Taxonomy term

For all the water stored in oceans, ice and other reservoirs at Earth’s surface, there’s likely even more in the planet’s interior, where it plays important roles in many geological processes, including the formation of magma and the lubrication of earthquake-producing fault zones. Uncovering just how much water is inside Earth — and the extent to which it moves back and forth between the surface and subsurface — has long been a challenge for scientists interested in understanding the planet’s water cycle. A new study peering beneath the Mariana Trench in the Western Pacific has revealed that some subduction zones might pull significantly more water into Earth’s interior than previously thought.

Geoscientists use the seismic waves produced by earthquakes to image the internal structure of the planet. Some seismic waves travel through liquids, while others do not, which is how we know, for instance, that Earth has a solid inner core and a liquid outer core.

Lapping waves or crashing surf may come to mind for most people when they imagine the sounds of the ocean. But the ocean has other voices as well, including one produced by interactions of waves with the seafloor along the continental slope. Unlike waves on the shoreline, this steady, low sound, or “hum,” is inaudible to the human ear and has even proven difficult to detect in recordings made by ocean bottom seismometers (OBS). But in a new study in Geophysical Research Letters, researchers analyzing OBS data have now clearly identified the hum for the first time, which may allow it to be used to develop a better picture of Earth’s interior structure.

On Nov. 14, 2016, a magnitude-7.8 earthquake struck near Kaikoura on New Zealand’s South Island, setting off a cascade of fault ruptures in the region. Within hours, seismic waves from the quake triggered a two-week-long slow-slip event on a section of the Hikurangi Subduction Zone between 250 and 600 kilometers north of the initial epicenter, as well as ongoing slow slip on the Hikurangi beneath New Zealand’s capital, Wellington, according to a new study in Nature Geoscience. Thanks to New Zealand’s advanced seismic and tectonic monitoring networks, the event is one of the best-documented examples of an earthquake triggering slow slip on distant faults.