Microseisms – Faint Earth Tremor
We may not be aware of it but the Earth is continuously oscillating thereby creating a hum and it is also known that the earthquakes and its seismic activity make their contribution to this hum.
In seismology, microseisms is defined as a faint earth tremor that is caused by natural phenomena which is used to refer to the dominant background seismic and electromagnetic noise indication on Earth that are the result of water waves in oceans and lakes.
Microseisms are detected and measured by means of a broad band seismograph which can be recorded anywhere on Earth. The short waves crash near the surface, creating weak microseismic waves that tend to combine with long and more powerful waves dragging over the ocean’s floor causing the constant hum.
However in the late 1990s it was discovered that the Earth does not oscillate just after the quakes but tends to vibrate at extremely low frequencies at all time. Microseismic activity that is recorded on the Earth is due to ocean waves and recent developments clearly indicate sources of microseisms in the energetic band within the periods from 3 to 10 s.
Ocean waves offer most of the energy which tends to feed the continuous vertical oscillations of the Earth in which period bands are normally recognised.
Two Mechanisms – Primary/Secondary
Two mechanisms are proposed to explain seismic wave generation – a primary mechanism wherein ocean waves circulating over the bottom slopes tend to generate seismic waves and a secondary mechanism which tends to rely on the nonlinear interaction of ocean waves.
It is shown here, that the primary mechanism clarifies the average power, frequency distribution where most of the variability in signals recorded through vertical seismometers for seismic period ranging from 13 to 300 s.
The secondary mechanism tends to explain seismic motion with periods shorter than 13 s and results build on quantitative numerical model which provides access to time varying maps of seismic noise bases.
The hum comprises of periods longer than 30 s while the primary and secondary peaks are focused around 15 and 5 s and the motions in bands are recorded on the planet provide the information on the earth structure, the ocean wave climate over the past century as well as the properties of short period ocean waves, though the use of seismic data could be limited since how and where ocean waves that rock the Earth is not clear especially for the hum.
The least understood part of Earth’s oscillation is the hum and the recently learned in seismometer records has been associated with ocean waves for periods shorter than 300 s.
Ocean Waves Cause of Constant Hum
A team of Japanese and Californian seismologist had suggested in 2004 that ocean wave were the cause of this constant hum and proposed that as the ocean waves moved in opposite directions and collided, it created the microseismic activity called Microseisms.
Fabrice Ardhuin, senior research scientist at Centre National de la Recherche Scientifique in France together with his team decided to put the theory to the test.
They started by researching and measuring seismic activity across the globe which included storms, earthquakes as well as the movements of ocean waves, throughout 2008. The two seismic during this time with magnitudes of 6.9 and 7.1 had taken place on 3 March and 20 March.
The largest hum happened together with the strongest seismic waves at the time of the Johanna storm on 10 and 11 March and generated waves with peak period of 16 seconds which reached the heights of up to 42 ft.
A spectrogram found energy level reach 10 mHz during the storm and lasted for 36 hours and on observing how much energy was generated by the storm, the researchers concluded that the hum was caused by the storm and was not the remnants of seismic activity from the events.
They got the understanding from the conclusion that `ocean waves provided major part of the energy which fed the continuous vertical oscillation of the solid Earth’ according to the researchers.
With the 2008 data, Mr Ardhuin together with his team noticed that ocean waves colliding generated seismic activity though these waves took only 13 seconds or less in completing a single oscillation.
They researched the movement of ocean waves at the bottom of the seabed and these waves are known as long since they travel from the floor to the coast and back again and as they travel over the ocean floor they tend to meet resistance as they pass over the steep continental shelves helping them to create long seismic waves which oscillate much slower.
This indicates that all these theories – earthquakes, colliding oceans and long waves are responsible in creating the Earth’s hum where the majority is from the latter.
These findings were printed in Geophysical Research Letters which is a journal of the American Geophysical Union – AGU. Mr Ardhuin had informed that on measuring and understanding long microseismic waves, they could learn more on how the Earth is structured and that they now know where the hum comes from and what can be done with it.