This is my favorite way of blowing off adrenaline after an earthquake. It's also fun to watch the reports roll in and see how different areas were affected.
https://earthquake.usgs.gov/data/dyfi/ USGS' "Did you Feel it Program" helps USGS figure out how seismic waves travel through the crust, which isn't uniform in density. Useful beyond seismographs.
Sure, they have seismographs, but they don't necessarily have full details on all the subsurface geology between you and the epicenter. Wave propagation can be complicated... depending on what's down there, the waves can intensified, dissipated, reflected...
If (e.g.) everyone in a small area reports strong shaking, while those in surrounding areas report less, that can indicate a need for further investigation as to whether the area that experienced strong shaking poses a specific risk.
> Does human reporting add resolution or other information to their data?
Yes, a lot. They only have so many seismographs. They ask a bunch of survey questions that get them a pretty good qualitative score of the experience in your exact location, and when merged together gives them a much better picture of how the waves propagated.
Also they ask you questions about damage, which they can't get from anywhere else.
With a standardized report format the data can complement modelling of ground motion intensity caused by an earthquake, such as https://earthquake.usgs.gov/data/shakemap/
I'm not an expert in seismometry, but I think there'd be quite a lot of challenges to overcome. Off the top of my head:
I don't know how precise iPhone accelerometers are, but modern seismometers measure on the nanometre scale.
There's also the issue of orientation. Seismometers measure X, Y and Z and are fixed in place, aligned with latitude and longitude or at an offset. It looks like you can get X, Y, Z from an iPhone accelerometer, but the phone could be in any number of different orientations. It would be necessary and difficult to rectify all those different orientations, as calculating magnitude on a horizontal or vertical component makes a difference.
Seismometers also have well known transfer functions, which are required because a step in calculating most magnitude types is simulating them as a particular type of seismometer, e.g. ml or local magnitude is taken after simulation as a Wood-Anderson seismometer.
Orientation wouldn't be that hard an issue to overcome, phones have magnetometers to act as a compass, and accelerometers which indicate the direction of gravity, so assuming you have a decent idea of where on earth the phone is located, you can work out its orientation in 3 space.
Precision can to some degree be made up for by quantity. If data from all the phones in an area skew the same way, it can stand out even if the signal would be impossibly lost in the noise from a single detector.
More problematic would be calibration. A phone at the top of a tall building vibrating like a tuning fork is going to have a radically different signature from one at the same altitude but in the pocket of a mountain climber, which in turn is going to be very different from one at sea level but close to a highway. If you had a sensor sitting in the same place for a long time, you could probably characterize it, but assuming these are phones people are going to pick up and move after a couple minutes I doubt you could do more than a very crude guess.
I got a second or two of warning once (also a fairly nearby quake). It's really odd to know in advance. Also used MyShake for a DYFI report just last night.
How would you distinguish between seismic events and someone walking by, running up and down stairs, or just bumping the table the phone is sitting on? seismometers are strategically located in places where humans won't accidentally affect their readings via normal daily activities.
You'd have to do some heavy filtering of the data while taking into account neighboring phones. I have no idea if any of this would work but it _could_ be really interesting to see.
