Slow earthquakes shift and slide without the shaking
If you live in a seismically active area, you probably overlook small earthquakes all the time. Smaller quakes occur all the time in areas like California, with 10 having occurred within 10 hours of this post being written, none of which exceeded a 2.1 magnitude. They are still detectable, because they’re near the surface and can be sensed by instruments like seismographs, even if no humans noticed them directly. However, scientists are realizing that earthquakes can take place much deeper in the earth, sometimes so gradually that traditional instrumentation might miss them.
Searching for subtle shifting
In 1999, some strange GPS readings from Vancouver Island and the Olympic Peninsula helped draw attention to what might be called “slow earthquakes.” Over the course of several weeks, the GPS stations shifted a half-inch against the larger tectonic movement in the area. This would have been perfectly normal for what you’d see with an earthquake at that location, except no seismic activity was ever recorded there. Without any shaking at all, it was hard to say that there had been an earthquake.
After confirming, repeatedly, that the equipment providing these readings was working correctly, geologists started looking deeper underground for an answer. They realized that there had been an earthquake, but the epicenter was 18 to 24 miles underground, deeper than the brittle crust that usually shakes and jumps in a quake. At that depth, the geology is a bit more of a sticky slush, not quite hard and crunchy, but not liquid enough to allow for perfectly smooth movement either. Things are just solid enough to grip and slip around every 14 months, but gently enough that it’s usually not a problem for people on the surface.
As this idea is being more carefully observed, we’re learning how varied a slow quake can be. Pressure gauges installed on the ocean floor near Japan in 2014 quickly detected a vertical heave of four to eight inches, spread out over the course of a few weeks. This movement was at a much more shallow depth than what had been seen in Canada, but otherwise fit the profile of a slower bit of seismic activity.
Slow quakes in the larger seismic system
The location of this movement was of some interest to researchers, as it had been the source of two tsunamis in 1947. The fact that significant movement was possible without the need for alarm along the Japanese coast in 2014 has helped built a hypothesis that these slower quakes might help prevent more dramatic tremors. The slow slips may be gentler because they dissipate built up energy over the course of weeks, rather than releasing it in one or two big bursts of destructive shaking. There’s also a chance that this kind of quake can offer predictive clues for other shaking, but a lot more data is needed before that can be confirmed. Beyond possible applications, geologists would like to figure why these quakes are so slow in the first place.
Source: Slow Earthquakes Are a Thing by Rachel Becker, Smithsonian