In the early hours of Sunday (October 6), around 4 am, a magnitude 4 earthquake struck near Ontario Airport in San Bernardino County, East Los Angeles. Many residents were awakened from their sleep by the tremors. Only a few minor aftershocks occurred after the initial earthquake.
The airport in Ontario is located approximately 1 mile from the epicenter. Some people posted on social media that when the earthquake hit, there were passengers waiting in the airport terminal, and in concern for safety, staff opened the passenger boarding gates for people to take cover. However, videos showed that some passengers seemed unaware, sitting still without moving.
Ms. Ni, who lives less than 8 miles in a straight line from the epicenter, felt the shaking in her sleep and was startled awake. She heard her two-story townhouse creaking from the vibrations. Immediately realizing it was an earthquake, she instinctively grabbed a few clothes, tried to wake up her husband, prepared to take cover, but the shaking quickly stopped. After waiting for a while without any further movement, she went back to sleep.
Ms. Choi, living in Pomona, was also awakened by the tremors during the night. Having recently moved to Southern California, she was already anxious about earthquakes, knowing the region’s seismic activity. She was surprised to personally feel this earthquake, “I was shaken awake, sounding like a big truck passing by. I looked outside the window, but there were no cars.” She was awakened once again around 2 a.m., suspecting it might have been a foreshock.
USGS image maps indicated that residents in nearby cities such as Chino, Eastvale, Riverside County, Orange County, and Los Angeles County also felt the tremors. Some residents shared on social media that they were too anxious about the possibility of a larger earthquake to go back to sleep after the initial earthquake.
According to the U.S. Geological Survey Agency, the earthquake initially reported at 3:51 am as a 4.3 magnitude was later revised to a magnitude of 4, with a depth of 3.54 miles. The epicenter was located about a third of a mile southeast of the intersection of Highway 60 and Archibald Ave, south of Ontario Airport. This was the fifth earthquake of magnitude 3 and above detected in Ontario City since early September.
Due to its shallow depth, the earthquake’s intensity was enough to awaken many people from their sleep. Some objects in high places indoors may have fallen, potentially causing minor damage like broken windows. The U.S. Geological Survey Agency stated that their earthquake warning system ShakeAlert had been activated and could be downloaded for free. This alert system is said to provide warnings approximately 1 minute before an earthquake, prompting people to take precautions. A resident mentioned that he was awakened by the alert 3 seconds before the earthquake began.
Recently, earthquakes have been frequent in the Los Angeles metropolitan area. In mid-August, a 4.4 magnitude earthquake occurred in the eastern part of Los Angeles City at noon, with a depth of 7.5 miles.
Some cities in Southern California are situated on the San Andreas Fault earthquake zone, which stretches about 1300 kilometers across the western part of California from the San Francisco Bay Area to Southern California and the northern and eastern fault lines of Lower California in Mexico. California experiences thousands of earthquakes annually, with the vast majority being small in magnitude. There are only a few hundred earthquakes of magnitude 3 and about 15-20 earthquakes of magnitude 4 and above each year.
The U.S. Geological Survey Agency mentioned that an earthquake can trigger subsequent tremors in the nearby region, known as an earthquake sequence. In most earthquake sequences, the subsequent earthquakes are smaller than the main shock. The initial earthquake is referred to as the main shock. Globally, there is about a 5% probability of a larger earthquake occurring in the vicinity within one week after an earthquake. Although numerous aftershocks occur, it does not necessarily indicate the occurrence of a larger earthquake.
Currently, seismologists are unable to predict whether a particular earthquake signifies a precursor to a larger earthquake. In August of this year, American seismologist Lucy Jones took to social media saying, “We have never found characteristics that make an earthquake more likely to be a foreshock.”