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In most of the earthquake footages filmed outside during the earthquake that hit Mexico City on September 9, 2017 there was a distinctive streak of hum in rapid succession that could be heard in the background. It sounded like someone is constantly hitting the tuning forks onto a light tower like 2 times per second.

You can find numerous footages on youtube, but I picked one and fetched the audio portion covering the hums, you can listen to the audio at this link here (sorry I did not know how to upload an audio to SE).

Here is the waveform of the sound

Mexico City Earthquake Waveform

What is that sound? What is causing this sound? If this was buildings or surrounding objects, why is there a pattern? Why is this sound heard in every footage no matter where they are filmed as long as they were filmed outside.

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Wailing sirens.

Examples 1, 2 where you see people come running out, i.e. exactly when you'd expect sirens, or 3, or 4 and 5 where they are explicitly mentioned.

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