Midnight tremors!

How a geophone works?

Mind your P's and S's

A pretty impressive M4.3 earthquake hit Medford Oklahoma at about 05:00 UTC today (160km N of Oklahoma City).   It shows up as a massive spike on the MAOK station

M4.3 event about 160km N of Oklahoma City recorded on 

station MAOK (upper) and our geophone site KEYWB(lower) 

On first glance it looks as though this event was missed my our geophone... however the MAOK station clock is running 5 minutes fast so we need to look at the wiggles occurring at 05:00UTC ...just when our geophone  trace skips from one line to the next (in jamaseis, click and drag the bit you want and click on Extract Selection) 

Zooming in on the time around 05:00 our geophone 

shows an impressive double-peaked signal 

The double peak is because earthquakes create two types of waves ... Primary P waves which are longitudinal waves (like sound waves in air) and Secondary S waves which are transverse waves (with a side-side particle motion).  The P waves travel a bit faster than the S waves so there is a time delay between the two sets of waves arriving

Because we know how fast P and S waves travel through the earth we can estimate how far away the event is by lining up P and S wave arrial times with the calculated travel times for P and S waves on a graph (in jamaseis you can do this by clicking the calculate distance tab in the zoomed data window) 

Graph of calculated P and S wave travel times vs distance with our seismogram overlaid, the seismogram is slid around on teh graph until the P and S waves line up with the curves and the distannce can be read off the Y axis.

 This gives us an estimated distance to the event of about 1.5 degrees.  (as every good sailor will tell you 1 degree = 60 nautical miles = approx 111 km (the circumference of the earth/360).)  So about 160km away !  

Reflections on Oklahoma 5+ quake

David: 'was not scared' ....'thought it was his dog'

 

 

Harry: 'picture shaking ...pool sloshing...I didn't know what was going on'

oops missed

Our intermittent 'system down' issue coincided with a shake , this one was before the one Paul reports we did get (post below this)...
Here the media update that alerted us :

 

And on our geophone trace - bottom half of screen - you can see the gap when it was not working is at the same time as the top section, which is the MAOK station, that does capture the quake!

Never mind - but thought we should be honest - we did miss one :-)

Another one hits

Over the weekend the trusty geophone sensor was moved to a new location.   Fortunately another small earthquake came along a few hours later just to reassure us that things were still working.

M2.9 earthquake about 50km from Oklahoma city

 This one was too small to be felt and quite a bit further away.   However a quick check with the Oklahoma Geological Survey seismic station in Oklahoma City, BCOK, shows a clear signal

Professional seismic station (lower figure) indicates where to look to find the seismic wiggle on our geophone (upper figure) 

Having a couple of seismic traces to compare side by side gives us the confidence to be sure that this small wiggle is due to the latest quake (scientists are not keen on the coincidence explanation)

Zooming in to the wiggle and filtering some of the noise away confirms that the geophone is still working in its new location (our geophone above, OGS professional sensor below)