So I’ve had this idea for an iPhone seismograph app that a person could use on a rollercoaster or taxi ride on the FDR or regional jet flight.

Here are a couple apps I’ve come across that are in the same vein:

Noise:
Decibel X is a free app for the iPhone (also available on Android) that is a noise meter. It pretty accurately measures noise on a decibel scale. I use it to monitor the noise levels in restaurants and workplaces in an effort to increase quiet. When I am recording podcasts I use it to ensure there’s little background noise. It’s also entertaining and instructive to measure sound levels outside in nature and urban areas. — KK  (from Cool Tools)

Potholes:

Consider Boston’s Street Bump smartphone app, which uses a phone’s accelerometer to detect potholes without the need for city workers to patrol the streets. As citizens of Boston download the app and drive around, their phones automatically notify City Hall of the need to repair the road surface. Solving the technical challenges involved has produced, rather beautifully, an informative data exhaust that addresses a problem in a way that would have been inconceivable a few years ago. The City of Boston proudly proclaims that the “data provides the City with real-time in­formation it uses to fix problems and plan long term investments.”

Yet what Street Bump really produces, left to its own devices, is a map of potholes that systematically favours young, affluent areas where more people own smartphones. Street Bump offers us “N = All” in the sense that every bump from every enabled phone can be recorded. That is not the same thing as recording every pothole. As Microsoft researcher Kate Crawford points out, found data contain systematic biases and it takes careful thought to spot and correct for those biases. Big data sets can seem comprehensive but the “N = All” is often a seductive illusion.
Source: https://rss.onlinelibrary.wiley.com/doi/full/10.1111/j.1740–9713.2014.00778.x

These couple ideas make me think that a Richter-scale app must be doable.