Amplitude Differences

Looking at the recent distant quake in Japan (~4600 miles from Seattle) I noticed that my max amplitude was measured as around 20 um/s. However, 4 other RS instruments within 20 miles of my home all consistently measured a max around half that (~10 um/s). Mine seems like the outlier. Mine definitely picked up the P-waves, but at double the amplitude. Does anyone know why that might be? I did verify that the ranges of each were at the same frequency (regional, 0.7-2.0 Hz).

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Hello Tomfinley1,

It’s good to see that you have verified that every trace was in the same frequency range, sometimes it can be overlooked, especially if the frequencies are set automatically.

There’s a lot of possible different factors that could have contributed to the phenomenon you are seeing between your signal and your close-by Shakes. Installation position, location, type of material upon where the Shake is positioned, soil type and density, depth to bedrock, structural footing and attachment to bedrock, bedrock type, etc.

Is this something that you have noticed happening constantly, or, for now, just for this particular earthquake?

This is something that I noticed. For my device the signal was quite pronounced:

I checked other devices, from about 5 to 30 miles away to see what they had seen.
It varied from just about discernible if you knew it was there, to a small signal, but nothing like I was seeing. I was about to put it down to something very local, then heard about the Japan quake, checked the timing, and it looked like that was it.

I have no idea why the difference. Even a device on the coast hardly twitched,

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Thanks for your reply. It wasn’t just for this quake. I think I noticed it because the signature of the signal (e.g., start time, shape) was almost identical across all regional seismographs, varying only by the amplitude scale. Maybe it’s just the soil and surface below each unit at different locations, as you suggested. I noticed that for other distant quakes, sometimes mine isn’t the outlier, and others have a matching signature, but with more or less amplitude. Thanks.

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