Any ideas what this is?

A couple of days ago, I just happened to be looking at the infrasound trace from my Shake/Boom, and saw this (as it happened - no sound or apparent vibration as far as I could tell:

That is a pretty impressive sound spike. Not seen anything like that before (in the three weeks or so that I have had the system up and running). So switched over to the Z trace to see if it left any impression there:

That too is quite impressive - much bigger than the random things I see on a daily basis. It doesn’t quite match up with the sound spike though … until we look at the frequency-domain plot:

All low frequency (>10 Hz) until about the time that the sound spike occurred, then there is a corresponding “spike” of higher frequencies.

I wondered if it was a purely local event, so took a look at the data from a device about 10 miles East from me (Z - only):

Looks like he got it too.

And another station maybe 20 miles East:

Given that it is detected as ground vibrations by three stations, each around 10 miles apart in an E-W line, it looks like a local earthquake to me. But what explains the sound spike??

For completeness, this is the frequency plot of the infrasound “spike” - it doesn’t give much away, other than it being low frequency:

I think the sound pulse is an explosion from a quarry or highway construction project. The seismic pulse at the same time is a ground vibration being excited by the sound wave. Unless your RShake is buried deep in the ground, it will respond to strong sound impulses. The surface of the ground is easily shaken around 20 Hz (depending on your soil type).

The rest of the seismic event is mysterious and I think unrelated to the sonic event. The seismic signature does not look like a typical quarry blast. The sudden onset and taper makes it look like a quake, but there is no separate P and S wave that would allow you to triangulate its location. The frequency spectrum indicates that it is not far away from you. But USGS did not record any quakes > M2.5 in that time range. So either it was weaker than 2.5 or something other than a quake. - or some sort of atypical ground movement that is not the usual quake mechanism.

Let us know if you ever figure out what caused it.



I think your analysis pretty much matches mine. I think it is a local quake (<2). The frequencies look right (to me) for that, and the fact that a couple of other local RS stations saw it too, but ones further out didn’t make me think that is right.

I have become fairly convinced that large spike at the end is unrelated (coincidental), and is something very local.

I am on top of a (minor) fault zone. It is shown by USGS as being inactive, but that doesn’t mean there are not minor tremors from time to time.

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