Athol Anderson, of Oamaru, asks :-

Does the Earth vibrate as a result of the total seismic, atmospheric, gravitational and human activity imposed on it?

Bob Hurst, a physicist working with the University of Canterbury's Ring Laser, responded.

Yes it does, although we usually refer to it in more specialized terminology. I work in the Cashmere Cavern under the Port Hills, in the University of Canterbury Ring Laser Project. (The February 2011 earthquake last year unfortunately closed us down for the time being.) Our ring lasers are large, extremely sensitive, rotation-detectors, and they are supplemented by seismometers. These instruments readily detect the "vibration" of the earth. We usually describe the disturbances in terms of seismic waves, but it's fine to call them low-frequency vibrations.

Canterbury people are by now very familiar with earthquakes. Quakes generate a rich mix of seismic wave types (P-, S-, Rayleigh, Love waves) that have different characteristics and propagation speeds. As felt, the quakes usually last only a few seconds, and even the large ones only 20-30 seconds or so. But the shaking and rotation as seen by the ring lasers and seismometers lasts much longer - many minutes. The instruments can easily detect large quakes at distances of thousands of kilometres. The recorded disturbances can last an hour! This is (mostly) not aftershock activity, but just the delayed arrival of seismic waves that have travelled sometimes complicated paths to get to the cavern. These include reflections from the boundary between the mantle and the core of the Earth, and even the ground surface on the other side of the Earth. Study of these waves is one of the main sources of information about the interior of the earth.

Weather plays a part. Practically anywhere on Earth, and at any time, sensitive instruments pick up a small continuous signal referred to as microseismic activity. The principal components as observed at Cashmere are the so-called double-frequency microseisms. They originate from waves (hence weather), and not from the surf at nearby beaches but out in the deep ocean. The seismic disturbances are small (a few microns or thousandths of a millimetre) and almost periodic. The double-frequency refers to the fact that they happen twice as fast as the ocean waves that generate them - their period is usually 5-6 seconds and the big waves 10-12 seconds. It required considerable ingenuity for the scientists of the 1950's to explain the frequency-doubling.

The effect of human activity is usually somewhat subdued, compared to these natural forces. Nearby heavy vehicles are sometimes detectable, but that disturbance is very localized. Blasting in quarries has sometimes showed up on our records. A striking example of what can (but we hope will not) be detected is underground nuclear testing. The Comprehensive Nuclear-Test-Ban Treaty Organization uses (amongst other sensors) a world-wide network of seismometers to monitor compliance. Two of the NZ network of seismic stations (Rata Peaks and Urewera) are designated as CNTBTO observation sites.