Science

More than a million tiny earthquakes revealed in Southern California

In between the “big ones,” millions of tiny, undetected earthquakes rumble through the ground. Now, a new study uncovers a decades worth of such “hidden” quakes in Southern California, increasing the number of quakes logged in the region tenfold. Such troves of quake data could shake up whats known about how temblors are born belowground, and how they can interact and trigger one another, researchers report online April 18 in Science.

The researchers used a technique called template matching to mine an existing archive of earthquakes, recorded by seismometers and other instruments in the region from 2008 to 2017. The team was searching for quakes of such small magnitude that their signals were previously too small to be separated from noise. The results boosted the number of earthquakes in the Southern California Seismic Network archive to 1.8 million.

Statistical analyses using this wealth of new data could help researchers suss out information about seismic activity that wouldnt have been possible previously. “You cant do statistics with small numbers,” says Emily Brodsky, a seismologist at the University of California, Santa Cruz, who wasnt involved in the new study.

She likens the usefulness of tiny quakes to that of fruit flies: Theyre like small but abundant laboratory model organisms. With large populations — whether of fruit flies or earthquakes — you can learn whats robust and whats a fluke; separating the two is a chronic problem in earthquake studies, Brodsky says.

Triggered swarm

A broader swath of Earth rumbled with aftershocks triggered by the 2010 El Mayor-Cucapah quake in Baja California, just south of the U.S.-Mexico border, than had been thought. The original earthquake archive (left) showed aftershocks as far as 175 kilometers from the epicenter. A new analysis (right) reveals tiny quakes as far as 275 kilometers away triggered by the main shock, increasing the number of recorded aftershocks by 142 percent (red dots indicate above-average seismicity). That suggests that changes in stress on nearby faults arent the only ways that earthquakes interact. Other subtle shifts, such as changes in fluid pressure near the edges of the fault zone, could be responsible for how one quake can trigger another.

Researchers have long suspected the tiny temblors existed. In 1944, scientists observed a peculiar relationship between earthquake magnitudes and frequencies: For every unit increase in magnitude, there are 10 times fewer quakes. That means that the vast majority of quakes are small. In fact, its difficult to distinguish many of them from other sources of shaking recorded by seismometers.

“These instruments are incredibly sensitive,” says study coauthor Zachary Ross, a seismologist at Caltech. “They record construction vibrations, vehicles, trains, air traffic, ocean noises from waves.” These sources of noise produce waves with similar amplitudes to those of quakes with magnitudes just above zero, he says.

Template matching, first proposed 15 years ago, separates the quake signals from all that noise. Scientists first take existing data for a larger quake that occurred in a fault zone. The series of wiggles recorded by a seismometer due to a given quake on a given fault follows a pattern as distinct as a fingerprint, because the waves are modified in a particular way by the rocks they pass through underground. Another quake occurring on the same fault will follow the same pattern — even if its much, much smaller.

“We look for something that has a near identical waveform,” Ross says. Previous studies have used this technique in a limited way, mapping out tiny aftershocks from a large quake, for instance. But the new study is the first to take on a whole regional seismic database.

The computationally intensive project took three years. But the rewardRead More – Source