July 6, 2012
Deep Tectonic Tremors Provide Clues About Shallow Earthquakes
Brett Smith for redOrbit.com - Your Universe Online
Although the earth is shaken by approximately 80,000 earthquakes every month, not many of them will send you running for the nearest doorway.However, scientists, led by a team at Karlsruhe Institute of Technology (KIT) in Germany, have recently started investing more time and research into understanding these subtle vibrations that occur deep in the Earth´s crust.
More specifically, these researchers are interested in tectonic tremors–a newly discovered type of seismic signal. They expect these tremors to give clues about types of earthquakes, including the more destructive earthquakes that take place at shallower depths.
“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,” said seismologist 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,” she said.
Because they were only discovered about a decade ago, there is very little known about tectonic tremors. They were first detected in Japan and have also been identified across the Pacific in northwestern North America. Since then, seismologists have measured tremors in many other places, including the San Andreas Fault in California.
Perhaps because of its historical background, the KIT researchers decided to collect tremor data along California´s infamous fault. In 2010, the German researchers–working in collaboration with scientists from the University of California, Riverside, and the US Geological Survey (USGS) in Pasadena–installed 13 seismic stations near Cholame, located about halfway between San Francisco and Los Angeles.
Each seismic station was comprised primarily of a broadband seismometer nested in an insulated hole in the ground, a small computer, and a solar panel for power. The seismometers used by the team are extremely sensitive to small ground motions and are ideal for detecting tremors and smaller earthquakes. The data recorded over a period of 14 months from 2010 to 2011 are presently being analyzed at KIT.
The subtle tectonic tremor signals present a unique challenge to researchers looking to understand the vibrations using automated techniques. In order to address the detection problem, the KIT researchers first wrote a new computer algorithm for the automatic isolation of tremor signals. Using their new technique, they found over 2600 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,” said Harrington.
Despite the latest advances in technology, seismology is still a long way from being able to predict earthquakes. However, seismologists hope that studying tremors will allow them to better estimate the danger posed by earthquakes by understanding what happens on a fault during a seismic event.
“We understand very little about what happens on a fault when it ruptures,” Harrington said. “The tectonic tremor generated on the deep part of a fault may provide clues about the behavior on the more shallow parts of a fault where more damaging earthquakes occur.”