How a seismometer works

Image from Wikimedia Commons. An old seismometer from Lick Observatory.

Earthquakes are a release of energy due to rock failing from the amount of stress and strain that are placed on it. This energy is released as a series of waves, or vibratory motion as the energy dissipates away from the source of the quake. Simplifying it a bit, there are S-waves, P-waves, Love Waves, and Rayleigh waves. Together, along with all the reflections and refractions that these waves can experience (due to what sort of material the wave encounters on its path) they generate a signal that can be measured. By measuring the signal and performing in depth analysis on it, geophysicists come up with ideas and theories about the specifics of what caused the earthquake.

One of the earliest seismic detection devices consisted of an ornate urn with copper balls that would fall off of the urn into the mouth of an awaiting toad (part of the ornate structure). Crude by today’s standard, but pretty brilliant considering that it was 132 AD. It provided a rough direction in which the quake originated.
Later designs incorporated a suspended mass that etched out a path on a lamp black coated drum with a stylus.(1) (lamp black – the soot from an oil lamp, almost pure carbon) It didn’t take long for paper and ink to take over the recording task.
With the advent of electronics, eventually seismographs consisted of a mass suspended on a pendulum structure, and a coil of wire moving in a magnetic field. The early ones used mechanical dampers in order to slow the movement if the mass. Failing to do so would result in the seismograph oscillating far beyond the time period of the quake and yield faulty data. (Remember, the mass is on a pendulum, and pendulums by their nature oscillate or swing back and forth.)

Enter the the torsion seismometer.

In 1925, Harry O. Wood and J.A. Anderson published a seismograph design that eliminated many of the problems with the pendulum siesmos.(2) It consisted of a wire under tension and a mass attached to one side of it. As the wave passed, the mass would swing back and forth along an arc beside the wire. In order to provide damping, magnets would counteract the movement and bleed off momentum from the moving mass. This design became the predominant tool for seismographs world wide. Highly sensitive, it had minimal resistance due to mechanical damping.

Years later, other designs came along, but in order to keep the seismological record consistent, the output of the newer gear had to be adjusted to match the sort of response that a Wood-Anderson seismograph would provide.

But that is only part of the problem.

How a quake is measured really depends on the structure of the crust, the type of gear measuring it, what size it was, and even what agency is doing the measuring. Here are a few of the scales listed in “Magnitude Scale and Quantification of Earthquakes” Hiroo Kanamori, 1983, Tectonophysics, 93.

ML Local magnitude, Richter (1935)
Ms Surface-wave magnitude, Gutenberg (1945a)
mB Body-wave magnitude, Gutenberg (1945b), Gutenberg and Richter (1956)
mb Short-period body-wave magnitude reported in “Earthquake Data Reports” and “Bulletin of International Seismological Center”
mbLg Lg-wave magnitude, e.g., Nuttli (1973)
MGR Magnitude used in Gutenberg and Richter (1954)
MR Magnitude used in Richter (1958)
MD Magnitude used in Duda (1965)
Mz Surface-wave magnitude determined from the vertical-component seismograms (e.g., Earthquake Data Reports)
Mv Surface-wave magnitude defined by Vanek et a!. (1962)
MJMA Magnitude scale used by the Japan Meteorological Agency
MM Moment magnitude by Brune and Engen (1969)
Mw Kanamori (1977)
ME Purcaru and Berckhemer (1978)
Mi Tsunami magnitude regressed against Mw ‘ Abe (1979)
Mc Coda (or duration magnitude), e.g., Bisztricsany (1959), Tsumura (1967), Real and Teng (1973)
MI Magnitude determined from intensity data and macro-seismic data, e.g., Nuttli and Zollweg (1974),
Nuttli et a!., (1979), Utsu (1979).
MK Kawasumi (1951).

A simple drawing for “Do It Yourself” building of a seismometer.

As you can see, there a lot of different scales. The ones that we see most often are Mw, M, and ML. mblg is used by IGN quite a bit. For the most part, they track along with each other in the mid scale, but at the low or high end they tend to diverge. Me was not mentioned in the list, but it figures prominently in trying to convert from one scale to the other… it’s the energy magnitude and is directly related to “A” which is the energy release in several of the formulas.
I’ll not bore you with the nitty gritty details about getting from one scale into another. I have been fighting this task for quite some time and have yet to find a reliable, reproducible method listed anywhere.

How I deal with energy release is to find a plot or published data by the agency that has the energy release and the magnitude listed, and then calibrating a conversion curve that matches what that agency uses. It’s a kludge, but it works and is stable across many quake reports within that agency. That’s the method that I used to come up with my cumulative released energy plots, which is different than using a canned formula published on a website. At least what I produce matches whatever methods the agency has adopted and has incorporated their adjustments.
Remember that “A” value that I mentioned? That’s the total energy of the quake. When spread across the fault face (where the fracture actually occurred) that will determine what the quake magnitude value is. (however it gets measured).
Some scales use the total amplitude of the trace movement; some measure the coda (how long it lasts). It’s part art, part science, and part nuanced thinking by some brilliant researchers.

GEOLURKING

1) Popular Mechanics Aug 1946
2) http://www.data.scec.org/Module/s3inset3.html
3) http://www.gps.caltech.edu/uploads/File/People/kanamori/HKtect83b.pdf

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