Archives for: November 2013
Anybody who has ever felt an earthquake will remember distinct jolts during the shaking. In strong temblors these sudden jolts can be sharp enough to throw a person off balance. In seismological terms they represent the arrivals of unique wave trains, like P- or S-waves, which are generated by the rupture in the earthquake source. Measuring and interpreting these onsets, as scientists call the jolts in a rather benign fashion, has been the bread and butter for studying the physics of earthquake sources and the structure of the Earth's interior for a very long time (see blog from January 1st 2012). That is why a discovery just over a decade ago surprised many seismologists. Japanese researchers found signals in the recordings of their dense sensor networks which had no onsets at all. Instead of producing a series of sharp peaks - the jolts - these recordings showed a gentle beginning and then rumbled on for minutes or even hours. In the end, the signal tapered off as sneakily as it began. Nowhere in those recordings could the researchers identify clear arrivals of distinct wave trains.
|Local earthquakes and tremor recorded at Tremorscope station TCHL. The top trace shows two small earthquakes (M2.1 and M1.8) which occurred just north of Parkfield, about 33 km from the station. The bottom trace (bottom trace) shows a 40 minute interval of tremor closer to the station but at a depth of 31 km. The data processing for both traces is the same.|
Similar signals were observed almost 90 years ago on Japanese volcanoes. There they were called volcanic tremor. Their cause: The movement of a fluid within the volcanic edifice, be it magma, hot water or steam, generates a murmur similar to the gurgling of a creek. But the new discovery had nothing to do with volcanoes. The murmur came from deep within the subduction zone east of Japan, where the Pacific plates dives into the Earth's mantle. Hence this type of signal got the name "non-volcanic" or "seismic" tremor. In the years that followed similar signals were observed in the Cascadia subduction zone off the coasts of Oregon and Washington. In 2005 two researchers at the Berkeley Seismological Laboratory, Bob Nadeau and David Dolenc, were the first to discover such seismic tremor in a strike slip fault. They identified the strange signal coming from deep along the San Andreas Fault just south of the tiny hamlet of Cholame in Central California.
In contrast to its volcanic sibling, the origin of the seismic tremor is by no means understood. It is possible that fluid deep below the brittle zone of an earthquake fault may play a role. But it was also observed that tremor episodes can be triggered by passing waves from other earthquakes. Looking through the vast archive of seismic recordings at the BSL, researchers found tremor episodes under Cholame which coincided with passing waves from strong local earthquakes, like the M 6.5 San Simeon quake of 2003 or the Parkfield earthquake of 2004 (M=6). Similarly, such tremor has been triggered by the waves of the strong M 9.0 Tohoku Earthquake, which struck Japan in March 2011 (see blog March 11, 2011 and the following).
Using the various sensors which are being installed in the Tremorscope network - described in the last blog entry - researchers hope to shed more light on these elusive rumbles and murmurs of the deep sections of the San Andreas Fault. (hra089)
|The drilling rig at work. (photo courtesy of Horst Rademacher)|
Drilling holes into the ground has never been a clean business. But operating a multi-ton drill rig on a windy ridge far from the amenities of everyday life takes special dedication. Grey drill-mud spatters out of the well. The wind scatters the mud all over the place and covers the drillers and their machines in thick, mucky goo. The noise of the machinery is deafening and the water to keep the mud circulating in the well has to be trucked in from miles away. The hole on very remote ranch land in northeastern San Luis Obispo County, that a Livermore based company has just started to drill, is the beginning of a new phase for studying the seismicity along the San Andreas Fault in Central California.
Under a contract from the BSL, the company will drill four 1000 ft. deep holes along the fault. These holes are part of a major research program called Tremorscope. Financed mainly by a grant from the Gordon and Betty Moore Foundation, Tremorscope will allow BSL scientists to investigate a different and hitherto poorly understood type of rumbling of the Earth called seismic tremor. When completed, each hole will be equipped with an array of sensors, all capable of measuring the slightest movement of the ground in various frequency ranges.
Most seismic sensors are put into action by installing them in small concrete vaults at the Earth's surface. Hundreds of these tiny vaults dot the landscape of California, especially along the major fault lines. But even far from any signs of civilization, like roads, railways, farms and factories, these surface sensors are so sensitive that they can still pick up movement of the ground unrelated to earthquakes and faults. One cause of what we call seismic noise is the lovely soughing of the wind in trees. It disturbs seismic measurements because the trees transmit the rustling of the wind through their roots into the ground, causing the Earth to shake ever so slightly. Seismometers easily pick-up this surface noise, which can mask the slight shaking of the ground caused by activity on a fault.
|As remote as you can get in Central California... (photo courtesy of Horst Rademacher)|
Going deep underground with the seismic sensors reduces such surface noise and as a consequence, their recordings of seismic signals are clearer. In addition, the deeper one drills into the Earth, the more solid the rock becomes. Seismic waves are much less attenuated and scattered by competent rock than by the loose sediments generally found near the surface. This also makes for a cleaner seismic signal and hence a better chance to study seismic activity.
Within the framework of Tremorscope, the four new boreholes will augment a network of four modern seismic stations, which were recently installed in surface vaults along both sides of the San Andreas Fault south of the tiny hamlet of Cholame. Analyzing the recordings of the eight stations together with the data from a multitude of existing sensors in the area, scientists hope to shed more light on seismic tremor. In our next blog, we will describe what is already known about this unique and elusive seismic signal. (hra088)