Slow-motion 7.0 earthquake drags on for 5 months under New Zealand

New Zealand's GeoNet, a public-private partnership that monitors the islands' seismic activity, has put up a blog post that describes a rather interesting event going on near the country's capital. Since January (and at last check, ongoing), the city of Wellington has most decidedly not been shaken by a magnitude 7 earthquake. And yet if you track the ground movement, that's about the size of the seismic energy that has been released.

How is that possible? We tend to pay attention to plate movements when they break things, especially when said things are entire cities. Those events occur when two plates get stuck as they try to move past each other and then release the built-up tension in a sudden lurch of motion. But not all tectonic shifts occur violently. Some plates move with such little friction that the resulting energy is released in ways that are barely perceptible.

Slow-slip earthquakes are somewhere in between. They tend to happen at subduction zones when plates get stuck as they slide past each other. But rather than releasing the energy in a sudden lurch, the tension goes through a period of smooth motion that can last for weeks or even months. New Zealand, which sits on top of a fault where the Pacific Plate slides under the Australian, has at least four regions that undergo slow-slip quakes, and all four of them seem to experience them very regularly, with periods ranging from two to five years between events. None of these are noticeable to the local residents but can be followed using GPS monitoring of ground locations.