A million tiny quakes shook Southern California — and no one knew
Many people in California worry about the next “Big One.” They are referring to an earthquake strong enough to cause widespread damage. But tiny, undetected quakes rumble through Southern California all the time. There were millions of them in the last decade alone, a new study finds. Data on these “hidden” quakes could shake up what scientists know about how the tremors develop and interact.
Researchers at the California Institute of Technology in Pasadena mined an existing archive of earthquake data. Seismometers and other instruments had recorded the data from 2008 to 2017. The researchers used a technique called template matching. This allowed them to pick out quakes that previously had been too small to pick out from background noise. And their efforts paid off.
The results boosted the number of earthquakes in the Southern California Seismic Network archive to 1.8 million. That’s 10 times more than previously identified. The team reported the new number online April 18 in Science.
All those new data will help earthquake researchers ask questions they previously couldn’t, says Emily Brodsky. She’s a seismologist at the University of California, Santa Cruz, who wasn’t involved in the new study. Scientists can use statistics, for instance, to calculate whether an observed difference is likely due to chance. But “you can’t do statistics with small numbers,” Brodsky explains.
She likens the tiny quakes to fruit flies. Both are small and abundant. And that makes them useful for science. Large populations — whether of fruit flies or earthquakes — can help tease out what’s a real effect from what’s a fluke. Telling the difference is a chronic problem in earthquake studies, Brodsky notes.
Researchers have long suspected the tiny temblors existed. In 1944, scientists observed a strange relationship between earthquake magnitudes (strength) and frequencies (number). That is, for every unit increase in magnitude, there are one tenth as many quakes. So the vast majority will be small. So small, in fact, that many end up hidden among other sources of shaking picked up by seismometers.
“These instruments are incredibly sensitive,” notes Zachary Ross. An author of the study, he’s a seismologist at the California Institute of Technology in Pasadena. Seismic sensors record a broad range of vibrations. These include tremors due to construction, vehicle traffic, trains, air traffic — even noises from ocean waves crashing into a beach. All produce waves similar in size to those of very small seismic quakes, he notes.
Ross and his colleagues tried an idea first proposed 15 years ago. Template matching homes in on patterns to separate quake signals from all that noise.
Seismometers record quakes and other shakes as wiggly lines. The series of these coming from a quake on a particular fault creates a pattern. It can be as distinct as a fingerprint. That’s because the seismic waves are modified in a certain way by the rocks through which they pass underground. The resulting pattern of wiggles is called a waveform. Another quake on the same fault will show that same waveform — even if it’s bigger or much, much smaller.
The scientists first used data from a larger quake in a fault zone to identify the pattern for a fault. Then, in the small wiggles, “we look for something that has a near-identical waveform,” Ross says. That pattern matching helped his team find signals of the tiny quakes.
Previous studies had used this technique in a limited way. For instance, it had been used to map the tiny aftershocks related to a large quake. This new study is the first to use this technique on a larger scale. Here, it’s been applied to a whole database of regional seismic waves.
The project demanded intense computing power. And it took three years. But the rewards are worth it, Ross says. His team’s new catalog fills in gaps between larger quakes. Those links could tie together events that might previously have seemed unrelated.
“We’re starting to complete the story about the interactions between these events,” Ross says. That includes observing how earthquakes might cluster throughout a region and time. They also can start to identify the tiny foreshocks that might develop before a larger quake. From that, he says, scientists may be able to learn about the physics of how a given earthquake is born.
Brodsky also notes that these data could impact efforts to link human activities to earthquakes. For example, a magnitude 5.5 quake struck Pohang, South Korea, in 2017. In March, a government-commissioned panel calculated that the quake was triggered by water injected underground from a geothermal power plant.
“Arguments for whether or not something is human-induced revolve around timing and location,” she says. “The thing is, very often there’s a delay.” But if that apparent delay was filled with a series of tiny quakes? That could change how people determine cause-and-effect. And that could be a game-changer.
aftershocks One or more smaller earthquakes which often follow a major earthquake.
ambiguous (n. ambiguity) An adjective used to connote uncertainty, such as in a situation or with an interpretation for which there are several possible explanations or outcomes.
archive (adj. archival) To collect and store materials, including sounds, videos, data and objects, so that they can be found and used when they are needed. The term is also for the process of collecting and storing such things. People who perform this task are known as archivists.
chronic A condition, such as an illness (or its symptoms, including pain), that lasts for a long time.
colleague Someone who works with another; a co-worker or team member.
database An organized collection of related data.
earthquake A sudden and sometimes violent shaking of the ground, sometimes causing great destruction, as a result of movements within Earth’s crust or of volcanic action.
earthquake magnitude A measurement of the intensity of the ground-shaking associated with an earthquake. The scale is logarithmic. So for every 1 point increase in magnitude (such as from 3 to 4), there is a 10 fold increase in ground motion (how far the land shakes back and forth) and a roughly 33-fold increase in the amount of energy released.
epicenter The underground location along a fault where an earthquake starts.
fault In geology, a fracture along which there is movement of part of Earth’s lithosphere.
foreshocks One or more smaller earthquakes which often come before a major earthquake.
fruit flies Tiny flies belonging to the species Drosophila melanogaster. Scientists often use these short-lived animals as a “guinea pig” for lab studies because they are easy to grow, can mature into adults in a short time and their bodies share many of the same traits and responses as more complex animals — including mammals.
geothermal plant A power plant that uses geothermal energy (the heat emanating from Earth's core) to run turbines that generate electricity, or regional system that pumps water heated with geothermal energy through a network of pipes to warms buildings.
network A group of interconnected people or things. (v.) The act of connecting with other people who work in a given area or do similar thing (such as artists, business leaders or medical-support groups), often by going to gatherings where such people would be expected, and then chatting them up. (n. networking)
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.
seismic wave A wave traveling through the ground produced by an earthquake or some other means.
seismology The science concerned with earthquakes and related phenomena. People who work in this field are known as seismologists.
seismometer (also known as a seismograph ) An instrument that detects and measures tremors (known as seismic waves) as they pass through Earth.
statistics The practice or science of collecting and analyzing numerical data in large quantities and interpreting their meaning. Much of this work involves reducing errors that might be attributable to random variation. A professional who works in this field is called a statistician.
technology The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.
temblor Another term for an earthquake or Earth-shaking tremor.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
waveform The mapped shape of a given wave.