Webcorder Basics

What is a "Webicorder"?

The Webicorder is a Web display of seismic data. The display looks like paper records made by machines called "helicorders". A plot of the data is called a seismogram.

How to convert from UTC time to Local Time?

Seismologists use UTC time which stands for Universal Time Coordinates.  That avoids confusion in describing what time an earthquake occurred.  Use the table below to convert to your local time.  So, if an earthquake occurred at 14:30 hours UTC and you live in the Pacific Time Zone and it is summertime (Daylight savings time) you will subtract 7 hours to get 7:30 local time.  Remember that we also use the 24 hour clock.  For example, 2:00 in the afternoon is is 12 + 2 or 14:00 on a 24 hour clock.

How do I read the Webicorder?

On the left side of the Webicorder plot you see the hour marks in Universal Time Coordinates (UTC).   Usually we display 24 hours of data on the webcorde and there are 24 lines.  You "read" the seismogram like a book, from left to right starting at the top. The earliest data starts in the upper left and then time increases to the right. Just like a book, when you get to the end of a line on the right side the seismogram continues on the left side of the next line down. Minutes are marked on the bottom of the display.

What does an earthquake look like?

When an earthquake occurs the seismogram will show ground motions that typically last from several tens of seconds to many minutes depending on the size and distance of the earthquake. The height of the recorded waves on the seismogram (wave amplitude) can help us determine the magnitude of the earthquake.

A recording of an earthquake has recognizable characteristics. Typically, one can recognize the arrival of different wave types: P (the fastest traveling waves), S (shear waves), and Surface waves.

One way to determine how far away an earthquake occurred is by examining the amount of time between the P Wave and the S Wave.  The more the separation, the farther away is the earthquake

Below is an event recorded on the Kids Zone Webicorder in Depoe Bay, Oregon.  Notice that there is virtually no separation between P and S waves.  This is because the epicenter for the earthquake was very close --- only 41 miles away!

Not all the wiggles seen on the seismograms are due to earthquakes. Anything that produces ground vibrations could be recorded. Some of the non-earthquake things are cars and trucks, ocean noise, and wind blowing nearby trees.

This description of the Webcorder was derived from The Pacific Northwest Seismograph Network at
http://www.pnsn.org/WEBICORDER/K20/about.html