Two strong earthquakes in two days, why are earthquakes frequent in California？

　　On July 4th and 5th, local time, strong earthquakes of magnitude 6.4 and 7.1 occurred in southern California, setting a new record for the strongest earthquake in the past 20 years. At present, both Lester County and San Bernardino County in Riziq, California have declared a state of emergency, but no deaths have been reported. Seismologists predict that the possibility of another earthquake of magnitude 7 and above will not be ruled out in the coming week.

　　Refresh the record of strong earthquakes in recent 20 years.

　　According to data from the US Geological Survey, the magnitude of the earthquake on July 4 was 6.4, and the magnitude on the evening of July 5 was 7.1, which was the strongest earthquake in Southern California in the past 20 years.

　　The earthquakes for two consecutive days are located in the same fault zone, and the epicentres are very close. They are located near the town of Riziq Lester in southern California, about 270 kilometers away from Los Angeles and surrounding cities, and about 380 kilometers away from Las Vegas, Nevada. The earthquake on the 5th made both Los Angeles and Las Vegas feel shocked. Residents nearby described the experience at that time as follows: "The house was shaking violently, just like an airplane was bumped by airflow" and "the earth was roaring".

　　Seismologists believe that because the two earthquakes are less than 48 hours apart and the epicentres are close, the magnitude 6.4 earthquake is only a foreshock, and the magnitude 7.1 earthquake is the main shock, and its intensity is six times that of the previous one, releasing 12 times as much energy.

　　According to the US Geological Survey, since the earthquake on the 4th, there have been more than 2,300 aftershocks in southern California, of which 300 have a magnitude of 3 or above, which makes people feel strong enough. There are also 50 earthquakes of magnitude 4 and above, which have enough energy to cause damage.

　　Experts predict more aftershocks will occur in the future. According to the prediction of USGS, the probability of an earthquake of magnitude 6 in the next few days is 50% or even greater. Some experts estimate that there is a 10% chance that Southern California will experience an earthquake of magnitude 7 and above again next week.

　　"Outbreaks" every 10 to 20 years?

　　Although California is called "paradise on earth" with its sunny beaches, it has also become one of the most dangerous places on earth because of frequent earthquakes.

　　In the early morning of April 18th, 1906, an earthquake of magnitude 8.3 occurred in San Francisco, which was one of the most serious natural disasters in American history. The epicentre is located on the San Andreas fault near San Francisco, and the fault slips as high as 6 meters.

　　After the earthquake lasted for dozens of seconds, San Francisco turned into rubble and triggered a fire that lasted for three days and three nights. According to statistics, in this earthquake and fire, 10 square kilometers of urban area was reduced to ashes, at least 3,000 people died and the direct economic loss was about 400 million US dollars.

　　After the San Francisco earthquake, Americans began to pay attention to earthquakes, gradually cultivated the awareness of earthquake disasters, and promoted the development of earthquake science. The last time California suffered an earthquake of magnitude 7 or above was 20 years ago. On October 16th, 1999, the Mojave Desert in Southern California was hit by an earthquake of magnitude 7.1, but it did not cause serious losses.

　　"In California, we expect an earthquake of magnitude 7 every 10 to 20 years," said Lucy Jones, a seismologist at the California Institute of Technology. On January 17, 1994, a 6.6 magnitude Northridge earthquake occurred in Southern California, the epicenter of which was located in the densely populated Los Angeles area, causing dozens of deaths and billions of dollars in property losses. On August 24, 2014, an earthquake of magnitude 6.0 occurred in the San Francisco Bay Area of California, injuring more than 100 people and damaging more than 2,000 buildings.

　　The boundary between plates is the "culprit"

　　California is prone to earthquakes because of its special geological conditions. California is located between the North American plate and the Pacific plate, with criss-crossing fault systems and other active systems under it, and the crust moves frequently.

　　It is reported that there are hundreds of known geological faults in California, and more than 70% of the population lives within 50 kilometers around the faults where strong earthquakes may occur. There are more than 200 large and small faults in Southern California, which can cause earthquakes ranging from 5.0 to 7.8.

　　Among them, the San Andreas fault is the eternal fear of Californians. Palm Springs, Los Angeles, San Francisco and other densely populated large and medium-sized cities are all located on the San Andreas fault. This fault is one of the regions with the most frequent stratigraphic activities on the earth, and it accumulates seismic stress every year. It is estimated that the fault system will have a large destructive earthquake every few decades.

　　The outside world is worried about whether the California earthquake will have a potential impact on the San Andreas fault, and will there be a stronger earthquake next?

　　Some seismologists believe that the idea that these two earthquakes will trigger the Santandrea earthquake is far-fetched. The fault near Lester County, Riziq is close to a fault named garlock. Although the fault finally intersects with Santandrea, the distance between them is far away.

　　However, some experts said that the San Andreas fault has been very quiet in recent years, but this does not mean that it has been dormant. In fact, the great earthquake of the San Andreas fault has been postponed. The longer this time drags on, the more stress is accumulated, and the more terrible the power of the earthquake may be.

　　A report by the Geological Survey last year warned that the risk of a major earthquake hitting California is increasing. In the next 30 years, the probability that California will experience an earthquake of magnitude 8 or greater will increase from about 4.7% to about 7%. (trainee reporter Yang Wei)