More than 20 tremors ranging from magnitudes 2.6 to 4.6 started early Monday morning were recorded in San Andreas Fault along the northwest of Palm Springs and the Salton Sea, and USGS warns that a more significant earthquake may happen in California this week.

USGS research geophysicist Morgan Pages told Los Angeles Times that this incident if only the fourth time in 88 years that such a swarm happen in the Salton Sea Area.

The most massive tremor recorded since Monday was at magnitude 4.6, which struck at 8:56 a.m. local time on Monday.

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San Andreas Fault Is Overdue for "a Big One"

The San Andreas Fault spans 750 miles and marks the boundary between Pacific and North American tectonic plates. It is one of the most dangerous faults in the world, and major cities like Los Angeles, San Francisco, and San Diego and several towns are within the said fault.

Southern San Andreas is a concern among scientists because it produces enormous quakes as big as magnitude seven or higher periodically but last event that the fault produced a quake that generated more than a magnitude 7 was in 1680, more than 300 years ago. Scientists believe that the fault generates such a major quake every 175 to 200 years. This would mean that San Andreas has not experienced a significant tremor a bit longer than the normal.

The USGS said that there is approximately 1-in-10,000 chance of a magnitude 7+ in a typical week on the southernmost San Andreas Fault. The recent earthquake swarms significantly elevated such probability, the USGS added,

Not having a major quake along the San Andreas Fault for more than a century has raised concerns that the "Big One" is bound to happen soon.

According to USGS, a magnitude 7.9 magnitude earthquake hit San Francisco in 1906 which killed over 3,000 people and left 225,000 homeless.

The chance that a magnitude seven quake or higher hitting on the fault in the next 30 years is around 20 percent.

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Possibilities in the Coming Days

The USGS predicts several scenarios in the coming days and weeks.

The most likely scenario, which the USGS says there is an 80 percent change is that the swarms will continue to happen until next week, but it will not be larger than magnitude 5.4.

Another scenario, which is at 19 percent is that a significant earthquake with magnitude ranging from 5.5 to 6.9, will hit in the next seven days. The earthquake is expected to cause damage around the Salton Sea region, and there would be aftershocks.

The least likely scenario that experts forecast has a 20 percent chance is that an earthquake of magnitude seven or above may happen. With this scenario, the USGS said: "While this is a very small probability, if such an earthquake were to occur, it would have serious impacts on communities nearby and would be followed by aftershocks that would increase the number of smaller earthquakes per day."

Earthquakes, the agency said, cannot be predicted. Its forecast is based on statistical analysis from previous quakes.

The agency said that the risk of a more large earthquake happening would stay high while the swarming will continue.

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