Faults can slip slowly over weeks, releasing the energy of a magnitude 7 earthquake without any shaking. GPS detects it, seismometers don't. These "slow slip events" transfer stress to neighboring faults - possibly triggering the big one.
Alicia.iverson, CC BY-SA 4.0, via Wikimedia Commons
The ground is moving
Deep beneath Washington State, sections of our planet's crust are slipping. Roughly every 14 months, a segment of the Juan de Fuca tectonic plate lurches a few centimeters deeper under the North American plate. This movement happens over two to four weeks and can release the same amount of energy as a magnitude 6.7 to 7.0 earthquake. Yet, on the surface, no one feels a thing.
This phenomenon is a "slow slip event" (SSE), sometimes called a silent earthquake. These events occur along the Cascadia subduction zone, a 620-mile-long fault stretching from Northern California to Vancouver Island. They happen at a depth of 25 to 40 kilometers (about 15 to 25 miles), in a "transition zone" on the fault. Here, the conditions are not right for the sudden, violent rupture of a typical earthquake. Instead, the plates slide past each other slowly and episodically.
The movement is imperceptible to humans, but not to science. A dense network of high-precision Global Positioning System (GPS) stations, part of the Pacific Northwest Geodetic Array (PANGA), constantly monitors the ground. These instruments can detect surface displacements of just a few millimeters. During a slow slip event, GPS stations on the coast temporarily reverse their normal direction of movement, shifting westward by up to 7 millimeters as the deep fault slips.
Tremor and stress
Slow slip events are not silent. They are accompanied by a faint seismic signal called non-volcanic tremor. This tremor is not the sharp jolt of a regular earthquake but a weak, continuous vibration with a low frequency, often compared to the rumbling of a freight train or the vibrations from a windstorm. The discovery of this tremor, and that its timing and location matched the GPS-detected slip, was a major advance. Canadian scientists Herb Dragert and Garry Rogers first identified this connection in 2002-2003, naming it Episodic Tremor and Slip (ETS).
The real concern with these silent events is how they affect the fault section directly above them. The upper, or shallower, part of the Cascadia fault is "locked." Here, the two plates are stuck together, building up enormous strain that will be released during the next great megathrust earthquake, which can reach magnitude 9.0. Each time the deeper part of the fault experiences a slow slip event, it transfers stress to this locked zone. The silent slip increases the shear stress on the locked segment, pushing it incrementally closer to failure. Some calculations suggest the probability of a large earthquake is 30 to 100 times greater during a slow slip event. These quiet, weeks-long adjustments deep underground show the immense pressures building toward Cascadia's next "big one."
💡Fun Facts
Slow slip events were first discovered in Cascadia around 2002 by scientists using a new network of high-precision GPS sensors to monitor crustal motion.
The non-volcanic tremor that accompanies slow slip is actually a swarm of thousands of tiny, low-frequency earthquakes, each too small to be located individually.
The network monitoring Cascadia, the Pacific Northwest Geodetic Array (PANGA), uses data from over 450 GPS stations to detect millimeter-scale ground movements.
Similar slow slip events have now been detected at other subduction zones around the world, including those near Japan, New Zealand, and Mexico.
