SAN FRANCISCO (CNN) — There's a big earthquake coming.
Tuesday's magnitude-4.0 earthquake in the San Francisco Bay Area caused relatively little damage, but it gave a glimpse of the potential geological threats the region could face.
Scientists believe there's strong evidence to suggest that a major quake will strike the San Francisco Bay Area; the only question is when.
Geophysicist Tom Brocher with the U.S. Geological Survey said scientists are confident that a major earthquake will happen on the Hayward Fault Zone, a 74-mile-long fault line that runs along the east side of the Bay Area, near cities such as Oakland and Fremont.
The zone is known for destructive earthquakes. "We know about it very well because it produced a big earthquake in 1868," Brocher said in an interview with CNN.
The quake Brocher is referring to was a magnitude-6.8 earthquake. Several people died in that event, which caused damage of around $350,000 — about $5 to $10 million in today's money.
"Anything larger than a magnitude-6 earthquake can be very dangerous if it is near a city," Brocher said.
The reason why geologists are concerned right now about earthquake safety is because they have been studying the 1868 earthquake and the 11 quakes that happened before it on the Hayward Fault and noticed that the earthquakes seemed to happen in loose patterns.
"The average is about 140 years and some (earthquakes) are only 90 years apart. Others are 160 years apart," he said.
It has been 147 years since the 1868 earthquake struck San Francisco.
"The most recent scientific study indicates that for the entire San Francisco Bay Area there is about a 70% chance of a magnitude 6.7 or greater in the next 30 years occurring on one of the several earthquake faults that run through the area," Brocher said.
In 1989, a magnitude-6.9 quake hit the Bay Area during baseball's World Series. The Loma Prieta earthquake caused 63 deaths, 3,757 injuries and an estimated $6 billion in property damage, according to the Geological Survey.
Since then, more than $30 billion has been invested by various businesses and companies to strengthen the city's infrastructure.
Brocher said most people will be fine after a quake — it's the potential lack of services such as water, gas and electricity that might cause problems.
But earthquakes do not happen like clockwork, according to Brocher. The most important thing people can do is to prepare by stocking up on water and emergency supplies.
It is a challenge for geologists to convey the seriousness of earthquakes, because large-scale quakes are relatively uncommon and quakes are far more difficult to predict than other natural disasters such as hurricanes, Brocher said.
"Earthquakes are rare enough that most people only experience one and they don't know what it really is like," Brocher said.
In April, a magnitude-7.8 earthquake hit Nepal claiming more than 8,000 lives. The powerful quake ravaged the country, separating families, decimating historical landmarks and leaving tens of thousands of people with injuries.
Earthquakes struck Peru, California, Chile and Iceland in 2014.