SCIENTISTS have developed a new technique to monitor movements beneath the Earth's surface, helping them better understand how earthquakes behave.The team, led by scientists from the University of Edinburgh, says that the new method, which uses data collected from earthquakes, potentially allows the Earth's seismic activity to be mapped more comprehensively.Scientists currently monitor underground movements, such as earthquakes and nuclear tests, using seismometers ' instruments that measure the motion of those events at the Earth's surface. This helps to indicate where they took place.Now, by analysing the seismic waves from two different earthquakes, the team has been able to simulate the seismic waves from one of the earthquakes as if they were recorded by a seismometer at the location of the second.The discovery allows earthquakes themselves to be used as virtual seismometers that record passing waves from tremors that happen elsewhere in the world.Using earthquakes in this way substantially increases the number of locations that could be used to detect seismic activity. And since earthquakes occur deep inside the Earth, using them also allows scientists to monitor seismic activity from far deeper than previously possible.The research, published in Nature Geoscience, was carried out in collaboration with the British Geological Survey and Utrecht University.Indeed, the Earth wobbles. Like a spinning top touched in mid-spin, its rotational axis fluctuates in relation to space. This is partly caused by gravitation from the sun and the moon. At the same time, the Earth's rotational axis constantly changes relative to the Earth's surface.On the one hand, this is caused by variation in atmospheric pressure, ocean loading and wind. These elements combine in an effect known as the Chandler wobble to create polar motion.Named after the scientist who discovered it, this phenomenon has a period of around 435 days. On the other hand, an event known as the 'annual wobble' causes the rotational axis to move over a period of a year. This is due to the Earth's elliptical orbit around the sun. These two effects cause the Earth's axis to migrate irregularly along a circular path with a radius of up to six metres.Capturing these movements is crucial to create a reliable coordinate system that can feed navigation systems or project trajectory paths in space travel. 'Locating a point to the exact centimeter for global positioning is an extremely dynamic process ' after all, at our latitude, we are moving at around 350 metres to the east per second,' explains Prof. Karl Ulrich Schreiber who directed the project in TUM's Research Section Satellite Geodesy.The study titled: 'How to detect the Chandler and the annual wobble of the Earth with a large ring laser gyroscope' was published in Physical Review Letters a journal of American Physical Society.The orientation of the Earth's axis relative to space and its rotational velocity are currently established in a complicated process that involves 30 radio telescopes around the globe. Every Monday and Thursday, eight to twelve of these telescopes alternately measure the direction between Earth and specific quasars. Scientists assume that these galaxy nuclei never change their position and can therefore be used as reference points. The geodetic observatory Wettzell, which is run by TUM and Germany's Federal Agency for Cartography (BKG), is also part of this process.In the mid-1990s, scientists of TUM and BKG joined forces with researchers at New Zealand's University of Canterbury to develop a simpler method that would be capable of continuously tracking the Chandler wobble and annual wobble. 'We also wanted to develop an alternative that would enable us to eliminate any systematic errors,' continues Schreiber. 'After all, there was always a possibility that the reference points in space were not actually stationary.' The scientists had the idea of building a ring laser similar to ones used in aircraft guidance systems ' only millions of times more exact. 'At the time, we were almost laughed off. Hardly anyone thought that our project was feasible,' says Schreiber.
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