When I was scholar at one class on "aircraft equipments" we also talked about weapons. The teacher mentioned that the military was developing weapons which were going to cause earthquakes. That was many years ago but whenever I hear about some earthquakes I am skeptical that maybe some of mega forces in world has madded that with reasons.
This may or may not be relevant to part of your question, but in 2006, exploratory drilling near Basel in Switzerland was considered responsible for a minor earthquake.
It even went to trial in 2009, and from what I can see, further drilling was suspended.
It's hard to know for sure because relevant research is classified. On April 28, 1997 the US Secretary of Defense William S. Cohen claimed in a DoD News Briefing that there are attempts to develop such weapons.
Others are engaging even in an eco- type of terrorism whereby they can alter the climate, set off earthquakes, volcanoes remotely through the use of electromagnetic waves.
Funny that you bring this up, just this evening some drunk in the subway claimed that recent tsunamis were caused by submarines.
For earthquakes, however, there is even research as to whether large-scale earthquakes may have been caused by human activity (not deliberately in this case).
Specifically, Kerr et al. argue that there may have been a Human Trigger for the Great Quake of Sichuan?. There is speculation that the man-made Zipingpu reservoir caused or at least triggered the quake, but data is bad.
Still, no one is near to proving that the Wenchuan quake was a case of reservoir-triggered seismicity. “There's no question triggered earthquakes happen,” says seismologist Leonardo Seeber of the Lamont-Doherty Earth Observatory in Palisades, New York. That fact and the new evidence argue that the quake-dam connection “is worth pursuing further,” he says, but proving triggering “is not easy.” And the Chinese government is tightly holding key data.
If Science is paywalled to you, here's a Discover magazine article discussing said paper.
Can the human make earthquakes?
Of course we can, it easy that many think:
- By geothermal energy production Thanks a Banjol for the link.
- By gas extraction by fracking Link here.
- By groundwater extraction reference here.
- By underground nuclear test like Small Earthquake Near Russian Test Site Leads to U.S. Charges of Cheating on Comprehensive Nuclear Test Ban Treaty, many of those are for "Subsidence/colapse crater" as you can check the wikipedia, rather than actual earthquakes.
Can any of those methods be weaponized?
Nope so much, even for the "already weaponized one", those are small geoengineering projects (reads as gigantic endeavors of massive size and cost) a bit off-place for a "earthquake bomb" (in layman's terms too little bang for the buck).