[irised] Where are the earthquakes?

[John or Jan Lahr]

Here's a question that I received recently from a teacher in Portland:

>I finally got the seismometer up and running at the end of January 
>and it has been running continuously since then.  I keep seeing 
>spikes with limited duration on the traces but have trouble tying 
>them to events I get from USGS on earthquakes occurring globally.  I 
>have followed the advice on the setting for gain and both 
>filters.  How much of a lag should I see from different parts of the 
>earth?  Anybody have any examples of earthquakes in the last month 
>and the traces and time shifts?

Here's my answer:

You will only be able to see the largest earthquakes around the 
world.  Alan Kafka has kept track of the events that were and were 
not recorded by his AS-1 system.  Check out the Figure 2 of his paper:
http://www2.bc.edu/~kafka/SeismoEd_SRL/SRL776_EduQuakes.htm to get a 
sense of how large an event will probably have to be to be seen on an AS-1.

The first P arrival will always be within 20 minutes of the origin 
time, but the surface waves, which may be all that you will be able 
to see, can be an hour or more later.  Printing out the travel time 
curve posted here may be very helpful:
http://neic.usgs.gov/neis/travel_times/ttgraph.html

When you see the USGS report of a large event, compare your record 
with the AS-1 that I'm running here in Corvallis, 
Oregon:  http://jclahr.com/science/psn/as1/heli/chor.html
With respect to earthquakes a great distance away, Portland and 
Corvallis are close together, so our seismograms will be quite similar.

You could also check out the professional station located here in 
Corvallis (COR).  Clicking on this record will cycle through three 
different versions:  unfiltered, short period filter, and long period 
filter.  If an earthquake looks huge on this record, then there is a 
chance that we'll also see it on our systems.  Of course equipment 
like that at COR would cost 10's of thousands of dollars!
http://www.jclahr.com/science/psn/cor/index.html

For example, on today's record of COR three earthquakes are clearly 
visible.  The amplitudes on the long-period record (the first record 
you see when visiting this site) are not huge, however, so these 
events are too small to be seen on our AS-1 records.

Cheers,
John