New earthquake category Nature 447, 76-79 (3 May 2007) | doi:10.1038/nature05780; Received 8 December 2006; Accepted 26 March 2007. A scaling law for slow

New earthquake category

Nature 447, 76-79 (3 May 2007) | doi:10.1038/nature05780; Received 8 December 2006; Accepted 26 March 2007. A scaling law for slow earthquakes

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Add ambient noise seismology

Episodic slow-slip and tremor activity

Dragert et al., Science, 2001


• Contineous GPS sites in southwestern British Columbia and northwestern Washington state have been moving landward as a result of the locked state of Cascadia subduction fault offshore.

• In the Summer of 1999, a cluster of 7 sites briefly reversed their direction of motion.

• No seismicity was associated with that event.



• The sudden displacements are best explained by ~2 centimeters of aseismic slip over a 50-kilometer-by-300-kilometer area on the subduction interface downdip from the seismogenic zone.

• Such a rupture is equivalent to an earthquake of moment magnitude 6.7.

• This provides evidence that the seismogenic locked part of the subduction is loaded in episodic pulses.

Obara, Science, 2002


Hi-net is a newly established seismic network, with each station consists of 3-component velocity seismometer, installed at the bottom of a 100-200 m deep borehole and whose sampling rate is 100 Hz.


• Small amplitude tremors that lasted from a few minutes to a few days.

• The predominant frequency of tremors ranged from 1 to 10 Hz, and was lower than that of ordinary earthquakes of similar size.


• Raw seismograms were transferred to RMS amplitude.

• Note that the envelop shapes of the tremor were very similar at different stations.

• In contrast to ‘normal earthquakes’, the tremor envelopes had gradual rise times.


• The center of the distribution of tremors determined for 1 hour is shown.

• The tremors were distributed along the strike of the subducting Philippine Sea plate over a length of 600 km.

• The epicentral distribution of the tremors corresponded to the seismicity with the depth range from 40-45 km.

• No tremors have been detected around the Kii channel, nor in the Eastern Shikoku Island.


• Frequency of tremors in 1hr windows.

• Usually the activity continued for several days and sometimes for a few weeks.

• Some tremors seem to be triggered by a nearby earthquake.

• Activity sometimes ended right after a nearby earthquake.


• Tremor episodes did not always remain in one region, but migrated.

Rogers and Dragert, Science, 2003


• Slip events occur down-dip from the currently locked, seismogenic portion of the subduction zone with a repeat time of 13-16 mounts.

• Were not accompanied by earthquakes and were thought to be seismically silent.


• A tremor on an individual seismograph is unremarkable and does not appear different from transient noise due to wind or cultural sources.

• It is only when a number of seismograph signals are viewed together that the similarity in the envelope of the seismic signals become apparent.


• If the one-to-one correlation between transient slip and seismic signatures proves to be robust, then the tremorlike signal can provide a real-time indicator for the occurrence of slip.

• Because slip events on the deep slab interface increase the stress across the locked plate interface, it is conceivable that a slip event could trigger a large subduction thrust event.

Obara et al., GRL, 2004


• Four episodes of step-like tilt can be identified that coincide with the time of tremor activity in that region.

• A tilt step of up to 0.1 micro radian is too small to be recognized as a meaningful signal by using only tilt data.

• The consistency between the tilt change and the tremor activity indicates that the two are coupled/related geophysical phenomena.


• The tilt steps are not instantaneous, but change gradually over an interval of several days.

• The four episodes can be classified into summer and winter groups.

• Winter: N-S component started 3 days before the E-W component.

• Summer: the two components started simultaneously and the N-S component continued 2 days after the E-W component stopped.


• Tilt vectors for the first and second stages are plotted on the bottom panels.

• Winter: South down changes to southeast down

• Summer: Southeast down changes to south down.


• Additional tilt-meter data are available for the Aug. 2002 event.

• The crustal tilt deformations were not detected by the GPS monitoring system.


• The source area of the slow slip seems to migrate gradually with the propagation of tremors.

• The observed data can be explained by slow slips with dislocations of 3 and 0.7 cm on two successive reverse faults.

• The total corresponding moment magnitude is of 6.0

• The faults are located just above the dipping seismic zone of the subducting slab.

• The up-dip end of the slip episode corresponds to the source area of the tremors.

Nadeau and Guilhem, Science, 2009



Thomas et al., Nature, 2009


• Non-volcanic tremor represent a different class of earthquakes?

• Tremor seems to represent shear failure on a critically stressed fault.


Documents

New earthquake category Nature 447, 76-79 (3 May 2007) | doi:10.1038/nature05780; Received 8 December 2006; Accepted 26 March 2007. A scaling law for slow