There seems to be no end of confusion surrounding the question, “Why do seismologists change
the magnitude of an earthquake? Magnitudes aren’t simply pulled out of a hat, yet the
downgrading or, in the case of larger earthquakes, upgrading of an earthquake’s magnitude is
understandably often misinterpreted. Are seismologists hiding something?
The Global Seismographic Network, along with thousands of other seismograph stations worldwide,
record seismic waves released by an earthquake. Data from each earthquake are recorded in
real-time, and either transmitted immediately by telecommunications to a data management
facility or stored for future analysis. Much of the stored data is accessible to anyone
who requests it. The National Earthquake Information Center operates a 24-hour service that issues
reports in near-real time for magnitudes greater than 3.0 in the U.S., or magnitude 5.0 anywhere
else in the world. These are just a fraction of the millions
of smaller earthquakes detected and located by seismic networks each year.
Obtaining an accurate preliminary magnitude can be difficult due not only to the complex
processes that occur deep within the Earth, but because there are over a dozen techniques
for calculating the magnitude of an earthquake. Some methods give approximate values within
seconds of the earthquake, while others require more complete data sets and extensive analysis.
Computers use the preliminary amplitude and arrival times for P, S, and surface waves
from multiple seismograph stations, to derive an initial magnitude and location. Since the
amplitude of a seismogram is largest near the earthquake, corrections must be made for
distance from the hypocenter, which is the place inside the earth where the fault actually
breaks. Seismograms recorded close to the earthquake can be a complicated superposition
of P, S, and surface waves, whereas distant seismometers are able to record a more-predictable
signature due to the P and S body waves that traveled through the mantle, and the slower
surface waves that travel near the earth’s surface.
Calculating an earthquake’s magnitude also requires measuring other characteristics of
the earthquake, such as the fault orientation, frequency, and rupture duration, with additional
adjustments made specifically for what kind of soil and local geology the station sits
on, and how the waves traveled through the earth Also factored in are the characteristics
of the seismograph that measures and records the earthquake including whether it records
3 components, or 3 directions of movement, and what type of waves the instrument is calibrated
to record. Each method of determining magnitude has its
own limitations. Two examples are the Richter and Moment Magnitude scales.
The Richter magnitude, recorded on a certain type of seismograph that was common in early
20th century when this scale was developed, relates the maximum amplitude of the seismic
waves, plotted against distance, which is calculated by S minus P arrival times. Connecting
the two gives the Richter magnitude. This scale is good for local smaller earthquakes
below magnitude five which produce most of their energy as short-period, high frequency
waves, but the scale is not as effective for larger earthquakes.
The Moment Magnitude is based on the lowest frequency data recorded on modern seismometers
and can be related to 1the distance the fault moved, 2the area of the moving fault, and
3the rigidity, or strength, of the rocks on both sides of the fault. It thus measures
the size of an earthquake in terms of the total energy it released. The larger the earthquake,
the more energy is released at lower frequencies that are associated with damaging surface
waves. The Moment Magnitude scale works well for large long-period, low-frequency, distant
earthquakes. This is the most common measure for earthquakes greater than magnitude five.
The 1989 Loma Prieta, California earthquake had an original Richter magnitude of 7.1.
In areas where structures were built on unconsolidated sediment, the shaking was severe. Several
years later, it was downgraded to a Moment Magnitude of 6.9 on the basis of more-detailed
studies of the various types of shock waves generated, combined with the extent of slippage
along the San Andreas Fault and new methodologies. Far from "downgrading" the power of past earthquakes,
the moment magnitude concept simply includes in its calculations the results of years of
research findings about how earthquakes behave. Revised estimates of magnitude do not mean
that the quakes were any less destructive. To complicate the initial magnitude picture,
large earthquakes are not simple local fault breaks, but are often are a cascading sequence
of breaks along a long fault rupture. The Tohoku earthquake was initially reported
as a magnitude 7.9 . However, that was just the onset of a 500-km-long rupture that was
to continue for 3 minutes. It was quickly upgraded to a magnitude 8.8, then to 8.9,
and then finally to a 9.0 Mw. Five years later, when all the data were in, it was ultimately
classified as Magnitude 9.1, the fourth largest earthquake ever recorded.
The reason it took so long to determine the magnitude is because the larger the earthquake,
the more complex are the seismic waves that are recorded from the earthquake. In addition,
surface waves can be more prominent farther away from an earthquake in contrast to body
waves that tend to die off, or attenuate, at greater distances from the hypocenter.
Determining an earthquake's exact magnitude once took weeks, but now seismologists, using
sophisticated computers can crunch the data in minutes to hours for small quakes, and
hours to days or weeks for Great earthquakes. Remember, if you see different magnitudes
reported for the same earthquake, it’s important to note that sometimes what appears to be
a smaller magnitude may be a different magnitude scale.
In closing, part of the confusion about magnitude lies in the fact that every earthquake has
one magnitude but many different intensities.