Seismology Instruction with the AS 1

Filtering

     Local and regional event (less than 10 degrees distance) earthquake seismograms are 

usually dominated by short period (near 1 s period) energy.  Teleseismic (distant earthquakes, 

10 to 180 degrees) recordings usually consist of lower frequency (longer period) energy and 

often have prominent surface wave arrivals of 12   30 s period.  The AS 1 seismometer has a 

natural period of oscillation of about 1 s.  To make the seismometer more useful for recording 

distant earthquakes, the long period (greater than 10 s) response of the instrument is enhanced.  

Because background noise is always present, fi ltering of the records is often desirable to 

enhance the earthquake signals.

  Local and Regional Events

   In the Extract Event Screen, from the File 

Menu Bar select Settings > Filter

   A useful fi lter is a bandpass fi lter with frequency 

cutoffs of 0.5 Hz and 3 Hz (isolating the short 

period energy:  2 s   .333 s period).  

   Enter the two frequencies in the Filter Dialog Box.

  Global Events

     For teleseismic or distant earthquakes, lower frequency fi ltering often enhances the 

seismogram, especially the surface wave arrivals that are prominent for shallow focal depth 

(0   70 km) earthquakes.  

   In the Extract Event Screen, from the File 

Menu Bar select Settings > Filter

    Suggested fi lter cutoffs for lower frequency 

(longer period) signal enhancement are 0.0001 Hz 

and 0.1 Hz  (isolating the longer period energy:  

10,000 s   10 s period).  

   Enter the two frequencies in the Filter Dialog Box.

  Default Low Pass Filter

   AmaSeis's default low pass fi lter is a display fi lter that removes high frequency 

noise.  We recommend that this be set to 1 Hz.

AmaSeis Version 1.0 Level 2005.05.19

4.12