Thread: Where are the earthquakes?

Started: 2007-02-12 17:50:06
Last activity: 2007-02-12 17:50:06
Topics: IRIS EPO
John or Jan Lahr
2007-02-12 17:50:06
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



06:34:53 v.22510d55