Scientists in the U.S. say the variations may trigger more destructive earthquakes, especially in heavily populated areas. And while the changes will alter the length of day by just a millisecond, they might also cause the release of energy undergrou

Thursday, January 25, 2018, 1:56 - A series of tremors shook the California coast Thursday, putting residents on alert for the Big One.

The first earthquake struck in the Santa Ana Mountains early Thursday morning, according to the U.S. Geological Survey (USGS). The magnitude 4.0 tremor was felt across much of Southern California, the Los Angeles Times reported.

Hours later, a magnitude 5.8 earthquake, struck 118 miles W of Ferndale in NorCal, with an aftershock of magnitude-5.0 following shortly after.

These shocks come amid a week of significant seismic activity along the Ring of Fire, as meteorologist Erin Wenckstern explains below.

All this recent activity has brought up the same question locals ask year after year when the ground shakes: What if California was hit by an 8.2 quake like Mexico?

Scientists have no problem affirming the possibility of a magnitude 8.2 earthquake in California. The San Andreas fault stretches 800 miles along California ground making it the longest in the state and one of the most dangerous in the world.

The last time it triggered a considerable earthquake in southern California was in 1857, when a magnitude 7.9 jolted the ground along the San Gabriel Mountains near Los Angeles. In 1906 it caused one of the most remembered quakes on record in San Francisco, a 7.8 which destroyed most of the city and claimed the life of 3000 people.

San Francisco Earthquake Aftermath, 1906; Creative Commons

The San Andreas fault is "locked, loaded and ready to roll"

According to Southern California Earthquake Center director Thomas Jordan, the most famous fault in the world has been quiet for a long time, too quiet, and is now "locked, loaded and ready to roll".

San Andreas fault line

Scientists warn that if we just follow some general rules of plate tectonic movement, with the Pacific Plate moving northwest of the North American Plate, earthquakes in the region should be retreating by about 16 feet of accumulated plate movement every 100 years. However, the San Andreas fault has yet to relieve the stress accumulated over more than a century.

Los Angeles would suffer the greatest impact

Although several other areas of the fault are long overdue for a large earthquake, scientists are especially concerned about the southern sector.

According to the USGS, every 150 years this section of the fault can produce a sizeable earthquake. If an 8.2 quake where to occur tomorrow, the second most populated city in the U.S., Los Angeles, would suffer a big blow.

The shacking would rupture the San Andreas fault from the Salton Sea near the Mexico border, all the way northwest to Monterrey. Some heavily populated counties like Los Angeles, San Bernardino or Riverside would be affected.

The impact for Southern California would be much greater than in Mexico, because the San Andreas fault runs through densely populated areas and is only 30 miles from downtown Los Angeles (see image below). A magnitude 8.2 earthquake along the San Andreas fault would produce an intensity level of 10 shaking -- which is extreme -- and could raise the death toll to 1800. These estimates come from a study from the U.S. Geological Survey for a 8.2 magnitude quake occurring along the San Andreas Fault near Los Angeles at 10 a.m. on calm dry November Thursday. Of those potentially killed, 900 would be fire related, 400 from steel-frame building collapse, more than 250 from other building collapse and 150 from transportation related accidents.

Potential impact on L.A.

It's impossible to predict an exact time frame to the next large earthquake in Southern California, but it is shocking to see that scientists agree on the fact that a considerable earthquake will rock Southern California within the next 30 years. For many Californians this is not new, it's the same old song, and many are beginning to get tired of hearing it year after year for decades.

The San Andreas fault has caused some relatively recent earthquakes, like the 1989 6.9 registered in the Loma Prieta area of northern California.

In Southern California, the largest most recent earthquake occurred in 1994, when a 6.7 shook Northridge, however it occurred along a nearby and different fault system than the San Andreas.

In the past major earthquakes in California have occurred when population density was much lower, but today things are very different, and close to 38 million people would be affected by a major earthquake along the San Andreas fault.

Watch below: Science Behind Earthquakes

How intense could it be?

After seeing Hollywood's San Andreas film where a 9.0 causes massive destruction across the state of California, one has to wonder how much of this will even be close to reality.

Earthquakes that strong have occurred in the past, but how much damage they eventually cause has a lot to do with the nature of the tectonic region. For example, in Chile or Japan the tectonics are different than in California, where two tectonic plates are sliding next to one another rather that above and underneath or away from each other.

Watch below: San Andreas Movie Trailer

Recent predictions limit the intensity of a San Andreas quake in Southern California to an 8.0, and there is a 7% probability that it could occur in the next 30 years. However, over the same period, the probability of a 7.0 or greater occurring does rise to a 75% probability.

The difference between a 7.0 and an 8.0 may not sound as much, but it is considerable, given that earthquake intensity and the resulting energy release grows exponentially.

USGS Fault Map

Whether it is a 7.0 or and 8.0, damage is inevitable, but the good news is that in Southern California the San Andreas fault runs under land and not under the ocean, so a major tsunami would not be an issue in major urban areas like Los Angeles or San Diego from the movement of this major fault alone.

Watch below: Pacific Northwest now in 'threat window' for potentially large earthquake