NOT a 6.8 Richter Magnitude

The 28 February 2001 Nisqually (Washington, USA) Earthquake

Joseph Hull, Seattle Central Community College

copyright 2001 Joseph Hull

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NOT a 6.8 Richter magnitude  (see answer at bottom)

Earthquake coverage by the media is always problematic ("fault lines", predictions using freaked-out pets, etc.) however the 2001 Nisqually  earthquake has received some abnormally strange press coverage.  The Seattle Times, for example, has called it "The Miracle Quake", and has invoked a smiley-face God to explain why there were no deaths and few serious injuries (perhaps an immediate benefit of electing the holier-than-thou Bush administration: quake relief).  The fact is that this earthquake was probably not a 6.8 on the Richter scale.

In their many press releases and press conferences, USGS and UW geologists and seismologists have quoted a magnitude of 6.8, and this magnitude has been widely reported in the print and electronic media ("6.8 Shocker" was the banner PI headline).  Like most news outlets, the Seattle Weekly referred to the 6.8 value as the Richter magnitude.  The Weekly also tried (and failed) to explain how the Richter scale works.  However, the number 6.8 for the 2001 Nisqually quake does not refer to the traditional Richter magnitude, but rather to a newer and completely different magnitude scale, called the moment magnitude.  Please bear with me while I explain the difference, and most importantly, explain why these two scales cannot be compared to each other.

The classical Richter magnitude (named for Charles Richter) is a measure of earthquake size based upon the amount of ground shaking as measured on a recording device (a seismometer).  Earthquake-generated waves passing through the Earth's interior ("body waves") or waves rolling and twisting across the ground ("surface waves") are recorded by machines.   The amplitude of the ground shaking (the height of the wiggles) is used to calculate the Richter magnitude.  The scale is logarithmic; each step up in magnitude equals 10 times the ground shaking.  The important point is that the Richter magnitude is based on ground shaking.

The moment magnitude (based on the physical entity known as the moment) is a measure of the amount of movement on the underground fault and the area of the fault that ruptured or broke.  Body waves and surfaces waves are produced by movement along planar breaks in the crust of the Earth (faults), when one fault block slides jerkily past another fault block.  The sliding of blocks past each other is not smooth and even; as the fault blocks move, waves are radiated outwards in all directions.  Instead of measuring the rumblings as Richter does, the moment magnitude measures the size of the fault break and the amount of fault block movement along that break.  The fault that produced the 2001 Nisqually earthquake had a rupture area of approximately 20 miles by 10 miles (200 square miles), and the maximum amount of slip or movement was probably about 3 feet (yeah, seems minimal, but you're moving monster blocks of the Earth).  Like the Richter magnitude, the moment magnitude is a logarithmic scale.  The important point is that the moment magnitude is based on fault movement, not on ground shaking.

These two scales cannot be directly compared, because they are based on completely different characteristics of the earthquake; the calculations have nothing in common.  In addition, there is no formula that says a Richter magnitude X equals a moment magnitude Y; one can be greater than or less than the other for the same earthquake event.  Typically, the moment number is numerically larger than the Richter number.  For example, the 1960 Chile quake had a Richter magnitude of 8.5 and a moment magnitude of 9.6.  The 1964 Alaska quake had a Richter magnitude of 8.3 and a moment magnitude of 9.2.  So if you wanted to puff up your pet earthquake a bit, you would say in a loud voice "MAGNITUDE 7" and in sotto voce, "moment".

Seismologists and geologists have not emphasized which magnitude scale they are using.  However, if you go to the USGS and UW websites, they clearly state "moment magnitude Mw."  In press releases and press conferences, seismologists and geologists have used a curious locution in describing this quake, which the media and public officials (including the Governor) have picked up on: "if this quake had been closer to the surface, it would have caused more ground shaking."  This statement only makes sense if you're talking about the moment magnitude, not the Richter magnitude.

So why not tell the media and the public straight up what kind of magnitude scale is being used?  Why this omission?  Why throw in the moment magnitude without identifying it as such?  Better yet, why not calculate the traditional Richter magnitude and use it?  What IS the Richter magnitude of the 2001 Nisqually earthquake, anyway?  As of this writing, no one is saying.  The Richter magnitude has disappeared.

The Richter magnitude of this quake is probably much less than a 6.8; my guess, a 6.2 or less (remember that logarithmic scale).  Practically everyone in the Puget Sound region knows this already, it's one of the main topics around water coolers and photocopy machines, what a poor excuse for a 6.8 the Nisqually quake was.  Are the seismologists deliberately puffing up this quake?  That makes no sense.  If  I was in their place, I would be yelling, "Hey, 2 billion in damage, and it's only a 5.9!"  Wait til the 8.3 hits!!  You better give us more money for research."

Another explanation for this obfuscation relates to Richter's problems as a magnitude index.  Many seismologists abandoned the Richter magnitude for scientific purposes a long time ago, because of the large number of corrections needed to calculate this number.  The Richter magnitude has, in many cases, outlived its usefulness as a quantitative measure of earthquake size, but the public is wired into the Richter scale.  Say "3.6" and "8.2" to anyone on the street, and they know what you're talking about.  What's a seismologist to do?  For years they calculated the Richter magnitude for public consumption, until 28 February.

I think this is a missed opportunity.  Give both numbers to the public and talk to them about the differences.  Treat the public like intelligent adults.

Or better yet:  bring back the Mercalli scale.

Joe Hull, 5 March 2001
 

ADDENDUM  12 March 2001

    The University of California Bekeley Seismological Laboratory is reporting the Richter magnitude:  6.3.  This is the "local" Richter magnitude (ML), for earthquakes less than 600 km/400 miles from the recording seismometer.  The local Richter magnitude is based on the height of the biggest wiggle, which is almost always from a surface wave.

For the Richter and Moment Magnitudes, go to http://www.seismo.berkeley.edu/seismo/eqw/eqw_01.02.28.html