#Richter Scale
8 messages in this thread
Gee, I wonder why I'm suddenly curious about the Richter scale….
I've heard contradictory things about what the scale measures. I've heard: a
change of 1 unit represents a 10 fold increase in energy
a change of 1 unit represents a 10 fold increase in ground movement
a change of 1 unit represents a 30 fold increase in energy
plus something I can't quite remember, but it gave a logarithmic equation for
energy that didn't work out to any of the above.
Can anyone clear this up for me?
The relationship of Richter Magnitude (M) to energy (W) takes the form:
log (W) = a + bM, b in the reference I saw was 1.5.
10
1.5
This means that a change of 1 in M will give a 10 increase in energy.
1.5
10 = 31.62
Obviously this number depends on the factor b. I am not certain if that has
changed in recent years.
Mark
Mark,
Thanks. Where would one go to find b?
It was 1.5 last time I looked. It may have been revised. a=11.8 b=1.5
according to Newmark and Rosenblueth, 1971.
Mark
Hi Mitchell, Mark's reply contains the quantitative answer. I thought I'd
dress it up a bit (professors tend to indulge)
Magnitude scales like the Richter scale [which, BTW, was intended only for
use in southern California; other parts of the country and world use DIFFERENT
magnitude scales–all are logarithmic, but they must vary to account for the
different kinds of rock (and thus different seismic wave velocities) around the
region] measure the amplitude of a needle on a seismograph. Because this is a
logarithmic scale, a M=5 EQ moved the needle 10 times as much as a M=4 did, and
100 times as much as a M=3 did. There is no upper limit on a magnitude scale.
Because seismologists use different waves and other information to calculate
magnitudes, you may hear different values quoted for the same quake. For
instance, the 1964 Alaska EQ had a surface wave magnitude of about 8 or 8.5 (or
something like that), but a moment magnitude of 9.2 (I think; sorry I don't
have any of the sources here at the moment).
Intensity scales (Modified Mercalli) measure the damage inflicted by an EQ,
and is a qualitative, subjective scale derived from reports of observors of the
quake. These are assigned values of Roman numeral I (not felt) to XII (damage
total, lines of sight completely distorted,
etc). Loma Prieta maxed out at IX, I'd guess from the tube that the Northridge
quake will do likewise.
The energy released by an earthquake (which you can calculate from the
equation supplied by Mark) increases ROUGHLY 30 times for each increment of 1
on a magnitude scale. Thus, a M=8 earthquake packs 30x30x30=27,000 times the
energy of a M=5 quake. Pretty sobering.
Let me know if I can supply any other info. Richard
Richard,
Thanks for all that info! I had no idea that the Richter scale was intended
only for California. I'm surprised I've never heard of the Modified Mercalli,
since it would seem from your note to be more relevant to the type of info
people are concerned about.
If you have a moment, I'd like to relate something that happened to me when I
was about 12. My family was visiting some museum that had a seismograph. I
was looking at it when suddenly the needle starting swinging wildly. I asked a
guard if that meant there was an earthquake — he said, "no it's probably just
a heavy truck outside." Later, when we were in our car going back home, we
heard on the news that there was a devastating quake in Alaska at about the
time I was looking at the seismograph. I have assumed that I was actually
seeing the quake as it happened. Does that seem likely — I really don't know
how a seismograph on the east coast would have reacted to an Alaskan quake.
Hi, Mitchell–
An Oklahoman TV station reported on detection of the CA quake and aftershocks
by a museum seismometer here in OK. The Alaskan quake was hundreds of times
more powerful (assuming it is the _big_ Alaskan quake you may have witnessed)
than those aftershocks, so I find it quite believable you were seeing the AK
quake. At that distance, it may have registered no more than a nearby truck,
so only a seismologist might have been able to tell the difference.
–Doug
Mitchell, Doug is right on as usual. The 1964 Alaska EQ was on Good
Friday, so that would make it late March or April, I forget the date. But it
was such a monster that the earth was ringing like a bell for days afterwards,
and it was measured at all stations around the globe.
Richard