What you need to know about earthquakes
Every year, there are about 500,000 earthquakes – 100,000 of which can be felt on the Earth’s surface. But how do earthquakes work? We explain.
Just the FAQs
An earthquake and dozens of aftershocks rattled Southern California, starting on Tuesday night and continuing into Wednesday morning. Could a bigger one be on the way?
As a general rule, the risk of a larger earthquake following a smaller one is relatively low. Approximately 5% to 10% of all California earthquakes are succeeded by a larger one within a week. The likelihood of a larger quake is influenced by the aftershock activity, although a high level of aftershocks does not guarantee a bigger quake.
This recent quake has indeed triggered aftershock activity. A magnitude 5.2 earthquake at 9:09 p.m. in Lamont, California, near Bakersfield, was felt as far away as Los Angeles, over 100 miles from the epicenter. Since then, the U.S. Geological Survey has recorded dozens of aftershocks, ranging from 2.5 to 4.5 in magnitude, none as large as the initial quake.
Experts caution that there is no reliable way to predict whether a large quake will be followed by a larger one until after it occurs. Lucy Jones, a noted seismologist, has stated, “We have never found any characteristic that makes a quake more likely to be a foreshock.”
What is an Aftershock? A Foreshock?
When an earthquake is followed by one or more smaller quakes in the same area, the subsequent quakes are termed aftershocks. Conversely, if a larger quake follows, the initial quake is labeled a foreshock. The largest quake in a sequence is referred to as the mainshock.
It is important to note that it is not possible to identify a mainshock until after the fact; one must wait to determine if a larger quake occurs.
Have Large Foreshocks Happened in California Before?
Historically, about half of California’s largest earthquakes have been preceded by foreshocks. The state’s heightened seismic activity makes it more likely for large quakes to occur in sequence, although the rarity of large earthquakes still makes it an unlikely event.
For instance, a sequence of smaller quakes that began on July 4, 2019, preceded two of the state’s largest earthquakes in two decades. The initial quake of magnitude 6.4 near Ridgecrest prompted evacuations and caused fires, followed by a 7.1 earthquake the next day.
Research into the 1857 Fort Tejon quake, one of California’s largest earthquakes, revealed at least two widely felt foreshocks of magnitude 5 to 6 in the hours prior to the mainshock. Similarly, the catastrophic 1906 San Francisco earthquake occurred roughly 20-25 seconds after a foreshock was felt throughout the city.
When Will ‘The Big One’ Hit?
The potential for a catastrophic earthquake, often referred to as “The Big One,” is a persistent concern in California. However, it is important to recognize that several devastating quakes are likely to occur over time.
The U.S. Geological Survey estimates that within the next three decades, there is a 46% chance of a magnitude 7 earthquake in the Los Angeles area and a 51% chance in the Bay Area. The likelihood of a 7.5 magnitude quake is 31% in Los Angeles and 20% in San Francisco over the same time frame.
As seismic activity continues to be a pressing concern, it is vital for communities in earthquake-prone areas to enhance their preparedness and resilience. This can be achieved through improved building codes, public education on earthquake safety, and the development of early warning systems. The integration of technology in monitoring seismic activity can also play a crucial role in mitigating the impacts of potential earthquakes.
