If this was a 6.5, an 8.5 or 9.5 will be off the charts destructive.
Anything larger than an M8.0 (Richter) is hard to accurately measure, because you get large ground movements. The December 26, 2004 Indonesian EQ saw lateral displacements in the hundreds of feet over long time spans. So, the idea is to be able to have a scale that quantifies total energy released, instead of just lateral or vertical accelerations. I agree that our current system is clunky:
Anything larger than an M8.0 (Richter) is hard to accurately measure, because you get large ground movements. The December 26, 2004 Indonesian EQ saw lateral displacements in the hundreds of feet over long time spans. So, the idea is to be able to have a scale that quantifies total energy released, instead of just lateral or vertical accelerations. I agree that our current system is clunky:
How Are Earthquake Magnitudes Measured?
The Richter Scale
 Figure 1 - Charles Richter studying a seismogram. |
There are a number of ways to measure the magnitude of an earthquake. The first widely-used method, the Richter scale, was developed by Charles F. Richter in 1934. It used a formula based on amplitude of the largest wave recorded on a specific type of seismometer and the distance between the earthquake and the seismometer. That scale was specific to California earthquakes; other scales, based on wave amplitudes and total earthquake duration, were developed for use in other situations and they were designed to be consistent with Richter’s scale. |
The Moment Magnitude Scale
Unfortunately, many scales, such as the Richter scale,
do not provide accurate estimates for large magnitude earthquakes. Today
the moment magnitude
scale,
abbreviated MW, is preferred because it works
over a wider range of earthquake sizes and is applicable globally.
The moment magnitude scale is based on the
total moment release of the earthquake. Moment is a product of the
distance a fault moved and the force required to move it. It is derived
from modeling
recordings of the earthquake at multiple stations. Moment magnitude
estimates are about the same as Richter magnitudes for small to large
earthquakes. But only the moment magnitude scale is capable of measuring
M8 (read ‘magnitude
8’) and greater events accurately.
Magnitudes are based on a logarithmic scale (base
10). What this means is that for each whole number you go up on the magnitude
scale, the amplitude of the ground motion recorded by a seismograph goes up
ten times. Using this scale, a magnitude 5 earthquake would result in ten times
the level of ground shaking as a magnitude 4 earthquake (and 32 times as much
energy would be released). To give you an idea how these numbers can add up,
think of it in terms of the energy released by explosives: a magnitude 1 seismic
wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake
releases as much energy as detonating 6 million tons of TNT. Pretty
impressive, huh? Fortunately, most of the earthquakes that occur each year
are magnitude 2.5 or less, too small to be felt by most people.
Magnitude scales can be used to desribe earthquakes so small that they are expressed in negative numbers. The scale also has no upper limit, so it can describe earthquakes of unimaginable and (so far) unexperienced intensity, such as magnitude 10.0 and beyond.
Magnitude scales can be used to desribe earthquakes so small that they are expressed in negative numbers. The scale also has no upper limit, so it can describe earthquakes of unimaginable and (so far) unexperienced intensity, such as magnitude 10.0 and beyond.
Here's a table describing the magnitudes of earthquakes, their effects, and the estimated number of those earthquakes that occur each year.
The Mercalli Scale
 Figure 2 - Giuseppe Mercalli |
Another way to measure the strength of an earthquake is to use the Mercalli scale.
Invented by Giuseppe Mercalli in 1902, this scale uses the observations
of the people who experienced the earthquake to estimate its intensity.
The Mercalli scale isn't considered as scientific as the Richter scale, though. Some witnesses of the earthquake might exaggerate just how bad things were during the earthquake and you may not find two witnesses who agree on what happened; everybody will say something different. The amount of damage caused by the earthquake may not accurately record how strong it was either. |
Some things that affect the amount of damage that occurs are:
- the building designs,
- the distance from the epicenter,
- and the type of surface material (rock or dirt) the buildings rest on.
Different building designs hold up
differently in an earthquake and the further you are from the
earthquake, the less damage you'll usually see. Whether a building is
built on solid rock or sand makes a big difference in how much damage it
takes. Solid rock usually shakes less than sand, so a building built on
top of solid rock shouldn't be as damaged as it might if it was sitting
on a sandy lot.
Some years ago an engineer came by our architectural office, I can't remember his purpose, if he was looking for work or what I couldn't remember.
ReplyDeleteIn any case he told us he would travel with a group of engineers to review the effects of earthquakes typically in North and South America. He was a part of what he called the windshield study locating faults along the Wasatch Front from the inside of a car. His believed that harmonics played the largest part in the destruction of buildings, depending upon the height of the building.
He said that after the last big one hit San Francisco, the portion of the bay bridge that was all concrete portion collapsed, and across the street from this reinforced structure was an old turn of the century theater that survived without any damage.