An analysis of 10 years of seismic data from Southern California has revealed almost 2 million previously “hidden” earthquakes – 10 times the number of seismic activities previously recorded. This equates to about 495 earthquakes every day across the southern part of the state, occurring every three or so minutes apart. Previous estimates suggested there were about 30 minutes between seismic events.
Now, before you head for the hills let’s just get one thing clear: this is in no way an indication of “the big one” yet to come.
Researchers were able to pick up on the 1.8 million undetected earthquakes due to a relatively new and finely tuned technique called “template matching.” The science community knew these earthquakes were happening but, until now, did not have the technology to pick up on small seismic events amid the background noise that appears in seismic data, such as shaking from traffic or construction.
“The reason these numbers are so large is because we have improved our ability to see the earthquakes. These small events are going to be happening all the time and sometimes there is more activity than others,” lead researcher Zach Ross told IFLScience in an interview.
It’s similar to looking at something before and after placing it under a microscope; once under the lens, the depth of that object is truly revealed.
“These smaller seismic events are just filling in the gaps between the larger ones, and it’s allowing us to see how these sequences evolve in better resolution,” added Ross.
Template matching – our “microscope” – uses slightly larger and more easily identifiable earthquakes as templates to show what an earthquake at a given location would look like. Two earthquakes that occur within 3 kilometers (2 miles) of each other, even years apart, will follow the same path through the Earth. Looking at the path that earthquakes travel from the source to a measuring instrument on the surface allows researchers to pick up on these minute events.
“Along that path, the seismic waves see exactly the same rocks and that means that when the waves finally arrive at the instrument that records the shaking, the shaking pattern is going to look exactly the same,” said Ross, whose team looked through an array of 200 powerful graphics processing units (GPUs) that worked endlessly for weeks in order to detect these new earthquakes and verify the findings.
Writing in Science, the authors note that the ability to detect these increasingly smaller earthquakes will mean the average time between observed events will “continue to decrease, revealing more complex dynamic behavior."
"These rich spatiotemporal patterns provide valuable constraints on the physics of earthquakes and faults, reflecting properties of the underlying fault structure, [and what might trigger earthquakes]," they add.