The Earth beneath your feet is \u201chumming\u201d all the time. Typically these vibrations are too faint and low-frequency for your ears to hear. But they can be detected by seismometers, the instruments designed to study the generally more powerful vibrations that emanate from earthquakes.<\/p>\n
Now researchers have used an array of seismometers in Japan to show that a group of tremors they detected had their origin in a violent \u201cweather bomb<\/a>\u201d storm on the other side of the planet off the coast of Greenland. There\u2019s a danger that this research could be misreported as an Atlantic storm causing an earthquake in Japan. The reality is that the Japanese scientists detected an intensification of the usual background hum. But these vibrations could prove useful in helping us to study the structure of the planet.<\/p>\n The Earth typically hums slowly, with most energy transmitted at about ten seconds per vibration. But this is mixed up in a noisy continuum of overlapping \u201cbackground noise<\/a>\u201d, vibrations lasting from less than a second to half a minute each. These come from many sources including ocean waves in general, weak earthquakes deep within the Earth, and the planet creaking as it is deformed by tides.<\/p>\n When researchers Kiwamu Nishida and Ryota Takagi analysed the Earth\u2019s hum on December 9-11 2014, as recorded by Japan\u2019s \u201chi-net\u201d array of seismometers<\/a>, they realised<\/a> they were picking up some unusual signals. By working out the direction and distance the vibrations had travelled, the researchers back-tracked them to their source and showed that they came from storm waves shaking the shallow, sloping sea-floor off the south-east coast of Greenland. These waves were particularly violent because the local atmospheric pressure at the time was plummeting, creating a so-called \u201cweather bomb\u201d.<\/p>\n This was the perfect storm to set up pressure waves that resonated between the sea surface and the sea floor, passing their energy to corresponding vibrations in the bedrock that could be detected as far away as Japan. Nishida and Takagi do not say they had detected earthquakes that were caused by the storm. They were well aware that this was just an intensification of the usual background hum.<\/p>\n The vibrations caused by the Greenland storm are not an earthquake. Most naturally-occurring earthquakes<\/a> happen near the boundaries between the tectonic plates into which the Earth\u2019s rigid outer layer is divided. The plates are moving relative to each other at speeds of a few centimetres per year, but at the fault surfaces where one plate grinds past another the motion is not smooth. Friction and irregularities bind the two sides together until enough strain has been built up to overcome the resistance. The fault then gives way in a near-instantaneous jolt \u2013 much more powerful than the humming caused by storm waves that the Japanese researchers found.<\/p>\n Tectonic earthquakes are not restricted to plate boundaries, however. They can occur, usually less powerfully, when ancient faults move a little, or the Earth\u2019s crust adjusts to the changing load of sediment upon it. A recent example was the magnitude 4.2 earthquake<\/a> in Kent, England in 2015.<\/p>\nWhy not an earthquake?<\/h2>\n