Anatomy of a Local Event

I received a USGS Earthquake Notification Service (ENS) message about an Oregon earthquake.


There was not much on the unfiltered helicorder record.

 


Since the event occurred nearby, I expected that there would be high-frequency energy.  The helicorder record above has had a high-pass filter with corner set to 1.2 seconds applied.

 


I highlighted the events and filled in the Setting/Event... dialog with parameters from the USGS ENS message.


The event is difficult to see in an unfiltered view..  It's the high frequency energy riding on low frequency noise.

 


After twice applying a high pass filter with corner set to 1.2 seconds, the local earthquake stands out quite well.


The event fits the travel time curves for a distance of 0.79 degrees.  Multiply by 111.2 to convert to kilometers.  0.79*111.2 = 88 km.

 

.  The time between P and S is about 12 seconds.  The rough rule of thumb for local events is Distance (km) = 8 x (S-P).  In this case,

8 x 12 = 96 km, in rough agreement with the computed distance of 88 km.

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