#Richter Scale
7 messages in this thread
I saw your question and wanted to refer you to the definition for Richter
magnitude I left under the topic "Richter Equation".
I think it might give you a little insight.
I think you are right to some extent about the TV "talking heads" but there are
some other reasons for describing it the way we do.
First the "Richter Scale" is a dimensionless ratio (very much like the decibel
scale someone else mentioned) therefore it has no units. However, the Richter
Scale has a very specific reference (see the Richter Equation message). There
are also other measures of earthquakes that seismologists have used for a long
time but the media doesn't grab ahold of. For instance, there is the "Modified
Mercalli Scale" which is a precise way of describing the effects of an
earthquake such as shaking, damage to structures, etc. Unfortunately Mercalli
measures depend on the location of the observer and changes from place to
place.
Relating all of this to your question, since there are no units to the Richter
scale and other scales exist you shouldn't just say an
earthquake is a 6.6. If you want to be precise you should say it was a 6.6 of
the Richter Scale.
But (sorry to repeat myself from last week), the precise term Richter scale
applies strictly only to southern California; earthquakes in other parts of the
world (e.g., midwest, Japan, Indonesia, even northern California) correct
travel times for different seismic velocities and do NOT use the Richter scale,
though they DO use a logarithmic scale that isn't too dissimilar from the
Richter.
Perhaps the "best" scale that has been proposed and is currently very
popular in the seismology community is a logarthmic scale based on the seismic
moment. These values, simply labeled M (but in bold), are independent of
seismometers and depend chiefly on the size of the rupture (i.e., the area of
the fault plane that broke) and to a lesser extent on the type of rock etc.
When recalculated, major earthquakes of the 1900s turn out to have moment
magnitudes greater than 9. I think the 1960 Chile and 1964 Alaska quakes were
9.5 and 9.2 (don't have the reference here though). This seems to be the most
useful scale out there, but sometimes I despair of the media or the populace
ever letting go of Richter. Though my class last semester seemed to get the
hang of it.
Richard
Hi, Richard.
The seismic moment is certainly the best measure for consistency. But doesn't it only
apply to surface rupture where measurements can be made? Otherwise aren't we
best served by systems based on wave amplitude? Atleast, for rapid
determination of location.
Even the moment is doesn't relate well to structural damage in less than
catastrophic quakes. Those quakes where resonance plays a role a better
served by spectral measurements. Two quakes with the same moment may have
significantly different results, ceteris paribus.
The public gauges severity, wrongly, from what the geologists report. Look at
the destruction in Asia Minor from quakes that would barely wake us from
slumber. The seismic measurements have no way of incorporating structual
integrity.
mark
Hi Mark, yes I think you're right: for rapid location calculations, wave
amplitudes are best. but for useful comparison (esp of major shocks, as you
noted), moment is the way everyone is going. And I don't think there has to be
a surface rupture; with a well-studied quake, e.g., intermediate depth
subduction zone, the rupture area can be determined pretty precisely.
Regarding intensity of damage: I think it would be very helpful to get more
media play for the Mercalli scale, esp in urban areas like the southland, Bay
Area, Seattle, and New Madrid seismic zone. Perhaps with luck we could even
get the public to recognize the difference between the two….yes, I know I'm
dreaming, but a lot of people here realize the increased risk posed by muddy
soils, hillside sites, and proximity to active faults.
Anyway, the semester started today. My EQ course is booked full at 40, up
from 15 in its initial run last semester. Northridge should guarantee an
interested class. Bye Richard
Well, this quake was easily a Mercalli VII at the epicenter.
Is the rupture zone measured by analysis of seismic echos? I read a
description of the rupture. Apparently the size is inferred from the depth
and horizontal location of the hypocenter of the aftershocks. It seems that
the aftershocks are assumed to occur at the periphery of a growing rupture
zone. Is this correct?
While the original hypocenter was 10km deep, the most recent aftershocks have
been as little as 3km deep.
Mark
Hi, Richard–
We will get to work on weaning the American public from the Richter scale
first thing after we get them to use the metric system and Celsius… 🙂
–Doug
Hmm, then I won't hold my breath!