The latest news report that I have seen now says that this was believed to be an actual earthquake that occurred seconds after a rock quarry blast.

However, scientists are stating that it was probably the blast that triggered the earthquake. And they are also saying that they don't understand how that could happen. The quarry operators have not yet decided to resume blasting. And the science people are talking about putting some USGS sensors at the quarry site so that they can do a better job of telling exactly where the fault zone is located etc.