Why has California been facing so many earthquakes recently?

Several earthquakes have shaken the Bay area in recent days and although it is a common event in California, the reasons why the great earthquake could occur in the region lie not only in the more than 300 faults detected in the area but also in a set of factors that make the region the perfect setting for such natural phenomena.

California experiences up to 100 earthquakes every day, but most are small in magnitude and cause minimal damage. The moment magnitude scale, which measures the movement of rock along the fault, is now used by scientists to accurately measure larger earthquakes, which can last for minutes and affect a much wider region.

Faults and Earthquake Risk in California

According to the United States Geological Survey (USGS), in order to understand the risk that different areas of the United States face from earthquakes, it is necessary to know where the faults are and how they behave.

However, "scientists can only know the existence of a fault when an earthquake occurs or when it has left a recognizable mark on the earth's surface." Once a fault has been identified, the next step is to determine how it behaves.

California experiences small earthquakes all the time: magnitude 3 every other day, on average. But not all act in the same way, and some bring more danger than others.

In the western part of the United States, hundreds of faults have been detected that have generated recognized and documented earthquakes in modern history. The volcanic activity and telluric movements of the continental and maritime tectonic plates of the Pacific Ocean in the area make the entire region called "The Ring of Fire", a ring characterized by the number of active volcanoes and earthquakes in a chain that connects the western part of all the Americas with eastern Asia and even Australia.

Risk factors for major Earthquakes

According to the USGS, Southern California, which is part of the Ring of Fire, "has the highest level of earthquake risk in the United States." In addition, there is a 9 per cent chance of a major earthquake occurring at any time because the Golden State lies on the North American and Pacific tectonic plates, including the well-known San Andreas fault.

Annual Southern California earthquake activity

Each year the Southern California area has about 10,000 earthquakes. Most of them are so small that they cannot be felt. Only several hundred are greater than magnitude 3.0, and only about 15-20 are greater than magnitude 4.0. However, if there is a large earthquake, the aftershock sequence will produce many more earthquakes of all magnitudes over many months.

The movements of huge blocks of the Earth's crust (tectonic plates) cause earthquakes in California. In the south of the state is the line that divides the Pacific plate and the North American plate, which moves horizontally: the North American plate does so to the east and the Pacific plate to the west.

Experts say these movements occur at a rate of 50 millimetres (2 inches) per year. In one of the already mentioned four plates in the south of the state -San Andrés, San Jacinto, Imperial and Elsinore- the much-announced Big One could be generated.

Almost two-thirds of the annual movements of the Pacific plate occur on the San Andres fault and some parallel faults (the San Jacinto, Elsinore and Imperial faults), according to Univision 32 reports. The San Andres stretches for 750 miles from north to south of California.

The sector in which it approaches San Jacinto is known as "the great bend", considered high risk due to its high population. This is in the San Bernardino mountains.

How many large earthquakes have hit California?

There have been about 70 large earthquakes in California since records began in 1812. However, there have been countless tremors and tectonic events of earthquakes happening in the state.

Large earthquakes triggered by smaller tremors

At least three times in modern California history, large earthquakes have been triggered by smaller tremors:

• Central and Southern California, 1857, Magnitude 7.8: The last megaquake in Southern California occurred on January 9 of that year, causing extreme shaking everywhere from southern Monterey County to Los Angeles counties and San Bernardino.

• Northern California, 1989, magnitude 6.9: The ground had trembled in the months before the Oct. 17, 1989, earthquake in the Santa Cruz Mountains that disrupted the World Series between the Oakland A's and San Francisco Giants. There had been a magnitude 5.4 earthquake two months earlier, and a magnitude 5.3 earthquake in June 1988.

• Southern California, 1992, Joshua Tree-Landers-Big Bear: Earthquakes after the 6.1 magnitude Joshua Tree on April 22, 1992 - strong enough to shake high-rise office buildings in downtown Los Angeles, more than 100 miles away, continued to migrate north. Thus began "the largest earthquake sequence in California in the last 40 years," according to a study published in 1993 and coauthored by Hauksson and Jones.

Although it's hard to predict exactly when earthquakes will strike or what their magnitude will be, the researchers urge local residents and officials to prepare now and adapt structures to withstand the seismic force before the next major earthquake -- or big quake.

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