(CNN) Several million earthquakes occur annually, but most are unnoticed because they're so small, the U.S. Geologic Survey says.
But three recent earthquakes -- on Thursday and Saturday morning in Japan and Saturday night in Ecuador -- have gotten lots of attention because of the great destruction.
Here are five things to know about those quakes.
1. Are the Ecuador and Japan earthquakes related?
It's way too early to tell, said Paul Caruso, a geophysicist with the U.S. Geological Survey.
"It's one day after the Ecuador earthquake and two days after the Japanese earthquake, so no real research has been done on these quakes as far as they're being connected," he said Sunday.
"Usually we don't think earthquake are connected across the ocean," Caruso said, but there's ongoing research in "remote triggering," the idea that a big quake can cause another quake a long distance away.
The distance between Japan and Ecuador: 15,445 kilometers, or about 9,590 miles.
Fast facts: Earthquakes
2. What about the Ring of Fire?
Both quakes occurred in this horseshoe-shaped area the National Geographic Society defines as "a string of volcanoes and sites of seismic activity, or earthquakes, around the edges of the Pacific Ocean."
From tile-shaking to tsunamis, how strong can a quake get?
Introduction
Earthquakes today are more commonly measured by moment magnitude, a scale based on the amount of seismic energy released by the quake. In the magnitude scale, each increase of one whole number translates to 32 times more energy.
Sources: CNN, US geological survey, California Institute of Technology (Caltech)
Micro
Magnitude: Less than 3.0
Average number by year: The U.S. Geological Survey estimates there are 1.3 million quakes per year that register a 2.0 to 2.9. Caltech says there are about 1,000 of those quakes per day and about 8,000 quakes per day that fall in the 1.0 to 1.9 range.
Damage: little to none
Additional note: A quake measuring 2.5 to 3.0 is generally the smallest earthquake felt by people.
Minor
Magnitude: 3.0-3.9
Average number per year: The U.S. Geological Survey estimates there are about 130,000 minor earthquakes each year. Caltech estimates there are 49,000.
Damage: Little to none
Light
Magnitude: 4.0-4.9
Average number per year: The U.S. Geological survey estimates there are 13,000 light earthquakes per year. Caltech puts the number at 6,200.
Damage: Moderate
Moderate
Magnitude: 5.0-5.9
Average number per year: 800, according to Caltech, or 1,319, according to the U.S. Geological Survey.
Damage: Considerable
StrongMagnitude: 6.0-6.9
Average number per year: 120-134
Damage: Severe
Notable quakes: About 5,500 people were killed in January 1995 by a magnitude 6.9 quake in Kobe, Japan.
The
Loma Prieta earthquake (magnitude 6.9) killed 63 people in October 1989 and caused an estimated $6 billion in damage in California's San Francisco Bay area. It is remembered for occurring just before a World Series game.
MajorMagnitude: 7.0-7.9
Average number per year: 17-18
Damage: Widespread, heavy
Notable quakes: A magnitude 7.3 quake killed 110,000 people in Asghabat, USSR, in October 1948.
About 70,000-100,000 people were killed in December 1908 by a magnitude 7.2 quake in Messina, Italy.
In October 2005, a magnitude 7.6 quake killed 86,000 people in Pakistan.
The
deadliest quake in American history registered 7.8 in 1906 and killed an estimated 3,000 people in San Francisco, California.
On April 25, 2015, an earthquake in Nepal
registered 7.8,
killing close to 9,000 people and causing $10 billion in damages.
GreatMagnitude: 8.0 and up
Average number per year: 1
Damage: Tremendous
Additional note: There is no upper limit to the magnitude scale, but the highest measurement ever recorded was a 9.5 (Chile, May 1960).
Notable quakes: A magnitude 8.0 quake killed 255,000 people on July 27, 1976, in Tangshan, China. It's the deadliest earthquake since 1900, when quakes were first measured.
Tsunamis resulting from a magnitude 9.0 quake
killed more than 200,000 people in December 2004. The quake struck the coast of Sumatra, Indonesia, and affected countries from Indonesia to African countries bordering the Indian Ocean.
