WASHINGTON - Thousands of times every day, drilling deep underground causes the earth to tremble. But don't blame the surprise flurry of earthquakes in Oklahoma on man's thirst for oil and gas, experts say.

The weekend quakes were far stronger than the puny tremors from drilling - especially the controversial practice of hydraulic fracturing. The weekend quakes didn't have the mark of man. They were a force of nature.

Hydraulic fracturing, called fracking, involves injecting millions of gallons of water, sand and chemicals deep underground to break up rock. While that may sound like it could cause an earthquake, experts say the process doesn't pack nearly the punch of even a moderate earthquake.

The magnitude-5.6 quake that rocked Oklahoma three miles underground had the power of 3,800 tons of TNT, which is nearly 2,000 times stronger than the 1995 Oklahoma City bombing.

The typical energy released in tremors triggered by fracking, "is the equivalent to a gallon of milk falling off the kitchen counter," said Stanford University geophysicist Mark Zoback.

In Oklahoma, home to 185,000 drilling wells and hundreds of injection wells, the question of man-made seismic activity comes up quickly. But so far, federal, state and academic experts say readings show that the Oklahoma quakes were natural, following the lines of a long-known fault.

"There's a fault there," said U.S. Geological Survey seismologist Paul Earle. "You can have an earthquake that size anywhere east of the Rockies. You don't need a huge fault to produce an earthquake that big. It's uncommon, but not unexpected."

But there's a reason people ask if the quakes are man-made rather than from the shifting of the Earth's crusts.

In the past, earthquakes have been linked to energy exploration and production, including from injections of enormous amounts of drilling wastewater or injections of water for geothermal power, experts said. They point to recent earthquakes in the magnitude 3 and 4 range - not big enough to cause much damage, but big enough to be felt - in Arkansas, Texas, California, England, Germany and Switzerland. And back in the 1960s, two Denver quakes in the 5.0 range were traced to deep injection of wastewater.

Still, scientists would like to know if human activity can trigger a larger event. The National Academy of Sciences is studying the seismic effects of energy drilling and mining and will issue a report next spring.

"This is an area of active research," said Rowena Lohman, a Cornell University seismologist. "We're all concerned about this."

One issue is that areas that are prone to earthquakes are also places where oil and gas flow along fractures, experts said. In some studies, scientists have taken earthquake data and, like detectives, tracked its causes to deep injections of lots of liquid under high pressure, such as ones that peaked at magnitude 3.3 at the Dallas-Fort Worth airport in 2008 and 2009, said USGS geophysicist William Ellsworth. The Switzerland quake was in the city, Basel, so it did cause damage, he and others said.

"How big an earthquake might we trigger? That is an open question at this point," Ellsworth said. "We do know we can trigger magnitude 5 earthquakes."

When lots of liquid is injected into the ground it changes the stress and pressure in a place that probably already was a fault, said Oklahoma Geological Survey seismologist Austin Holland. Putting the liquid in is similar to injecting water between two adjacent bricks, it allows them to slide more easily and "the water under pressure is helping push the bricks apart ever so slightly," Holland said.

Holland, who has documented some of the biggest shaking associated with fracking, compared a man-made earthquake to a mosquito bite. "It's really quite inconsequential," he said.

Hydraulic fracturing has been practiced for decades but it has grown rapidly in recent years as drillers have learned to combine it with horizontal drilling to tap enormous reserves of natural gas and oil in the United States.

About 5 million gallons of fluid is used to fracture a typical well. That's typically not nearly enough weight and pressure to cause more than a tiny tremor.

Earlier this year, Holland wrote a report about a different flurry of Oklahoma quakes last January - the strongest a 2.8 magnitude - that seemed to occur with hydraulic fracturing. Holland said it was a 50-50 chance that the gas drilling technique caused the tremors.

That is the largest tremor associated with fracking in the scientific literature, experts say. And the strongest of this weekend's natural quakes, magnitude-5.6, released nearly 16,000 times the energy of the worst from that January flurry.

An industry-funded study into a 2.3-magnitude tremor in June near a fracking site in England linked the drilling activity to the quake, but it was a "worst case scenario" of fluid injection into the exact wrong place in a fault, said German geologist Stefan Baisch, lead author of the study.

But wastewater from hundreds of wells is often collected and disposed of deep underground through so-called injection wells. In Lincoln County, Okla., where the recent earthquakes hit, there are approximately 1,982 active oil and gas wells, according to Matt Skinner, spokesman for the state agency that oversees oil and gas production. There are 181 injection wells.

These wells pump wastewater often much deeper underground, all day and all night, for years. The weight and pressure from all of this fluid has been known to cause relatively large earthquakes, including recently in Arkansas, home to another large shale gas field.

After a swarm of small earthquakes hit north-central Arkansas near a formation called the Fayetteville Shale, the state issued a temporary moratorium last year on new injection wells. The state found that three wells were operating near an unknown fault and were likely contributing to earthquakes. The state shut those wells and banned future ones near the fault.

Oil and gas production can lead to tremors another way: When drillers suck all the oil from underground and leave nothing to fill the gap for where the oil was, the emptying reservoir can collapse. If this happens at all, it usually happens slowly over decades. It triggered a series of earthquakes in Los Angeles county in the 1930s, according to the Lawrence Berkeley National Laboratory.

Now, water is injected into depleting wells to maintain pressure. The water also helps keep oil flowing.

Fahey reported from New York.