Seismic Tremor: Rumbles Without the Jolts
November 18, 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.
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)