728: That’s how many earthquakes have been catalogued in the Human-Induced Earthquake Database, which covers tremors set off since 1868 from activities like mining or oil and gas extraction. Most have been small shakes — magnitude 3 and 4. But there have been whoppers too, including a quake in China in 2008 that reached 7.9. That was set off — according to one finding — by filling a reservoir. Quakes often have predictable causes, such as fracking, though only 4 percent of quakes were linked to that hot-button procedure. Others came from nuclear tests or the construction of skyscrapers. Despite the database, compiled by British researchers, it’s still surprising how little we know about what moves the earth beneath our feet.Alex Kingsbury can be reached at firstname.lastname@example.org.