Earth's 'Hum' Helps Probe Planet's Interior

Hi ya! Keeping out of mischief. hmmm? Wait...don't tell me! Too much information! Fill your coffee mug and grab one of those hum...mungous muffins, why don't'cha? Say...did you know that the Earth hums? Apparently it does.

The hum, called seismic noise, is generated by sources such as storm-driven ocean waves and is detectable everywhere on Earth. It could help scientists analyze the innards of the planet worldwide.

Traditionally, researchers peer into the interior of the Earth by analyzing seismic waves generated by earthquakes. The way seismic waves zip through the planet depends on physical properties of the Earth's innards, such as rock composition, temperature and pressure. As such, the way the waves behave offers useful clues about details of Earth's geology that are otherwise largely hidden from view.
With these waves, seismologists produce images in a way similar to medical imaging.

The problem with this strategy is that it depends on earthquakes. Large earthquakes are rare. Quakes also mostly recur in specific places, which leads to some areas being imaged well but leaving others relatively obscure. In addition to seismic waves from earthquakes, the interior of the Earth is pervaded by seismic noise, a collective hum resulting from the bombardment of Earth's surface by a variety of sources, such as the swelling of oceans during storms. Seismic noise was regarded as useless and even problematic since it hides slight earthquake signals.

However, in recent years, by analyzing large amounts of seismic data collected over time, investigators successfully followed ambient seismic noise waves as they rippled across Earth's surface. Now scientists reveal they can also use ambient noise to image Earth's deep interior. The advantage of this strategy is that "ambient noise imaging can be applied in regions without earthquakes.

Using this data, the researchers imaged the transition zone separating the upper and lower layers of the Earth's mantle, the main layer just below Earth's crust. The top of the mantle was about 9 miles (15 kilometers) thick and 255 miles (410 km) from the Earth's surface, while its bottom was about 2.5 miles (4 km) thick and 410 miles (660 km) from the Earth's surface. The differences between top and bottom are due to changes in crystal structure resulting from how pressure varies according to depth.

Scientists hope that ultimately, ambient seismic noise will also probe all the way down to the core-mantle boundary. Personally, all I need to know is that this spinning, orbitting planet of ours causes gravity and that keeps us from flying off into deep, dark space. Though the more we know about what happens down in the core, the better equipped we might be to deal with anything Earth throws at us...and it sure does throw some nasty things our way, doesn't it?

See ya, eh!

Bob