However, thankfully many of these go undetected because they hit remote areas or have such small magnitudes that they are practically imperceptible.
Now new European research, headed by the Karlsruhe Institute of Technology (KIT) in Germany, is investigating these subtle seismic rumblings, in the hope that they will give us clues about the behaviour of all types of earthquakes, including the more destructive earthquakes that occur at shallower depths.
The rumblings being investigated belong to the tectonic tremor, a less hazardous form of seismic activity that occurs far deeper into the earth's core than the devastating earthquakes that occur much closer to the earth's surface.
One major difference between tectonic tremor and earthquakes is that tectonic tremor causes relatively weak ground shaking and is not cause for immediate concern.
'Both earthquakes and tremor have the same cause. They result from the relative movement on fault surfaces, a result of the motion of the tectonic plates,' explains seismologist Dr Rebecca Harrington, who heads a research group at KIT. 'While earthquakes at our research site in California typically occur at depths of up to 15 km below the surface, tectonic tremor signals are generated at depths ranging from approximately 15 to 35 km.'
We know little about tectonic tremor because they were only discovered about a decade ago. This knowledge gap prompted KIT researchers to collect tectonic tremor seismic data in California, which is now being evaluated in order to better understand this relatively new seismic phenomenon.
Credit: NASA, DTAM project team/Wikipedia
While seismologists admit that we are still a long way from being able to predict earthquakes, we have however improved our ability to estimate the danger posed by earthquakes by understanding what happens on a fault during a seismic event.
According to Dr Harrington, research of tectonic tremor may play an important role in understanding fault behaviour: 'We understand very little about what happens on a fault when it ruptures. The tectonic tremor generated on the deep part of a fault may provide clues about the behaviour on the more shallow parts of a fault where more damaging earthquakes occur.'
California is an excellent place to start research as tectonic tremor was first detected in subduction zones in the Pacific Northwest in North America and in Japan. Subduction zones are the areas where one tectonic plate moves under another tectonic plate. Since their discovery seismologists have discovered that tremor occurs in many other places, including the San Andreas Fault in California. The San Andreas Fault marks the boundary where the Pacific Plate and the North American Plate drift past each other; during this process earthquakes are generated.
Back in mid-2010, KIT researchers working together with scientists from the University of California, Riverside, and the US Geological Survey, Pasadena, installed 13 seismic stations near Cholame, located approximately halfway between San Francisco and Los Angeles. Each seismic station was then equipped with a broadband seismometer in a thermally insulated hole in the ground, a small computer and a solar panel for power. The broadband seismometers used are extremely sensitive to small ground motions and were therefore considered ideal for detecting tremor and small earthquakes. The data recorded over a 14-month period is currently being analysed at KIT.
Work however began before the installation of the seismic stations. Because tectonic tremor signals have a unique character that differs from earthquakes, they were more difficult to detect using traditional automated techniques. To overcome this challenge, KIT researchers first developed a new algorithm for the automatic isolation of tectonic tremor. Using their new technique, they found over 2,600 tremor events that are now being studied in detail. 'In addition to detecting tremor, we will determine their size or magnitude of the individual events. In order to do so, each of the tremor events must be precisely located,' says Dr Harrington.
KIT geophysicists are also comparing the tremor and earthquake recordings in California with earthquake recordings at the Mount St. Helens volcano, located in the Cascadia subduction zone in the US state of Washington. Between October 2004 and January 2008 the Mount St. Helens volcano continuously erupted with a gradual extrusion of magma which resulted in the gradual building up of a new lava dome. This gradual eruption prompted a series of earthquakes on newly formed faults from which data was collected.
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