On March 11, 2011 a 9.0-magnitude earthquake struck off Japan's coast --
the most powerful to ever hit the country. It triggered a series of tsunamis and led to 16,000 deaths and the meltdown of the
Fukushima nuclear reactor.
It may seem more than coincidental that the quakes occurred a few days apart. But it's also true that the large majority of the world's earthquakes -- about 90% -- occur in the Ring of Fire, per the National Geographic Society.
3. How do the Ecuador and Japanese quakes compare in power?
The Ecuador earthquake was almost 16 times stronger than the Saturday morning quake in Japan.
That figure comes courtesy of the "Try It Yourself" calculator on the USGS website. Ecuador was hit by a magnitude-7.8 quake, Japan by a magnitude-7.0 quake. Plug those numbers into the calculator, and you come up with 15.848. (The Thursday Japan quake was magnitude-6.2.)
If you're a math geek and want to see the formula, it's on the USGS website.
How you can help
4. Is this Ecuador's strongest earthquake?
Powerful quake hits Ecuador
Soldiers move an injured person onto a military airplane at an air base in Manta on Tuesday, April 19. A magnitude-7.8 quake struck off Ecuador's central coast on Saturday, April 16, flattening buildings and buckling highways. It's the deadliest quake to strike the South American country in decades.
A relative kisses a photo of Kexly Valentino attached to her coffin in Montecristi on April 19. Kexly and her mother and brother died in the quake.
Rescuers search for survivors in Manta on April 19.
Maria Victoria, 89, is comforted by her daughter Mariana in Estancia Las Palmas on April 19. The elderly woman was injured when a column fell on her.
A Manta family wakes up April 19 after sleeping outside their home destroyed in the quake.
Firefighters remove a body from a destroyed building in Portoviejo on Monday, April 18.
A truck moves the belongings of a family from Pedernales to Jama over a road destroyed in the quake on April 18.
Pallbearers carry a coffin to a cemetery as relatives mourn the loss of loved ones in Portoviejo on April 18.
A man, his home destroyed by the earthquake, sleeps in a boat docked along the shore in La Chorrera on April 18.
Ecuadorian President Rafael Correa kisses a group of children after meeting with local authorities at the emergency center in Portoviejo on Sunday, April 17.
Rescue workers search the rubble of a collapsed building for victims in Guayaquil, Ecuador, on April 17.
Volunteers remove a body from a destroyed house in Pedernales, Ecuador, on April 17.
Vehicles from a car dealership hang on a precipice caused by an earthquake induced landslide in Portoviejo, Ecuador, on April 17.
People walk among the debris of a collapsed building in Pedernales, Ecuador, on April 17.
A woman cries as she stands next to a destroyed house in Pedernales, Ecuador, April 17.
People make their way through debris from a collapsed building in Pedernales, Ecuador, on April 17.
A building has collapsed in Portoviejo on April 17.
People search through the rubble of destroyed homes in Pedernales on April 17.
A destroyed vehicle lies under debris in Portoviejo on April 17.
A man takes in the damage in Portoviejo on April 17.
A rescue worker searches a destroyed house in Pedernales on April 17.
Residents remain on the streets for fear of aftershocks in Pedernales on April 17.
Rescue workers try to pull out survivors trapped in a collapsed building in Manta on April 17.
Police check out a car crushed under a collapsed overpass in Guayaquil.
Rescue workers stand near a destroyed car in Guayaquil.
Residents walk on a street amid destroyed buildings in Guayaquil on April 16.
An hotel leans after the earthquake in Manta on April 16.
Damage is seen inside a store in Guayaquil on April 16.
No. On January 31, 1906, a magnitude-8.8 quake occurred off the coast of Ecuador and Colombia, generating a tsunami that killed 500 to 1,500 people, according to the USGS website. The quake was first measured at magnitude-8.2.
On May 14, 1942, a magnitude-7.8 earthquake occurred 43 kilometers south of the site of the recent quake.
The Saturday quake is the deadliest to hit the nation since March 1987 when a 7.2-magnitude temblor killed 1,000 people, according to the USGS.
5. How often do quakes of this power occur?
Quakes with a magnitude between 7.0 and 7.9 occur about 15 times a year, according to information collected by the USGS over the decades.
The bigger ones -- magnitude-8 and over -- happen about once a year.