(.) – In southern California there is much more action than we thought.
In a new study, researchers from the California Institute of Technology and the University of California at San Diego discovered nearly two million previously unidentified small earthquakes that occurred between 2008 and 2017. That translates to approximately 495 earthquakes per day in southern California, or an earthquake every three minutes.
An average person walking down the street would not notice these little tremors, which range in magnitude from -2.0 to 1.7.
In general, people experience earthquakes greater than 3.0, according to the U.S. Geological Survey.
Using larger earthquakes to find smaller ones
Even researchers studying earthquakes had a hard time detecting these “hidden” earthquakes.
Background noise, such as jolts from car traffic or building construction, appears in seismic data and can mask signals from smaller earthquakes.
To detect these elusive tremors, the scientists used a technique called template matching. Using seismic data from known earthquakes, they identified patterns as to what an earthquake signal should look like. Using this information, the researchers then scanned seismometer records to find the small earthquakes.
The template matching method has been used in seismology since about 2006, but was mainly limited to the analysis of small data sets for a couple of weeks.
“The computational burden of using this method is heavy,” said Zach Ross, principal investigator of the study. “It requires large computers, which limit their use to small pieces of data. When we started this project, we wanted to apply this on a scale… significantly larger than anything that has been done before. ”
Scientists now have a wealth of data to learn from it
The expanded earthquake data will help scientists understand how serial earthquakes and the previously unknown tremors that precede major earthquakes evolve.
“The reason we are interested in smaller earthquakes is because we do not have enough earthquakes on file to observe the long-term evolution of earthquakes and faults,” said Ross. “The (smallest earthquakes) begin to fill all the gaps among the largest.”
Marine Denolle, a seismologist at Harvard University who was not involved in the study, said she is excited about future studies that will generate this new data.
“This is the largest catalog of earthquakes of all time,” Denolle told .. “This will help us find where the earthquakes come from, and they can highlight new fault systems that we couldn’t see before and reveal new tectonics that we weren’t aware of before.”