Human activity (e.g. oil production, underground nuclear testing) does impact seismological events to some degree so butterfly effects are possibly there too.

If he can leave a bunch of computers in the past and there is a strong enough incentive to keep them running indefinitely... could he have software on those specific machines that will produce an output every so many years?

Would there be a plausible defense against the stewards of the machines being able to reproduce the computers state and fast-forwarding it on a faster computer?

Measurement of earthquakes is susceptible. If there's extra reasons to pay attention to and record earthquakes, that could easily influence the development of measurement technology, which would change the historical record of earthquake data.