Earthquake Sensor - Share your Projects!

According to Peak ground acceleration - Wikipedia, 0.25 stands for instrumental intensity VII with a “very strong shaking”, that intensity 3 you use stands for Japan JMA seismic intensity,
i do not know if you guys in NZ measure intensity according to JMA, Richter or Mercalli or any other.

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I personally use the table from the link above, not the JMA table. But since your houses are wooden, that vibration could easily be reflected to the stick on the wall. Our houses are 99% concrete here in Turkey (most are poorly constructed as we did see from the recent earthquake), they tend to poorly reflect the vibration compared to wooden houses, so the energy collects and breaks the concrete and cause collapses.

In fact the best sensor to use would be OMRON D7S earthquake sensor, which is expensive compared to this, but i had non satisfactory results with that sensor.

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The main issues are: you would need very sensitive measurements, you would need to synchronize the data in some way to get location information, and you'd need to tune the system for different areas. All of this really needs to be done to differentiate between an intruder and the building setting or wind or temperature changes, and even then the system would not work in many solidly built places. This Bosch sensor, which was linked earlier, is targeted to detect opening vault doors and such, and in that limited situation it probably works fine.

I work with vibrations pretty much every day. Every time you step on the ground, you put the same amount of energy into the ground, but the vibrations you generate are different based on the hardness of the contact (I'm electrical not mechanical, so I may be using the wrong terms). Tap shoes onto a hardwood floor will make a fast, high acceleration. Slippers onto a carpet will make a low amplitude shock spread over a wider time.

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The way these waves propagate will be different based on what they're propagating through. Walls, furniture, and beams will interfere with the propagation and cause reflections or absorption. So, you might get a solution working fine in one situation, but failing when the source is on the other side of an invisible support beam.

Here is a worst case situation of me in my winter boots jumping next to a 16g accelerometer. The floor is cheap hardwood overlay over (I think) concrete. Pretty hard to see any vibrations, and I was right next to the unit.

On the other hand, here is the same unit on a pretty flimsy table, with me tapping the table with my fist: Lots of shaking.

If you can measure the vibrations and figure out the reflections through the area, if you want to localize the source of the vibrations, you need to synchronize your data and apply something called beamforming. The resolution you can get out of beamforming depends on the synchronization, wave propagation speed, and the number and spacing of your sensors. Waves propagate very quickly in mechanical systems.

Like I said earlier, in some situations vibration sensing for security may work, but it needs to be tuned carefully. Here's a neat related blog on analyzing footbridge traffic with some accelerometers.

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