# RS4D Accelerometers

Hello, I believe that my RS4D accelerometers are not functioning as they should be. When I am on swarm looking at my data, the geophone data (EHZ AM) looks great, I can make out the p-wave and the s-wave. But when I open the accelerometers ENE AM and ENN AM, they are just straight lines of data that don’t pick up any seismic activity whatsoever. I stomped next to my shake to see if it would even pick up the slightest thing but it didn’t. The accelerometer ENZ at least is able to pick up when I stomp next to it but no earthquakes, and it for some reason makes it’s data go off of it’s segment that it is supposed to be on. By that I mean the data starts to overlap other data. So my question is if my shake is defective or I am doing something incorrectly. I hope I manged to formulate my paragraph correctly and that it is understandable. I’m only in 8th grade.

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Hi Ezra, thanks for your interest! Your paragraph was perfectly understandable, and this is a great question. My response gets into a bit of complicated stuff, so please let me know if you don’t understand anything about it.

Here’s a diagram that might help explain a little about what’s going on if you’re a visual learner like me:

``````     weaker shaking < ------------------------------- > stronger shaking

# geophone measurement scale: ( | indicates limit of human sensing)
100 nanometers/sec [--------------|----] 2 cm/sec (geophone will clip)

# accelerometer measurement scale:
(noise floor) 10 mm/sec^2 [|--------------------] 19.62 m/s^2 (2g)
``````

This is actually normal behavior for the Shake accelerometers. The geophone (EHZ) is what’s called a “weak motion” instrument, meaning that it will pick up vibration much weaker than what humans can feel. The geophone “clips” when shaking veolocity is greater than about 2 cm per second. Clipping happens when the mass inside the geophone hits the housing at the end of its track and bounces back (you can hear this if you pick up the Shake and shake it—do this with power off obviously). When the geophone clips, it has reached the upper limit of vibration that it can measure.

The accelerometers (ENE, ENN, ENZ) are “strong motion” sensors. The scale at which they can sense vibration is shifted higher, so the “noise floor” of what they can sense is higher than the geophone’s. You can think of the accelerometer noise floor as basically the lower limit of what humans can feel. So if there are any “felt” earthquakes, they should show up on the accelerometers. On the upper end, our accelerometers can measure vibration all the way up to twice the acceleration due to gravity (2g or 19.62 m/s2).

Here’s a good example of an earthquake in which the geophone picked up the vibration really well, but it just barely registered on the accelerometer channels. I was asleep for this one, but I am not certain I would have felt it even if I were awake:

And here’s a stronger earthquake, where the accelerometer channels are able to capture a bit more of the signal. This one I definitely did feel, for nearly 45 seconds:

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The accelerometer chips sometimes show some “wander” in their measurements, and SWARM doesn’t deal well with this wander. The wander is normal and can be corrected by using a high-pass filter that removes lower frequency vibration (rsudp does this automatically).

This did answer my question thank you. But around what magnitude would it be necessary for the accelerometers to pick up. Because I have had a 4.0 earthquake happen about a mile away and it still didn’t pick anything up.

There a few factors that go into the answer for that question, the main one being “could you feel it at the time?”

If you could feel the quake and it still didn’t show up on the accelerometer channels then perhaps there is something wrong. When was the quake? I can go back and look at it in the data.

The shaking was very noticeable. But I was wrong, the distance was about 6 miles.
Here is the link to the quake:
https://earthquake.usgs.gov/earthquakes/eventpage/ci38824959/executive
P.S. I am in Menifee, CA