Earthquakes are common: many hundreds occur around the world each day, but it’s only those of significant size, or which cause significant damage, which make the news. What do seismologists mean when they talk about an earthquake having a magnitude of, say, 6.5, and how do they determine how big one tremor is compared to another?
Early Attempts to Measure Earthquake Size
Earthquakes are a feature of the earth’s existence and, thus, have always had an impact on humans. The first attempts at measurement were undertaken by the Chinese around 2000 years ago (Dewey and Byerly); but it wasn’t until the latter quarter of the nineteenth century that serious and repeated attempts to measure earthquakes were undertaken.
Seismographs have evolved with technology but still follow the basis principles of the 1890s – a rigid frame to which is attached a weight of some kind. On the weight, which is attached to the frame in a manner restricting movement to one plane only, there is a pen. The frame itself is attached to solid ground. Any movement leads to the pen leaving a trace on a rotating drum.
The Mercalli Scale
The Mercalli Scale was the first attempt to quantify the size of an earthquake. Developed by the Italian scientist Giuseppe Mercalli in 1897 (and later moderated), it was contemporaneous with the early development of seismography. The scale was based upon direct observation and ranged from I (for an event which was ‘not felt’ by humans, though presumably recorded on a seismograph) to XII (which was an episode of total devastation).
Though useful in its time, the Mercalli Scale is clearly flawed. Any scale dependent on human reporting, especially of a dramatic event, is likely to be unreliable: it also fails to allow for other factors such as the fact that greater damage may occur to structures at a distance from and earthquake than to those nearby. The scale also had no application in remote and unpopulated areas.
The Mercalli Scale has been modified on a number of occasions and is still in use, although as the US Geological Survey puts it, it ‘has a more meaningful measure of severity to the nonscientist than the magnitude’. This is because it is a measure of the intensity which is felt by the people who actually experience the earthquake event.
The Richter Scale
The Richter Scale, which was developed by American Charles Richter in 1927, was the first mathematical quantification of the size of an earthquake using the amplitude of the seismic waves which it produces and, thus, the amount of energy released. The scale relies on identifying the maximum amplitude of the waves via a network of seismometers. The calculation of the magnitude (and energy released) relies on a series of complicated formulas.
To be effective, the Richter Scale (and any other measure of an earthquake’s size) depends on a network of scientific instruments because an earthquake will have impacts over a large area. This network helps to identify the epicentre of the earthquake (the point on the earth’s surface directly above the earthquake itself).
Richter’s scale is logarithmic. This means that each step on the scale represents an event that releases ten times as much energy as the step below – so that an M7 earthquake is ten times greater than an M6. The scale is open-ended and can identify extremely small tremors (with negative magnitude). The largest earthquake to have been recorded so far is had a magnitude of 9.5 and took place in Chile in 1960.
Beyond Richter: Other Information on Earthquake Size
Earthquakes are extremely complex: the more geologists study them, the more complex the issues become as more influencing factors are included – for example, depth, local geology, fault movements and so on. Even the Richter Scale, which is a commonly understood shorthand for earthquake size, is now regarded as perhaps too simple.
Earthquakes generate more than one type of wave. The Richter Scale is based on the largest wave, of whatever type, but the waves have different properties. Scientists now use two different magnitude readings, for surface waves (which travel along the earth’s surface) and body waves (which travel through it).
Even here there are problems – both surface waves and, in particular, body surface waves are now thought to underestimate the energy generated by larger earthquakes (USGS). A third type of wave, a Love wave, has been proposed as an alternative, but this solution is also problematic (Love waves are long-period waves and most earthquake damage is caused by shorter period waves).
It’s clear that the evaluation of earthquake size and intensity – whether it be defined as energy released or damage caused – has many parameters and that there is no single satisfactory measurement. For the layman, however, the simple magnitude of the Richter Scale or, for those involved, the Mercalli Scale, provide an adequate measure of the relative size and scale.
Sources and Further Information
Ailsa Allaby and Michael Allaby (eds), Oxford Dictionary of Earth Sciences, Oxford University Press 1999
James Dewey and Perry Byerly The Early History of Seismometry (to 1900) Bulletin of the Seismological Society of America 1969
Peter J Smith and Hazel Rimer The Physical and Chemical Properties of the Earth Open University 2001
William Spence, Stuart A. Sipkin, and George L. Choy “Measuring the Size of An Earthquake” USGS Earthquakes Hazard Program, earthquake.usgs.gov, accessed 20 July 2010
USGS “The Modified Mercalli Intensity Scale” earthquakes.usgs.gov accessed 20 July 2010
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