Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end.

The Intensity Scale differs from the Richter Magnitude Scale in that the effects of any one earthquake vary greatly from place to place, so there may be many Intensity values (e.g.: IV, VII) measured from one earthquake.

Each earthquake, on the other hand, should have just one "Magnitude", although the several methods of estimating it will yield slightly different values. (E.g.: 6.1, 6.3.)

Ratings of earthquake effects are based on the following relatively subjective scale of descriptions:
Modified Mercalli Intensity Scale
from FEMA

I. People do not feel any Earth movement.
II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.

IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.

V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.

VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings.

XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.


A problem with intensities:

These are magnitudes as listed in United States Earthquakes, Earthquake History of the United States.

They are a measure of energy and are on a logarithmic (sliding) scale. A difference of one magnitude represents ten times the wave amplitude. But the energy released is thirty time different. So a two magnitude increase equates to 30 x 30 times the power.

For example, approximately 900 times more energy is released in an earthquake of Magnitude 7 than in an earthquake of Magnitude 5.

Each increase in magnitude of one unit is equivalent to an increase of seismic energy of about 1,600,000,000,000 ergs.


1,600,000,000,000 ergs is only 160,000 Watts. It doesn't seem all that much:

http://www.unitconversion.org/unit_converter/power-ex.html (Beware commas.)


Still with me?

Now suppose it is all wrong. A time difference of less than a minute for two quakes to occur at exactly the same place, gives a power output of the summ of both quakes. So the output of two quakes of 3.5M (say) in the same location, some 30 seconds apart eachhave an force of X Megawatts.

Their combined forces are 2X Mw. (I'm hopeless at maths so I am not even going to try to work it out.)

But if they occur at exactly the same time in the same location they are granted a value of 7M. Which is plainly ridiculous.

The equivalent in Storm values is when a Low of some 980 millibars combines with another Low of similar pressure and form a new Low singularity of 970mb or whatever. Then they join up with more Lows (as in the case of the recent Baltic one) and their pressure drops to 950 mb.

Then a quake occurs some 60 degrees around the corner and the Low dissipates.

Does that sound daft to you? It would give geologistst appoplexy if I was posting this on Usenet and they had forgotten to block me with their newsreader filters.

I use the NEIC lists which uses an aggregate scale just called Magnitude:
7.3Mag. Date = 2008/11/24. Time = 09:02:59
Latitude = 54.198. Longitude = 154.316. Depth = 491.7. SEA OF OKHOTSK


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