More than 400 tremors followed a magnitude 5.2 earthquake near Bakersfield that rocked Southern California earlier this week.
The earthquake struck the southern end of the Central Valley, about 90 miles north of downtown Los Angeles, just after 9pm Tuesday and was felt in parts of Kern, Los Angeles and Ventura counties. Over the next two days, 74 smaller earthquakes, measuring 2.5 or greater, were recorded within 10 miles of the epicenter of Tuesday night’s quake.
Including much smaller earthquakes (those measuring less than 1.0 magnitude), there were more than 400 in the two days that followed and within a 10-mile radius of Tuesday’s quake, according to the U.S. Geological Survey.
Aftershocks, Main Shocks and Precursors
Aftershocks are smaller earthquakes that follow a so-called “main” shock, which appears to be the case with Tuesday’s 5.2-magnitude event, according to California seismologist Lucy Jones. Aftershocks occur in the same area as the main shock and often follow such an earthquake because of faults underneath.
“Typically, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the main shock,” according to the U.S. Geological Survey.
In addition to aftershocks and mainshocks, there are also foretremors, earthquakes that occur before a larger quake. It appears Tuesday’s quake was not a foretremor, Jones said that night. In the two days since, there have been no further tremors of a magnitude similar to Tuesday’s.
Jones said Tuesday there is a 5 percent chance an earthquake is a precursor, and that chance diminishes in the days that follow.
“The risk of this being a pre-quake is decreasing rapidly over time,” Jones said of the Kern County quake. “Within three days, the risk will be virtually gone.”
“A fairly active sequence”
About a quarter of all aftershocks occur within an hour of the main quake, Jones said. There were at least two aftershocks greater than magnitude 4.0 within an hour of the 5.2-magnitude quake this week.
“It’s a pretty active sequence for this very short period of time since the event,” Jones said.
Jones said Tuesday’s quake occurred in the same area as a 7.2-magnitude earthquake in 1952. However, she said it does not appear the two quakes are linked to the same fault.
