Hello

I live in Oslo, 8.23 km NW of the site of the terrorist bomb that went
off 22 July.  The explosion could be strongly felt here, and seconds
later heard.  I have three 4.5Hz geophone and I've now studied the
data from that day.  Both the N and E geophones show the event
clearly, while the Z geophone shows little.

The P wave turns up at 15:25:22.9 local time and the S wave at
15:25:24.5.  Assuming that the S wave travels at 40% the speed of the
P wave, this places the explosion at 16:25:21.8.  I can see the
arrival times best when I apply a lowpass filter, such as this one at
2 Hz:

http://voksenlia.net/nytt/2011/bombe-20110722b.jpg

Later, there is a big signal at 15:25:45.4.  This is the arrival of
the sound wave, which was heard like a distant thunder.  This is 23.5
seconds after the explosion (as indicated by the P and S waves).
Assuming that the distance is 8.23 km (trusting Google Earth), the
sound has travelled 350 m/s, which sounds reasonable.  I assume that
the shock wave was supersonic just after the explosion giving it a
slight head start compared to the normal speed of sound.

There is an interesting weak signal at 15:25:42.3 which shows up
better on an unfiltered plot:

 http://voksenlia.net/nytt/2011/bombe-20110722c.jpg

There is a hill (Holmenkollen hill) between my house and the bomb
site.  I think this weak signal is the seismic signature of the blast
wave hitting the other side of the hill 1.4 km away from my house and
6.8 km away from the blast.  At least the timings fit that hypothesis
well.

-- 
Steinar Midtskogen
http://voksenlia.net/met/
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