Purpose:

As the first in class assignment, we will look at some actual seismograms.

The seismograms used  are for two earthquakes in the central United States.  They are pen-and-ink recordings.  Historically seismograms have use smoked paper, photpgraphic paper and pen-and-ink recording technologies.  Later continuous magnetic recording was used.  The current technology records everything digitally, but a computer display.  For example,

Data from station SNZO (South Karori, New Zealand)

last updated at

Wed 01/21/09 07:19 MST (Wed 01/21/09 14:19 GMT)


SNZO seismogram

This page is URL: http://aslwww.cr.usgs.gov/Seismic_Data/telemetry_data/SNZO_24hr.html

As you can see in this display, this displays 24 hours of recordings.  Time increases from top to bottom and from left to right. You will also note an earthquake signal starting at about 02:24. This earthquake had a magnitude of 4.9 and had the coordinates:

  MAG UTC DATE-TIME
y/m/d h:m:s 		LAT
deg 		LON
deg 		DEPTH
km 		 Region
MAP  4.9   2009/01/21 02:19:16   -22.841    170.902  35.0   SOUTHEAST OF THE LOYALTY ISLANDS

There is also evident of smaller earthquakes on the same seismogram.

To study earthquakes and to study the structure of the Earth, we must be able to measure the arrival times of earthquakes on seismograms.  This means that we must be able to read TIME from the seismogram.  Note that the seismogram consists of lines and that we can always measure distances from a printed copy. However, the number of centimeters per hour (or 10 minute span here) depends on the printer that you use.  The one piece of information that we have is that the vertical lines (or trace offsets on other seismograms) are places every 10 minutes or 600 seconds in this display.

To determine the arrival time of a seismic signal, we can measure down from the top to get the correct hour, and the move horizontally to get the minute and second.  For example, the first downward blip on the seismogram is
  
2009/01/21 02:23:34

You might wonder how I was able to read the time to the nearest second from this display. The answer is that I ran a program and got the data from the USGS National Earthquake Information Center in Golden, Colorado. I then filtered and displayed the digital data with my gsac program to work with the following image:


t.png
The time that I gave above corresponds to the arrival marked iP_D.

To convert a distance to a time we must determing the ratio of  TIME/DISTANCE, e.g., SECONDS/MILLIMETER. You will shortly see why I use millimeters for the unit of time when we work with real seismograms.

Assignment:

Define seismogram.
Define seismoscope.
Define seismomomenter.