Tectonic tremor is a new type of seismic signal that seismologists started studying only within the last few years. It is less hazardous than earthquakes and occurs at greater depths.
Tectonic tremors cause relatively weak ground shaking, and while they may last longer than earthquakes, they do not represent any direct danger, researchers said.
"Both earthquakes and tremor have the same cause. They result from the relative movement on fault surfaces, a result of the motion of the tectonic plates," seismologist Rebecca Harrington of the Karlsruhe Institute of Technology in Germany said.
"While earthquakes at our research site in California typically occur at depths of up to 15 kilometers (10 miles) below the surface, tectonic tremor signals are generated at depths ranging from approximately 15 to 35 kilometers (10 to 18 miles)," she said.
Tectonic tremor signals have a unique character that differs from earthquakes, making them more difficult to detect, researchers said.
Researchers have recorded 2,600 tremor events that are being studied in detail.
"We understand very little about what happens on a fault when it ruptures," Harrington said. "The tectonic tremor generated on the deep part of a fault may provide clues about the behavior on the more shallow parts of a fault where more damaging earthquakes occur."