How do you determine that that specific earthquake was directly caused by reinjection? Is it possible that reinjection is creating more frequent, smaller quakes, and avoiding stress buildup that would be released as a larger quake later? Do they get credit against the latter for the former? Is there an amount of earthquake insurance available commercially to cover "all restitution costs"?

You don't. It's a general fund paid in by all operators in an area. The funds should be sufficient to cover necessary indemnification.

Sure, the small quakes likely are relieving bigger ones tens of thousands of years hence, so we don't care about that.

The rules would be, you want to drill and re-inject to make some money, make sure it's profitable enough that you can pay into an EQ indemnification fund. Actuaries would set rates.

I agree with this. The model would be the US Vaccine Injury Compensation Fund. The idea is all drillers pay an excise tax into a fund, and this fund acts as insurance for anyone who can show damages by an earthquake in a drilling area.

More like millions of years hence, if they haven't been active for 300 million years.

You legislate it. Figure out how much the earthquakes are costing and tax drillers.

You need lots of tracking equipment, lots of claims for damage and finally lots of lawyers. It's the lawsuit way of doing society, case by case. Think of all the lawyer salaries and the great boost to the economy!

We do similar for flood insurance. These quakes are not the devastating kind. They range in the <M4 scale, so it'll be mostly toppled chimneys and adornments, perhaps some brick walls might collapse. The damages are typically in the "minor", possibly escalating to "moderate" but not "major", so there would not be armies of lawyers trawling for claimants. But this is still a nuisance which needs to be addressed for property and infrastructure owners as well as an incentive for the industry to find better solutions to disposing drilling water byproducts.

>Since science is now establishing a connection between drilling and quakes

Science funded by middle eastern princes trying to stop their competition with manufactured environmental outrage?

While it's good to be skeptical, in this case it's the USGS[1] who have made the attribution, albeit a bit late after years of insistence by others.

[1]https://earthquake.usgs.gov/research/induced/