Magnitude 4 earthquake rates may forecast larger future earthquakes

A new forecasting model suggests an uptick in the frequency of magnitude 4 earthquakes along certain faults can predict large earthquake events of magnitude 6.7 or larger.

A few years ago, while studying seismic patterns along California's most active fault lines, a team of researchers led by Boston College seismologist John Ebel identified several faults that hosted an average of at least 0.5 magnitude 4 earthquakes per year from 1997 to 2016.

Researchers estimated the next major earthquake in California was likely to originate from one of the following eight faults: the San Andreas Fault, the Southern San Andreas Fault, the Calaveras Fault, the Little Lake Fault, the Maacama Fault, the Anza section of the San Jacinto Fault, the San Bernardino Section of the San Jacinto Fault and the offshore San Clemente Fault.

Since scientists first identified the aforementioned faults, there has been exactly one major earthquake, the magnitude 7.1 Searles Valley earthquake that struck in July 2019 -- part of a series of quakes known as the Ridgecrest earthquake sequence -- on a fault line adjacent to the Little Lake Fault.

At this week's meeting of the Seismological Society of America, Edel is scheduled to detail the implications of the Searles Valley earthquake for his forecasting model.

"My forecast was not exactly fulfilled," Ebel said in a press release. "Strictly speaking, the 2019 earthquake did not fall on the fault that I forecast, but it did occur in the area that I forecast."

Because major earthquakes strike, on average, once every five to six years in California, Edel's forecast model probably won't be put to the test for another couple of years.

And while the model doesn't predict when a major quake is likely to strike on a dangerous fault line, the 2019 quake does suggest the model is able to accurately pinpoint areas of seismic risk.

Edel first started studying magnitude 4 earthquake patterns while studying the ends of older rupture zones on the East Coast.

"I wondered if I could find where the ends of old earthquake ruptures were by figuring out where the modern magnitude 4s are," he said.

Returning to the most seismically active part of the West Coast, Edel and his research partners traced the seismic histories of the many California fault lines known to host magnitude 4 earthquakes.

"The first thing I noticed there was the rate of magnitude 4s was a lot higher before a large earthquake than it was after a large earthquake occurred," Ebel said. "And then the other thing I noticed was that those magnitude 4s were scattered all along those faults that were eventually to have the big earthquake."

Researchers looked for similar seismic patterns in Japan. They found that the 2016 magnitude 7.3 Kumamoto earthquake and the 2016 magnitude 6.2 Central Tottori earthquake both occurred on fault lines that had experienced an uptick in magnitude 4 earthquakes during the preceding decades.

However, the 1995 magnitude 6.9 Kobe earthquake occurred on a relatively quiet fault line, suggesting that pattern may not always hold.

Edel said he hopes his presentation will inspire other seismologists to test the forecasting model in other parts of the world with reliable seismic records.

Related Links
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