Can you make a seismograph with an accelerometer?
Many MEM's accelerators have a frequency response like this:


Source: https://www.analog.com/en/analog-dialogue/articles/choosing-the-most-suitable-accelerometer-for-your-application-part-2.html

Which is most responsive after 1kHz. Earthquakes have frequency response of less than 20Hz. This means most cheap accelerometers will not be able to detect earthquakes well as the signal will be very low and the MEM's accelerator won't be able to respond to most small earthquakes (big ones are another matter). 

As far as footsteps go, most floors have a long wavelength (think floor joists) and also have low frequencies, cement floors would be very hard to pick up on vibrations from footsteps. Microphones probably wouldn't have too hard of a time however, but also pick up much more noise.

A peizo electric floor mat might be a good way to go also.