AmaSeis: How toExtract and Save an Earthquake signal

 

 

1) We know that there has been an earthquake at about 1100 hours (UTM). I recognize a typical surface wave signal here during the 11th hour. It is hard to see because it is without filtering, so I値l enhance long period waves. (See Low- and high-pass filter figures below.)

 

To enhance signal, pull-down "Helicorder" and open the box above, I値l increase the gain from 20 to 200 and set the low-pass filter to 12 seconds and the high-pass filter to 25 seconds.

 


With these settings periods shorter than 12 seconds and greater than 25 seconds will be attenuated.




2) WHERE WAS THE EVENT? The event is much more obvious now, with these filter settings. Because this even is more that 7 days old, I looked up its location on this web site: http://neic.usgs.gov/neis/qed/ .
Had this been a more recent event, I would look it up here: http://earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/quakes_all.php .

 

I知 sure this is a large earthquake, that the first arrival is early in the 11th hour, so the origin time must be later than 10:40 because the longest time possible between the origin time and the first arrival is 20 minutes. It must be the one from Peru:

 

 

3) SELECT THE SIGNAL: Using the mouse and cursor, I値l highlight the trace, starting at the origin time (10:48:57) and continuing through the largest amplitude surface waves,

 

 


and then click on the blue squiggly seismogram icon

 

 

 

 

 

and the following window opens up:

 

 

4) RECORD THE EVENT ON THIS COMPUTER. Since I know where this event is located, I値l enter that information under Settings > Event Dialog

 

 

At the top of the window, AmaSeis has now entered the earthquake痴 location, magnitude, and the distance to my station.

 

5) SAVE THE EVENT. Next, use the File/Save As dialog box to save this event in SAC format. Select the folder AmaSeis:

 

 

 

 

 

 

 

 

 

 

 

 


The file will be in the directory C:\AmaSeis on your hard disk.

 

6) Want to make the image even better? A filter may be applied to enhance this event. Under Control/Filter set up a low-pass filter to enhance periods longer than 12 seconds.

 

 

 

 

 

 

 

 

 


After applying this filter twice, the trace looks much 田leaner.箱 Even so, save the trace prior to filtering so that different filter choices can be made in the future. You will have at least two copies of the same even in your folder.

 

 

7) Want to have an instant printable image for your class?You can use the Alt/PrintScrn keys to capture this image in the paste buffer. Past it into an MSWord document, print it out and post it on a world map.

 

8) OK, let's look at P and S wave arrival times:Next click on the travel-time curve icon, the inclined blue line with the "P":

 

 

 

 

 

 


With your mouse/cursor grab and slide the seismogram up and down, keeping the start aligned with the Y-axis, as that represents the origin time. The P and S phases on the seismogram fit the travel time curve when the distance is about 72 degrees, as expected.