So Long, Old Magnitude
"Hey," said my spouse, handing me a page of the Sunday paper. "Take a look. What's going on?" Shortly thereafter, I called Charlotte Rowe, Deputy State Seismologist at the Geophysical Institute. "Hey," I said. "What's going on?"
These buck-passing questions rose from an article by Hawaiian journalist Jan TenBruggencate, in which he reported the demise of the Richter scale for describing the size of earthquakes. Scientists are phasing it out, he wrote.
"Actually, we rarely use it to report on earthquake size," Rowe said. Rats. Not only had I barely begun to understand the Richter scale, I believed I'd heard quotes---recent quotes---attributed to Rowe herself giving earthquake magnitudes according to that scale.
"Not really," she said. "For one thing, I'm careful not to use the term 'Richter scale'--we get too many visitors coming through the lab asking us to show them the Richter scale, as if it were a machine of some sort," Rowe said. "It's actually the calculated Richter magnitude of a quake, and it's not the calculation we generally use. We just say 'magnitude.' But journalists love Richter, and they insist on putting earthquake magnitudes in terms of that so-called scale."
I was glad Rowe couldn't see me turning red at the other end of the phone line. She went on to explain that Charles Richter's 1930s-era method of measuring earthquake magnitude was accurate for typical California seismic events because California's earthquakes usually are medium-sized and are caused by breaks in the comparatively shallow crustal rocks. This combination of size and depth produces a release of energy that can be estimated quite accurately from a certain set of waves generated by the quake and measured by seismometers. The Richter magnitude can be calculated fairly quickly by measuring the extent of those waves as they are represented by the jagged lines on a seismogram, and normalizing the readings to a specific distance from the epicenter.
In Alaska, the earthquake hypocenters are often deep, caused by the subduction of the Pacific oceanic plate beneath the North American continental plate. Here, the earthquakes can be too big to be accommodated by the few waves measured to produce a Richter magnitude number. The great Good Friday earthquake of 1964, for example, was quickly assigned a Richter magnitude of 8.4 to 8.6. But that number was too small, tremendous though it is, compared to recent damaging seismic events in California. (Remember, each whole-number increase in Richter magnitude represents a 30-fold increase in the energy of the earthquake. The Northridge event, magnitude 6.8, near Los Angeles this past January was almost 900 times---30 times 30 times smaller---than the 1964 Alaska earthquake, going by the size of the same set of earthquake-generated waves.)
Nowadays, the magnitude of the 1964 earthquake is generally given as 9.2, often followed by a Mw notation. That's a symbol all of us may see with increasing frequency, even though now it's pretty well limited to scientific publications. The Mw stands for moment magnitude, and it represents a number integrating more of the waves energy released by an earthquake can produce. For example, the greater size now assigned to the 1964 quake takes into account very long-period waves Rowe calls "whole-earth oscillations," waves that took days to stop reverberating through the planet.
Rowe convinced me; moment magnitude is a more appropriate and accurate way of representing the comparative size of earthquakes. That's scientifically speaking, of course. When it comes to assessing how they feel, I think the Modified Mercalli intensity scale can't be beat. The Mercalli scale isn't concerned with how much energy an earthquake actually releases but rather with how strongly it is felt and how much damage it causes. Appropriately for such a quaint scale, its magnitudes are given in Roman numerals, from (I) for a quake not generally felt to (XII) for total destruction.
