TRUE CONFESSIONS FROM A MAGNITUDE-WEARY SEISMOLOGIST
I hate the Richter scale. It feels almost sacrilegious, but I have to say it. Even more, I wish that the public had never heard of it. I am sick of trying to explain logarithms to the reporter who never understood logarithms in middle school and understands them even less today. I dread the phone call from the next middle-school teacher who does understand logarithms and is determined to use the Richter scale to "make logarithms come alive for her students", a teacher who will undoubtedly argue with me when I tell her a magnitude 6 has about 32 times the energy of a magnitude 5. Even when the lay public is not involved, the Richter scale offends me. My scientific soul despises something so arbitrary and empirical. I want meaning; I want connection to a physical reality. I want units.
Many years ago, Hugo Benioff proposed describing earthquake size with energy. It never took hold in part because we could never agree just how much energy is released. I have hoped that seismic moment could deliver us from The Scale. It has everything a scientist could love, a physical reality that connects field geology to geodesy to seismology. Just tell them the moment and we won't have to explain that this earthquake is really much larger than that other earthquake, even though M 7 does not seem that much more than M 5. But deliverance has not come. We seismologists all cling to The Scale even though calculating seismic moments is now routine. The first thing we do with a seismic moment is use it to calculate a moment magnitude, and we are back where we started, without units, saying, "Well, actually the energy goes up as the square root of 1,000 ...." What is this power that The Scale holds over us?
I think it is the power of ten. With our fingers in front us, we have a visceral understanding of a number less than ten, but not more. Look how hurricane categories, the UV index, and the Beaufort scale for wind have picked up the same simple approach. Richter understood early on what other fields are now realizing--the public wants numbers that can be counted on our hands. Richter's original approach used only whole units, or occasionally half units, of magnitude. This made magnitude seem more like the UV index and less of a scientific concept. If we can't get rid of magnitude, maybe we could at least give up the decimals.
While The Scale panders to this desire for simplicity, it does a disservice to earthquake education. The public does relate magnitude to the fingers on our hands and thus does not comprehend the true differences in the sizes of earthquakes. Explanations of logarithms and energy radiation just confirm the prejudices that scientists are incomprehensible nerds. We are leaving the public to make decisions about their safety without the recognition of the much greater destructiveness of a great earthquake on the San Andreas compared to a moderate event such as Northridge. Our explanations are so confusing that many give up on listening to us and believe what they want. Rumors continue, such as the one common in 1994 that the Northridge earthquake was really a magnitude 8 but we lowered the number (6.7 isn't that far from 8, is it?) so the victims would not get a rebate off their taxes.
I believe these problems will continue and the public will not understand the nature of earthquakes as long as we use magnitude as our primary description of earthquake size. It is not going to be easy to change the habits of decades, but we need to start. The USGS has recently developed a policy that moment magnitude is the preferred value whenever it is available. I advocate taking this policy to its logical conclusion and making moment itself the preferred expression of earthquake size.
As a first step, when asked what magnitude means, we should describe it is an arbitrary number scaled from the moment. If forced, we can add that it scales with the logarithm of the moment to the 1.5 power, which should convince most people to drop the subject. But this won't be enough. Before we have any chance of convincing people to give up The Scale, we have to make seismic moment more user-friendly. Trying to explain "one point two times ten to the twenty-six dyne-centimeters" in a sound bite is enough to get most seismologists running back to a two-handed magnitude. We need a unit a little closer to the ordinary human experience. So I propose we describe seismic moment with the Aki, in honor of the recently retired inventor of the seismic moment, Prof. Keiiti Aki.
I would define the Aki to be equal to 1021 dynes-cm (or 1014 N-m for the SI police). The smallest earthquakes routinely recorded by most networks (M 2.0), would be about 0.01 Akis, a barely damaging earthquake (M 5) would be 400 Akis, Northridge would be 120,000 Akis, and the great Chilean earthquake would be two hundred billion Akis. Essentially every earthquake we would ever talk about would involve a range of numbers similar to our monetary system. From a penny for our thoughts to a 1980's United States federal deficit, these are numbers that may not fit on our hands but are ones we can live with.
With the Aki in hand, we can discuss earthquakes and earthquake hazard with a nonseismologist without the phrases "scales with the logarithm of the ground velocity" or "increases by the square root of 1,000" ever having to be said. We can forget MW = (log M0)/1.5 - 10.7 (or should it be MW = [log M0 - 16.1] /1.5?). We can put units on our graphs, just like other scientists. We can tell a reporter that the second earthquake was 2,000 Akis while the first was 2 Akis and not need to explain that makes it a thousand times larger. We can communicate.
To send a letter to the editor regarding this opinion or to write your own opinion, contact Editor John Ebel by email or telephone him at (617) 552-8300.
Posted: 3 August 2000