From: jh507@shakegeosc.psu.edu (John Hammer for GEOSC 507)
Subject: Re: To John Hammer
Organization: Penn State University, College of Earth & Mineral Sciences
Date: Sun, 15 Nov 1992 18:06:58 GMT

Sorry, I should have explained things a little better.

Bob, ( a guy how works in the same lab as I do), was running
an earthquake search on the NEIC Global Hypocenter Data Base
which we have on CD here in our geophysics computer lab.  I 
remembered your request and thought that such a limited search
would be very quick to perform, so I did it while he had the CD
in.

The Global Hypocenter Data Base combines several earthquake 
catalogs and lets the user search for events in a given area of
the world, in a given time window, of a given magnitude, and so
on.

The info that I passed along to you was in the State Seismicity
Files catalog. (This is one of many that the CD contains. )
The State Seismicity Files are maintained by the USGS.

I then looked up the event on the 
SEISMICITY MAP OF THE STATE OF NEW MEXICO
MAP MF-2035
by C.W. Stover, B.G. Reagor, and S.T. Algermissen.
  Published by the Department of the Interior,
                   U.S. Geological Survey, 1988

The map lists the source of information for this event as the 
U.S. and Geodetic Survey
Seismological Bulletin MSI 1 through MSI 316,
January 1934 through May 1967.

I don't know how you would get this source, but I would suggest 
contacting the USGS ( U.S. Geological Survey ). 

The NEIC, ( National Earthquake Information Center ), might be
the one to try.
Their address is:

United States Geological Survey
National Earthquake Information Center
Box 25046, MS 967
Denver Federal Center
Denver, Colorado, 80225


The number I quoted you in the first post was a Mercalli
Intensity IV.  This is a measure of how the earthquake was "felt".
It is based on the perceptions of people near the event, and on
structural damage inflicted by the event.  From the above map, a
description of an Intensity IV:

Felt indoors by many, outdoors by few.  Awakened few, especially
light sleepers.  Frightened no one, unless apprehensive from previous
experiences.  Vibration like that due to passing of heavy or heavily 
loaded trucks.  Sensation like heavy body striking building or falling
of heavy object inside.  Rattling of dishes, windows, doors;  glassware
and crockery clink and clash.  Creaking of walls, frame, especially in
the upper range of this grade.  Hanging objects swung, in numerous
instances.  Disturbed liquids in open vessels slightly.  Rocked 
standing motor car noticeably.

 
To calculate a Richter magnitude from an event you need to have a 
recorded waveform.  Since the instruments recording in this time were
not spread across the country as they are today, one would not expect
the event to have been recorded at a seismic observatory.  There are
methods for converting Intensity to magnitude, but I am not familiar 
enough with them to explain them.  I BELIEVE, (someone correct me if I'm
wrong), that the conversion is sensitive to the depth and orientation
of the rupture as well as the site response.  The site response simply
put is how the local geology responds to the energy of the seismic
waves.  ( An example might be Mexico City.  It behaved differently than 
the surrounding area to a earthquake some distance away. )
It does not seem as if the event's depth is well known in this case.
A shallow event is typically felt more than a deeper event of the same
magnitude.  So the deeper the event, the larger it could have been.
There would be an upper limit to the size if it was not recorded on the 
west coast.  Here in the east, many people convert intensity to 
magnitude for old, large events.  I think they feel these are reasonable
because of the population density.  ( Maps of the areas of different 
intensities are more accurate in highly populated areas. )  I do not 
personnally work with earthquake Mercalli Intensities, so you should
find a better source of info on this.


Hope this clears some things up, and helps with your work,

John Hammer
EMAIL: hammer@geosc.psu.edu
Graduate Student
Penn State Geosciences
Disclaimer:  I'm a graduate student (Enough said)
Newsgroups: sci.geo.geology